BASE METALS SPECIAL — SX-EW process changes picture at Chilean deposits

If necessity is the mother of invention, five copper projects in northern Chile can call her Grandma. All are in production, or headed that way, because metallurgical technology has improved their feasibility as mines.

Mother of the projects is solvent extraction-electrowinning technology (SX-EW), a hydrometallurgical technique that bypasses conventional flotation and smelting in favor of bacterial or chemical leaching to produce a copper solution that can go straight to the refinery.

Rio Algom (ROM-T) will be using the technology at its Cerro Colorado deposit near Iquique, Chile, to win copper from chalcocite and oxide ores. Because the copper is largely tied up in chalcocite, Rio has had to adopt a bacterial-leaching attack at the start of its process.

The ores are rolled in water, then delivered by conveyor to a leach pad where they are drenched in a weak sulphuric acid solution. A common bacterium, Thiobacillus ferrooxidans, acts on the chalcocite and makes it possible for the copper to react with the weak acid. The reaction produces dissolved copper sulphate in water — at a concentration around 3-4 grams per litre — which goes to the next stage of the process, organic complexing.

In the complexing stage, the dilute copper sulphate solution is mixed with kerosene containing an organic solvent that bonds with the copper, drawing it out of the water. Then the mixture is allowed to stand, so that the water separates from the kerosene-based organic solution that now contains the copper.

The organic solution is then pumped to the second reaction vessel, where it is mixed with strong sulphuric acid. The acid strips the copper from the organic solution, which can then be returned to the complexing stage to pick up more copper. The sulphuric acid solution, with about 45 grams copper per litre, goes to a tankhouse where it is recovered electrolytically, producing top-quality cathode copper.

Rio Algom estimates a cost of US51 cents per lb. of copper, compared with US63 cents per lb. for combined flotation, smelting and refining.

Not far away, the Quebrada Blanca open-pit mine, which started up in 1995, is using much the same process. The operating company is owned 47% by Cominco (CLT-T) and 29% by Teck (TEK-T), with the remainder held by Chilean interests, including Minera Pudahuel, which developed the mine’s proprietary SX-EW technology.

Quebrada Blanca is a notable success story, despite a few start-up problems, having produced 21,900 tonnes of copper in 1995 despite cold weather that hampered the leaching process. Chile’s Institute of Mining Engineers presented its 1995 award for innovation to Quebrada Blanca in recognition of the mine’s technical progress.

The Ivan mine of Minera Rayrock (MRA-T) is another SX-EW project. The main deposit is relatively small, but its grade — 2.5% copper — is unusually high. After some initial teething problems, Ivan is close to its planned production level of 10,000 tonnes per year, and costs are around US60 cents per lb.

SX-EW is also moving another project on the Chilean Altiplano. The Collahuasi mine, a joint venture of Minorco and Falconbridge (FL-T), has a reserve of 2 billion tonnes grading 0.83% copper, in two large porphyry systems.

The partners will exploit the sulphide ores by conventional milling but have gone with SX-EW for a 50,000-tonne-per-year operation on the oxide ores.

Unlike Cerro Colorado, the project will go the chemical-leaching route, using dilute sulphuric acid on the leach piles. Collahuasi is at a higher altitude and reaches the freezing point every night of the year. Temperatures drop to minus 20C in winter, and the bacteria refuse to work in that kind of weather.

At this project, the SX-EW plant is a supplementary process, producing 50,000 tonnes per year, while the conventional mill produces concentrate with a total of 330,000 tonnes contained copper. But the SX-EW plant will allow the project to exploit 53 million tonnes of oxide ores grading 1.31% copper that the conventional milling approach can not touch.

Ready to go in November, the Andacollo project, a joint venture of Canada Tungsten (CTG-T) and Chilean iron-ore producer Compania Minera del Pacifico, will use an SX-EW plant to exploit a 35-million-tonne reserve grading 0.74% copper. Earlier feasibility studies that based production on conventional milling found that mercury and arsenic in the sulphide ores would mean costly smelting penalties. The oxide ores, on the other hand, have already been stripped of their mercury and arsenic by weathering, and an SX-EW operation does not have to take them into account. The partners are estimating a production cost around US56 cents per lb.