Aspects of the electrochemistry of the chemelec cell

The Chemelec Cell is an electrochemical reactor which achieves the recovery of metals from electroplating effluent by electrodeposition at concentrations too low to be considered for conventional electrolysis. The combination of a mesh-type configuration of electrodes together with the use of a non-conducting (inert) fluidised bed electrolyte engenders a sufficiently high rate of mass transport in the cell for efficient deposition to be carried out at metal ion concentrations below 1.0 gl-1. Two approaches have been made to the investigation of the system. First, experiments have been conducted using a laboratory scale Chemelec Cell in which the electrodeposition of nickel from Watts electrolytes has been carried out under a wide range of conditions of electrode potential, pH and metal ion concentration. Relatively long-term experiments have been attempted in which the cell is operated under conditions of constant electrode potential, temperature, pH and fluidisation and the fall of nickel ion concentration plotted as a function of time. Measurement of the charge passed (also as a function of time) has allowed access to the cathode current efficiency. Polarisation data, comprising two regions of electrode potential in which the current density varies semi-logarithmically with a large apparent Tafel slope (570 mV/dec.) separated by a region of some 300-500 mV in which the current density is almost independent of potential, have been obtained under these diffe rent conditions. An interpretation of these somewhat unusual characteristits has been sought. Secondly, more fundamental studies concerning the electrode kinetics of cadmium in various electrolytes have been carried out. The techniques of faradaic impedance, linear sweep voltammetry and the rotating disc have been applied and the results of these experiments interpreted. The presence of a hydroxide film on the electrode has been demonstrated and this obscures the true kinetics. Finally, the observed electrode kinetics and the experimental results obtained from the operation of the Chemelec Cell are compared and critically discussed.