Tungsten Concentrate Decomposition Method – Nitric Acid

Calcium tungstate is the main mineral of tungsten scheelite. When calcium tungstate reacts with nitric acid, it has bigger equilibrium constant, concentration constant is 757. It is no worse than hydrochloric acid.

Calcium tungstate reacts with nitric acid chemical formula:
CaWO₄(S)+HNO₃=H₂WO+Ca(NO₃)₂

Compared with hydrochloric acid decomposition method, nitric acid has lower corrosion for equipment, so stainless steel equipment is enough. However, tungstic acid membrane can also generates during nitric acid decomposition process, it is covered on the surface of undecomposed scheelite, thus affects leaching rate of scheelite.

To overcome cover of tungstic acid, hot ball mill is used to clear its impeded function. However every sword has two blades. Nitric acid can promote decomposition effect of scheelite, and its consumption is lowered by 120% from theoretical value. But after ball mill decomposition reaction, a lot of tungstic acid is missed, its solubility is only 0.01~0.014g/L, polytungstate takes a larger amount in solution.

Tungsten scheelite in China has low grade and complex composition. Using nitric acid decomposition method will cause a lot of tungsten to disperse and waste. Due to impeded function of tungstic acid membrane, hot ball mill machine is required to promote decomposition rate of scheelite. Nitric acid will pollute environment unless its waste liquid is processed. Compared to hydrochoric acid, its only advantage may be the low volatility and corrosion property.

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