GIBRALTAR

Gibraltar’s mine and mill were officially opened in mid-1972. The plant immediately went into overdrive, beating the concentrator’s nameplate capacity of 30,000 tons per day (today the nominal capacity is about 40,000 tons) by more than 20% in that first year. This still can happen today, says Mine Manager Art Brown. It depends, in the main, on ore hardness as measured by the ore work index. At a work index of 12, the mill can process 42,000 tons per day. “Sometimes we barely have to grind the ore,” Brown says. Pump capacity in the “bottom end” of the concentrator — the tailings pumps — limits maximum throughput to 42,000 tons per day.

The concentrator is fairly standard. Grinding consists of three circuits each with an Allis-Chalmers rod mill and a ball mill. The ground product reports to a standard cell flotation circuit or rougher flotation. This produces a 10%-12% concentrate that is reground and fed through a column flotation circuit for upgrading to 29%. After the material is thickened and dewatered, B.C. Rail transports the concentrates to Vancouver for transhipment and delivery to the Orient.

Copper production from the low- grade piles (which run from 0.141% to 0.153% copper) begins with a bacteriological leach by ferro-oxidans. These bacteria occur naturally in sulphide mine water and flourish under conditions of low ph and moderate temperatures. A nominal flow of 3,000 gal of acidified water adjusted to a ph of 2.3 (for optimum “bug” growth) over the low-grade piles dissolves copper and iron sulphates. The copper is removed from the leach solution by solvent extraction. The extraction plant consists of conventional mixer/settler units. The reagent is a liquid ion exchange material from a class of chemicals known as aldoximes. The organic phase in solvent extraction consists of 6% aldoximes dissolved in high-grade kerosene. The copper transfers to the aldoximes, which, with the kerosene, floats to the top in mixer-settlers.

From this second phase (the first was the aqueous phase through the dump), the now-loaded organic is stripped, and the copper is recovered in a 32-cell electrowinning process. The copper is electrowon on stainless steel blanks. “As you can see, we have three interlinking loops,” says Mill Superintendent Dean Lindsay. “No. 1 is the aqueous phase through the dump, No. 2 is the organic phase and No. 3 the electrolyte and electrowinning. When you consider that we get the copper from out of a dump, it’s pretty good.”

The more conventional portion of the Gibraltar complex begins with open pit mining. Gibraltar draws ore from three pits. The pits yield harder ore now, in the neighborhood of a 14-to-16 work index, from the Granite Lake and Pollyanna pits. So daily throughput will fall to the lower end of the average production rate this year and next. Pollyana is the stripping pit and Granite Lake the ore pit. “Generally we design a pit which will supply the mill with ore for two years. Then we’ll pioneer (begin stripping) another pit for another 2-year ore supply,” says Mine Superintendent Doug Bailey. He added that each pit goes through several stages of development and mining, although the initial stage usually extracts the richest ore. “We could mine it all out in one stage,” he explains, “but we have to maximize the economics of the mine. By taking the richest ore first, we get early cash flow.” The third pit, Gibraltar East, will be stripped in 1991 for mining in 1992. This is the third stage (or push-back) for the Gibraltar pit.

The pits are drilled off with Bucyrus Erie 55R (12 1/4-inch) and 45R (9 7/8-inch) rotary drills. The mine also uses a Marion M4 (12 1/4-inch) drill. The benches are 45 ft high and Bailey says the anfo and slurry supplied by ici Explosives (c-i-l) give good fragmentation. P & H 2100 (15 cu yd) and P & H 2300 (25 cu yd) shovels load the 100- and 170-ton Unit Rig haul trucks. The bulldozers in the pit include D8N track-type Caterpillar tractors and four 824 rubber-tired Caterpillar dozers for shovel cleanup. An Allis-Chalmers 45x 74-inch crusher is near the mill, while another crusher sits near the rim of Gibraltar East pit. Eventually the pit-rim crusher will be moved to the bottom of the pit when mining begins in Granite Lake in 1996. To control groundwater, a system of perimeter wells has been drilled. Flygt pumps operating at a total rate of 350 gal per min work year-round to keep the pit floors dry.

Ever since its inception nearly two decades ago, Gibraltar has had severe ups and downs. During the troughs in the copper price cycle, Gibraltar usually runs in the red. “We’re very, very sensitive to copper prices,” says Brown. “We’re probably the lowest- grade profitable copper producer in the world.” Current reserves stand at 198 million tons grading 0.315% copper and 0.009% molybdenum. The cut-off grade is 0.2%. The average millhead grade last year was 0.32%. Millfeed was 6 million tons to produce 30 million lb of copper in concentrate. The strip ratio is 1:1. Production costs per ton of ore mined have averaged 51 cents over the past three years.

Over the years, recurring labor disputes have also worked against consistent performance. Says Brown: “We do have a good workforce, but the union has made life difficult for us.” Employees are represented by the Canadian Association of Industrial, Mechanical and Allied Workers.

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