Terrazas passes prefeasibility

A solvent extraction-electrowinning (SX-EW) project for the Terrazas copper-zinc project in Chihuahua state, Mexico, is economic, according to a feasibility study commissioned by Summo Minerals (SMA-T).

The deposit, an oxide copper-zinc body, would be mined by open-pit methods at the daily rate of 15,000-20,000 tonnes. The resource figure used in the feasibility study — 58.3 million tonnes grading 0.35% copper and 0.57% zinc — would cover an 11-year mine life; the pit design has a stripping ratio of 0.62:1. A number of prospects exist in the surrounding area and provide potential to extend the mine life or provide a base for expansion of the project.

The study, by consulting firm Jacobs Engineering, expects annual production of 18,000 tonnes of copper cathode and 27,000 tonnes of zinc ingot. Jacobs estimated the cash cost of production at US$770 per tonne copper and US$550 per tonne zinc, or US$860 for copper and US$480 for zinc if production costs are split pro rata between the two metals.

SX-EW copper projects are nothing new, though leachable zinc projects are. Metallurgical tests on Terrazas mineralization indicated 85% recovery rates for both metals, but the assumed recovery in the study is 83% for copper and 77% for zinc.

The feasibility plan’s metallurgical design includes a 3-stage crushing circuit near the pit, which would provide a 19-mm crushed product for trucking to leach pads 1.2 km away. The leach pad, designed for 60 million tonnes of material in a set of five 10-metre lifts, would drain to a leach solution pond, where leach solution would be pumped to the solvent extraction plant.

At the plant, a conventional copper extraction circuit using an organic reagent would feed an acid stripping circuit, from which copper could be recovered by conventional electrolysis. The zinc would largely remain in the leach solution during the solvent extraction phase, and 23% of the leach solution would be collected for a zinc electrowinning circuit. Remaining leach solution from both the copper solvent extraction circuit and the zinc electrowinning circuit would be returned to the leach pad.

Capital costs to bring Terrazas into production are estimated at US$136 million. The figure assumes that the mining fleet and conveyor-stacker system will be bought new.