Effects of coal rank,Fe304 amounts and activation temperature on the preparation and characteristics

MINING° Science directSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20(2010)0872-0876ww.elsevier.com/locate/jcumtEffects of coal rank, Fe3 O4 amounts and activationtemperature on the preparation and characteristicsof magnetic activated carbonYANG Mingshun, XIE Qiang, ZHANG Jun, LIU Juan,WANG Yan, ZHANG Xianglan, ZHANG QingwuSchool of Chemical and Environmental Engineering, China University of Mining Technology, Beijing 100083, ChinaAbstract: Coal-based Magnetic Activated Carbons(CMACs)were prepared from three representative coal samples of variousranks: Baorigele lignite from Inner Mongolia; Datong bitumite from Shanxi province; and Taixi anthracite from Ningxia Hui Auto-nomous Region. Fe3O4 was used as a magnetic additive. A nitrogen-adsorption analyzer was used to determine the specific surfacerea and pore structure of the resulting activated carbons. The adsorption capacity was assessed by the adsorption of iodine andmethylene blue. X-ray diffraction was used to measure the evolution behavior of Fe O4 during the preparation process. Magneticproperties were characterized with a vibrating-sample magnetometer. The effect of the activation temperature on the performance ofCMACs was also studied. The results show that, compared to Baorigele lignite and Taixi anthracite, the Datong bitumite is moreappropriate for the preparation of CMACs with a high specific surface area, an advanced pore structure and suitable magneticFea Oa can effectively enhance the magnetic properties and control the pore structure by increasing the ratio of meso-An addition of 6.0% Fe 3 O4 and an activation temperature of 80C produced a CMAC having a specific surface area, anand ,adsorption, a methylene blue adsorption and a specific saturation magnetization of 1152.0 m2/g, 1216.7 mg/g, 229.5 mg/gKeywords: magnetic activated carbon; coal rank; Fe3O4; activation temperature1 Introductionof lignite(from Baorigele, Inner Mongolia), bitumite( from Datong, Shanxi province)and anthracite(fromTons of Spent Activated Carbon(SAC) are derived Taixi, Ningxia Hui Autonomous Region)along withfrom a variety of applications, including electrode various amounts of Fe3O4. Efforts were made to de-materials, gas adsorption and water purification, gold termine the effects of coal rank, magnetic additivesrecovery from cyanide solutions and catalyst car- and activation temperature on CMAC's performancerier1-91.These SACs challenge general separation as characterized by pore structure, specific area andmethods and cause economic losses. Coal-based magnetic properties. Of special importance in thisMagnetic Activated Carbon(CMAC) manifests its study is the idea that it could facilitate the effectiveefficiency and economy in the recovery of spent ac- recovery of spent activated carbonstivated carbons by the magnetic separation approachMany researchers have prepared CMAC's using var2 Experimentalus methods(6, 0-l However there is still room forimproving the adsorption capacity and magnetic per- 2.1 Preparation of magnetic activated carbonsformance of current CMAC's. Also, the effects ofcoal rank, magnetic additives and activation tem-Table 1 shows the proximate analysis of the threeoal samples, labeled Cl, C2 and C3 for the lignite,perature on the performance of CMAC's remain un- bitumite and anthracite, respectivelycertainIn this study, CMAC,s were prepared from samplesTable 1 Proximate analysis of raw coals中国煤化工FcMeceived 11 March 2010: accepted 22 May 2010487851.22Corresponding author. Tel: 86 10 62331014C2YHCNMHG30. 3 697E-mail address: dr-xieg@cumtbeducndoi:10.1016s16745264(09)6029820.54YANG Mingshun et alEffects of coal rank, Fe O4 amounts and activationThe three coal samples were ground into fine parti- safe to conclude that sample Char-C3-6 graphitcles smaller than 0.15 mm in diameter: 90%(by more easily than the other two. The values ofweight)of every sample passed a 200 mesh(0.076 shown in Table 2 further demonstrate this.mm)screen. Fe3O4 was used as the magnetic additivea powdered coal sample was mixed well with asighed portion of Fe3 O4 that varied from 0 to 8%in⊥ Peak of doo2% increments. Coal tar and deionized water werealso used in the formulation The mixture was thencompressed into a cylinder. These cylinders were airdried and then carbonized in a tube furnace at a heat-Char-C3-6ing rate of5°/ min frc261C and then maintained for 45 minutes. The resultingChar-CI-6chars were then activated at a temperature from 850to 940C in 30C increments. In this way a series of2030405060708090activated carbons was obtained. The resulting prod-ucts, the so-called CMACs, are distinguished thisFig. 1 XRD patterns of the resulting charsy: the sequence (CMAC-Coal sample)and theTable 2 Value of dooo ofamount of Fe3 O4. For example, CMAC-C1-6 represents the CMaC prepared by using lignite, Cl, with aFe]O4 amount of 6%(by weight)Char-C1-6Char-C2-624.2.2 Characterization of the magnetic activatedcarbonsNote: a Read at the peaks of dooon: b Calculated by the followThe iodine and methylene blue adsorption of theCMACs were measured referring to China StandardsGB77027-2008 and GB7702.6-2008The effect of coal rank on the pore structure of theNitrogen-adsorption isotherms were obtained with CMAC's is shown in Table 3. The SBet of thea nitrogen-adsorption analyzer(model NOVA-1200. CMAC's falls in the order: CMAC-C2-6>CMAC.Quanta Chrome, USA). The specific surface area wasC3-6>CMAC-CI-6. Sample CMAC-C2-6, derivedthen estimated from the Bet (Brunauer, Emmett,from bitumite, manifests advanced pore structureTeller)equation and the total pore volume was calcu- with a specific surface area, SBET, of 1152.0 m"ig duelated assuming a relative pressure P/P-0.995. The to its favorable index of burn-off (58.32%) 4 Howmicropore volume was determined by the t-plot me. ever, samples CMAC-C1-6 and CMAC-C3-6 havethod, the mesopore volume and the pore size distribu-lower SBEt and an unsuitable mesopore ratio. Astion by the BJH(Barrett, Joyner, Halender)me- shown in Table 1, when compared with the other twosamples the bitumite sample( C2)has a reasonableratio of volatile to fixed carbon capable of facilitatingThe crystal structure of the Fe]04 at each period the formation and enlargement of pores", especially-precursor, carbonized and activated-was moni-tored by X-Ray Diffraction (XRD, model D-MAx- mesopores that constitute 40. 509 of the total. AsRB, Rigaku, Japan)to investigate its conversion and shown in Fig. 2, the diameter of the mesopores isevolution behavior. The parameters of the XRD were mainly in the range of 3.0-5.0 nm. In contrast, theCu, Ka and l=0.154 nmlignite(C1) sample shows a higher ratio of meso-A Vibrating Sample Magnetometer (VSM, model pores. The anthracite CMAC (C3)contains micro-07, Lakeshore, USA)was employed to characterizemagnetic properties that included coercivity, magnetization, residual magnetism and specific magnetic005AC-C1-6AC-C3-63 Results and discussion3.1 Effect of coal rankThe effect of coal rank on carbonizing behaviorcan be determined from the d(ooz) peaks in the XRDpatterns of the chars. A sharp and narrow peak indi中国煤化工cates a higher degree of graphitization and vice versal3). As shown in Fig. 1, the dooz, peaks becomeCNMHGore distnbution ot the CMAcssharper and narrower as the coal rank increases. It isMining Science and TechnologyVoL 20 No 6Bum-off(%) SBer(m/g)Pore volume (Mesopore percentage(%) Average pore size(nm)CMAC-O1-621.63485.5043650.15250.28406506CMAC-C2-6 58.32115200675804021027374050CMAC-C3-6 54.17993.505086043440074314.59lodine and Methylene Blue(MB)were used as ad- separate routes of micropore formation and enlargesorbates to evaluate adsorption capacity. The relative ment that depends on the degree of catalysis. Whenamounts of iodine and methylene blue adsorbed indi- the content of Fea O4 is less than 6.0% there are fewercate the relative ratio of micropores and mesopores, active catalyzing sites and the distribution of Fe3O4 isrespectively. As shown in Fig 3, the CMAC-C2-6 unsymmetrical. Then a portion of the micropores en-prepared from bitumite has a higher iodine and me.large into mesopores and macropores, so the iodinethylene adsorption compared to the other two samples, adsorption decreases but the methylene blue adsorp-which is consistent with the results shown in Table 3. tion increases. The adsorption capacity deteriorates athigher Fe3O4 amount(8.0%), because the catalysislodinebecomes extensive and the speed of enlargement exceeds the regeneration rate of the micropores. Besides,the steric hindrance of excess Fe]O4 on the pore vo-Table 4 Adsorption capacity with various amounts of Fe3O41■Adsorption6558849.85646MB(mg/g)295108.2539Fig 3 Adsorption capacity of the CMAC'sFig. 4 shows clearly the crystal structure and con3.2 Effect of the amount of added Fe3O4version of Fe3 O4 for each period of the CMAC-C3-6transformation. As indicated in Fig. 4a, the sampleA previous investigation had suggested that Fe3O4 with 6.0% Fe3O4, CMAC-C3-6, may be distinguishedwas preferable to other iron containing magnetic ad- from the sample without Fe]O4, CMAC-C3-0, byditives including Fe, Fe2 O3 and Fe2(C204)3.Con- noting the peaks from Fe3 O4 and FeO Fe304 still ex-sequently, of the special interests is to further study ists in the Cmac after activation so it is obvious thatthe effect of Fe304 amount on the performances of the Fe3 O4 will cause the magnetism of the CMAC(Feoresulting CMACsis non-magnetic). However, as shown clearly in Fig.Compared with the other two coal samples, the an- 4b, the amount of Fe3 O4 tends to decrease as the reacthracite coal from Taixi was selected as the precursor tion proceeds. At the same time FeO appears as a newfor its ultra low ash content and trace iron element phase, in a rapidly growing amount, during the finalwith the aid to elucidate the effect of Fe3O4 on the activating period. During carbonization only a smallpreparation and characteristics of CMAC'sportion of Fe3O4 is converted to Feo(this occurs atThe adsorption capacities of CMAC's prepared 600C). While as the terfrom the C3 sample using various Fe3O4 amounts are diffuses during the activating period, the reactionslisted in Table 4. The optimum adsorption occurs at become increasingly vigorous, leading to a more re-6.0% Fe3O4 amount. The adsorbed amounts of iodine ducing environmenand methylene blue have increased by 17. 1% and a hydrogen and carbon monoxide, which could defiremarkable 352.7%, respectively, compared to nitely convert Fe3O4 into FeO, Fe203 and Fe asCMAC-C3-0. However, higher or lower Fe3O4 con- shown in Fig. 4b>. Metallic Fe, as explainedtent decreases adsorption capacity, in agreement with above, can provide catalyst sites that generatee304 reduction during the activating period as paration due to its magnetic prope g magnetiseveral previous reports. Metallic Fe, formed by, pore distributions. It also helps during magnetnown in Fig. 4b, can catalyze the surface reactionThe magnetic properties of the CMAC's were ex-between carbon and steam. This would cause micro- amined because the conversion of fe3 O4 to Feopores to be enlarged into mesopores or macropores. show中国煤化工n. The hysteresisThe latter could function as a bridge to allow more loopsin Fig. 5 and thesteam into the interior of the specimen thereby facili- magnCNMHole5tating the formation of more micropores. As a result, saturation magnetization of CMAC-C1-6, the samplethe pore distribution is controlled and regulated along with 6% Fe O4 addition is 13. 2 times higher than thatYANG Mingshun et alEffects of coal rank, Fe O4 amounts and activationof CMAC-C1-0, the sample with no Fe3O4 addition. of simple separation tests using a magnet, shown inOnce susceptibility exceeds 1.26x10'-75x10 Fig. 6, demonstrate that it is relatively easy to sepa-m/kg, the substance can be separated magnetically rate CMAC-C3-6 due to its high magnetic suscepti-by a magnetic field of 800-1600 kAm. The results bility of 3. 073x10 m/kgCMAC-C3-6⊥FeOActivating peCMAC-C3-0Fe: OPrecursor period100(a)X-ray diffraction of the CMACs and FeO(b)Conversion of Fey Oa during each period: CMAC-C3-6Fig 4 XRD pattems3.3 Effect of activation temperatureCMAC-C3-6The effect of the activation temperature on the ad-sorption capacity of CMAC's prepared from coal C3CMAC-C3-0(the Taixi anthracite)was investigated by program900-6000-300ming the temperature fron850to940°cin30℃cField(Gincrements at a constant steam flow rate of 0.77Initially, as the temperature rises from 850 to 880Cboth the iodine and methylene blue adsorptions in-crease slightly. TheeasesFig 5 Magnetization hysteresis loopas the temperature is further increased from 880 toCMAC-C3-6 and CMAC-C3-0940°CSince the reaction between carbon and steam isendothermic higher temperatures theoretically shiftthe reaction toward product, leading to advanced porestructure and a suitable ratio of micropores and me-spores. The iodine and methylene blue adsorptionreaches 849.7 and 108 2 mg/g respectively as thetemperature increases. However, the intensity of theburn-off becomes uncontrollable as the temperaturerises further. As reported in previous investiga-become the predominant pore structure of the resulting activated carbon. Then the iodine and methyleneblue adsorptions drop significantly as a consequenceFig 6 Magnetic separation tests(a)CMAC-C3-6,(b)CMAC-C3-0 and(c)magnetTable 5 Magneticrties of CMAC- C3-6and CMAC-C3-0magnetization magnetism (G)(107m3/kg)598850CMAC-O300.18310037424786403中国煤化工一CMAC-C3-6241580.236427101307CNMHGFig. 7 Iodine and methylene blue adsorptionctivaton at vanous temperaturesMining Science and TechnologyVoL 20 No,64 Conclusionsration of magnetic activated carbon. NanotIts applications, 2009, 929: 183-1881)Compared to Baorigelen Wang C L, Huang H Q, Wei s Q Cheng X Z, Jiang WB,aIXIZhao Y. Feasible study on gold extraction by magnetianthracite the Datong bitumite was more favorablecarbon- in-pulp process. Gold, 1995, 16(6): 27-31.(Infor preparation of CMAC's having high specific areaChinese)and advanced pore structure, especially mesopore[8] Tseng HH, Wey M Y Study of SOz adsorption and ther-2)Fe Oa both enhances the magnetic properties ofal regeneration over activated carbon-supported copperthe activated carbon and controls pore distributionoxide catalysts. Carbon, 2004, 42(11): 2269-2278due to the catalytic function of metallic Fe, which isactivated carbon content in TiOr-loadethe product of Fe3O4 reduction during the activatingarbon on photodegradation behaviors ofd dichloroperiod.methane. Journal of Photochemistry and photobiology A)Added Fe3O4 is mostly converted into FeOChemistry,1997,103(1/2):153-157during activation, while residual Fe 04 and any Fe [10] Demircan Z, Tekol E, Tanyolac D, Ozdural A RPaformed are sufficient to create magnetic propertiesra-magnetic polyvinylbutyral particles containingappropriate for magnetic separationvated carbon as a new adsorbent. Chem Eng co2003,19058):831-842.4)The specific surface area, the iodine and [11] Liu Z C, Ling L C, Qiao WM, LuCX, Wu D, Liu L.methylene blue adsorption and the specific saturationEffects of various metals and their loading methods onmagnetization of CMAc prepared from Datongthe mesopore formation in pitch-based spherical actibitumite with added (6.0%)Fe3 O4 and an activationvated carbon, Carbon, 1999, 37(4): 1333-1335temperature of 880C were 1152.0 m?/g, 1216.7 [12) Gong G Z, Xie Q, Zheng Y F, YeSE Chen Y FRegula-mg/g, 229.5 mg/g, 4.623 emwg, respectivelytion of pore size distribution in coal-based activatedcarbon. New Carbon materials, 2009, 24(2): 141-146[13]Xie Q. Bian B X. Principles of Control over Coal CarAcknowledgementsapplication in Preparation of ActivatedUniversity of Mining TechThe research is supported by the National Naturalnology Press, 2002. (In Chinese)Science Foundation of China(No. 20776150)and the [14] Xie Q, Zhang X L, Li L T, Jin L Porosity adjustment ofpractice. 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