Improvements in causticisation of green liquor produced in the alkaline processes of the manufacture of paper pulp

摘要

581,579. Electrolytic recovery of caustic soda. ASSOCIATED PULP & PAPER MILLS, Ltd. April 11, 1944, Nos. 6683 and 6684. Convention dates, April 27, 1943, and May 19, 1943. [Class 41] Caustic soda is recovered by electrolysis from the green liquor obtained by dissolving in water the sodium carbonate formed when black liquor is evaporated and burnt. The electrolysis is preferably performed in a mercury-type cell, although diaphragm-type cells may be used. The green liquor is passed through a separator, Fig. 1 (not shown), to remove suspended solids and is then introduced into the primary compartment of a mercury cell.' The liquor decomposes, giving carbon dioxide at the anode and sodium at the cathode, the sodium immediately forming an amalgam with the mercury. The amalgam passes to the secondary compartment where the sodium forms caustic soda upon reacting with the electrolyte from the primary cell. This electrolyte is only partially denuded of soda in order to keep the current efficiency high, and may alternatively, Fig. 2 (not shown), be returned to the dissolvers, water being used in its place in the secondary cell. Fig. 3 (not shown) illustrates a modification in which part of the black liquor is passed to the secondary cell to regenerate the caustic soda. Where electrolytic cells are used, Fig. 5, the electrolysis may be controlled to remove only about 50 per cent. of the soda, in which case the effluent is evaporated to produce crystals which are filtered and passed back to the dissolvers. If the " sulphate " process is used, Fig. 6, hydrogen sulphide is also given off at the anode, the mixture of gases being passed into an absorption tower where the hydrogen sulphide is absorbed in a solvent from which it is easily regenerated. The hydrogen sulphide then passes to a second absorption tower where it is mixed with the caustic solution to produce the digested liquor for the wood-pulping operation. Other methods of separating the hydrogen sulphide from the'carbon dioxide together with examples of voltage, current density, and composition of anodes,and electrolytes, are given.