Sodium hydrosulfite dihydrate density is a critical factor in determining the quality and purity of alkali metal hydrosulfites used as reductive bleaching agents for textile fibers. The reductive action of sodium hydrosulfite dihydrate in the dyeing process reduces excess carbonyl and alcohol groups that cause color problems and improves the colour fastness of textile materials.
Typical densities of commercially available sodium hydrosulfite dihydrate are from about 85% to 90% by weight, and are typically obtained as crystalline powder or as a refrigerated 150 g/L solution stabilized with alkali. This white crystalline solid is soluble in water and has a sulfur dioxide-like odor.
The present invention provides for high purity solutions of sodium hydrosulfite in which the concentration of alkali metal thiosulfate ions is reduced to insignificant levels by adding a stabilizing amount of an aqueous solution of a stabilizing agent. The stabilizing agent may be an alkali metal hydroxide or an alkali metal carbonate compound.
A slurry of needle shaped crystals of sodium hydrosulfite dihydrate is produced in a crystallizing zone having a residence time from about 30 minutes to about 240 minutes and a temperature range from about -5deg C. to about +10deg C. The crystallizing zone is operated to maintain supersaturation in the aqueous solution at below about 5 grams per liter.
The slurry of needle shaped crystals is conveyed from the crystallizing zone to a separating zone employing non-gravitational separation means. The separating zone, which is operated to maintain overflow to underflow ratios of from about 1:1 to about 2:1, rapidly separates larger sodium hydrosulfite dihydrate crystals from a mother liquor. The separating zone may utilize a variety of suitable separation means, including but not limited to a filtration system, a centrifuge, and a hydrocyclone.
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