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It is known that lead anodes used in the industrial extraction of copper by electrolysis (electrowinning) suffer corrosion as a result of accidental or intended current interruptions. In order to improve understanding of the corrosion and protection of such anodes, the effects of the concentrations of copper, sulphuric acid, cobalt, iron, manganese, chloride and an organic additive (guar) on the corrosion of lead have been studied by means of weight loss tests and surface analysis techniques (X-ray photoelectron spectroscopy, X-ray diffraction, and wavelength dispersive spectroscopy). The rate of corrosion of lead during current interruptions increases with increasing concentration of sulphuric acid and copper, whereas it decreases markedly in the presence of cobalt and iron and, to a lesser extent, in the presence of chloride and the organic additive. Manganese is the only impurity whose presence does not reduce the rate of corrosion; it is also the only element which precipitates in significant amounts on the lead anode surface under the conditions studied. A method is proposed to establish the optimum anodic protection current density during current interruptions in electrowinning cells. Three current density ranges have been found, of which the 'high' protection range could be caused by the degree of compactness acquired by the PbO 2 layer at applied anodic current densities in excess of 60 A m 22 .

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This work is an experimental and theoretical study of antimony speciation and the solubility of Sb2O5, Sb2O3 and Sb2(SO4)3 at 298 K in aqueous sulphuric acid solutions (0 to 6 kmol/m3). A thermodynamic model was developed to quantify the solubility products and aqueous metal speciation. Main dissolved species were H3SbO3(aq) and H3SbO4(aq). The solubility of antimony increases with increasing solution acidity and with the presence of hydrogen peroxide in the solution. The following standard equilibrium constants at 298 K were obtained: Log K0(Sb2O5) = 11.7±0.5, Log K0(Sb2O3) = 32±1, and Log K0(Sb2(SO4)3) = 55±2.

The dependence of the corrosion rate on cell current density (CD) for three anode materials (PbCa-Sn, RuO 2 -IrO 2 /Ti and IrO 2 -Ta 2 O 5/ Ti) in a laboratory scale copper electrowinning cell has been studied by means of short term weight loss tests, scanning electron microscopy observations and energy dispersive spectroscopy analysis. The lead anodes (Pb-Ca-Sn) corroded at all the studied cell CDs, and their corrosion rate increased with increasing cell CD. The precious metal oxide anodes (RuO 2 -IrO 2 and IrO 2 -Ta 2 O 5 ) only exhibited corrosion at the highest tested cell CD (1000 A m 22 ), and their corrosion rates were about a quarter of the lead corrosion rate at the same cell CD. The electrocatalytic properties of the three anode materials were characterised by means of potentiodynamic experiments. The overall results pointed to IrO 2 -Ta 2 O 5 /Ti as the best anode material of choice, although plant tests would be required before deciding on any specific commercial use. List of symbolsA electrode surface area/m 2 Cu EW copper electrowinning I current/A m Cu mass of deposited copper during the test/kg m corr corroded anode mass/kg r corr corrosion rate/g h 21 cm 22 or kg s 21 m 22 SEC specific energy consumption/kWh kg 21 t time/s V cell , V cell, ave cell voltage/V; average cell voltage/V W electrical energy/J or kWh