| 摘要 |
Sodium sulphite is recovered from spent semi-chemical pulp liquor by oxidizing said liquor under the vapour pressure of the reaction mixture and in the liquid phase in a reaction zone, reacting calcium bisulphite (added as such or formed in situ by introducing sulphur dioxide and an alkaline-reacting calcium compound such as the carbonate, oxide or hydroxide or the sulphite which is a by-product of the process) with the sodium-containing liquor formed in said reaction zone, separating the precipitated calcium sulphate, treating the filtrate with an alkaline-reacting calcium compound to form sodium sulphite and calcium sulphite and recovering the former. In a modification the sodium sulphate is separated after the formation of sodium sulphite. The oxidation is preferably performed by pumping the liquor into a tower reactor at 200-2000 lb./sq. in. and heated initially to 100-330 DEG C., preferably 235-250 DEG C. and passing a gas containing free oxygen, e.g. air, thereinto at a rate sufficient to ensure complete oxidation of the liquor. The oxidized liquor is then passed through a flash chamber where gases are vented and some concentration takes place. When calcium bisulphite is produced in situ, the mixture should be maintained at above room temperature, preferably at 12-25 DEG C, or more, and suitable pHs are less than 5, preferably 3-4.5. In the second stage wherein an alkaline calcium compound is added, a suitable temperature is 70 DEG C. and a pH of 7-12 is generally obtained. |
