Reductive leaching of stibnite (Sb2S3) flotation concentrate using metallic iron in a hydrochloric acid medium I: Thermodynamics

The reductive decomposition of a stibnite (Sb2S3) flotation concentrate using elemental iron was investigated in hydrochloric acid. The leaching process which proceeded via both reductive and non-oxidative chemical decomposition reactions achieved desulphurisation in excess of 95% to produce elemental and dissolved antimony species, ferrous ions and hydrogen sulphide gas. A complexing-hydrolysis mechanism was used to describe the non-oxidative chemical dissolution of stibnite in hydrochloric acid. The parameters pH and iron-to-stibnite flotation concentrate ratio exhibited strong interactions such that an increase or decrease in either parameter without a corresponding adjustment to the other negatively or positively affected the process efficiency. This was principally due to the hydrogen evolution side-reaction that was strongly dependent on both parameters. Stibnite decomposition was maximised at pH around 0.44 and iron-to-concentrate ratio of 0.7. Subsequent cyanidation of the reductive leach residue resulted in only 27% of the gold extraction compared to 11% obtained by direct cyanidation of the flotation concentrate. However, an oxidative leach of the same residue followed by cyanidation achieved gold recoveries in excess of 95%, demonstrating perhaps that the sulphide matrix is not solely responsible for the refractoriness inherent in stibnite ores. While the reductive leach pre-treatment of stibnite appears to be a less efficient process with respect to the liberation and subsequent recovery of the precious metal, gold, it offers technologically superior aspects when coupled with a subsequent oxidative leach step as the former eliminates problems associated with elemental sulphur formation.