Mechanistic study on the depression of calcite by sodium hexametaphosphate in sodium oleate system

Abstract

Sodium hexametaphosphate (NaHMP), a common depressant for calcium-containing minerals, is widely used in flotation practice. In this work, the depression mechanism of NaHMP on calcite was thoroughly investigated by using experiments and DFT calculations. Accordingly, the micro-flotation results revealed that NaHMP possessed noticeable depression effect on calcite, with the calcite recovery decreasing from 94.21% without NaHMP to 17.29% with NaHMP concentration of 1 mg/L at pH 8. The surface Zeta potential suggested that NaHMP could be strongly adsorbed on calcite, while no absorption band of hexametaphosphate was detected in the FTIR spectra of calcite treated by NaHMP, but a remarkable weakening of the absorption peak of sodium oleate (NaOL) was detected in the presence of NaHMP, which was also reflected in the XPS spectra. In addition, the significant chemical shifts in the binding energies of Ca 2p and P 2p electrons observed from the XPS high-resolution spectra suggested that the hexametaphosphate groups (HMP) were chemisorbed on calcite. DFT calculations further revealed that the sorption of HMP on calcite was more preferable than that of oleate ion (OL) in thermodynamics, which explained the beneficiation practice that NaOL was difficult to float calcite in the presence of NaHMP.