Anode electro winning

Oxygen-Evolving Anode. Research efforts to iacorporate the coated metal anode for oxygen-evolving appHcations such as specialty electrochemical synthesis, electro winning, impressed current, electrodialysis, and metal recovery found only limited appHcations for many years. 

In electro winning, the cathode reaction is the same as for electrorefining (see eq. 31). However, because of the use of insoluble anodes, oxygen is released at the anode. 

The current efficiency in most gold electro winning operations is only 5-10%. The majority of current is used in oxygen reduction. As the anode reaction consumes OH- ions, the local pH lowers. On the cathode, metal is deposited and hydroxide ions are formed. As a net result, the metal concentration and pH of the electrolyte decrease. The decrease of pH is critical especially in cyanide solutions as anode corrosion increases and at pH <9.0, the formation of lethal HCN gas begins [63]. 

To achieve this pH, a diaphragm is needed to separate the electrolyte near the cathode (catholyte) and the electrolyte near the anode (anolyte). The anolyte is acidified during operation as water decomposes to oxygen and protons as the anodic reaction [2]. The need for a diaphragm adds to the operational complexity and difficulty of conventional electro winning cells. 

For the electro winning of nickel, a tubular anode constructed from titanium with a mixed metal oxide coating is employed. In a standard design EMEW cell, the anode is approximately 50mm in diameter. The cathode is a stainless steel tube approximately 151 mm in diameter. Both the anode and cathode are fixed in the cell. A thin stainless steel starter sheet is rolled and inserted in the cell to deposit nickel. The deposited nickel cathode is removed from the cell as a tube. The stainless steel starter sheet then springs off the deposit to release the nickel cathode. 

In nickel electro winning and refining and some cobalt plants, filter cloth style separators between anodes and cathodes are used to allow catholyte pH control and separate capture of anode gas. In those cases, short circuits due to nodules and dendrites also damage the separators which may ultimately lead to poor catholyte pH control and a loss of current efficiency. 

This mechanism was proposed by Diard et al. [1992] to evaluate EIS for cathodic production of hydrogen by the metallic electrodeposition reaction and anodic oxidation reaction of metals (e.g. iron) close to their corrosion potentials. A similar mechanism was found by Rerolle and Wiart [1996] studying the oxygen evolution during zinc electro winning. 

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