Role of sodium carbonate in scheelite flotation - A multi-faceted reagent

Even though sodium carbonate is a reagent frequently used in flotation, its role is mostly described as a buffering pH modifier and a pulp dispersant. In the case of scheelite flotation, literature has shown that sodium carbonate improves flotation performance but little is said as to selectivity against gangue minerals. There is a consensus that sodium carbonate neutralizes calcium-bearing minerals through surface carbonation but this does not explain why it should depress said minerals in a specific order. Furthermore, the addition of depressants such as sodium silicate or quebracho could be triggering new mechanisms. Through batch flotation testwork on a skarn scheelite ore, single mineral flotation and contact angle measurements, this article aims at demonstrating that sodium carbonate is a multi-faceted reagent, which serves as a buffering pH modifier, a pulp dispersant precipitating calcium ions in suspension and a depressant for calcite and calcium-bearing silicates. Based on the kinetics, calcium surface site density, calcium activity and surface reactions, fluorite would be the first mineral to be depressed, then calcite, then silicates and finally scheelite and possibly apatite if sodium carbonate is over-dosed, likely independently from pH and definitely independently from the co-pH modifier. Sodium silicate acts mostly synergistically and partially antagonistically with other depressants, notably sodium silicate and queb-racho.