Process and plant for producing tower acid by recycling for the chemical digestion of wood within a sulphite process

摘要

In a process for producing by recycling tower acid for the chemical digestion of wood within a sulphite process producing sulphite waste liquor, which is incinerated after evaporation to give a so-called thick liquor, a flue gas containing sulphur dioxide being taken off. The flue gas is purified in a scrubbing tower by means of circulated scrubbing water containing calcium carbonate or calcium hydroxide, and the sulphur dioxide is converted to calcium monosulphite, which is taken off as a suspension from the scrubbing water circulation. The tower acid is obtained from the calcium monosulphite. The calcium monosulphite is converted in the suspension with sulphuric acid to give a calcium disulphite Ca(HSO3)2 and gypsum. The gypsum is filtered off. The tower acid is taken off in aqueous solution.