| 摘要 |
1,141,419. Crystallization of urea. STAMICARBON N.V. 27 Sept., 1967 [30 Sept., 1966]. No. 44034/67. Heading C2C. A process for the continuous crystallization of urea from a biuret-contaminated solution of urea in water comprises subjecting the urea solution to a first vacuum crystallization at 40-70‹ C. to crystallize and remove from 90 to 95% of the supplied urea in the form of crystals, and subjecting the mother liquor to a second vacuum crystallization at 90-110‹ C. to crystallize and remove further urea crystals, and discharging the final mother liquor containing from 21 to 28% by weight of biuret. The final mother liquor obtained may be recycled to a urea synthesis zone or the final mother liquor may be treated whereby the biuret content therein is lowered, and the thus obtained mother liquor depleted in biuret may be recycled to the first urea crystallization stage. The biuret content of the final mother liquor may be depleted by heating it at 130- 160‹ C. under an ammonia pressure of 1 to 5 atmospheres, or by diluting and cooling it and isolating the biuret crystals thus formed. |
