(Digital Presentation) The Role of Iron in the Zinc Electrodeposition from Chloride Media for Recovering Zinc from Spent Pickling Liquors

Abstract

Electrodeposition of Zn from chloride media had been studied by potentiodynamic method to understand the characteristics of Zn recovery from spent pickling liquor (SPL). Various concentration, pH, agitation speed and impurities contamination were examined in the series of experiments. The influence of Zn concentration in the range of 30-150 g/dm3 relevant to the real SPL were studied along with acidity level change (pH 1.5-5.5). It was found that the cathodic deposition started with the uniform patern followed by sponge-like deposit and as the concentration of the electrolyte near the cathode surface decreased, the dendritic deposition start to grow, especially at the edge of the cathodes.It was found that the effect of iron concentration on the polarization curves of zinc from SPL is complex. Initially it has a negative effect on the generated cathodic current because of the enhancement of hydrogen bubble formation. At a higher concentration range, however, iron deposits at a relatively higher rate. Further increased iron concentrations may make the composition of the Zn-Fe deposit dominantly in favor of iron, resulting in a hydrogen dominated cathodic mechanism. It is also characterised by the loss of the dendritic structure – attributed mostly of zinc deposition when hydrogen bubbles are not blocking the cathode. As the gas evolution becomes more characteristic, the deposit tends to become more powdery. Under such conditions the cathode becomes smoother and the active surface is reduced. Increasing the stirring speed, the powder was easily detached from the cathode surface. Iron, however can enhance the cathodic deposition process of zinc. With 30 – 60 g/dm3 iron in the 90 g/dm3 Zn solution, an increased iron concentration resulted in a significant increase in the mass of the deposited zinc. This may be interpreted by a beneficial effect of the increased hydrogen evolution enhancing the zinc ion transport to the electrode surface. However further increments of iron had a contrary effect, possibly by the locally increased – and generally detected – pH of the solution indirectly causing a superficial precipitation of hydroxides. Technically pure zinc could be deposited from only low-iron (< < 30 g/dm3) zinc solutions (of 90 g/dm3 Zn) at relatively vigorous stirring speeds. High iron concentration in the solution is definitely unsuitable for obtaining the aimed quality of cathode zinc.