EXPLORATION ’93 — Western Copper-Thermal pilot operation

Joint-venture partners Western Copper Holdings (TSE) and Thermal Exploration (ASE) produced about 800 lb. of copper cathode during the first month of their pilot-plant project, near this northern community.

The bulk metallurgical test, to run from October through December, is designed to simulate actual production from the Carmacks copper deposit, about 25 miles from Carmacks and 100 miles north of Whitehorse. What is needed now is a good blast of Yukon winter to determine the effects of leaching in cold weather. It is also necessary to work out final plans for full-scale development using solvent extraction-electrowinning (SX-EW). “We’re here to demonstrate we can leach year-round,” said Robert Quartermain, president of Western Copper. “So far, the test is showing that heat generation is taking place within the heap, and we are producing high-purity copper cathode without technical difficulty. The work we’re doing will be an integral part of our feasibility study and permitting.”

Western Copper and Thermal each have a half interest in the project, formerly known as Williams Creek. Results of the bulk leach test and completion of the feasibility study (by Kilborn Engineering) are both expected by year-end. If these prove positive, production could begin in early 1995, although the joint venture makes it clear this would be predicated on a copper price that would provide an acceptable rate of return. Mining would be seasonal (8-10 months), but plans call for year-round leaching and processing. “This project has the potential to be Canada’s first stand-alone SX-EW copper mine,” Quartermain said, pointing out that the technology has been used successfully since the early 1960s.

In the past decade, copper recovered by SX-EW has doubled. It now accounts for 14% of refined output in the Western World. The process offers lower capital costs, and lower operating costs — often 50% less than conventionally produced copper because no milling, smelting or refining is required. In addition, the closed-circuit process is considered environmentally benign.

But this type of project would be new to the Yukon, which prompted Quartermain and Thermal President Dale Corman to provide a tour of the pilot plant to local politicians, native groups and The Northern Miner. The Yukon government is keenly interested in the project, having granted $100,000 under the Canada-Yukon Economic Development Agreement. The grant helped fund the leach test, estimated to cost $500,000. About half the small work-force needed to run the operation was hired from the local native group. The test program is being conducted by Lakefield Research, based on metallurgical work supervised by Vancouver-based Beattie Consulting and Texas-based Brown & Root Braun. Technical assistance is also being provided by Teck (TSE), the major shareholder of Western Copper with a 42% interest. Information from the test will be used to prepare operating estimates for mining and crushing costs, acid consumption, heat generation within the heap, and copper recovery and production.

Work began with the extraction of a 300-ton bulk sample grading 1.36% copper from three trenches crossing the deposit. This sample was taken from the No. 1 zone which hosts an open-pit, undiluted reserve of 12.5 million tons grading 1.15% copper and 0.015 oz. gold per ton (proposals for gold recovery may be developed at a later date).

The oxide reserve is contained within the zone’s global reserve of 22 million tons grading 1.06% copper and 0.013 oz. gold. As many as 13 additional mineralized zones are known to occur on the property, several of which contain copper mineralization similar to the No. 1 zone. There is almost no overburden, and the work index is expected to be low because there is no quartz in the rock. Bornite and chalcopyrite constitute the primary copper mineralization, which has been weathered to oxidized equivalents. The 300-ton sample was crushed to three-quarters of an inch and placed within a 20-ft.-high and 16-ft.-wide crib built within a gravel pit just outside Carmacks. A polyvinyl chloride liner and a secondary liner were installed underneath the plant, and all drainage settled into a nearby, lined pond. Project engineer Kenneth McNaugton explained that ore was placed into the crib in 4-ft. lifts (six lifts per column), with 50 temperature probes well-distributed within the crib’s R-40 insulated walls. The drip-emitters were buried 3 ft. below surface. When loaded, the temperature of the crushed material rose from just above freezing to 35C, which was due to the chemical reaction between the ore and the sulphuric acid leach-solution. The solution is not being heated, although this option has not been ruled out. “The current temperature is 20C within the pile,” McNaugton said. “We are not seeing any degradation of our temperature profile, and everything is going beyond our expectations.”

The outside temperature had dipped to as low as minus 12C at the time of our visit (Oct. 19), but the partners are anxious to see how the pilot plant will operate under even colder conditions. They say the total leach cycle will take 90-120 days under current conditions, with 50% extraction in the first 20 days.

After the crushed ore is treated with strong sulphuric acid, the stack is continuously washed until all the recoverable copper has been dissolved (metallurgical work indicates recoveries of 80-85%). The pregnant leach solution containing 5 grams copper per litre is collected in the bottom of the leach pad and transferred to the SX-EW plant. By a process using an organic reagent and gravitational separation, it is turned into a solution containing 50-60 grams of pure copper sulfate per litre.

The solution is then transferred to the electrowinning plant, where highly pure (99.99%) copper cathode sheets are plated out by simple electrolysis. The entire process is fascinating to watch, and not too uncomfortable thanks to a mist-suppression system.

Quartermain said a full-scale operation would consume about 150 tons of sulphuric acid per day. The partners are looking at building a plant on the site to produce their own acid, using sulphur trucked in from Alberta. “This looks as if it is the most attractive alternative,” Quartermain said. “But we are also looking at bringing in acid, in concentrated form, from Skagway.”

An on-site acid plant would cost about $11 million to build, and this item is already included in the project’s capital costs, estimated at $46 million. Energy options include diesel, or the more favored plan of bringing in a power line to the mine property where the full-scale SX-EW plant would be situated.

A preliminary economic analysis indicates copper can be produced for an initial operating cost of 65 cents per lb. An average annual production rate of 28-30 million lb. of cathode copper is projected, over a minimum 9-year period. An operation of this size would provide about 100 jobs. The partners are completing the terms of an economic agreement with the Little Salmon Carmacks First Nation. And the Yukon government is re-surveying a road up to the Carmacks Copper turnoff, before reconstruction slated for next year.

Yukon Economic Development Minister John Devries said the government supports responsible mine development in the territory. “Our mandate is to create wealth and opportunities in the Yukon, and we would like to see this project go into production,” he said.

The government’s pro-mining stance does take into account the environmental aspects of any mine proposal. To that end, the joint venture has been collecting baseline data for the past two years and no problems are anticipated. The mine proposal incorporates measures meant to ensure there will be no solutions or metal discharged into the environment, particularly the Yukon River, a salmon habitat about five miles from the proposed mine site. These types of concerns are further reduced by the arid nature of the region.