Application of LAICPMS to trouble shooting mineral processing problems at the Getchell mine, Nevada

A study has been conducted on Au-bearing complex sulfide ore feed and autoclave tailings from the Getchell mine, Nevada. In this study products at different stages of an autoclave process were studied and identification of the source of low recovery in some Au-bearing sulfide ores located. All samples were studied to assess the occurrence of gold and to identify poor performance ores under the standard operating conditions of the autoclave mill.The results show that in the high recovery samples, gold occurs predominately as chemically included atoms within arsenian pyrite. This pyrite is exposed on crushing and oxidises almost completely within the autoclave. Typically recoveries are above 90% for such ore.Where poor recovery is observed, two modes of occurrence are observed in the feed that may prevent complete oxidation of the arsenian pyrite:
- Arsenian pyrite occurs as fine inclusions (<40 μm in size) within quartz and illite, even with fine grinding it appears from residues. Such pyrite is not exposed and thus does not get oxidised.
- Trace amounts of gold observed by LAICPMS with a mixed illite/smectite clay, montmorillionite and free carbon indicating potential for preg-robbing by clays (particularly the High exchange capacity smectite group clays) and carbon.To overcome these problems, the options include:
- Fine grinding would release the gold-bearing pyrite.
- Addition of an oxidant such as MnO may remove carbon; or
- Surface suppressant to depress surface cation exchange capacity on clays and carbon may remove problems of “preg-robbing”. The test sample in which MnO was added showed an improvement of >13% in gold recovery from a carbon-rich sample.