Processing of Oxide Ore

Oxide ores are generally processed using hydrometallurgy. This process uses aqueous (water-based) solutions to extract and purify copper from copper oxide ores at ordinary temperatures, usually in three steps: heap leaching, solvent extraction, and electrowinning.

Heap Leaching and Solvent Extraction of Oxide Ore

Heap Leaching is the process of using percolating chemical solutions to leach out metals. Heap leaching is very commonly used for low-grade ore, which would otherwise not be economical to send through a milling process. Following mining, transporting, and crushing to a consistent gravel or golf ball-size, the crushed ore is piled into a heap on top of an impenetrable layer, on a slight slope.  The leaching reagent (dilute sulfuric acid) is sprayed through sprinklers on top of the heap pile and allowed to trickle down through the heap, where it dissolves the copper from the ore. The resulting “pregnant” leach solution of sulfuric acid and copper sulfate is collected in a small pool. The copper compound can now be seen at concentrations of between 60-70%.

The second step is solvent extraction, in which two immiscible (un-mixing) liquids are stirred and allowed to separate, causing the cooper to move from one liquid to the other. The pregnant leach solution is mixed vigorously with a solvent. The copper migrates from the leach solution into the solvent. The two liquids are then allowed to separate based on solubility, with copper remaining in solution in the solvent, and impurities remaining in the leach solution. The leftover leach solution is then recycled, by adding additional acid and sending it back to the sprinklers in the heap leaching process.

Electrowinning is the final step in processing oxide ore into copper cathodes.

The last step is called electrowinning, a type of electrolysis. An electrical current passes through an inert anode (positive electrode) and through the copper solution from the previous step, which acts as an electrolyte. Positively-charged copper ions (called cations) come out of solution and are plated onto a cathode (negative electrode) as 99.99% pure copper.