Process for manufacturing sodium carbonate perhydrate particles and coating them with sodium borosilicate

A process for preparing a coated sodium carbonate perhydrate stable in detergent formulations comprising

• (a) preparing sodium carbonate perhydrate by the steps of:
• (i) uniformly distributing an aqueous hydrogen peroxide solution containing about 50% to about 75% by weight hydrogen peroxide, and an effective amount of 1-hydroxyethylidene-1,1-diphosphonic acid to provide a concentration of about 0.5% to 10% by weight in step (a) (iii) onto a substantially dry particulate reaction mixture, the reaction mixture initially consisting essentially of anhydrous sodium carbonate with a pore volume of at least 0.29 ml/g,
• (ii) concurrently balancing the heat of hydration and heat of perhydration of the sodium carbonate in the reaction mixture with the heat of evaporation of water and adding sufficient sensible heat to maintain the reaction mixture between 50.degree. C. and 80.degree. C. and to maintain the reaction mixture substantially dry by evaporating water added as the aqueous hydrogen peroxide solution, and
• (iii) withdrawing at least a portion of the reaction mixture as a particulate sodium carbonate perhydrate containing about 12% to about 14% active oxygen, and about 0.5% to about 10% 1-hydroxyethylidene-1,1-diphosphonic acid, and
• (b) coating sodium carbonate perhydrate particles withdrawn in step (a) (iii) by the steps of:
• (i) suspending the particles of sodium carbonate perhydrate in a gas stream thereby substantially eliminating solid-solid contact between the particles,
• (ii) contacting the suspended particles with a plurality of fine drops of an aqueous solution of sodium borosilicate, the sodium borosilicate consisting essentially of a mixture of from 25 to 75 parts by weight sodium silicate and from 75 to 25 parts by weight sodium metaborate to provide a SiO.sub.2 :B.sub.2 O.sub.3 weight ratio of from 1:1 to 2:1, and
• (iii) concomitantly evaporating substantially all the water added by the aqueous sodium borosilicate solution at a sufficient rate to avoid moistening or hydrating the suspended particles, to provide thereby a coated sodium carbonate perhydrate containing from about 3% to about 10% coating by weight, the coated particles having a bulk density of from 800 to 1,050 kg/m.sup.3 and containing from 10.5% to about 14% by weight active oxygen, the coated particles being capable of releasing substantially all of the active oxygen into water within two minutes.