High-altitude bioleaching — Chilean projects nearing

At a time when most conventional copper producers are being battered by sagging prices, two Canadian companies are hoping to take advantage of cheaper solvent extraction-electrowinning (SX-EW) technology to produce high-quality copper cathode.

Rio Algom (TSE), through its 100%-owned Chilean subsidiary Compania Minera Cerro Colorado S.A. (CMCC), is developing the 40,000-tonne-per-year Cerro Colorado project in the foothills of the Andes, about 120 km inland from the northern port city of Iquique.

Only 110 km south of Rio’s project, but at a much higher elevation (4,300 metres above sea level), Cominco (TSE) and partners Cominco Resources International (TSE) and Teck (TSE) are developing the 75,000-tonne-per-year cathode project known as Quebrada Blanca.

During recent site visits, The Northern Miner learned that both development projects are progressing on budget and slightly ahead of schedule. At the time of the visit, Rio had just finished stripping 41 million tonnes of overburden to expose the orebody, which has reserves of 79 million tonnes grading 1.3% copper. The primary crusher was in operation and the secondary and tertiary crushers are expected to be on-line within a month. If all goes according to plan, the company should be producing cathode copper by early December.

CMCC President Pedro Campino says that once it reaches full production in 1994, the mine will be “one of the most efficient operations in Chile, employing only 309 people.” At current mining rates, the deposit will yield about 90 million lb. of London Metal Exchange (LME) grade-A copper cathode per year, for a period of 23 years. Total operating costs are estimated at US52.3 cents per lb.; with depreciation, costs are projected to be about US60 cents per lb.

However, even as the current mine takes shape, Rio is contemplating a 50% increase in production at a capital cost of $35 million. Should the expansion take place, output from Cerro Colorado will eclipse the company’s 33.6% share of production from the Highland Valley Copper mine in British Columbia, where last year Rio netted nearly 127 million lb. of copper.

Financing for the US$285.6-million project will come from $80 million in advanced metal sales, $100 million in bank loans, and $106.5 million in equity from Rio.

In the late 1970s and early 1980s, Falconbridge and its affiliates explored Quebrada Blanca and the adjoining Collahuasi property. The major decided not to develop the former, but opted to retain the latter.

Quebrada Blanca was then returned to its owners, Empresa Nacional de Mineria (ENAMI), a Chilean state mining corporation. Lacking the money and expertise to develop it, ENAMI put the project out for tender and, in late 1989, accepted a joint bid by Cominco and its partners.

Two years later, Teck agreed to finance 45% of the project in return for a 29.25% working interest. Cominco and Cominco Resources retained 38.25% and 9% interests respectively, with ENAMI holding 10% and Sociedad Minera Pudahuel (SMP), a Chilean mining company, owning the balance.

Being much larger than Cerro Colorado, Quebrada Blanca is being developed at a somewhat slower pace. To date, about 80% of the 24 million tonnes of overburden to be stripped, have been removed.

Crushing is slated to commence in January, 1994, with cathode production expected to begin by March. Total cash costs are estimated to be less than US50 cents per lb.

Quebrada Blanca is a typical copper porphyry deposit hosted by a quartz monzanite intrusion. The orebody is football-shaped, with an outer zone of secondary chalcocite ore overlying a low-grade, chalcopyrite-rich core. Minable reserves of secondary mineralization are 89 million tonnes grading 1.3% copper. Primary reserves (the low-grade, chalcopyrite-rich core) are estimated at 225 million tonnes grading 0.5% copper. At this time, there are no plans to mine the primary reserves.

At the end of September, expenditures for the US$360-million project totaled US$216 million. Financing will consist of $250 million in bank financing, $55 million in shareholder loans, and $55 million in equity.

Both projects will employ bio-

leaching technology developed by SMP at a small copper mine south of Santiago. With this process, ore is crushed, agglomerated and stacked on leach pads. Acidic solution is sprayed over the heaps to assist naturally occurring bacteria, known as Thiobacillus ferroxidans, to break down copper sulphide minerals. This one-celled bacterium breaks down the sulphides and generates weak sulphuric and sulphurous acids, which take the metal into solution as sulphates and sulphites, often leaving a precipitate of ferric iron.

The metal bearing solutions are then collected and processed in an SX-EW plant to produce 99.99%-pure copper cathodes.

In Cominco’s case, SMP was granted a 5% carried interest in Quebrada Blanca, in return for use of the patented technology (they also hold an 8.5% participating interest in the project). Rio will pay SMP a 0.5% royalty on all copper production for four years.

Within the mining community, there has been some skepticism about the ability of bioleaching to work at high altitudes and low temperature. To waylay these concerns, Cominco has been testing the process in pilot plants for more than three years, with considerable success. Pilot-scale tests by Rio indicate the process is very reliable, yielding recoveries of 92%.