The dilute nitric acid was added into ammonium paratungstate (referred APT) solution at the appropriate reaction conditions of pH and temperature, and ammonium paratungstate gradually transformed into ammonium metatungstate. Ammonium metatungstate solution through a nanofiltration membrane device, after concentrated and washing process, pure ammonium metatungstate solution was prepared. At a certain time interval, sampling and analysis the solution and concentration of tungsten and nitrate ions of the permeate; and the pressure of the permeate is directly read by the pressure gauge.

 

Raw Material Quality Requirements

Because of the high solubility of ammonium metatungstate, in the course of preparation, the ability of removing impurities is poor. The quality of product highly dependents on the quality of raw materials. According to the ammonium paratungstate product quality requirements, choose the appropriate quality of ammonium paratungstate as raw materials. The experiment used the international zero-grade pure ammonium paratungstate.

 

Temperature Control of Dissolution Process 

In the process of dissolution, since the solubility of ammonium paratungstate increases with increasing temperature, while the increasing temperature of is also conducive to the conversion of  the ammonium paratungstate turning to ammonium metatungstate. Therefore in this process, we should try to increase the reaction temperature. However, the reaction temperature should not exceed 100 ℃, or it will cause water evaporation and increasing energy consumption, so the  temperature of dissolution process can be controlled in 80 ℃ ~ 95 ℃.

 

Liquid Ratio

Because of the low solubility of ammonium paratungstate, in the neutralization conversion process, the weight ratio of ammonium paratungstate added and deionized water (solid-liquid ratio) is smaller, ammonium paratungstate is more easily to dissolve and transform. But the smaller the solid-liquid ratio is, tungsten trioxide content of liquid conversion is lower, which reduce the equipment utilization effect. Excessive solid-liquid ratio will not only affect ammonium paratungstate dissolving, but also inhibit the dissolution of ammonium paratungstate and affect the clarity of the dissolving liquid due to the effect of ammonium ions common-ion effect in solution. Therefore, in order to easily dissolve and ensure the clarity of the dissolving liquid, minimize the amount of deionized water and improve the solid-liquid ratio.