Zinc improves economics at Boleo — International Curator seeks partner for development

Armed with a prefeasibility study that indicates its El Boleo copper-cobalt deposit in Baja California Sur, Mexico, is economic, International Curator Resources (ic-t) plans to hold discussions with potential development partners.

The preliminary feasibility study concludes that the deposit is economic at current metal prices and that operating costs could be covered with the co-product credit from cobalt and zinc. Earlier studies had proposed selling zinc carbonate precipitate to smelters, or treating the zinc as a waste product, but the new work sees zinc credits as a major source of revenue.

The proposed open-pit design has a reserve of 70 million tonnes with an average grade of 1.29% copper, 0.09% cobalt and 0.62% zinc. That reserve has been outlined using an economic cutoff equivalent to 1% copper, but in practice the copper grade has been the main control on the pit design.

Curator’s president, Philip Wright, says “we will take the cobalt and zinc we get with that copper orebody.”

The pit has a high stripping ratio (18-to-1), but the pit material is rippable and allows for an inexpensive truck-and-shovel operation. Actual mining costs are expected to clock in at around US$7 per tonne of ore.

The reserve is part of a much larger resource of 445 million tonnes grading 0.71% copper, 0.06% cobalt and 0.69% zinc, determined from about 80,000 metres of drilling in 855 holes.

The deposits are stratabound in nearly flat-lying clayey sediments atop a conglomerate unit; there are both oxide and sulphide zones in the mineralization. In the oxidized zones, the copper is held in atacamite, chrysocolla and malachite, and the zinc and cobalt are dominantly in manganese oxides. Sulphide zones carry chalcocite, carrolite and sphalerite.

The metal association, the geology and the mineralogy all suggest a comparison with the copper-cobalt deposits of Zambia and Katanga.

Mined from 1886 to 1985 by French interests, Boleo produced 19 million tonnes of direct-shipping ore grading around 4% copper.

More zinc resources are known to exist on the property, but they have not been adequately defined for a resource calculation.

The Boleo mineralization is metallurgically complex. Recoveries of 81% copper, 69% cobalt and 59% zinc are expected, but only with a process that includes two leaching stages, precipitation, flotation, roasting and solvent extraction.

The current metallurgical plan calls for a semi-autogenous grinding mill, followed by a 2-stage acid-leaching circuit that first strips copper in oxidizing conditions, then strips cobalt in reducing conditions. The next step precipitates the metals from solution as sulphides. A copper-zinc-cobalt concentrate is floated from the precipitate, and can be roasted and finally re-leached for solvent extraction-electrowinning.

The method demands the addition of acid-consuming ore at the precipitation stage. In addition, ore rich in manganese dioxide is added after the precipitation stage, to remove iron from tail water; the water, containing dissolved metal from this ore, is then recycled through the process.

Waste treatment

Tailings solids are routed to a pond that currently has a capacity to match the mine life. Expansion of the pond is possible if further resources are upgraded to minable reserves.

Previous plans had intended that zinc would be precipitated and the zinc-carbonate precipitate sold for direct smelting; but zinc remaining in solution had to be treated as waste and removed at a substantial cost. “By planning to recover the zinc by electrolysis, we have seen a healthy improvement in project economics,” says Wright.

The plant will need to import sulphur, and sulphuric acid and hydrogen sulphide plants are part of the blueprint. Most of the metallurgical process can use seawater, though desalinated ocean water (produced in a reverse-osmosis plant) will be used for those processes that cannot stomach sea water. The plant will also provide a domestic water supply for the operation.

Despite the complex metallurgy, cash costs of copper production are expected to be covered by the co-product credits. Curator is predicting a cash cost of minus US16 cents per lb., assuming a cobalt credit of US$12 per lb. and a zinc credit of US65 cents per lb. Without the cobalt credit, the cash cost is US58 cents per lb. of copper. Because the process costs cover production of electrolytic copper, there are no smelting or refining charges, and the operation is expected to see the full value of its metal.

The capital cost is estimated at US$440 million, the bulk of which — US$300 million — will go into the metallurgical plant. This capital cost estimate is believed to be within 20% of the final figure.

The mine plan sees annual production of 45,000 tonnes of copper, 2,800 tonnes of cobalt, and 18,000 tonnes of zinc over a 17-year mine life. Copper and cobalt production is projected to decline and zinc production to rise over that 17-year period.