| 摘要 |
Sodium carbonate monohydrate is prepared from light soda ash by a continuous process comprising maintaining in a crystallizing zone a slurry of (a) an aqueous mother liquor saturated with sodium carbonate and having dissolved therein 3-17% by weight of sodium chloride based on the weight of the mother liquor and (b) 30-65% by weight of solid sodium carbonate monohydrate crystals, based on the entire weight of the slurry, the slurry being maintained at a temperature below, but not more than 20 DEG C. below, the transition temperature for conversion of sodium carbonate to anhydrous sodium carbonate and being agitated so that there is at least 30% by weight of solid sodium carbonate monohydrate in any part of the slurry; continuously feeding light soda ash to the slurry at a rate of 800 to 4800 gm. per hour per litre of slurry; continuously discharging part of the slurry and classifying the discharged slurry into a second slurry containing smaller crystals of sodium carbonate monohydrate and a third slurry containing larger crystals of sodium carbonate monohydrate; recycling to the crystallizing zone at least part of the second slurry and separating crystals of sodium carbonate monohydrate from the third slurry and recycling at least part of the mother liquor to the crystallizing zone. The monohydrate may be dehydrated to produce dense soda ash. Alternatively the light soda ash may be added as a slurry prepared by (a) mixing light soda ash with mother liquor (b) evaporating sodium carbonate or sodium carbonate-sodium chloride solutions at temperatures above the transition temperature or (c) decomposition of sodium bicarbonate suspended in an aqueous solution containing sodium carbonate, sodium bicarbonate and sodium chloride. The slurry may be added above, at, or slightly below the monohydrate/anhydrous sodium carbonate transition temperature. Temperature control may be achieved by, for example, controlling the temperature of incoming materials by circulating through a water-cooled heat-exchanger, by blowing air across the surface of the slurry, or by circulating part of the slurry through a flash tank maintained at a pressure below that of the crystallizer. |
