Electrowinning of copper from

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

Fig. 12.9 Electrowinning of copper from AN-H2O-H2SO4 solution (electrolysis at 0.6 V and 0.01 A cm-2) [15],...

Figure 5 A simplified flowsheet and materials balance for the recovery of copper from oxidic and transition ores by heap leaching, solvent extraction and electrowinning.

The process for electrowinning of copper is schematically shown in Fig. 12.9. If copper(I) sulfate in AN-H20-H2S04 solution is electrolyzed using a platinum electrode as anode and a copper electrode as cathode, one-electron processes occur at the two electrodes (Cu1 —> Cu2 at the anode and Cu1 —> Cu° at the cathode). Compared with the conventional electrowinning from the aqueous acidic solution of copper(II) sulfate (water oxidation at the anode and Cu2+ —> Cu° at the cathode), the electric power consumed is only about 10% and high-quality copper can be obtained. It is of coruse necessary to return Cu2+, generated at the anode, to Cu+. But various methods are applicable to it, e.g., by the contact with coarse copper. 

Cementation, the process by which a metal is reduced from solution by the dissolution of a less-noble metal, has been used for centuries as a means for extraction of metals from solution, and is probably the oldest of the hydrometallurgical processes. It is also known by other terms such as metal displacement or contract reduction, and is widely used in the recovery of metals such as silver, gold, selenium, cadmium, copper and thallium from solution and the purification of solutions such as those used in the electrowinning of zinc. The electrochemical basis for these reactions has been well established414 and, as in leaching reactions, comprises the anodic dissolution of the less-noble metal coupled to the cathodic reduction of the more-noble metal on the surface of the corroding metals. Therefore, in the well-known and commercially exploited44 cementation of copper from sulfate solution by metallic iron, the reactions are... 

Since the mid-1960s, solvent extraction has been applied on a commercial scale to the recovery of copper from aqueous solutions obtained from the leaching of acid-soluble copper ores with dilute sulfuric acid. At present, several plants producing up to 200 t of copper per day by an integrated leach—solvent extraction—electrowinning route are in operation throughout the world. 

The PLATSOL M treatment of a nickel rich concentrate and a pyrrhotite 3 cleaner concentrate was demonstrated. The PLATSOL autoclave process was combined with precious metal precipitation, copper concentrate enrichment, copper removal, iron/aluminum removal, mixed hydroxide precipitation of nickel and cobalt and finally magnesium removal. The copper concentrate enrichment process proved to be a novel addition to the NorthMet flowsheet. The copper in solution from the autoclave processing was recovered by metathesis on the copper concentrate in the enrichment process. The solvent extraction and electrowinning of copper process applied to precious metal free PLATSOL solutions is no longer required by using this metathesis process route. This development provides maximum operational flexibility for treatment of the NorthMet ore. 

Cementation is also an efficient way of putifyiag a pregnant solution by removing impurities that are more noble than the metal being processed. An example is the cementation of copper, cadmium, cobalt, and nickel from ziac solutions prior to electrowinning. 

Besides copper passing into the solution as ions, the total concentration of the solution in respect of S04 ions (not discharged) and Cu2+ ions (copper is depositing on the cathode) remains constant. The electrolysis merely transfers copper from the anode to the cathode. With a platinum or a carbon anode, it is found that the color of the solution fades as copper is deposited, whereas with copper electrodes, the color does not change. The two situations, in fact, represent respectively the electrowinning and the electrorefining processes introduced later in the section on process classification. 

A number of electrolytic processes are used for the industrial production of metals. Some metals such as zinc, copper, manganese, gallium, chromium, etc. are electrowon from aqueous baths. Another common electrolytic process used is molten salt electrolysis. The most important application of molten salt electrolysis till now has been in the electrowinning of metals. Today aluminum, magnesium, lithium, sodium, calcium, boron, cerium, tantalum, and mischmetal are produced in tonnage quantities by molten salt electrolysis. As a representative example, the electrowinning process for aluminum is taken up. 

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