Extraction of tin from scrap by chemical and electrochemical methods in alkaline media

Extraction of tin from tinned plates serves the double purpose of preparing steel scrap for re-melting and producing secondary tin. The best route to reach these goals is offered by hydrometallurgy. In order to compare the efficiency of chemical and electrochemical removal of tin coatings, the mechanisms of the principal processes were investigated. Laboratory analysis demonstrated that tin dissolution rates in oxygenated NaOH solutions are controlled by the oxygen transport to the reaction interface. Increased sodium hydroxide concentration (>0.1 M) has a negative effect on tin dissolution because of reduced oxygen diffusivity. The efficiency of electrolysis in alkaline stannate/stannite electrolytes strongly depends on the applied current density and temperature. Under the applied experimental conditions, cathodic extraction of tin (in an appropriate spongy form) could be stabilised at current efficiencies higher than 80%. Measurement of the anode potential was found to yield a proper end-point indication for de-tinning tinned steel plate scrap in anode baskets. The results demonstrate the practical advantages of electrochemical processing in alkaline media.