Silver cementation from nitrate solutions onto copper in classical and forced convection systems

During the electrorefining of silver, impurities such as copper dissolve from the anode but do not deposit at the cathode unless their individual concentrations exceed 60 g/L. Increasing copper content of the electrolyte decreases the efficiency of the system and copper starts to deposit on the cathode along with silver. Since there is no electrochemical technique to stop this copper build-up, the electrolyte must be replenished with a fresh and copper-free electrolyte, periodically. Although there are various procedures to recover silver from these solutions, cementation is the most suitable and common method from the viewpoint of economics and simplicity. On the other hand, AgNO₃ forms when silver is used for the dissolution of gold, known as "quartation process" by the plants that recycle jewelry scraps. Dissolved silver is recovered by the cementation with high purity copper. During the process, the cement silver layer, encapsulating the copper granules instigates a blockage in the system and does not allow the process to advance properly. In this study, the influences of required amount of cementator copper, stirring speed (80, 180, 400, 600 min^-1), pH (0-5.5), the cementation period (60-360 minute), and the temperature of the electrolyte (25-65°C) on the recovery of silver from silver nitrate solutions by copper are investigated. The effects of various conditions of the forced convection system on cement silver morphology and cementation efficiency are also examined. Experimental results show that the optimum cementation efficiencies can be reached when 2.5 times the stoichiometrically required amount of electrolytically pure copper granules and plates are used as cementator. Cementation efficiency, although seems to be unaffected by the pH (between 1 and 3), increases when the pH is lowered to 0.5. However, this increase in efficiency must be counterbalanced with an amplified copper consumption due to the chemical dissolution. Therefore, the pH value for the solution is predicted to be between 1 and 3 in order to eliminate the problems in the cementation reaction. Cementation speed increases with increasing temperature, and the silver obtained is fine-grained showing good adhesion characteristics. The cementation rate and period show strong dependence on the velocity of stirring in silver cementation from AgNO₃ solutions by using copper.