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After the vanadium-containing solution was purified pentavalent vanadium plurality vanadate present. As the acidity of the solution increases, the vanadate will precipitate as vanadium, commonly known as red cake. The hydrolysis of vanadium depends mainly on the acidity, temperature, vanadium concentration and the influence of impurities. Precipitated precipitates also have different polymerization states due to changes in pH and vanadium concentration. The relevant mechanism is not uniform in terms of understanding. It can be roughly outlined as follows: Figure 1 and Figure 2 show the nature of the vanadic acid aqueous solution:
Vanadium concentration / (mol · L - 1 ) | Solution pH | Main vanadium ion | Hydrolyzate |
Low, 10 - 4 | Acidic |
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low | 4~8 |
| |
High, 50×10 - 3 | 2 to 3 |
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High, 50×10 - 3 | 1 to 6 |
|
|
High, 50×10 - 3 | 10~12 |
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High, 50×10 - 3 | 13～ |
|
When the pH is about 1.8, the solubility of V 2 O 5 is the smallest, about 230 mol/L. The concentration relationship between V 2 O 5 and H 2 SO 4 is as follows:
[H2SO4]/(g·L - 1 ) | 2.3 | 12.0 | 17.1 | 21.2 |
V 2 O 5 /(g·L - 1 ) | 0.24 | 0.78 | 1.14 | 2.04 |
Table 1 lists data for a set of V 2 O 5 -H 2 SO 4 -H 2 O systems.
Table 1 V 2 O 5 -H 2 SO 4 -H 2 O system equilibrium data
30 ° C | 75 ° C | |||||
V 2 O 5 /% | H 2 SO 4 /% | Density / (g·cm - 3 ) | Precipitates | V 2 O 5 /% | H 2 SO 4 /% | Precipitates |
1.63 | 7.3 | 1.066 | 1 | 1.48 | 17.43 | 1 |
4.79 | 23.5 | 1.219 | 1 | 2.00 | 24.18 | 1 |
7.4 | 37.26 | 1.370 | 1 | 5.06 | 33.0 | 1 |
4.41 | 45.01 | 2 | 5.48 | 38.02 | 2 | |
5.5 | 54.36 | 1.519 | 2 | 5.27 | 41.01 | 2 |
9.14 | 60.42 | 1.661 | 2 | 5.13 | 46.56 | 2 |
5.44 | 66.76 | 3 | 8.09 | 52.31 | 3 | |
1.59 | 74.67 | 3 | 9.08 | 57.33 | 3 | |
6.21 | 73.26 | 4 | 10.8 | 60.20 | 4 | |
0.276 | 80.41 | 1.727 | 4 | 7.51 | 4.98 | 4 |
0.053 | 99.16 | 1.817 | 4 | 7.52 | 70.50 | 4 |
9.26 | 40.49 | 1.440 | 12 | 0.13 | 93.44 | 4 |
10.49 | 62.22 | 1.734 | 23 | 6.10 | 34.30 | 12 |
1.50 | 77.48 | 1.714 | 34 | 8.29 | 49.53 | 23 |
11.96 | 57.56 | 34 | ||||
2V 2 O 5 ·2 H 2 O, 2SO 3 ·8H 2 O pink, amorphous, brownish red, needle-like;
3V 2 O 5 ·H 2 O, V 2 O 5 ·2SO 3 ·3H 2 O Light yellow, needle-like, red, columnar;
4V 2 O 5 , V 2 O 5 ·5SO 3 ·4H 2 O Yellow, needle-like, yellow, crystalline.
The factors that have an important influence on the hydrolysis of vanadium are temperature, acidity, vanadium concentration and impurity content.
Figure 1 Figure 2 V 2 O 5 solubility versus pH (25 ° C)
1-V 2 O 5 / , lg =-0.82-pH; 2—no precipitation of V 2 O 5
Lg =-0.04-pH; 3-V 2 O 5 / , lg =-4.44+pH;
4—No precipitation of V 2 O 5 , lg =-3.00+pH; 5— / ,
pH=1.03-0.333 lg ;6— / , pH = 2.62;
7— / , pH=7.38+lg
Figure 2 Relationship between vanadium in aqueous solution and vanadium concentration and pH (25 ° C)
First, the temperature
The vanadium hydrolysis precipitation should be carried out above 90 ° C, preferably in a boiling state. The relationship between precipitation rate and time under different temperatures and acidity is shown in Fig. 3.
Figure 3 Relationship between precipitation rate and time
:I-0.855; II-0.954; III-1.16; IV-1.18
Second, vanadium concentration
The solution contains V in an amount of 5-8 g/L. If the concentration is too high, the crystal nucleation is too fast, and it is easy to form a loose filter cake, adsorbing more impurities and free water. The red cake composition xa y is large in xNa 2 O·yV 2 O 5 ·z H 2 O. When the concentration of vanadium in the solution is low, there is a negative impact.
Third, the impact of impurities
Phosphorus forms a stable complex with vanadium, H 7 [P(V 2 O 5 ) 6 ], and forms a phosphate precipitate with Fe 3 + and Al 3 + , which contaminates the red cake. For this purpose, the P content after purification is required to be less than 0.15 g/L. When the acidity is high, the solubility of FePO 4 and AlPO 4 can be improved, and the pollution of red cake by phosphorus can be reduced.
When the concentration of silicon, chromium , aluminum , or iron is high, the colloidal precipitate formed by hydrolysis hinders the growth of the V 2 O 5 crystal, and the hydrolysis rate is slow, and the resulting red cake is difficult to settle and filter. Properly increasing acidity can improve such adverse effects.
Chloride ions can accelerate the rate of vanadium hydrolysis precipitation. The sodium sulfate content of 20 ~ 160g / L, the vanadium hydrolysis precipitation rate decreased, mainly to extend the nucleus gestation period. Excessive sodium chloride or sodium sulphate will reduce the V 2 O 5 content of the red cake.
Fourth, stirring
The hydrolyzed precipitation of vanadium is a hydrolysis reaction process with heat and mass transfer. Therefore, it is necessary to maintain a suitable stirring speed and reach a critical suspension state without any dead angle. Industrial mechanical stirring vanadium cans are cylindrical, with an inner diameter of 2 to 5 m and a volume of 4 to 5 m 3 . The inner wall of the tank is lined with acid-resistant tiles or diabase . A stainless steel stirrer is installed in the center. A stainless steel steam heating pipe is arranged near the tank wall.
Hydrolyzed vanadium is a batch operation. First, add 25% of the pre-vanadium precipitation solution, start stirring, add the required sulfuric acid, and then heat it to above 90 °C by steam. Continue to add the remaining 75% of the vanadium pre-fluid. Finally, the concentration of free acid and vanadium in the solution is analyzed, and the acidity or the pre-vanadium pre-solution is adjusted to make the vanadium content in the final solution less than 0.1 g/L. Stop heating, stir, and then let stand for 10-20 min and then filter to obtain a red cake. Depending on the scale of production, the filter unit can be a suction filter, a filter press or a drum vacuum filter.
The red cake must be dried to remove moisture, then melted at a temperature of 1073 to 1173K, and cast into a sheet shape as a raw material for the ferroved iron.
Hydrolyzed vanadium is widely used in the early stage, but the content of V 2 O 5 produced by red cake is only 80%-90%, the purity is low, the acid consumption is large, and the amount of sewage is large, so it is basically ammonium. Replaced by salt vanadium.
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