Investigation of the depressants involved in the selective flotation of scheelite from apatite, fluorite, and calcium silicates: Focus on the sodium silicate/sodium carbonate system

Flotation tests were performed on a skarn ore to attain the selective separation of scheelite (CaWO4) from fluorite, apatite, and silicates. Fluorite and apatite constitute common gangue minerals in scheelite ores. Due to their similar surface properties and good floatability, the separation of scheelite from fluorite is particularly difficult. These ores are usually processed at basic pH using sodium silicate as a gangue mineral depressant and a fatty acid as the collector. Sodium silicate was not efficient enough to depress the fluorite in the studied ore. Thus, different depressants were tested in the flotation of scheelite using a commercial mixture of fatty acids. The effect of adding metallic salts [FeSO4, Al(NO3)3, and Zn(SO4)] prior to sodium silicate was therefore investigated. The sole use of organic molecules (starch, lignin sulfonate, tannin, carboxymethyl cellulose, and citric acid), instead of sodium silicate, was also studied. This led to a global depression of the minerals, including scheelite, and did not improve the selectivity of the separation. The addition of metallic cations did not significantly enhance the selectivity of the flotation; however, iron sulphate slightly increased the sodium silicate performance. Overall, among all the studied depressants, the combination of sodium carbonate and sodium silicate presented the best efficiency. This system was then studied through the design of experiments methodology. Strong synergistic effects existed between the two reagents, mainly impacting the scheelite and fluorite recovery as well as the scheelite grade. This was attributed to surface carbonation of the minerals, subsequently leading to a better depression by sodium silicate. The effect of sodium carbonate on the apatite and silicates was small. This was attributed to spontaneous carbonation and the presence of complex crystallographic structures, respectively. The efficiency of the sodium carbonate/sodium silicate combination was limited (maximum WO3 enrichment ratio = 16.48) due to the lack of selectivity of the depressants. The proposed optimization of the flotation separation of scheelite from gangue minerals maximizes both the WO3 grade and recovery.