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For many years it has been considered impossible to dissolve tin anodically in dilute sulfuric acid. Even if the appropriate experiments are made with solutions containing 20 g of granulated tin per liter of sulfuric acid, only one-third is dissolved, which leads to a high content of quadrivalent tin compounds.
Processes for the production of tin (II) sulfate which operate without membranes cannot avoid this, as it is only possible to bring the deposited tin back into solution by reversing the polarity, or if the concentration of the electrolyte liquid is lowered. This is not economic.
The invention provides an electrolytic process for producing tin (II) sulfate in which it is possible to obtain almost 100% current yields. In this process, a special diaphragm is used which is made of inorganic, fire- and acid-resistant material. These are sintered diaphragm plates of mullite (a flame-resistant, alkaline earth aluminum silicate) with wall thicknesses in excess of 5 mm, pore widths of less than 2 mm and a pore volume of over 45%.
These diaphragms are placed into a rectangular PVC cell, in which the two outer spaces (cathode space) and an inner space limit each other by means of partitions. The cell is filled with 112 liters of sulfuric acid solution, each having a concentration of 250 g/l H2 SO4 and 30 g decaethyleneglycol-mono-nonylphenol per liter. The solution is then reacted for 48 hours to form tin (II) sulfate. After this time, the tin-containing coating at the anode becomes white and falls off.