Abstract:
The leach solution obtained by leaching of zinc containing ores typically has to be highly purified before it can be used as electrolyte for the electrowinning of zinc. Cobalt is a troublesome impurity in the sense that not only has it even at relatively low concentrations a very significant negative impact on the zinc electrowinning process, but that is also difficult to remove by the zinc cementation process typically used for this purpose. The aim with the present work was to better understand the arsenic activated cementation of cobalt using zinc powder to enable the optimization of an industrial purification plant. Thermodynamic based calculations confirmed that the role of arsenic in the process is to allow for the precipitation of the cobalt at more positive potentials as cobalt arsenide and that it should be possible to remove the cobalt to very low concentrations with zinc cementation. The kinetics of cobalt cementation was studied using batch cementation experiments using different sizes and quantities of zinc dust and by varying the temperature. The nature of the cementation products was characterized using scanning electron microscopy and energy dispersive spectroscopy. It was found that the cobalt cementation could be described by a first order rate equation but with a faster initial stage with an activation energy of 43 kJ/mol followed by a much slower temperature insensitive second stage. Activating species such as copper, cadmium and arsenic cemented faster than the cobalt on the zinc. The rate of cobalt cementation was increased by using the same mass of finer zinc, increasing the temperature and recirculation of some of the cemented cobalt. It was shown that the zinc dust consumption and/or the minimum temperature required to achieve the required cobalt removal could be reduced by recirculation of the cobalt cement from the early stages of a train of backmix reactors or by using zinc dust with a finer size distribution.
