Tailings yield gold for operators

A pair of tailings retreatment projects continue to yield gold in the Kirkland Lake, Ont., area, while a similar project at Timmins, Ont., has been shut down and a fourth in the Northwest Territories has also been closed.

The Kirkland Lake projects are those of Eastmaque Gold Mines (TSE), in production since January, 1988, and of LAC Minerals (TSE), which was started up in July, 1989.

Eastmaque is a year-round operation and employs a fleet of three suction dredges to recover tailings from the bed of Kirkland Lake. The slurry produced by the dredges is pumped to the mill 2,000 ft. away. The dredges, of which only two are in use at any one time, are fitted with rotary cutter heads and are capable of digging to a depth of 45 ft. below surface. Mill capacity reached a maximum of 2,700 tons per day over peak periods in 1989 but average capacity is generally 200-400 tons less.

The final Eastmaque product is a sulphide flotation concentrate assaying 1-2 oz. gold per ton which is trucked 55 miles to the Noranda smelter. Tailings reserves quoted at the end of 1990 were 2.1 million tons grading 0.035 oz. The 1990 cash cost for producing an ounce of gold, including royalties, was US$334 per oz., up from US$272 in 1989.

Gold recoveries have ranged from a high of 67% to the current low of 43%. General manager Rolly Lemire noted that recoveries can fluctuate over a broad range depending on what part of the lake the tailings are taken from and from what elevation. They can also be affected by the proportion of fine slimes in the feed, by the presence of carbonaceous matter (decomposing vegetation) and by the presence of very fine gold tellurides which are difficult to float and tend to pass off with the tailings.

Gold production from startup in 1988 to the end of 1990 totals 44,805 oz. from 1.66 million dry tons of tailings, for an average recovered grade of 0.027 oz. Capital cost of the project was $7.4 million.

LAC’s tailings treatment is similar to Eastmaque’s in so far as tailings are dredged year-round. However, the output, at a nominal 750 tons per day (602 tons average in 1990), is much less; in addition, the tailings are processed by a dedicated plant at LAC’s nearby Macassa mill. There is no flotation pre-treatment and the whole of the tailings is ground to about 96% minus 44 microns in a parallel, tower mill-ball mill circuit. The tower mill is responsible for about one-third of the output. After grinding, the tailings are leached for 60 hours in their dedicated tanks, after which the pulp is combined with Macassa’s pulp flow and delivered to CIP precipitation. For 1990, 13,885 oz. gold were produced for a cash cost of US$245 per oz. Recovery was 71.8% from a feed grading 0.088 oz. Tailings remaining to be processed total 3.1 million tons grading 0.07 oz.

The tailings originated from the Lake Shore mine, a high-grade operation in its day.

Inspiration for the reprocessing of Canada’s gold mine tailings was the mammoth project of East Rand Gold and Uranium Corp. (ERGO), of South Africa’s Transvaal. ERGO produces about 375,000 oz. gold per annum from the throughput of 38 million tons, or more than 100,000 tons per day. (The plant also recovers 375,000 lb. uranium and more than half a million tons of pyrite annually for sulphuric acid manufacture.)

In the expectation of duplicating ERGO’s success, a Canadian counterpart, ERG Resources, part of the former Pamour group of companies, was organized to acquire and process 144 million tons of tailings grading 0.0128 oz. This material was in the Timmins area and there was the strong probability of increasing the available tonnage at a later date. About 200 million tons of tailings have accumulated at a number of locations in the Timmins area over the last 70 years.

Production started in August, 1988. The plant was shut down, according to plan, in November to avoid sub-zero temperatures and continued through a scheduled 9-month year in 1989. The operation did not start up the following year and was officially placed on a care-and-maintenance basis in April, 1990. Daily production rates at the planned level of 44,000 tons per day were found to be achievable but recoveries were far below the 45% level registered in test work.

The flow sheet was uncomplicated. A high pressure water gun reduced the compacted tailings to a slurry which was first screened of trash and then pumped to the flotation plant. A ratio of 1-to-10 sulphide concentrate was ground to an unusually fine 80% minus 11 microns to expose the finely divided metal particles and ensure an 80-85% cyanide recovery. Sixteen hours of cyanide leaching followed, then 24 hours carbon adsorption in the CIP circuit. Conventional carbon stripping, electrowinning and carbon reactivation followed.

Had ERG produced its forecast 112,000 oz. in 1989, the company would have paid back its $77 million investment in three years, it would have been launched on a production career of at least 17 years, employed a full-time payroll of 50-60 people and produced in excess of one million ounces of gold. The reality for ERG is quite different. It was rejected as one of Pamour’s assets (Pamour had a 61.6% ownership) when that company was absorbed by Royal Oak Mines (TSE) and it is now owned by a numbered Ontario company. A buyer is being sought for the company’s assets by Westpac, ERG’s Australian bankers. Despite ERG’s failure, the tailings of the Timmins region are a major gold resource; they were originally promoted as that by ERG several years ago and that assessment, according to people in the know, remains equally valid today. On the other side of the country, Royal Oak’s retreatment plant at Yellowknife, N.W.T., has been closed indefinitely. The project, operated formerly by Giant Yellowknife Mines (also part of the Pamour group), featured high-pressure water monitors which produced 8,000 tons tailings per day; the slurry was treated in a straight forward CIL plant. There was neither regrinding nor flotation processing in the circuit. The plant was closed down at the end of its third, 6-month operating season in October, 1990.