All underground mines in Jerritt Canyon are less than 1000 feet in depth. Low profile 35 and 40-ton tractors are used to remove ore, which is then stacked near the mine portals for grade sampling. After sampling, ore is transported by 150-ton haul trucks to the mills, where the feed is dried and roasted. Gold is recovered using conventional carbon-in-leach processing. High quality gold bars are then shipped for commercial refining and sale.
The mineral processing operation at Jerritt Canyon is one of only three processing plants in Nevada that uses roasting in its treatment of refractory ores. Initially, Jerritt Canyon was designed to process oxide and mildly refractory gold ores. In 1989, the roasting circuit was added to the process for the treatment of highly refractory ores which are now being mined and processed at Jerritt Canyon.
The milling process begins with crushing run of mine (ROM) ore to minus five inches in a 42 inch x 48 inch jaw crusher. The crushed ore is then conveyed to a gas-fired drier where the crushed material is dried to about 1% moisture. The dried ore is then conveyed to the secondary crushing circuit, which consists of a single 4.25 foot Symons standard cone crusher operated in open circuit.
The crushed ore is then advanced to the tertiary
crushing circuit, which consists of two Symons 4.25 short head cone crushers operated in closed circuit with vibrating screens to produce a minus 3/8 inch product, which is then conveyed to a 2,000 ton fine ore bin that feeds the dry grinding circuit. The dry grinding circuit consists of a 18.5 foot diameter x 14.5 foot long Fuller grate-discharge ball mill driven by a 2,500 hp motor and operated in closed circuit with a Fuller O’Sepa air classier to produce a final ground product sized at about 80% passing 105 microns, which is stored in a 2,000 ton roaster feed bin.
The ore roasting circuit at Jerritt Canyon consists of two Dorr-Oliver two-stage fluid-bed roasters, each rated at 2,000 tpd. Ore is withdrawn from the feed bin by a variable speed screw feeder and lifted with a bucket elevator to the roaster feed system and dropped into the roaster. The roasting process almost completely destroys all of the refractory constituents which makes cyanidation more effective.
The fluid-bed roasting system has no moving parts and can generally be operated at high availability. The use of oxygen as opposed to air as the fluidizing-combustion gas reduces the size of scrubber and achieves high conversions of sulfides and carbon at lower combustion temperatures than would be required by traditional air roasting.
After the ore exits the roaster it drops into quench tanks. The solution is then pumped to a thickener prior to advancing to the CIL cyanidation circuit, where the gold is leached and recovered using procedures standard to the industry.