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Electrowinning is the process of extracting metals from a solution with the help of electricity. In an electrowinning cell, new metals are not dissolved from the anode as in electrolytic refining. The anode is inert and isn't consumed by the process.
Depending on the electrolyte different gases will evolve at the anode at the same time as metal is deposited at the cathode. To protect the anode from the process care must be taken when selecting the anode material.
- Sulphate media often uses lead or lead alloys as the anode material.
- Chloride media often uses titanium or graphite
- Nitrate media often uses graphite
Electrowinning of copper from copper sulphate
Copper is reclaimed in cells with lead anodes. To protect the anodes from corrosion (a compact layer of lead oxide is formed) the current density shouldn't go below 60 A/m2. The electrolyte is a solution of copper(II) sulphate (40g/l) in concentrated sulphuric acid (180g/l). Operating current density is 250-350 A/m2 and the anode is covered by a passivating PbO2 layer.
Anode process : 2H2O -> O2 + 4H+ + 4e- Cathode process : 2Cu2+ + 4e- -> 2Cu
If the current is switched off the PbO2 turns into PbSO4 at the same time as metallic lead is oxidized to PbO. Therefore it is important to remove the anode if the cell is switched off.
Electrowinning of copper from copper chloride
The electrowinning of copper from acidic chloride leach solutions is difficult compared to the sulphate based process since chlorine often is produced at the anode.
One way to avoid the chlorine gas is to use a Copper(I) solution, the anode process will turn Copper(I) chloride into Copper(II) chloride while metallic copper is deposited at the cathode.
- Lin et. al 1999, Makita and Kawakami 1990, Filor 1987, Palmer et. al 1988 (Google books, Handbook of Copper Compounds and Applications)
- The electrowinning of copper from a cupric chloride solution, Lin 1991 (downloaded)
- Electrodeposition of copper from pure cupric chloride hydrochloric acid solutions, Kekesi 1997 (downloaded)
- Copper electrowinning: theoretical and practical design (pdf, downloaded)
From the beginning graphite or lead alloy anodes were used for chloride based electrolytes but erosion from the chlorine gas made it a problematic process. Then came the development of DSA anodes (The history of progress in dimensionally stable anodes) and nowadays they are used in electrowinning in chloride-based solutions and recovery of metals from dilute solutions. Now, many plants exist that are using titanium coated anodes for copper electrowinning. DSA anodes are also used for zinc electrowinning (some advantages, operates at higher current densities, the elimination of sludge, ability to operate at a reduced anode voltage at 300 mV less than Pb-alloy anodes).
Electrowinning of gold and silver from cyanide solution
Electrowinning of zinc from basic solution
(Main article : Making zinc from alkaline batteries)
Electrowinning of zinc from sulfate solution
(Main article : Electrowinning zinc from a sulfate solution)
- GRF : Advices for electrowinning base metals
- Chemguide : Basic electrochemistry calculations
- Selective Electrowinning of Silver and Gold From Cyanide Process Solutions
- Copper electrowinning: theoretical and practical design
- Handbook of Copper Compounds and Applications Pg 352-353
- Davenport, Extractive Metallurgy of Copper