Precipitation behavior of impurity elements during evaporation crystallization of ammonium paratungstate

Ammonium paratungstate (APT), mainly made by evaporation of ammonium tungstate solution, is the most important intermediate product in tungsten smelting. Its chemical quality directly affects the quality of subsequent tungsten products. With ammonium tungstatesolution obtained from the synergistic decomposition of scheelite by sulfur-phosphorus mixed acid as raw material, we studied the precipitation law of main impurity ions during the evaporation and crystallization process of APT. The concentration upper limits for each impurity in the ammonium tungstate raw material liquid for the preparation of APT-0 grade products were determined. It not only provides a data basis for impurity removal for the ammonium tungstate purification process, but also ensures that the product quality of the evaporation crystallization process meets the standard. The impurity K is easier to be precipitated than Na. The precipitation rate of impurities Cu and Mg and their contents in APT increased with the growth of their concentrations. The precipitation mode of impurity Mo was different at initial and later stages of evaporation crystallization. The precipitation rate of impurities P and Si decreased with the increase of their concentrations, and the precipitation rate of P was higher than that of Si. Impurity S existed in the form of (NH₄)₂SO₄ with high solubility, so it is rarely precipitated except for being adsorbed and entrained. According to GB/T 10116-2007, when c(Na) < 52 mg/L, c(Cu) < 18 mg/L, c(Mg) < 17 mg/L and c(Mo) < 30 mg/L in the solution, APT product can meet the zero-order requirements. However, the requirements for impurities K, P, Si and S in the solution must be more stringent in order to reach the product standard.