Reaction behavior and mechanism of anatase in digestion process of diasporic bauxite

Available online at www.sciencedirect.comTransactions of骂PScienceDirectNonferrous Metals总s ScienceSociety of ChinaEI SEVIER PressTrans. Nonferrous Met. Soc. China 20(2010) 142-146www.tnmsc.cnReaction behavior and mechanism of anatase in digestion process ofdiasporic bauxiteLI Xiao-bin(李小斌), FU Wei-an(付伟岸), ZHOU Qiu-sheng(周秋生),LIU Gui-hua(刘桂华)，PENG Zhi-hong(彭志宏)School of Metallurgical Science and Engineering, Central South University, Changsha 410083, ChinaReceived 15 December 2008; accepted 29 March 2009Abstract: Based on the study of infuence of temperature, digestion time, amount of CaO added and composition of aluminatesolution on reaction behavior of pure anatase in high-pressure digestion process of bauxite, reaction mechanism of anatase waspreliminarily determined. Anatase first reacts with caustic soda to produce Na2TiO3, then the resultant Na2TiO3 reacts with3CaO Al2O3:6H2O resulting from the reaction of CaO with sodium aluminate solution to produce CaO-2TiO2rH2O which eventuallyconverts into CaTiO3. Higher temperature, concentration of free Na2Ok (the caustic soda uncombined with aluminate anions in theform of Na2O) and molar ratio of CaO to TiO2 are favorable to the conversion of CaO:2TiO2rH2O to CaTiOz. And Al(OH)4" showsthe function of a catalyzer in the reaction of anatase with caustic soda with or without CaO added during the digestion process ofdiasporic bauxite.Key words: anatase; hydroxy calcium titanate; calcium titanate; high-pressure digestion; reaction mechanismWith the addition of lime in the digestion process,1 Introductiontitanium-containing minerals react with calcium-bearingcompounds to produce calcium-titanium compounds,There is 2%- 4% of TiO2 mainly in the form ofsuch as CaO-TiO2 and CaO:2TiO2H2O[6]. Theseanatase, rutile or brookite in almost all kinds 0compounds usually form substantial scale which is ratherbauxite[1-3]. In the digestion process at relatively lowdifcult to be cleaned up on the suface of heattemperature for gibbsitic bauxite, chemical reactionsexchanger during the bauxite digestion process so as tobetween these titanium-containing minerals andincrease energy consumption and cause wastage ofcomponents of aluminate solution will not take place.equipments productivity[7].But as for the digestion of diasporic bauxite in China,Up to now, although some research work washigher digestion temperature ranging from 210 C tocarried out on the composition, crystal morphology and270 C is required. In this case, titanium containinglattice parameters of calcium-titanium compounds in theminerals will react with components of aluminateed mud[5], it is still unclear about the formationsolution. According to the results reported in Refs.[4- -5],conditions of calcium-titanium compounds and reactionsodium titanates with different compositions producedmechanism of titanium-containing minerals with theby the reaction of titanium-containing mineral withadditive of lime in the alkaline aluminate solution.caustic soda appear in a dense thin film covering on theTherefore, it is of great importance to theoretically makesurface of bauxite particles. The film can preventclear the reaction behavior and mechanism of anatase indiaspore from contacting with caustic solution, whichBayer high-pressure digestion process of diasporicsignificantly lowers the extraction rate of alumina frombauxite for scale control and removal to reducediasporic bauxite. In order to eliminate the inhibitionmanufacture cost and improve equipment efficiency.effect of titanium-containing mineral, lime is alwaysHoweyer. as there are so many other mineralsadded to the original slurry in the high pressure digestion中国煤化工auxite ore, such asprocess for diasporic bauxite in the alumina industry.limesfYHCNMHGgoethite, etc, theFoundaton ltem: Project(2005CB6237-02) supported by the National Basic Research Program of ChinaCorresponding author: ZHOU Qiu-sheng; Tel/Fax: +86-731-88830453; E mail: csqszhou@163.comDOI: 10.10161003-63690960111-4LI Xiao-bin, et a/Trans. Nonferrous Met. Soc. China 20(2010) 142-146143chemical reactions and influencing factors are complexIt was reported that the apparent activation energyduring high- pressure digestion process of diasporicof de-titanization in diasporic bauxite digestion slurry isbauxite, which is one of the reasons that the reaction84.4 kJ/mol in the temperature range of 180 - 260 C, andmechanism of anatase in high-pressure digestion processsurface chemical reaction is the controlling step of theof bauxite has not yet been revealed clearly. So, it isde titanization reaction in this temperature range[9- -11]necessary to employ pure titanium-containing mineral toTemperature and time have great influence on reactionexplore the reaction mechanism of titanium-containingability and solubility of all minerals present in bauxite,mineral in high-pressure digestion process of diasporicand on the composition of scale and scaling speedbauxite. With consideration of the presence state and theduring the bauxite digestion process. Composition ofsequence of reaction ability of titanium-containingscale formed by Ti-containing minerals varies withminerals with caustic soda[8]， anatase, the mosttemperature and time, such asCaO-2TiO2:H2O,representative titanium-containing mineral present intitanium- containing hydrogarnet, and perovskite.diasporic bauxite in China, was chosen as the startingFig.l shows the influence of temperature on w(Ti).material in this research work.It is obviously that w(Ti) first increases slowly to reach2 Experimentalts maximum and then decreases sharply with theincrease of temperature. When temperature is lower than2.1 Materials180 C, w(Ti) increases with the increase of time; whenTiO2 powder was a chemical reagent in the phase oftemperature is higher than 180 C, w(Ti) first increasesanatase; NaOH and Na2COz were reagents withand then decreases with the prolongation of time.analytically pure grade; Sodium aluminate solution wassynthesized in laboratory by heating the mixture of1001- -20 minNaOH and Al(OH)3 with analytically pure grade; and-40 minCaO was prepared by calcination of Ca(OH)2 with8●- 80 minanaltically pure grade at 800 C for 2h.2.2 MethodsDigestion operations of anatase were carried out in40high-pressure autoclaves (made in Machine Factory ofCentral South University, China) with mixed nitratemolten salts as heating medium. The resultant slurry20from digestion process was abruptly cooled down bycold water, then filtered by vacuum pump, and washed200220240260with boiling water three times. The filter cake was driedTemperature/Cat 95- -105 C, and finally weighed by electrical balanceFig,1 Infuence of temperature on w(T) of residue resultingwith a accuracy of0.1 g. Then 0.50 g of flter cake wasfrom reaction of anatase with alkaline aluminate solutionput into 100 mL hydrochloric acid solution with a(p(Na20)-233.11 g/L, p(Al20;)=183.01 g/L, m(CaO)m(TiO)=volume ratio of hydrogen chlorine to water of 1, thenheated at 100 C for 15 min in thermostat and filtered1.0)while it was hot. The concentration of titanium dissolvedin hydrochloric acid wasmeasured by H2O2Fig.2 represents XRD patterns of residue obtainedcolorimeter with 7230G spectrometer (Shanghait different digestion temperatures for various time.Analytical Apparatus Company, China).When digestion temperature is lower than 200 C, theSymbol o(Ti) represents mass ratio of TiO2main phases of residue are CaO-2TiO2H2O anddissolved in hydrochloric acid to the total amount of3CaO*Al2O;:6H2O with a small amount of CaTiO3 asTiO2 present in the anatase added. Na2Ok representsshown in Fig.2(a), and CaO 2TiO>:H2O converts intocaustic soda (NaOH) in the form of Na2O in alkalimore stable CaTiOz at higher digestion temperature andaluminate solution, and free Na2Ok represents the causticlonger time, as shown in Fig.2(b).soda uncombined with aluminate anions in the form ofCaTiOz in the phase of perovskite is the most stableNa2O. Na2O。represents sodium carbonate (Na2CO3) inphase of titanium-containing compounds in residue in thethe form of Na2O.digesti中国煤化工，which is also themainthe surface of heat3 Results and discussionexchari YHc N M H sallied CaTiO,3.1 Infuence of temperature and time on reactiondissolves completely only in hydrofluoric acid, and withbehavior of anatasea small solubility in boiling hydrochloric acid[12-14],144LI Xiao-bin, et al/Trans. Nonferrous Met. Soc. China 20(2010) 142-146while poorcrystallized CaTiO3， Na2TiO3 andCaO-2TiO2H2O can completely dissolve in the boiling100间)hydrochloric acid solution. With increasing the digestiontemperature and time， moreCaTiO3 with bettercrystallization degree is produced, which leads to the80 tdecreases of w(Ti) rapidly. So, it should be noted that70w(Ti) is not always a reliable parameter to characterizereaction degree of titanium-containing mineral, which is50different from the common knowledge.50-- Free Na2O.=179.94 g/L40-- Na2O.-243.96 g/L(a▲-CaO-2TiO2*H2O30L100 120 14016018200 220■- 3CaO*Al2O3.6H2Op(Al2O3)/(gL)" (b)80十|:70 t50 t23040 5(68026/(°)|(b)●-CaTiO310120200240p(Na2QO)(gL")Fig.3 Influence of composition of sodium aluminate solutionono(Ti): (a) 180 C，30 min, m(CaO)Vm(TiO2)=l.0; (b)p(Al2O3)=183.01 gL, 180 C, 30 min, m(CaO)/m(TiO2)=1.03.3 Influence of molar ratio of CaO to TiO2 onireaction behavior of anatase in sodium aluminatesolution040SDuring the high-pressure digestion process,20/<*)Na2TiO3 isolates diaspore from caustic soda so as toFig.2 XRD patterms of residue prepared under differentinhibit digestion of alumina in the diasporic bauxite.conditions ((Na2O)- 233.11 g/L, p(Al2O3)=183.01 g/L, m(CaO)/Calcium aluminate hydrate (3CaO-Al2O;6H2O) isn(TiO2)=1.0): (a) 170 C, 40 min; (b) 250 C, 80 minrapidly produced once CaO is added to the aluminatesolution, then other reactions may take place. As for the3.2 Influence of composition of sodium aluminatefamiliar compounds, CaO-2TiO2rH2O and CaTiO3,solution on reaction behavior of anatasetheoretical molar ratios of CaO to TiO2 are 0.5 and 1.0.When concentration of Na2Ok or free Na2Ok is fixed,Researchtransformationinfluence of concentration of Al2O3 on reaction behaviorsilicon-containing and titanium containing minerals inof anatase is shown in Fig.3(a). o(Ti) is almosthigh-pressure digestion proves that TiO2 mainly convertsinvariable with the increase of concentration of Al2O3into CaO-2TiO2rH2O when molar ratio of CaO to TiO2 iswhile concentration of free Na2Ok is fixed. However,less than 0.76, and that CaO-2TiO2rH2O becomeswhen concentration of Na2Ox is fixed, o(Ti) decreasesunstae calcium-titaniumwith the increase of concentration of Al2O3. It may becom中国煤化dded15-19.explained that the concentration of free Na2Or decreasesYHCN M H G between o(Ti) andwith the increase of concentration of Al2O3 in themolar ratio of CaO to TIO2. 0(I1) increases with thealkaline aluminate solution. And it is the same case inincrease of molar ratio of CaO to TiO2 with relativelyFig.3(b). The reason will be discussed later in detail.low concentration of free Na2O%, whereas w(Ti) firstLI Xiao-bin, et al/Trans. Nonferrous Met. Soc. China 20(2010) 142- 14645increases and then decreases with the increase of molarshows the XRD pattern of residue obtained under theratio of CaO to TiO2 under the condition of highfollowing conditions: the molar ratio ofCaO to TiO2 is 3,concentration of free Na2Ok. For example, w(Ti)and the concentation of free Na2On is 220 g/L. It can bedecrcases from 84.70% to 35.63% with molar ratiorevealed that CaO 2TiO>2H2O changes into more stablecorrespondingly ranging from 0.5 to 3.0 whenCaTiO3 which is hard to dissolve in hydrochloric acid.concentration of free Na2Ok is fixed at 220 g/L. Fig.53.4 Reaction mechanism of anatase in high-pressure100digestion processIn order to explore the reaction laws and itsmechanism of titanium-containing mineral during8high-pressure digestion process of diasporic bauxite,digestion experiments of anatase in different solutions60were carried out at 180 C for 60 min. Experimentalresults are listed in Table 1.- Free Na20 50.00 g/l.From results listed in Table 1 and the reactionFree Na20L 110:00g/Lbehaviors of anatase discussed above, reaction- Free Na2O.220.00 g/Lmechanism of anatase in sodium aluminate solution20during the high-pressure digestion process can be0.51.01.2.0 2.5 3.0m(CaO)(TiO2)presumably determined as follows.Fig.4 Influence of molar ratio of CaO to TIO2 on w(Ti)1) Neither soda (Na2CO3) nor Ca(OH)2 can react(p(Al2O)=180.94 g/L, 180 C for 30 min)with anatase under the experimental conditions. However,anatase can react with NaOH to form Na2TiO3 whichmay further react with Ca(OH)2 to form CaTiO3ifCaO is3CaO-Al2O-6H2Oadded to the system, as shown in reactions (1) and (2):-CaTiO22NaOH+TiOz =Na2TiO3+H2O(1)Ca(OH)2+Na2TiO;=CaTiO3+2NaOH(2)2) Aluminate anion seems to remarkably catalyzereactions of anatase with caustic soda solution. Results ofexperiment 3 and experiment 4 show that, under thealmost same concentrations of free Na2O2(Na2Ok 116.00g/L), w(Ti) increases from 3.05% to 37.88% in the .presence of aluminate anion without CaO, and o(Ti) also20/(*)increases from 7.16% to 84.87% in the presence ofFig.5 XRD patterm of residuc prepared with high molar ratio ofaluminate anion when CaO is added with molar ratio ofCaO to TiO2 and high concentration of free Na2O%CaO to TiO2 of 1. So, the conclusion can be drawn that(0(Al2O3)=180.94 g/L, m(CaO)/m(TiO)=-3.0, p(ree Na2O)=220aluminate anions plays catalysis role in the reactiongL, 180 C, 30 min)process. The catalysis mechanism of aluminate anionsTable 1 Reaction behavior of anatase in different solutionsMolar ratio ofExperiment No.SolutionCompositionw(Ti)/%Product in solid phaseCaO to TiO20AnatasePure waterAnatase, Ca(OH)zSodium carbonatep(Na20)=117.31 g/LAnatase, CaCO3Anatase, Na2TiO33Caustic sodap(Free Na2O)=115.95 g/L7.16Anatase,CaTiO3, Ca(OH)2p(Na20)=243.96 g/L37.88p(Free Na2O)-116.00 g/L_0Al2O36H2O,p(Al203)-210.65叭中国煤化工10H50.CaTiO,_4Sodium aluminatepNa2O)= -329.98 g/LMYHC N M H G:-l20:6H2o,p(Free Na2OJ)-220.00 g/L55.05CaTiO3 .p(Al20)=180.94 g/L146L1 Xiao-bin, et a/Trans. Nonferrous Met. Soc. China 20(2010) 142-146could be explained as follows:References2NaAl(OH)+TiO2=Na2TiO3+2Al(OH)3+H2O(3)NaOH+Al(OH)=NaAl(OH)4(4)[] ZHAO Hengqin, LI Jie, WANG Li-zhuo, LIU Ye-xing. Review ofcurreat situation on bauxite resources aod production technology ofThe total reaction can be expressed asalumina in China [J]， Conservation and Utilization of MineralResources, 2001(5): 38- 42. 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From the discussion mentionedof a gibbitic bauxite with hydrocbloric acid [0 Hyomtllugy,above, conclusion can be made that elevating the1999, 51(1): 131-138.digestion temperature, prolonging the duration time,[12] BI Shi- wen, L Dian-feng, YANG Yi-hong The study on the reactionof rutile in Bayer liquor [Cy/Light Metals. Warrendale, Pennsylvania,increasing concentration of free Na2Ok and molar ratio of1996: 43-48.CaO to TiO2 favor the conversion of CaO-2TiO2H2O to[13] GU Song-qing. CAO Rongj-jang, CHENG Xin min. CaO 2TiO2H2OCaTiO3.in Bayer digesting process [D Chinese Journal of Rare Metals, 1987,6(3): 161-166. (in Chinese)[14] PAWLEK F, KHEIRI M J, KAMMEL H C R. The leaching bechavior4 Conclusionsof bauxite during mchanochermical treatment [CV/Light Metals.Warrendale, Pennsylvania, 1992: 91-95.1) In sodium aluminate solution, aluminate anion[15] AI zijin. Study on bchavior of si and π during indrectly heaingprocess for bauxites from different areas [D Noferrous Metals:has a catalysis function for reaction of anatase withExtractive Mtallurgy, 1995(6): 33- 36. (in Chinese)caustic soda.[16] CHEN L, LI Man. Phase transformation of Si-containing and2) During high-pressure digestion process oTi-containing minerals during high-pressure leaching process forbauxites [小Nonferrous Metals: Extractive Metlungy, 1991(5);:diasporic bauxite, the reaction degree of anatase depends25-29. (in Chinese)on the concentration of free Na2Or rather than the[17] CHEN Li, LI Man. Phase transformation of Si-containing andconcentration of Al2O3, so long as there is aluminatebauxite from Guizhou U]. Light Mctals, 1991(6): 18-22. (anions in alkali solution.3) Anatase first reacts with free Na2Ok to form [18] CHEN Li, u Yong. The fect of lime on high pessure digstion ofNa2TiOz which further reacts with 3CaOAl2O:-6H2O tobauxite in Guizhou [J]. Journal of Guizhou University of Technology,produceCaO-2TiO2H2O, andCaO-2TiO2:H2O[19中国煤化工essofdgstionofOuitoueventually changes into CaTiOz.Moreover, higherUaiversity of Technologytemperature, longer time, higher concentration of freeMHCNMHGNa2Ok and molar ratio of CaO to TiO2 all favor the(Edited by YANG Hua)conversion of CaO-2TiO2H2O to CaTiOz.