PROCESS FOR RECOVERING SULPHUR DIOXIDE FROM OFF GASES

摘要

Sodium sulfate is purged from a sulfur dioxide removal system involving contact of a sulfur dioxide-containing gas with a solution containing sodium sulfite to absorb sulfur dioxide from the gas. The spent absorbing solution is regenerated by desorbing sulfur dixoide and recycled for further use. To avoid an unduly large build-up of sulfate in the system, at least a portion of the absorbing-desorbing medium, e.g. spent absorbing solution, containing sodium sulfate and a relatively large amount of sodium bisulfite is treated to reduce the amount of water in the medium so that there is precipitated therefrom up to about 10 weight percent undissolved solids containing sodium sulfate in greater concentration than would otherwise be obtained in the absorption-desorption cycle. The insolubles containing sodium sulfate are removed from the liquid, and the liquid can be returned to the sulfur dioxide removal system. In one preferred aspect of the invention, up to about 75 weight percent of the entire stream of spent absorbing solution is treated to form from this solution a slurry having up to about 10 weight percent undissolved solids which are relatively rich in sodium sulfate content. In the invention the sodium sulfate-containing solids can be separated from the liquid which is then subjected to a desorption operation to produce sulfur dioxide, and the latter operation can be conducted while maintaining at least about 25 weight percent undissolved solids in the desorption zone.