Pyrrhotite oxidation and its influence on alkaline amine flotation

In the production of ground calcium carbonate (GCC) for the paper industry, pyrrhotite dramatically reduces the GCC brightness and the removal of pyrrhotite through froth flotation is essential. The present study aims to study the effect of pyrrhotite oxidation on flotation recoveries during typical GCC flotation (i.e. alkaline pH, CaCO₃ saturation, amine collector). EDTA extraction and measurements of Eh (redox potential) showed a significant difference in pyrrhotite surface oxidation state when comparing exposure times of 40 and 60 min, the latter being significantly more oxidised. Microflotation results show that when pyrrhotite is exposed to air extending for more than 5 min at pH 8, the recoveries drop significantly. At higher pH, recoveries were generally low at all exposure times tested. Flotation recoveries showed strong correlation with zeta potential measurements. Bench scale flotation experiments on a sulphide bearing marble, confirmed that pyrrhotite oxidation significantly lower the GCC quality at low collector concentrations. By increasing the amine concentration, the flotation performance became independent of pH and exposure time.