A tin salt aqueous solution containing stannous ions (Sn.sup.2+) is generally used as an electrolyte containing tin ions. This aqueous solution containing stannous ions is very unstable since stannous ions are easily exidized into stannic ions (Sn.sup.4+) by oxygen in the air or oxygen generated during electrolysis.
As such stannic ions form in the electrolyte, the concentration of stannous ions in the electrolyte decreases and the current efficiency of electrolysis is reduced. Furthermore, it gives rise to the serious problem that the stannic ions become insoluble substances such as stannic acid, etc. which are likely to become colloids and therefore great difficulty is encountered in removing such insoluble substances by filteration. These insoluble substances attach onto the surface of the product and cause pitting thereon.
Therefore, in order to obtain good quality products, it is essential to control the concentration of stannic ions in the electrolyte.
With regard to preventing insoluble precipitates from forming, it is effective to add a chelating agent to the electrolyte to dissolve Sn.sup.4+. In this case, however, Sn.sup.4+ is accumulated in the electrolyte, leading to an increase in the concentration of Sn.sup.4+ and, as a result, electrolytic coloring or electro-plating tend to become difficult. Therefore, a tin salt aqueous solution in which the concentration of stannic ions has increased by oxidation has had to be abandoned.
The regeneration of the electrolyte by the reduction thereof has been effected. One of such regeneration methods is an electrolytic reduction method as described in Japanese Patent Publication No. 19856/1978. In accordance with this electrolytic reduction method of stannous ions and stannic ions which are present in the electrolyte, the stannous ions are first reduced into metallic tin and thereafter the stannic ions are reduced into metallic tin. Therefore, although this method is excellent, a very long processing time is required and equipment and operation costs are increased.
Additionally, as the regeneration method of an electrolyte, a method of reducing stannic ions into stannous ions by adding those metals which are baser than tin has been carried out. According to this method, however, since the added metal is present in the electrolyte as a metal ion, it may exert bad influences on electrolytic processing. It is, therefore, necessary to remove such metal ions, but the removal operation of the metal ions is very complicated. Thus this method is not preferred from practical and economic standpoints.
The inventors have studied to develop a method which removes the disadvantages in the prior art as described above and which permits the reduction of stannic ions by a simplified procedure in a short time and the formation of a regenerated electrolyte having a stable composition. As a result, it has been found that when metallic tin is added as a reducing agent to the electrolyte, as necessary, after the pH of the electrolyte has been controlled to the acidic region, the metallic tin and stannic ions undergo the oxidation-reduction reaction, promptly both changing into stannous ions.