Sustainable and efficient leaching of tungsten from scheelite using the mixture of ammonium phosphate, ammonia and calcium fluoride

Abstract

The traditional scheelite treatment process, which involves the sodium hydroxide decomposition and anion exchange, discharges a large amount of high-salinity wastewater, resulting in severe environmental pollution. In this study, ammonium tungstate solution was prepared directly by scheelite leaching with a mixture of ammonium phosphate, ammonia and calcium fluoride. The effects of ammonium phosphate and calcium fluoride dosages, initial ammonium concentration, temperature, liquid-solid ratio and holding time on the leaching efficiency of scheelite were investigated. The leaching efficiency of scheelite reached 98.6% under the following conditions: the dosages of ammonium phosphate and calcium fluoride were 1.6 and 2 times their theoretical amounts, respectively. Other conditions included: initial ammonia concentration of 30 g/L and L/S ratio of 2:1 at 180 °C and a holding time of 3 h. In addition, the leaching reaction sequence of scheelite and the formation pattern of calcium fluorophosphate were inferred. After purification of the ammonium tungstate solution, a high-purity ammonium paratungstate was prepared by evaporative crystallization. This study presents a new approach for the green and efficient extraction of tungsten from scheelite.