Key Factors Affecting Vanadium Precipitation in Ammonium Metavanadate Production
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Where Vanadium Precipitation Sits in the Flowsheet
In industrial production the pH value of the leaching solution is about 8-9. The precipitant ammonium chloride and concentrated sulfuric acid are added to adjust the pH value to between 2-2.5. Vanadium can then be precipitated in the form of ammonium metavanadate, but the precipitation is affected by several conditions that must be controlled together rather than one at a time.
Operators should treat these variables as a set. Raising acidity, for example, speeds the reaction but also opens the door to side reactions, so the gain from one parameter can be cancelled by the loss from another.
Concentration of the Vanadium Solution
The higher the concentration of vanadium in the stock solution, the faster the growth rate of the crystal nucleus during the hydrolysis of vanadium precipitation. Because the precipitation rate is faster, the crystal nucleus grows faster as well, so the crystal structure is incomplete, the precipitated particles are not tight, more impurities are adsorbed, more water is carried, and the quality is poorer. If the operation is not well controlled, sticking can also occur. In short, the higher the concentration of vanadium, the faster the precipitation rate and the lower the grade of the precipitate. In general the concentration of the stock solution is controlled at 15-20 g/L.
Influence of Acidity
Experiment shows that the precipitation of vanadium begins when the pH value is 3.6. As the acidity increases, the reaction rate of vanadium becomes faster. However, as the acidity increases to a certain value, it is accompanied by the occurrence of side reactions, that is, the reverse dissolution of polyvanadate ammonium. The practical target window of pH 2-2.5 is therefore a compromise between a workable reaction rate and an acceptable loss of already precipitated vanadium.
Ammonium Salt Amount and Precipitation Time
Ammonium salt decomposes easily in alkaline solution, so in industry the pH value of the solution is first adjusted to between 4-5 and the ammonium salt is then added. Under stirring, the pH is adjusted to about 1.9-2.1, and the batch is heated for precipitation.
Precipitation time has to match the reagent addition. When the ammonia addition coefficient is fixed at 0.65 and the pH value is equal to 2.2, the precipitation rate can reach more than 98% once the time reaches more than 25 minutes. In actual production the precipitation rate can be guaranteed by controlling the time for more than 30-40 minutes, which leaves margin for variations in mixing and temperature.
Effect of Temperature
When the other conditions are exactly the same, an increase in temperature accelerates the precipitation rate of vanadium from the solution. Temperature also has a great influence on the reverse precipitation rate, the physical form and the grade of polyvanadate ammonium. Precipitation carried out at low temperature gives fine apparent particles, a red colour and a high grade; precipitate obtained at high temperature has large particles, a dark colour and a correspondingly lower grade.
Because colour and particle size are visible early in the batch, they are useful indicators for the operator: a shift towards large, dark particles signals that conditions have drifted, and the grade of the final product will follow.
Typical Product Specification
| Product name | Vanadium Pentoxide |
|---|---|
| Grade | Industrial Grade |
| Color | Brown / Black |
| Purity | 98% |
| Shape | Flake |
| Melting point | 690 °C |
| Solubility | Slightly soluble in water, insoluble in ethanol, soluble in strong acid and strong base |
FAQ
Q: What pH range is used for vanadium precipitation?
The leaching solution is adjusted from about 8-9 down to 2-2.5, with concentrated sulfuric acid and ammonium chloride as the precipitant.
Q: What stock solution concentration is recommended?
The stock solution is generally controlled at 15-20 g/L of vanadium; higher values speed precipitation but lower the grade of the precipitate.
Q: At what pH does precipitation start?
Precipitation of vanadium begins when the pH value is 3.6, and the reaction rate accelerates as acidity increases.
Q: Why is ammonium salt added after pH adjustment?
Ammonium salt decomposes easily in alkaline solution, so the pH is first adjusted to 4-5 before addition and then lowered to about 1.9-2.1 under stirring.
Q: How long does precipitation take?
At an ammonia addition coefficient of 0.65 and pH 2.2, more than 98% precipitation is reached beyond 25 minutes; production practice controls the time at more than 30-40 minutes.
Q: How does temperature change the product?
Low temperature precipitation gives fine, red, high-grade particles, while high temperature precipitation gives large, dark particles of lower grade.
Q: What are the main properties of the finished pentoxide?
Industrial grade vanadium pentoxide of 98% purity is supplied as brown to black flakes, melts at 690 °C and is soluble in strong acid and strong base.
