In-situ study of the kinetics of phosphoric acid interaction with calcite and fluorapatite by Raman spectroscopy and flotation

Abstract

In the reverse anionic flotation of phosphate ores using phosphoric acid, it is often suggested that a selective surface passivation of the phosphate minerals by calcium phosphate occurs. Several calcium phosphate species have been proposed in the literature for this surface passivation. In this study, in-situ Raman spectroscopy was used to monitor as a function of time the interaction of phosphoric acid with calcite and fluorapatite at a fixed pH of 4.5 or at fixed phosphoric acid concentrations in order to identify what species have formed in conditions similar to those used in flotation. X-ray photoelectron spectroscopy was also used to confirm and identify more clearly the species formed at the minerals surface. Although a calcium phosphate (1:1) species was observed on both minerals, its presence on calcite was highly dependent on phosphoric acid concentration and pH as the latter controls both calcium dissolution and calcium phosphate precipitation. The results of mineral flotation have confirmed the importance of phosphate concentration in solution, originating from phosphoric acid addition and/or mineral dissolution, in the formation/precipitation of calcium phosphate which can turn on or off the flotation of calcite or fluorapatite, or both.