The Limiting Phenomena at the Anode of the Electrowinning of Zinc from Zinc Chloride in a Molten Chloride Electrolyte

The objective of this research is to investigate the possibilities and technological viability for the electrowinning of zinc from zinc chloride. This research contributes to development of an alternative process, because it provides: ⢠A clear understanding and overview of the present zinc industry and future developments. ⢠A thorough literature investigation, leading to: o Understanding the reasons to abandon the proposed process route previously used (molten salt electrowinning in particular). o Definition of the practical problem and focusing the research on the anode, where the main loss of energy occurs. ⢠An ingenious experimental set-up enabling visual observations of the electrowinning process that contributed to a creative approach of interpreting the bubble phenomena at the anode. ⢠A basic model that enables a systematic analysis of the experimental data. Zinc is extracted from ores and products when they have served a useful life, via physical separation and extractive metallurgy. An analysis of both hydro- and pyrometallurgical processing routes is provided, which leads to a motivation to develop an alternative process for the winning of zinc. The incentives for an alternative process can be summarised as: ⢠Growing environmental awareness. ⢠Economical considerations, such as reduced energy consumption. ⢠Possibility to process complex zinc-containing materials. A dry chlorination process can possibly meet these criteria. In order to position molten salt electrowinning of zinc in the current state of technology and research in this area, commercially operated molten salt electrowinning processes and new developments are discussed. The decomposition potential of zinc chloride is low compared to zinc sulphate, thus theoretically the electrical energy consumption per kilogram of zinc can be reduced. The main properties of molten salts which make them attractive for electrolysis compared to aqueous electrolytes are these high conductivities and diffusivities. In addition, it is possible to extract zinc in the molten state. Zinc chloride itself is not an appropriate electrolyte, because it has a low conductivity, high viscosity, high vapour pressure, is very hygroscopic and has a high solubility for zinc metal, thereby reducing the current efficiency. An electrolyte with more suitable properties is prepared by the addition of alkali chlorides. When reviewing the existing literature data, it appeared that the energy consumption lies in the range between 1.8 and 8.8 kWhâ¢kg-1 zinc. The electrowinning of zinc from zinc chloride has received attention for about a century and has been performed under different conditions, on different scales and with different cell designs. Experimental results have been used to validate and discuss the existing knowledge critically, indicating that scale-up causes an increased cell potential. It is proven that the anodic current density has been evaluated incorrectly in literature, since the effective anode area has not been defined correctly. On the basis of a systematic experimental study into the cathodic reaction of these results it can be concluded that processes occurring at the cathode are not the reason for the relatively high energy consumption of the electrowinning process. The current efficiency is sufficiently high (generally > 98%) and the electrochemical study proved that the process is ohmically limited at the conditions employed. The cathodic current density can be extremely high without experiencing mass transfer limitations, whenever freezing of the electrolyte is prevented. When the electrolyte composition is prepared to have a melting point sufficiently above the operating temperature (e.g. by increasing the ZnCl2 concentration, or by the addition of LiCl), very high current densities can be applied at relatively low potentials, i.e. j > 1 Aâ¢cm-2, at Ecell < 2.5 V and current efficiencies > 95%. On increasing the anodic current density, the energy consumption of the process rises considerably, due to a higher cell potential. The experimental results prove that the bubble phenomena at the anode give considerable rise to the energy consumption of the electrolysis of zinc chloride under the experimental conditions employed. Because the chlorine bubbles growing at the anode surface area tend to stick at the surface, and because of the large volumetric production of gas, a large ohmic overpotential for the anodic reaction emerges. This conclusion could be drawn on the basis of electrochemical measurements and could be visualised in the see-through furnace. Different concepts have been applied in order to be able to remove the chlorine bubbles from the surface, but none of these were successful. To minimise the energy consumption for the electrowinning process, the ratio of the anodic to cathodic surface area ratio (A:C ratio) is the most important parameter. The ratio of the effective conductivity to the specific conductivity of the electrolyte was determined to be about 0.4 and the anodic overpotential contributes for about 90% to the total overpotential for an A:C ratio of about 1. The anodic overpotential is only about 50% of the total overpotential when the A:C ratio is about 24. A model has been developed to give clear insight into the contribution of the different terms to the total ohmic drop and it has been determined that, in general, the largest part of the ohmic drop is due to the layer of chlorine adhering to the anode. The ohmic drop has been modeled by representing the interelectrode gap as three layers, i.e. a layer of gas of certain thickness attached to the electrode causing coverage, the plume (a rising mixture of gas and electrolyte) and a layer of pure electrolyte. The dimensions of the plume are characterised by the plume angle, which has been measured by image analysis. The effects of anode material, anode height, pressure and temperature on the electrowinning have been determined. The velocity of the plume has been modeled by solving the momentum balance, to determine whether the rising velocity of the plume could affect the layer of bubbles adhering to the anode and covering the anode. The model cannot be used to demonstrate the interaction between the two layers, but provides a good and reliable estimate of the plume velocity in the vertical direction. In general, it can be concluded that the electrowinning of zinc from zinc chloride is technically feasible. This work has clearly demonstrated that the bottle-neck of the process is the anodic reaction, but that it can be more attractive than the conventional aqueous process with regard to energy consumption