Extraction of Alumina from Coal Fly Ash with Sulfuric Acid Leaching Method

第1卷第2期过程工程学报Vol 11 No. 22011年4月The Chinese Joumal of Process EngineeringApr.2011Extraction of Alumina from Coal Fly Ash with Sulfuric Acid Leaching MethodLI Laj-shi(李来时),WUYu- sheng(吴玉胜), LIU Ying-yin(刘瑛璞), ZHAI Yu-chun(翟玉春Shenyang Aluminum and Magnesium Engineering and Research institute, Shenyang, Liaoning 110001, China2. School of Materials Science and Engineering, Shenyang University of Technology, Shenyang, Liaoning 110870, ChinaAbstract: Coarsey-Al2O, powder was prepared through acid leaching, solid-liquid separation, crystallizing and aluminumsulfate decomposing processes using coal fly ash and concentrated sulfuric acid as raw materials. The metallurgy level Al2Ocan be obtained by disposing coarse y-Al2O, powder with Bayer process. The optimum conditions of acid leaching processwere determined by experiments. The extraction rate of Al2, can be stabilized at about 87% under these conditions. Thethermodynamics and kinetics of sulfuric acid leaching process were studied with irreversible thermodynamicsKey words: coal fly ash; alumina; sulfuric acid; leaching; thermodynamics; kineticsCLC No. TF821; X705 Document Code: A Article No. 1009-606X(2011)02-0254-051 INTRODUCTIONresidue, and the use of the residue is more. Furthermoreit needs less equipment and energy. The residue fromCoal-burning power plants consume pulverized the sintering processes is calcium hydrate garnet, whichlid fuels and produce a large amount of coal fly ash can only be used in cement industry. And the main(CFA)as residue!. CFA causes severe environmental component in the residue from the present process inpollution and lots of health problems for the local this work is silica, which can be used to produce whiteresidents, so research on comprehensive utilization of carbon black and other chemical products. TheCFA is very necessary and important. In the current experiments of each unit involved in the process werestate,the utilizations of CFA are carried out mainlycarried out, the optimum conditions of each unitarchitectural material field2-, but they are low-value obtained and the thermodynamics and kinetics ofle main sulfuric acid leaching process studied with irreversiblecomponents in CFA are alumina and silica. Metallurgy thermodynamics in this research.lumina is the raw material of aluminum, its value ismuch higher than that of normal architectural material. 2 EXPERIMENTALUsing CFA to prepare alumina is one of high-valueutilizations of CFA. There are some extraction processes2.1 Materials and Reagentsof alumina from CFA nowadays, mainly beingThe main composition of CFA is given in Table 17) soda-limeThe concentrated sulfuric acid and caustic soda are inprocess/9, acid leaching process with ammoniumtechnical purity. Redistilled water is used in thefluoride 0. a number of problems in these processes experiments. The other reagents are in analytical purity.have existed, such as large volume of residue causedTable 1 Composition of the fly ash from somewherefrom the sintering processes-normally 8 to 10 times ton Shanxi Provinceinitial CFA, it would cause the secondary pollution, and Component Alo Sion Feo Cao tio, Mpo MndHF caused from ammonium fluoride added processwhich is harmful to workers and local residents2.2 Experimental and Analytical InstrumentsCFA from somewhere in Shanxi Province, China,A 4-mouth flask and heating jacket(2NHW-1000was processed with concentrated sulfuric acid in this mL with precision of l C)were used in acid leachiwork. This process need not add ammonium fluoride, so experiments. The phase composition and microstructureit is safer than previous acid leaching processes. of samples were analyzed by XRD(D/max-2500PCCompared with limestone sintering process and Rigaku, Japan) and SEM(SSX-550, Shimadzu, Japan)ime sintering process, this process causes less Their V凵中国煤化工 weight loss wereReceived date: 2011-01-19Acby National Natural Science Foundation of China(No. 50804031)CNMHGBiography: LI Laj-shi( 1980-), male, native of Shijiazhuang City, Hebei Province, PhD, senior engineer, major in physical chemistry of metallurgyTel: 13889301618, E-mail: lilaishia hotmail com.第2期LI Lai-shi, et al. Extraction of Alumina from Coal Fly Ash with Sulfuric Acid Leaching Methoddetermined by SDt 2960 Simultaneous DSC-TGA (TAThe optimum conditions of acid leaching processwere determined through single factor and orthogonal2.3 MethodologyThe CFA sample pulverized by mechanical methodThe CFA, intermediate product(aluminum sulfate),and concentrated sulfuric acid were put into the 4-mouth y-Al2O3 and a-Al2O3 were identified by XRDflask in proportion, heated with the heating jacket andThe main components, including alumina, siliconagitated with an agitator. The overdosed sulfuric acid oxide, ferric oxide, in CFA and the residue of acidwas separated from the solids with filtration and the leaching were detected with XRF device.intermediate product aluminum sulfate was on theContents of Al. Si and Fe in solutions weresurface of leached CFA(see Fig. 1). The aluminum detected with chemical analysis methods[ 12.sulfate was dissolved by hot water and separated fromThe acid leaching process was studied withsulfate crystals were obtained by concentrating the thermodynamics, icthe acid leaching residue with filtration. The aluminum classical thermodynamics3-5)and irreversiblealuminum sulfate solution. The y-Al2O, powder was 3 RESULTS AND DISCUSSIONobtained through sintering the aluminum sulfate crystals.The emitted gas SO,, which was the other product of 3.1 Optimum Conditions of Acid Leaching Processsintering aluminum sulfate, was absorbed byThe results and analysis of orthogonal experimentsconcentrated sulfuric acid to prepare sulfuric acid. And are in Table 2.the metallurgy level alumina was produced with BayerThe optimum conditions of acid leaching processprocess, which was the mature process to produce are the temperature of 200-210C, leaching time of 80metallurgy level alumina in alumina industry, by min, volumetric ratio of acid to CFA of 5: 1 and agitationdigesting the coarse y-Al2O3 powder that had been speed of 300 r/min The extraction rate of Al2O3 can beprepared beforestabilized at about 87% under these conditionsTable 2 Results and analysis of orthogonal experimentsmperature, T(℃)e,t(min) Volumetric ratio of acid to CFA Stirring speed(r/min) Extraction rate of Az O(%)803:l220405;1220603:2208030078.2339.7677700081267100349.13326003.2 SEM AnalysisFigure 1 is the SEM morphology of solid phaseafter acid leaching and filtrating acid. It shows that theintermediate product (aluminum sulfate) fromconcentrated sulfuric acid leaching of CFA was on thesurface of residue In order to separate aluminum sulfatefrom the residue, it need be dissolved by hot water.3.3 XRD AnalysisFigure 2 shows the XRD spectra of products. Theresults implied that the intermediate product fromsulfuric acid leaching of CFA was aluminum sulfate the中国煤化工product from sintering of aluminum sulfate was y-Al2O3,CNMHGand the final product from Bayer process of y-Al2O3digesting was a-Al2O, at 1200CFig 1 SEM image of solid phase after filtrating acid过程工程学第11卷△G1r=21.3817h7+86×10372+458×10r110572-20837-9855,a-A,o,△G+4r=1063157ln7383×10272154×105r18△Gr=62.1327n7-1.2l×10-272-1.02×10372.16×10673-550.67+93812△G0r=697487hn748810372-5414107+1.28×10°7-7784T+201351Reactions(1-4)are the normal acid-base reactionsFig 2 XRD spectra of product samplesand their AGT values are far lower than zero at thenormal temperature, so they are possible to take place at3.4 Influence of Leaching Time on Extraction Rate the experimental temperatureof AluminaAG5. values of reaction(5)at differentTable 3 lists the relationship between the acid temperatures were calculated and its binominal formulaaching time and extraction rate of alumina, a under wasthe same conditions(473 K, volumetric ratio of acid to△Ge0=69587-121.87ash of 5: I and stirring speed of 300 r/min).Table 3 The relationship between leaching time andThe formula fit to the range from 298 to 553 K wasextraction rate of alumina△G3r=△Gr+RThnLeaching time(mJ is entropy of the reaction. It is supposed thatLeaching time(min)80 5060 7090 120- W(s).rPH2soa(yp for Eq (14), because the sulfuric acid84682283984.1857880in the acid leaching process is concentrated sulfuric acidThe results in Table 3 show that the extraction rate It is considered that little sulfuric acid vapor is producedof alumina increases with leaching time. And the results when the surface vapor pressure of sulfuric acid is lessare prepared for kinetic analysis of acid leaching.than 0.005 MPa, that is, Jos,r0.05. The balance3.5 Thermodynamics Analysis of Acid leachingtemperature of reaction(5)is 474 K when J(s).0.05In the acid leaching process, the followingAG(.). of reaction(6)at different temperatures wasreactions may take placecalculated and its binominal formula was△G6=169398-28447,(15)AlO3+3H2SO40=Al(SO4)3( +3H20(bFe2O3ot3H2SO4oFe2(SO4)3(*3H2O(gh(2) The formula fit to the range from 298 to 553 K:△G6r=△G6 r+RTIn6xFeOo+H2(@+HoUghis approximately considered that J(6.u0=CasO4o+H2O(4)(pSow /p)(PHO..-(psow/p)2 for Eq-(15)inH2SO4=H2SOag)that little SO gas is produced when the sulfuric acidH2SO40=H2O(g+SOugh(6) surface SO, vapor pressure is less than 0.005 MPa, thatThe standard Gibbs free energy (AG) values of is, J(.7<0.0025. The balance temperature of reaction(6)above reactions were obtained with reference andis 507 K when J(6),70.0025calculation. The results are expressed as followsThe sulfuric acid vapor and SO3 gas are notxpected to form in the acid leaching process, so the△G-1182457n7+82×1074+temperature of acid leaching reaction should be les562×10571-10707-12500 7 thanV山凵中国煤化工3.6△dmr=2083647m7-47×10372+132×10T+CNMH.actions are typical132×1°7-141457+34872,(8) liquid-solid reactions, the main reaction is reaction(1),第2期LI Lai-shi, et al. Extraction of Alumina from Coal Fly Ash with Sulfuric Acid Leaching Method257which takes place on the surface of CFA particles. The prior stage the reaction is controlled by intemalproduct, aluminum sulfate, is not dissoluble in diffusion, and its kinetic equation is expressed asconcentrated sulfuric acid and sticks on the surface ofCFA particles. It is supposed that the distribution of[11-a)]-0.00499alumina is uniform in the CFA particles, the holes willreplace the alumina after the acid leaching and thealuminum sulfate dissolving and the diameters of CFAarticles are invariable. So the acid leaching processcould be considered that the size of solid phase does notvary with the reaction rate and the product is on thesurface of the solid particles. The acid leaching reactionmay follow the invariable size shrinking unreacted-coremodel. Fig 3 demonstrates the reaction model.Product layerTime(min)Unreaction coreFig 4 Simulation of control of diffusion versusDd laverIf the reaction was controlled by interface-chemicalreactoncalculated by taking the experiment data into Eq (17)As a result, Fig. 5 was obtained. It shows that the curveof the first power equation does not fit to theig. 3 The unreacted-core model of sulfuric acid leachingexperimental data well, and the curves of the second,Considering that the sulfuric acid leaching reaction third, forth and fifth power equations fit to theis mainly controlled by internal diffusion, external experimental data. So the second power equation can bediffusion and interface-chemical reaction, the agitation considered to meet the fitting requirement, its kineticin the system can eliminate the effect of extermalquation is expressed asdiffusion in the acid leaching reaction. So the sulfuracid leaching reaction is mainly controlled by internal=(1-a)21[2918×103A+1618×10-02].(20)diffusion and interface-chemical reactionAccording to irreversible thermodynamics theory,the rate equation of the leaching reaction controlled byinterface-chemical reaction was calculated, that isd0-a)°1(1)+(mn+A(m)+,(17where k=3MAlyo (roPA12o, ), 41,I2, 43, "" are coefficientsThe second power simulation curve of Eq- (17)The third power simulation curve of Eq (17and can be calculated though computer simulatione forth power simulaton curve of Eq (17)method,A=△G=△G+Rn. And the rate equation ofThe fifth power simulation curve of Eq (17)the leaching reaction controlled by internal diffusion20406080100120was calculated. that isTime, t(min)1-(1-a)=k,Fig 5 Simulation of control of interface-chemical reactionwhere k is constant and can be calculated throughversus the experimental datacomputer simulation methodhe prior stage(before 40 min) the reactionIf the reaction was controlled by intermal diffusion, was中国煤化工 a diffusion andtaking the experiment data into Eq (18), Fig 4 was interfobtained. It shows that the curve fit to the data in the 40 r..CNMHGlater stage(afterrug controlled byprior stage(before 40 min). So it is concluded that in the interface-chemical reaction only258过程工程学第114 CONCLUSIONS[7] Zhang B Y, Zhou F L The Limestone Sintering Process to ProduceAlumina with Fly Ash [J]. Light Metals, 2007, (6): 17-18, 27(in( 1)The metallurgy alumina was prepared byChinese).concentrated sulfuric acid leaching of CFA[8] Tang Y, Chen F L. Extracting Alumina from Fly Ash by Soda Lime(2)The feasibility of acid leaching process wasSintering Method []. Mining and Metallurgical Engineering, 2008,demonstrated with thermodynamic calculation28(6): 73-75 (in Chinese)()The optimum conditions of acid leaching [9] Liu Y Y,Li L s, Wu Y, et al. Further Utilization of Flyprocess of CFA were obtained through experiments,Ash--Extracting Alumina J. Light Metals, 2006, (5): 20-23(inthey were the temperature of 200-210C, leaching timeof 80 min, volumetric ratio of acid to CFA of 5: 1 and[10] Femandez A M, Ibanez L, Lavona M A, et al. Leaching of Aluminumstirring speed of 300 r/min. The extraction rate of AlOin Spanish Clays, Coal Mining Wastes and Coal Fly Ashes bySulphuric Acid [A]. Welch B J. Light Metals 1998 [C]. San Autoniocould be stabilized at about 87% under these conditionsTMS Aluminum Committee. 1998. 121-130.(4) The kinetic equations of leaching process were [11] Cheng J L, Tao Z N. 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