GB742079A - Process for manufacture of a granular perborate having a small bulk density - Google Patents

Abstract

Granular alkali metal perborates, especially those of sodium and potassium, with reduced bulk density are prepared by forming from perborate crystals having an average size of at most 0.1 mm., an aqueous paste of such water content that it is not capable of flowing, then breaking down the paste to pieces capable of passing through a sieve of 1.5 mm. inside width, and finally drying the pieces in known manner. The starting perborate crystals may suitably be prepared by reacting hydrogen peroxide of at least 10 per cent concentration with a metaborate solution of at least 35 per cent concentration in such amounts that the calculated concentration of the crystallizable perborate in the resulting solution amounts to at least 10 grams per litre. According to an example, a 40 per cent sodium metaborate solution containing colloidal magnesium silicate was treated with a 20 per cent hydrogen peroxide solution and the resulting crystal paste was cooled to 4 to 12 DEG C and the mother liquid sucked off. The maximum grain size was 0.006 mm. The paste was then washed with water to leave a paste with a 42 per cent by weight water content. The paste was then pressed through a sieve of 1.5 mm. inside width and the resulting granules air dried. After drying the granules were again pressed through the sieve. The bulk density was 430 grams per litre. The moist crystal paste may if necessary be dewatered to the required degree, normally to a 40 to 45 per cent by weight water content, by washing the paste with a water soluble organic solvent such as methyl alcohol, ethyl alcohol or acetone. Stabilizers such as magnesium silicate referred to above, sodium stannate, metastannic acid, nitric gel or ethylenediamine tetracetic acid, nitrilotriacetic acid their alkali metal salts may be added to the perborate. If desired the dried perborate granules may be heated from 40 to 80 DEG C to further reduce the bulk density by removing water of crystallization.