Dupont develops copper-gold process

A new, cost-effective, integrated process for the economic production of gold and copper from ores containing cyanide-soluble copper has been developed by DuPont.

The company says this technology can also be applied to improve the economics of primary copper production as well as gold production. The process utilizes cyanide leaching to produce both gold and copper in forms readily accepted by industry. Only common industrial reagents are used. The company describes it as an integrated process, from leaching through to metal production, as opposed to an add-on cyanide recycle system. It has been developed as a result of large-column testing.

The specific benefits of the technology will depend on the ore and the status of the project to which it is applied. These benefits include: * economic production of gold from lower-grade, copper-bearing zones previously considered uneconomic;

* the ability to convert “waste” rock into economical ore, enabling (1) replacement of reserves at low or no cost and (2) acquisition of inexpensive property options on waste rock;

* improved economics for some oxide copper ores, as a result of (1) the recovery of gold values which solvent extraction-electrowinning (SX-EW) would leave behind, and (2) alkaline operation, which permits the processing of highly alkaline ores that otherwise would consume large, sometimes uneconomic, quantities of acid;

* on-site production of gold and copper, which could be more economic than shipping concentrate to a smelter;

* recycling of cyanide from leach solutions with little or no acid use; * recovery of copper and gold values from cleaner tails in a copper flotation plant; and

* recovery of copper and gold values from rougher concentrates so that a higher-grade chalcopyrite copper concentrate can be produced. The technology includes a set of integrated cyanide leach and recovery processes that recover gold and copper, and recycle cyanide. It can be used on copper ores or concentrates, with or without gold present. The technology can be applied to either heaps or mills. Proper physical and chemical setup of the leaching plant and integration with the recovery processes are an indispensable part of the technology.

In the area of gold recovery, the gold can be recovered separately from the copper using either conventional technology or DuPont’s proprietary technology. Gold can also be recovered in a gold-copper dore. The new technology is used to recover copper at standard grade copper specifications, as well as used to recover cyanide with little or no acid use. The total investment for copper, gold and cyanide recovery will be about equal to that for the SX-EW portion of an acid heap copper-leaching and recovery plant.

As for reagent and power costs, the following consumption numbers apply to a typical case where no acid is used in the process:

Na2C03 1.0 kg/kg Cu

NaCN 0.3 kg/kg Cu

Power 1.4 kW hours/kg Cu

Other miscellaneous operating costs should total in the range of US10-20 cents per kg copper recovered. (Note that power costs are lower than they would be for equivalent copper recovery via SX-EW.)

The main reagent cost is in the consumption of cyanide as a result of the formation of cyanate (from the leaching of oxidized copper minerals) and the formation of thiocyanate (from the leaching of sulphidic copper minerals). Cyanide consumption can range

from 0.1 to 0.5 kg NaCN/kg Cu, depending on the mineralogy.

The company expects to be able to reduce costs associated with this process even further (especially reagent consumption) as research continues. Reagent consumption on any particular ore can be determined from lab and pilot-plant tests on target resources.

Chalcopyrite will not leach well in the process. Overall recovery of copper from other minerals will be a function of the specific minerals present, as well as the usual factors of liberation and residence time.

The technology can also be applied to remove and recover copper and cyanide from effluents, thereby reducing concentrations to desired levels.