The leaching of copper from ores has been practiced for many centuries, but the large-scale recovery of high purity copper by hydrometallurgical processing has only recently be achieved. Leaching of copper from weathered pyrite was performed in Spain as early as the 1700's . Recovery of copper was achieved primarily by cementation onto iron. Iron cementation produced a copper-iron sponge that required further processing. Cementation and precipitation were the main recovery methods until earlier this century and produced impure copper products. Electrolysis or electrowinning of 99.15 to 99.85 percent copper from vat leaching of high grade copper ore was proven feasible in the United States in 1914 . However, the large-scale hydrometallurgical production of high purity copper from low-grade ores was still decades away.

In 1968, the first commercial plant to utilize solvent extraction to concentrate and purify leach solutions was operated in Miami, Arizona by Ranchers Exploration and Development Co. Ltd. Solvent extraction had been used for numerous years in analytical laboratories for the separation and concentration of compounds for subsequent analysis. The use of amine solvent extraction was proven for the recovery of uranium from leach liquors in 1956. The recovery of copper from leach solutions was not possible on a large scale basis until the development of hydroxyoximes. The first hydroxyoximes, such as LIX 63 and 64, had serious limitations. The second generation, LIX 64N, exhibited much higher selectivity of copper over iron, faster kinetics, and improved phase separation. Future generations have only improved upon these characteristics. The development of these extractants has led to development of the solvent extraction electrowinning process (SX/EW).

DocCopper recommends Hydroxyoximes and Copper Hydrometallurgy as an excellent book describing in detail the organic chemistry involved in copper extractant chemicals.

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The process starts with the blasting and crushing of the ore. The use of crushing is dependent on the head grade of the ore. Reducing the size usually increases the leaching rate and thus recovery occurs more rapidly. However, the cost of crushing must be overcome. Low grade ores are sometimes leached as run-of-mine product. Prior to the irrigation of heaps on prepared pads or stockpiles of waste and low-grade ore with dilute sulfuric acid, a cure will sometimes be used. The cure solution is typically higher strength acid which is allowed to react with the ore for several days. The result of the cure is a quicker recovery of copper with less total acid consumption.

Materials that are most amenable to leaching with sulfuric acid are oxides and secondary sulfides. Leaching of stockpiles (formerly called dump leaching) which contain primary sulfides usually involves bacterial assisted leaching and has leach times measured in years.

After leaching, the pregnant leach solution (PLS) is collected and pumped to a solvent extraction plant. Solvent extraction usually involves several extraction and stripping stages. In extraction, the PLS is mixed intimately with an organic phase containing the copper extractant, HR. The copper is preferentially extracted into the organic phase according to the equation (Eq. 4). The copper bearing organic phase separates from the copper depleted aqueous solution or raffinate because of the immiscibility and the density of the two solutions. The raffinate that has been replenished with acid is returned to the heap or waste stockpile to leach more copper. The organic phase is cycled to the stripping stage. In stripping, the organic phase is mixed with an aqueous solution of higher acidity to reverse the previous equation (Eq. 5). This causes the copper to leave the organic phase and re-enter the aqueous phase. This process results in conversion of the organic extractant into its acid form or regeneration. The regenerated organic is then recycled back to the extraction phase. The aqueous solution that is rich in copper and has very low concentrations of impurities is pumped to a tankhouse for copper recovery.

Extraction: CuSO₄ + 2 HR CuR₂ + H₂SO₄(Eq. 4)

Stripping: CuR₂ + H₂SO₄ ® CuSO₄ + 2 HR(Eq. 5)

The recovery of copper from the rich electrolyte occurs by electrowinning. Electrowinning consists of the plating of copper onto the cathode and the evolution of oxygen at the anode. The chemical reactions involved with these processes are shown in equations 6 and 7, respectively. Plating of copper occurs on copper starter sheets, titanium blanks or stainless steel blanks. To ensure suitable purity, morphology is critical and additives are sometimes included in the electrolyte to help produce a smooth and dense deposit. The anodes are typically alloys of Pb-Ca or Pb-Ca-Sn. Anodes are quasi-inert (i.e. dimensionally stable) in the electrolyte and provide an adequate surface for oxygen evolution. For more information about anodes click here.
Purities of greater than 99.999% have been produced using the SX-EW process.

Cathode: CuSO₄ + 2 e Cu^o + SO₄^2-(Eq. 6)

Anode: H₂O 2 H⁺ + 0.5 O₂ + 2 e(Eq. 7)

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