Characterizing the leaching of sperrylite (PtAs2) in cyanide-based solutions

The mineral sperrylite (PtAs2), unlike other Pt minerals, has shown to be resistant to cyanide leaching, even at elevated temperatures of 50 °C. This has prevented further development of a two-stage heap leaching process, of which the second stage is cyanide leaching for a Platreef ore, due to a considerable portion of the Pt being present as sperrylite.

Through a comprehensive set of leaching tests, this study has shown that sperrylite leaches slowly in cyanide due to eventual passivation. The addition of ferricyanide to the cyanide solution results in significantly more Pt being leached (up to 16 times) than with cyanide only under identical process conditions. However, over time passivation occurred in this system also, but can be seemingly be overcome by filtering, rinsing and re-suspending the sperrylite. This was further investigated using a reactor system continuously fed with a ferricyanide-cyanide to leach a fixed bed of sperrylite mineral. Again, gradual passivation of the mineral phase was observed, which was fully reversible following a day of feeding with distilled water.

XPS analysis of fresh and leached mineral indicated a depletion of As on the mineral surface, which tallied with an observed preferential leaching of As during leaching, at least initially. As a result, it is postulated that a less cyanide-soluble compound of the form PtAs(X) forms, where x is < 2. The ease with which the leaching process can be restarted after rinsing the mineral indicates that the adsorption of solution species is the key step in forming a passivating layer. This adsorption is potentially favoured by the gradual surface transformation.

Finally, an electrochemical study confirmed that the ferricyanide-cyanide system facilitated an electron transfer reaction at the sperrylite surface with the likely oxidation of As and reduction of the ferricyanide. Gradual passivation of the surface was also observed in this system.