Rare metal vanadium is an important strategic materials, mainly used in steel industry, national defense, cutting-edge technology, chemical industry and textile industry and other fields. Vanadium is rich in resources and widely distributed in the world, but there is no rich ore that can be mined separately, but it is symbiotic with other minerals at a low grade. At present, the raw materials for the production of vanadium in the world are mainly vanadium slag which is by-produced in the smelting process of vanadium-iron magnetite. The vanadium resources in China are mainly in the form of vanadium-iron magnetite and vanadium-bearing coal . Vanadium-bearing coal is a kind of vanadium ore resource unique to China. It has abundant reserves and has great advantages for the extraction and smelting of vanadium. However, the vanadium recovery rate of traditional flat kiln sodium roasting-water immersion process has high production cost and salt roasting. Harmful gases such as Cl 2 and HCI released during the process seriously pollute the environment. The state has therefore forced the closure of hundreds of small enterprises that use NaCll as an additive and have no pollution control measures to produce vanadium products in flat kiln. Although the air roasting and calcification roasting process avoids the pollution problem of harmful gases such as Cl 2 and HCl, it depends on the mineral phase structure and chemical composition of the stone coal, and the process adaptability is poor. In recent years, with the rapid growth of China's steel output, the demand for vanadium has gradually increased, and the research on extracting vanadium from stone coal has attracted people's attention. The extraction of vanadium from stone coal is an important development direction for the comprehensive utilization of stone coal, and it is also a new direction for the development of China's vanadium smelting industry. Therefore, the development of a new low-cost, high-environment, high-recovery vanadium extraction process is imminent.
I. Analysis of ore phase and chemical composition
(1) Analysis of ore phase
The samples were obtained from a certain area in Guizhou and were divided into blocks and powders. The lithofacies identification and electron microscopy analysis were carried out. The results are as follows.
1, the main phase. Gangue main secondary phase of metallic iron, vanadium iron, potassium aluminum silicate, graphite.
2. Microscopic features. The gangue is granule-shaped, mostly quartz , with a particle size of about 0.05 mm, and a few are vanadium-containing silicon silicate. The metal iron is in the form of large and small particles, generally having a particle size of between 0.015 and 0.02 mm, and a small number of up to about 1 mm. The vanadium-containing element (vanadium carbide) can be seen in the metal iron. The graphite is in the form of a strip and its content is about 10%. The vanadium-containing silicon aluminum ferrite is in the form of fine particles, generally in the range of 0.015 to 0.025 mm.
(2) X-ray diffraction analysis results of ore
The X-ray diffraction analysis results of the ore are shown in Fig. 1.

Figure 1 X-ray diffraction analysis results of stone coal
(3) Chemical composition of ore
The stone coal mine was first crushed into small pieces of 2 to 3 cm in diameter, and then crushed to a particle having a diameter of 0.5 mm by a crusher . Finally, it was dry-ground by a ball mill to a target of 200% for chemical analysis. The analysis results are shown in Table 1.
Table 1 Analysis results of main chemical components of ore
ingredient
V 2 O 5
C
SiO 2
Al 2 O 3
MgO
CaO
Na 2 O
content
3.26
7.60
53.03
16.62
1.22
0.49
0.59
ingredient
K 2 O
Fe 2 O 3
FeO
TFe
MnO
Cr
Cr 6 +
content
3.36
2.70
2.38
3.84
0.0019
0.064
Trace amount
ingredient
S
P 2 O 5
As
TiO 2
Zn
Cu
Mo
content
0.70
0.19
0.043
0.98
0.018
0.019
0.087
ingredient
Ni
Pb
Cd
Loss on ignition
Fixed carbon
Ash
Volatile
content
0.034
0.0004
0.0012
14.71
5.62
85.29
9.09
Second, theoretical research on vanadium extraction from stone coal
The selection of vanadium extraction process for stone coal should be comprehensively investigated according to the composition of stone coal in different regions, the occurrence state of vanadium and the valence state. The oxidation of vanadium in stone coal is the basis and necessary condition for the conversion of vanadium. Therefore, before the development of the vanadium extraction scheme, the valence, solubility, oxidation and conversion of vanadium in stone coal should be studied in depth.
(1) The occurrence of vanadium in stone coal
The material composition of vanadium-bearing coal is relatively complicated, and the occurrence state of vanadium varies. Classified according to the state of vanadium, there are mainly vanadium-containing mica type (carbonaceous rock type), vanadium-containing clay type (siliceous rock type) and an intermediate type between the two. The vanadium phase analysis results of the test ore samples are shown in Table 2.
Table 2 Results of raw ore-like vanadium phase analysis
Vanadium phase
Iron oxide and clay
Mica mineral
Insoluble aluminosilicate
TV
V 2 O 5 content
Occupancy rate
0.586
17.98
2.626
80.55
0.048
1.47
3.26
100.00
It can be seen from Table 2 that vanadium in the ore is mainly present in the mica minerals in the state of adsorption, and a small amount replaces Fe 3 + into the oxidized minerals such as iron oxide and clay minerals in the form of isomorphism in the same phase, and has a very small amount in the form of isomorphism. Substituting A 3 + into the poorly soluble aluminosilicate phase.
(2) Valence state of vanadium in stone coal
The material composition of vanadium-bearing coal in several provinces in the south of China is complex, and the occurrence and variation of vanadium are diverse. Understanding these issues is of great guiding significance for the formulation of a reasonable process for the extraction of vanadium from stone coal. The results of valence analysis of vanadium in stone coal show that only V 3 + and V 4+ are present in the local coal ore, and V 2 + and V 3 + are rarely found. Except for some places where the V 4 + in the stone coal is higher than V 3 + , the vanadium in the stone coal is mostly V 3 + . The vanadium valence state analysis results of the test ore samples are shown in Table 3.
Table 3 Distribution ratio of vanadium in different valence states
Vanadium valence
V 3 +
V 4 +
V 5 +
TV
Vanadium content
Occupancy rate
0.627
34.34
0.527
28.86
0.672
36.80
1.826
100.00
It can be seen from Table 3 that the content of vanadium in the three valence states is not very different, but it is mainly in the form of pentavalent, and the content of trivalent vanadium is equivalent to that of pentavalent vanadium, and vanadium in stone coal studied in most literatures. The valence situation is quite different. According to the analysis in Table 2, the V 3 + moiety replaces Fe 3 + , A1 3 + , etc. into the oxidized minerals such as iron oxide ore, clay minerals, and the insoluble aluminosilicate phase in a homogeneous phase. In the mica minerals, V 4 + and V 5 + are almost entirely present in the mica minerals in the state of adsorption.
(3) Solubility of vanadium in different valence states in stone coal
1, V 3 + . V 3 + in stone coal is present in the clay mineral dioctahedral sandwich layer, partially replacing A1 3 + . This aluminosilicate structure is relatively stable. Generally, V 3 + in stone coal is difficult to be dissolved by water, acid or alkali. Unless HF is used to destroy the crystal structure of the clay mineral, it is considered that V 3 + is not substantially leached. Only after V 3 + is oxidized to a high price, vanadium in the stone coal may be leached.
2, V 4 + . V 4 + in stone coal may be present in the form of oxide (VO 2 ), vanadyl ion (VO 2 + ) or vanadate. VO 2 can replace part of Al 3 + in the illite-type clay mineral dioctahedral lattice, and this part of V 4 + can not be leached by water, acid or alkali. Free VO 2 + insoluble water in stone coal, but soluble in acid, producing vanadium oxylate VO 2 + , stable, blue.
VO 2 +H 2 SO 4 =VOSO 4 +H 2 O
(2) V 5 + . The V 5 + ionic radius is too small to exist in the clay mineral dioctahedron. V 5 + in stone coal is mainly present in the form of free V 2 O 5 or crystalline (xM 2 O·yV 2 O 5 ) vanadate, and is easily soluble in acid.
Third, oxygen pressure direct acid leaching to extract vanadium
The new technology of extracting vanadium by direct acid leaching of stone coal oxygen pressure is a full wet process developed by Kunming University of Science and Technology, as shown in Figure 2. This method is mainly aimed at the shortcomings and shortcomings in the vanadium extraction technology of stone coal, grasping the core technology and key technology in the vanadium extraction technology of stone coal, researching and developing the extraction of vanadium from stone coal under pressure field or pressurized condition. On the basis of strengthening metallurgical conditions, the large radiation increases the recovery rate of vanadium, and at the same time, it has no exhaust emission and protects the environment.

Figure 2 Process of vanadium extraction by direct acid leaching of stone coal oxygen pressure
(1) Comparative test of aerobic and anaerobic
1. Test conditions. Time 4h, temperature 150 ° C, H 2 SO 4 dosage 25%, liquid-solid ratio 1.2:1, particle size -200 mesh, additive (ferrous sulfate) 5%.
2. Test results. Three parallel tests were carried out under aerobic and anaerobic conditions, respectively. The leaching rate results are shown in Table 4.
Table 4 Leach rate results for aerobic and anaerobic comparison tests
Test conditions
Number of trials
average value
1
2
3
Aerobic test
Anaerobic test
77.30
34.02
75.27
36.51
74.23
35.69
75.60
35.41
It can be seen from Table 4 that the leaching rate under aerobic conditions is much higher than that under anaerobic conditions, indicating that oxygen plays a significant role in the reactor. Since there are vanadium in the form of V 3 + which is difficult to be dissolved by water and acid in the ore, after passing through oxygen, O 2 dissolved in the aqueous solution oxidizes Fe 2 + to Fe 3 + , then Fe 3 + then V 3 + is oxidized to V 4 + which is soluble in acid. Therefore, the leaching rate of vanadium under oxygen-passing conditions can be greatly improved compared with the anaerobic conditions.
(2) Effect of leaching time on vanadium leaching rate
1. Test baseline conditions. The temperature was 150 ° C, the amount of H 2 SO 4 was 25%, the liquid-solid ratio was 1.2:1, the particle size was -200 mesh, and the additive amount was 5%.
2. Test results. Taking time as a variable, 5 points (1h, 2h, 3h, 4h, 5h) were tested. The test results are shown in Figure 3.

Figure 3 Effect of time on vanadium leaching rate
It can be seen from Fig. 3 that the leaching rate of vanadium increases with time, but after a certain time (3h), the vanadium leaching rate decreases, but the decrease is slow. The peak value of vanadium leaching rate is between 3h and 4h. The reason why the leaching rate of vanadium has decreased may be that, as time goes by, in a closed container, the ore is agglomerated, vanadium is wrapped, and the leaching rate is lowered. Therefore, it is more practical to select the leaching time between 3h and 4h.
(III) Effect of leaching temperature on vanadium leaching rate
1. Test baseline conditions. At 4 h, the amount of H 2 SO 4 was 25%, the liquid-solid ratio was 1.2:1, the particle size was -200 mesh, and the additive was used at 5%.
2. Test results. The temperature was used as a variable, and 5 points (120 ° C, 135 ° C, 150 ° C, 165 ° C, 180 ° C) were tested. The results are shown in FIG. 4 .

Figure 4 Effect of temperature on vanadium leaching rate
As can be seen from Figure 4, the higher the temperature, the higher the leaching rate of vanadium. Mainly because the higher the temperature, the faster the reaction rate, the larger the amount of vanadium leached in the same time (4h), so the leaching rate is high. However, the temperature cannot be increased indefinitely, and its influence on the leaching rate must have an extreme point, and the energy consumption, the production cost, and the bearing capacity of the industrial production must be comprehensively considered. The choice of temperature, only from the level of leaching rate, should choose the high temperature as much as possible, but in the case of multi-stage leaching, the leaching rate is not much different, then the low temperature should be chosen to help reduce energy consumption and meet the needs of industrial production.
(4) Effect of sulfuric acid dosage on vanadium leaching rate
1. Test baseline conditions. Time 4h, temperature 150 ° C, liquid-solid ratio 1.2:1, particle size 200 mesh, additive dosage 5%.
2. Test results. Taking the amount of sulfuric acid as a variable, 5 points (15%, 20%, 25%, 30%, 40%) were tested, and the results are shown in Fig. 5.

Figure 5 Effect of sulfuric acid dosage on vanadium leaching rate
It can be seen from Fig. 5 that the amount of sulfuric acid has a great influence on the leaching rate of vanadium, and the leaching rate of vanadium is on the rise. The leaching rate of vanadium is not much improved between 25% and 30%. It indicates that the larger the concentration of sulfuric acid, the larger the H + concentration, and the greater the probability of entering the mica lattice, which is conducive to destroying the structure of mica, so that the leaching rate of vanadium is higher.
(5) Effect of liquid-solid ratio on vanadium leaching rate
1. Test baseline conditions. Time 4 h, temperature 150 ° C, H 2 SO 4 dosage 25%, particle size -200 mesh, additive dosage 5%.
2. Test results. The test was carried out by taking 5 points (1.1:1, 1.2:1, 1.5:1, 2.0:1, 3.0:1) with the liquid-solid ratio as a variable, and the results are shown in Fig. 6.

Figure 6 Effect of liquid-solid ratio on vanadium leaching rate
The effect of liquid-solid ratio on leaching rate and the effect of sulfuric acid dosage on leaching rate are somewhat similar. The lower the liquid-solid ratio, the higher the relative concentration of sulfuric acid and the higher the leaching rate of vanadium. It can be seen from Fig. 6 that the leaching rate of the first point of 1.1:1 is lower than the second point of 1.2:1, which may be due to the fact that the liquid-solid ratio is too small, the pulp slurry is too high, and the sulfuric acid activity is lowered, resulting in the vanadium outgassing. The rate is reduced. ”
(6) Effect of mineral particle size on vanadium leaching rate
1. Test baseline conditions. Time 4h, temperature 150 ° C, liquid-solid ratio 1.2:1, H 2 SO 4 dosage 25%, additive dosage 5%.
2. Test results. The test was carried out by taking five points (-150 mesh, -200 mesh, -250 mesh, -300 mesh, -350 mesh) with the particle size as a variable, and the results are shown in Fig. 7.

Figure 7 Effect of raw material particle size on vanadium leaching rate
It can be seen from Fig. 7 that when the original ore size is 150 mesh-250 mesh, the leaching rate of vanadium is basically maintained at about 77.3%; but when the ore size is less than -250 mesh, the leaching rate of vanadium begins to decrease; when the ore size is less than - The leaching rate of vanadium at 300 mesh showed a significant decrease. It indicates that the fine particle size will cause the agglomeration of the raw materials during the leaching process, resulting in a decrease in vanadium leaching rate. Therefore, the ore size should not be too low in this test face. Considering the actual grinding problem, the original ore size should be controlled at 150 mesh to 250 mesh.
(VII) Effect of the dosage of ferrous sulfate on the leaching rate of vanadium
1. Test baseline conditions. The time was 4 h, the temperature was 150 ° C, the amount of H 2 SO 4 was 25%, the liquid-solid ratio was 1.2:1, and the particle size was -200 mesh.
2. Test results. Using the additive (ferrous sulfate) as a variable, 5 points (15%, 20%, 25%, 30%, 40%) were tested. The results are shown in Fig. 8.

Fig. 8 Effect of FeSO 4 dosage on vanadium leaching rate
It can be seen from Fig. 8 that the addition of ferrous sulfate makes the leaching rate of vanadium under the same conditions greatly improved, and the vanadium leaching rate gradually increases with the increase of the additive amount, but the increasing trend is slower, when the dosage exceeds At 8%, it basically does not increase. At the same time, the addition of ferrous sulfate will cause more metal iron ions in the leachate, which is not conducive to the subsequent vanadium extraction process. Therefore, the amount of additives should not be excessive. It can be seen from Fig. 8 that when the amount of the ferrous sulfate additive is 5% of the amount of the stone coal mine, the vanadium leaching rate can be increased by 8.07 percentage points compared with the case where no additive is added under the same conditions. Therefore, comprehensive consideration, the amount of additives is about 5%.
(8) Two-stage leaching comprehensive test
Based on the above test results, five groups of two-stage leaching tests were carried out under the optimum test conditions to verify the leaching rate of vanadium. The results are shown in Table 5.
The leaching conditions are as follows:
1. A section of leaching conditions. Constant temperature time 3h, leaching temperature 150 ° C, sulfuric acid dosage 25%, solid-liquid ratio 1.2:1, particle size -200 mesh, additive dosage 3%.
2. Two-stage leaching conditions. Constant temperature 4h, leaching temperature 150 ° C, sulfuric acid dosage 35%, solid-liquid ratio 1.2:1, particle size -200 mesh, additive dosage 5%.
Table 5 Two-stage leaching test results
Numbering
1
2
3
4
5
Total leaching rate
90.81
91.71
90.96
92.96
90.99
It can be seen from Table 5 that the total leaching rate of vanadium in the two-stage leaching test of the five groups reached more than 90%, indicating that it is feasible to carry out the direct acid leaching of vanadium under the above conditions.
Fourth, the conclusion
(1) The leaching rate of vanadium under oxygen-passing conditions is much higher than that of vanadium under non-passing oxygen test conditions, which proves that vanadium oxynitride is a practical and feasible process route.
(2) The addition of ferrous sulfate additive during the oxygen pressure acid leaching process can further increase the leaching rate of vanadium. When the amount is 5% of the amount of stone coal mine, the vanadium leaching rate can be compared with that when no additive is added under the same conditions. Increase by 8.07 percentage points.
(3) The research shows that the optimum process parameters of vanadium-bearing coal oxy-pressure acid leaching vanadium extraction process are leaching time 3~4h, leaching temperature 150°C, liquid-solid mass ratio 1.2:1, sulfuric acid dosage 25%～35%, The ore particle size is -200 mesh, and the additive dosage is 3% to 5%.
(4) The test stone coal mine sample is leached by two stages of oxygen-pressurized sulfuric acid, and the vanadium leaching rate can reach more than 90%.
(5) The new technology of direct acid pickling and vanadium extraction with stone coal oxygen pressure has the advantages of short process flow, simple operation, high vanadium leaching rate and low environmental pollution. It is an environmentally friendly new vanadium extraction technology with good development prospects. Oxygen pressure acid leaching and non-polluting vanadium extraction process is the proper trend of China's stone coal vanadium extraction process reform.

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