Metallurgy & Processing
Historically, the recovery of gold from auro-stibnite (gold and antimony) ores has been relatively poor. The poor recoveries were primarily due to a lack of understanding of the metallurgical properties of auro-stibnite ores, in particular the flotation properties of stibnite and its re-agent consumption during the cyanide leaching process to extract gold.
However, these processing issues are now well understood and modern process flows produce very good recoveries of gold and antimony. In respect of Blue Spec Shear style ores, this is demonstrated by gold recovery rates from the most recent processing of ore from the Golden Spec deposit which ranged between 89-91% for gold .
Based on previous production from Golden Spec, Northwest expects recoveries from the treatment of Blue Spec Shear ores to exceed 90% for gold and 60% for antimony in concentrate (in line with average industry recovery rates for antimony).
Auro-stibnite ore can be treated with a conventional process flow comprising gravity gold recovery, flotation of antimony to a bulk concentrate and sequential leaching of gold and antimony.
Gold and antimony are well suited to gravity separation because of their relative densities. Effective gravimetric separation results in the coarsest-grained gold being removed from the circuit upstream of the subsequent cyanide leaching step (where coarse-grained gold requires a long retention time) .
After separation of the gold, antimony may be produced as either antimony concentrate (60% antimony) or antimony metal (>99% antimony). An additional step of electrowinning is required to produce antimony metal. Conventional electrowinning results in even higher recoveries of gold and antimony.
Northwest's Treatment Plant
Northwest has a proven treatment plant located at its Blue Spec Shear Project. The treatment plant was installed by Chase Minerals NL in 1988 at a cost of approximately $9 million to process ore from the Golden Spec deposit. The plant was specifically designed by Minproc to deal with auro-stibnite ore.
During operation, the plant achieved very good recoveries of gold and antimony with gold recovery ranging between 89-91%. The unit processes employed for gold and antimony recovery from ore include:
- two stage crushing
- conventional ball milling
- cyclone classification with gravity separation on the circulating load
- float cells
- carbon in pulp (CIP) adsorption
The plant is currently configured for a throughput of around 40,000 TPA - a capacity which reflected the fact that very high-grade ore was processed. However, the existing crushing units of the plant have a capacity of around 100,000 TPA. Accordingly, through the installation of additional tanks and ball mill, the plant's throughput can be increased to levels approaching capacity without the need for extensive redesigning. The tailings dam used during previous production can also be expanded to deal with increased plant capacity.
The plant operated for a period of around 6 months before Chase Minerals was forced into administration following the 1988 stock market crash. The plant was subsequently operated for another 15 months by Fiminston Mining between 1991-3.
This infrastructure will significantly reduce the capital expenditure required to bring high-grade gold deposits identified along the extent of the Blue Spec and Red Ribbon Shears into production in a shorter period than a greenfield project.
Page last updated: 09.08.2007
