Getchell Mine Development & New Mill

Fluor Daniel provided process design, feasibility study, detailed design, procurement, construction management and startup assistance for the development of an open pit mine and construction of a new mill utilizing autoclave technology to process 3,000 tons per day of refractory gold.

Three pressure oxidation autoclaves were installed, each fed by a positive displacement diaphragm pump. This was the first commercial use of an oxygen scrub/recycle system. For treating refractory gold ore in pressure oxidation autoclaves, the process includes semi-autogenous grinding, autoclave oxidation and carbon-in-leach recovery of gold. Although flotation, bioleaching and several other oxidation processes were examined, pressure oxidation of the ore was chosen as the most economical method to recover the gold.

Client's Challenge

Originally prospected in 1933, the known gold deposits within the Getchell Trend are Carlin-type, sediment-hosted, replacement deposits containing micron gold. The Getchell deposit is a complex sulfide ore containing substantial amounts of orpiment and realgar. Gold is finely disseminated in a pyrite matrix which must be oxidized before the gold can be recovered.

After acquiring the property in 1983, FirstMiss Gold carried out extensive drilling programs to expand the ore reserves and investigated several alternative metallurgical processes.

Fluor's Solution

Fluor was commissioned in August 1984 to undertake a technical and economic study of the project. This led to further mine planning and metallurgical investigations, which culminated in a preliminary feasibility study in June 1985 and a final feasibility study in August 1987. Fluor updated the database, re-estimated the ore reserves independently, and modified and optimized the open pit designs. Mining sequences, schedules and mining costs were re-estimated and incorporated into the final feasibility study.

Fluor continued to provide mining and computer services including the transfer, plotting and storage of Getchell sampling data for operations. Fluor supervised the grinding test-work program, selected samples, interpreted the results and recommended grinding circuit design and equipment sizes and subsequently completed detail design for the mine, mill and service facilities in late 1988.

Conclusion

The mill was completed and production resumed in 1989, combining a traditional cyanide leach circuit with pressure oxidation. Sulfide ore was run through the first pressure oxidation autoclave in April 1989, followed by the start-up of the other two autoclaves by June. Overall gold recovery exceeded 89%.