The effect of sodium hydrosulfide on molybdenite flotation in seawater and diluted seawater

Abstract

Seawater is used in copper-molybdenum flotation plants when fresh water resources are limited. To minimize the negative effect of process seawater on molybdenite flotation, diluting process seawater with fresh water is a common practice. This study was designed to identify whether the dilution of process seawater in copper-molybdenum flotation plants benefits molybdenite flotation where NaHS is added to depress chalcopyrite. Seawater was prepared based on the concentrations of salts in the process seawater used in a copper-molybdenum flotation plant, and diluted with different proportions of de-ionized water. The concentrations of NaHS chosen to test in molybdenite flotation were determined by conducting chalcopyrite flotation to depress chalcopyrite. It was found that the concentration of NaHS which maximumly depressed chalcopyrite also depressed molybdenite to some extent in both seawater and diluted seawater. Water chemical equilibrium calculation, Cryo-XPS analysis and SEM measurement indicated that the depression mechanisms of NaHS on molybdenite were different in seawater and diluted seawater. Significant dilution of seawater weakened the buffer effect of seawater on pH and raised the pH in flotation significantly after the addition of NaHS so that Mg(OH)2 and CaCO3 precipitates formed to depress molybdenite. Insignificant dilution of seawater had little effect on flotation pH after the addition of NaHS, but the adsorption of HS− and the formation of SO42− on micro-edges of basal plane surfaces of molybdenite at a high NaHS concentration decreased the hydrophobicity of the basal plane surfaces, leading to reduced molybdenite flotation recovery.