Electrowinning Cell Process
Eluates containing soluble gold and silver from either a ZADRA or an AARL type elution are suitable for recovery in the Kemix Electrowinning cell.
Electrowinning is a process used to recover metals (eg. gold and silver) from concentrated solutions by applying a voltage across electrodes immersed in a concentrated solution.
The positive terminal from the rectifier is connected to the anode where the oxidation reactions occur and electrons are generated. These generated electrons are consumed at the cathode which is connected to the negative terminal of the rectifier. The reduction reaction results in the deposition of metal on the cathode. The following electrode reactions take place during electrolysis of an alkaline gold cyanide solution:
Cathode : Au (CN)¯² + ē¯ → Au + 2CN¯
Anode : 2H₂O → 4H+ + O2 + 4ē¯
In cyanide solutions, gold is present as a stable auro-cyanide complex anion with a relatively high cathodic potential (E0). The cathodic shift demands a higher cell voltage and consequently other cathodic reactions like the formation of H2 and the reduction of O2 can also occur. These additional reactions consume current and reduce the current efficiency of the gold electrowinning process.
Caustic is required in the electrolyte to firstly maintain the conductivity of the electrolyte and secondly to ensure that the pH value remains sufficiently high thus minimising anode corrosion. The recommended caustic strength in the eluate should range between 1 to 2 % NaOH and the conductivity of the electrolyte should be above 1.7 S/m.
High extraction efficiencies can be achieved at elevated eluate temperatures of approximately 70°C. This is due to a combination of lower dissolved oxygen content, reduced solution viscosity and increased ionic mobility at higher temperatures. However, a maximum electrolyte operating temperature of 90°C can be tolerated in the Electrowinning cell.
Kemix Electrowinning cells are manufactured from grade 316L stainless steel and polypropylene. It is engineered as a non-complex, robust and easy to operate unit having minimal maintenance requirements.
The Electrowinning cell is provided with a sloping channel type launder into which any accumulated cell sludge can be drained away without removing the fixed anodes. Flow level within the Electrowinning cell is maintained by an adjustable ‘V’ notched outlet weir in the tank.
The Electrowinning cell consists of a predetermined number of anodes and cathodes arranged in a sandwich configuration.
The anode and cathode bus bars are connected to the positive and negative terminals direct current (DC) Rectifier respectively.
The anode frame comprises a stainless steel support bar connected to 316L stainless steel strips and the positioning within the tank is accomplished by a polypropylene support frame (anode spacer frame).
The anode bus bars are connected in parallel and the anode frames are fastened onto the anode bus bars through specially machined insulated bushes that ensure the anode frames always stay electrically insulated from the tank shell.
The cathodes take the form of a holder for woven stainless steel or mild steel mesh with a cross-sectional area of 0,5 m2 and are 40 mm thick.
The cathodes are electrically connected through a single bus bar fitted with stainless steel threaded pins. The pins fit into a stainless lug mounted on the cathode with ample surface area contact to prevent a localised heat build-up.
The Electrowinning cell’s hinged hood has a locking facility to increase the security associated with the cathode sludge.
An air ventilation connecting flange is situated at the discharge end of the Electrowinning cell to which ventilation ducting and an extraction fan can be connected. The ventilation ducting should be constructed from polyvinyl chloride (PVC) fibreglass reinforced rolled tubing in graded internal diameters to maintain an even airflow through the manifold.