Direct spray pyrolysis of aluminum chloride solution for alumina preparation

J. Cent. South Univ. (2014)21: 4450- 4455包SpringerDOI: 10.1007/511771-014-2448-8Direct spray pyrolysis of aluminum chloride solution for alumina preparationLU Guo-zhi(吕国志), ZHANG Ting-an(张廷安)，WANG Long(王龙)，MA Si-da(马思达)，DOU Zhi-he(豆志河), LIU Yan(刘燕)Key Laboratory of Ecological Utilization of Multi-metal Intergrown Ore of Ministry of Education(School of Materials and Metallurgy, Northeasterm University), Shenyang 1 10004, ChinaC Central South University Press and Springer-Verlag Berlin Heidelberg 2014Abstract: The effects of pyrolysis mode and pyrolysis parameters on Cl content in alumina were investigated, and the aluminaproducts were characterized by XRD, SEM and ASAP. The experimental results indicate that the spray pyrolysis efficiency is higherthan that of static pyrolysis process, and the reaction and evaporation process lead to a multi-plot state of the alumina products byspray pyrolysis. Aluminum phase starts to transform into )-Al2O3 at spray pyrolysis temperature of 600 °C, which is about 200 °Clower than that of static pyrolysis process. The primary particle size of y-Al2O3 product is 27.62 nm, and Cl content in aluminaproducts is 0.38% at 800 °C for 20 min.Key words: spray pyrolysis; aluminum chloride solution; )-Al2O; phase transformationand are also useless for Al and Si separation of coal fly1 Introductionash.Alkali sintering and acid leaching methods wereCoal fly ash is mainly emitted from heat-engineeffective method for Al and Si separation during aluminapower station. Piling and storing treatment of coal fly ashproduction using coal fly ash. Alkali sintering processwill lead to occupancy of vast lands [1], and seriousinvolves the following steps: desilication of fly ash, .pollution of soil, air and water. Comprehensivecalcinations, digestion and carbon dioxide decompositionutilization of coal fly ash has been studied since 1970s [2][10-11]. For alkali sintering process, high extractionefficiency of aluminum can be obtained, which requiresenvironmental contamination have been paid morehigh energy consumption and produces more solidattentions around the world. In addition, coal fly ash withwastes. Separation of silica from aluminum can beabundant Al2O3 [3], is a kind of potential raw material forachieved by acid leaching method, but in traditional acid-extracting aluminum.leaching process, AlCl3 solution was treated by NaOH orOn the other hand, annual production amount ofNH;:H2O precipitation method for Al(OH)3, and Al2O3coal fly ash in China has reached above 30 million tonsvas obtained by roasting process [12- 14], recycling ofby the end of 2011. Although bauxite resources in Chinawater and acid was the main problem of this method.are abundant, the reserves of high-grade bauxite areIn order to solve the water and acid recyclinglimited, and the majority (>95%) are diasporic bauxite problems during alumina production process with acidwith high contents of aluminum and silica. Suchleaching, a direct spray pyrolysis using AICl3 solutioncomposition combination presents less economicmethod was presented in this work. Various techniquesattraction due to the high levels of reactive silica [4- -6].have been used to prepare alumina from AlCl3. .There have been many intensive digestion methods forSHAMALA et al [15] studied the optical, electrical anddiasporic bauxite, such as the roasting pretreatment- -lowstructural properties of Al2O3 thin films prepared b:temperature digestion technology, mining addedspray pyrolysis and by electron beam evaporation.sweetening technology, and activated digestionAUGILER-FRUTI et al [16] deposited Al2O3 thin filmstechnology [7-9]. These methods cannot solve the Alspray pyrolysis and reported electricaland Si combination form of direct digestion treatment,characterization from C-V and I- V measurements ofFoundation item: Projects(U1202274, 51004033, 5 1204040) supported by the National Natural Science Foundation of China; Proiect(2012AA062303)supported by the National High Technology Research and Development Prograt中国煤化工! supprted by theNational Science and Technology Support Program of China; Project(L2014096:ment of LiaoningProvince, China; Project(N130702001) supported by the Fundamental Research FIYHCNMHGnaReceived date: 2013-10-28; Accepted date: 2014- 03-31Corresponding author: ZHANG Ting-an, Professor, PhD; Tel: +86- -24- 83687715; E-mail: zta2000@ 163.netJ. Cent. South Univ. (2014) 21:4450- 44554451metal-oxide-semiconductor structure. NASUTION et al[17] used CVD method for alumina preparation fromAlCl3 vapor. These researches were mainly focused onthe alumina preparation from AlCl; powder or vapor. Inaddition，alumina is widely used in industry inmechanical parts， abrasives， and insulators [18].Especially, because of its large surface area, low cost,thermal stability, good mechanical strength and volatileacidity, multi-plot )-Al2O3 has been utilized as carrier for6catalyst or adsorbent [19]. Typically, )-Al2O3 is obtainedby dehydroxylation of )-AIOOH prepared byprecipitation from aluminum chloride, nitrate, and sulfateor by hydrolysis of aluminum alkoxides [20- 21]. To theFig.1 Pyrolysis experimental set-up: 1- Ceramic crucible; 2一best of our knowledge, there is less report on the directTube furnace; 3- Flow control valve; 4- Solution tank; 5一spray pyrolysis for )-Al2O3 or the other alumina productsPressure gauge; 6- -Air cylinderfrom AICl3 solution.The objective of this work is to propose a directJapan). X-ray diffraction pattern was detected on aspray pyrolysis process of AICl3 solution. The effect ofPW3040/60 X-ray diffractometer (Philips, Holland) withdirect pyrolysis parameters on reaction efficiency ofCu Ka radiation from 10° to 90° by 2 (°)/min. Thealumina preparation from AlCl3 solution waschlorine content in alumina products was determined byexperimentally studied, and phase transformation andchemical titration.apparent morphology changes of alumina products wereNitrogen adsorption isotherms were obtained usingtested by XRD, SEM and BET.an accelerated surface area and porosimetry system(ASAP2020, Micromeritics, USA). The samples were2 Experimentaldried in vacuum at 150 。C for 3 h prior to measurement.The surface areas of alumina products were calculated2.1 Materialsfrom the isotherms using the BET equation.All chemical reagents of analytical grade wereproduced from Tianjin Kemiou Chemical Reagent Co.,3 Experimental results and discussionChina and used directly without further purification.3.1 Effect of pyrolysis parameters2.2 Experimental proceduresThe AlCl3 pyrolysis reaction with H2O and O2 isThree kinds of pyrolysis experiments werelisted as follows:conducted for comparing the effects of pyrolysis mode2AlCl;+3H2O= =Al2O3+6HCl(1)on pyrolysis efficiency and phase transformation 0alumina product. Static pyrolysis experiments of2AlCl;+3/2O2=Al2O3+3Cl2(2)aluminum chloride crystal (15 g) and solution (15%,The OG' of reaction (1) is -327.8 kJ/mol at 298 K, and .mass fraction, 40 mL) were carried out in a mufle- 476.5 kJ/mol at 1000 K. In the temperature range offurnace, and a pyrolysis set-up ilustrated in Fig. 1 was 298- 1600 K, the OG° of Eq. (1) is always lower than thatused for the spray pyrolysis study. The tube furnaceof Eq. (2), so when steam exists in the pyrolysis system,made of steel with quartz inner chamber height of 90 cmthe decomposition thermodynamic conditions of AICl3 isand inner diameter of 7 cm was connected to abetter than that of reacting with O2.programmable temperature controller which is capable ofThe CI contents in alumina products are presentedproviding control of the heating rate, target temperaturein Fig. 2 for spray pyrolysis of aluminum chlorideand the heating time. Each aluminum chloride solution solution, static pyrolysis of aluminum chloride crystal(15%, mass fraction, 40 mL) was placed in the solutionand solution, respectively. There are two processes oftank, which was kept at room temperature. Air was usedaluminum chloride solution: evaporation (sublimation)as a carrier gas for aluminum chloride solution of and pyrolysis reaction. The two processes occur0.2 MPa. Aluminum chloride solution was fed into thesimultaneously in the temperature range of 400- 1000 。Creactor by the pressure effect of flowing air through theBy comparing Fig. 2(a) with Fig. 2(b), the pyrolysis rateflow control valve. The spray time was about 20 min forof aluminum C中国煤化工r than that ofeach spray pyrolysis experiment.aluminum chlo1HCNMH G。content inThe alumina products were characterized usingalumina produc..... .e crystal atSSX- 550 scanning electron microscope (SEM, Shimadzu,800 °C for 20 min is less than 1% and for alumina4452J. Cent. South Univ. (2014) 21: 4450-4455aluminum chloride crystal by static pyrolysis. There are30(a)two plausible reasons: 1) the droplet in spray reactor is1-400 °Cdispersed into 30 - 70 um, and the heat transfer efficiency25is much higher than that of static pyrolysis; 2) the steam2(partial pressure of spray pyrolysis process is higher than●-1000。Cthat of the others, which is benefit for enhancing the国15pyrolysis rate and efficiency.E 103.2 Phase transformation of alumina productsA series of XRD analyses were carried out forinvestigating the influence of pyrolysis mode an0 20304050 60parameters on phase transformation of alumina products.Pyrolysis time/minThe results are shown in Figs. 3- -5, and the peakparameters of alumina products are listed in Table 1.5.5-b)|Results in Fig. 3(a) indicate that AlCl3 phase changes.- 600°Cinto amorphous structure in pyrolysis temperature range4.5---800。Cof 500- 900 °C for 20 min. Combined with CI content inalumina products, amorphous alumina is the maincomposite in static pyrolysis products when the pyrolysisE 2.5-temperature is lower than 900 °C for 20 min [17]. Thecrystallinity of alumina products increases with pyrolysis1.5time, characteristic peaks of )-Al2O3 appear at (222),s。(400) and (441) respectively at 800 °C for 60 min, and0.5Lthe primary particle size of )-Al2O3 calculated by5080100120 14060Scherrer equation is 36.81 nm [22]. When the staticpyrolysis temperature of aluminum chloride crystal gets1.8(c) Is 1.6-●- AICI31.4-吕1.2-500 °C1.0-唱0.8-600°C目0.6-800°C营0.4-900 °C60070080090010000204020/()Fig. 2 Effect of pyrolysis mode and parameters on Cl content inalumina products: (a) Static pyrolysis of aluminum chloride|(b)crystal; (b) Static pyrolysis of aluminum chloride solution; (c)- 0-ALO,Spray pyrolysis of aluminum chloride solution1000 °Cproducts from aluminum solution it is 1.13% at 800 °Cfor 150 min. This indicates that at the beginning of900。Cpyrolysis reaction of aluminum chloride solution, theevaporation process occurs fiercely, but after evaporation,800 °Cthe residual AlCl3 reacts with O2. The pyrolysis ratedecreases slightly, so the pyrolysis efficiency is lower700 °Cthan that of aluminum chloride crystal.20 3(中国煤化工80 9Meanwhile, for spray pyrolysis process, CI contentin alumina products at 800 °C for 20 min is 0.38%,Fig. 3 XRD palJMHCNMHG_. pyuuuww :om aluminumwhich is lower than that of alumina products fromchloride crystal by static pyrolysis: (a) 20 min; (b) 60 minJ. Cent. South Univ. (2014) 21: 4450-44554453--Al2O;、- -a-AL2O,.-a-ALO;800 °C,1000 °C, 20 min150 min800°C,900 C, 20 min60 minmin800C.20min700 °C,700 °C, 20 min|nin0240 608(1004050820/(°0/(°)Fig. 4 XRD patterns of alumina products from aluminumFig. 5 XRD patterns of alumina products from aluminumchloride solution by static pyrolysischloride solution by spray pyrolysis1000 °C for 60 min, characteristic peaks of a-Al2O3products is lower than that of pyrolysis process ofappear at (104), (110), (006) and (202), the )-Al2O3aluminum chloride crystal.content in products calculated by quantitative analysisThe phase transformation rate of alumina productsformula [18] is 81.66%.during spray pyrolysis process is higher than that of theFor static pyrolysis process of aluminum chlorideothers. The intensities and FWHM of )-Al2O3 in aluminasolution, the unique characteristic peak of )-Al2O3products at spray pyrolysis temperature of 800 °C forappears at (441) at pyrolysis temperature of 800。C for20 min are close to those of static pyrolysis at60 min, which indicates that the crystallinity of aluminatemperature of 800。C for 60 min with aluminumTable 1 Analysis results of XRD patterns of alumina productsPyrolysis mode Pyrolysis conditions Crystal phase(hk)20/(0)HeightFWHM()1/% .22237.8000191.840.708548.42800 °C, 60 miny-Al2O340045.9997276.510.551069.7844167.5386396.240.9446100.0037.9900207 .980.157439.28Static pyrolysis)-Al2O345.7266292.50).787255.24of aluminum4467.4960529. 53chloride crystal1000 °C, 60 min10435.3615201.540.137838.0639.5726123.4923.32a-Al2O300643.5486142.95).236227.002057.6726138.230.236227.4537.6753933.790.519643.8745.90201319.300.45461.9867.56292128.541.188of aluminum 800 °C, 150 min1032.4247129.580.90926.09chloride solutiona-Al2O3 .11039.7864371.0517.430042.7951317.470.454614.9237.8900168.06).944634.35800 °C, 20 miny-Al2O3 .4.8093166.8434.1067.5732363.430.960074.27Spray pyrolysis37.9262645.3050.8545.6702129.6810.2267.597662508) 736239.7235.2950中国煤化工43.4821YHCNMH G99.5120257.6053 .986.730.177177.754454J. Cent. South Univ. (2014) 21: 4450-4455chloride crystal. The )-Al2O3 content in alumina productsstructure of AICl3 reacting with O2[17], and for sprayat spray pyrolysis temperature of 1000。C for 20 min ispyrolysis, the product is in multi-plot state. In spraydecreased to 45.50%， which indicates that the spraypyrolysis process, the solid reactants precipitate firstlypyrolysis method can enhance the phase transformationfrom liquid phase and transform into round particlesrate of alumina products. Moreover, the primary particle(Fig.6(C)), then the circular alumina particles bond withsize of )-Al2O3 products (obtained at spray pyrolysisthe others in the bottom of the reactor. So, if thetemperature of 800 °C for 20 min) is 27.62 nm.pyrolysis reactor is big enough, spherical alumina couldbe prepared by direct spray pyrolysis process. The3.3 Morphology analysisreaction and evaporation process in the bottom of reactorSEM images shown in Fig. 6 indicate that aluminalead to a multi-plot state of the alumina products. Theproduct from aluminum chloride crystal by staticcrystallization process of alumina by static pyrolysis is:pyrolysis is in laminar state, which is a typical productAICl3 (solution or AICl;6H2O)- →AICl;- →Al2O3，andAlCl3 is transformed into alumina directly in spraypyrolysis process. So, crystallization rate of alumina byspray pyrolysis is higher than the others.Figure 7 shows the N2 adsorption/desorptionisotherm of alumina product by static pyrolysis at 800 °Cfor 60 min and spray pyrolysis at 800 °C for 20 min. Theadsorption curve of static pyrolysis product displays atype IV with type C hysteresis loops of slit-pore structurein static pyrolysis product which coincides with the SEMresults [23]. For spray pyrolysis product, the hysteresisloops (type A) can be explained by the product with5 umcylinder-shaped pores. In spray pyrolysis process, thetransit time of droplet through the reactor is less than6)10 s, and part of reactants transform into )-Al2O3 and 7-AlOOH through this step. The dehydration process in thebottom of reactor leads to the cylindrical pore structureof alumina product. Moreover, the average pore diameterof the spray pyrolysis product at 800 °C for 20 min is .about 8 nm.The surface area of spray pyrolysis alumina productfrom aluminum chloride solution at 800。C for 20 min is78.35 m2/g, lower than that of static pyrolysis product at800 °C for 20 min with aluminum chloride crystal (asshown in Fig. 8). Surface areas of alumina products aredecreased with pyrolysis temperature and timeincreasing.口■Adsorption200 F。● Desorption50Static pyrolysis product自100t50 tFig. 6 SEM images of alumina products by different pyrolysisSDrav Dvrolvsis productmodes: (a) Static pyrolysis product of aluminum chloride中国煤化工crystal; (b) Spray pyrolysis product of aluminum chlorideYHCNM HG1.solution; (c) Products at top of reactor (Pyrolysis temperature800 °C; pyrolysis time 20 min)Fig. 7 N2 sorption isotherm curves of alumina productJ. Cent. South Univ. (2014) 21: 4450-44554455ferite adition [D]. Hydrometallurgy, 2010, 104(2):313- 316.7] WANG Yi-yong, ZHANG Ting-an, CHEN Xia, BAO Li. Effects ofStatice pyrolysis ofmicrowave roasting on leaching behavior of diaspore ore []. The140aluminum chloride crystalChinese Journal of Process Engineering, 2007, 7(2): 317-321. (inStatic pyrolysis of20 Faluminum chloride solution口Spray pyrolysis of[8] KAKROUDI M G, FOGAING E Y, GAULT C, HUGER M,00 tCHOTARD T. Effect of thermal treatment on damage mechanical80behaviour of refractory castables: Comparison between bauxite andandalusite aggregates [] Jourmal of the European Ceramic Society,602008, 28(13): 2471-2478.)] ALEX T C, KUMAR R, ROY S K, MEHROTRA S P. Strred bead40mill grinding of gibbsite: surface and morphological changes [J].Advanced Powder Technology, 2008, 19(5): 483- 491.2010] GRZYMEK J, DERDAKA W A. Method for obtaining aluminumoxide, US 4149898 [P].1979 -02- -21.1] QIN Jian-guo. A technique of preparing silicon oxide and aluminawith fly ash, CN 2007 10061662.X [P]. 2007-04- 03. (in Chinese)12] SEIDEL A, ZEMMLES Y. Mechanism and kinetics of aluminumand iron leaching from coal fly ash by sulfuric acid []. ChemicalPyrolysis parameterEnineringSeience 19853122:38353 238525Fig. 8 Surface areas of alumina products by different pyrolysis13] SEIDEL A, SLUSZNY A, SHELEF G, ZIMMELS Y. Self ihibitionmodesof aluminum leaching from coal fly ash by sulfrie acid [D].Chemical Engineering Science, 199, 72(): 195- 207.4 ConclusionsChemistry of the direct acid leach, calsinter, and pressure digestion-acid leach methods for recovery of aluminum from fly-ash [].1) Aluminum phase starts to transform into )-Al2O3Resources and Conservation, 1982, 9(1/2/3/4): 271-279.at spray pyrolysis temperature of 600 °C, which is about [15] SHAMALA K s, MURTHY L C s, RAO K N. Studies on opical200 °C lower than that of static pyrolysis process. Sprayand electrical properties of Al2O; thin films prepared by spraypyrolysis and electron beam evaporation [J]. Materials Science andpyrolysis efficiency and phase transformation rate ofEngineering B, 2004, 106(3): 269- -274.alumina products are higher than those of static pyrolysis[16] AUGILAR-FRUIT M, GARCIA M, FALCONY C. Optical andelectrical properties of aluminum oxide thin flms prepared by spraypyrolysis []. Applied Physics Letters, 1998, 72(2/3): 1700-1706.2) When the spray pyrolysis temperature is 800 °C17] NASUTION I, VELASCO A, KIM H J. Atmospheric pressureand spray pyrolysis time is 20 min, the primary particlechemical vapor deposition mechanism of AlO3 film from AlCl; andsize of 7)-Al2O3 product is 30.78 nm, and Cl content inO2[J]. Jourmal of Crystal Growth, 2009, 311(2): 429 -434.product is decreased to 0.38%, which is lower than that18] OKADA K, NAGASHIMA T, KAMESHIMA Y, YASUMORI A.Effect of crystallize size of boehmite on sinterability of aluminaof alumina products from aluminum chloride crystal byceramics [J]. Ceramics International, 2003, 29(5): 533-537.static pyrolysis.PARK T, LIM J s, LEE Y, KIM S. Catalytic supercritical water3) Alumina product from aluminum chloride crystaloxidation of waste water from terephthalic acid manufacturing_ acidmanutacprocess [] The Journal of Supercritical Fluids, 2003, 26(3):by static pyrolysis is in laminate state, and for spray201-213.pyrolysis the product is in multi-plot state., CHADWICK D. Efcect of pH of piptationon onthe preparation of high surface area alumina from nitrate solutions [J]ReferencesIndustrial and Engineering Chemistry Research, 1998, 37(2): 405- -[1] WU Cheng-you, YU Hong-fa, ZHANG Hui-fang. Extraction of[21] NOGUCHI T, MATSUI K, ISLAM N M, HAKUTA Y, HAYASHIH. Rapid synthesis of )-Al2O; nanoparticles in supercritical water byaluminum by pressure acid-leaching method from coal fly ash [J].continuous hydrothermal flow reaction system [] The Joumal ofTransaction of Nonferrous Metals Society of China, 2012, 22(9):Supercritical Fluids, 2008, 46(2): 129- 136.228 1- 2288.[2] AHMARUZZAMAN M. A review on the utilization of fly ash [].[22] ZHANG Qing-hong, GAO Lian, GUO Jing-kun. Effects of sulfateProgress in Energy and Combustion Science, 2010, 36(3): 327- -363.ions and hydrolytic temperature on the properties of TiCl4-derivednanostructured TiO2[J]. Joumal of Inorganic Materials, 2000, 15(6):[3] WANG Fu-yuan, WU Yuan zheng. Fly ash uilization handbook [M].992- 997. (in Chinese)Beijing: Elecric Power Press, 200: 4- 23. (in Chinese)[4] CHEN Wan-kun, PENG Guan-cai. Intensifed digestion technology23] QI Hong. Preparation of composite microporous silica membranesusing TEOS and 1, 2-bis(triethoxysilyl) ethane as precursors for gasof bauxite [M]. Beijing: Metallurgical Industry Press, 1998: 23-95.(in Chinese)separation [J]. Chinese Journal of Chemical Engineering, 2011, 19(3):404-409.[5] YANG Zhong-yu. Alunina production technology [M] Beiing:(Edited by YANG Bing)Metallurgical Industry Press, 1998: 112- -156. (in Chinese)[6] LI Xin-hua, GU Song-qing, YIN Zhong-in, wU Guo-bao, ZHAIYu- chun. Regulating the digestion of high silica bauxite with calcium中国煤化工YHCNMH G