Flow and mass transfer modelling for copper electrowinning: development of instabilities along electrodes

Abstract

A computational fluid dynamics (CFD) model has been developed to simulate the copper electrowinning (EW) process, and applied to model the flow and mass transfer in the inter-electrode gap for a single plate pair, with geometrical and operation parameters typical of industrial EW operation. The CFD model predicts a recirculation zone in all cases, driven by oxygen bubbles rising along the anode, with the electrolyte deflecting at the upper free surface and recirculating down to the base of the electrode space. The CFD model results showed that laminar natural convection driven by concentration-related density deficiency is dominant along the lower part of the cathode. Strong eddies arise along the cathode where copper depletion becomes large enough to drive buoyancy instabilities: the instability is analogous to the waves formed in natural convection on a vertical heated plate for Prandtl number much greater than 2. Limiting current density can be increased by decreasing the boundary layer thickness which can be achieved by increasing the velocity past the cathode, or by triggering flow instabilities such as the buoyancy-generated ones described above or more conventional shear-driven instabilities. Higher up the cathode, the natural convection profile becomes completely broken up by the recirculating down-flow. Similar instabilities also form close to the anode due to build-up of oxygen bubbles, the fluctuating velocities associated with the anode instabilities being much higher than those at the cathode.