The effect of grinding chemistry on cyanide leaching of gold in the presence of pyrrhotite

- The galvanic interactions between grinding media and pyrrhotite was studied.
- 30% chromium media behaved more like ceramic media.
- 21% chromium media behaved more like forged steel.
- A correlation between galvanic current and grinding media oxidation was described.
- Cyanide-based leaching can serve as a diagnostic for grinding-media-ore interactions.

In this paper, the role of grinding media type on cyanide-based gold leaching in the presence of pyrrhotite was investigated. Pyrrhotite commonly occurs in many gold ores together with pyrite and/or arsenopyrite. A synthetic ore was prepared by mixing pyrrhotite with gravity gold concentrate and quartz. Pyrrhotite is of interest to grinding and leaching chemistry because this sulfide mineral is the highest cyanide- and oxygen-consuming iron sulfide and, contrary to pyrite, tends to behave as a reactive anode. The results showed that when the grinding of the ore was performed using the ceramic media, the gold extraction process was up to three times more efficient than that when the forged steel media was used. The reason is that the galvanic interactions between the forged steel media and pyrrhotite resulted in the formation of iron hydroxide; the formed iron hydroxide converted free cyanide to ferrocyanide that cannot dissolve gold (i.e. 75% of free cyanide was converted to ferrocyanide). Additionally, the galvanic interactions also significantly reduce the dissolved oxygen content and thus cyanide gold leaching is more difficult. In contrast, when grinding was performed with the ceramic or high chromium (30%Cr) media, an insignificant amount of free cyanide was decomposed (i.e. 1% of free cyanide was converted to ferrocyanide while 2.5% of that was converted to thiocyanate). For that reason, cyanide gold leaching is very efficient when ceramic or high chromium media was used during the grinding of the ore. It can be expected that the leach behaviour would be different if leaching is preceded by pre-oxidation, as is often done for pyrrhotite bearing ores. In addition, a theoretical model was developed to describe the correlation between the galvanic current and the grinding media oxidation, as measured by using EDTA extraction technique and dissolved sulfur analysis by HPLC during grinding. The electrochemical data were correlated with the amount of the grinding media oxidation during grinding with different media type. A linear relationship between the galvanic current and grinding media oxidation was obtained. It was found that the higher the galvanic current, the higher the amount of iron hydroxide was produced during grinding. Bench scale cyanide leaching can therefore serve as a useful diagnostic to determine the extent of galvanic interactions between media and minerals in grinding mills (similar to bench scale flotation which has also been used as diagnostic tool).