This invention relates to the detinning of scrap tin plate in an aqueous solution of sodium salts. In the past, an aqueous solution of 12%-14% sodium hydroxide by weight and 11/2%-2% sodium nitrite by weight has been used in batch detinning operations, the solution being heated to a temperature of approximately 210.degree. F. Heretofore it was common practice to use sodium nitrite instead of the less expensive sodium nitrate because, in the batch operations, sodium nitrite is faster, and the batch process as heretofore carried out was found to be very slow in any event. A batch of scrap tin plate was immersed in the heated solution until all of the tin thereon had been changed to sodium stannate. This required an immersion period of approximately two and one-half hours. The detinned scrap was then removed from the detinning bath and a fresh batch of scrap tin plate was immersed therein. This was repeated until the amount of sodium stannate in the solution approached the saturation point for sodium stannate in the solution under the above-noted conditions. At that time the solution was pumped to a clean tank and the sodium stannate was removed by electrolysis or by adding sulphuric acid or other acids to the solution for the production of tin oxide.
The above-noted process can be speeded up by raising the temperature of the solution, but since the solution is already operated near its boiling point, the boiling point of the solution would have to be first raised by increasing the concentration of sodium salts therein. This was not done in the past, however, because increasing the concentration of sodium salts reduced the percentage of sodium stannate that could be held in the solution and caused the sodium stannate to precipitate out of the solution and fall to the bottom of the tank, where it mixed with dirt and other foreign matter and became almost impossible to recover.
In accordance with this invention, however, it has been discovered that the immersion time for the scrap tin plate can be reduced from two and one-half hours to a period as short as eight to twenty minutes by using the combination of higher concentrations of sodium salts, higher temperatures, and continuous removal of the precipitated sodium stannate out of the solution in a centrifuge or filter press.
Accordingly, the principal object of this invention is to provide a detinning process which is significantly faster than those heretofore known in the art.
Another object of this invention is to provide a detinning process which may be a continuous process.
A further object of this invention is to provide a detinning process in which tin is removed from the detinning solution by continuously precipitating out sodium stannate crystals and separating them from the solution.
An additional object of this invention is to provide a detinning process of the above-noted character which is less expensive than those heretofore known in the art and which permits the efficient use of the less expensive sodium nitrate instead of the more expensive sodium nitrite.
Yet another object of the invention is to provide a detinning process in which scrap tin plate is continuously conveyed through a detinning bath rather than being inserted therein in batches.