GB924245A - Sodium carbonate manufacture - Google Patents

Abstract

Sodium carbonate monohydrate and anhydrous sodium carbonate are prepared by carbonating a caustic soda liquor containing less than 20% by weight of NaOH and more than 6% by weight of Na2CO3, but preferably less than 12% NaOH and more than 14% Na2CO3, at a temperature above 30 DEG C. with sufficient carbon-dioxide-containing gas to convert substantially all the NaOH to Na2CO3, and then reacting the carbonated liquor from the carbonation stage in a crystallisation stage with a caustic soda liquor containing at least 56% and less than 63% by weight of NaOH if sodium carbonate monohydrate is to be produced, and at least 63% by weight of NaOH if anhydrous sodium carbonate is to be produced, without the application of heat from any external source, and separating the precipitated product in each case. The caustic soda liquor which is carbonated is a mother liquor from a preceding crystallisation stage. Where anhydrous sodium carbonate is to be produced the liquor from the carbonation stage may be divided into portions, one of which is reacted in the crystallisation stage with a caustic soda liquor containing at least 63% by weight of NaOH whereby anhydrous Na2CO3 is precipitated and separated to give a first mother liquor and a magma, and said first mother liquor is thereafter mixed with the second portion of liquor from the carbonation stage whereby sodium carbonate monohydrate is precipitated and separated from a second mother liquor, which is recycled to said carbonation stage, said monohydrate being thereafter returned to said crystallisation stage or to the magma from that stage, whereby in either case it is converted by heat into anhydrous Na2CO3 which is precipitated and separated by suitable means. The monohydrate may also be converted to anhydrous Na2CO3 in a separate vessel by reaction with concentrated caustic soda liquors. The crystallisation stages may be operated at a temperature between 8 DEG C. and 108 DEG C., preferred ranges being 50 DEG C. to 80 DEG C. in the straightforward processes, and 50 DEG C. to 100 DEG C. in the modification where the carbonated liquor is divided into two portions. There may be one or more crystallisation vessels, some of which may operate at reduced pressure. Scaling in the carbonation stage, which usually occurs in the upper part of the carbonator, may be prevented by having two carbonators in series and using each alternately as top or bottom of the system.