GB911664A - Improvements in or relating to the preparation of sodium perborate - Google Patents

Abstract

Sodium perborate is prepared by suspending a bed of particulate sodium perborate in a vertical column in a rising stream of liquor consisting of sodium metaborate solution saturated with respect to sodium perborate, introducing streams of aqueous hydrogen peroxide and aqueous sodium metaborate solution into the liquor at a point in or below the suspended bed in such amounts as to produce a supersaturation of sodium perborate on the liquor suspending the bed, particulate sodium perborate being continuously or intermittently removed from the bed. Preferably, the process is made continuous by recycle of the liquor leaving the top of the column to the base thereof the point of re-entry being preferably below the point of entry of the hydrogen peroxide. The supersaturation of the liquor in the zone occupied by the bed with respect to sodium perborate may be produced or enhanced by cooling, preferably by cooling a portion of the vertical column, in which case the <PICT:0911664/III/1> streams of hydrogen peroxide and sodium metaborate solution are preferably introduced into the column at points below the cooled portion. A bed of particulate sodium perborate at a temperature of say 15 DEG -25 DEG C. is suspended in a vertical column 2 by a rising stream of liquor having a linear velocity of say 0,777-0,933 cm./sec. and of concentration with respect to sodium metaborate such that under the temperature conditions of the reaction the requirement of sodium perborate for saturation is a minimum. Stoichiometric amounts of aqueous hydrogen peroxide and sodium metaborate solution of concentration say 300 gms./litre and 600 gms./litre respectively are introduced into the liquor in column 2 through inlet pipes 8 and 9 respectively, the column being preferably constricted at the point of entry of hydrogen peroxide from pipe 8 to minimize the formation of sodium perborate encrustation. Cooling coil 9 cools the combined liquor as they pass through the bed 1 causing supersaturation thereof with respect to sodium perborate which thus precipitates in the bed. At the top of the column cone 2a causes lowering of the velocity of the rising stream of liquor which thereafter does not suspend the particulate sodium borate and passes off from the column through overflow cone and pipe 4 to be recycled to the column through filter 5, reservoir 6, pump 3 and orifice 7. Any suspended sodium perborate which is carried over with the liquor is removed in filter 5. Crystals of sodium perborate are withdrawn periodically through pipe 10 at the bottom of the column, the frequency of withdrawal and thus the time of retention of the crystals in the bed determining their size. The ratio of the rate of circulation of liquor through the system to the combined feed rates of hydrogen peroxide and sodium metaborate solution is preferably within the range 75:1 to 50:1. The preferred concentration of sodium metaborate in the circulating liquor is 15-20 gms./litre.