Lin Wang,Weizao Liu,Jingpeng Hu,Qiang Liu,Hairong Yue,Bin Liang,Guoquan Zhang,Dongmei Luo,Heping Xie,Chun Li*

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

1.Introduction

Carbon dioxide capture(utilization)and storage(CCS/CCUS)is recognized as one of the options to tackle the increase in the atmospheric concentration of CO2for climate change mitigation[1].Although geological and oceanic sequestration of CO2has a huge storage capacity,proper geological conditions are required,such as depleted oil and gas fields and geographical conditions,e.g.,close to deep sea for the CO2sources,as well as inevitable use of expensive monitoring equipment to ensure that CO2does not escape into the atmosphere for a very long term[2,3].Mineral carbonation is a potential CO2sequestration method,in which calcium and magnesium oxides in various minerals,especially silicate minerals,react with CO2and for mthermodynamically stable carbonates.The silicate rocks are abundant in nature to the extent that in theory the potential for CO2storage by mineral carbonation is larger than the potential for storage of CO2by other methods[4].

Mineral carbonation consists of direct and indirect methods.The direct method mimics the weathering process in nature but proceeds at a much faster rate,enhanced by a variety of methods,including activation pretreatment of minerals[5],high temperature and pressure[6,7],and high gravity[8].However,the products of direct mineral carbonation are generally a mixture,which is hard to separate for further utilization or selling as a valuable product.Indirect CO2mineralization is ordinarily comprised oftwo successive steps:(1)extracting Ca and Mg from minerals with an acidic or weakly acidic additives and(2)carbonation of the Ca-and Mg-rich solutions or solids with CO2in a basic or weakly basic environment.The method is now receiving widespread attention because of the relatively mild reaction conditions,high carbonation conversion of Ca and Mg,and purer,thus more valuable,by-products[9–11].

Recently,the main raw materials for mineral carbonation are calcium-and magnesium-containing natural minerals[9,12,13]and industrial solid wastes[14–16].Comparatively,the industrial solid wastes are easily available and cheap with high reactivity and usually appear where a large quantity of CO2is emitted.Hence,there are many advantages for sequestration of CO2with industrial solid wastes.

Steel making slags are the main solid wastes in the iron and steel industry with total CaO+Mg Ocontent≥40 wt%,including blastfurnace slag and steel slag emitted in the iron re fining and steel making process,respectively[17].Currently,approximately 300–1000 kg blast furnace slag is discharged per ton of iron production,depending on the grade of the iron ores and process conditions employed[18].Approximately 100–150 kg steel slag is produced per ton of steel production[19].In 2015,the global output of steel was approximately 1.6 billion tons[20].If all of the Ca and Mg elements in steel making slag worldwide were utilized to fix CO2,theoretically 200–680 million tons of CO2could be safely sequestered annually.Although the amounts are small in comparison with the capacity of natural mineral resources,using steel making slags to store CO2on site could provide an inexpensive way to reduce the CO2emissions for individual iron and steel plants significantly.

Ti-bearing blast furnace(TBBF)slag is a special kind of blast furnace slag,which is generated during the smelting of the vanadium titano magnetite in a blast furnace for iron production.TBBF slag usually contains 10 wt%–25 wt%of TiO2in addition to the conventional calcium,magnesium and aluminium silicates.Currently,approximately 20 million tons of the TBBF slag is discharged in China annually.The TBBF slag is difficult to use and thus must be stockpiled,resulting in serious environmental pollution as well as a waste of the precious titanium resources.Although numerous methods,including high temperature carbonization and low temperature chlorination[21],selective enrichment growth and separation process[22,23],acid processes[24,25],alkali processes[26,27]and iron-silicon-titanium alloy processes[28,29],have been proposed,those methods focus mainly on the recovery of titanium and pay almost no attention to the efficient utilization of the remaining components of the slag.Some of these methods may even produce secondary pollution.Recently,Xue et al.[30–32]proposed an ammonium sulphate roasting method for simultaneous recovery of Ti and Al,in which 91.7%of the titanium and 97%of the aluminium were extracted.However,large quantities of ammonium hydroxide and acidic leaching residue containing calcium sulphate and silicon dioxide were discharged and are difficult to utilize.

Recently,we suggested a novel,facile pathway for simultaneous mineralization of CO2with the Ca and Mg components in the TBBF slag and extraction of titanium and aluminium[33].In the process,the TBBF slag was roasted with recyclable ammonium sulphate and converted into sulphates of various metal elements in the slag.Highadded-value Ti-and Al-rich products can be achieved through stepwise precipitation of the leaching solution of roasted slag.The NH3produced during the roasting was used to capture CO2from flue gases.The solutions of NH4HCO3and(NH4)2CO3thus obtained are used,respectively,to carbonate the CaSO4-containing leaching residue and the MgSO4-rich leaching solution after the recovery of Ti and Al.The process flow sheet is shown in Fig.1.Present study focuses on the process parameters and efficiency of the roasting,mineral carbonation and recovery of titanium and aluminium.

2.Experimental

2.1.Materials

Fig.2.The XRD patterns of the water-quenched slag before(b)and after(a)annealing in 850°C.

Fig.1.The process flow sheet of combined mineralization of CO2 and recovery of Ti and Al from the titanium-bearing blast furnace slag.

The TBBF slag used in this study was water-quenched slag,provided by Panzhihua Iron and Steel Group Company Limited(Sichuan,China).X-ray diffraction(XRD)analysis indicated that the slag was amorphous as shown in Fig.2b and could be converted into crystalline state after annealing at 850°C(Fig.2a).The major crystal mineral constituents were perovskite(CaTiO3),titanium-bearing diopside(Ca(Ti,Mg,Al)(Si,Al)2O6),diopside(Ca(Mg,Fe,Al)(Si,Al)2O6),calcium silicate(CaSiO3)and calcium aluminate(Ca3Al2O6).The chemical composition of the slag is listed in Table 1.From the literature[34],approximately 20%–30%of the titanium is low valence and distributed in the titanium-bearing diopside.The remaining titanium was mainly in the perovskite.The particle size distribution of water-quenched slag is shown in Fig.3.The slag had a wide particle size distribution with a D50 of 38 μm and 99%of the particles were less than 100 μm.The morphology of the slag is presented in Fig.4.Clearly,the slag consisted of smooth-faced irregular particles.

Table 1 Chemical composition of the TBBF slag used in this study(wt%)

Fig.3.The particle size distribution of the TBBF slag.

The chemicals,including ammonium sulphate(AS),sulfuric acid,ammonia,ammonium bicarbonate and ammonium carbonate were of analytical grade.

2.2.Experimental methods

The purpose of roasting is to conver tall the metal components in the slag into their sulphates,which are readily separated for further enrichment and carbonation.Thus,the optimum sulfation process parameters must be determined.In each roasting experiment,the TBBF slag sample was thoroughly mixed with AS at a specific mass ratio.The mixture,placed in a crucible,was then heated at a rate of 10 °C·min?1to the required temperature,ranging from 310 to 410°C,and annealed for 120 min(except for the experiment for the effect of reaction time)in a muffle furnace(KSL-1200X,Heifei Kejing Materials Technology Co.,Limited,Heifei,China).Then,the crucible was withdrawn and cooled naturally to room temperature.

The roasted slags were leached with a 2.5 wt%H2SO4at a liquid-tosolid mass ratio of 3:1 and 55°C for 1 h and filtered.A leaching residue rich in CaSO4and SiO2and a leaching solution rich in TiOSO4,Al2(SO4)3,MgSO4and AS was obtained.The concentrations of Ti,Aland Mg ions in the leaching solution were measured and the conversions calculated for evaluating the efficiency of the sulfation roasting.CaSO4was slightly soluble in an aqueous solution and distributed between the leaching solution and the residue.Clearly,high conversion of Ti,Al and Mg components must correspond to high conversion of Ca since nearly all the phase constituents in the slag contained calcium,as shown in Fig.2b.Besides,since an appreciable quantity of metal elements in the slag free of sulfation roasting with AS can be dissolved in a dilute acid solution,the conversion here actually reflected the contributions of both the roasting and acid leaching.

The sulfation ratios(η1)of magnesium,titanium,or aluminium ions in the TBBF slag were calculated using Formula(1).

where m(g)is the mass of the TBBF slag used in the roasting experiments,α1(wt%)is the content of titanium,aluminium,or magnesium in the TBBF slag,V(L)is the volume of leaching solution,and c1(g·L?1)is the concentration of titanium,aluminium,or magnesium ions in the leaching solution.

The sulfation ratio(η2)of calcium in the TBBF slag was calculated using Formula(2).

where c2(g·L?1)is the concentration of calcium ion in the leaching solution,m1(g)is the mass of leaching residue,α3(wt%)is the content of calcium existing in the form of CaSO4in the leaching residue,and α2(wt%)is the content of calcium in the TBBF slag.

Fig.4.The SEM patterns of the TBBF slag.

The leaching solution was first subjected to thermal hydrolysis for recovery of titanium.In each test,100 ml of the solution was placed in a 250-ml three-necked glass reactor fitted with magnetic stirring,a thermometer and a re flux condenser,and heated by using an oil bath with temperature fluctuation of±1 °C.The solution was heated at a rate of 5 °C·min?1to 102 °C under stirring and annealed until a precipitate occurred.This moment was considered the beginning of the hydrolysis.After a certain period of time,the reactor was taken out,and the slurry was cooled slowly and aged at 50 °C for 30–60 min.Then,the slurry was filtered.The concentration of titanium in the filtrate was measured and its hydrolysis ratio calculated.The hydrolysate was chemically analysed and characterized after calcining at 1000°C for 2 h.

The Ti-depleted leaching solution was further employed to recover aluminium.In each test,a certain amount of 13 wt%ammonia water was introduced to 50 ml of the leaching solution at room temperature,aged for 40 min and filtered.The pH and concentrations of Al and Mg in the filtrate were measured and the precipitation ratios calculated.The precipitate obtained at the optimal pH value was chemically analysed and characterized.

The leaching residue was reacted with saturated NH4HCO3solution at a molar ratio of NH4HCO3:CaSO4of 3:1 and 50°C for 120 min and filtered.The carbonation solid product was chemically analysed and characterized.

The CO2sequestration capacity M1(kg CO2·t?1)for the leaching residue per ton of TBBF slag was calculated using the following formula:

where m2(g)is the mass of carbonation product from the leaching residue of m1g,and α4is the content(wt%)of carbon in the carbonation product.

The Ti-and Al-depleted leaching solution was reacted with saturated(NH4)2CO3solution.Since a small amount of CaSO4dissolved in the leaching solution,this part of Ca would also participate in the carbonation reaction together with Mg.Thus,the reaction proceeded with a molar ratio of(NH4)2CO3to(Ca2++Mg2+)of 3:1 at a temperature of 40°C for 1 h.After filtration,a solid carbonate mixture and a filtrate rich in AS were obtained.The carbonation product was chemically analysed and characterized.

The CO2sequestration capacity M2(kg CO2·t?1)for the Ti-and Aldepleted leaching solution per ton of TBBF slag was calculated using the following formula:

where m3(g)is the mass of carbonation product from the Ti-and Aldepleted leaching solution,and α5(wt%)is the content of carbon in the carbonation product.

Therefore,the total CO2sequestration capacity E(kg·t?1)per ton of BF slag can be calculated as follows:

2.3.Analysis and characterization

The concentrations of titanium,aluminium,calcium and magnesium ions in the leachates were determined by inductive coupled plasma atomic emission spectroscopy(ICP-AES)(Spectro ARCOS ICP,Germany).For determination of the chemical compositions of the leaching residue and the Al-rich precipitate,the solid sample was melted with sodium dioxide(Na2O2)and sodium hydroxide(NaOH)at 750°C for 30 min and then leached with dilute hydrochloric acid.The concentrations of titanium,aluminium,calcium and magnesium ions in the solution thus obtained were analysed via ICP-AES.To determine the content of calcium sulphate in the leaching residue,1 g of the residue was dissolved in 500 ml deionized water,and then the concentration of Ca in the solution was measured by ICP-AES.

XRD analyses were performed using a DX-2007 X-ray diffraction spectrometer(Danton,China)operating with a Cu Kαradiation 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.

Allelements in the TBBF slag were analysed using X-ray fluorescence spectrometry(XRF-1800,Shimadzu,Japan)with a Rh Kαradiation source.

A thermogravimetric analyser(TGA/DSC,2/1600,Mettler Toledo,Switzerland)and a mass spectrometer(GSD 320 T3,Pfeiffer Vacuum,Germany)were combined to investigate the thermal decomposition behaviour of the carbonation product of the Ti-and Al-depleted leaching solution under a nitrogen atmosphere at a flow rate of 30 ml·min?1and a heating rate of 10 °C·min?1.The CO2amount fixed in the carbonation product in the form of CaCO3and MgCO3was analysed by a high frequency infrared-ray carbon and sulphur analyser(TL851–6,Lida,China).

The surface morphology of the solid samples was observed using a scanning electron microscope(SEM,JSM-7500F,JEOL,Japan)at an accelerating voltage of 5 kV.The relative elemental content of samples was analysed with a combined energy-dispersive X-ray spectrometer(EDS,IS250,Oxford,Japan).The content of elements C,N,H and S in the Al-rich precipitation product was measured by an element analyser(EA)(Euro EA 3000,LEEMAN LABS,INC.,USA)with their accuracy of C≤±0.1%,H≤±0.1%,N≤±0.05%and S≤±0.1%,respectively.

The particle sizes of the TBBF slag were examined using an Omec LS-POP(9)Laser Particle Size Analyser(Zhuhai,China).

3.Results and Discussion

3.1.Sulfation roasting of TBBF slag

3.1.1.Effect of roasting temperature

Fig.5 shows the effect of roasting temperature on the sulfation of titanium,aluminium and magnesium elements in the TBBF slag.

Fig.5.Effect of the roasting temperature on the sulfation of titanium,aluminium and magnesium components(roasting:AS-to-slag mass ratio 2:1,reaction time 120 min;leaching:2.5 wt%dilute sulfuric acid,liquid-to-solid mass ratio 3:1,55°C,1 h).

The sulfation of the titanium component increased with the rising temperature.The sulfation was only 76%at 310°C and almost levelled off with its equilibrium value of approximately 87%beyond 350°C,while the sulfation of aluminium and magnesium components remained almost unchanged over the whole temperature range with their conversions of 85%and 92%,respectively.From the literature[35],the reaction of AS with silicate minerals can be divided into two stages,i.e.,decomposition of AS into NH3and NH4HSO4and then the digestion of silicate minerals by NH4HSO4.Therefore,the decomposition of TBBF slag by AS is essentially an acidolysis reaction.As a comparison,a blank experiment was conducted.The TBBFslag with out roasting with AS was leached directly with 2.5 wt%dilute sulfuric acid at 55°C for 1 h and the leaching of titanium,aluminium and magnesium was measured to be 10%,29%and 54%,respectively.Clearly,the leaching of the three components resulted primarily from the diopside and titaniumbearing diopside since most of the magnesium and aluminium components and small amounts of titanium(20%–30%)in the TBBF slag were distributed in the diopside minerals,and CaTiO3was much more acid resistant than the diopside minerals[24].The results indicated that even at low temperatures,there were significant amounts of diopside minerals decomposed by dilute sulfuric acid,which could explain why the conversion of aluminium and magnesium became insensitive to the temperature employed during the roasting,which was much higher than the direct acid leaching temperature.Therefore,an increased roasting temperature mainly enhanced the conversion of CaTiO3in the present study.

Fig.6 shows the XRD patterns of roasted slag at different temperatures.The major crystal products were CaSO4and the compounded sulphates of aluminium combined with AS.The diffraction peaks of CaSO4increased with the rise in temperature,probably due to the elevating CaTiO3conversion,and beyond 350°C,remained almost unchanged while the sulphate of aluminium at low temperatures was(NH4)3Al(SO4)3and converted to be NH4Al(SO4)2athigh temperatures,indicating that the conversion of aluminium seems to change little.The results were in good agreement with the values implied by Fig.5.The sulphates containing titanium and magnesium species were not observed,probably due to low crystallinity.

Fig.6.XRD patterns of the roasted slag at different temperatures.

3.1.2.Effect of the mass ratio of AS to TBBF slag

Assumption that the reactions of the metal oxides(abbreviated as MOor M2O3)in the TBBF slag with AS can be written as Eqs.(6)and(7).

The theoretical mass ratio of AS to TBBF slag was calculated to be～1.9 based on the chemical compositions listed in Table 1.

Fig.7.Effect of the AS to TBBF slag mass ratio on the sulfation of titanium,aluminium and magnesium components(roasting:temperature 350°C,reaction time 120 min;leaching:2.5 wt%dilute sulfuric acid,liquid-to-solid mass ratio 3:1,55°C,1 h).

Fig.7 shows the effect of the mass ratio of AS to TBBF slag on the sulfation of titanium,aluminium and magnesium elements.The sulfation ratios of titanium,aluminium and magnesium increased with the increasing mass ratio,the significant increase occurred at the mass ratio less than 2:1,which is close to the theoretical value.

Further increasing the mass ratio,the sulfation of both magnesium and aluminium almost approached equilibrium while the sulfation of titanium increased slowly due to the more rapid reaction of diopside minerals with AS than the reaction of CaTiO3.Taking into consideration the energy consumption,mainly in the regeneration and recycle of AS,and economy of the whole process,the optimal mass ratio was chosen to be 2:1,where the titanium extraction was up to 87%,although a higher mass ratio would result in higher titanium recovery.

Further,a comparison experiment for leaching of the slag roasted at the mass ratio of 2:1 with deionized water instead of 2.5 wt%H2SO4demonstrated that the extractions of Ti,Mg and Al were 4%,92%and 84%,respectively,and the slurry thus obtained was difficult to filter,indicating the hydrolysis of the titanium sulphate in the roasted slag might occur.To restrain the titanium hydrolysis during the water leaching,a minimummass ratio of6:1 was required due to the presence of some amounts of NH4HSO4and(NH4)3H(SO4)2in the roasted slag at high mass ratios(Fig.8).Clearly,the leaching with a dilute acid solution can avoid the use ofa high AS to TBBF slag mass ratio employed by other studies[30,32].

Fig.8.XRD patterns of the roasted slags at different AS to TBBF slag mass ratios.

Fig.8 shows the XRD patterns of the roasted slags at different mass ratios.The phase constituents of roasted products became more complicated with increasing mass ratio.At a mass ratio larger than 4:1,NH4HSO4and(NH4)3H(SO4)2appeared in the products,and the double salts formed by the metal sulphates and AS changed towards higher molar ratio of NH4+-to-Mz+(Mz+=Ca2+,Al3+,Fe3+).Obviously,the dosage of ammonium sulphate was then excessive.

3.1.3.Effect of the roasting time

Fig.9 shows the effect of roasting time on the sulfation of titanium,aluminium and magnesium components in the TBBF slag.The sulfation of aluminium and magnesium reached a plateau at times beyond 60 min while the sulfation of titanium increased monotonically,although its incremental range gradually decreased.A roasting time of 120 min was chosen to be the optimal parameter.

Fig.9.The effect of the roasting time on the sulfation ratio of titanium,aluminium and magnesium(roasting:temperature 350°C,AS-to-ore mass ratio 2:1;leaching:2.5 wt%dilute sulfuric acid,liquid-to-solid mass ratio 3:1,55°C,1 h).

Fig.10.XRD patterns of the roasted slag at different times.

Fig.10 shows the XRD patterns of the roasted slag at different times.The sulphate of magnesium,MgSO4·7H2O,was observed in the initial stage.The occurrence of the crystal water was probably attributed to strong hygroscopicity of MgSO4,which would absorb ambientmoisture during sample storage,preparation and testing.With the increasing reaction time it became amorphous,maybe forming a compound sulphate of magnesium and ammonium ions with low crystallinity.The diffraction peaks of CaSO4increased with the rise of reaction time,due to the elevating CaTiO3conversion,and beyond 120 min remained almost unchanged while the compound sulphate of aluminium and ammonium ions was(NH4)3Al(SO4)3in the initialstage and itgradually changed to NH4Al(SO4)2with the extension of reaction time.The conversion reaction can be expressed using Eq.(8):

The NH4HSO4produced would further digest the TBBF slag.

Based on the study above,the optimal roasting parameters were determined as follows:roasting temperature of 350°C,AS to TBBF slag mass ratio of 2:1,and reaction time of 120 min.The roasted slag under the optimal condition was leached with a 2.5 wt%dilute sulfuric acid and the sulfation of titanium,aluminium,magnesium and calcium were measured as 87%,84.4%,92.6%and 85%,respectively.Fig.11 shows the XRD pattern of the leaching residue.The primary crystal phases were calcium sulphate and unreacted perovskite.The existence of perovskite was responsible for the not too high titanium extraction.The content of calcium sulphate was measured to be 69.4 wt%.The chemical composition of the residue is presented in Table 2.A small amount of Al2O3was detected,which was also responsible for the not too high aluminium extraction.We guess the Al2O3present in the residue might be mainly in the form of Ca3Al2O6,which was even more resistant to acid digestion than perovskite in this study.

Fig.11.XRD pattern of the leaching residue.

Table 2 Chemical composition of the leaching residue(wt%)

3.2.Recovery of titanium and aluminium components from the leaching solution

The leaching solution obtained under the optimal roasting conditions consisted of 15.1 g·L?1Ti4+,8.4 g·L?1Al3+,5.6 g·L?1Mg2+and 0.8 g·L?1Ca2+.The solution was subjected to thermal hydrolysis as described in the 2.2 section,Experimental methods,and the hydrolysis ratios of titanium at different times are shown in Fig.12.The hydrolysis at 4 h reached 95.7%,close to the value in the commercial sulphate process for TiO2production[36].

Fig.12.Hydrolysis ratios of titanium at different times.

Fig.13.SEM photo of the 4 h hydrolysate.

Fig.14.XRD patterns of the hydrolysates:(a)dried at 45 °C;(b)calcined at 1000 °C.

Fig.13 shows the SEM photo of the 4 h hydrolysate.The product particles were uniform and spherical,with a size distribution of 300–500 nm.Fig.14 shows the XRD patterns of the hydrolysates before and after calcining at 1000°C.The primary crystalline phase in the hydrolysate was anatase,and was partially transformed into rutile after calcination.Chemical analysis indicated that the calcined product consisted of～98 wt%TiO2and ～2 wt%SiO2,demonstrating that a high-purity titanium dioxide was obtained.

The mother liquor after titanium hydrolysis(the Ti-depleted leaching solution)was further subjected to ammonia neutralization for selective precipitation of Al3+.The precipitation ratios of Al3+and Mg2+at different pH values are shown in Table 3.99.7%of the Al3+was precipitated while co-precipitated Mg2+was only 0.89%at pH=5,considered the optimal precipitation conditions.The XRD analysis showed thatthe Al-rich precipitate was amorphous.The chemical composition of the precipitate is listed in Table 4.The precipitate contained large quantities of Al2O3(ca.49.9 wt%)and a small amount of impurities such as titanium,iron and silicon.The LOI reached 40.25%,while the EA analysis didn't detect N element,indicating that the Al-rich precipitate contained a large amount of OH?and free water.Therefore,we inferred that the major Al-containing phase was basic aluminium sulphate.The Al-rich precipitate was calcined at 850°C,and its XRD pattern is shown in Fig.15.The main crystal phase was Al2O3.The purity of Al2O3and the mass ratio of Al2O3to SiO2were measured to be 70.8 wt%and approximately 49,respectively,so this material could be used as a raw material for electrolytic aluminium production.

Table 3 Variation of precipitation of Al3+and Mg2+with pH value

Table 4 Chemical composition of the Al-rich precipitate(wt%)

Fig.15.XRD pattern of the precipitate after calcining at 850°C.

3.3.Mineral carbonation of TBBF slag

3.3.1.Carbonation of the leaching residue

Recently,carbonation of various CaSO4-containing industrial solid wastes using CO2under NH3media or NH4HCO3obtained by NH3capture of CO2in flue gases was reported[37–39].In this study one of the main phase constitutions of the leaching residue was also CaSO4.Carbonation of the leaching residue was conducted with saturated NH4HCO3solution at 50°C with the molar ratio of NH4HCO3to CaSO4of 3:1.The conversion of calcium sulphate in 120 min was measured as 99.7%.Fig.16 shows the XRD pattern of the carbonation product,and CaCO3was the primary crystalline phase.

Fig.16.XRD pattern of the carbonation product of the leaching residue.

Fig.17 shows the SEM photos of the carbonation product.Combined with the EDS analysis,the irregular smooth lump particles were amorphous SiO2,while the regular minute particles with particle size of 2–4 μm were crystalline CaCO3.

The content of carbon in the carbonation product was determined by a high frequency infrared-ray carbon and sulphur analyser and the CO2sequestration capacity M1of the leaching residue per ton of TBBF slag was calculated to be 170 kg·t?1TBBF slag from Formula(3).

Chemical analysis showed that the carbonation product consisted of 61.7 wt%of CaCO3,31.3 wt%of SiO2and 2.8 wt%of CaTiO3,and can be utilized as the raw material for cement production for the replacement of natural limestone and silica.

3.3.2.Carbonation of the Mg-rich leaching solution

Carbonation of the Ti-and Al-depleted leaching solution with(NH4)2CO3solution was carried out at 40°C and the molar ratio of(NH4)2CO3to Ca2++Mg2+of 3:1 for 60 min.The conversion ratios of Mg2+and Ca2+reached 91%and～100%,respectively.XRD analysis indicated that the main crystal product was(NH4)2Mg(CO3)2·4H2O as shown in Fig.18.The SEM photos of the product are shown in Fig.19.Combined with the EDS analysis,the majority of precipitates were compact,column-like(NH4)2Mg(CO3)2·4H2O crystals of dozens of microns while a small amount of minute particles of several microns were calcium carbonate.Based on the results,the carbonation reactions of the leaching solution can be expressed as follows:

Fig.18.XRD pattern of the carbonation product of the magnesium-rich leaching solution.

TG-Mass was employed to analyse the thermal stability of the carbonation product,and the result is shown in Fig.20.There are three obvious temperature ranges of mass loss:30–200 °C,200–500 °C and 500–750 °C.In the first temperature range,there was a large amount of NH3,CO2and H2O.Obviously,the presence of this material is related to the decomposition of(NH4)2CO3and removal of the adsorbed and/or crystal water.Therefore,low-temperature decomposition can be adopted to recover the ammonia for reuse from the carbonation product.The appearance of CO2in the second temperature range was attributed to the decomposition of MgCO3.Both the results in the first two temperature ranges were in good agreement with the literature[40].In the last temperature range,a distinct CO2peak,which was attributed to the decomposition of calcium carbonate[41],was observed.

Fig.17.SEM photos of the carbonation product of the leaching residue.

Fig.19.SEM photos of the carbonation product of the magnesium-rich leaching solution.

Fig.20.TG-Mass curves of the carbonation product of the magnesium-rich leaching solution.

Based on the analysis of TG-Mass,the amount of CO2fixed in the carbonation product of the Mg-rich leaching solution in the form of CaCO3and MgCO3was measured using a carbon and sulphur analyser following decomposition ofthe(NH4)2CO3at200°C.The CO2sequestration capacity M2(kg·t?1)of the leaching solution per ton of the BF slag was calculated to be 69.7 kg CO2·(t slag)?1.

The contents of MgCO3and CaCO3in the calcined product were measured to be approximately 95.6 wt%and 4.4 wt%,respectively.The product can be used as raw material in light magnesium carbonate production for the replacement of natural dolomite.

Accordingly,approximately 82.1%of calcium and 84.2%of magnesium in the TBBF slag can be utilized to chemically sequester CO2in this process,and their sequestration capacity reached 239.7 kg CO2per ton of TBBF slag.

4.Conclusions

In this paper,a novel,facile method for mineralization of CO2coupled with recovery of TiO2and Al2O3was proposed,in which the TBBF slag was roasted with recyclable AS at a mass ratio of AS to TBBF slag of 2:1 and 350°C for 2 h followed by leaching in a 2.5 wt%H2SO4.The sulfation of calcium,magnesium,titanium and aluminium components reached 83%,92.6%,87%and 84.4%,respectively.The leaching solution was subjected to hydrolysis at 102°C for 4 h with a titanium hydrolysis ratio of 95.7%and the purity of titanium dioxide in the calcined hydrolysate reached up to 98 wt%.The Ti-depleted leaching solution was neutralized to pH=5 with ammonia and approximately 99.7%of the aluminium was selectively precipitated with the coprecipitated magnesium less than 1%.The NH3produced during the roasting is used to capture CO2from the flue gases.The NH4HCO3and(NH4)2CO3thus obtained are used to carbonate the CaSO4-containing leaching residue and the MgSO4-rich leaching solution,respectively.In this process,approximately 82.1%of Ca and 84.2%of Mg in the TBBF slag were transformed into its carbonates with a totalCO2sequestration capacity of 239.7 kg per ton of TBBF slag.

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