Yi Luo ,Xiaokui Che, *,Xinglan Cui, *,Qi Zheng ,Lei Wang

a National Engineering Laboratory of Biohydrometallurgy,GRINM Group Corp.,Ltd.,Beijing 101407,China

b GRINM Resources and Environment Tech.Co.,Ltd.,Beijing 101407,China

c General Research Institute for Nonferrous Metals,Beijing 100088,China

Keywords:Selective leaching Vanadium V-Ti magnetite concentrate Pellets Sulfuric acid

ABSTRACT A novel method of pellet calcification roasting-H2SO4 leaching was proposed to efficiently separate and extract vanadium (V) from vanadium-titanium (V-Ti) magnetite concentrates.The leaching rate of V is as high as 88.98%,while the leaching rate of impurity iron is only 1.79%.Moreover,the leached pellets can be used as raw materials for blast furnace ironmaking after secondary roasting.X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy dispersive X-ray spectrometry (SEMEDS)analyses showed that V3+was oxidized to V5+after roasting at 1200°C,and V5+was then leached by H2SO4.X-ray diffraction (XRD)analyses and single factor experiment revealed a minimal amount of dissolved Fe2O3 during H2SO4 leaching.Therefore,a high separation degree of V and iron(Fe)from V-Ti magnetite concentrate was achieved through H2SO4 leaching.Compared with the traditional roastingleaching process,this process can achieve a high selectivity of V and Fe,and has excellent prospects for industrial production.

1.Introduction

Vanadium(V),which is characterized by valuable chemical and physical properties,has been applied in various industries such as chemical,steel,aviation,and battery [1-6].In China,V element is mainly extracted from vanadium-titanium(V-Ti)magnetite,which has vast reserves and high utilization value [7].The basic method for V extraction is applying a converter blow and oxidizing the V-bearing hot metal to obtain V slag for subsequent V extraction[7-13].Nevertheless,this process has a low recovery efficiency of V and causes serious environmental pollution [14-19].

Direct V extraction from V-Ti magnetite concentrates has attracted attention due to its high V recovery [20-24].Li et al.[21] realized an 82.04% V leaching efficiency through K2SO4/KCl roasting-leaching.Wang [25] and Li [26] employed CaCO3roasting-leaching to directly extract V from V-Ti magnetite concentrates.However,two crucial issues arouse regarding the direct V extraction from V-Ti magnetite concentrate.First,iron (Fe) could also be leached into the solution because of its high content.Fe3+would affect V extraction and reduce the purity of V products[27,28].Second,the concentrate powder was sintered under high-temperature roasting,which was not conducive to the subsequent leaching.

Solvent extraction and ion exchange are commonly used to extract and concentrate V from the pregnant solution [29].However,massive impurity iron ions are leached together with V,which has an adverse effect on the extraction of V in the solution.Fe3+can easily be precipitated as colloidal Fe(OH)3,which is not only unfavorable for ion exchange but also causes the third phase and emulsification during the solvent extraction process [30].Meanwhile,iron ions can be partially extracted with V,reducing the purity of the vanadium-rich solution and V-products.Therefore,it is essential to separate V and Fe selectively during the leaching process.Many works showed that sulfuric acid (H2SO4)had a good effect on Fe separation [31-36].Huang et al.[32]adopted high-pressure oxidative H2SO4to leach nickel converter slag and achieved the selective leaching of valuable metals with the extraction rates of ＞97% for Co and Ni,95% for Cu,and ＜0.4%for Fe.Xu et al.[34] used H2SO4leaching to realize high Zn(＞97%)and low Fe(＜0.77%)extraction as well as silicon dissolution.In the present study,sulfuric acid was used as a selective leaching agent to separate V and Fe from the V-Ti magnetite concentrate.

Basing on the literature review,this work proposes pellet calcification roasting with subsequent H2SO4leaching to directly extract V from V-Ti magnetite concentrate.Firstly,the concentrates are formed into pellets to prevent the powder from sintering and sticking to the wall at high temperature.Secondly,calcification additives are introduced during roasting.In ironmaking stage,V is converted by alkaline regulator into calcium vanadate,which can be leached by using acid.Sulfuric acid can selectively separate V and Fe,and pellet leaching is an economical and reasonable method for V extraction.The leached pellets after secondary roasting can meet the strength requirements for blast furnace smelting and can be efficiently utilized as iron-making feed material.

In this work,a novel method of pellet calcination roastingsulfuric acid leaching is proposed to selectively leach V from V-Ti magnetite concentrate into liquor,but maintains Fe in the pellets,according to the selectivity of sulfuric acid.The maximal V leaching rate can be as high as 88.98%,and only 1.78%of the Fe impurity is leached.This process is simple and brings little harm to the environment,with a high comprehensive recovery rate of Fe and V,and has excellent prospects for industrial production.

2.Experimental methods

2.1.Materials

The V-Ti magnetite concentrate was obtained from Panzhihua,Sichuan,China and used as the research object.All chemicals such as CaCO3and H2SO4were of analytical grade.The aqueous solution used in the experiment was prepared from distilled water.

2.2.Experimental procedure

The separation process for V and Fe is shown in Fig.1,and the specific operating procedures are as follows.

Fig.1.Flow chart of vanadium and iron separation.

2.2.1.Pelletizing

2.0 kg V-Ti magnetite concentrate powder and 40 g bentonite(2% of concentrate) were weighed by an electronic balance and mixed in porcelain plate for 10 min.As an additive,CaCO3with the mass fraction of m(CaCO3)/m(concentrate) of 0%-10% was mixed with the concentrate.

The pelletizing process had three phases,namely,pellet formation,growth,and compaction,and was conducted in a disc granulator with a radius of 250 mm,a rotational speed of 20 r/min,and an inclination angle of 45°.Finally,the wet pellets were dried at 70°C for 12 h to obtain dry pellets.

2.2.2.Pellet calcification roasting

In each roasting experiment,100 g of dried pellets were roasted in a muffle furnace from 25 to 500-1300°C for 0-4 h under a heating rate of 10°C/min.The muffle furnace was not completely closed down to provide an oxidizing atmosphere.The temperature of the pellet was lowered to room temperature for the subsequent leaching.

2.2.3.Sulfuric acid leaching

The roasted pellets were leached in 1-15 v/v%H2SO4solution at a specific ratio of L/S (LH2SO4solution/Mpellets;mL/g) of 1-5 and a leaching temperature of 30-60°C for 1-7 d.After leaching,pregnant solution and leaching pellets were collected separately through vacuum filtration.The pregnant solution was analyzed to calculate the leaching efficiency using Eq.(1).

where β and C are the leaching efficiency and the concentration of V and Fe in the leaching solution(g/mL),respectively;v the volume of the leaching solution(mL);ω the grade of V and Fe in the raw sample;and m the mass of the raw sample (g).During leaching,each experiment was repeated three times and averaged to minimize errors and determine reproducibility.

2.3.Analytical methods

X-ray fluorescence(XRF)spectrometry was used to analyze the chemical compositions of the concentrate.V and Fe contents in the leaching solution was detected by an inductively coupled plasma optical emission spectrometer (ICP-OES,Agilent Technologies 700).The phase compositions of the concentrate and roasted pellets were identified through X-ray diffraction (XRD,Panalytical Empyrean).V and Fe phase changes were analyzed through X-ray photoelectron spectroscopy (XPS,Thermo Scientific,USA).Scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDS) was employed for microscopic observation and elemental analysis.The phase of V in the V-Ti magnetite concentrates was determined by the chemical method shown in Fig.2.

3.Results and discussion

3.1.Characterization of the concentrate

Table 1 presents the chemical composition of V-Ti magnetite concentrate.The concentrate is a low-quality material for V extraction because the obtained V2O5has a grade of only 0.53% and the Fe2O3is up to 70.36%.Thus,the selective separation of V and Fe is important.The V phase of the concentrate is analyzed.Fig.3 shows that V mainly exists in the magnetite phase in the form of V-Fe spinel (FeV2O4) and accounts for 97.19%,and the remaining phases only account for 2.81%.

Table 1.Main chemical composition of the concentrate.

Fig.4 shows the XRD pattern of the concentrate.The concentrate mainly contains Fe3O4and FeTiO3,and CaCO3is the external calcification additive.No V-containing minerals are found because the V grade is lower than the XRD detection range.

Fig.2.Phase analysis process of vanadium in V-Ti magnetite.

Fig.3.Phase analysis of vanadium.

Fig.4.XRD pattern of V-Ti magnetite concentrate.

3.2.Results of roasting and leaching experiments

Varying amounts of CaCO3addition from 0%to 10.0%were used to determine the effect of CaCO3addition on the leaching efficiency of V and Fe.The results were shown in Fig.5.The V leaching efficiency increased firstly and then decreased with the increasing addition of CaCO3and reached the maximum at 5.0% CaCO3addition.Excessive CaCO3addition produced low-melting calcium salts,which wrapped around V and hindered its oxidation.When the addition of CaCO3was increased from 0%to 10.0%,the Fe leaching efficiency also increased from 0.17% to 2.84%.The decomposition of excessive CaCO3transformed the inside of the pellets into a porous structure,which increased the contact area between the pellets and H2SO4and consequently promoted Fe leaching.Therefore,CaCO3addition of 5.0%was selected as the optimal condition to maximize V and Fe separation.This conclusion was consistent with Li’s experimental results [26].

Fig.5.Effect of CaCO3 addition on the leaching efficiency of V and Fe.(Conditions:roasting temperature of 1200 °C,roasting time of 3.0 h,leaching time of 6 d,leaching temperature of 50 °C,10.0% H2SO4,and liquid/solid ratio of 3:1).

Fig.6.Effect of roasting temperature on the leaching efficiency of V and Fe.(Conditions:CaCO3 dosage of 5.0%,roasting time of 3.0 h,leaching time of 6 d,leaching temperature of 50 °C,10.0% H2SO4,and liquid/solid ratio of 3:1).

Fig.7.Effect of roasting time on the leaching efficiency of V and Fe.(Conditions:CaCO3 dosage of 5.0%,roasting temperature of 1200 °C,leaching time of 6 d,leaching temperature of 50 °C,10.0% H2SO4,and liquid/solid ratio of 3:1).

The effect of roasting temperature on V and Fe extraction was studied by applying various temperatures in the range of 500-1300°C,and the experimental results were shown in Fig.6.When the roasting temperature was increased from 500 to 1200°C,the V leaching rate also increased from 8.75% to 85.78%,indicating the influence of roasting temperature on V leaching efficiency.The V leaching efficiency reached its peak at the roasting temperature of 1200°C and dropped sharply to 50.57% when the roasting temperature was further increased to 1300°C.This phenomenon occurred because the formation of new silicate compounds(Ca2MgSi2O7)prevented the leaching of V as confirmed by the subsequent XRD analysis in Section 3.3.The Fe leaching rate was significantly affected by the roasting temperature.Fig.6 presented that the Fe leaching efficiency was reduced from 9.50% to 1.40%as the roasting temperature was increased from 500 to 1300°C.Based on these results,1200°C was chosen as the optimal roasting temperature.

Fig.7 showed the influence of roasting time on the leaching efficiency of V and Fe.When the roasting time was increased from 0.5 to 4.0 h,the V leaching efficiency fluctuated from 80.10%to 90.04%.A maximum leaching efficiency of 90.04% was obtained at the roasting time of 3.0 h,at which only a minimal amount of Fe was leached,and the maximum Fe leaching rate was less 1.92%.This finding proved that the proposed method achieves a good separation of V and Fe under all roasting time conditions.Hence,the most suitable condition of roasting time was set at 3.0 h.

The effects of leaching time from 1 to 7 d and leaching temperature from 30 to 60 °C were examined.The relationships of V leaching efficiency versus leaching time at different leaching temperatures were presented in Fig.8.When the leaching time was extended,the V leaching efficiency increased gradually at first and then slowed down after reaching the highest level at 6 d.With 6 d as the optimal leaching time,the V leaching efficiency displayed a rapid growth from 71.79% to 89.82% when the leaching temperature was increased from 30 to 50°C.When the leaching temperature was further increased to 60°C,the V leaching efficiency showed no apparent change.Hence,6 d and 50°C were selected as the optimal leaching time and temperature,respectively.The selectivity of H2SO4leaching was reflected in the harmful Fe impurity which was as low as 1.89% under this condition.

Sulfuric acid concentration was varied from 1.0% to 15.0% to investigate its effect on V and Fe leaching efficiency,and the results were presented in Fig.9.Two different stages were observed for V leaching efficiency.In the initial stage (1.0%-7.5%),the V leaching efficiency rapidly increased from 25.34% to 81.88% with the increase of sulfuric acid concentration.In the second stage (7.5%-15.0%),the V leaching efficiency increased gradually and reached the highest level of 88.98% when the sulfuric acid concentration was 10%.Meanwhile,the Fe leaching efficiency increased from 0.42%to 2.04%when the sulfuric acid concentration was increased from 1.0%to 15.0%.For a reduced acid consumption,the most suitable concentration was proposed as 10.0%.

Fig.8.Effect of leaching time and leaching temperature on the leaching efficiency of V and Fe.(Conditions:CaCO3 dosage of 5.0%,roasting temperature of 1200 °C,roasting time of 3.0 h,10.0% H2SO4,and liquid/solid ratio of 3:1).

Fig.9.Effect of sulfuric acid concentration on the leaching efficiency of V and Fe.(Conditions:CaCO3 dosage of 5.0%,roasting temperature of 1200 °C,roasting time of 3.0 h,leaching time of 6 d,leaching temperature of 50°C,and liquid/solid ratio of 3:1).

Fig.10.Effect of liquid/solid ratio on the leaching efficiency of V and Fe.(Conditions:CaCO3 dosage of 5.0%,roasting temperature of 1200 °C,roasting time of 3.0 h,leaching time of 6 d,leaching temperature of 50 °C,and 10.0% H2SO4).

The effects of different liquid/solid ratios on V leaching efficiency were studied,and the experimental results were shown in Fig.10.The V leaching efficiency reached 88.98% with the solidliquid ratio ranging from 3:1 to 5:1.However,when a high ratio was used,the V concentration was low,and the acid consumption was high.This condition might impede V recovery in subsequent processes.Therefore,the best liquid-solid ratio was selected as 3:1.

Based on the optimal leaching conditions,it revealed the concentration of V and Fe in the pregnant solution in Table 2.The results showed that V concentrate was up to 803 mg/L,and Fe concentrate was 2633 mg/L.The leaching efficiency of V was 88.98%,while the leaching efficiency of Fe was only 1.79%,showing that V could be effectively separated from Fe.Finally,solvent extraction or ion exchange could be adopted to separate V from Fe from the leaching solution.

Table 2.Concentrates of vanadium and iron in the leaching solution.

3.3.Characterization of the pellets

The strength of pellets in the three stages of roasting,leaching,and secondary roasting was measured by a universal testing machine,as shown in Fig.11.Under the optimal calcination conditions of 5.0% CaCO3and roasting at 1200°C for 3.0 h,the crushing strength of the roasted pellets reached 2555 N.After H2SO4leaching at 50°C for 6 d,the crushing strength of the leached pellets decreased to 501 N.After the leached pellets were secondarily roasted at 1200°C for 10 min,their strength can reach 2600 N,which satisfied the strength requirements for blast furnace smelting.

XRD analyses were conducted on the concentrate,500-1300°C roasted pellets,and leached pellets under optimal conditions to characterize the phases during roasting and leaching.The Fe3O4,FeTiO3,and CaCO3in the concentrate were oxidized and decomposed completely after roasting at 600,500,and 700°C,respectively (Fig.12).At 500 °C,Fe2O3appeared,and the height of its diffraction peak increased with the calcination temperature (Eq.(2)).At 900 °C,the diffraction peaks of Fe2TiO5appeared after calcium roasting (Eq.(3)).This phenomenon was consistent with Tian’s research about the high-temperature oxidation process of ilmenite [36].At 1300 °C,Fe2TiO5diffraction peak disappeared,and Ca2MgSi2O7peaks were formed (Eq.(4)).According to Zheng et al.[23],molten silicate could prevent the conversion of V3+into V5+and decrease the V leaching efficiency.This phenomenon was consistent with the experiment result in which V leaching efficiency was only 51.14% at 1300 °C.

Fig.12k showed no significant difference in the diffraction peaks and intensity between leached pellets and roasted pellets at 1200°C,indicating that the Fe2O3and Fe2TiO5did not react with sulfuric acid during leaching.The only difference was the appearance of the diffraction peaks of Ca(SO4)(H2O)2formed due to the reaction of sulfuric acid with calcium vanadate to form calcium sulfate precipitation as shown in Eq.(5).

Owing to its low content,there was no V peak observed in the whole XRD diffraction pattern.Hence,XPS analysis was performed to further investigate variations in the valence state of V.Fig.13 showed the detailed XPS spectra of V in the concentrate,roasted pellets,and leached pellets.According to the XPS-NIST X-Ray Photoelectron Spectroscopy Database [37],only the V3+peak wasdetected at the binding energy of 516.78 eV(V2p3/2)(Fig.13a).This result proved that the V in the concentrate existed in the form of V3+and was consistent with the hypothesis.Fig.13b showed that after roasting at 1200°C,V5+and a very weak peak of V4+were detected at the binding energies of 517.18 and 515.66 eV,respectively.This finding revealed that V3+was completely oxidized mainly to the highest valence state (V5+).Fig.13c presented the detailed V XPS spectrum of leached pellets.Only a weak V diffraction peak was detected at 516.47 eV,proving that the V content was decreased significantly.In general,V3+was oxidized to V5+,which was then leached by sulfuric acid to extract V.

Fig.11.Crushing strength of roasted pellets,leached pellets and secondary pellets.

Fig.12.XRD pattern of pellet samples.Note that (a) indicates concentrate,(b)-(j)the roasted pellets,and (k) the leached pellets.

SEM-EDS was applied for the analysis of elemental composition and distribution in the concentrate (with 5% CaCO3),roasted pellets,and leached pellets to further study the reaction mechanism of calcified roasting and leching.The results were shown in Fig.14.Fig.14a showed that Fe and V were highly correlated,indicating that V existed in the V-Fe spinel in the form of V3+.This result was also consistent with the phase analysis of V (Fig.3).The distribution of Ca was different from that of Fe and V because CaCO3was introduced as an additive.In Fig.14b,V was distributed differently from Fe,proving that the V-Fe spinel was oxidized and decomposed.The distribution area was close to the V area after roasting,indicating that the overlapping particles were calcium vanadate(Eq.(6)).After sulfuric acid leaching(Fig.14c),the signal of V was distributed randomly in the leached pellets,and V was not detected in the surface scan spectrum.Therefore,V was leached thoroughly by the sulfuric acid.The distribution of Fe signal remained unchanged in the view field,thus verifying the variation on Fe content.Combined with the test results,these findings proved that calcification roasting and subsequent sulfuric acid leaching were effective in selectively leaching V while keeping Fe inside the pellets.

Fig.13.Detailed V XPS spectra.

Fig.14.SEM images with EDS element mappings of concentrate,roasted pellets,and leached pellets.

4.Conclusions

A method of selective V extraction through pellet calcination roasting with subsequent sulfuric acid leaching was proposed to separate Fe and V from a V-Ti magnetite concentrate.The optimal roasting and leaching conditions for V extraction were studied.In addition,the mechanism of the selective leaching V was investigated in detail.The following conclusions could be drawn.

(1) The optimal roasting and leaching conditions were leaching time of 6 d,leaching temperature of 50°C,leaching liquidsolid ratio of 3,roasting time of 3.0 h,roasting temperature of 1200°C,and 10.0% H2SO4.The maximal V leaching efficiency was up to 88.98%with only 1.79%of leached Fe impurities.Finally,the leached pellets could reach 2600 N strength after the secondary roasting,thus meeting the quality requirements for blast furnace production.

(2) XRD,XPS,and SEM-EDS analyses of the concentrate,roasted pellets,and leached pellets revealed that V existed in the V-Ti spinel,which was oxidized and reacted to Fe2O3and calcium vanadate after calcification roasting.During leaching,H2SO4reacted with calcium vanadate but not with Fe2O3.Therefore,the efficient separation of Fe and V from V-Ti magnetite concentrate was achieved.

Acknowledgements

This work was funded by the National Science Foundation of China (No.51704028) and the Key R&D Program of Yunnan Province (No.2018IB027).