Copper Oxide Metallurgical Recovery

From the mine, leachable ore (containing copper-oxide minerals) is placed directly on lined leach pads where a weak acid solution is sprayed on the surface of the heap. This solution percolates down through the pile of ore and dissolves the acid-soluble sopper. The copper bearing solutions are collected and pumped to a solvent extraction plant where organic chemicals remove the copper from the solution. The resulting electrolyte solution is transferred to the electrowinning process where copper is plated out as a 99.99% pure copper cathode.

Dump leaching refers to percolation leaching of copper from run-of-mine low-grade ores that have been piled on native ground. These dumps typically cover hundreds of acres and contain millions of tons of low-grade ore. The leaching cycle covers a span of many years. The percentage of copper produced by leaching operations has increased and this trend is expected to continue. Copper leach dumps typically cover hundreds of hectares, are more than a hundred meters high, and contain millions of metric tons of leach ore. 

Dump leaching and heap leaching are distinguished by the use of liners. Dump leaching refers to the leaching of lowgrade ore that has been deposited directly on the ground. The pregnant leach solution is typically collected in unlined natural drainage basins. In contrast, heap leaching refers to the leaching of ore that has been deposited on specially prepared pads. Dump leaching operations are always constructed in the immediate vicinity of the mine site. Leach sites are selected to minimize haulage costs and to utilize the natural drainage patterns of the native terrain for collection of the pregnant liquor solutions. Leaching of copper from massive dumps of sulfide ore is accomplished bacterial activity and, often, by the addition of sulfuric acid. Ferric sulfate, the major. lixiviant, forms in the presence of oxygen and bacterial activity. The bacteria generate acid in situ, which prevides acid for acid-consuming reactions, including oxygen reduction. Frequently, only makeup water is needed in copper dump operations because the oxidation of the sulfide minerals generates sufficient acid to dissolve the copper and maintain an active bacterial population. More effective leaching reagents have been identified, but they are generally much more expensive and their impact on the environment is uncertain.

Copper is recovered from pregnant leach liquors either by cementation or by solvent extractionlelectrowinning. These processes remove copper from solution and allow other dissolved substances to accumulate. The recovery process itself may add other substances to the leach solutions. The cementation process uses scrap iron to precipitate copper from the pregnant solution. The iron replaces copper in solution, and this iron-rich solution is subsequently recycled to the top of the leach dump. Upon exposure to the atmosphere, the dissolved iron oxidizes to form insoluble salts, which precipitate on the surface of the dump and restrict the flow of solution. 

Solvent extraction uses a complexation mechanism whereby copper is chelated; the copper is then stripped from the organic carrier by a strong acid solution. Kerosene is a common carrier used in most solvent extraction operations, and it may appear in small quantities in the raffinate recirculated the dump.