MINING IN MEXICO — SX-EW copper production grows in

The fall in the copper price in recent months has refocused investors attention on the copper market. In discussions on the subject, a number of people have raised questions about solvent extraction-electrowinning (SX-EW), which seems to be a mystery even to some professional mining investors.

In this article, we briefly describe the process of SX-EW, which has become an increasingly important method of producing copper metal. Phelps Dodge (NYSE) in the U.S. is now the largest producer in the world and in 1992 SX-EW accounted for 45% of copper production.

The ability to extract metal from a rich solution by electrolysis was one of the first applications to which electricity was put. However, it was not until the early 1960s that a reagent was developed to extract copper from oxide leach solutions. It then took another 20 years for the methods to be fully understood but by then, SX-EW plants were operating in the U.S., Chile, Zambia, Peru, Australia and Mexico.

In the past decade, copper recovered by SX-EW methods has more than doubled and now at around 1.2 million tonnes per year accounts for nearly 14% of Western World refined output, which totalled 8.82 million tonnes in 1992. In coming years, the proportion is expected to rise further, having an impact on the copper cost curve and the need for new smelting-refining capacity. SX-EW is predominantly used to treat low-grade copper oxide deposits which by their nature tend to be on or close to the earth’s surface. These weathered ores often need little drilling and blasting to break up the rock, and only minor crushing to reduce the ore to a size suitable for heap leaching. The next stage of the process is similar to that for low-grade gold ores, except that the solution sprayed on the rock dumps is weak sulphuric acid rather than cyanide. The sulphuric acid solution percolates through the pile of broken rock, dissolving copper oxides in its travels.

Under the base of the dump is an impermeable plastic sheet which has been positioned to be on a moderate angle that allows the pregnant leach solution (including copper sulphate) to flow to a central collecting point. The pregnant solution (which is an intense blue color) is piped to a feed pond before being pumped to the solvent extraction tanks. In the first stage, it is mixed with kerosene and an active extractant designed to attract only copper ions; impurities such as iron, molybdenum, lead and zinc are not attracted to this reagent.

This mixture is then physically separated in a settling tank where the barren aqueous phase (called raffinate) is recycled and reacidified for sprinkling on the broken ore dumps. The loaded kerosene is transferred from the extraction section to the stripping section where it is mixed with a highly acidic copper-rich electrolyte which strips the copper ions from the kerosene and becomes even richer in copper. This enriched electrolyte is then pumped to the electrowinning tankhouse.

“Electrowinning” is an electrochemical process in which the copper-rich electrolyte is passed through large tanks in which insoluble anodes (commonly lead) and cathodes (stainless steel) are suspended. Copper in solution is electrically attracted to the cathodes with a plating cycle of some seven days to grow sheets of pure copper weighing about 100 lb. each on either face of the blank cathode. The copper sheets are stripped mechanically from the blanks.

In the early days, this copper was not pure enough to be used in anything but brass mills and was sold at the same price as scrap. Today, it is estimated that some 78% of electrowon copper cathode production is high grade and many brands are registered for sale on the London Metal Exchange and the Commodity Exchange of New York (plus-99.995% pure).

Electrowon copper is now commonly used by wire mills and even has some uses such as continuous cast wire rod which requires the highest purity (99.999%) metal. There is every reason to believe that levels of quality from SX-EW plants should continue to improve in coming years.

It is not hard to see how the cost of SX-EW copper can be so much lower than copper produced from the more conventional hardrock mining/

milling/smelting/refining process. Financial costs are reduced from the start of the process. There is little or no drilling and blasting of ore, no milling, no smelting or refining, reduced labor and power costs, lower capital costs, premium product and consumption of by-product sulphuric acid. Cash operating costs for SX-EW can range from US20-50 cents per lb. compared with US40 cents-$1 for more conventionally produced copper. In addition to the financial aspects, the SX-EW process is closed circuit and has minimal toxic discharge while also consuming environmentally unfriendly sulphur. — From a recent issue of “International Mining Outlook,” a publication of international securities firm S.G. Warburg & Co.