J. Cent. South Univ. Technol. (2011) 18: 1940-1944

DOI: 10.1007/s11771-011-0926-9

Extraction of vanadium from stone coal by modified salt-roasting process

WANG Ming-yu(王明玉), WANG Xue-wen(王学文), SHEN Ji-feng(沈继峰), WU Ri-na(乌日娜)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: In order to reduce the pollution of Cl₂ and HCl released during extracting vanadium from stone coal by sodium chloride roasting, a modified salt-roasting process was proposed by adding calcined lime in roasting process followed by H₂SO₄ leaching. The effects of parameters including roasting temperature, roasting time, addition mass ratio of NaCl, calcined lime upon leaching rate of vanadium and curing rate of chlorine were investigated, and the effects of leaching time and leaching temperature on leaching rate of vanadium were also studied. The results show that the vanadium leaching rate and the curing rate of chlorine are 67.3% and 51.5% (mass fraction), respectively, at roasting temperature of 750 °C, roasting time of 4 h, 15% sodium chloride and 8% (mass fraction) calcined lime, leaching temperature of room temperature, and leaching time of 4 h.

Key words: vanadium; stone coal; salt-roasting; chlorine curing

1 Introduction

China is rich in mineral resources of vanadium. More than 87% vanadium existing in stone coal, is widely deposited around the Southern China [1]. With the rapid growth of vanadium products demand, the study on extraction of V₂O₅ from stone coal attracted more and more attentions.

The traditional vanadium extraction technique from stone coal is roasting with a source of sodium chloride under oxidizing conditions in a flat kiln, and then vanadium in stone coal will be converted to soluble sodium salt [2]. After roasting, the calcine is leached with low acidity solution. Due to the simple flow and high recovery of vanadium, this technology has been widely applied to treat stone coal in many places for over 40 years. However, the released exhaust gas containing Cl₂ and HCl pollutes the environment seriously. In recent years, several other processes have been proposed to recover vanadium from stone coal, such as oxidizing roasting-alkaline leaching [3], dilute sulfuric acid leaching [4], calcium salt roasting-sulfuric acid leaching [5], direct acid leaching-solvent extraction [6], pressure acid leaching [7-8], sulfuric acid with villiaumite leaching [9-10] and low temperature sulphating roasting-water leaching [11]. However, there is no an all-purpose process for the extraction of vanadium from stone coal. Among all the above extraction processes, only direct acid leaching-solvent extraction process has been taken for industrial application in the northwest region of China [12], while the recovery is low (about 60%). Although the vanadium leaching rate of other extraction processes is high (>70%), they are still not applied to industrial application because of high cost or rigorous reaction system. For example, the vanadium leaching rate of pressure acid leaching can reach 76%, based on the condition of reaction system with the special material (titanium) and high temperature    (180 °C). Therefore, the sodium chloride roasting-low acidity leaching technique is still used to extract vanadium from stone coal in some places.

In order to reduce the pollution of exhaust gas during sodium chloride roasting, and the difficulty of treating the exhaust gas, a modified sodium salt roasting process is presented. About half of chlorine can be cured and the rest of chlorine is easy to be absorbed by lime pellets deposited in the chimney flue in this process [13].

2 Experimental

2.1 Materials and procedure

The stone coal used in experiment was taken from Tongren District in Guizhou Province, China. XRF (X-Ray Fluorescence) analysis result of stone coal is listed in Table 1. As shown in Table 1, the Ca content in raw stone coal is very low. The reagents including sodium chloride used for roasting, sulfuric acid used for leaching and sodium hydroxide, ammonium ferrous sulfate etc used in chemical analysis were of analytical grade. Additive agent was industry calcined lime and the main composition is 95.41% CaO, 1.98% MgO, 0.82% Al₂O₃, 0.36% Fe₂O₃ and 0.82% SiO₂ (mass fraction).

Table 1 XRF analysis results of main composition in raw stone coal (mass fraction, %)

After crushed and ground, the particle size of stone coal and calcined lime is less than 0.089 mm accounting for 80% and 85%, respectively. Experiments were carried out in 100 g raw stone coal scale. The charge was composed of a weighed amount of stone coal, sodium chloride and calcined lime. This charge was put in a crucible and heated gradually in an electric muffle furnace at a predetermined temperature with the door open to maintain an oxidizing atmosphere.

The leaching procedure was performed in a beaker, the liquid-to-solid ratio was 2 mL/g, the adding sulfuric acid was 3.5 mL, and the leaching temperature was room temperature. The contents of vanadium in the stone coal samples and the leached residue were analyzed by ammonium ferrous sulfate titration. Titration with ammonium sulfocyanate was used to determine the content of chlorine in the residue and the concentration of chlorine in the solution. The curing rate of chlorine is expressed as

                                                       (1)

where p_Cl means the curing rate of chlorine, Q_Cl,R means the quantity of Cl in residue, Q_Cl,S means the quantity of Cl in solution, and Q_Cl,A means the total quantity of added Cl.

2.2 Experimental principle

The main reactions taking place in the roasting process with sodium chloride addition can be expressed as follows:

y(V₂O₅)_c+2xNaCl+(x/2)O₂→xNa₂O·yV₂O₅+xCl₂↑    (2)

y(V₂O₅)_c+2xNaCl+xH₂O→xNa₂O·yV₂O₅+2xHCl↑   (3)

where (V₂O₅)_c is a generic term for the vanadium bearing oxide in stone coal. The numerical values x and y in the product xNa₂O·yV₂O₅ are correlative with roasting condition, and the product can be NaVO₃, Na₄V₂O₇ and Na₃VO₄. When calcined lime is added, the following reaction will take place:

2HCl+CaO=CaCl₂+H₂O↑                                           (4)

CaO+H₂O=Ca(OH)₂                                                                                      (5)

2Ca(OH)₂+2Cl₂=2CaCl₂+2H₂O↑+O₂↑                        (6)

It can be seen from Eq.(4) and Eq.(6) that Cl₂ and HCl can be absorbed by calcined lime to produce CaCl₂, which can reduce chlorine element volatilization during roasting. In practice, CaO can also react with vanadium to produce calcium vanadate, and the reaction may be expressed as follows [14]:

V₂O₅+CaO=Ca(VO₃)₂                                                                                  (7)

V₂O₅+V₂O₃+4CaO+O₂=2Ca₂V₂O₇                            (8)

V₂O₅+V₂O₃+6CaO+O₂=2Ca₃(VO₄)₂                                             (9)

Soluble sodium vanadate in roasted stone coal is easy to be leached out with water, and for calcium vanadate, it can be leached out by sulfuric acid solution, and the reactions can be expressed as follows:

Ca(VO₃)₂+2H₂SO₄=CaSO₄↓+(VO₂)₂SO₄+2H₂O      (10)

Ca₂V₂O₇+3H₂SO₄=2CaSO₄↓+(VO₂)₂SO₄+3H₂O      (11)

Ca₃(VO₄)₂+4H₂SO₄=3CaSO₄↓+(VO₂)₂SO₄+4H₂O   (12)

3 Results and discussion

3.1 Effect of roasting temperature on vanadium leaching and chlorine curing

The roasting temperature was varied in the range of 700-900 °C, with an interval of 50 °C. The roasting time was constant of 4 h, the mass ratio of stone coal, sodium chloride and calcined lime was 100:15:8. The leaching condition was constant, and the results are illustrated in Fig.1. It can be seen that, at around 800 °C, the vanadium leaching rate is maximal. A further increase in temperature leads to obvious decrease in vanadium leaching, which may be attributed to the sintering of stone coal. With increasing temperature, curing rate of chlorine decreases rapidly. This may be attributed to the increased volatilizing of NaCl and CaCl₂. The vanadium leaching rate is 70.1%, and 67.3% at 800 °C and 750 °C, respectively, while the curing rate of chlorine are 35.9% and 51.5% respectively. The decrease of the curing rate of chlorine is more obvious compared with the increase of recovery of vanadium. Thus, the optimum temperature of 750 °C is chosen for further experiments.

Fig.1 Effect of roasting temperature on vanadium leaching and curing of chlorine

3.2 Effect of roasting time on vanadium leaching and chlorine curing

In order to study the effect of roasting time on vanadium leaching and chlorine curing, the roasting time was varied from 2 h to 6 h, with an interval of 1 h, while keeping sodium chloride and calcined lime addition constant. The experimental results are shown in Fig.2. It is observed that there is a rapid increase in vanadium leaching rate with the increase in roasting time up to 4 h. With further increase in roasting time, there is a fall in vanadium leaching rate. The longer the standing time of stone coal in high temperature zone is, the more severe the enwrapped stone coal will be, which is unfavorable for the leaching of vanadium. For the curing of chlorine, with the increase of roasting time, curing rate of chlorine decreases rapidly. Thus, roasting time of 4 h is found to be optimum for maximum extraction of vanadium.

Fig.2 Effect of roasting time on vanadium leaching and curing of chlorine

3.3 Effect of addition of NaCl on vanadium leaching and chlorine curing

The effect of sodium chloride addition on leaching of vanadium and curing of chlorine was studied for a period of roasting for 4 h at 750 °C, with 8% (mass fraction) calcined lime addition. The NaCl addition was varied from 9% to 17% (mass fraction), and the results obtained are presented in Fig.3. The graph shows that, with more than 15% (mass fraction) of NaCl in the feed mixture, the increase of vanadium leaching rate is not obvious. Thus, 15% (mass fraction) of NaCl addition is chosen for extraction of vanadium. The curing rate of chlorine increases with the increase of NaCl addition, which may be caused by two factors: one is the increase of the released Cl₂ and HCl during roasting process absorbed by the calcined lime to form CaCl₂, and the other is the increase of unreacted NaCl with the increase of NaCl addition.

Fig.3 Effect of NaCl addition on vanadium leaching and curing of chlorine

3.4 Effect of addition calcined lime on vanadium leaching and chlorine curing

The effect of calcined lime addition was studied for a period of roasting for 4 h at 750 °C, with 15% (mass fraction) of NaCl addition, and the results obtained are presented in Fig.4. It can be seen that the curing rate of chlorine increases with the raise of calcined lime addition. However, with an increase in calcined lime, the vanadium leaching rate decreases. In roasting process, the more the calcined lime is added, the more the unreacted calcined lime is remained in roasted stone coal. Under the condition that the amount of sulfuric acid used for leaching is invariable, the increase of unreacted calcined lime can consume more H₂SO₄ to join in the neutralization reaction. In addition, the more the calcined lime is added, the more the CaSO₄ will be produced which can wrap the stone coal particles and hinder the leaching of vanadium.

Fig.4 Effect of calcined lime addition on vanadium leaching and curing of chlorine

3.5 Effect of leaching time on vanadium leaching

The effect of leaching time on vanadium leaching was studied, and the results are presented in Fig.5. The roasted stone coal used for leaching was got at 750 °C roasting for 4 h, and the mass ratio of stone coal, sodium chloride and calcined lime was 100:15:8. As shown in Fig.5, the vanadium leaching rate increases with the extending of leaching time, But after the leaching time of 4 h, the vanadium leaching rate rises slowly. Thus, 4 h is chosen as the leaching time in experimental process.

Fig.5 Effect of leaching time on vanadium leaching

3.6 Effect of leaching temperature on vanadium leaching percent

Figure 6 shows the relationship of the leaching temperature and vanadium leaching rate as well as silicon concentration. It can be seen that the effect of leaching temperature on vanadium leaching rate and silicon concentration in leach liquor is not obvious in leaching temperature range from room temperature to 60 °C. With the increase in leaching temperature, the vanadium leaching rate and silicon concentration obviously decrease. The concentration of SiO₂ in leach liquor is 5.90 g/L at room temperature leaching, while only 1.99 g/L at 95 °C. When the leaching temperature is higher than 60 °C, the silicon can be condensed and precipitated. The precipitate can adsorb vanadium from the liquor [15], which results in the decrease of vanadium leaching, and thus high temperature is unfavorable for the leaching of vanadium.

Fig.6 Effect of leaching temperature on vanadium leaching and silicon concentration

4 Conclusions

1) The modified salt-roasting process can not only extract vanadium effectively from stone coal but also reduce the environmental pollution.

2) The vanadium leaching rate of 67.3% and the chlorine curing rate of 51.5% (mass fraction) are obtained under the experimental condition of roasting temperature of 750 °C, roasting time of 4 h, 15% (mass fraction) sodium chloride and 8% (mass fraction) calcined lime addition to stone coal, leaching temperature of room temperature, and leaching time of  4 h.

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(Edited by DENG Lü-xiang)

Foundation item: Project(2007GH001) supported by Development and Reform Commission of Guizhou Province, China

Received date: 2010-12-31; Accepted date: 2011-03-28

Corresponding author: WANG Ming-yu, Professor, PhD; Tel: +86-731-88830247; E-mail: wmydxx@163.com