Depressants in scheelite flotation - Mechanism of sodium carbonate and acidified water glass and the application of process mineralogy

The European Union classified tungsten as a Critical Raw Material already in 2011, due to its high economic importance and high supply risk. Tungsten occurs under two main mineral forms, scheelite (CaWO4) and wolframite ((Fe,Mn)WO4), with scheelite’s importance increasing as wolframite resources are progressively depleting. Interest in scheelite is growing fast, as publications show: 15 % of all publications on scheelite flotation since the 1950s were published in 2019. A polar salt type mineral, scheelite is semi-soluble and exhibits a negative charge, almost regardless of the flotation conditions. It is mostly hydrophilic but can easily be floated using chemical reagents, usually at a high pH of 9 to 10. Scheelite flotation has run into serious difficulties when it is associated to a carbonaceous gangue. Other calcium-bearing minerals, such as calcite (CaCO3), apatite (Ca-phosphate) and fluorite (CaF2) all exhibit similar flotation properties and are therefore classified as semi-soluble salt-type minerals. These minerals will tend to float better than scheelite under the same circumstances and not only increase reagent consumption but heavily contaminate the concentrate, making it harder and more expensive to process for the smelter. Several depressants can be used to remedy this problem, the most used one being sodium silicate. However, this reagent is imperfect and its effect can be improved by modifying it or by combining it with other depressants. As a consequence, the focus of this work is to understand the mechanism of two important depressants in scheelite flotation, sodium carbonate and acidified sodium silicate, and linking said mechanism to mineralogy. A third depressant, colloidal silica, is studied from a performance point of view.