GIBRALTAR’S DUMP LEACHING: An insight into acid effluent control

Since 1972 Gibraltar Mines has operated a medium-sized, copper-molybdenum open pit in the Cariboo country of British Columbia.

To improve the economics of a low-grade operation, Placer Dome (the majority shareholder) installed a dump leach system six years ago. While there is nothing new in heap leaching, Placer did go out on a limb in two respects. First, this was to be a dump leach rather than a heap leach. The dumps, upwards of 200 ft. high, were not engineered for leaching, they were simply taken as they stood. Second, winter temperatures in the Cariboo, frequently drop to -35 and -40 C. for several weeks during the long winter.

On the positive side, the dumps are underlain by thick beds of impervious till thus voiding the need for a geomembrane. The system went ahead in 1986 and has proved to be an unqualified success, producing up to 25,000 lb. of high-purity cathode copper per day.

For Gibraltar the benefits were immediate — a fresh source of revenue. For the mining industry as a whole, controlled dump leaching may well become the means for coping with the major environmental problem of the day — the generation of acid from mine wastes.

Gibraltar mills well over its nameplate capacity of 30,000 tons per day; 35,400 tons were treated daily in 1990. Grade for the same year was 0.30% copper and 0.018% molybdenum disulphide. Mineralized rock bearing less than the cut-off grade of 0.20% copper finds its way onto the wastedump, and it is this material that is leached. Rock grading less than approximately 0.13% copper is allocated to the final waste dump. The reason for the approximation is not only the copper price, which has an immediate impact, but the rock’s content of natural carbonate. Carbonate is good news for the environment, but it is a heavy consumer of acid and can be a make-or-break cost item.

The Process

Present practice at Gibraltar is to level the section of the dump to be leached and dozer-rip the surface. A grid of 2-inch HDPE pipe is laid to deliver leaching solution on 20-ft. centres. Leaching starts with spent electrolyte from the electro-winning cells. The solution is maintained at pH 2.2 and fresh sulphuric acid is invariably required when a new dump is being brought on line. The acidic solution sprays through simple perforations in the 2-inch branches and is distributed naturally by the prevailing winds during the warm weather months. When freezing temperatures arrive, the solution is drawn through a T and valve fitted on the 2-inch lines. The controlled flow of solution flows through an 8-ft. downpipe and then drips to the bottom of a 10-inch-diameter hole drilled in the pile. These holes are drilled prior to installation of the pipe grid and were originally 20 ft. deep, the depth at which it was thought freezing would not cause problems. As it has turned out, the heat generated by the biochemical leaching reaction is far greater than anticipated and 10-ft. holes have been found to be entirely adequate.

On a recent visit to Gibraltar by The Northern Miner Magazine, we were told by Mill Superintendent, Paul Blythe, that pile temperatures of 20dC registered. at depths of 20 ft. – 40 ft., while the outside air scored minus 35dC. Total sprayed area at Gibraltar is a little over six million square feet. Each spray grid covers 150,000 sq.ft.

Forty five grids are presently installed with eight or nine running at any one time and Gibraltar uses a run and rest cycle of 14 days spraying followed by 56 days of rest. The 100 million tons of leachable rock runs approximately 4% sulphides.

Leach solution is sprayed at the rate of 240 (US) gallons per hour per 1,000 sq. ft. and the pregnant solution carrying 0.9 grams copper and 1.5 grams ferric iron per litre is pumped to the solvent extraction plant.

The Environmental Result

What is of commanding interest is that chaotic dumps can be leached. Unlike Gibraltar, for whom leaching is a commercial enterprise, there will often be no recoverable metals and leaching will be employed solely to reduce the sulphur content of the rock pile The pregnant liquor carrying ferric iron spiked with thiobacillus ferrooxidans would be recycled to the dump until the solutions were sufficiently concentrated for secondary treatment. The net benefit to the mining company would be completion of the (mandatory) dump monitoring program in five or 10 years compared to the 50 – 75 years needed if nature alone were left to do the oxidation.

A second avenue for researchers is currently under study at Gibraltar. This work resulted from the opening of the initial dump where leaching started in 1986. A 30-ft.-deep cut was made earlier this year and a number of intriguing phenomena were revealed. Not the least of these is that coatings extracted from the leaching solutions have been deposited upon the exposed copper and iron sulphides and prevented little more than superficial oxidation from taking place. These species have consequently been removed from active acid generation. Not encouraging observations for Gibraltar where copper recoveries are in the scheduled 40% range but of major significance environmentally — can the growth of these coatings be controlled, can they be accelerated?

Also to be considered by researchers are the possibilities of using low-cost additives to leach solutions, such as common salt or the soluble fertilizers should phosphorus or ammonia assist in the solubilization or coating processes.