A new process for the efficient decomposition of low-grade scheelite based on the formation of insoluble calcium fluorophosphate

With the exhaustion of wolframite and scheelite concentrates, low-grade scheelite has gradually become the main raw material of the tungsten industry. The current processes used for leaching low-grade scheelite with sodium hydroxide or sodium carbonate in an autoclave have the disadvantages of low tungsten leaching efficiency and the requirement of a large leaching reagent dosage. In this study, a new process for leaching low-grade scheelite by using a mixture of sodium phosphate, sodium hydroxide and calcium fluoride is proposed. In the leaching process, scheelite is converted into calcium fluorophosphate while transferring WO42− from the mineral to the solution. To explore the conditions favourable for the formation of calcium fluorophosphate during leaching, thermodynamic analysis of the Ca-W-P-F-H2O system was carried out. The results show that calcium fluorophosphate can be formed over a wide range of pH values. Increasing the concentration of total phosphorus and total fluorine in the solution is beneficial to the decomposition of scheelite. The effects of various parameters on the leaching of low-grade scheelite were studied, and the optimum process parameters were determined. A total of 98.21% of the tungsten can be leached from low-grade scheelite using 2 times the theoretical amount of sodium phosphate, 1.5 times the theoretical amount of calcium fluoride, a sodium hydroxide to ore mass ratio of 14%, and an L/S ratio of 4:1 at 453 K for 3 h, with a residual phosphorus concentration of only 0.12 g/L in the leaching solution. Mineralogical analysis of the leaching residue shows that most of the associated mineral calcite in the low-grade scheelite is not decomposed, which indicates that selective and efficient decomposition of scheelite is achieved by using this novel process.