GB882694A - Purification of sodium carbonate - Google Patents

Abstract

<PICT:0882694/III/1> Sodium carbonate is purified by cooling a slurry of unhydrated solid sodium carbonate from a temperature above 112 DEG C. to below 112 DEG C. but not less than 32 DEG C. thereby forming an aqueous slurry of solid sodium carbonate monohydrate which is separated to produce a pure product. The initial slurry of unhydrated sodium carbonate is preferably prepared by forming a slurry of sodium carbonate monohydrate, heating to above 112 DEG C. and generally from 115-150 DEG C. to convert the monohydrate to the unhydrated salt and thereafter cooling to below 112 DEG C. as above. In order to prevent coalescence it is preferable to keep the solids content of the slurry below 75% by weight during cooling. As shown in the Figure, crude soda ash was slurried in water and treated in tank 10 with elemental chlorine to improve the colour and then pumped to a reactor 18 where the slurry is heated above the transition point from the monohydrate to the unhydrated salt (112 DEG C.) and preferably at 120 DEG C. for about five minutes. The slurry of anhydrous sodium carbonate thereby formed is withdrawn at intervals through a retention tank when the temperature is maintained at 70-100 DEG C. thereby converting the solid anhydrous sodium carbonate to the desired monohydrate form. Water is added to maintain the solids concentration below 75% by weight and the slurry is preferably maintained in the tank until over 60% by weight of the monohydrate crystals have a size over 30 mesh. The monohydrate is separated in a centrifuge 30 and the solids passed through a dryer at 150 DEG C. where anhydrous sodium carbonate is formed. The mother liquor is recycled.