Patent GB-645701-A (GB)
Title: Improvements in or relating to the manufacture of thiourea
Filed: 1947-07-15
Abstract: Thiourea is obtained by heating ammonium thiocyanate, isolated from the conversion mixture by aqueous extraction, and mother liquors containing unchanged ammonium thiocyanate, are dehydrated and recycled to the heating stage, at least the latter portion of the water being removed therefrom by passing a stream of inert gas or vapour, e.g. air, nitrogen, ammonia, through the apparatus. Thus about 80 per cent of the water may be removed from the mother liquor at temperatures below 100 DEG  C. or even 110 DEG  C., decomposition being then negligible by, e.g., vacuum evaporation, dehydration being continued as instanced above.  Vapour is preferably obtained by distillation with a liquid water entrainer such as xylene, benzene, toluene, kerosene, cyclohexane, mesitylene, ethyl benzene and chlorinated hydrocarbons such as monochlorobenzene, which also acts as a vehicle so that by agitating, over-heating and the formation of films in the material undergoing dehydration is prevented.  50-80 parts of xylene per part of non-aqueous material in the charge may be used, xylene and water distilling over therewith being separated and the xylene returned to the dehydration unit.  The temperature of dehydration may be altered by pressure adjustments and when complete the solid ammonium thiocyanate that separates may be melted by an increase of pressure and heated to below 155 DEG  C., e.g. 150-152 DEG  C., until sufficient thiourea is formed to substantially lower the melting point of the mixture, e.g. for 1 to 4 hours, and then to below 145 DEG  C., e.g. 140-143 DEG  C., for 8-10 hours, when heating is preferably stopped when the rate of formation of impurities and thiourea is approximately equal.  Preferably the pressure is increased during dehydration so that no solid separates and in this case superheated xylene vapour may be led in from outside the unit. The aqueous extraction is preferably carried out by removing the entrainer from the conversion mix and running the latter into water, which is preferably mother and wash liquors obtained as indicated below, to give a concentrated solution which is boiled with charcoal, filtered, and cooled rapidly to obtain small crystals in a slurry to which chopped ice is added when thiourea is obtained by filtering and washing, mother and wash liquors being returned for treatment in the above fashion.  The total amount of water required is approximately 6-7 parts per 8 parts of conversion mixture, an optimum of 33 parts to 40 being quoted, which approximates to the ammonium thiocyanate saturation temperature at the temperature employed.  At least 50 per cent, e.g. 60 per cent, of the water is added as ice.  The temperature of the pre-icing step is below 25 DEG  C., preferably 0 DEG  C. or below, a temperature of -10 DEG  to -18 DEG  C. then being obtained on icing.  In contrast to prior art processes no double salt of ammonium thiocyanate and thiourea is obtained.  If required, the conversion mix may be solidified before extraction and added to water in a solid form.  Charcoaling may be omitted but is of value in a recycling process because build up of impurities is prevented.  In a modification, mother liquors containing up to about 8 per cent of thiourea are concentrated so that ammonium thiocyanate crystallizes out, is separated and dehydrated as above, thiourea being recovered from the second mother liquors, e.g. by crystallization at below 0 DEG  C., or these mother liquors may be returned to the quenching step.  Mother liquors from the above cold filtration step are dehydrated by vacuum evaporation and entrainment distillation to leave 5-7 per cent of water and the emulsion cooled to below the ammonium thiocyanate but above the thiourea saturation temperature until the former crystallizes out, e.g. at about 45-95 DEG  C. Ammonium thiocyanate is isolated and entrainer removed from the mother liquor from which thiourea is obtained or which is preferably returned to the quenching stage.  Raw ammonium thiocyanate may be dehydrated at any convenient point in the system.  Specification 400,916 is referred to.