Qing Lin,Guoquan Zhang,Kun Wang,Dongmei Luo,Siyang Tang,Hairong Yue

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

Keywords:Vanadium Titanium Roasting Leaching

ABSTRACT Compared with traditional sodium or calcification roasting process for vanadium extraction from raw vanadium slag(V-slag),ammonium sulfate(AS)roasting could reduce about 470°C roasting temperature and avoid Cl2,HCl,sodium-containing waste-water and waste gypsum discharging.To reduce the amount of AS added in vanadium extraction process,an efficient AS two-stage cyclic roasting and acid leaching process was proposed.The result of TG analysis indicates V-slag could be decomposed in 275-380 °C using AS roasting process.Using 2.03:1 total mass ratio of AS to V-slag,90.86% V and 80.54% Ti could be extracted after 380 °C roasting for 30 min and 8% initial concentration of H2SO4 leaching at 70 °C for 100 min.XRD analysis indicates V-containing spinel phase in the 1st stage leaching residue would be efficiently decomposed by the cyclic two-stage roasting and leaching process.Furthermore,the valence of V(III) in raw V-slag was not changed after the 1st AS roasting stage,but a part of V(III) in the 1st leaching residue was oxidized to V(V) after 2nd roasting process.

1.Introduction

The market supply structure of vanadium (V) has been unbalanced for a long time as a result of extensive demand and shortage of V production [1,2].V is widely used in national defense,aerospace,battery manufacturing and industrial catalysis[3],however,by weight,V occupies only 0.02%[4]of the earth’s crust.V usually coexists with titanium (Ti),chromium,tungsten,molybdenum,lead,copper,carbon and phosphorus in vanadium-titanium magnetite,sphalerite or black shale [5-7].Because the large reserves of vanadium-titanium magnetite resources and relatively concentrated distribution [8-11],it is reported that over 60% of V in the market was produced from V-slag,a by-product of vanadiumtitanium magnetite during the process of converter steelmaking,which is a mixture of V,Ti,iron (Fe) and other silicates [12-14].Therefore,the efficient extraction of V,Ti,Fe from V-slag has been studied over the past years.

As V-containing spinel phase in V-slag is packaged by fayalite and difficult to be destroyed by conventional methods [15,16],the roasting pre-treatment of V-slag plays a key role in extracting V from V-slag [17,18].The traditional method to extract V from V-slag is sodium chloride roasting and water leaching.This process roasts the slag at 850 °C or above temperature and makes the spinel phase change to soluble sodium salt [19-26].During this process,large amount of sodium chloride additives was consumed[27].Meanwhile,some toxic gases like Cl2and HCl was also produced [28].Large amount of leaching residue and leachate were formed after sodium roasting-leaching,resulting intractable solid and liquid wastes [29-31].Taking into account the shortcomings of the sodium roasting process,the industry is currently using calcified roasting-acid leaching to extract V from V-slag [32,33].This process was roasted with calcium oxide or carbonate at about 950 °C to generate V containing calcium salt [34].Then,this roasted slag was leached by H2SO4[34].Calcification roasting process cannot achieve the extraction efficiency,and discharge of waste gypsum.

AS roasting is a green method of extracting valuable mineral components,which has been used for the extraction of elements such as titanium,aluminum,nickel and cobalt.For example,Wanget al.[35] obtained the extraction and separation of 87% titanium by roasting titanium-bearing blast furnace slag with AS at 350 °C.Liuet al.[36] treated the blast furnace slag with AS and recovered 76% aluminum in the form of NH4Al(SO4)2·12H2O with a purity of 99.6%.Tanget al.[37] roasted AS mixed with waste nickel-cobalt oxide ore at 350 °C,and then water leached the roasted slag to achieve 98.5% nickel-cobalt.

In our previous research,AS was suggested as a new leaching agent to extract V and Ti from V-slag.In this route,water quenched V-slag was roasted with AS at 370°C and followed by leaching with diluted sulfuric acid [38].During roasting process,the additive AS decompose NH3which was used as a base to precipitate V.Under the optimized roasting conditions,the extraction ratios of V and Ti reached 91% and 77%,respectively [38].This new process could recover V and Ti without changing the rotary kiln that roasts V-slag in industry,however,this route was hindered by two factors to realize industrialization in an energy efficient way.The first difficulty is that an energy-consuming water quenching process,operated at 1500°C for 180 min,was used to increase the reactivity.Furthermore,a high dosage of AS(8:1 of mass ratio of AS to V-slag)was need in roasting process to realize considerable extraction efficiency [38].The excess AS would increase the energy input in the subsequent operation to recycle AS.And the roasted product was difficult to separate from our laboratory-scale muffle furnace on account of the high viscosity of AS at roasting temperature.

To extract V and Ti from V-slag more efficiently,a cyclic twostage AS roasting-leaching process was proposed.Using lower AS to V-slag mass ratio in roasting process,the first-stage roasting slag was obtained and leached by sulfuric acid.After collecting the firststage leaching residue and leachate,the first-stage leaching residue and new AS additive were taken as the second-stage roasting material,and all of the first-stage leachate was used as leaching agent of second-stage leaching process.The specific experimental flow chart was shown in Fig.1.Compared with previous reports,this process could reduce the addition of AS in roasting process.Using low-proportion AS as roasting additive could not only avoid the sintering formation of the reactants in the muffle furnace,but also increased the relative proportion of V-slag in a single feed to expand the yield.As more V and Ti was enriched in the final leachate,the subsequent separation process would also consume less energy consumption than previous laboratory research.This paper investigated the optimal process conditions and mechanism involved for extracting V and Ti from V-slag using cyclic twostage AS roasting-leaching process.

2.Materials and Methods

2.1.Materials

The particle size less than 74 μm V-slag was obtained from a plant in Sichuan,China.All sample were dried at 110 °C for 720 min before experiment.Table 1 shows the chemical composition of the V-slag determined by XRF (X-ray fluorescence spectrometry).As shown in Fig.2,XRD (X-ray diffraction) patterns indicates that main phases of the V-slag were (Mn,Fe)(V,Cr)2O4,Fe2TiO4,Fe2SiO4and Ca(Fe,Mg)Si2O6.Chemical reagents such as AS,sulfuric acid,and nitric acid are of analytical grade.Ultrapure water with a resistivity greater than 18.25 MΩ·cm-1was laboratory-made.

Table 1 Chemical compositions of the V-slag.

Fig.3 shows the SEM (scanning election microscopy) of the Vslag,the particles were not uniform.The surface of large particles were smooth,dense and amorphous,and small particles were clumped onto the surface of large particles with outside shape.The main components on the surface of large particles could be seen through EDS (energy-dispersive X-ray spectrometer) analysis as Fe2SiO4.The small particles are mainly spinel phases like FeV2O4and Fe2TiO4,which was consistent with the XRD analysis results in Fig.2.

2.2.Methods

In first-stage roasting process,a certain mass ratio of V-slag and AS was mixed and settled in a 100 ml crucible placed in muffle furnace (KSL-1200X,HKMT Co.,Limited,China).The heating rate of muffle furnace was stetted as 10 °C·min-1.After required time,all samples were taken out and cooled to the room temperature in a desiccator.In first-stage leaching process,the roasted slag were leached by specific concentration sulfuric acid in a threeneck flask.A water bath was provided to keep the leaching temperature at ±0.5 °C accuracy.After first-stage roasting and leaching operation,the first-stage residue and leachate were obtained by filtering.In second-stage roasting,the first-stage leaching residue was mixed with new AS as raw material using the same roasting condition of first-stage process.Then,using all of the first-stage leachate as second-stage leaching agent,the second-stage obtained roasted samples were leached.The contents of elements in the leaching solution were measured by the (1) formulation:

Extraction ratio of V,Ti,or Fe:

wherem1represents the mass of the V-slag,g;w1is the mass fraction of V,Ti or Fe in the V-slag,% (mass);V1is the volume of the first-stage or second-stage leachate,ml;c1is the concentration of V,Ti or Fe in the test leachate,g·ml-1.

Fig.1.Specific experimental flow chart of the two-stage process for treating the V-slag.

Fig.2.XRD analysis of the V-slag.

2.3.Characterization and analysis

The extraction efficiency of V,Ti and Fe in the leaching solution was measured and confirmed by chemical reagent titration,plasma atomic emission spectroscopy(ICP-OES,ICAP7400,Thermo Fisher Scientific,USA) and energy disperse X-ray spectrometry(EDS,Aztec X-Max80,Thermo Fisher Scientific,USA).Phase compositions of solid samples were identified by X-ray diffraction analysis(XRD,X’Pert Pro MPD DY129,Shimadzu,Japan)using Cu Kα(λ=0.154056 nm)radiation.The XRD data was analysised 5°-80°,0.02°step by Jade software.Microscopic observation and elemental analysis were illustrated by scanning electron microscopy (SEM,Aztec X-Max 80,Thermo Fisher Scientific,USA) equipped with energy disperse X-ray spectrometry.The physical and chemical changes during the roasting process were characterized by the synchronous thermal analysis system (TGA/DSC2/1600,METTLER TOLEDO).The valence state change of V in two-stage roasting and leaching process was confirmed by X-ray photoelectron spectroscopy (XPS,AXI Sultra DLD,UK).

3.Results and Discussion

3.1.TG analysis of V-slag decomposition by AS

Fig.4 shows TG analysis of the V-slag,AS and mixture of 1.5:1 mass ratio of AS and V-slag.The V-slag gained a certain degree of mass because of oxidation when heating from 30 °C to 800 °C.And it indicates the decomposition of AS was mainly divided into two stages.In the first stage,AS was decomposed into NH3and NH4HSO4(ABS),whose reaction speed was relatively slow.In the second stage,ABS was decomposed into (NH4)2S2O7(s) [35] and water vapor at a significantly fast reaction rate which much faster than oxidize this V-slag.

Fig.3.SEM images and EDS analysis of the V-slag.

Fig.4.TG curves of the AS,V-slag and AS to V-slag mass ratio of 1.5:1.

The weightlessness curve of the mixture of AS and the slag was different from that of AS,which started with less weightlessness obviously from about 275 °C.It indicates that a part of ABS and the V-slag began to react,leaving this ABS particle fixed in the solid phase,thereby slowing down the weightlessness efficiency.When the temperature was about 340°C,the weightlessness efficiency of the mixture is 10.32%compared to 275°C.The weightlessness difference at this point between the two samples was the largest,indicating that the relative proportion of ABS involved in the oxidation reaction reached the maximum at this time,which may be the fastest reaction temperature for roasting process.When it comes to 380-400 °C,the weightlessness efficiency is as high as 14.60% in the heating range of 20 °C.The weightlessness efficiency of the mixture tends to consistent with that of AS.This result indicates the decomposition of ABS was absolutely dominant and no ABS participated in the destruction of the V-slag structure in 380-400 °C.In this temperature range,the oxidation roasting reaction of V-slag reached equilibrium.AS would start be wasted and valuable metals in the slag would not be extracted efficiently in this state.And the presence of reaction equilibrium also proved that the proportion of roasting additives was sufficient.Thus,a suitable temperature range for decomposing V-slag by AS roasting was 275-380 °C.

3.2.Determination of the first-stage roasting-leaching optimal conditions

3.2.1.Effect of roasting temperature on V,Ti and Fe extraction

The non-variable factors in all factors experiment were set as roasting temperature 380 °C,roasting mass ratio 1.5:1,roasting time 30 min,leaching temperature 70 °C,leaching time 100 min,liquid-solid ratio L/S (ml·g-1)=5:1,sulfuric acid mass concentration 8%.Fig.5 shows the influence of first-stage roasting temperature on the extraction of V,Ti,and Fe from V-slag.With the increasing of roasting temperature,Fig.5(a) shows the extraction efficiency of V,Ti,and Fe increased obviously in 260-340°C.When it came to 380°C,81.87%of V,70.89%of Ti,and 76.21%of Fe were leached.However,the extraction efficiency increased slowly from 380 to 420 °C.When it was 460 °C,an obvious decrease of V,Ti and Fe extraction efficiency was attributed to the excessive consumption of the decomposition ratio of ABS at these high temperatures,which could not match the reaction rate of the V-slag.This high temperature led to the waste of additive AS and was not conducive to the recovery of valuable metals like vanadium.Thus,380 °C was selected as the optimal temperature to decompose Vslag using AS roasting,which was also consistent with the TG analysis results in Fig.4.

Fig.5(b)shows the XRD patterns of the first-stage roasting slag at 380°C.It indicates that the slag phases changed after AS roasting and oxidation reaction.A part of diffraction peak of spinel phase was destroyed and its of AS disappear completely.A part of V existed in the acidic soluble forms of NH4V(SO4)2,and all Ti were in forms of TiOSO4.Some V were still in forms of (Mn,Fe)(V,Cr)2O4in the roasting slag.However,it was proved the feasibility of the AS roasting extracting V,Ti.Fig.6(a) shows that the extraction efficiency did not significantly increase by increasing the mass ratio of AS to 5:1.The conclusion could be drawn that even if the roasting reaction reached equilibrium at first-stage,oxidation was not completed.

3.2.2.Effect of AS mass ratio and roasting time on V,Ti and Fe extraction

Fig.6(a)shows the extraction efficiency of V,Ti and Fe gradually increased and reached the plateau at 1.5 mass ratio.With the mass ratio increased from 1.5 to 5,the extraction efficiency of V,Ti and Fe only increased by 3.86%,6.51% and 5.39% respectively.Compared with using extra 3.5 mass ratio of AS,the extraction efficiency was increased little and it also extruded the equipment space.The excessive AS would cause serious sintering in crucible,which was very unfavorable for industrialization.Thus,1.5:1 mass ratio of AS to V-slag was selected to realize the full utilization of AS under the ideal extraction efficiency.

As shown in Fig.6(b),the extraction efficiency of V increased with the roasting time increasing,but none obvious changed after 30 min.There was a slight decrease in Ti extraction,which may be attribute to the decomposition of part of the TiOSO4as the roasting time increases.However,the extraction efficiency of Fe increased little in the interval of 10-120 min.Therefore,30 min was chosen as the best roasting time in this research.

3.2.3.Effect of leaching conditions on V,Ti and Fe extraction

Fig.7 shows the effect of leaching conditions on V,Ti and Fe extraction.It indicates that with the increasing of leaching temperature,leaching time and acid concentration,the extraction rate of V,Fe,and Ti increased.The liquid-to-solid ratio will promote the leaching of V and Ti,but has no effect on the extraction rate of Fe.When it was 80 °C,the extraction rate of Ti distinctly reduced due to its hydrolysis and precipitation.When the leaching time was 100 min,the extraction rate of the three valuable metals start to increase slightly.When the liquid-to-solid ratio reached 5:1,the extraction rate of V and Fe almost no longer increases with L/S increasing,and the extraction rate of Ti increases very slowly.Because the hydrogen ions in the acid solution first reacted with the iron oxide on the surface of the roasting slag,a low extraction rate of V and Ti would be obtained at a 2%-6%concentration of sulfuric acid.Furthermore,the hydrolysis of Ti would occur at a relative low concentration of sulfuric acid and its hydrolysate may be re-encapsulated in the solid phase.Thus,a leaching condition of 70°C leaching temperature,100 min leaching time,5:1 L/S,8%concentration of sulfuric acid was the best leaching experiment conditions.

Fig.5.(a) Effects of first-stage roasting temperature on extraction efficiency of V,Ti and Fe,(b) XRD patterns of roasting slag at 380 °C.

Fig.6.(a) The effect of roasting mass ratio on the extraction efficiency of V,Ti,Fe,(b) The effect of roasting time on the extraction efficiency of V,Ti,Fe.

3.3.Result of the second-stage roasting-leaching process

In this condition,the first-stage leaching residue was used as the return material for second-stage roasting and leaching,and the final extraction efficiency of V,Ti,and Fe are 90.86%,80.54%,and 80.35%,respectively.The first-stage leaching residue,which was the return material,accounts for 14.10% of the total feed.Counting the amount of AS in the whole roasting process,the mass ratio of the actual roasting additive to the V-slag was 2.03:1.The mass of the second-stage leaching residue is only 7.37%of the total feed,and the concentration of V,Ti and Fe in the second-leachate were 7.70,4.96 and 19.27 g·L-1,respectively.Table 2 shows that two-stage roasting and leaching process not only reduces the amount of AS,but also increases the single feeding amount of Vslag in the production process by 133.1%,which greatly improves the efficiency of V-slag recovery.

Table 2 Comparison of water quenching process that made in our previous research.

4.Mechanism of the V-slag Roasted by Ammonium Sulfate

Fig.8(a) shows that the XRD patterns of second-stage roasting slag was different from Fig.5(b).There was no spinel phase in the second-stage roasting slag,which indicates the extraction efficiency of V and Ti could be increased by two-stage roasting.Furthermore,NH4V(SO4)2,(NH4)2V6O16·1.5H2O and NH4VO(SO4)2appeared in the XRD patterns,indicating some V(III) was oxidized V(V).Fig.8(b) shows (Mn,Fe)(V,Cr)2O4,Fe2TiO4and Fe2SiO4were not destroyed completely in the first-stage roasting-leaching.The XRD patterns of the second-stage leaching residue indicates the diffraction peaks of spinel and fayalite phases at 61.72° disappeared entirely indicating that the phase of the V-slag was destroyed.The main components of residue were SiO2,Fe2SiO4and Ca(Fe,Mg)Si2O6after two-stage process.

Fig.7.The influence of leaching factors on the extraction efficiency of V,Ti and Fe;(a)-leaching temperature;(b)-leaching time;(c)-L/S ratio;(d)-acid concentration.

Fig.8.XRD patterns of (a) the second-stage roasting slag,(b) the first-stage leaching residue and the second-stage leaching residue.

Fig.9.SEM images and EDS analysis of the first-stage leaching residue and the second-stage leaching residue.

Fig.9 was the SEM image and EDS shown the first-stage leaching residue and the second-stage leaching residue.The particles of first-stage leaching residue present either dense or amorphous.According to EDS analysis,the particles are mainly V-containing spinel phase(9-1)or Ti-containing spinel phase(9-2).The proportion of spinel phase is different for particles of different sizes,while the amorphous substance were mainly silica and fayalite (9-3).The average particle size of the second-stage leaching residue was smaller.It was mainly Si with little V and Fe in the secondstage leaching residue.According to EDS results of the secondstage leaching residue,the final extraction efficiency of V,Ti and Fe in this two-stage process were 91.90%,82.76% and 82.60%,respectively.Although EDS analysis show 3.19 %V was in selected microscopic area,XRD analysis cannot detected the V-containing phase.

Compared with V-slag,the apparent morphology and content of first-stage and second-stage leaching residue were all changed.The particle size is getting smaller in the two-stage reaction.V-slag was destroyed layer by layer,leaving the spinel phase exposed and oxidized gradually.

Fig.10 was the XPS spectra of solid samples of V-slag,first(second) stage roasting and leaching sample.The main optoelectronic detected line corresponding with the C1s (its biggest binding energy of 284.8 eV).The spectra of V were different in those sample and its analysis was shown in Table 3.It shows it was V(III) in the V-slag,and the peak values generated are 515.8 eV and 517.2 eV.The XPS fitting result of shows it was also V(III) in the first-stage roasting slag and leaching residue,and their peak position was 515.8,516.8 or 517.2 eV.This indicates that the some soluble V(III) generated in the firststage roasting process entered into the solution after the first-stage leaching.

Table 3 Results of analyses for V 2p3/2 also shown in Fig.10.

The XPS results indicate the valence of V changed after the second-stage roasting.It was V(III) in 516.8 eV and V(V) in 517 or 517.7 eV in the second-stage roasting slag.However,only V(III) existed after second-stage leaching.This indicates that V(III)could be oxidized to V(V) in the second-stage roasting process,but it was not completely oxidized.The V(V) would be leached in the second-stage leaching and leaving the V(III) in the secondleaching residue.

Fig.10.XPS spectra of(a)all samples;(b)V-slag;(c)first-stage roasting slag;(d)first-stage leaching residue;(e)second-stage roasting slag;(f)second-stage leaching residue.

5.Conclusions

Two-stage low-proportion AS roasting technology was proved to be a feasible and efficient way to extract V and Ti from V-slag.It proposes to use the 1st roasting-leaching slag as the raw material of 2nd roasting,and use the 1st roasting-leaching liquor as 2nd leaching agent.After AS and V-slag were roasted at 380 °C for 30 min at a mass ratio of 1.5:1,the 1st roasted sample was leached for 100 min in an 8%sulfuric acid solution with a liquid-solid ratio(ml·g-1)of 5:1 at 70°C.For the 2nd roasting process,the 1st leaching residue was mixed with 1.5:1 mass ratio of AS and roasted at the same condition of 1st roasting.Then,the obtained 2nd roasting sample was leached by 1st leachate.The total extraction efficiency of V,Ti and Fe reached 90.86%,80.54% and 80.35%,respectively.The valence of V(III) in raw V-slag was not changed after the 1st AS roasting,but a part of it was oxidzed to V(V) after the 2nd AS roasting.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was funded by National Natural Science Foundation of China(22008161).Thanks to the Engineering Experimental Teaching Center,School of Chemical Engineering,Sichuan University for the ICP-OES technical assistance.