GB640976A - Improvements in the manufacture of thiourea - Google Patents

Abstract

Thiourea is prepared by progressively introducing free cyanamide, i.e. in the crystalline or fused state, into an aqueous solution of alkali sulphide which is maintained saturated with hydrogen sulphide. The excess hydrogen sulphide prevents side reactions by converting any base formed into the corresponding sulphide and is maintained by passing a current of the gas through the reaction vessel during operation. Temperatures instanced are 30-60 DEG C., particularly 50 DEG C. Thiourea may be isolated by crystallization from the reaction medium and the mother liquor returned for further processing. In examples: (1) molten cyanamide at 45 DEG C., and (2) crystallized cyanamide is fed into a closed vessel containing aqueous ammonia maintained saturated with hydrogen sulphide at 45-50 DEG C. and when reaction is complete the mixture is cooled to 15 DEG C., crystals of thiourea collected and mother liquor continuously recycled, in (3) sodium sulphide is prepared and used in place of the ammonium sulphide of (1), and in (4) a continuous process is described in which molten cyanamide is run into aqueous ammonium sulphide saturated at 20 DEG C. with thiourea and after reaction at 45 DEG C. the mixture is passed to a cooling vessel at 20 DEG C., the precipitated thiourea filtered off and the mother liquor recycled. Specifications 634,463 and 636,600 are referred to.