The effect of polytetrafluoroethylene on pressure oxidation of nickeliferous pyrrhotite

Nickeliferous pyrrhotite tailings, from Vale's Clarabelle Mill, were oxidized at 140 °C and 690 kPa (using compressed O2) in an electrolyte with an initial composition of 50 g/L H2SO4 and 2.25 g/L of Fe2(SO4)3. These conditions are similar to those employed in the Sherritt Zinc Pressure Leach Process that promotes the formation of elemental sulfur. The kinetics of oxidation was tracked by measuring the extraction of nickel to the electrolyte as a function of time. As expected, the pyrrhotite oxidation kinetics under these conditions was inhibited by the occlusion of the pyrrhotite particles by molten elemental sulfur formed during the leaching process. Addition of a commercially available granular form of PTFE increased the degree of leaching of nickel from the pyrrhotite, increasing the ultimate extraction of nickel at low and high impeller speeds from ca. 65% to over 90% after 90 min. This effect was attributed to the PTFE particles acting as a solid sorbent for the molten sulfur, which mitigated the effect of pyrrhotite occlusion and allowed oxidation to continue. The results of the use of PTFE on pyrrhotite oxidation kinetics are similar to those with addition of lignin sulfonate.

Research Highlights
► PTFE addition during pressure oxidation of nickeliferous pyrrhotite improves extraction of nickel.
► PTFE acts as a solid dispersant for molten sulphur during pressure oxidation of pyrrhotite tailings.
► PTFE performs as well as lignin sulfonate as a dispersant for sulphur during POX of pyrrhotite.