To help gold mine operators determine the best size and depth of tailings pond for their operations, Environment Canada has spent about 16 years developing a mathematical model that predicts the change in cyanide concentration as the result of natural degradation in a tailings pond. By entering information (such as the type of pond desired, the time of year being evaluated, the desired surface area and mean depth of the pond, initial pH of pond water, and initial cyanide concentration), the program will calculate cyanide concentrations of effluent from the pond. By changing the surface area and depth, operators can come up with the best design. Copies of the program will soon be available to the mining industry, says Abbas Saidi of Environment Canada in Burlington, Ont.
One tailings pond that was modelled on the computer to test its accuracy was that of Echo Bay Mines’ Lupin mine, in the Northwest Territories. The values of effluent cyanide concentration predicted by the computer program showed good agreement with the observed values during a 12-month period from June,1984, to May, 1985, says Ljuba Simovic of the Wastewater Technology Centre in Burlington, Ont., one of the developers of the program. The tailings pond at the Lupin mine is about 6 km from the main plant site and covers an area of about 750 ha. In the 2-year period from 1986 to 1988, cyanide concentration of effluent from the area averaged 0.17 mg of total cyanide per L, well below the 1 mg per L required by federal law.
Natural degradation is not the only method of treating cyanide-rich effluent. It is only the least expensive. Close to half (23) of the 50 gold mills in Canada use more sophisticated, chemical treatments, says James Scott of Environment Canada in Ottawa. What follows is a description of these methods and the experiences of some of the mines that use them. For more information on the Environment Canada computer program, circle reply card #19 Inco’s SO2/air Process
The most widely-used chemical treatment method for the detoxification of cyanide is the Inco SO2/air process. It was patented in 1984 and has been used in 12 mills so far. Six more will be commissioned by the end of this year, says Dr Eric Devuyst of Inco Research in Mississauga, Ont.
Pilot plant tests were conducted at the Campbell Red Lake mine in Red Lake, Ont., in 1982. Then a full-scale operation designed by Inco scientists was commissioned at the McBean mine in Kirkland Lake, Ont., in June, 1984. Its performance was evaluated over a 4-month period in 1985. “The system was somewhat over-designed for the application but was a good test case,” Devuyst says.
The process involves the oxidation of cyanides to cyanate and the precipitation of base metals. Copper is necessary in sufficient quantity to act as both a catalyst and a precipitant for any ferrocyanide not oxidized in the reaction. The optimum pH for the reaction is usually about 9. Sulphur dioxide can be used in several different forms.
At the McBean mine, 93.6% of the barren bleed from the mill’s Merrill- Crowe zinc precipitation process was treated by the SO2 process before being discharged to the tailings area. A 4-compartment flotation cell was used as a reactor. Copper sulphate was added to the feed in the first compartment, while sodium metalbisulphite and lime slurry, along with air, were added to the second compartment. Total capital costs were $167,000. Flow to the tailings pond averaged about 800 L per minute at a pulp density of abou t 50%. On average, the SO2 process was effective in removing more than 99.5% of the total and weak acid dissociable cyanide present in the wastewater. More than 90% of the dissolved metals, including iron, was removed. The mean concentration of residual total cyanide in treated barren bleed was 0.69 mg per L. Total operating costs were 52 cents per tonne of ore milled (in 1986 dollars).
The process has also been used at the Equity Silver mine in Houston, B.C., the Mt Skukum mine in the Yukon, the MacLellan mine in Lynn Lake, Man., the Ketza River mine in the Yukon, the Johnny Mountain mine in B.C., and the Colosseum mine in California, among others.
Inco is considering automating the process to avoid operating problems. A hefty operators manual is used now and a hot line to Inco scientists is also provided to clients. “You don’t have to think,” Devuyst says. “You just have to do what is written in the manual.” For more information on Inco’s SO2 process, circle reply card #20 Degussa’s Hydrogen Peroxide Process
Another chemical used to treat gold mill effluent is hydrogen peroxide. The largest supplier of this chemical is Degussa of West Germany, so it should come as no suprise that this multi-national corporation is marketing a detoxification process. The first system was installed at the OK Tedi mine in Papua New Guinea in 1984. Since then, 12 mills in Canada have decided to follow suit, says Gregg Vickell of Degussa Canada.
Hydrogen peroxide is a clear, colorless and odorless liquid that can be shipped at concentrations up to 70% by weight and stored for long periods of time. It can also oxidize weak acid dissociable cyanide and reduce ferricyanide to ferrocyanide. After copper is added, the ferrocyanide can be precipitated. With the addition of another non-toxic reagent marketed by Degussa, called TMT 15, residual copper can also be precipitated. This reagent is used at the Hope Brook mine in southwestern Newfoundland. The oxidation reaction involving hydrogen peroxide operates at a ph which is typical of most gold mill waste streams.
Another feature is the absence of toxic gases. Also, no aeration is necessary and capital costs are low.
The hydrogen peroxide process has been applied to a variety of milling applications including tailings pond overflow and barren bleeds. For more information on the hydrogen peroxide process, circle reply card #21 Hemlo Gold Process
Operators of the gold mill at the Golden Giant mine in Hemlo, Ont., have come up with an innovative effluent treatment process of their own for which patents have been applied.
By adjusting the ph of the leaching circuit to between 8.8 and 9.4 and by maintaining a free cyanide concentration of 0.1 g per L, the company has discovered that the dissolution of antimony has been reduced significantly in its 3,000-tonne-per-day mill. Also, overall cyanide consumption has dropped to 0.36 from 0.95 kg per tonne of ore. This work alone reduced the comp
any’s effluent treatment costs. But persistent work led to the discovery of a new process.
Hemlo Gold tested both the SO2 and the hydrogen peroxide processes. But the potential of an SO2 leak or spill in the vicinity of the shaft collar or the fresh air intake of the mine was deemed an unacceptable risk, says Ernest Goodwin of Hemlo Gold. And acceptable effluent quality was not consistently attained using hydrogen peroxide.
So an alternative was sought. In the course of that development work, a process that employs ferrous sulphate was discovered. The process involves a non-hazardous aqueous solution of copper and ferrous sulphate that is added to the waste water at a controlled pH of 6 to 7. Heavy metals are then co-precipitated from solution by the ferric hydroxide formed from the solution.
Hemlo Gold has also rerouted its reclaim water to take advantage of a natural detoxification process occurring in the grinding circuit. By redirecting the overflow from the grinding thickener directly to the effluent treatment plant, reagent requirements in the plant have been reduced by about 40%, Goodwin says.
Hemlo Gold has also doubled the surface area of its tailings pond to enhance natural degradation by ultraviolet light. This has also helped reduce cyanide levels in the mine’s effluent. For more information on the Hemlo Gold process, circle reply card #22 CANMET’s Modified AVR Process
The AVR (Acidification/Volatilization/Regeneration) process was used by Hudson Bay Mining & Smelting to recycle cyanide in Flin Flon, Man., from 1935 to 1978, Environment Canada’s Scott says. The process was also used by AMOK Ltd. in a gold recovery project at the Cluff Lake mine in Saskatchewan. But air-stripping of hydrogen cyanide gas from the total volume of waste barren solution has become too expensive, so the Canada Centre for Mineral and Energy Technology (CANMET) has been working to reduce operating costs.
Research scientists at the Extractive Metallurgy Lab of CANMET in Ottawa have found an inexpensive way to recover most of the hydrogen cyanide generated in the AVR process. This is done simply by liming. The remainder is recovered by the more expensive air-stripping method.
Economic evaluation of the modified process by F. J. Kelly, project analyst for CANMET, indicates the process would have a high rate of payback. Based on a feed rate of 1 cu m per min, total capital costs for a modified AVR plant are estimated to be about $1.9 million. Three costly items (a centrifuge, a filter press and a calcining kiln) account for about half of the total capital costs. Annual operating costs would be about $899,200 or $1.78 per cu m of effluent treated, Kelly says. But some $1.5 million worth of cyanide would be recovered, giving a payback of about $650,000 per year. For more information on the modified AVR process, circle reply card #23
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