Gold miners in the nation’s biggest gold camp might be mining some pretty lean ore over the next 2-3 years. Instead of mining ore that averages no less than 0.065 oz gold per ton, as they do today, miners in the Timmins camp will likely be able to make a buck taking mineralization grading as little as 0.04 oz gold per ton. This significant drop in mining cut-off grades, which will likely be unique to this camp, will develop thanks to a new mill being built by ERG Resources. It will be located just north of Pearl Lake and employ up to 70 people full-time for the next 20 years. ERG, which is controlled by Pam our Inc., has decided to put up a hefty $65 million for the new mill and other facilities. The mill will be custom- designed by Kilborn Engineering to treat large volumes of low-grade gold tailings averaging 0.013 oz gold per ton at a very low unit cost. Once in operation using the tailings, Pamour will be able to add sub-economic gold mineralization from underground and open pit mines to the mill feed, giving the new ore what ERG’s president and chief executive officer, Ronald Sweetin, calls a “joy ride through a very low-cost mill.” It will still cost the same to mine the rock and bring it to surface, but low milling costs should turn it into ore.
Massive tailings deposits, accumulated over 75 years of mining in the area, have enabled ERG to justify the expenditure. A knowledge of how mine managers tended to run mines in the past was one of the interesting elements in the decision to build the mill. Although they would never admit it, some mine managers used to downplay tonnage figures for the sake of showing high ore grades and mill recoveries in their reports. With that in mind, Pamour director Dennis MacLeod, who was then a senior executive with East Rand Gold and Uranium, a company that retreats gold tailings in South Africa, recognized the value of the 162 million tonnes of mill tailings sitting on surface in and around Timmins — tailings which, according to historical reports, were not supposed to contain much gold. In fact many local residents saw them only as an environmental problem.
Two years of drilling by consultants C. Von Hessert and Associates on a 200-ft grid and, in some cases, on a 60-ft grid convinced ERG that the deposits contained significant quantities of gold. Three hundred drill holes, ranging in depth from 2-34 m, also showed that the deposits are fairly homogeneous in terms of grade and size distribution. The results showed there are some two million ounces of gold in 27 seperate tailings areas in the Timmins area. Within a 6-km radius of the ERG mill site are 22 areas with 1 million ounces of recoverable gold. “Not a bad little prize,” Sweetin says. These results convinced the company to go ahead with the project. The bulk of the material — 140 million tonnes — is controlled by Pamour with Dome controlling the other 22 million tonnes. These figures translate into grades averaging 0.013 oz gold per ton with some sweeter pockets containing actual mill concentrates grading as high as 0.1 oz gold per ton. For the most part the deposits consist of fine-grained tailings which average about 25 microns in size. They are deposited rather consistently over a relatively small geographic area in large, beach-like deposits. About 10% of the total tonnage was deposited in Gillies Lake; the rest in specially designed impoundments covering large, sometimes cleared areas of land. Tailings were deposited on land within berms constructed to retain the slurry. Coarse particles, which tend to drop out of the tailings stream first, were used to build up the berm while finer particles, which tend to flow further, fanned out into the depression between the berms. The result is a beach-like deposit.
The fact that there is so much of this material in such a small geographic area will enable ERG to build a mill that can handle a million tonnes of tailings per month. Construction has started and production is slated for late next year. Site preparation was underway at the time of this writing and foundations for the mill were scheduled to be poured early in August. The construction project is expected to employ 220 people.
In terms of today’s technology, milling the old tailings will be entirely conventional. Overall recoveries are conservatively estimated to range from 30-72% depending on the grade of the area being mined. Average recoveries will be 46%. A pyrite concentrate from a flotation circuit will provide 100,000 tonnes of feed to: a ball mill re-grind circuit; a conventional cyanid e leaching circuit; and a carbon-in-pulp elution, stripping and electrowinning circuit. Extensive metallurgical studies by Lakefield Research, in Peterborough, Ont., have determined an optimum mass pull of 10-12% for initial flotation and an optimum 8-12-micron size for economic regrinding in one of two 2,225-kW ball mills.
The flotation circuit will produce about 900,000 tonnes of tailings. This very low grade material (probably about 0.006 oz gold per ton) will end up back in a new tailings area to be created by ERG to the northeast of the mill. It is estimated to cost $12 million to construct. Its location has been selected to minimize environmental problems associated with the disposal of gold mine tailings containing pyrite and traces of residual cyanide. Exposed to oxygen, pyrite in the tailings oxidizes slowly over a number of years and undergoes hydrolysis to create weak acid run-off, usually with a ph of 4.5-5.0. Low ph redisolves a host of cationic metallic contaminants in solution, together with sulphates and other dissolved anions. Once mobilized, these substances can get into waterways upsetting the food chain of aquatic life. Most of the tailings areas in Timmins provide little protection for the environment. Not only will ERG clean up an environmental problem in Timmins by moving old tailings to a safer area; it will leave opportunities for future companies. “We or someone else will probably come along, probably in as little as 20 years’ to recover even more gold from our tailings,” Sweetin says.
EGR plans to recover 38,000 oz of the yellow metal in 1988 and an average of 82,000 oz per year in the following three full years of operation. While milling will be relatively conventional, getting the old tailings to the mill will take some pretty novel technology — novel for this country anyway. About $8 million will be spent on captial equipment to reclaim the tailings. High-pressure (300 lb per sq in) water jets, delivered by machines called water-monitors and designed to nullify the kinetic energy of the high-pressure water, will be fired at the tailings deposits from 30-40 m away. While these units are not manufactured in Canada, ERG plans to push for local manufacture to maximize Canadian content.
The fine-grained material will be undercut by the water jets and the tailings-laden slurry will flow to a sump where it can be pumped to the mill. A backhoe will be used to dig trenches around each tailings area to direct the flow of tailings slurry to a central sump. Two stages of screening, with the smallest opening about 8 mm, will remove most of the “crap” that gets thrown into tailings areas over the years (old 45-gallon drums, timber, logs, moose antlers and tires, for example) and all the organic material which can foul gold milling circuits. Conventional centrifugal pumps will pump the slurry down grade, probably in a 20- or 36-inch pipeline to the mill.
Four skid-mounted water monitors can be controlled by one operator and ERG plans to have two or three operating faces from which to draw material. Climatic conditions will dictate how many months of the year slurry will be available for milling, but Sweetin expects to be operating for at least eight months annually. As soon as it is cold enough to make ice sculptures in early winter, it will be too cold to mine tailings. The opposite will be true in late winter. Since the mill will not be running during freeze-up, large, heated buildings will not be needed. Some of the mill circuitry will be exposed to the elements —
a feature that should keep down costs.
Mill feed will come initially from the huge McIntyre tailings disposal area and later from what used to be Gillies Lake. These two areas contain sufficient material to satisfy mill requirements for 5-6 years. As feed from more remote tailings areas is required and longer pipelines are needed, low-gr ade ore from underground and open pit operations will likely supplement mill feed, Sweetin says.
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