GB895690A - Improvements in or relating to the preparation of sodium carbonate - Google Patents

Abstract

<PICT:0895690/III/1> In a continuous process for the production of sodium carbonate from an aqueous liquor containing NaCl, Na2CO3, and NaOH by carbonating the NaOH with a gas containing CO2, and crystallizing Na2CO3, the carbonating and crystallizing are carried out in separate zones, the concentration of NaOH in the liquor supplied to the carbonating zone being such that on carbonating no crystallization of Na2CO3 takes place, an additional amount of NaOH solution is added to the carbonated liquor during transfer to the crystallizing zone, and the liquor supplied for carbonating is part or all that remaining after crystallization. CO2 and water vapour are fed into carbonator 2 and scrubbed at 35 DEG -100 DEG C. with a spray of mother liquor containing Na2CO3, NaCl, and 1-10% NaOH from storage tank 4. Alternatively the carbonator may be a packed gas washing tower. The carbonated liquor is passed into mother liquor cycling by pump 5A through steam heat exchanger 6 and pipe 9 from crystallizing section A of evaporator-crystallizer 1 to evaporator section B, and concentrated caustic soda solution, e.g. electrolysis cell liquor containing 50% NaOH, is added at 12. Reduction of pressure in B by vacuum equipment 7 causes evaporation of water, whereby the liquor is concentrated and super-saturated with Na2CO3 . H2O. The partially evaporated liquor is passed to the bottom of crystallizer A to form a crystal suspension, which is removed as a slurry by pump 5B to centrifuge 3 for recovery of Na2CO3 . H2O crystals. The mother liquor from 3 passes to storage tank 4 whence mother liquor may be returned to crystallizer A as well as carbonator 2. Mother liquor may be withdrawn directly from crystallizer A through pipe 10 to tank 4, or passed directly to the carbonator 2. Mother liquor may be purged at a suitable point in the cycle to prevent chloride build up.