Conventional electrowinning involves placing an anode and cathode into a bath of slowly circulating or stagnant electrolyte. The metal ions eventually make their way to the cathode via diffusion to be plated. With emew electrowinning, the electrolyte is circulated rapidly past the anode and cathode at a very high flow rate, significantly enhancing mass transfer and allowing for improvements in efficiency and recovery.
“Novel electrochemical cell designs are emerging as alternative process units for various applications in industry. Most of the advances in electrochemical cell design are in the field of improving mass transfer. Improved mass transfer allows increased specific production capacity, reduced footprint, increased selectivity, and plating from lower concentrations. One of the designs gaining acceptance in industry is the annular/tubular/cylindrical electrochemical cell. It is concluded that swirl flow significantly improves mass transfer over axial flow in annular electrochemical cells due to the slower formation of the diffusion layer.”
Source: Mass Transfer in Annular Cylindrical Electrochemical Cells, Etienne Roux
Each emew cell comprises a 150mm or 200mm diameter precision cylinder made of stainless steel. The cathode area in each cell is 0.5m² for the 150mm cell, or 1m² for the 200mm cell. The ANODE runs through the center of the cell. The inside surface of the cylinder is the CATHODE.
There are two basic emew cell designs. One produces metal plate (1) commonly referred to as “cathode”, the other produces metal powder (2). In the emewPowder cell, the anode has a larger diameter and a smaller electrode gap. There are also emewPolish fitted with high surface area cathode, and emewClean, which is a hybrid of emewPlate and emewPolish.
Multiple cells are fitted together (in a series or parallel) to form a frame, with multiple frames assembled to form a modular, scalable emew production plant.
The emew cell in action
(3) With emewPlate cells, a starter sheet (usually made of stainless steel), is inserted into the body of the emew electrowinning cell. The metal being recovered is plated onto this removable/reusable starter sheet when current is passed between the ANODE and the CATHODE.
(4) The electrolyte is pumped through the cell from the bottom. Power is applied to the cell between the ANODE and CATHODE. Metal then begins to plate, or forms as powder, on the CATHODE.
Harvesting The Metal
emewPowder cells are harvested automatically using a flush-through methodology. This flush harvesting is PLC controlled and fully automatic. The metal powder is collected in a conical bottom separator tank, dewatered using a filter or hydrocyclone and dried.
emewPlate cells are harvested by removing the upper casing and extracting the cathode with an overhead hoist equipped with a uniquely designed harvestor head. The reusable starter sheet is then replaced and the electrowinning process starts again. The time to harvest each plating cell is approximately 30-60 seconds.