GB848986A - Regeneration of solvents used in the preparation of isocyanates - Google Patents

Abstract

Alkyl phthalates or other solvents which have been used in the production of isocyanates and whose boiling points are higher than those of the corresponding isocyanates are recovered and regenerated from the reaction mixture after the isocyanate has been distilled therefrom and after the used solvent has been drawn from the distilling apparatus, by subjecting the used solvent to a heat treatment at 150 DEG -200 DEG C. to cause precipitation of impurities which are then separated by filtration or centrifuging. The impurities which are removed include residual isocyanate compounds. The thermal purification treatment may be associated with a treatment with about 2% of a substance containing -OH or -NH groups capable of reacting with I the isocyanate compounds remaining in the used solvent and converting them into insoluble compounds. Substances which may be used for this purpose are water, acids, bases, alcohols, and amines. Alcohols such as methanol and ethanol, amines such as methylamines and aniline, and ammonia, are preferred. After filtration or centrifuging, the regenerated solvent may be re-used in the isocyanate production and it may be recycled several times. If desired, the solvent may be clarified by treating it with active carbon or activated earth. In an example, a cooled solution of tolylene diamine in butyl phthalate is added to a solution of phosgene and butyl phthalate; the mixture is heated and a further quantity of phosgene is added; nitrogen is bubbled through the mixture; the mixture is subjected to vacuum distillation and the used solvent is subsequently drawn off and filtered; the filtrate is heated for 90 minutes at 180 DEG -200 DEG C. and the precipitate which forms is removed by filtration; the butyl phthalate filtrate contains only 0,2% of NCO groups. Other examples describe the use of ethyl phthalate and its regeneration by addition of aniline followed by heating, by addition of ethanol and water followed by heating, and by addition of bleaching black and activated earth followed by the heat treatment, the liquid being then separated from the solids by filtration. Octyl phthalate may be used as solvent instead of ethyl or butyl phthalate.