Improvements in or relating to the purification of caustic soda solutions

摘要

In purifying concentrated caustic soda solutions by the addition of sodium sulphate to form an insoluble complex salt with sodium chloride and caustic soda, the removal of the complex salt from the solution and the recovery of sodium sulphate from the complex salt, an accumulation of sodium carbonate in the system is prevented by dissolving a portion of the recovered sodium sulphate in water, adding sulphuric acid to the sodium sulphate solution, treating the complex salt with the solution to dissolve sodium chloride and caustic soda therefrom while leaving at least a portion of the sodium sulphate undissolved, separating the undissolved sodium sulphate from the solution and thereafter re-using at least a portion of the recovered sodium sulphate in the process for the purification of further amounts of caustic soda solution. Preferably, the sodium sulphate solution should be saturated and sufficient sulphuric acid should be added to give a neutral solution. The acid-treated solution may be filtered before it is brought into contact with the complex salt. The complex salt may be treated with the solution at a temperature above 34 DEG C. Preferably, the conditions should be such that the solution obtained after treating the complex salt with the neutralized sodium sulphate solution contains 16--26 per cent caustic soda. In an example, 40 per cent of the recovered sodium sulphate is used for the treatment of the complex salt the remainder being used for treating the caustic soda solution. Specifications 299,995, [Class 1 (ii)], and 452,218 are referred to.