Evaluating the effect of molecular properties of polyacrylamide reagents on deposit growth in copper electrowinning
By Coetzee, C.; Tadie, M.; Dorfling, C.
Published in Hydrometallurgy
2020
Abstract
Copper electrowinning reagents include levellers which are included to control deposit growth characteristics. These additives are made up of high molecular weight molecules such as the polysaccharide type (e.g. Guar gum) and less common polyacrylamide (PAM) type. Adsorption of these molecules at the surface of the deposit facilitates deposit levelling. To improve performance of these molecules it is necessary to consider the chemical design of these additives through an understanding of the relationship between chemical characteristics and additive performance. The focus of this work is the consideration of molecular weight as a controlling parameter for cathode smoothness. In this work, voltammetry was used to compare the polarisation effects of industrial PAM flocculants of varying molecular weight with one another and with Guar gum; the polarisation effect is a proxy for inhibition and adsorption of additives on the surface. Copper growth was evaluated using scanning electron microscopy, and qualitatively evaluated using morphological characterization descriptors previously reported in the literature. It has been shown that inhibition intensity or polarisation of PAM additives increases with decreasing molecular weight resulting in deposit growth which on a microscale indicates superior levelling capabilities. The controlling mechanism of the molecular weight range of the additives is confirmed through electrowinning experiments. The lowest molecular weight PAM showed superior reduction in surface roughness compared to Guar and higher molecular weight PAMs. It is shown that molecular weight is a relevant design consideration for additive performance in electrowinning.