Recent Progress in Milling and Gold Extraction At Giant Yellowknife Gold Mines Limited

The gold at Giant Yellowknife is intimately associated with a complex mixture of sulphides, including pyrite, arsenopyrite, stibnite, and a wide variety of antimony sulphosalts. Direct cyanidation has never been feasible. Recovery is a lmost entirely dependent upon flotation, roasting of flotation concentrate, and cyanidation of c~lcine. In November 1958, an Edwardstype roaster and an early prototype, two stage Fluosolids roaster, were replaced by a large two-stage F luosolids roaster of improved design. The new roaster was started at "safe" medium temperatures, but early in 1959, temperatures in both stages were gradually and deliberately lowered to determine whether continuous autogenous roasting could be maintained under exacting low temperature conditions necessary for good extraction. Operation is smooth and continuous with fir st and secona stage temperatures controlled at 925° and 875°F respectively. Plant cyanide extraction compares very favourably with extraction from low temperature laboratory roasts. Average recovery by roasting and calcine cyanidation for July to October 1958 was only 58 per cent. Mill tonnage was only 750 tons per day. For July to December 1959 recovery, on the same basis, was 78.4 per cent and mill tonnage averaged 1,030 tons per day. Grinding and flotation techniques are vitally important. Concentrates must be as coarse and granular as possible to maintain bed fluidity. Fine grinding once practised is scrupulously avoided. Equally good flotation recovery at 60 per cent minus 200 mesh is obtained with a more middling concentrate at slightly lower ratio of concentration. Unit cells recently installed in the grinding circuit have improved roaster stability -and reduced dust losses noticeably. Low temperature roasting brought more cyanicides to the calcine circuit. A two stage wahing circuit, including a preliminary fresh water grind, had to be improvised quickly. Washing is now adequate. Initial extraction rates are very high and spe- cial measures for quick removal of high grade solution from solids are taken to avoid reprecipitation of gold, known to occur at times, particularly in thickeners. Plant modifications for operation at the increased tonnage are still incomplete. Precipitation capacity is being increased so t hat lower grade solutions can be precipitated and more solution circulated. Other modifications are pending. This paper should be regarded purely as a Progress Report.
Keywords: antimony, arsenic, ball mill, Lower, stibnite, Calcine, Concentrate, Concentrates, flotation, Gold, Roasters, Roasting, Temperature
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