简介概要

Mechanisms of aid-leaching reagent calcium fluoride in the extracting vanadium processes from stone coal

来源期刊：Rare Metals2013年第1期

论文作者：Fei Wang Yi-Min Zhang Jing Huang Tao Liu Yi Wang Xiao Yang Jie Zhao

文章页码：57 - 62

摘要：The mechanisms of aid-leaching reagent calcium fluoride in the extracting vanadium processes were researched by comparing the blank extraction with the extraction with calcium fluoride as aid-leaching reagent. The leaching experiments were carried out under the conditions of 95 ℃, 6h, 1.5 mlg-1 , 15 vol % H2SO4 , and 5 % calcium fluoride （mass ratio of CaF2 to stone coal）. The concentrations of hydrogen ion were adjusted from initial 1.82 and 4.79 mol L -1 to the pH of 1.80 ± 0.05 by 200 g L-1 lime milk, respectively. The solvent extraction experiments were carried out under the organic phase using 15 vol% D2EHPA, 5 vol% TBP, and 80 vol% sulfonated kerosene and O/A phase ratio of 1/4. The leaching rate of vanadium and six-stage countercurrent solvent extraction rate of vanadium can be remarkably improved from 66.71 to 92.97 % and 81.15 to 98.81 %, respectively, when calcium fluoride was added as aid-leaching reagent. In the leaching process, fluorine can enhance the breakage of the structure of vanadium-bearing minerals. Meanwhile, fluoride ion can combine with iron ion to generate acid-insoluble iron fluoride hydrate （Fe2F57H2O） and consume all ferric ions and a portion of ferrous ions. In adjusting pH value process, the massive impurities are decreased to form KAlSO4 and FeAlF5 . In solvent extraction process, fluorine can prevent the formation of ferric hydroxide colloids and eliminate the formed unstable ferric hydroxide colloids and the silicon-containing colloids to promote the sufficient contact between the aqueous phase and the organic phase and the sufficient reaction.

Rare Metals 2013,32(01),57-62

Mechanisms of aid-leaching reagent calcium fluoride in the extracting vanadium processes from stone coal

Fei Wang Yi-Min Zhang Jing Huang Tao Liu Yi Wang Xiao Yang Jie Zhao

College of Resources and Environmental Engineering, Wuhan University of Science and Technology

作者简介：Yi-Min Zhang e-mail: zym126135@126.com; wfwust@126.com;

收稿日期：22 February 2012

基金：financially supported by the Key Science and Technology Support Program (No. 2011BAB05B01) from the Ministry of Science;Technology of China and the Special Science and Technology Program (No. 201009013) from the Ministry of Environmental Protection of China;

Mechanisms of aid-leaching reagent calcium fluoride in the extracting vanadium processes from stone coal

Abstract：

The mechanisms of aid-leaching reagent calcium fluoride in the extracting vanadium processes were researched by comparing the blank extraction with the extraction with calcium fluoride as aid-leaching reagent. The leaching experiments were carried out under the conditions of 95 ℃, 6h, 1.5 mlg-1 , 15 vol % H2SO4 , and 5 % calcium fluoride (mass ratio of CaF2 to stone coal). The concentrations of hydrogen ion were adjusted from initial 1.82 and 4.79 mol L -1 to the pH of 1.80 ± 0.05 by 200 g L-1 lime milk, respectively. The solvent extraction experiments were carried out under the organic phase using 15 vol% D2EHPA, 5 vol% TBP, and 80 vol% sulfonated kerosene and O/A phase ratio of 1/4. The leaching rate of vanadium and six-stage countercurrent solvent extraction rate of vanadium can be remarkably improved from 66.71 to 92.97 % and 81.15 to 98.81 %, respectively, when calcium fluoride was added as aid-leaching reagent. In the leaching process, fluorine can enhance the breakage of the structure of vanadium-bearing minerals. Meanwhile, fluoride ion can combine with iron ion to generate acid-insoluble iron fluoride hydrate (Fe2F57H2O) and consume all ferric ions and a portion of ferrous ions. In adjusting pH value process, the massive impurities are decreased to form KAlSO4 and FeAlF5 . In solvent extraction process, fluorine can prevent the formation of ferric hydroxide colloids and eliminate the formed unstable ferric hydroxide colloids and the silicon-containing colloids to promote the sufficient contact between the aqueous phase and the organic phase and the sufficient reaction.

Keyword：

Vanadium; Stone coal; Aid-leaching reagent; Calcium fluoride; Mechanisms;

Received： 22 February 2012

1 Introduction

Vanadium is an important byproduct that is used almost exclusively in ferrous and nonferrous alloys due to its physical properties such as high tensile strength,hardness,and fatigue resistance[1].In China,the gross reserves of vanadium in stone coal,which is an important vanadiumbearing resource and is also called black shale,are 118million tons in terms of V₂O₅,and this accounts for more than 87%of the domestic reserves of vanadium[2].However,the amount of V₂O₅extracted from stone coal only accounts for 30–40%of total V₂O₅output of China in2010,which is not commensurate with the reserves of vanadium in stone coal in China.Hence,the need for vanadium will continue to increase and it is acknowledged that meeting the world’s increasing demand for vanadium can only be met by extracting vanadium from stone coal in the near future[3].

However,vanadium is always found in combination with various minerals[4]and,for the most part,exists in the crystal lattice of the aluminosilicate minerals as isomorphism[3,5].At present,there are some environmental and hopeful flowsheets,including direct acid leachingsolvent extraction,blank roasting-acid leaching-solvent extraction,and oxidizing roasting-alkali leaching.Especially,the extraction process of blank roasting-acid leaching-solvent extraction attracts increasing attention,because roasting without salinity and acid leaching are regarded as environment friendly and high efficient,and solvent extraction is a well-known process of enriching vanadium from acid leach solutions for its advantages of large treatment capacity,high metal recovery rate,and fast equilibrium speed.Some studies suggest that it is helpful to enhance the leaching of vanadium by adding fluoride,for example,HF,Na F,and Ca F₂,to acid leaching solution[6].However,the mechanism of leaching process with fluoride has not been well-known until now.Besides,during solvent extraction,the formation of interphase filth(also called the third phase)and the unstable operation limit the industrial application of solvent extraction[7–12].The aim of the article is to research the mechanisms of fluorine in the extracting vanadium processes when calcium fluoride is added as aid-leaching reagent.The processes include acid leaching,adjusting p H,and solvent extraction.

2 Experimental

2.1 Materials

The stone coal used in this study was obtained from Jiangxi Province,China.Before the experiments,the ore was crushed to 0–3 mm,roasted in muffle furnace(KRY-10)at700°C for 60 min to remove carbon,and then grinded to the particles less than 74 lm accounting for 75%.The obtained ore was called as raw ore in the work.The chemical composition of the raw ore analyzed by inductively coupled plasma-atomic emission spectroscopy(ICP-AES)is shown in Table 1 and the mineral analysis by X-ray diffraction(XRD)is shown in Fig.1.The main mineral was quartz and the main vanadium-bearing minerals were biotite,muscovite,and polylithionite.

The calcium fluoride was supplied by Shanghai Shanpu Chemical Co.,Ltd.The organic phase used in the experiments was taken from a vanadium plant in Hubei Province,China.The organic phase consisted of 15 vol%D2EHPA,5 vol%TBP,and 80 vol%sulfonated kerosene.All the other reagents and chemicals used were analytical reagent grade.

2.2 Methods

The whole extracting vanadium process is shown in Fig.2.The aim of the article is to find out the mechanism of fluorine in the processes,including acid leaching,adjusting p H,and solvent extraction.

Leaching experiments were carried out in SZCL-2A-type magnetic and controlling temperature stirrers at H₂SO₄concentration of 15 vol%,liquid/solid ratio(L/S)of 1.5 ml?g^-1,and temperature of(95±1)°C for 6 h.One of the experiments was added with 5 wt%calcium fluoride,whereas the other was not.Then,the slurries were separated by vacuum filtration.The obtained leachates were analyzed for chemical compositions;the residues were analyzed for XRD patterns.

The pH of the solutions was adjusted by lime milk of200 g?L^-1concentration to 1.80±0.05 to improve the solvent extraction rate and decrease the vanadium loss rate[13].

In consideration that the p H of the aqueous phase is decreasing gradually and affecting the process of solvent extraction[13],the organic phase was washed with water until the p H of water was over 2.80.Countercurrent solvent extraction experiments were carried out by magnetically churning the organic and aqueous phases for 8 min and resting in separating funnels at room temperature(298±1)K for 5 min.

The vanadium concentration in the aqueous phase was determined by ferrous ammonium sulfate titration using2-(phenylamino)-benzoic acid as an indicator[14].Fluorine concentration was determined by fluorine ion selective electrode[15]and the iron concentration was determined by 1,10-phenanthroline spectrophotometry[16].The content of other elements in the aqueous phase was analyzed using ICP-AES method.

Fig.1 XRD pattern of the raw ore

Table 1 Main chemical composition of the raw ore 下载原图

Table 1 Main chemical composition of the raw ore

Fig.2 Flowsheet of the whole extracting vanadium from raw ore

3 Results and discussion

3.1 Mechanism in leaching

To research the mechanism of fluorine in acid leaching vanadium,experiments were carried by adding calcium fluoride as aid-leaching reagent,whereas leaching temperature,contact time,liquid/solid ratio,and calcium fluoride were kept constant as 95°C,6 h,1.5 ml?g^-1,and5%(mass ratio of Ca F₂to stone coal),respectively.The only difference of the other one was not adding calcium fluoride.Every experiment adopted 100 g raw ore.According to the results of experiments illustrated in Fig.3,when the calcium fluoride was added,leaching efficiency of vanadium increases from 66.71 to 92.97%.Meanwhile,the contents of aluminum and potassium increase conspicuously in the leachate with fluoride.Considering that the vanadium-bearing minerals are mainly biotite,muscovite,and polylithionite(Fig.1),the fact is stated that the primary function of fluoride ion is to enhance the dissolution of the mica[17].Besides,the contents of total iron ion decrease substantially in the leachate with fluoride.Especially,all ferric ions and a portion of ferrous ion are removed.

Fig.3 Main chemical composition changes of leachates

The analyses of leaching residues by energy spectrum(EDAX)through scanning areas(Fig.4)and XRD(Fig.5)shows that a new mineral iron fluoride hydrate(Fe₂F₅?7H₂O),which has been testified to exist[18,19],is generated.The reaction[20]between fluoride ion and iron ion during leaching process ought to be as follows:

Figures 3,4,and 5 prove that fluoride ion could react with all ferric ions,what is more,with a part of ferrous ion,to form Fe₂F₅?7H₂O,which is insoluble in acid.In sum,fluorine can enhance to destroy the structure of the biotite,muscovite,and polylithionite,generate insoluble Fe₂F₅?7H₂O,and decrease the content of the ferric ion and total iron.

3.2 Mechanism in adjusting pH

The leachates by lime milk were neutralized to the p H of 1.80±0.05 from initial 1.82 and 4.79 mol?L^-1,respectively,so that the solvent extraction rate of vanadium could be improved and the vanadium loss rate of adjusting p H value could be decreased.As shown in Fig.6,the results of adjusting p H indicate that impurities are lessened remarkably,particularly Fe,Al,and K.Analyzing the residues of adjusting p H,Fig.7 reports that the main minerals in residues are Ca SO₄and KAl SO₄.In addition,in the residues with fluorine,Fe Al F₅,which has been certified to existence[21],is formed to make for the decrease of a portion of Fe,Al,and F,whereas,in the residues without fluorine,Fe(OH)₃is formed to make for the decrease of a portion of Fe.It suggests that adjusting p H with lime milk can remove a great deal of impurity elements.

3.3 Mechanism in solvent extraction

Experiments were carried out by comparing the solvent extraction from fluorine-containing solution with solution without fluorine to analyze the mechanism of fluorine in solvent extraction.The effect of fluorine on solvent extraction of vanadium is shown in Table 2 with an O/A phase ratio of 1/4.Every experiment adopted 30 ml feed solution.The results indicate that solvent extraction with fluoride ion can improve the one-and six-stage solvent extraction rate remarkably from 26.15 to 55.42%and81.15 to 98.81%,respectively.Furthermore,it is discovered in the process of solvent extraction that the massive interphase filth(also called the third phase)is formed without fluorine,whereas none of interphase filth is formed with fluorine and the aqueous phase and the organic phase separate conspicuously and quickly.

Fig.4 EDAX spectra of a leaching residues with Ca F2and b leaching residues without Ca F2

Fig.5 XRD pattern of(1)leaching residues with Ca F2and(2)leaching residues without Ca F2

Fig.6 Main chemical composition of(1)adjusting p H solution with fluorine and(2)adjusting p H solution without fluorine

Fig.7 XRD pattern of(1)adjusting residues with fluorine and(2)adjusting residues without fluorine

Table 2 Extraction rate of different counter current extraction stages 下载原图

Table 2 Extraction rate of different counter current extraction stages

Fig.8 Composition of(1)solvent extraction solution with fluorine and(2)solvent extraction solution without fluorine

The composition of different-stage raffinates in Fig.8shows that(1)the ferric ion always exists no matter how little iron ion is and(2)two kinds of elements,vanadium and iron,are descended obviously,whereas the others,especially fluorine,remain unchanged.It proves that ferrous ion could be transformed into ferric ion[22]due to strong churning and slow oxidation.Meanwhile,it stated that sodium sulfite is not necessary to be reductant for solvent extraction.Therefore,it makes more sense to decrease the content of total iron than to reduce ferric ion.

By comparing the huge difference of extraction rate of vanadium whether fluorine is contained,it could be analyzed that(1)the concentrations of ferric ion of feed solution with fluorine and without fluorine for solvent extraction are 109.804 and 1,063.529 mg?L^-1,respectively.According to the solubility product of difficult soluble electrolyte Fe(OH)₃and ion product of water[23],the lowest p H is 2.05 and 1.72,respectively,if ferric ion is hydrolyzed.Because the aqueous p H is 1.80±0.05 and the organic p H is over 2.80,ferric ion can be hydrolyzed to form ferric hydroxide colloids to prevent the extraction process of ion exchange and organic phase and aqueous phase separation.However,fluoride ion can prevent the formation of ferric hydroxide colloids and eliminate the formed unstable colloids as fluorine is one of the most active nonmetallic elements.(2)In the process of solvent extraction,the silicate in aqueous phase might form silicon-containing colloids,which exists between the organic phase and the aqueous phase to prevent the extraction process of ion exchange and the aqueous phase and organic phase separation[10,11].However,fluoride ion can eliminate them,owing to the formation of hydrogen fluoride and its destructive capability,and promote the sufficient contact between the aqueous phase and the organic phase and the sufficient reaction.

Therefore,fluoride ion can benefit solvent extraction through preventing and eliminating the unstable ferric hydroxide colloids and the silicon-containing colloids.It is necessary that the content of total iron must be decreased regardless of the ferric ion or ferrous ion to gain the pure product.It is worth noting that the extraction raffinates can be directly made into acid to leach vanadium or be treated by a two-step excess lime milk-sulfuric acid titration method to reduce fluoride ion to 10 mg.L^-1or lower to reuse or discharge[24].

4 Conclusion

The leaching rate of vanadium and six-stage countercurrent solvent extraction rate of vanadium can be improved remarkably from 66.71 to 92.97%and 81.15 to 98.81%,respectively,when calcium fluoride was added as aidleaching reagent.Fluorine benefits extracting vanadium are as follows:

In leaching process,fluorine can enhance the breakage of the structure of vanadium-bearing minerals to increase the leaching rate of vanadium.Meanwhile,fluoride ion can combine with iron ion to generate acid-insoluble iron fluoride hydrate(Fe₂F₅?7H₂O)and consume all ferric ions and a portion of ferrous ion.

In solvent extraction process,fluorine can prevent the formation of ferric hydroxide colloids and eliminate the formed unstable ferric hydroxide colloids and the siliconcontaining colloids to promote the sufficient contact between the aqueous phase and the organic phase and the sufficient reaction.Meanwhile,it proves that ferrous ion can be transformed into ferric ion due to strong churn and slow oxidation,and it makes more sense to decrease the content of total iron than to reduce the ferric ion into ferrous ion.

Adjusting p H value using lime milk cannot only gain the necessary p H value for solvent extract but also decrease the massive impurities,especially Fe,Al,and K,due to the formation of KAl SO₄and Fe Al F₅.

Acknowledgments This project was financially supported by the Key Science and Technology Support Program(No.2011BAB05B01)from the Ministry of Science and Technology of China and the Special Science and Technology Program(No.201009013)from the Ministry of Environmental Protection of China.