Nanobubble generation and its applications in froth flotation(part Ⅳ): mechanical cells and speciall

Availableonlineatwww.sciencedirect.comMININGScience DirectSCIENCE ANDTECHNOLOGYMining Science and Technology 20(2010)0641-0671www.elsevier.com/locate/jcumtNanobubble generation and its applications in froth flotation(part I): mechanical cells and specially designedcolumn flotation of coalFAN Maoming, TAO Daniel", HONAKER Rick, LUO ZhenfuDepartment of Mining Engineering, University of Kentucky, Lexington KY 40506, USACPT, Eriez Manufacturing Co, Erie PA16514,USASchool of Chemical Engineering and Technology, China University of Mining Technology, Xuzhou 221116, ChinaAbstract: Coal is the worlds most abundant fossil fuel. Coal froth flotation is a widely used cleaning process to separate coal frommineral impurities. Flotation of coarse coal particles, ultrafine coal particles and oxidized coal particles is well known to be difficultand complex. In this paper, the nanobubbles' effects on the flotation of the varying particle size, particle density and floatabilitycoal samples were evaluated using a bank of pilot scale flotation cells, a laboratory scale and a pilot scale specially designed flota-tion column. The parameters evaluated during this study include the flow rate ratio between the nanobubble generator and the con-ventional size bubble generator, the superficial air velocity, collector dosage, frother concentration, flotation feed rate, feed solidsconcentration, feed particle size, and the superficial wash water flow rate, etc. The results show that the use of nanobubbles in abank of mechanical cells flotation and column flotation increased the flotation recovery by 8%-279 at a given product gradeNanobubbles increased the flotation rate constants of 600-355, 355-180, 180-75, and 75-0 microns size coal particles by 98.0%,98.4%, 50.0% and 41.6%, respectively. The separation selectivity index was increased by up to 34%, depending on the flotationfeed characteristics and the flotation conditionKeywords: coal; froth flotation; cavitation; nanobubble: flotation kinetics1 Introductionbubbles. froth flotation is known to be the most efficient and the most cost-effective fine particle separa-With the introduction of automation and moderni- tion techniques. However, it is efficient only forzation of mining methods, fine and ultrafine coal par- ticles within a narrow size range. Flotation efficiencyticles content has been increasing, which now makes decreases sharply with ultrafine and relatively coarseup more than 20% of coal shipments by weight. Utili- coal particles. It is also well known that froth flota-ties and coal companies need more-effective flotation tion does not work well with oxidized, low rank orprocesses to remove mineral impurities from fine coal high clay content coals. Recent studies have shownparticles and recover the high-quality coal fractions that the low flotation efficiency of ultrafine particlesthat often lost during conventional coal separation is mainly due to the low probability of bubble-particleprocesses. On the other hand, when coal is ground collision -. Enhanced flotation of fine and ultrafineultra fine(below ten microns), almost all mineral coal can be achieved by increasing collision and at-impurities are liberated from coal,s molecular matrix. tachment probabilities and decreasing the detachmentThe successful flotation of ultra fine coal or diffiprobability. Recent studies have shown that the flota-ult-to-float coal particles will greatly promote en- tion process efficiency can be significantly enhancedergy conservation and environmental protection.by use of microbubbles or nanobubbles to increaseFroth flotation is often used in the coal industry to the collision and attachment probability and decreaseeparate minus 0.6 mm or minus 0.15 mm coal fines the detachment probability-21. Extensive stucby floating coal particles away from the associated have proven that fine bubbles have great impacts onmineral matter in the presence of finely dispersed air gas holdup in flotation column, which is very esseneceived 11 January 2010: accepted 22 March 2010中国煤化工 eral based processCorresponding author. Tel: 1 8592572933- can significantlyE-mailaddress:man@eriez.com:dao@engr.uky.eduCNMHGconsNanobubbles are characterized by an inherently highMining Science and TechnologyVol 20 No, 5robability of collision with particles, a high prob- conventional froth flotation than the particles within aability of attachment, and a low probability of de- particle size range between 10 um for minerals (50tachment due to their tiny size, low ascending veloc- um for coal)and 100 um(for coal 500 um)becauseity and rebound velocity from the particle surface, of their lower inertia, which enables them to followand therefore, are very effective in enhancing flota- the hydrodynamic streamlines and avoid being cap-tion recovery of fine/ultrafine particles Nanobubbles, tured by the bubbles. Particles larger than 100 um foralso known as picobubbles in Lature, refer to tiny minerals(for coal 500 um)are difficult to float due tobubbles mostly finer than a few hundred nanomethe disruption of bubble-particle aggregates in theters". They can be produced using ultrasonic or hy- turbulent field in the flotation vessel and the difficultydrodynamic cavitation method627-30).Nanobubblin transferring particles from the collection zone togenerated by cavitation preferentially form on the the froth zone. The upper size limitation mainly desurface of hydrophobic particles, i.e., coal in coal pends on the probability of detachment, Pp.Previousflotation. This is because 1)work of adhesion Wa studies have revealed the following generalities: 1)between a solid particle and water, which is equal to Pp increases with an increase in particle size andA(l+cose( is water surface tension and 8 is con- density: 2)The maximum particle size that can betact angle), is al ways smaller than work of cohesion floated is a function of the degree of hydrophobicityof water We or 2/4. 2)Wa decreases with increasingsolid surface hydrophobicity measured by contactLower limitation due to Pc Upper limitation due toangle. In other words, the cavitation process can render selective adhesion of air bubbles to hydrophobicparticles without bubble-particle collision that is nec器essary for conventional flotation process. This hasprofound impacts on the flotation process efficiency,especially for ultrafine particles, because the collisionm mineralsbetween ultrafine particles and bubbles is often therate determining step in flotation 31-32. In addition,101001000nanobubbles generated on coal surface serve as aecondary collector, improving the probability of ad-hesion and reducing the need for the hydrophobizingFig 1 Effect of coal and mineral particle density onne upper particle size limit and the lower particlechemical or collectorsize limit of the effective flotation( 3This study was carried out to investigate the feasi-ing coarse particles, fine/ultrafine par-Particle size significantly affects the stability ofticles and difficult-to-float coal particles by incorpo- bubble-particle aggregates. Schulze developed a therating nanobubbles in mechanical cells flotation and ory on the upper particle size of floatability from thecolumn flotation Nanobubbles were produced using theoretical and experimental investigations on stabilpecially designed hydrodynamic cavitation nano- ity of bubble-particle aggregates in flotation. Bybubble generators. Process parameters such as super- considering the kinetic energy of aggregates in theficial air flow rate, slurry split ratio between the turbulent field of a flotation machine, the upper partinanobubble generator and conventional bubble gen- cle size limit for flotation is dictated by the resultanterator, collector dosage, frother concentration, feed of forces acting on a bubble and particle aggregaterate and feed solids concentration, etc, were studied such as gravity, buoyancy, hydrostatic pressure, cap-for their effects on froth flotation recovery, product illary compression, tension, and shear forces inducedgrade, flotation kinetics, and flotation selectivityby the system. Schulze developed two simple ap-proximating equations comprised of solid density,2 Scientific discussionfluid density, surface tension of the liquid, contactangle and a parameter depending on energy dissipation"machine acceleration"for the calculation of theParticle density, particle size and surface hydro-phobicity play important roles in coal and mineral upper particle size of floatability under the turbulentfroth flotation. Particle size fractions that are effec- hydrodynamic conditions in a flotation machine(371.Atively treated using froth flotation vary among the dimensionless characteristic number analogous to thedifferent minerals based on the solid density of the bond number is introduced in order to characterizefloatable material as shown in Fig. 1 351. For example, Schulze, Bensley and Nicol show that turbulent con-the effective flotation particle size fraction for min.he drastically reduceeral particles (p5.0 g/cm)is from 10 to 100 um,中国煤化工materialwhile the particle size fraction for coal (P16-1.3g/cm)is from 50 to 500 um. Particles finer than 10 PeCNMHGrbulent conditionsum for minerals(50 um for coal)are less suited to of that found in non-turbulent conditions 40-41. How-FAN Maoming et alNanobubble generation and its applications in froth flotation.ever, Soto et al. speculated that the poor recovery of that nanobubble increased P2Os recovery by up tocoarse material is strictly a result of detachment be- 10%-30% for a given Acid Insoluble(A I )rejection,cause small particles have a higher rate of flotation depending on the characteristic of phosphate samplesand crowd out coarse particles from the surfaces of The improvement effect of nanobubble on thethe air bubbles. Honaker et al. and Patwardhan et hard-to-float particles was more significant than thatal. studied the product mass flux under carrying-ca- on the easy-to-float particles, especially at lower col-pacity limited conditions 43-44. A decrease in the lector dosages. Nanobubbles reduced the collectorroduct mass flux under a carrying-capacity limited dosage by 1/3 to 1n2 Nanobubbles almost doubleddetachment of the coarsest particles, thereby causing creased the flotation selectivity index by up to 258 Dcondition is considered as a result of the selective the coarse phosphate flotation rate constant and irthe concentrate to become finer and, hence, resultingStudies show that tiny bubbles generated by hy-in a reduced product solids rate. They further advo- drodynamic cavitation were found to change the sur-cate the use of separate circuits for fine and coarse face characters of minerals, increase contact angle ofprocessing in an effort to optimize the conditions solids and hence attachment force, bridge fine partinecessary for increased recovery.cles to form aggregates, minimize slime coating, re-The top size of particles that can be recovered by move oxidation layers on particle surfaces, andflotation depends on the combination of mineral consequence reduce reagents consumption5-16l. Fig2properties and flotation devices. Turbulence in flota- schematically shows that nanobubbles on particletion devices is one of important factor that affects the surface activate flotation by promoting the attachmentstability of bubble-particle aggregate. Even in the of larger bubbles since attachment between nanobub-flotation of fine particles, the bubble-particle detach- bles or gas nuclei and large bubbles is more favoredment can significantly influence the kinetics of flota- than bubble/solid attachment. In other words,nano-tion taking place in mechanical cells by intensive bubbles act as a secondary collector for particles, re-turbulent agitation). There are two contradictory ducing flotation collector dosage, enhancing particleoperation goals for conventional flotation cells 42 1) attachment probability and reducing the detachmentconducting enough agitation to maintain particlesprobability. This leads to substantially improved flo-suspension, shear and disperse air bubbles, and pro- tation recovery of poorly floating fine and ultrafinemote bubble-particle collision; 2)providing a quies- particlescent system for optimal recovery to reduce detach-ment and minimize entrainment Coarse particle flctation is more difficult because increased agitation isrequired to maintain particles in suspension andcoarse particles are more likely to detach under turbulent conditions 46In the part I of this series of papers, the test resultdepicted that nanobubbles can reduce the conven-tional-sized flotation bubble rinsing velocity, increasethe bubble-particle sliding time, decrease the tangential velocities of particle sliding on bubble surface,and thus increase the bubble-particle attachmentprobability and decrease the detachment probabilityFurther research results in the part II of this series ofFig. 2 Schematic illustration of nanobubble's effectpapersindicated significant recovery improvements offine and ultrafine particle froth flotationcoal and phosphate particles froth flotation could beachieved using cavitation generated nanobubblesl8. More efficient attachment of particles and im-particles was attributed to the nanobubbles selectively bubbles co-exist with conventional-sized flotationgenerated on or attached to the coal and phosphate bubbles. The potential of cavitation generated pi-particles which increased the surface hydrophobicity, cobubble in improving fine particle froth flotationelevated the probabilities of coal and phosphate parti- recovery has been demonstrated by tests on fine silicacle-bubble collision and attachment, and reduced the (-5 um)and precipitated zinc sulfide!ol. The advan-probability of detachment. The test results published tage of cavitation generated nanobubble incorporatingin the part Ill of this series of papers demonstrated with conventional-sized bubble in flotation has beenthat significant recovery improvement of coarse expl中国煤化工ntribute to the in-phosphate flotation was achieved using cavita- creasthe tiny bubblestion-generated nanobubble though its effects differ forC N M H Grticles may causeamong the four testing phosphate samples. The flocculation by a bubble-bndging mechanism, result-laboratory-scale flotation column test results indicate ing in increasing the collision probability with theMining Science and TechnologyVol 20 No 5bubbles;b)particles frosted with nanobubbles may 3.2.3 Ash analysispresent a surface favorable to attachment to flotationCharacterization of the coal sampleized bubblesformed by taking representative samptested samples to be prepared and pulverized at minusperimental150 um and then subjected to analytical analyses forash content3. 2. 4 Release analysisRelease analysis is the procedure used to obtainThree coal samples, A, B and C, were acquired best possible separation performance achievable byfrom three different coal mines in West Virginia and any froth flotation process. As such, the performanceused for a bank of three 10 L mechanical cells, 50.8 of a flotation process can be monitored and improvedmm in diameter column, and 152 mm in diameter with the idea to approach the theoretical flotation re-column flotation tests, respectively. Dry coal samples sponse curve given by the release analysis curve for aa and C were crushed and screened into varying size certain feed material. This goal is analogous to thefractions for mechanical cells and 152 mm in diame- gravity-based washability analysis. The releaseter column flotation tests. Sample B was coal slurryanalysis test was carried out in a conventional labo-3.2 Coal sample analysisratory flotation cell and conducted in two-phases with3.2.1 Particle size distribution analysisdistinctly different goalshe first stage separates the hydrophobic materialThe particle size distribution of the three tested away from the hydrophilic material by doing multiplecoal samples was determined by wet sieve: the sepa- cleaning phases from the initial feed. The sample wasration of fines from the coarse portion of the sample introduced in a laboratory Denver flotation cell of 5Lwhile suspended in water solution introduced to a capacity at a 5% feed solid content by weight. Aftertesting sieve. The water medium was used to negate mixing collector(Fuel Oil No. 2)and frother(Shurstatic charges, break down agglomerates and lubricate Flot 948)was injected at minimum doses. The fuel oilnear-size particles. After the fines have been washed was added and conditioned for 2.5 minutes prior tothrough the sieve, the residue is oven-dried and re- the addition of the frother. Flotation was continuouslyweighed3.2.2 Density fraction analysisperformed in phase I to float all the hydrophobic ma-terial from the cell. when needed, frother was addedThe density fraction test was used to developto allow a continuation of flotation. The product waswashability curve to estimate the theoretical maxi- continuously collected in a separate container. Whenmum mass yield and recovery expected while all of the floatable material was collected, the re-achieving a given product grade for a certain coalmaining material was placed in a separate containerThe separation performances predict the theoretical which was labeled as tailings. The floated materialperformance, which means the recovery achieved was placed back into the flotation cell and refloatedwhile producing a given product quality can not benisprocess was repeated three times to ensure regreater than the recovery achieved by density frac- move of all the hydrophilic particles from the hydro-tionationThe density fraction tests were performed on the The second stage has the goal of separating parti-cles into fractions of degrees of surface hydrophobeused as the heavy liquid to increase the density of the ity. For coal, the more hydrophobic coal contains thesolution( specific density of 2.95 RD) with distilled least amount of ash-forming material with the ashwater. The required medium density (i.e, 1.3, 1. 4, 1.5, content increasing with a decline in hydrophobicity1.6 1 8 and 2.0 RD)was achieved by mixing LMT Fractionated samples are obtained by controlling airflow rate and rotator revolutions under starvationdure followed the ASTM D4371 standard. The coal conditions. Lower air flow rates and rotator revolu-was submerged in the lowest density medium first. tions lead to higher floatable particles reporting firstin the medium, the floated material was removed us- 1). Then an incremental increase in air flow rateing a hand held screen. The material that sank to the and/or rotator revolution was allowed to obtain thebottom was recovered by filtration. The sink material next most floatable particles, which contained a largerwas then submerged in the medium with the nexthighest density. This process was repeated through all in air rate or rotati, ntrate 2). A progressive increaseamount of ash(conceof theeed assures the total flotationrom 1.3 toof2.0 RD. The float products and final tailings were中国煤化工 particles by a fracrinsed, filtered, dried and weighed. Each sample wasessCNMHGan float The sam-analyzed for ash content using the ASTM D3172ple were titered, dried and weighed followed bystandard procedureanalysis of the ash content.FAN Maoming et alNanobubble generation and its applications in froth flotatio3.3 Bank of mechanical cells flotation testrespectivelyTo assess the impact of adding nanobubbles onTable 1 Levels of variables for a six factor five-level designcoal separation performance, experiments were confor a bank of three 10-liter mechanical cells flotation testsducted in a bank of three 10-liter mechanical cells asshown in Fig. 3. During the tests with nanobubbles, variableCode LowMiddleMiddle Highrt of the slurry in the third cell was pumped througha nanobubble generator with 3. 2 mm inner neck diameter and 12 mm inner pipe diameter, and then fedCollector (kg/t)Aback to the first cell. In this recycling, nanobubblesFrother(mg/L)10152023took place preferentially on the hydrophobic coal par-Feed particle sizeC00480.1280.26804080488ticle surface. The nanobubbles formed on coal parti- Feed solidscle surface remained attached while those on hydro-philic particle detached, which was a selective proc-ss that enhanced flotation separation efficiency. thehydrophobic particles had higher collision probability w rate(/min)2.46610813.2with nanobubbles, higher attachment probability, andlower detachment probability, as explained earlier,Table 2 Six-factor five-level central composite design fora bank of three 10-liter mechanical cells flotation testsresulting in greater flotation rate constant and flota-Std Run- Level of fac181-1.00-1.00-1.00-100-1001003520000000000000001.00-100-100-100100-1000000.00001.001.001Slurry pumped from3960.000000000000000.00217100100100-1.00100-1.00000000000slurry enterin89-100100-1.00100100-1.00lotation cell2210-100-1001.001.001.00100Feed entering notation cellFlotation product3411000000000000001.57Fig 3 Bank of three 10-liter mechanical cellsl1121.00-1.00-100100100-1.00The effect of nanobubbles on flotation performance6131.001.00100-1.00of varying size ranges and density fractions of the00100100coal sample A was evaluated. A series of exploratory2815000001.570.00000000tests were performed on the coal sample A using a1516-1.00-100100100100-1.00six-factor five-level central composite experimental 7 17 1.00-1.00-I00-1.001.001918-1.00-1.00-1001.00-1.00-1.00design with the Design-Expert software acquired31190.000000000001.570.00from Stat-Ease Inc, Minneapolis, MN. The specific00000-1.57levels of individual variables are indicated in Table 1. 36 21 0.00 0.00 0.00 0.00The six process parameters include collector dosage,00000000000frother concentration, feed mean particle size, feed 24 23 1.57 0.00 0.00 0.00 0.00 0.00solids concentration, retention time, nanobubble1024100-1.001.001001001.00slurry flow rate. Each numeric factor is varied over 5 2525 0.00 -1. 57 0.00 0.000.00 0.00levels: plus and minus alpha(axial points), plus and126-100-1001.001.00minus 1(factorial points)and the center point. The30270000000001570.00000levels of process variables were coded as"-1.57", 40 28 0.00 0.00 0.00 0.00 0.00 0.00“-1”“0"and“+1”,“+1.57", respectively, where2291001.00represents the low or middle-low level, "O"represents100-100100-100-1.00-10000-1570.00000represents th00-1.00-100-100or high level of the factors. The details of designed26330001.570000.000000experiments are shown in Table 2.1234100.001001.001001.003.4 Column flotation test23To assess the effect of nanobubble on column flo-中国煤化工∞0100100tation performance of coal, experiments were per- 5formed in a 50.8 mm in diameter laboratory scale 14CNMH-1001.000o0-100-100flotation column and a 152 mm in diameter pilot340-100100-1.00-1.001001.00scale flotation column with coal samples B and C,Mining Science and TechnologyVol 20 No, 53.4.1 Laboratory scale column flotationpump. A microprocessor series 2600 Love ControlsFig. 4 shows the laboratory scale continuously op- received signals from a pressure transducer located aterated flotation column utilized for fine coal flotation the bottom of the column. The signal adjusts thetests. The cylindrical column was made of Plexiglas pump speed that controls the underflow flow rate andof 50.8 mm in diameter. The length of the collection the desired froth level.and froth zones typically used in the test program Prior to each test, the feed slurry was conditionedwere 210 and 30 cm, respectively. With a diameter of for 15 minutes with fuel oil No. 2 which was used to5. 1 cm, the length-to-diameter ratio was around 51: 1 enhance the hydrophobicity of the coal surfaces.which provided near plug-flow conditions. Wash wa- Conditioning was conducted in a sump that waster was added in the froth zone at a depth that was 1/3 equipped with a mixer and four baffles placed verti-of the froth zone height below the overflow lip. The cally and separated by an equal distance along thecavitation tube and the static mixer, both are compact circumference of the sump. The feed slurry waand have no moving parts, were used to generate pumped to a feed tank which utilized a recirculatingnanobubbles and conventional sized bubbles, respec-line to ensure suspension of all solids. A peristaltictively. The pipe diameter of the cavitation tube is 12 pump was used to draw a pre-determined amount ofmm and the neck diameter is 3.2 mm. frother and air feed into the flotation column. Unless otherwisewere injected into the stream prior to the static mixer. specified, all column flotation tests were performedThe feed slurry entered the column in the upper pulp under the following conditions: froth depth: 30 cm;zone, 45 cm below the overflow lip. After being fed superficial gas flow rate: 1.5 cm/s; tailing recirculatinto column, coal particles collected by rising bubbles ing flow rate ratio between the nanobubble generatorascend to the top. Those that settle to the bottom of and the conventional size bubble generator: 60%the column are pumped through the cavitation tube collector dosage: 0.23 kg/t fuel oil; frother concentra-and the static mixer to have more chances for recov- tion: 15 mg/L MIBC; superficial wash water flow rateery. The slurry jet outof the neck of the Venturi cavi- 0.3 cm/s; superficial feed slurry flow rate: 0.75 cm/station tube at a speed of 6 to 10 m/s causes hydrody- feed slurry solids concentration: 9%. A period of timenamic cavitation in the stream with nanobubbles equivalent to three particle retention times was al-formed preferentially on coal particle surface. The lowed to achieve steady-state conditions. After reach-nanobubbles formed on hydrophobic coal particle ing the steady-state, samples of the feed, product andsurface remained attached while those on hydrophilic tailing streams were collected simultaneously. Theparticle detached, which was a selective process that samples were filtered, dried, weighed and analyzedenhanced flotation separation efficiency. The hydrofor ash content. The standard deviation of clean coalphobic particles had higher collision probability with ash and yield is less than 0.3% and 1%0, respectivelynanobubbles, higher attachment probability, and 3.4.2 Pilot scale column flotationlower detachment probability, as explained earlier,Based on the laboratory scale results, furtherresulting in greater flotation rate constant and flota- evaluation of the effects of nanobubbles on the coaltion recovery. Hydrophilic particles moved outward flotation performance was carried out in a speciallyand downward and were eventually rejected from the designed pilot-scale flotation column with coal sam-bottom. The tailing flow rate was adjusted with a ple C Fig. 5 shows the 152 mm in diameter flotationcolumn used in this study. The cylindrical columnwas made of glass. The pipe diameter of the nano-bubble generator is 50.8 mm and the neck diameter is12.7 mm. Frother was injected into the bottom of thecolumn while the air was injected into the streamprior to conventional size bubble generator. The feedpoint was located at 50 cm below the overflow lip ofthe column. The description of the flotation 152 mmin diameter column operation is similar to that of 50.8mm in diameter flotation column as provided previ-PressureGas flowmeterFeed pump feedA number of flotation experiments were performedat varying flotation feed rates. The column flotationStatic mixerTo recycling pumptests of coal sample C were performed under the fol-Cavitation tubeFlotation tailinlowing conditions: froth depth: 45 cm; superficial gasConditionflow中国煤化立gW以ate ra-tio bd the coFig4 Laboratory flotation column with adjustable seriesCNMHG%, collector dosor parallel configurations of the nanobubble generatorage: 0.Jll, Iruuier cuncentration: 15 mg/Land the conventional size bubble generatorMIBC; superficial wash water flow rate: 0.3 cm/sFAN Maoming et alNanobubble generation and its applications in froth flotationfeed slurry solids concentration: 10%4 Results and discussion4.1 Particle size and density fraction distribu-Flotation concentrateFlotation feedFig. 6 shows the particle size and ash distributionpal samples A and B. It can be clearlyfrom Fig. 6 that the ash contents of both samples de152 mm in diametercrease as the particle size increases. The significantincrease in ash content at fine particle size fractionscould be resulted from more efficient desliming oflow-density coal at finer sizes and also could be iSlurry pumped fromdicative of preferential liberation of the ash-formingcomponents. There are significant differences begenerator andtween the two samples in particle size distributionand ash content, Sample B has a much wider particlesize distribution than the sample A. Fig. 6b indicatesNanobubble generatethat almost all coal particles (about 98%)wereFlotation tailingsmaller than 1180 microns(16 mesh). A small portionFig 5 Specially designed 152 mm in diameterof coal particles(6.83%)were smaller than 43 mi-crons(325 mesh). The feed ash content was 30. 28%.-Cumulative mass ?9emulative ashIndividual ashParticle size(microns)(a)Sample A(b)Sample BFig 6 Particle size and ash distribution of tested coal samplesFig. 7 shows the ash content of four particle sizes increasing the density, more significantly when theof the sample A as a function of density fractions. density increases from 1. 25 to 1.5 g/cmand from 1.9Coal is composed of macerals and ash-forming min- to 2.25 g/cm. The liberation of the intermedi-erals. The densities of ash-forming minerals are usu- ate-density particles with density between 1.5(or 1. 4)ally significantly higher than those of combustible and 1.9 g/cm'is not complete and thus it is difficultmatter. Therefore, the coal ash content increases as to separate these coal particlesHt75-0 microns cumulative ma600-355 microns cumulative massor600-355 microns cumulativeicons cumulative asla°14161.8202222141618202.22(a)600-355 and 355-180 microns fractions(b)180-75 and 75-0 microns fractionsFig. 7 Density and ash distribution中国煤化工4.2 Release analysis resultsCNMHG-355,35-18Fig 8a shows the flotation product yield of the coal 180-1be seen from theample a as a function of product ash content of the figure that at the product ash content of 7.5% theMining Science and Technologyproduct yields of 600-355, 355-180, 180-75, and flotation combustible recoveries of 600-355, 355-75-0 microns are 56%, 75%, 78%, and 39%, respec180,180-75,and75-0 microns are70%,93%,97%tively. The yield of coarse 600-355 microns fraction and 89%, respectively. The flotation combustible re-is 22% lower than that of the 180-75 microns fraccovery of coarse 600-355 microns fraction is 19%0-tion. The yield of fine 75-0 microns fraction is only 27% lower than that of the other three particle sizehalf of the 180-75 microns fractions yield. Fig. 8b fractions. Therefore, the particle size has significantpicts the flotation combustible recoveries of these effects on coarse and fine/ultrafine coal flotationfour particle size fractions as a function of product combustible recovery and product yieldash content. At the product ash content of 7.5% the2603m102030405060(a)Mass yie(b)Combustible recoveryFig. 8 Release analysis results of the coal sample Asample C. At the product ash content of 7.5%, the ery significantly decreases with increasing the cov-Fig.9 depicts release analysis result of the coal clearly seen from Fig 10 that the combustible recov-flotation product yield and the flotation combustible cle density. The coal particle floatability of a givenecovery are 31% and 559, respectively, which are coal particle size range decreases with increasing thesignificantly lower than those of the coal sample A. coal particle density. First, because the organic com-The product yield increases from 31% to 56% as the ponents those are most amenable to bubble attach-product ash content increases from 7.5% to 15%. ment decrease and the inorganic mineral mattersignificant and it is difficult to separate sw fash components which mostly resist bubble attachmencontent particles, indicating that the liberation is not increase as the coal particle density increases. Second,the bubble volume or the bubble number required forfloating a lower density coal particle of a given particle size is smaller than that required for floating ahigher density coal particle. In other words, the in-crease in coal particle density of a given particle sizeresults in decrease of the amount coal surface areaCombustible recoveryavailable for bubble attachment which reduces coalparticle/bubble attachment probability, and increaseProduct ash(%)detachment probabilityFig. 10 shows that the use of nanobubbles re-Fig 9 Release analysis results of the coal sample Cmarkably increases the various density coal flotation4.3 Bank of mechanical cells flotation testcombustible recoveries of 600-355 and 75-0 micronsparticles. Fig. 10a indicates that the presence of4.3.1 Effect of nanobubbles on flotation of various nanobubbles increases the 600-355 microns coal par-density fractions' coal particlesticles combustible recoveries of the various densityig. 10 illustrates how the coal particle density arnd fractions: minus1.3,1,3~1.4,1.4-1.5,1.5-1.6,andnanobubbles affect the flotation performance of 600- 1.6-1.8 g/cm'by 13 1%, 22.2%0, 27.6%0, 22.5%, and355 and 75-0 microns particle size, respectively. In 11.9%, respectively. The nanobubbles increase thethese figures, the partition factor refers the ratio of the effective separation density when the partition factormass of a given density fraction in the flotation prod- is 50% from 1.44 to 1.54 g/cm. That means nano-uct to the mass of the corresponding density fraction bubbles have more significant effect on the mediumin the flotation feed. The combustible recovery in Fig.中国煤化工 and high density10 was calculated by dividing the mass of combusti- fractofnano-ble material recovered in a given density fraction by bulCNMHGonS coal particlesthe mass of the combustible material in the flotation combustible recoveries of the various density fracfeed of the corresponding density fraction. It can be tions: minus 1.3, 1.3-1.4, 1.4-1.5, 1.5-1.6, 1.6-1.8,FAN Mand 1.8-2.0 g/cm by 14.6%, 25.7%, 29.7%, 33.8%, nanobubbles have more remarkable effect on the30.6% and 9.5%, respectively. The nanobubbles in- 75-0 microns finer particles than the 600-355 mi-crease the effective separation density when the parti- crons coarser particles. More high density fractiontion factor is 50% from 1. 42 to 1.61 g/cm. Similarly particles of the 75-0 microns finer particles werethe nanobubbles have more significant effect on the floated than that of the 600-355 microns coarser pamedium density fractions than on the low and high ticlesdensity fractions. It can be seen from Fig. 10 that thePartition factor with nanobubblesPartition factor without nanobubble012141.618202241.21416182.0(a)600-355 micron75-0 micronsFig. 10 Effect of nanobubbles on coal flotation recovery and separation densityFig. 11 is the comparisons of the yield and the presence of nanobubbles increases the 75-0 micronscombustible recoveries with and without nanobubbles coal particles flotation yield and combustible recov-for 600-355 microns coal particles and 75-0 microns ery by up to 10% and 38%, respectively. The nano-coal particles, respectively. At a given product ash bubbles have more significant effect on the combus-content, Fig. lla indicates that the presence of nano- tible recoveries of 600-355 microns coarse coal par-bubbles increases the 600-355 microns coal particles ticles and 75-0 microns fine coal particles in the me-flotation yield and the combustible recovery by up to dium product ash range than in the lower or the18% and 35%, respectively. Fig. 1lb shows that the higher product ash range.-o- Recovery with nanobubblesFig. 11 Effect of nanobubbles on yield and combustible recovery4.3.2 Effect of nanobubbles on flotation particle tribution of flotation concentrate samples collectedSIze rangefrom each flotation cell as a function of particle size.Fig. 12 reveals the effect of nanobubbles on the Fig. 13a indicates that the 600-180 microns coarseflotation product particle size distribution of 600-180 coal flotation product mean particle size increasesmicrons and 180-0 microns flotation feed in presence from cell 1, cell 2, to cell 3. The use of nanobubblesand in the absence of nanobubbles. It can be clearly increases the mean particle size of cell 1, cell 2 andseen from cumulative particle size distribution curves cell 3 flotation products by 65, 90 and 105 microns,in Fig. 12a that the use of nanobubbles increased the respectively. Nanobubbles remarkably improved the600-180 microns coarse coal flotation product mean coarse coal particle flotation recovery. On the con-particle size from 250 to 325 microns, which means trary, Fig. 13b reveals that the 180-0 micronsthat nanobubbles greatly improved the coarse coal fine/ultrafine coal flotation product mean particle sizeparticle flotation recovery. Fig. 12b depicts that the decreases from cell 1, cell 2, to cell 3. The presencepresence of nanobubbles decreased the 180-0 mi- of nano hlmean particle size ofcrons fine/ultrafine coal flotation product mean part中国煤化工 oducts from108tocle size from 103 to 69 microns, significantly im- 80,proved the fine/ultrafine coal particle flotation recovCNMHGvely Nanobubblesrafine coal particleflotation recovery. Therefore, nanobubbles can widenFig. 13 shows the cumulative undersize mass dis-flotation size range.Mining Science and TechnologyVol20 No, 5扫己80-o- Cumulative with nanobubbles00200300400500600306090120150180(a)product of 600-180 microns flotation feed(b)Product of 180-0 microns flotation feedFig. 12 Incremental and cumulative particle size distribution of flotation product with and without nanobubbles80240300360420480540600306090120150180rticle size(microns)Fig. 13 Cumulative size distribution of each flotation cell product with and without nanobubbles4.3.3 Effect of nanobubbles on flotation kineticsabout 27. 0%0, 24.790, 17.9%, 18.3% higher than in theThe investigation of nanobubbles'effect on flota- absence of nanobubbles for 600-355, 355-180tion rate constant was performed with four coal parti- 180-75, and 75-0 microns particle size fractions,cle size fractions: 600-355, 355-180, 180-75, and respectively75-0 microns in the above described three 10-literThe fact that the curve with nanobubbles is alwaysmechanical cells. Fig. 14a shows the effect of nano- above the other curve indicates that the presence ofbubbles on the recovery-flotation time curves in nanobubbles improved the flotation kinetics. Assumwhich the combustible recoveries for 600-355, ing perfect mixing and first-order kinetics conditions,355-180, 180-75, and 75-0 microns particle size the flotation rate constants shown in Fig. 15b werefractions were plotted against the flotation time. The determined from the data in Fig. 15a, in which thecurves indicate that the combustible recovery in the flotation recovery(R) for each particular size range ispresence of nanobubbles was significantly higher plottedthe flotation time. Fig. 15a depicts thethan in the absence of nanobubbles, which means the curves of In(l-R) versus flotation time for four coalpresence of nanobubbles increased the flotation re- particle size ranges of 600-355, 355-180, 180-75covery of combustible matter. It can be clearly ob- and 75-0 microns. Fig. 15b shows the flotation rateserved from Fig. 14b that nanobubbles have more constants, the slope of each plot shown in Fig. 15a,significant effect on the flotation combustible recov- are 0.50, 0.61, 0.94, and 1.01 min for 600-355ery before 1.5 minutes flotation time than that after 355-180, 180-75, and 75-0 micrns size range coal1.5 minutes flotation time. Nanobubbles increased the particles, respectively, when no nanobubbles werecombustible recovery of 75-0 microns particle size present In the presence of nanobubbles, the flotationfraction by 20.5%, 18.3%,10.4%,8.8%, 6.8% and rate constants of coal particles in these four particle4.8% at the flotation time of 0.5, 1.0, 1.5. 2.0. 3.0, size fractions increased to 0.99, 1.21. 1.41, and 1.43and 4.0 minutes, respectively. The difference of the min, respectively The flotation rate constants withcombustible recovery decreased as the cumulative nanobubbles are 98.0%, 98.4%, 50.0% and 41.6%flotation time was increased from 0.5 to 4 minutes. higher than without nanobubble for 600-355, 355-Fig. 14b also depicts that nanobubbles have more 180, 180-75, and 75-0 microns coal particles, reremarkable effect on the flotation combustible recov- spectively. This indicates that the use of nanobubblesery of the coarse coal particle size fraction and the increticle flotation rate,ultrafine coal particle size fraction than on that of the whi中国煤化工medium particle size fractions. For an example, when tionCNMHGthat the flotaticthe cumulative flotation time is I minute, the com- rate constant increases witn decreasing coal particlebustible recovery in the presence of nanobubbles was size. The test results are in agreement with previousFAN Maoming et alNanobubble generation and its applications in froth flotation51research results about the effects of very small bub- ics47-711bles on the fine mineral or coal flotation kinet-o-600-355 microns--355-180 microns15035 microns wI地e180-75 microns+355-180 microns without nanobubble00-355 microns without nanobubbleFlotation time(min)Fig 14 Effects of nanobubbles on the flotation kinetics of various size coal partiw--355-180 without nanobubbleF-10-o- 600-355 with nanobubblesw 180-.75 with nanobubblesH Without nanobubble00051015202530lotation time(min)Fig. 15 Effects of nanobubbles on the flotation rate constant4.3.4 Effect of nanobubbles on overall performance than that of 75-0 microns particle size fraction coalFig. 16 reveals the effect of nanobubbles on flota- The flotation yield of 180-75 microns particle sizetion product yields of four coal particle size ranges of fraction coal are about 15%, 10%, 4%, and 2% higher600-355, 355-180, 180-75, and 75-0 microns. It can in the presence of nanobubbles than in the absence ofbe observed from Fig. 16 that the effect of nanobub- nanobubbles at the product ash content of 490, 5%0les on the yields of coal particle flotation decreases 7.5%, and 10%, respectively. The use of nanobubblesth decreasing the feed particle size and increasing increases the 75-0 microns coal particle flotationthe product ash content. Fig 16a shows the presence yield by about 9%,8%, 39, and 2% at the productof nanobubbles increases the 600-355 microns coal ash content of 3%, 5%, 7.5%0, and 10%, respectivelyflotation product yield by about 14%, 22%, 16%, It can be concluded from Fig. 16 that the use of13% at the product ash content of 5%, 7.5%, 10%, nanobubbles increased the various size coal particleand 12.5%, respectively. The nanobubbles increases flotation yield at a given product ash content. Thethe 355-180 microns coal flotation product yield by flotation product yield curves with nanobubble areabout 17%, 16%, 12%, 7% at the product ashmuch closer to the corresponding release analysisof 5%, 7.5%, 10%, and 12.5%, respectively. Ficurves than those without nanobubbles, which meandepicts that the overall product yield of 180-that the presence of nanobubbles greatly improvedcrons particle size fraction coal is remarkably higher the flotation yieldnanobubble中国煤化工Product ash(%)(a)600-355 and 355 180 micronsCNMHGFig. 16 Effect of nanobubbles on flotation yieldMining Science and TechnologyVol 20 No5Fig. 17 depicts the flotation combustible recovery tion combustible recovery. It can be obviously seenas a function of flotation product ash content for by comparing Fig. 17 with Fig. 16 that nanobubbles600-355, 355-180. 180-75 and 75-0 microns size have much more remarkable effect on the flotationfraction coal particles with and without nanobubbles. combustible recovery improvement than on the flota-The flotation combustible recovery in all size frac- tion product yield improvement, especially for thetions in the presence of nanobubbles was remarkably 70-0 microns fine or ultrafine particle size fractionhigher than in the absence of nanobubbles, indicating coalthat the presence of nanobubbles increased the flota-600-355 microns without nanobubble75-0 microns withProduct ash (%Product ash(%)(a)600-355 and 355-180 microns(b)180-75 and 75-0 micrFig 17 Effect of nanobubbles on flotation recoveryFig. 17a shows the presence of nanobubbles in- without nanobubbles. The wetting film separating thecreases the 600-355 microns size fraction coal parti- colliding convention-sized bubbles and nanobubblescles flotation combustible recovery by 18%0, 27%, is more unstable than the film between the conven-20%6, 16% at the product ash content of 5%, 7.5%0, tion-sized bubbles and coarse coal particles. The sig10%, and 12.5%, respectively. The use of nanobub- nificant difference in diameter between the conven-bles increases the 355-180 microns size fraction coal tion-sized bubble and nanobubble leads to a huge90,9% at the product ash content of 5%0,7.5%0, rupture of water films separating convention-sized10%, and 12.5%, respectively. Fig. 17b indicates that bubbles and nanobubbles. Thus the presence ofthe flotation combustible recovery of 180-75 microns nanobubbles on coal particles surfaces accelerates theparticle size fraction coal are 18%, 13%0, 5%, andwater film rupture and bubble-particle collision. Inhigher in the presence of nanobubbles than in the ab- other words, nanobubbles on the surface of coal par-ence of nanobubbles at the product ash content of ticle act as a secondary hydrophobizing agent to en-4%, 5%6,7.5%, and 10%, respectively. Nanobubbles hance the flotation rate constant.increase the 75-0 microns coal particle flotationRejecting ash is one of the main goals in coal frothcombustible recovery by 21%,18%,8%, and 5% at flotation. Fig. 18 reveals the flotation ash rejection ashe product ash content of 3%, 5%0, 7.5%, and 10%0, a function of the flotation combustible recovery withrespectively. By comparing these combustible recov- and without nanobubbles for 600-355, 355-180ery curves in the presence of nanobubbles and in the 180-75, and 75-0 microns size coal particles. It canabsence of nanobubble with the corresponding release be clearly seen from Fig. 18 that the flotation ash re-analysis curves, it can be clearly seen that the flota- jection without nanobubble decreases rapidly withtion combustible recovery curves with nanobubble further increasing the flotation combustible recoveryare much closer to the corresponding release analysis after the flotation combustible recovery reaches aboutcurves than those without nanobubble, indicating that 75%, 80%, 90%, and 80% for 600-355, 355-180,the use of nanobubbles significantly improved the 180-75, and 75-0 microns size fractions, respectivelyflotation combustible recovery of the various size The flotation combustible recovery at the"elbowfraction coal particlespoint "is 5%-10% higher in the presence of nanoIt can be seen from the above results that the most bubbles than in the absence of nanobubble. Fig. 18significant improvement in flotation performance can clearly shows that the ash rejection of 600-355 mi-be observed in the 355-600 microns coarse coal par- crons size coal particles is about 11%,7.5%, 6%, andticles flotation. This is because the presence of nano- 5% higher in the presence of nanobubbles than in thebubbles on the coarse coal particle surfaces facilitates absence of nanobubble at the flotation combustibleparticle attachment to conventional-sized bubbles. rece中国煤化工50%, respectivelyThe nanobubbles on the coarse coal particles do not Nand-I ash rejection ofsufficient buoyancy to float the particle by theCN MHGcoal parIves. But the surface of a coarse coal particle 14%, 12.5%, 10%, and 6.5% for the flotation com-with nanobubbles is more hydrophobic than bustible recoveries of 91.5%, 85%, 80%, and 70%FAN Maoming et alble generation and its applications in froth flotation653with nanobubble are much closer to the correspond-Ingbubbles, which means that the use of nanobubblesremarkably improved the flotation ash rejection of thevarious size fraction coal particlesFig. 19 depicts the flotation selectivity index as afunction of flotation product yield with and withoutnanobubbles. The flotation selectivity index, S, whichindicates the flotation selectivity, is determined byS=R-R2, where R is the recovery of the flotation combustible recovery wanted to float and R, isFig. 18 Effect of nanobubbles on flotation ash rejectionthe recovery of ash content to be rejected. A higherrespectively. At the combustible recoveries of 95%, selectivity index S suggests a better selectivity for the90%, 80%, and 10%, the corresponding air rejection flotation process. Flotation selectivity is an importantof 180-75 microns fraction coal particle flotation is parameter for wide particle size range coal particl22%, 4%, 2.5%, and 1.5% higher with nanobubbles flotation because of the great variation among flota-than without nanobubble. It can be clearly observed tion rate constants of various size coal particles. Thefrom Fig. 18 that nanobubbles have less effect on the flotation time required by the coarsest size fraction isash rejection of 70-0 microns finer coal particle flo- too long for the finer sizes. The attempts to improvetation than that of the other three coarser size frac- coarse particle recovery using longer flotation timetions. The flotation ash rejection of 70-0 microns and higher dosage of collector often lead to dimin-fine/ultrafine coal particles is increased by 2.5%, ishing the finer size particles flotation selectivity in1.5%, and 1%at the flotation combustible recoveries addition to incurring a higher flotation operation cost.of 84%, 80%, and 75%, respectively. by comparing The reduced flotation selectivity of finer size fracthese ash rejection curves in the presence of nano. tions often produces undesirable high ash product andbubbles and in the absence of nanobubble with the results in unacceptable overall product qualitycorresponding release analysis curves, it can be ob- 19 indicates that the use of nanobubbles is aously seen that the flotation ash rejection curves approach to increasing flotation selectivity.80-75 with nanobubblesH-600-355 without nanobubbleProduct yield(%)Fig. 19 shows the flotation selectivity index S for 3%, and 1% at the product yields of 80%, 70%6, 609,the four coal particle size fractions of 600-355, and 50%, respectively. The presence of nanobubbles355-180, 180-75, and 75-0 microns with and with- increase the coal particle flotation selectivity index ofout nanobubbles. It can be clearly observed from Fig. 75-0 microns size fraction by 5%0, 4%, 2%0, and 1%19 that the presence of nanobubbles has a positive at the product yields of 379, 35%, 30%, and 25%impact on the flotation selectivity of all four particle respectively. Therefore, the effect of nanobubbles onsize ranges. The 600-355 microns coal particles flo- the flotation selectivity decreases with decreasingtion selectivity indexes with nanobubbles are 14%, particle size90, 7%, and 5% higher than without nanobubble atAs discussed previously, nanobubbles are preferenthe product yields of 70%, 65%, 60%, and 50%, re- tially generated and adsorbed on hydrophobic coalspectively. The use of nanobubbles increases the particle surface rather than on the hydrophilic ash355-180 microns coal particles flotation selectivity forming mineral particle surface, which increases theindexes by 13%,11%, 10%, and 5% at the product coal particle hydrophobicity and enlarges the flotationyields of 78%,70%, 60%0, and 50%, respectively. rate中国煤化工 s and the hydro-Less flotation selectivity improvement is observed for philicThis may resultthe finer size fractions of 180-75 microns and 75-0 in theCN Gon coal flotationmicrons. Nanobubbles increase the 180-75 microns selectivity, There are two major reasons why nano-coal particle flotation selectivity index by 6%, 5%o, bubbles are preferentially generated on the surface ofVol20 No5hydrophobic coal particle: 1)the adhesion force be- bubble on the flotation performance was carried outtween hydrophobic coal particle surface and water is using a six-factor five-level central composite exmuch weaker than that between the hydrophilic min- perimental design with the Design-Expert softwareeral particle surface and water. The rupture of wa. The six process parameters include nanobubble slurryter-coal interface is much easier than the water-coal flow rate, collector dosage, frother concentration,interface. The boundary water layers on hydrophilic flotation feed mean particle size, feed solids concen-mineral particle surface are ordered to a higher extent tration, and flotation retention time. Response surfaceand more stable than those on hydrophobic surfaces methodology was used to analyze the six factor five-of coal particles; 2)hydrophobic coal particles may level central composite experiment data. Responseentrap gas in crevices, thus greatly reducing cavita. surface and contours were generated for the flotationtion threshold and considerably enhancing cavitation combustible recovery, product yield, product ashefficiency by acting as the main source of gas nuclei. content and flotation selectivity index as a function ofIt is also believed that most of the nanobubbles the studied six process parameters. Figs. 20-23 depictformed on hydrophobic coal surface remain attached the effects of the six studied parameters on theto particle while those on hydrophilic mineral particle flotation combustible recovery. Figs. 24-27 show thesurface are more readily detached from the particle, effects of the six studied parameters on the flotationwhich is a selective process that enhances flotation product yield. Figs. 28-30 indicate the impacts of the4.3.5 Six-factor five-level central composite experi- content. The effects of the six studied parameter aperformancestudied parameters on the flotation product ashmental testthe flotation selectivity index are shown in Figs.The comprehensive study of the effects of nano- 31-35Recovery (%)13.23=002zNanobubble now(Lmin)000.190.190.340.500.66081Collector(kg/t)Fig 20 Effects of nanobubble and collector on the flotation recoveryFig. 20 shows the effects of the nanobubble flow the contours of the flotation combustible rerate and collector dosage on the flotation combustible shown in Fig. 20 that when the nanobubble flow raterecovery when the frother concentration, flotation increases from 0 to 9.9 L/min, the flotation combustifeed mean particle size, feed solids content, and flota- ble recovery increases from 71% to 92%, 73%totion retention time were 15 mg/L, 0. 27 mm,8.0%, 93%, 74% to 93%, 75%to 93%, and 76% to 93%atand 5.0 min, respectively. It can be clearly seen from the collector dosages of 0. 19, 0.34, 0.50, 0.66, andthe response surface and the contours of the flotation 0.81 kg/t, respectively. This means that nanobubblescombustible recovery shown in Fig. 20 that the flota- have more significant effect on the flotation combus-tion combustible recovery considerably increases tible recovery at the lower collector dosageith increasing the nanobubble flow rate and slightly the higher collector dosage. At a givenincreases with increasing the collector dosage when combustible recovery, more than half of thethe nanobubble flow rate less than about 9.9 L/min.aan be saved by using nanobubblesslight decrease in flotation combustible recovery atFig. 21 depicts the effects of the frother concentra-the high collector dosage and the high nanobubble tion and the nanobubble flow rate on the flotationflow rate is probably because of overdosing the col- combustible recovery when the collector dosage, flo-lector. The point of the highest flotation combustible tation feed mean particle size, feed solids concentra-ecovery is located at the area of the high level of tion, and flotation retention time were 0.50 kg/t, 0.27nanobubble flow rate and the middle level of the mm,中国煤化工 y. The significantcollector dosage. At the collector dosage of 0.19 kg/tthe flotation com-the flotation combustible recovery increased from bustiCNMHGbe seen in Fig. 2171% up to 94%as the nanobubble flow rate increased The flotation combustible recovery remarkably in-from O to 13. 2 L/min. It can be clearly observed from creases with increasing the nanobubble flow rate andFAN MaoNanobubble generation and its applications in froth notationslightly increases with increasing the frother concen- tration. At the frother concentration of 15 mg/L, thetration. The area of the highest flotation combustible flotation combustible recovery increased from 74%recovery was attained at the high level of nanobubble up to 93% as the nanobubble flow rate increased fromflow rate and the middle level of the frother concen- 0 to 9.9 L/miRecovery (%)Frother(mg/L)Fig. 21 Effects of nanobubble and frother on the flotation recoveryFig. 22 indicates the effects of the flotation feed flotation feed mean particle size from 0. 16 to 0.05particle size and the nanobubble flow rate on the flo- mm. The highest flotation combustible recovery wastation combustible recovery when the collector dos- obtained at the high level of nanobubble flow rate andage, frother concentration, flotation feed solids con- the mean flotation feed particle size of about 0. 16 mmtent, and flotation retention time were 0.50 kg/t, 15 It can be observed from the contours of the flotationmg/L,8.0%, and 5.0 min, respectively. As can be ob- combustible recovery shown in Fig. 22 that when theviously seen in Fig. 22, both the nanobubble flow rate nanobubble flow rate increases from 0 to 13. 2 L/minand flotation feed particle size have remarkable im- the flotation combustible recovery increases fronpacts on the flotation combustible recovery. At a 60% to 84%, 69% to 90%, 74% to 949, 77% to 96%given mean flotation feed particle size, the flotation and 77.5% to 96% for the mean flotation feed particlecombustible recovery significantly increases with size of 0.49, 0.38, 0.27, 0.16, and 0.05 mm, respecncreasing the nanobubble flow rate. At a given tively. This means that the impact of nanobubbles onnanobubble flow rate, the flotation combustible re- the flotation combustible recovery improvement decovery increases considerably as the flotation feed creases with decreasing the mean flotation feed partimean particle size decreases from 0.49 mm to 0. 16 cle size from 0.49 to 0.05 mmmm, but decreases slightly with further decreasing the13.20.270.38Mean size(mm)Fig 22 Effects of nanobubble and flotation feed particle size on the flotation recovery23a depicts the effects of the flotation feed time and the nanobubble flow rate on the flotationcontent and the nanobubble flow rate on the combustible recovery when the collector dosage,flotation combustible recovery when the collector frothdosage, frother concentration, mean flotation feed中国煤化工 on feed particlecontent were 0.50particle size, and flotation retention time were 0.50 kg/t,CNMHGo, respectively.Itke/, 15 mg/L, 0.27 mm, and 5.0 min, respectively. can be obviously seen from Fig. 23 that the flotationFig. 23b shows the impacts of the flotation retention combustible recovery increases considerably withMining Science and TechnologyVol 20 No 5increasing the nanobubble flow rate.c nowFig. 23 Effects of nanobubble, flotation feed solids concentration and retention time on the flotation recoveryFig. 24 depicts the effects of the nanobubble flow ble flow rate is probably because of overdosing therate and collector dosage on the flotation product collector. It can be observed from the contours of theyield when the frother concentration, flotation feed flotation product yield shown in Fig. 24 that when themean particle size, feed solids content, and flotation nanobubble flow rate increases from 0 to 9.9 L/minretention time were 15 mg/L, 0. 27 mm,8.0%, and 5.0the flotation product yield increases from 54% tomin, respectively. It can be clearly observed from the 74%, 56% to 74%, 58% to 73%0, 61% to 73%o, andresponse surface and the contours of the flotation 63% to 73% at the collector dosages of 0.19, 0.34product yield shown in Fig. 24 that the flotation 0.50, 0.66, and 0.81 kg/t, respectively. Therefore,product yield considerably increases with increasing nanobubbles have more significant effect on the flothe nanobubble flow rate and increases with the col- tation yield at the lower collector dosage than at thelector dosage when the nanobubble flow rate less than higher collector dosage, At a given flotation productabout 9.9 L/min. A slight decrease in flotation prodtuct yield, more than half of the collector can be saved byyield at the high collector dosage and high nanobub- using nanobubbles.o340.500.0anobubble mow. 9.9 13.20.8)0.190.340.500.660.8Fig 24 Effects of nanobubble flow rate and collector dosage on the flotation yieldFig. 25 shows the effects of the nanobubble flow frother concentration. At the frother concentration ofrate and the frother concentration on the flotation 15 mg/L, the flotation product yield increased fromproduct yield when the collector dosage, flotation 56% up to 73% as the nanobubble flow rate increasedfeed mean particle size, feed solids content, and flota- from 0 to 9.9 L/mintion retention time were 0.50 kg/t, 0.27 mm, 8.0%Fig 26 depicts the effects of the flotation feed par-and 5.0 min, respectively. It can be observed from Fig. ticle size and the nanobubble flow rate on the flota-25 that the flotation product yield increases consis- tion product vield when the collector dosage, frothertently with increasing the nanobubble flow rate from con中国煤化工> ontent, and flota-0 to 13. 2 L/min and slightly increases with increasintionthe frother concentration. The highest flotation prod- andC Gbe observed fromuct yield was obtained at the area of the high level of Fig. 26, the mean flotation feed particle size and thenanobubble flow rate and the middle level of the nanobubble flow rate have considerable impacts onFAN Maoming et alNanobubble generation and its applications in froth flotationthe flotation product yield. At a given particle size, product yield shown in Fig. 26 that when the nano-the flotation product yield significantly increases with bubble flow rate increases from 0 to 9.9 D/min, theincreasing the nanobubble flow rate. At a given flotation product yields increase from 34% to 56%6,nanobubble flow rate, the flotation product yield in- 46% to 66%, 59% to 72%6, 66% to 74%, and 70% tocreases as the flotation feed mean particle size de- 71% for the mean flotation feed particle size of 0.49,creases. The highest flotation product yield was 0.38, 0.27, 0.16, and 0.05 mm, respectively. The ef-obtained at the high level of nanobubble flow rate and fect of nanobubbles on the flotation product yieldthe low level of the mean flotation feed particle size. decreases with decreasing the mean flotation feedIt can be observed from the contours of the flotation particle sizeFig. 25 Effects of nanobubble flow rate and frother dosage on the flotation yield132Yield (%迟Effects of nanobubble flow rate and flotation feed particle size on the flotation yieldFig. 27 depicts the effects of the flotation feed sol- tent. In the presence of nanobubbles, 10% to 16%ids content and the nanobubble flow rate on the flota- product yield improvement was achieved at varioution product yield when the collector dosage, frother solids content from 4.9% to 11.1%concentration, mean flotation feed particle size, andFig. 28 shows the effects of the nanobubble flowflotation retention time were 0.50 kg/t, 15 mg/L, 0.27 rate and collector dosage on the flotation product ashmm, and 5.0 min, respectively. Fig. 27 shows that the content when the frother concentration, flotation feedflotation product yield increases considerably with mean particle size, feed solids content, and flotationincreasing the nanobubble flow rate. It can be seen retention time were 15 mg/L, 0. 27 mm, 8.0%0, and 5.0rom the contours of the flotation product yield that min, respectively. It can be observed from the rehen the nanobubble flow rate increases from 0 to sponse surface and the contours of the flotation9.9 L/min, the flotation product yields increase from product ash content shown in Fig. 28 that the flotation61% to 77%0, 59%to 759, 59% to 729, 59% to 71%0, product ash content increases with increasing theand 59%to 69% at the solids contents of 4.9%, 6.5%6, nanobubble flow rate and the collector dosage8.0%,9.6%6, and 11. 1%, respectively. However the Probal中国煤化工 r caused a slightflotation product yield remains essentially constant decrentent at the highabove 9.9 U/min nanobubble flow rate Nanobubbles collectCN MHGubble flowhave more significant effect on the flotation yield at Fig. 28 depicts that when the nanobubble flow ratethe lower solids content than at the higher solids con- increases from 0 to 9.9 min, the flotation productMining Science andVol 20 No 5increases from 5.7% to 8.2%, 6.3% to nificant effect on the flotation product ash content atto 8.1%, and 7.5% to 7.9%, and 76% to the lower collector dosage than at the higher collector93% at the collector dosages of 0. 19, 0.34, 0.50, and dosage0.66 kg/t, respectively. Nanobubbles have more sigYield (%E6.6Fig. 27 Effects of nanobubble and flotation feed solids concentration on the flotation product yield0.340.500.6Fig. 28 Effects of nanobubble flow rate and collector dosage on the flotation ash contentFig. 29 depicts the effects of the nanobubble flow flotation feed particle size. This is because nanobub-rate and the frother concentration on the flotation bles improve the flotation recovery of the high ashproduct ash content when the collector dosage, flota- content coarse particles. The flotation product ashtion feed mean particle size, feed solids concentration, content decreases with increasing the nanobubbleand flotation retention time were 0. 50 kg/t, 0.27 mm, flow rate when the flotation feed mean particle size is8.0%, and 5.0 min, respectively. The flotation product finer than 0. 20 mm because nanobubbles improve theash content increases with increasing the nanobubble flotation selectivity of fine and ultrafine particles. Itow rate and the frother concentration from low level can be seen from the contours of the flotation productto middle level. After that, there is a slight decrease in ash content shown in Fig. 30 that when the nanobub-the flotation product ash contentble flow ratfrom 0 to 9.9 L/min, the flotaFig. 30 depicts the effects of the flotation feed par- tion product ash content increases from 5. 1% to 9.0%6ticle size and the nanobubble flow rate on the flota-.9% to 8.6, and 6.9% to 8.1% for the mean flota-tion product ash content when the collector dosage, tion feed particle size of 0.49, 0.38, and 0.27 mm,frother concentration, flotation feed solids content, respectively. The effect of nanobubbles on the flotaand flotation retention time were 0.50 kg/t, 15 mg/L, tion product ash content decreases with decreasing8.0%, and 5.0 min, respectively. As it can be observed the mean flotation feed particle size when the particlefrom Fig. 30, the flotation product ash content in- is coarser than 0.2 mm. As the nanobubble flow ratecreases with increasing the nanobubble flow rate increases from 0 to 9.9 L/min, the flotation productwhen the flotation feed mean particle size is coarser ashthan 0.20 mm. The area of the highest flotation prod- 6.8中国煤化工toarticle size of 0.16t ash content is located at the high level of andCNMHGnanobubble flow rate and the high level of the meanFAN Maoming et alNanobubble generation and its applications in froth flotation72515Nanobubble flow(L/minFrother(mg/L)Fig. 29 Effects of nanobubble flow rate and frother dosage on the flotation ash contentAsh content (10923000.00.050.160.270.38049Fig. 30 Effects of nanobubble flow rate and flotation feed particle size on the flotation ash contentFig. 31 shows the effects of the collector dosage high level of collector dosage and the middle-highand the frother concentration on the flotation selectiv- level of the frother concentration. as the frother con-index when the flotation feed mean particle size, centration and collector dosage increase from mid-feed solids content, and flotation retention time were dle-high level to high level, the flotation selectivity0.27 mm,8.0%, 5.0 min, and 6.6 L/min, respectively. index decreased obviously. Generally, coarse coalIt can be observed from Fig. 31 that the flotation se- froth flotation requires high level of collector dosage,lectivity index increases with increasing the collector which is detrimental to fine/ultrafine coal particledosage from 0. 19 to 0.66 kg/t and frother concentra- flotation because of the decrease in flotation selectivtion from 13 to 19 mg/L. The highest flotation selec- itytivity index was obtained at the area of the middle-0.190.340.500660.81Collector(kg/t)Fig. 31 Effects of collector dosage and frother dos n中国煤化工Fig. 32 depicts the effects of the nanobubble flow meN MHGtent, and flotationrate and collector dosage on the flotation selectivity retention15mgL,0./mm,8.0%,and5.0index when the frother concentration, flotation feed min, respectively. The response surface and the con-Mining Science and TechnologyNoStours of the flotation selectivity index shown in Fig. from 58% to 70%6, 71%, and 72% at the collector32 indicate that the flotation selectivity index consid- dosages of 0.19, 0.34, and 0.50 ke/t, respectivelyerably increases with increasing the nanobubble flow Nanobubbles increase the flotation selectivity indexrate. At a given nanobubble flowrate, the flotation by more than 12% at varying collector dosages. It canelectivity index increases slightly as the collebe concluded by comparing Fig. 31 and Fig. 32 thatdosage increases from 0. 19 to 0.66 ke/t. However, the nanobubbles are beneficial for wide size range coalflotation selectivity index decreases with further in- particle flotation, solving the problem that the condicreasing the collector dosage. Fig. 32 depicts that tions that favor coarse coal flotation are detrimentalwhen the nanobubble flow rate increases from 0 to to the rejection of ash in the fine and ultrafine co9.9 w/min, the flotation selectivity index increases fractionSelectivity index(%)日9.96650.00.190340.500.660.8Fig. 32 Effects of nanobubble and collector dosage on the flotation selectivity indexFig. 33 shows the effects of the nanobubble flow 33 indicate that the flotation selectivity index consid-ate and frother concentration on the flotation selec- erably increases with increasing the nanobubble flowtivity index when the collector dosage, flotation feed rate. At the frother concentration of 15 mg/L, the nlo-mean particle size, feed solids content, and flotation tation selectivity index increases from 58% to 65%0retention time were 0.50 kg/t, 0.27 mm, 8.0%, and 5.0 70%, and 72% as the nanobubble flow rate increasesmin, respectively. The response surface and the con- from 0 to 3.3, 6.6, and 9.9L/min, respectivelytours of the flotation selectivity index shown in Fig13.2Selectivity index (a)Fig. 33 Effects of nanobubble and frother dosage on the flotation selectivity indexFig. 34 depicts the effects of the flotation feed flotation feed mean particle size. It can be clearlymean particle size and the nanobubble flow rate on seen from the contours of the flotation selectivitythe flotation selectivity index when the collector dex shown in Fig. 34 that when the nanobubble flowdosage, frother concentration, flotation feed solids rate increases fromo to 9.9 L/min, the flotation seleccontent, and flotation retention time were 0.50 kg/t, tivity index increases from 38% to 60%, 47% to 66%615 mg/L,8.0%, and 5.0 min, respectively. The flota- 58% to 72%, 68% to 77%, and 77% to 82% for thetion selectivity index increases with increasing the mea中国煤化工f049038027,nanobubble flow rate and decreasing the flotation 0.16The effect of nano-feed mean particle size. The area of the highest flota- bublCNMHGy index decreasestion selectivity index is located at the high level of with decreasing the mean flotation feed particle sizenanobubble flow rate and the low level of the At a given nanobubble flow rate the flotation selecFAN Maoming et altivity index decreases as the flotation feed mean improve the coarse particle flotation recovery, thusparticles are more difficult to float and some coarse lectivity inder ease the coarse particle flotation se-particle size increases because the coarser coal remarkably incparticles are unliberated. Nanobubbles considerablySelectivity index (%9132050.160.270.380Mean size(mm)Fig. 34 Effects of nanobubble and flotation feed particle size on the flotation selectivity indexFig. 35a depicts the effects of the flotation feed tion, mean flotation feed particle size, and the flota-solids content and the nanobubble flow rate on the tion feed solids content were 0.50 k/t, 15 mg/L, 0.2flotation selectivity index when the collector dosage mm, and 8.0%, respectively. It can be clearly seenfrother concentration, mean flotation feed particle from Fig. 35 that the flotation selectivity index insize, and flotation retention time were 0.50 kg/t, 15 creases considerably with increasing the nanobubblemg/L, 0.27 mm, and 5.0 min, respectively. Fig. 35b flow rate from 0 to 9.9 L/min. There is no observableshows the impacts of the flotation retention time and increase in the flotation selectivity index as furtherthe nanobubble flow rate on the flotation selectivity increasing the nanobubble nlow rate from 9.9 to 13. 2index when the collector dosage, frother concentraNanobubble flow(LininFig. 35 Effects of nanobubble, flotation feed solids concentration, and retention time on the notation selectivity indexIn summary, the above six-factor five-level central creases the flotation selectivity index, which indicatescomposite experimental test results provide proof that that the overall coal flotation performance is considnanobubbles can considerably improve the flotation erably improved. The flotation selectivity index is ancombustible recovery, the flotation product yield, and important parameter in froth flotation of wide particlethe flotation selectivity index at varying collector size range coal. In practice, attempts to improvedosage, frother concentration, flotation feed mean coarse coal or hard-to-float coal particle recoveryparticle size, feed solids concentration, and flotation using higher dosage of collector often lead to dimin-tention time. Because of the significant improve- ishing flotation selectivity in addition to incurring ament of flotation recovery of coarse and hard-to-float higher flotation operation cost.medium ash content coal particles, nanobubbles The improved coal flotation selectivity by nano-slightly increase the flotation product ash content at a bubtgiven collector dosage, frother concentration, flota- pret中国煤化工 at nanobubbles aretion feed mean particle size, feed solids concentration, photCN MH Gthan on the hydroand flotation retention time. The application of philic impurity mineral particle surface, which selec-nanobubbles in coal froth flotation significantly in- tively increases the coal particle hydrophobicity andVo.,20No.5enlarges the flotation rate difference between the hy- flotation feed rate, feed solid content, and flotationdrophobic coal particles and the hydrophilic impurity feed particle size. Unless otherwise specified, allmineral particles. The nanobubbles are preferentially column flotation tests of coal sample B were perforgenerated on the surface of hydrophobic coal parti- med under the following conditions: froth depth: 30cles because the adhesion force between hydrophobic cm; superficial gas flow rate: 1.5 cm/s; tailingcoal particle surface and water is much weaker than recirculation flow rate ratio between the nanobubblethat between the hydrophilic mineral particles surface generator and the conventional size bubble generator:and water. The rupture of water-coal interface is 60%0, collector dosage: 0.23 kg/t fuel oil; frothermuch easier than the water-mineral interface. The concentration: 15 mg/L MIBC; superficial washboundary water layers on hydrophilic mineral surface water flow rate: 0.3 cm/s; superficial feed slurry floware ordered to a higher extent and more stable than rate: 0. 75 cm/s; feed slurry solids concentration: 9%6those on hydrophobic surfaces of coal particles. Also The test results are presented and discussed in thethe hydrophobic coal particles may entrap gas in followingcrevices,thus greatly reducing cavitation thresholdFig. 36a indicates the effect of nanobubbles onand considerably enhancing cavitation efficiency by combustible recovery and product as at varying su-acting as the main source of gas nuclei. It is also be- perficial air velocity in the flotation column. It can believed that most of the nanobubbles formed on hy- clearly seen from Fig. 36a that the presence of nano-drophobic coal surface remain attached to particle bubbles significantly increased the flotation combuswhile those on hydrophilic mineral surface are more tible recovery, especially at lower air velocities. Forreadily detached from the particle, which is a selec- example, at 0.5 cm/s air velocity, the recovery wastive process that enhances flotation performance. The almost 30% higher in the presence of nanobubbles. Atnanobubbles preferably generated on or attached to the highest air velocity of 2.5 cm/s, the difference inhydrophobic coal surfaces remarkably improve three recovery was about 8%. It is noticed that the productkey steps of the froth flotation progress: bubbleparti- ash was slightly higher when the nanobubbles werecle collision, bubbleparticle adhesion, and bub- present in the column. In order to evaluate the effectble-particle detachment. The collapse of some nano- of nanobubbles on the flotation separation efficiency,bubbles on the oxidized coal surface may result in the the separation curve, i. e, combustible recovery vs.damage of hydrophilic oxidized coal surface, which product ash curve, is shown in Fig. 36b. The fact thatshould increase the oxidized coal hydrophobicity, the separation curve obtained with nanobubbles isimprove its adsorption properties in relation with flo- above the one without nanobubble indicates that thetation collectors and thus improve its flotation perof nanobubbles improved the separation effiformances. Therefore using nanobubbles is a better ciency. The improvement was more significant atapproach to improve the flotation performances of lower product ashes. For example, the combustiblewide particle size range coal and hard-to-float coalrecovery with nanobubbles was more than 28%4.4 Column flotation of coal sample Bhigher at a product ash of 6%. The minimum im-provement in combustible recovery created by nano-Extensive evaluation of the effects of nanobubbles bubbles was about 8% at a given product ash, whichon the flotation performance of coal sample B was is still very significant for the coal industry>.Figcarried out in a specially designed laboratory-scale 36c depicts the flotation selectivity index as a funcflotation column. A number of flotation experiments tion of the flotation combustible recovery, which waswere performed at varying superficial air velocities, obtained at varying superficial air velocity with andrecycling slurry flow rate ratio between the nanobu- without nanobubbles. The presence of nanobubblesbble generator and the static mixer, collector dosage, increased the coal flotation selectivity index by 5%tofrother concentration, superficial wash water rate, 7% at varying flotation combustible recovery.Product ash wtth nanobubblesWithout nanobubbleH Product ash without nanobubble0.51.01.52025Superficial air velocity(cm/s)Product ash中国煤化工Fig. 36 Effect of nanobubbles on the combustible recoveryCN MH Gindex atvarying superficial air velFAN Maoming et alNanobubble generation and its applications in froth flotationThe higher flotation recovery observed with nano- the combustible recovery slightly decreases. The flo-bubbles was not solely caused by higher bubble sur- tation selectivity index increases from 52% to 68%asface area. Because the data shown in Fig. 36 clearly the tailing re-circulating flow rate ratio from 0%toindicate that while more bubble surface area at higher 60%. After that, there is a slight decrease in flotationir flow rate without nanobubbles can increase flota- selectivity index. The combustible recovery vstion recovery, it was achieved at the expense of product ash curve shown in Fig. 37b was drawn basedhigher product ash. The separation efficiency curve of on the data shown in Fig. 37a. It can be concludedcombustible recovery vs. product ash shown in Fig. from Fig. 37 that the best separation was achieved36b reveals that nanobubbles improved separation with about 60% tailing flow passing through thesharpness, not just combustible recoverynanobubble generator under the given conditions em-Fig 37a depicts the combustible recovery, flotation ployed for these tests. It should be noted that the op-selectivity index, and product ash as a function of the timum re-circulating slurry ratio should depend ontailing re-circulating flow rate ratio between the feed particle size distribution. More slurry should gonanobubble generator and the static mixer that gener- to the conventional bubble generator when the feedated conventional size bubbles. A flow rate ratio of contains more coarse coal particles. This is becausezero means there were no nanobubbles in the flota- nanobubbles or their aggregates may not have enoughtion column and a 100% flow rate ratio implies that levitation to lift coarse coal particle in water and thusall tailing slurry flow went through the nanobubble more conventional sized bubbles are needed. Thegenerator. It can be observed from the figure that the hydrophobicity of coal particles also affects the flowcombustible recovery increases from 65% to 85% rate ratio, the lower hydrophobicity coal particleswith increasing the tailing re-circulating flow rate need more nanobubbles to improve their floatabilitys,ratio from o% to 60%o. with further increathus requires the higher nanobubble flow rate ratiotailing recirculating flow rate ratio from 60% to 100%,Selectivion recovery7.577798.18.38.5Flow rate ratio (%Product ash(%)Fig 37 Effect of the nanobubble flow rate ratio on the combustible recovery, product ash, and flotation selectivity indexFig. 38a indicates the combustible recovery and bles is always substantially above the one withoutproduct ash as a function of collector dosage with and nanobubbles whereas the product ash was slightlywithout nanobubbles. It can be seen from the figure higher in the presence of nanobubbles. The separationthat the presence of nanobubbleIncreaseperformance curve shown in Fig. 38b suggests thatbustible recovery by 11% to 25% at varying collector the flotation combustible recovery was 12%0-219dosage from 0.05 to 0.50 kg/t More significant im- higher for a given product ash when nanobubblesprovement in flotation recovery was observed at were employed. Fig 38c indicates that the flotationlower collector dosages, indicating that the nanobub- selectivity index with nanobubbles is more than 6%bles produced on coal surface at lower collector dos- higher than that without nanobubble at a given com-age plays a bigger role as a secondary collector. The bustible recoverycombustible recovery curve generated with nanobub--e without nanobubble10求中国煤化工Mwbo8090100Collector dosage(kg/Product ashCNMHGrecovery(%)Fig. 38 Effect of nanobubbles on combustible recovery, product ash and flotation selectivity index at different collector dosageMining Science and TechnoloFig. 39 shows the effects of nanobubbles on flota- 16.6%, 17.5%, 18.7%, 21.8%, 21.2%, and 23.0% attion combustible recovery, product ash content and the frother concentration of 5, 10, 15, 20, 25, and 30flotation selectivity index performed at different mg/L, respectively. Fig. 39b depicts that the nano-frother concentration Similar to the above discussed bubbles increased combustible recovery by 17%0-test results obtained at varying collector dosages, a 20% at a given flotation product ash. It can be seenpronounced increase in combustible recovery was from Fig 39c that the flotation selectivity index withobserved at a given frother dosage when nanobubbles nanobubbles is 13%0-15% higher than that withoutwere introduced, Fig. 39a indicates that the use of nanobubble at a given flotation product ashnanobubbles increased combustible recovery byRecovery)- With nanobubblesbubble 11Forhter concentration(mg/L)Product ash content(%)g. 39 Effect of nanobubbles on the combustible recovery, product ash and flotation selectivity index atvarying frother concentrationFig 40a indicates the effect of nanobubbles on the present than when the nanobubbles were absent in theombustible recovery and product as at varying su- column. Fig 40b shows the combustible recovery vsperficial wash water velocity in the flotation column. product ash curve for evaluating the effect of nano-It is well known that the superficial wash water ve- bubbles on the flotation separation efficiency. The uselocity has significant influence on column flotation of nanobubbles increased the combustible recoveryperformance. It can be observed from Fig. 40a that by 5%0-18% at the flotation product ash content ofthe presence of nanobubbles increased combustible 6%-10%. Fig. 40c depicts the flotation selectivityrecovery by 5%, 11%, 22%, 21%, 17%, and 9% at the index as a function of the flotation combustible re-superficial wash water velocity of 0.1, 0. 2, 0.3, 0.4, covery obtained at varying superficial wash water5, and 0.6 cm/s, respectively. Nanobubbles have velocity with and without nanobubbles. The presencenore significant effect on the combustible recovery at of nanobubbles increased the coal flotation selectivitythe middle and middle-high level of superficial wash index by 4%0-8% at varying flotation combustiblewater velocity. It also can be noticed that the product separation selecivil anobubbles enhanced flotationwater velocity than at the low level superficial wash recovery, indicatingash was slightly higher when the nanobubbles were∞ With nanobubbleso With nanobubblesProduct ash without nanobubbleWithout nanobubblewithout nanobubble5060708090I00Fig. 40 Effect of nanobubbles on the combustible recovery, product ash and flotation selectivity index atvarying superficial wash water velocityFig. 41a depicts the flotation combustible recovery, tention time. Combustible recovery decreased as feedproduct ash and flotation selectivity index as a func- sIttion of the superficial feed slurry velocity with and resiv凵中国煤化工 uced flotwithout nanobubbles. The superficial feed slurry ve- frorCNMH Gobubbles increasedlocity is a very important parameter for flotation the flotation combustible recovery by 11%, 18%column operation because it affects the flotation re- 22%, 26%, and 33% at the superficial feed slurry ve-locity of 0. 25, 0.5, 0.75, 1.0, and 1. 25 cm/s, respec- when nanobubbles were employed. Fig. 41c indicatestively. More significant combustible recovery im- that the flotation selectivity index with nanobubblesprovement by nanobubbles can be observed at higher is 3%-10% higher than that without nanobubble at asuperficial feed slurry velocity. This is because given combustible recovery. The most considerablenanobubbles increased the coal particle flotation rate influence of nanobubbles on the flotation combustibleconstant. The separation performance curve shown in recovery and flotation selectivity improvement ooFig. 41b suggests that the flotation combustible re- curred at the superficial feed slurry velocity of 0.75covery was 3%0-12% higher for a given product ash§without nanobubble-o- with nanobubble◆ Without nanobubble012perficial feed slurry velocity(em/s)Product ash (%)Product ash(%)41 Effect of nanobubbles on the combustible recovery, product ash and flotation selectivity index atvarying superficial feed slumy velocityFig 42a indicates the impact of nanobubbles on the Fig. 42b that the flotation combustible recovery wasflotation combustible recovery, product ash and flota- 24%, 23%0, 22%0, 15%,14%, and 12% higher withtion selectivity index at different feed slurry solids nanobubbles than without nanobubble at the flotationconcentration. It can be clearly seen from the figure product ash contents of 6%6, 7%0,8%, 9%, 10%, andthat the flotation combustible recovery decreased 11%, respectively. Fig. 42c indicates that the use ofwith increasing flotation feed solids concentration, nanobubbles increased the flotation selectivity indexdue to the fact that less bubble surface was available by 19%6, 18%, 17%6, 129, 11%, and 9% at the flotaor each particle at higher solids concentration. tion product ash contents of 6%, 7%, 8%, 9%, 10%However, the flotation combustible recovery curve and 11%, respectively. It can be concluded from Fig.with nanobubbles is always substantially above that 42a and Fig. 42c that nanobubbles play the biggestwithout nanobubbles, suggesting that use of nano- role in the flotation combustible recovery and flotabubbles significantly increased the flotation combustion selectivity at the highest flotation feed solidstible recovery. The flotation combustible recovery concentration. This is because nanobubbles remarka-was 10%, 16%, 22%, 24%, 25%, and 26% higher bly increased the bubble surface available for eachwith nanobubbles than without nanobubble at the particle at higher solids concentration. The better seflotation feed solids concentration of 5%6.7%.9%lectivity or separation sharpness observed with11%, 13%, and 15%, respectively. The product ash nanobubbles at varying solids concentration may bewith nanobubbles was slightly higher than without attributed to the preference of nanobubbles to coalnanobubbles at varying tested solids concentration. particles. In other words, nanobubbles not only inFig 42b shows the flotation combustible recovery vS. creased flotation recovery, it also improved flotationproduct ash content curve that was drawn based on selectivity. This is because nanobubbles generatedthe data shown in Fig. 42a. It can be observed from using the cavitation principle form preferentially on-o- with nanobubbles-e- without nanobubbleFeed solids concentration(%)Product as中国煤化工 ct ash content(%))Fig. 42 Effect of nanobubbles on the combustible recovery,THsCNMHGndexvarying feed slurry solids concentrationng Science and TechnologyVol 20 No5the surface of more hydrophobic particles that have ery of coarse particles since smaller bubbles producelower work of adhesion Wa or A(1+cos0(d is water more stable froth and higher water recovery. The highsurface tension and 6 is contact angle). Our funda- recovery of ultrafine coal particles observed in themental study on the effect of nanobubbles on differ- presence of nanobubbles can mostly be attributed toent coal particles has confirmed this theorythe fact that the bubble-particle collision probabilityFig. 43 shows effect of nanobubbles on the flota- was much higher with nanobubbles and the directtion combustible recovery, product ash content and formation of nanobubbles on coal surface by hydro-flotation selectivity index of the varying size coal dynamic cavitation rendered ultrafine coal particleparticles. The data were obtained by the particle size flotation possible even without the bubble-particleanalysis of the flotation feed sample, the clean coal collision process which is known to be the rate de-product and tailing samples. It can be clearly seen termining step for ultrafine particle flotation. Fig 4from the figure that the flotation performance is indicates that product ash was slightly higher whenstrongly dependent on particle size. Fig. 43 clearlynanobubbles were present, which is a result of highershows that use of nanobubbles considerably increased recovery of particles of lower floatability or higherrecovery of both ultrafine and coarse coal particles In ash content. Fig. 4b clearly indicates that use offact, the recovery was increased by up to 40% for nanobubbles expanded the particle size range for ef-ultrafine particles and 55% for coarse particles. It is fective flotation separation. The most significant im-believed that nanobubbles formed on coal surface provement in flotation selectivity index was observedreduced the high detachment probability of coarse with particles smaller than 0.08 mm and larger thanparticles by making coal more hydrophobic and more0.4 mm. For example, the flotation selectivity indexresistant to hydrodynamic turbulence. More stable was increased by about 35% with 30 microns parti-froth produced by nanobubbles also helps the recov- cles and 30% with 1.5 mm particles-o- With nanobubbles量Particle size(mm)Fig. 43 Effect of nanobubbles on the flotation combustible recovery, product ash and flotation selectivity indexof varying size coal particles4.5 Column flotation of coal sample Cwith the corresponding release analysis curves, it canbe clearly seen that the flotation combustible recov-Fig. 44 shows effect of nanobubbles on the product ery curves with nanobubble are much closer to theyield and combustible recovery of coal sample C at corresponding release analysis curves than thosethe collector dosage of 0.3 kg/t, frother concentration without nanobubble, indicating that the use of nano-of 15 mg/L, the superficial gas flow rate of 1.5 cm/s, bubbles considerably improved the flotation combus-feed slurry solids concentration of 10%, and the tail- tible recovery of the coal sample C.ing recirculating flow rate ratio of 60% between thenanobubble generator and the conventional size bub-ble generator. It can be seen from the figure that thepresence of nanobubbles increases the flotation com-bustible recovery by 12%, 16%0, 26%, and 20% at theproduct ash contents of 5%0, 7.5%, 11%, and 15%,respectively. The use of nanobubbles increases coalflotation product yields by 7%,9%, 15%, and 12%atthe product ash contents of 5%, 7.5%, 11%, and 15%respectively. The most significant effect on the prod中国煤化工4050uct yield and combustible recovery can be observedat the product ash content of about 11%. By comparCNMHGing these combustible recovery curves in the presenceFig. 44 Erect ot nanodudbles on flotation yield andof nanobubbles and in the absence of nanobubbleble recoveryFAN Maoming et alNanobubble generation and its applications in froth flotation叫二恤油4)The coal sample C that contains considerablequantities of intermediate-ash content particles ismuch more difficult to float than the coal sample AAt the prodproduct yield and the flotation combustible recoveryare 31% and 55%, respectively, which are signicantly lower than those of the coal sample A.5)The coal particle floatability of a given coalample a particle size fraction decreases with in-Product ash(%)creasing the coal particle density. Because the organicFig. 45 Effect of nanobubbles on flotation selectivitycomponents those are most amenable to bubble at-tachment decrease as the coal particle density inFig. 45 depicts the flotation selectivity index as a creases. the bubble volume or the bubble numberfunction of product ash content with and without required for floating a lower density coal particle of ananobubbles. It can be clearly observed from Fig. 45 given particle size is smaller than that required forthat the presence of nanobubbles remarkably in- floating a higher density coal particle. In other words,creases the flotation selectivity at varying product ash the increase in coal particle density of a given particlecontent. The coal flotation selectivity indexes with size results in decrease of the amount coal surfacenanobubbles are 12%, 15%, 23%, and 16% higher area available for bubble attachment which reducesthan without nanobubble at the product ash contents coal particle/bubble attachment probability, and in-of 5%, 7.5%, 11%, and 15%, respectively. Less flota- crease coal particle mass which increases the coaltion selectivity improvement is observed when the particle detachment probabilityproduct ash is low or high. This further proves that6 The use of nanobubbles remarkably increasesthe use of nanobubbles can significantly improve coal the various density coal flotation combustible recov-flotation yield, combustible recovery and the selectiv- eries of 600-355 microns and 75-0 microns sample Acoal particles. The presence of nanobubbles increasesthe 600-355 microns coal particles combustible re-5 Conclusionscoveries of the various density fractions: minus 1.3,13-14,14-1.5,1.5~1.6,and1.6-1.8gcm3by1)The particle size and ash distribution are sig131%,22.2%,27.6%,22.5%,andl1.9%, respecnificant different between the two tested coal samples tively, indicating more significant effect on the me-A and B. However the ash contents of both samples dium density fractions than on the low and high den-much wider particle size distribution than the sample separation density from 1.44 to 1.54 g/cm.Thepresence of nanobubbles increases the 75-0 microns2)The ash content of the coal sample A increases coal particles combustible recoveries of the variouswith increasing the density, more significantly when density fractions: minus 1.3, 1.3-1.4, 1.4-1.5the density increases from 1. 25 to 1.5 g/cmand from 1.5-1.6, 1.6-1.8, and 1.8-2.0 g/cm by 14.6%, 25.7%01.9 to 2.25 g/em. The liberation of the intermediate- 29.7%, 33.8%, 30.6%, and 9.5%, respectively. Thedensity particles with density between 1.5(or 1. 4) nanobubbles increase the effective separation densityand 1.9 g/em is not complete and thus it is difficult from 1.42 to 1.61 g/cm. The nanobubbles have moreto separate these coal particlesconsiderable effect on the 75-0 microns finer parti3)The particle size has significant effects on cles than on the 600-355 microns coarser particlescoarse and fine/ultrafine coal flotation combustible More high density fraction particles of the 75-0 mi-recovery and product yield of the coal sample A. The crons finer particles were floated than that of therelease analysis indicates that at the product ash con- 600-355 microns coarser particlestent of 7.5% the product yields of 600-355, 355-1807)At a given product ash content, the nanobubbles180-75, and 75-0 microns are 56%, 75%, 78%, and have more significant effect on the combustible re-39%, respectively. The yield of coarse 600-355 mi- coveries of 600-355 microns coarse sample A parcrons fraction is 22% lower than that of the 180-75 cles and 75-0 microns fine coal particles in the me-microns fraction. The yield of fine 75-0 microns dium product ash range than in the lower or thefraction is only half of the 180-75 microns fraction's higher product ash range. The presence of nanobub-yield. The flotation combustible recoveries of blescreases the 600-355 microns coal particles600-355,355-180,180-75,and75-0 microns are flota70%,93%,97%, and 89%, respectively. The flotation 18%中国煤化工 recovery by up tobbles increase thearse 600-355 microns 75fraction is 19%-27% lower than that of the otherCN MH Gon yield and com-uy up w 1v70 and 38%, respecnree particle size fractionstively.Vol20 No, 58)Nanobubbles increase the 600-180 microns ash contents of 3%, 5%0, 7.5%, and 10%, respectively.coarse sample A coal flotation product mean particle The presence of nanobubbles increased the flotationsize from 250 to 325 microns, indicating that nano- combustible recoveries of 600-355, 355-180, andbubbles greatly improved the coarse coal particle flo. 180-75 microns size fraction particles by 27%, 19%tation recovery. The presence of nanobubbles de- and 5%, respectively, at the product ash content ofcreases the 180-0 microns fine/ultrafine coal flotation 7.5%. Nanobubbles have the most significant im-product mean particle size from 103 to 69 microns, provement effect on the 355--600 microns coarse coalsignificantly improved the fine/ultrafine coal particle particles flotation because the presence of nanobub.flotation recoverybles on the coarse coal particle surfaces facilitates9)Nanobubbles widen the flotation particle size particle attachment to conventional-sized bubblesrange. For the 600-180 microns coarse coal sample a The nanobubbles on the coarse coal particles do notflotation, the use of nanobubbles increases mean par- have sufficient buoyancy to float the particle byticle size of the first, second, and third cell products themselves, but the surface of a coarse coal particlein a bank of three 10-L mechanical cells by 65, 90 coated with nanobubbles is more hydrophobic thanand 105 microns, respectivelyOn the contrary.without nanobubbles. The wetting film separating thenanobubbles decrease the 180-0 microns fine/ultra- colliding convention-sized bubbles and nanobubblesfine coal flotation product mean particle size of the is more unstable than the film between the convenfirst, second and third cell from 108 to 80. 96 totion-sized bubbles and coarse coal particles. The sig-and90 to 62, respectively.nificant difference in diameter between the conven-10) Nanobubbles have more significant effectstion-sized bubble and nanobubble leads to a hugethe sample A flotation combustible recovery before differential capillary pressure, which facilitates the1.5 minutes flotation time than that after 1.5 minutes rupture of water films separating convention-sizedflotation time. Nanobubbles increased the combus- bubbles and nanobubbles. Thus the presence oftible recovery of 75-0 microns particle size fraction nanobubbles on coal particles surfaces accelerates theby 20.5%, 18.3%, 10.4%0,8.8%, 6.8%, and 4.8% at water film rupture and bubble-particle collision. Inthe flotation time of 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0 other words, nanobubbles on the surface of coal parminutes,respectively. The improvement of the com- ticle act as a secondary hydrophobizing agent to enbustible recovery decreases as the cumulative flota- hance the flotation recoverytion time increases from 0.5 to 4 minutes. when the4)The use of nanobubbles increases the flotationcumulative flotation time is l minute, the combustible combustible recovery at the"elbow point"on therecoveries in the presence of nanobubbles are 27.0%, curves of flotation ash rejection vs flotation combus24.790,17.9%, and 18.3% higher than those in the tible recovery of varying size sample a particles bybsence of nanobubbles for 600-355, 355-180, 5%-10%. The ash rejection of 600-355 microns size180-75, and 75-0 microns particle size fractions, coal particles is about 11%, 7.5%0, 6%, and 5% higherrespectivelywith nanobubbles than without nanobubble at thee 11)The presence of nanobubbles improves the flo- flotation combustible recoveries of 83%, 75%, 70%,ion kinetics, Nanobubbles increase the flotation and 60%, respectively. At the flotation combustiblerate constants of 600-355, 355-180, 180-75, and recovery of 90%, nanobubbles increased the flotation75-0 microns sample A particles by 98.0%0, 98.4%, ash rejections by 13. 5% and 22% for 355-180 and50.0%, and 41.6%, respectively, which leads to the 180-75 microns size fraction coal particles, respecincreased capacity of a given flotation machinetively.12) The use of nanobubbles increases the various15)The use of nanobubbles is a feasible approachsize sample a coal particle flotation yield at a given to increasing flotation selectivity, especially for flota-product ash content. The presence of nanobubbles tion of particles over a wide size range. The flotationincrease the 600-355 microns coal flotation product time required by the coarse size fraction is too longyield by 14%, 22%, 16%, and 13% at the product ash for the fine sizes. The attempts to improve coarsecontents of 5%0, 7.5%0, 10%, and 12.5%, respectively. particle recovery using longer flotation time andAt the product ash content of 7.5%, nanobubbles in- higher dosage of collector often lead to diminishingrease the flotation product yields of 355-180, the finer size particles flotation selectivity in addition180-75, and 75-0 microns size particles by 16%0, 4%, to incurring a higher flotation operation cost. Theand 3%, respectivelyreduced flotation selectivity of finer size fractionse. 13)Nanobubbles have more considerable effects often produces undesirable high ash product and rethe flotation combustible recovery improvement sults in unacceptable overall product quality.Nanothan on the flotation product yield improvement, es- bublct on the flotationpecially for the 70-0 microns fine or ultrafine particle sele中国煤化工 of sample A. Thesize fraction of sample A. Nanobubbles increase the 60CN Dotation selectivity75-0 microns coal particle flotation combustible re- indeo ae 170, 9%, 7%, and 5%0coveries by 21%, 18%,8%, and 5% at the product higher than without nanobubble at the product yieldsNanobubble generation and its applications in froth flotationof 70%, 65%,60%, and 50%, respectively. At the a given product ash Nanobubbles improve flotationproduct yield of 70%, the use of nanobubbles in- performance more significantly at lower superficialreases the 355-180 and 180-75 microns coal parti- gas velocity and lower collector or frother dosagescles flotation selectivity indexes by 11% and 5%, re- The effect of nanobubbles on flotation performancespectively. The effect of nanobubbles on the flotation depends on coal particle size. Most significant im-selectivity increases with increasing particle size. provements in flotation recovery is observed withNanobubbles increase the flotation selectivity be- ultrafine(smaller than approximately 0.08 mm)andcause nanobubbles are preferentially generated and coarse coal particles(larger than 0.4 or 0.5 mm). Theadsorbed on hydrophobic coal particle surface rather flotation recovery is increased by almost 40% andthan on the hydrophilic ash-forming mineral particle 55% with 30 microns and 1.5 mm coal particles, re-urface, which increases the coal particle hydropho- spectively. Use of nanobubbles not only increases thebicity and enlarges the flotation rate difference be- flotation recovery but also improves the flotation se-tween coal particles and the hydrophilic ash-forming lectivitymineral particles19)Column flotation of the coal sample C further16)The six-factor five-level central composite ex- proves that the use of nanobubbles can significantlyperimental test results provide further proof that improve coal flotation yield, combustible recoverynanobubbles can considerably improve the flotation and the flotation selectivity. The presence of nano-combustible recovery, the flotation product yield, and bubbles increases the flotation combustible recoverythe flotation selectivity index at varying collector by 12%, 16%0, 26%, and 20% at the product ash con-dosage, frother concentration, flotation feed mean tents of 5%, 7.5%, 11%, and 15%, respectively. Theparticle size, feed solids concentration, and flotation most significant effect on the product yield and com-retention time. The application of nanobubbles in coal bustible recovery is at the product ash content offroth flotation significantly increases the flotation aboutThe flotation combustible recoveryselectivity index, indicating that the overall coal flo- curves with nanobubble are much closer to the corre-tation performance is considerably improved. The sponding release analysis curves than those withoutnanobubbles preferably generated on or attached to nanobubble, indicating that the use of nanobubbleshydrophobic coal surfaces remarkably improve three considerably improved the flotation combustible re-key steps of the froth flotation progress: bub- covery of the coal sample C. The presence of nano-ble-particle collision, bubble-particle adhesion, and bubbles significantly increases the flotation selectivbubble-particle detachment. The collapse of some ity at varying product ash contents. The coal flotationnanobubbles on the oxidized coal surface may result selectivity indexes with nanobubbles are 12%, 15%,in the damage of hydrophilic oxidized coal surface, 23%, and 16% higher than that without nanobubble atwhich may increase the oxidized coal hydrophobicity, the product ash contents of 5%, 7.5%, 11%, and 15%,improve its adsorption properties in relation with flo- respectivelytation collectors and thus improve its flotation per-formances. Therefore using nanobubbles is a better Acknowledgementspproach to improve the flotation performances ofwide particle size range coal and hard -to-float coalThe authors gratefully acknowledge the Center for17)The following conclusions can be drawn fromdvanced Separation Technologies (CAST), thethe column flotation tests of the coal sample B Slurry Florida Institute of Phosphate Research(FIPR),andrecirculating ratio in the nanobubble generator and the National Natural Science Foundation of Chinaconventional-size bubble generator is an important (Nos. 50921002 and 90510002)for the financialfactor for the performance of nanobubbles. For the support. Special thanks to the Director of CAST. Dcolumn and coal sample employed in this study, the Roe-Hoan Yoon and the project manager of FIPR,Droptimum ratio is 60% for the nanobubble generator Patrick Zhang for their valuable advice and support.and 40% for the conventional bubble generatorThe authors wish to thank the faculty and staff inNanobubbles significantly increase flotation recovery the Mining Engineering Department, CAER(Centerand separation efficiency under various conditions. for Applied Energy), ERTL(Environmental ResearchFor example, at 0.5 cm/s air velocity, the recovery is Training Laboratories) and KGS(Kentucky geologi-almost 30% higher in the presence of nanobubbles. cal Survey) Lab. The authors also wish to thank MsThe use of nanobubbles increases recovery by about Jin Xiaoyan for her excellent editing12%0-25% at a given collector dosage. The separationMaoming Fan would like to express hisperformance curve suggests that the combustible re- thanks to his wife Rui Zhao, his son Xingyucovery with nanobubbles is 12%0-21% higher than Fan fothat without nanobubble for a given product ashV山中国煤化工吗gess18)A17%-24% increase in combustible recovery gratituCNMHG, Dr Daniel Taois produced at a given frother dosage and a 17%-21% his dissertation committee members, Dr. Rick Qimprovement in combustible recovery is observed for Honaker, chair and professor of Mining EngineeringDepartment at UK, Dr. M. Pinar Menguic, professorTechnology Advance and Adoption. Littleton: Society forof Mechanical Engineering, and Dr. B. K. ParekhMining, Metallurgy, and Exploration, 2010senior research engineer and Editor- in-chief of Inter- [13] Tao D, Fan M, Honaker R Enhanced fine coal columnnational Journal of Coal Preparation and Utilization,flotation using cavitation concept. In: Proceedings ofXVI intermational Coal Preparation Congress.for their guidance and motivation. His sincere thankston,2010.are extended to Dr. Eric Grulke, Associate Dean and [14] Fan M. Picobubble Enhanced Flotation of Coarseprofessor of Chemical Engineering. He thanksPhosphate Particles [Ph D. dissertation]. LexingtonZ.A. Zhou, Alberta Research Council and ProfessorUniversity of Kentucky, 2008Zhenghe Xu at University of Alberta, Canada. He [15] Kohmeunch J. Christodoulou L Fan M, Mankosa Malso wishes to thank his colleagues in CPt and Erieztation. In: Proceedings of the 42d annual canadiaManufacturing CoMineral Processors Operators Conference. OntarioMaoming Fan's deepest thanks go to his first Ph.D.Canadian Mineral Processors 2010.advisor, Professor Chen Qingru, an 85-year-old Aca- [16] Zhou Z A. Xu Z, Finch JA, Hu H, Rao S R. Role ofdemician of Chinese Academy of Engineering. Pro-hydrodynamic cavitation in fine particle flotation. 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