Scientific ResearchStudy on Filtration Performance of Silica/Alumina Slurry in the Process of RecyclingMother Liquor of NaY ZeoliteGuo Yaoqing; Ma Yuelong; Deng Jinghui(Research Institute of Petroleum Processing, Beijing 100083)Abstract: In order to recover the SiO2 contained in the mother liquor in the course of NaY zeolite synthesisto minimize pollution, the influence of various preparation conditions on the filtering velocity of gel slurywas studied using the SiO/A1,O, gel recovered from the NaY mother liquor in the laboratory. The results ofstudy had shown that at a SiO,/Al,O, ratio in the feed equating to 9:1 the SiO2 recovery rate and Al,O,utilization rate were high with a faster flow velocity of the filtrate. The pH value of the system had greatimpact on the flow velocity of filtrate. Between the two methods for regulating the pH value, the one forformation of silica/alumina gel slurry by addition of sulfuric acid prior to adding aluminium sulfate in thesolution could secure a faster filtration velocity. The filtration velocity was decreased in tandem withincreasing SiO, concentration in mother liquor, meanwhile an increase in dry filter cake yield.Key words: NaY zeolite; mother liquor; filtration performance; Si/Al ratio; gel; waste utilization; laboratoryscale1 IntroductionAl,O, gel slurry differs significantly because of the differ-ence in colloidal state of silica/alumina gel. This researchThe synthesis of NaY zeolite generates a large amount of project has conducted a study on the factors affecting thesynthetic mother liquor, which contains a lot of silicate ions filtering rate of the silica/alumina gel slurry and investigatedand caustic soda. The conventional treatment for handling the recovery and utilization rate of SiO2/Al,O, contained insoluble silicate waste liquid is the addition of an acidic the slurry to provide reference data necessary for filteringsolution, which can react with soluble silicate to form silica silica/alumina gel slurry in the commercial unit.gel slurry or a gel slurry containing other components. Thewater is then separated from the gel slurry through separa- 2 Experimentaltion and filtration with the gel filter cake being recovered toreclaim the soluble silica. Recovery and utilization of the said 2.1 Raw materials used in the experimentssynthetic mother liquor has assumed great importance withrespect to realization of“zero discharge”of silica, pollution Aluminium sulfate solution with a mass concentration ofreduction, as well as reduction of production cost.Al,O, equating to 84g/L;WR Grace & Davison Company has published an open infor- NaY mother liquor, technical grade, with a mass concentra-mation on the technique for recovery of mother liquorl"I, while tion of SiO, in mother liquor equating to 35g/L.the catalyst factory of Lanzhou refinery has given a success-ful example for realization of this technique in commercial 2.2 Preparation of silica/alumina gel slurryscalel2-3I. Because different NaY compositions are appliedduring synthesis of zeolite, the NaY mother liquor contains The silica/alumina gel was prenared through precipitate for-various concentrations of SiO,, and the filtering rate of SiO/ mation following中国煤化Itherliquor. TheYHCNMHG19.China Petroleum Processing and Petrochemical TechnologyNo. 1, March 2004aluminium sulfate solution was added slowly into the NaY value of Q can be obtained via calculation based on the val-mother liquor under high-speed stiring, and the mixture after ues of a and b. The velocity for filtering different silica/alu-homogenously stirring was subjected to aging for 2 hours in mina gel slurries can be obtained through comparing the val-a water bath heated to a temperature of 80C.ues of Q corresponding to the same volume of the filtrate.2.3 Determination of the filtering rate of silica/2.4 Determination of grain size of SiO2/Al,O3alumina gel slurrygelCurrently the horizontal belt-type continuous vacuum filter The E type laser particle analyzer (MasterSizer E) manufac-is used in the commercial scale for performing the filtering tured by Malvern Company of England was applied to mea-process. In this experiment in order to study the filtering rate sure the grain size distribution. The focal distance of lensof the silica/alumina gel slurry, a Buchner funnel was applied was 300 mm.to simulate the operation of horizontal belt-type continuousvacum filter.3 Results and DiscussionSince the fltering process is conducted at a low vacuumun- 3.1 Effect of SiO,/AI,O3 feeding ratioder atmosphere pressure, the filter cake is compressible. According to Rush' s basic equation for filtering!4.5I the math- The mother liquor containing 35 g/L of SiO2 was used as theematical deduction has led to the following relationship be-object of investigation. The effect of Si/Al feeding ratio ontween the instantaneous flow velocity Q and the filtrate vol- the pH of slurry and fltrate flow velocity is shown in Figure 1.ume V:It can be seen from Figure 1 that when the feeding ratio of1/Q=dt/dV=2aV+bSiO,/Al,O3 was equal to 14- &, the silica/alumina gel slurrywas alkaline in nature, and the flow velocity of filtrate de-inwhich a=[ μ a,(Ap)"c]/2A',b= μR /A Spcreased with a decreasing Si/Al ratio. When the feeding ratioof SiO,/A1,O, was equal to 7.0- 5.5, the silica/alumina gelIn this equation:slurry was neutral or slightly acidic in nature, and the flowQ一instantaneous flow velocity of filtrate, m/s; .V - Volume of filtrate moved over time, m);1.414Filtering duration, s;A一Surface area of filter cake perpendicular to flow1.2-direction, m;△p - Pressure exerting on both sides of filter cake with a1.0-10 。thickness ofL, Pa;0.8μ - Viscosity of filtrate, cP;xR ,一Resistance of filter medium, mil;0.6a.- Specific resistance of filtering pressure intensity on0.4unit filter cake, m/kg;n一Filter cake compressibility coefficient obtained by ex-).2 Iperiment().0.0ume of filtrate discharged, kg/m3SiO,/AIO,A graph is ploted using /V versus V as the coordinates, and Figure 1 Effect of Si/AI feedine ratio on slurrythe values of a and b are obtained through linear ftting. The中国煤化工YHCNMH G ..Scientific Researchvelocity of filtrate was minimum. When the feeding ratio of cause a surplus of aluminum sulfate in the feeding material.SiO/AI,O3 was equal to 4.5- 2.0, the silica/alumina gel slurry When the SiO,/Al,O, ratio was equal to 6- 8, the filtrate flow .was strongly acidic in nature, and the flow velocity of filtrate velocity was lower and could not be tolerated in the opera-increased gradually with a declining Si/Al ratio. This phe- tion despite a higher recovery rate of SiO, and a utilizationnomenon was caused by the formation of electric double layer rate of Al,O、When the SiO,/AI,O, ratio was in excess of 9,inside the gel micellae under the alkaline environment. The the aluminum sulfate solution would not suffice to providemicellae were not able to aggregate because of the repelling the acidity needed for complete precipitation of SiO2, result-action between electric charges, and were loosely piled u ing in a drop in the SiO2 recovery rate. Hence it is evident thatwith large porosity in the filter cake, which was conducive to the SiO2 recovery rate and Al,O utilization rate were maxi-filtration. When the silica/alumina gel slurry was neutral or mized at a SiO,/AlO, feeding ratio of around 9 because of theslightly acidic, the electric double layer was compressed or higher flow velocity of the SiO,/AI,O, gel slurry.even vanished, and the micellae were densely packed withsmall porosity in the fiter cake, rsulting ina slow flow veloc- 3.2 Effect of the pH value of SiO,/AI,O, gelity of the filtrate. When the silica/alumina gel slurry was slurrystrongly acidic, a counter electric double layerl7l appeared inthe micellae, which were loosely piled up, leading to an in- The experiment had revealed that the pH value of this systemcreased flow velocity of the filtrate. The change in porosity had a significant impact on the flow velocity of filtrate. Aof the filter cake can be seen from the SEM micrographs enu- mother liquor containing 35 g/L of SiO, and a silica/aluminamerated in Figure 2.gel slurry formed at a SiO,/Al,O, feeding ratio of 9 were se-lected as the targets for investigation. Two different modesThe effect of SiO,/AI,O, ratio in the feedstock on the recov- of sulfuric acid addition were adopted to study the change inery of SiO, and the utilization rate of Al,O, is shown in Figure recovery of SiO, and utilization of Al,O, and the difference in3. It can be seen from Figure 3 that at a SiO2Al,O, ratioof the flow velocity of silica/alumina gel slurry. The results ofless than 6, the Al,O3 utilization rate was lower, because the investigation are presented in Figure 4 and Figure5. Mode 1Al2O3 in gel could be again dissolved in the strongly acidic refers to the adjustment of pH value at first, which requiredenvironment, or the mother liquor could not provide suffi-the addition of sulfuric acid followed by adding aluminumcient caustic-neutralized aluminum bisulfate, which could sulfate solution to form the silica/alumina gel. Mode 2 refers:X-20.00科PhanLae HenbcFigure 2 SEM micrographs offilter cakes formed at different SiO[Al,O3 feeding ratiosa- SiO,/A1,O_=9; b- SiO,/A1O,=6; c- SiO,/A1,O,=2中国煤化工MYHCNMH G51.China Petroleum Processing and Petrochemical TechnologyNo. 1, March 2004to the after-adjustment of the pH value, which required add- refer to Table 1.) A heck of a lot of large colloidal particlesing aluminum sulfate solution first to form the silica/alumina had made the porosity of filter cake larger, which was condu-gel followed by the addition of sulfuric acid.cive to an increased filtration velocity. In addition, the in-creased filtration velocity was also detected when the pHIt can be seen from Figure 4 that the mode for pH adjustment value of the SiO,/Al,O, gel slurry system reached 9- 40, whichdid not have significant impact on recovery and utilization of might be stemmed from the reduced viscosity of the slurrySiO/A1,O,caused by dilution of the SiO/A1,O, gel slurry during adjust-ment of the pH value along with reduction of the virtual massIt can be seen from Figure 5 that the flow velocity of the SiO2 fraction of slurry c.Al,O3 gel slurry prepared by Mode 1 was slightly faster thanthat of the gel slurry prepared by Mode 2, because the vol- 3.3 Influence of SiO, mass concentration in theume of colloidal particles exceeding 100 μ m contained in Na Y mother liquorthe SiO/AI,O3 gel prepared by prior adjustment of pH valuewas more than that of the SiO/Al,O, gel prepared by after- Recovery of the mother liquor aims at the reclamation of SiO,adjustment of the pH value (as specified by Mode 2). (Please contained in the mother liquor. The concentration of SiO,100一 10contained in the mother liquor differs depending upon theratio of raw materials used for synthesis of NaY zeolite andthe amount of water consumed on zeolite washing. Hence it90+980 ITable 1 Effect of the mode for adjusting pHvalue on SiO[Al,O, grain size distribution70 I-70ModelMode2pHvalue| >100μm,v% pHvalue| >100um, v%6- 6(11.3126.0611.1422.279.6925.919.6622.9650- s(9.239.3223.018.5325.868.8622.27.2323.887.320.72SiO/Al,O,5.325.936.4523.14Figure 3 Effect of Si/Al feeding ratio on recov-5.429.945.0323.36ery of Si/Al8京98t980.66人9J901210pH value of systemFigure 4 Effect of pH value on recovery andFigure 5 Effect of pH value on filtrate flowvelocityutilization of SiO ,/AI,O;1rmode1,-日mode1,士mode2,去mode2中国煤化工YHCNMH G52.Scientific Researchis necessary to study the filtration velocity of mother liquor was 10 g/L, the flow velocity of filtrate was increased, result-with different SiO, contents. Since the difference in SiO, con- ing in a low yield of dry cake because of the low solid concen-centration contained in the mother liquor can affect the con- tration and small amount of filter cake obtained despite acentration of solids in the SiQ/Al,O3 gel slurry, and thus can higher throughput.have its impact on the mass of dry filter cake on unit time andunit area, namely the dry cake yield. Therefore, along with It can be seen from Figure 7 that the recovery and utilizationinvestigation on the filtration velocity of SiO/AI,O, gel slurry of SiO,/Al,O, were quite low at a lower SiO, concentration inthe yield of filter cake should be taken into consideration.the mother liquor, which was associated with the penetrationphenomenon at the beginning of filtration. When this tech-The effect of SiO, concentration in the mother liquor on the nique is applied in the commercial scale, it is necessary toflow velocity of filtrate, on the yield of dry filter cake, on the consider not only the filtration velocity of SiO/AI,O, gel slurry,recovery of SiO2 and on Al,O, utilization rate was studiedbut also to pay attention on the recovery and utilization ofusing the SiO,/AI,O3 gel formed at a SiO/AI,O, feeding ratio SiQ,/AIl,O,of 9 as the object of investigation. The study results arepresented in Figure 6 and Figure 7. .The efct of SiO, concentration in mother liquor on the dis-tribution of SiO,/A1,O3 gel grain size is shown in Figure 8. ItIt can be seen from Figure 6 that the flow velocity of filtrate can be seen from Figure 8 that the size of SiO,Al,O、colloidalwas small at a SiO, concentration of 5g/L in the mother liquor,and this value was of no significance for practical application.100厂00,When the SiO, concentration in the mother liquor was in therange of 10- 70 g/L, the flow velocity of filtrate decreasedand the yield of dry filter cake increased gradually with ang 9898increasing SiO, concentration in the mother liquor. This wasresulted from the increase of solid concentration in the SiO/96204(6(80Al2O3 gel slurry, as soon as the SiO2 mass concentrationMother liguor concentration, g/Lincreased. When the SiO2 concentration in the mother liquorFigure 7 Effect of SiO, concentration on2.5r70recovery of SiO2[Al,O3502.0十八11.5-40g 10-1.0101000.5Particle diameter, μmFigure 8 Effect of different SiO2 concentra-6tions in mother liquor on grain size ofSiO,Al,0, gelFigure 6 Effect of SiO2 concentration on flowSiO2 concentration in mother liquor: 1- 5g/L; 2- t5g/L; 3- 35g/L;velocity of filtrate中国煤化工YHCNMH G53.China Petroleum Processing and Petrochemical TechnologyNo. 1, March 2004particles and their distribution could differ depending on the (2) The pH value of the system had significant impact on thedifference in the SiO2 mass concentrations in mother liquor. flow velocity of filtrate. Between the two modes adopted forWhen the SiO, mass concentration in mother liquor reachedadjusting the pH value, the SiO/AI,O3 gel slurry formed after5 g/L, the porosity of filter cake obtained was small and was adjusting the pH value first, which required the first additionapt to block the filter cloth because of the smaller diameter of of sulfuric acid followed by adding aluminium sulfate solution,SiO/A1,O3 colloidal particles and narrower range of their grain could attain a faster filtration velocity.size distribution. The diameter of SiO,/Al,O, colloidal par-ticles tended to increase gradually and the range of grain size (3) The mother liquor containing different SiO, mass concen-distribution was broadened with an increasing SiO2 concen- trations had shown different performance of filtration. Thetration in mother liquor. When the mass concentration offlow velocity of filtrate decreased with an increasing SiO,SiO, was increased, the polycondensation reaction was concentration in the mother liquor, resulting in an increasedaccelerated, and the primary colloidal particles could collide dry cake yield.with each other to form a gel before further growth of colloi-dal particles, resulting eventually in non-homogeneity of col-loidal particle sizel7. During the filtration process the tinyReferencesparticles could occupy the voids among the accumulated [1] Curtis H. Elttr, Baltimore, Md. Preparation of Zeolite.larger particles to reduce the porosity of filter cake, resulting [P]US Pat. Appl. 4164551,1979in an decreased filtration velocity caused by the increased [2] Li Zhongxian, Wu Jianqiang, Mao Xuewen. Commercialresistance in the filter cake.Application of Technology for NaY Zeolite Synthesis withFull Recycling of Silica. Petroleum Processing andSince the filter cloth applied in the laboratory was different Petrochemicals, , 1998, 29( 10): 24 26from that adopted in the industry in terms of their mesh size,[3] Liu Conghua, Gao Xionghou, Ma Yanqing. Study on theduring application of this technique in the commercial scale Mechanism of Zeolite Y Formation in the Process of Liquorthe problems of clogging and penetration through filter cloth Recycling. Microporous and Mesoporous Mater, 1998, 25should be taken into consideration. Moreover, the filtration (1- 3):1-6velocity of mother liquor containing different silica concen-[4] Ouell (editor), Theory and Practice of Filtration. Beijing,trations would differ a lot. Application of this technology in National Defense Industry Press, 1982, 153- 162the commercial scale should be conducted in line with the [5] Xu Li, Zhu Qixin, Lu Shuqun et al . Some Problems Relatedactual conditions of the catalyst factory engaged in manu- with the Study of Theory and Practice of Filtration. Chemical .facture of the zeolite.Machinery, 2000, 27(5), 287- 291[6] L. Swarowski, Solid-Liquid Separation, 20 edition, Beijing,4 ConclusionsChemical Industry Press, 1990,. 68- 70[7] Guo Zhaoquan and Yang Keqin (editor-in-chief). Basics(1) When the ratio of SiO,/Al,O, in the feeding materials was of Technology for Manufacture of Petroleum Refining Cata-equal to around 9, the flow velocity of SiO,/Al,O、gel slurry lysts (Medium Grade), Bejing, Hydrocarbon Processing Press,was higher, leading to higher recovery and utilizationofSiO! 1989, 121-+26 and 145- 146Al,O,中国煤化工MHCNMH G54.
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