Soapstock

Soapstock is sludge resulting from alkaline refining of vegetable oils and fats in the fat-processing industry. Categorized as recyclable waste, it has a complex and unstable composition that depends on the oil or fat nature or properties (acid number, co-substances, etc.), refining method, and process accuracy. Fatty acid composition of butter fat and tallow (hydrogenated fats).

Similar to the fatty acide composition of recyclable raw materials. Contains an aqueous solution of soaps, oil, dyes, mineral and mechanical impurities, etc. Approximate composition of soapstock generated during refining of sunflower oil, cotton oil, and hydrogenated fat, per cent by mass: fat 8–50, including soap 8–30; neutral fat 1–20; water; degradable non-fat components 50–92; small content of unreacted NaOH, NaCl, colors, phosphatides, proteins, and carbohydrates.

To extract free fatty acids from soapstock, it's treated with mineral concentrates or an alkali (to saponify neutral fat) and then distilled. Fatty acids extracted from butter fat and tallow soapstocks are used in production of soap, glycerin, and stearin. Linseed oil soapstock is used in production of drying oil and oil drying agents.