Preparation of synthetic rutile via selective sulfation of ilmenite with(NH4)2SO4 followed by targeted removal of impurities☆

Weizao Liu,Xiaomei Wang,Zhenpu Lu,Hairong Yue,Bin Liang,Li Lü,Chun Li*

College of Chemical Engineering,Sichuan University,Chengdu 610065,China

1.Introduction

Titanium dioxide,the mostimportant white pigment,is widely used in paints,plastics,rubber,paper,ceramic,medicine,food,cosmetic and textile industries[1].There are two commercial processes for producing TiO2,namely the sulfate and chlorination processes.In the sulfate process[2],ground ilmenite is first digested with concentrated sulfuric acid,yielding a Ti-containing solution that is later purified and hydrolyzed.The as-prepared hydrolysate is then calcined to yield TiO2.This process is lengthy,costly and produces large quantities of less marketable ferrous sulfate(3-4 tons per ton of TiO2)and～20 wt%waste sulfuric acid(8-10 tons per ton of TiO2),resulting in severe environmental problems.In the chlorination process[3],high-grade titanium slag or rutile is chlorinated in a fluidized bed reactor at high temperatures.The TiCl4thus obtained is oxidized into titanium dioxide after purification.This process can produce more superior TiO2pigment with less discharged waste.Currently,more than 60%of TiO2pigments are manufactured by the chlorination process worldwide.However,natural rutile reserves are only one-tenth ilmenite[4].The shortage of natural rutile has been encouraging research on the conversion of ilmenite into synthetic rutile.

The traditional methods for producing synthetic rutile from ilmenite consist of the smelting,Becher and acid leaching processes.In the smelting process[5],the iron in ilmenite is reduced into metallic iron and melted at over 1600°C for separation from the titanium containing phases(the so-called titania slag).In the Becher process[6,7],the iron in ilmenite is first reduced to metallic iron at approximately 1150°C.The metallic iron is then air oxidized in an ammonium chloride solution and separated from the titanium oxides using hydrocyclone.The acid leaching processes[8]consist of HCl and H2SO4leaching of ilmenite.Because the direct leaching of ilmenite is slow,various enhancement measures are employed;these include mechanical activation[9-11],high-temperature carbonthermal reduction[3],and preoxidation followed by reduction[12]of ilmenite prior to leaching,high temperature and pressurized operation[13],and the addition of reductant[14,15]during the dissolution.Recently,Lahiri[16]proposed an alkali process in which ilmenite was first roasted with alkali followed by water leaching to remove soluble impurities;then,the residue was reductively leached in a solution of ascorbic and oxalic acids to obtain synthetic rutile.

In metallurgy,reactions of ammonium sulfate with metal oxides are frequently applied for bene ficiation and extraction of metal oxide from various low-grade metal ores[17,18].Here,we report a route for preparing synthetic rutile in which ilmenite is firstsul fated with ammonium sulfate at low temperatures to form mixed sulfates of ammonium and various metal ions associated in ilmenite.The Ti-bearing sulfate in the sulfated ilmenite is then selectively decomposed to TiO2at medium temperature.A series of leaching units are then designed to remove the impurities from the selectively thermal decomposed slag(abbreviated hereafter as STDS),i.e.,water leaching the non-decomposed FeSO4,MgSO4and CaSO4;dilute acid leaching a small amount of the Fe2O3from partial decomposition of FeSO4and alkali leaching SiO2.

In this study,the enrichment process parameters of the main units were systematically investigated.The intermediate and final products were characterized and the relative reaction mechanism was discussed.

2.Experimental

2.1.Materials

An ilmenite from Panzhihua,Sichuan,China,provided by the Titanium Company of Pangang Group Corp.,was employed.The X-ray diffraction(XRD)analysis of the ilmenite is shown in Fig.1a.The major mineral constituent is hexagonally structured FeTiO3.No other crystal phases are detected,which is probably due to the low contents of impurity phases in the ilmenite or the formation of solid solution between the impurity phases and ilmenite[19].The chemical composition of the ilmenite listed in Table 1 indicates that the major impurities are SiO2(4.93%)and MgO(3.42%).During the experiments,the-45 μm fraction of the ore was utilized.The chemicals,including ammonium sulfate,ferric sulfate,ferrous sulfate and hydrochloric acid,were of analytical reagent grade,while titanyl sulfate was of chemical reagent grade and all were used as received without further purification.

Fig.1.The XRD patterns of(a)the as-received ilmenite and(b)the sulfated ilmenite prepared by roasting at 360°C for 120 min with a(NH4)2SO4/ore of 14.

Table 1Chemical composition of Panzhihua ilmenite used in this study(wt%)

2.2.Selective sulfation of ilmenite

In each sulfation experiment,the ilmenite sample was thoroughly mixed with ammonium sulfate at a specific mass ratio.The mixture placed in a crucible was then heated at a rate of10 °C·min-1to a required temperature,ranging from 250 to 420°C,and annealed for a certain period of time in a horizontal tube furnace(GSL-1600X,Heifei Kejing Materials Technology Co.Limited,Heifei,China)under nitrogen at a N2(99.99%) flow of 0.1 L·min-1.After the reaction,the crucible was pulled out halfway,and the sulfated ilmenite was cooled to room temperature in the presence of a flow of nitrogen.In each thermal decomposition experiment,the sulfated ilmenite was further roasted at a required temperature,ranging from 440 to 570°C for a certain period of time,under nitrogen.After the reaction,the STDS was cooled to room temperature in the presence of a flow of nitrogen.

To evaluate the sulfation of ilmenite and thermal decomposition of the Fe and Ti-containing sulfates,the sulfated ilmenite and STDS were,respectively,subjected to 10%H2SO4and water leaching at 50°C.

Based on the Ti and Fe leaching,the sulfation and decomposition percentages were calculated using the following formulas:

where η (%)is the conversion of the sulfation;c1(g·L-1)andV1(ml)are,respectively,the TiO2or Fe concentration and volume in the H2SO4leaching solution;w1(wt%)is the TiO2or Fe mass percentage in ilmenite;andm1(g)is the mass of ilmenite sample used in the sulfation experiment.φ(%)is the decomposition percentage of the Fe and Ti-containing sulfates,c2(g·L-1)andV2(ml)are,respectively,the TiO2or Fe concentration and volume in the water leaching solution.

2.3.Leaching for removal of impurities

Leaching experiments were performed in a 100 ml,three-necked glass reactor fit with magnetic stirring,a thermometer and a re flux condenser.The reactor was heated in an oil bath with temperature fluctuation of±1 °C.

In the waterleaching experiment,the STDS was dissolved in distilled waterata liquid/solid ratio of10 ml·g-1and 50 °Cfor1 h to remove undecomposed sulfates,such as FeSO4,MgSO4and CaSO4.In the acid leaching experiment,the water leached residue was dissolved in a dilute HCl solution of 1 wt%-10 wt%at a liquid/solid ratio of 50 ml·g-1and temperature ranging from 15 to 98 °C for up to 180 min to remove a small amount of the Fe2O3,generated by the decomposition of FeSO4.In the alkali leaching experiment,the HCl leached residue was further dissolved in a 5 wt%NaOH solution at a liquid/solid ratio of 50 ml·g-1and 102 °C for 2 h to remove the SiO2impurity.The asprepared residue was thoroughly rinsed with water and calcined at 1000°C for 1 h to obtain synthetic rutile.

The total removal efficiency of Fe and loss of TiO2after HCl leaching were calculated using Formula(3):

whereλ(wt%)is the removal efficiency of Fe or the loss of TiO2;c3(g·L-1)andV3(ml)are,respectively,the TiO2or Fe concentration and volume in the HCl leaching solution.

2.4.Analysis and characterization

The concentrations of titanium and iron ions in the leachates were determined by redox titrations of ammonium ferric sulfate(NH4Fe(SO4)2)and potassium dichromate(K2Cr2O7),respectively.For determination of the chemical composition of synthetic rutile,the product sample was melted with sodium dioxide(Na2O2)and sodium hydroxide(NaOH)at 750°C and then leached with dilute hydrochloric acid.The titanium concentration in the resulting solution was analyzed by the aforementioned redox titration method,and the iron,calcium,silicon,manganese,and aluminum concentrations were analyzed via ICP-OES(Spectro ARCOS ICP,Germany).

XRD analyses were performed using a DX-2007 X-ray diffraction spectrometer(Danton,China)operating with a CuKαradiation source that was filtered with a graphite monochromator at a frequency of λ=1.54 nm.The voltage and anode current were 40 kV and 30 mA,respectively.The continuous scanning mode with a 0.03 s interval and 0.05 s set time was used to collect the XRD patterns.

Thermal analysis of the samples was performed using Simultaneous Thermogravimetry-Differential scanning calorimetry(TG-DSC,Netzsch,STA 449F3,Germany).

3.Results and Discussion

3.1.Sulfation of ilmenite

Fig.2 shows the effect of the roasting temperature,which varied from 250 to 420°C,on the sulfation of titanium and iron.Clearly,the roasting temperature significantly affected the conversion.At 250°C,the sulfation percentages of Ti and Fe were only 26%and 37%,respectively.With increasing reaction temperature,the sulfation rapidly increased,reached a maximum at 360°C with titanium and iron conversions of 94%and 92%,respectively,and then slowly decreased.It was observed that the sulfation of titanium was remarkably lower than that of iron at 420°C.This is because ferrous sulfates have better heat stability than the titanium-containing sulfate,which started to decompose into acid-insoluble TiO2at 400°C[20].

Fig.2.The effect of the sulfation temperature on the conversion ratio of iron and titanium at(NH4)2SO4/ore=14 and reaction time 120 min.

Figs.3 and 4 show the effects of the roasting time and mass ratio of(NH4)2SO4to ilmenite on the sulfation at 360°C,respectively.Clearly,with increasing time,both the sulfations of iron and titanium rose monotonously and plateaued at a time beyond 120 min.Similarly,with the increasing mass ratio,both the sulfations of iron and titanium increased and leveled off at a mass ratio in excess of 14.

Fig.3.The effect of the sulfation time on the conversion ratio ofiron and titanium at360°C and(NH4)2SO4/ore=14.

Fig.4.The effect of the mass ratio of(NH4)2SO4 to ore on the conversion ratio of iron and titanium at 360°C and reaction time 120 min.

Therefore,optimum parameters for sulfation were determined to be a temperature of 360°C,mass ratio of(NH4)2SO4to ilmenite of 14,and reaction time of 120 min.Under these conditions,the sulfation of ilmenite reached～95%.

Fig.1b shows the XRD spectra of sulfated ilmenite obtained under the optimal conditions.The diffraction peaks of ilmenite were completely missing and the primary productphases were NH4Fe(SO4)2,TiOSO4,NH4HSO4and SiO2.Since the majority of the iron in Panzhihua ilmenite is in a ferrous state,we infer that NH4Fe(SO4)2was formed due to the oxidation of(NH4)2Fe(SO4)2during the cooling process in the tube furnace,even under the N2flow.(An experiment was conducted by roasting(NH4)2Fe(SO4)2·6H2O at 360 °C and then cooling to room temperature under N2flow.XRD result showed that the roasted product was NH4Fe(SO4)2).

Studies[21,22]on the sulfation of iron oxides and TiO2by ammonium sulfate showed that these reactions could be divided into at least two steps.The first step was decomposition of(NH4)2SO4to NH3and NH4HSO4.The resulting NH4HSO4then digested the metal oxides into corresponding metal sulfates and/or mixed metal and ammonium sulfates.Therefore,the main sulfation reactions of ilmenite in the present study can be represented as:

In addition,itis well known that the chlorination TiO2process is very sensitive to the CaO and MgO contents in rutile or titania-rich slag.The main alkaline earth impurities present in Panzhihua ilmenite exist as titanaugite(CaMgSi2O6)[23].A thermodynamic calculation using HSC Chemistry 5.0(commercial software from Outotec,Finland)shows that the following sulfation reaction of titanaugite occurs even at room temperature with an equilibrium constant of over 1020:

3.2.Selectively thermal decomposition of sulfated ilmenite

(NH4)2Fe(SO4)2decomposed to FeSO4at approximately 430°C[24],and FeSO4further decomposed to Fe2O3at 500°C[25,26].Both MgSO4and CaSO4have excellent thermal stability with decomposition temperatures of 1100 °C[27]and 1200 °C[28],respectively.Although the decomposition of TiOSO4is reported to start at 540°C[29],our preliminary work showed the decomposition temperature of TiOSO4would decrease dramatically to ～430 °C in this system.The reason for this will be discussed later.Therefore,selectively thermal decomposition of the(NH4)2Fe(SO4)2and TiOSO4in the sulfated ilmenite to FeSO4and TiO2,respectively,which are readily separated in subsequent leaching,is feasible by controlling the roasting temperature between 430 °C and 500 °C.The two decomposition reactions can be expressed as Eqs.(7)and(8).

Fig.5 shows the effect of the reaction time on the decompositions of FeSO4and TiOSO4at480°C under nitrogen.Clearly,the decompositions of titanium and iron sulfates to their oxides increased with time.At 30 min,over 60%TiOSO4were decomposed while the FeSO4decomposition was only 5%.After 180 min,almost all TiOSO4were decomposed,while the co-decomposed FeSO4reached up to 23%.

Fig.5.The effect of the roasting time on the decomposition of iron and titanium sulfates at 480°C under nitrogen.

Figs.6 and 7 show the effect of temperature on the decomposition of sulfated ilmenite under nitrogen and air atmospheres,respectively.Clearly,both the decompositions of titanium and ferrous sulfates increased with increasing temperature in spite of the atmospheres.It can be observed that both TiOSO4and FeSO4in the sulfated ilmenite decomposed more rapidly in N2flow than in air flow.For example,at 500°C,the extent of decomposition of titanium and iron were 99.8%and 42.7%,respectively,under nitrogen,while the corresponding values declined to 81.2%and 2.9%under air.Clearly,to selectively achieve thermal decomposition of the sulfated ilmenite,the optimal roasting temperature is 480°C in a N2atmosphere;in those conditions,nearly all TiOSO4were decomposed with the FeSO4co-decomposition of 23%,while the optimal roasting temperature increased to 570°C in air atmosphere in which 98%of the TiOSO4was decomposed with the 12%co-decomposition of Fe2(SO4)3.

Fig.6.The effect of the roasting temperature on the decomposition of iron and titanium sulfates under nitrogen for 180 min.

Fig.7.The effect of the roasting temperature on the decomposition of iron and titanium sulfates under air for 180 min.

Fig.8.The XRD patterns of(a)the decomposition product under nitrogen at 480°C and(b)the decomposition product under air at 480°C.

To understand the effect of atmosphere on the thermal decomposition of sulfated ilmenite,XRD patterns of the decomposition products of the sulfated ilmenite under different atmospheres were detected as shown in Fig.8.The primary product phases were anatase TiO2and FeSO4under nitrogen as well as anatase TiO2and Fe2(SO4)3under air.Obviously,under air,FeSO4was oxidized to Fe2(SO4)3,which has a higher heat stability than FeSO4[26].Thus,the decomposition temperature of iron rises when exposed to air.A comparison experiment on the thermal decomposition of pure TiOSO4at 500°C under N2and air for 3 h showed that the extent of decomposition was very close(～35%).As a result,a dramatic elevation of decomposition of TiOSO4in the sulfated ilmenite under N2must be relative to the presence of FeSO4in this system.

To justify the inference,the thermal decomposition of the pure and mixed sulfates,respectively,was investigated using TG/DSC.Fig.9a shows the TG plots of different sulfates,which were heated at a rate of 10 °C·min-1to 850 °C in nitrogen.The weight losses of different sulfates at ≤300 °C and 500-720 °C were,respectively,attributed to dewatering and desulfurization.The desulfurization of TiOSO4,FeSO4and their mixture nearly started at～550 °C,while this value increased to ～650 °C for Fe2(SO4)3.More precise temperatures could be obtained from their DSC and/or DTG curves,as shown in Fig.9b and c.The peak temperatures of DSC and DTG curves were very close to each other,and there were errors within 2°C for all sulfates.Pure TiOSO4and Fe2(SO4)3decomposed,respectively,at 627 °C and 714 °C,while the thermal decomposition of pure FeSO4proceeded via two stages with two evidentendothermic peaks at605 and 699°C.XRDanalysis indicated that the intermediate and final products for FeSO4decomposition were,respectively,Fe2(SO4)3and hematite Fe2O3.This result was in agreement with Masset[30].The decomposition of a mixture of 50 wt%TiOSO4and 50 wt%FeSO4was observed to mainly occur at 565,592 and 708°C,which corresponds to the decomposition of TiOSO4to TiO2,FeSO4to Fe2(SO4)3,and Fe2(SO4)3to Fe2O3,respectively.Clearly,compared with pure sulfates,the decomposition temperature of TiOSO4mixed with FeSO4was significantly decreased by 62°C,while the second-stage decomposition temperature of FeSO4mixed with TiOSO4was slightly increased by 9°C.The detailed reasons for these observations will be discussed hereafter.Table 2 lists the decomposition reactions of the pure and mixed sulfates that were mentioned above and their decomposition temperatures.

Fig.9.(a)TG curves of various sulfates;(b)DSC curves of various sulfates;(c)DTG curves of various sulfates;(d)decomposition rule of mixed sulfates(TiOSO4+FeSO4,TiOSO4+Fe2(SO4)3)at 480°C for 180 min under nitrogen.

Table 2The desulfurization reactions of different sulfates

Fig.9d shows the effects ofthe iron sulfate ratio in mixed TiOSO4and FeSO4or Fe2(SO4)3on decomposition of the three sulfates at 480°C under N2for3 h.As seen,in the mixed TiOSO4and FeSO4system,the decomposition ofTiOSO4remarkably accelerated with an increasing FeSO4ratio.The 3 h decomposition of pure TiOSO4was only 17%,while the value dramatically increased to 90%at the 50 wt%FeSO4.Conversely,the decomposition of FeSO4decelerated with increasing TiOSO4ratio.The decomposition of pure FeSO4was 44%,while the value decreased rapidly to 8%at the 50 wt%FeSO4.In other words,FeSO4can promote the decomposition of TiOSO4,while TiOSO4can prevent FeSO4from decomposition.For the mixed TiOSO4and Fe2(SO4)3system,however,the decomposition of TiOSO4accelerated slightly with increasing Fe2(SO4)3ratio,while Fe2(SO4)3was hardly decomposed at any Fe2(SO4)3ratios.These patterns were observed because the FeSO4mixed with TiOSO4can be oxidized to a higher thermalstability Fe2(SO4)3by oxygen generated from the decomposition of TiOSO4.As a result,the decomposition of FeSO4mixed with TiOSO4was inhibited,and the decomposition reaction of TiOSO4was promoted due to the immediate in situ consumption of the oxygen from the decomposition product of TiOSO4.Therefore,the decomposition temperatures of TiOSO4in the sulfated ilmenite in nitrogen were lower than those in air(see Figs.6 and 7).

3.3.Targeted leaching

In the leaching experiments,the STDS obtained under optimal sulfation and selectively thermal decomposition conditions in N2was employed.As mentioned above,the main phases in the STDS are TiO2and FeSO4,and there were low levels of Fe2O3,MgSO4,CaSO4,and SiO2.Based on differences in the physicochemical properties of these products,a series oftargeted leaching stages was designed to selectively remove the impurities from TiO2.First,water leaching was employed to remove all water-soluble FeSO4and MgSO4as well as a significant fraction of CaSO4at a liquid/solid mass ratio of 10 at 50°C.The water leaching residue thus obtained was dissolved to remove Fe2O3with HCl:

Fig.10.Effects of the temperature,HCl concentration and leaching time on iron removal and titanium dissolution loss.

The effects of the leaching temperature,HCl concentration and leaching time on Fe removal and TiO2dissolution loss are shown in Fig.10.With increased leaching temperature,the removal efficiency of iron monotonously increased and reached 94.5%at 98°C,while the TiO2dissolution loss increased first and then decreased with a minimum TiO2loss of less than 0.5%at 98°C.This is because the TiO2in the STDS underwent a dissolution-precipitation(hydrolysis)mechanismduring acid leaching[31].With increasing temperature,hydrolysis of the dissolved TiO2became obvious.Thus,the optimal leaching temperature was selected as 98°C.With increasing HCl concentration from 0 to 10 wt%,the Fe removal efficiency increased from 77%to 95%and the TiO2loss increased from 0.2%to 1%.To minimize the TiO2loss,an HCl concentration of 2.5 wt%was chosen as the optimal condition.With increased leaching time,the removal efficiency of iron increased and reached 94.2%at 120 min;after that time period,the iron removal was almost unchanged.On the other hand,it was observed that the maximum TiO2dissolution occurred at 30 min.Obviously,the dissolution rate of TiO2surpassed the hydrolysis rate of dissolved TiO2within 30 min,while the hydrolysis rate was in excess of the dissolution rate after 30 min.As a result,120 min was selected as the optimal leaching time.

Fig.11a shows the XRD pattern of the leaching residue under the optimal leaching conditions(2.5 wt%HCl,98°C and 120 min).No iron-containing crystal phases,except anatase TiO2,were observed.The chemical compositions of the residue after calcination at 1000°C are presented in Table 3a.As seen,SiO2with content as high as 9.06%was confirmed to be the major impurity,which might be a by-product of the sulfation of titanaugite and could exist in the amorphous form due to the lack of diffraction peaks from SiO2containing phases being observed in Fig.11a.As a result,removal of SiO2by NaOH leaching of the acid leaching residue in 5 wt%NaOH solution was conducted at 102°C for 1 h.The residue after NaOH leaching was washed with water,dried and calcined at 1000°C for 60 min.XRD analysis indicates that this final product was in a pure rutile phase,as shown in Fig.11b.The chemical composition of the final product,listed in Table 3b,shows that the TiO2grade of the synthetic rutile reached 92.86 wt%with a total MgO+CaO that was less than 1.5 wt%.

Fig.11.XRD patterns of(a)the 2.5 wt%HCl leaching residue at 98°C for 120 min and(b)after calcinations at 1000°C.

Table 3Chemical composition of synthetic rutile(a)before NaOH leaching and(b)after NaOH leaching(wt%)

The particle size analysis(Fig.12)shows that the d50 of the prepared synthetic rutile was only 1.4 μm,and all particles were less than 10 μm,which is remarkably smaller than the minimum size required for the fluidized chlorination in the chloride TiO2process.Further studies are required to coarsen the minute synthetic rutile,which can probably be achieved by pelletizing and then sintering.Alternatively,the minute synthetic rutile can be directly chloridized in a molten salt chlorinating furnace or pipe reactor.

3.4.Flow sheet

Fig.12.Particle size distribution of the obtained synthetic rutile.

Based on the results obtained in the study,a concept flow sheet for preparing synthetic rutile from ilmenite is proposed,as shown in Fig.13.The ilmenite is first selectively sulfated with ammonium sulfate.The STDS thus obtained is continuously subjected to water leaching,dilute acid leaching and alkali leaching,respectively,for the removal of water-soluble sulfates,Fe2O3and SiO2.Aftercalcination,an additional grain agglomeration is needed to increase the particle size.A synthetic rutile product assaying over 92%TiO2with a sum of CaO and MgO that was less than 1.5%can be obtained after calcination of the alkalileaching residue.

Fig.13.Schematic flow sheet for the preparation of synthetic rutile(F: filtrate and R:residue).

The water leaching solution is neutralized to precipitate Fe(OH)2and Mg(OH)2,step by step,by controlling the solution pH value via bubbling the exhaust gas emitted during the selective sulfated roasting,which contains a high level of NH3and a small amount of SO2/SO3.The mother liquor thus produced is oxidized to transform(NH4)2SO3to(NH4)2SO4,which is evaporated and cooled to crystallize(NH4)2SO4for recycling.The acid leaching solution is recycled to increase its FeCl3concentration;then,it is evaporated and cooled to crystallize FeCl3.The alkali leaching solution is also recycled to increase its Na2SiO3concentration,which is evaporated and cooled to crystallize Na2SiO3.

4.Conclusions

In this paper,a novel,facile method for preparing synthetic rutile from ilmenite is proposed.Its major steps include selectively sulfating ilmenite and targeted leaching of the impurities associated with ilmenite.The process of selectively sulfating ilmenite was realized via roasting ilmenite with ammonium sulfate at low temperatures,which is followed by selective thermal decomposition of the sulfated ilmenite at elevated temperatures.The targeted leaching consisted of water,dilute acid and alkali leaching to remove the water-soluble sulfates,Fe2O3and SiO2,respectively.A synthetic rutile with TiO2content of over 92 wt%and total MgO+CaO less than 1.5 wt%was obtained after calcination of the alkali leaching residue.In the present process,the ammonium sulfate and hydrochloric acid used can be recycled and byproducts like Fe(OH)2,Mg(OH)2,and FeCl3can be obtained.Additionally,it was found that,compared with decomposition of pure sulfates,the decomposition of FeSO4mixed with TiOSO4under N2was inhibited due to its oxidation to higher thermal stability Fe2(SO4)3by oxygen from the decomposition of TiOSO4.Atthe same time,the decomposition of TiOSO4was promoted by in situ immediate consumption of the oxygen by FeSO4.The synergistic effect might be responsible for enhanced selectivity of the thermal decomposition of sulfated ilmenite.

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