Sodium sulfite: A promising reagent in the electrochemical oxidation of metallic silver

Abstract

Among the complexing agents suitable for the electrochemical silver recovery, a promising one is sodium sulfite forming silver complexes with high stability constants: according to our data, log β1 ~ 5.3, log β2 ~ 8.2. This reagent is accessible and cheap, but nevertheless it almost never has been used for these purposes. We studied the stability of the sodium sulfite solutions in air and showed a possibility of using them for a long period. Using the methods of potentiodynamic polarization and potentiostatic electrolysis, we have studied characteristics of the process of metallic silver oxidation in the sulfite media. The anodic polarization process of the silver dissolution is shown to occur with 100% yield with respect to the consumed current. Simultaneously, on the cathode deposited up to 90% of dissolved silver. An urgent problem of chemical technology is the extraction of noble metals, particularly silver, from the electronic and jewelry scrap and similar sources. The traditional methods of the silver extraction (1-3) are based on its dissolution in the presence of oxidants in acid solution (perchloric acid, sulfuric acid, nitric acid) or complexing agents (cyanide, thiocyanate, thiourea). From these solutions, silver is isolated as insoluble salts or is reduced to metal. The main disadvantages of these techniques are the high consumption of reagents and non-selectivity of dissolution: besides silver copper, iron, and other components of the raw material are dissolved. Significantly less expensive are the methods that use electrolytic anodic oxidation of silver in specially selected electrolytes (4). Processing a material containing large amount of basic metals (copper, iron, nickel, etc.) should be performed under conditions providing selective transfer of silver in the solution. For this purpose are used the electrolytes containing thiourea (5), thiocyanate (6, 7), amino acids (8), etc. Silver can be deposited and isolated on the cell cathode. Note that the earlier proposed thiourea me- dium (2) still is widely used in the gold and silver hydrometallurgy. The study of the process of elec- trolytic oxidation and reduction of silver has been carried out in (9-11). A promising complexing agent in the processes of electrochemical oxidation of silver is sodium sulfite, the side product in processing of sulfide ores used in the manufacture of chemicals. The stability of the