COMMENTARY — Treatment for refractory ore

Newmont Gold has been building a refractory ore treatment plant, or roaster, in conjunction with its existing milling facilities in Nevada. It is part of Newmont’s expansion, which includes deepening the Gold Quarry pit and developing two small open-pit mines, MAC and Tusc. There is also a new haulage road.

Work began in May, 1993, and as many as 1,000 construction workers have been involved with the project. The roaster facility is in the South Area of Newmont’s operation on the Carlin trend in Nevada.

The ore to be treated is refractory. Refractory ores are those that have not been naturally oxidized; they are encased in sulphides and may contain carbon. The roaster will dry-grind, crush and treat 8,000 tons of ore per day from Newmont’s Gold Quarry, Post and underground mines before the ore is processed by conventional cyanidation gold recovery circuits. The roaster prepares these ores for treatment using normal cyanide gold recovery processes. Half of Newmont’s 17.8 million oz. in reserves at Carlin are refractory. These ores have an average grade of 0.17 oz. per ton, nearly 10 times that of oxide ores. The expected recovery rate per ton for the ore is 88%, higher than that of any of Newmont’s other mills in the Carlin area. The roaster consists of a 3-stage crushing section followed by drying, grinding, ore separation and two parallel roasting sections. Crushed ore, containing about 9% moisture, is dried to 1% moisture and finely ground in a 2-compartment rotating mill. It is then transferred to feed bins for two circulating fluid bed roasting circuits, where temperatures reach 1,020 F, burning off the carbon and sulphur pyrites and allowing for conventional cyanide recovery. A 935-ton-per-day oxygen plant is providing the gas for the combustion reaction needed for the roasters. Gold Quarry is Newmont’s largest operating mine, with a peak production rate of 280,000 tons per day. The oxide and refractory ore zones combined contain roughly 208 million tons of reserves and resources.

While the capital costs per ton of capacity for a roaster are higher than those for an autoclave (the alternative technology), the roaster’s operating costs are expected to be lower.

— From a recent issue of the Nevada Mining Association’s “The Nevada Miner.”