Patent GB-899182-A (GB)
Title: Method of producing urea crystals
IPC: C05C9/00
Filed: 1959-05-06
Abstract: <PICT:0899182/III/1> Urea is crystallized from solution as fine crystals by rapid cooling, e.g. from 150 DEG -212 DEG F. to 135 DEG -185 DEG F., the crystal suspension is held at a substantially constant temperature, e.g. 180 DEG F., for a period of 1,5-2 hrs. to allow some crystals to grow at the expense of others, and the resulting crystals are separated from the remaining solution, 27% of the urea may be crystallized from solution.  The crystals may grow to such a size that not more than 2% are held on a 20 mesh screen.  The urea may be crystallized from a solution obtained by the interaction of ammonia and carbon dioxide at high temperature and pressure and containing, e.g. 0,3-1,0% of biuret.  The crystallized urea may contain not more than 0,04% of biuret.  The solution may be purified before crystallization by treating with carbon and aerating.  Up to 50% of the urea may be recovered in a cyclic process.  As shown, urea solution is passed from a line 11 into a temperature adjustment tank 1 having a cooling or heating coil 12 and stirrer 13.  Any recycled solution passes into the tank from a line 14 and evolved ammonia passes off via line 15.  The temperature-adjusted solution passes via a line 16 to a jacketed cooler 2 having scraping blades 2a and an inlet 17 and outlet 18 for a cooling medium.  The resulting suspension of fine crystals passes to a holding tank 3 having a stirrer 20 and a steam-heating coil 39.  The resulting suspension of enlarged crystals passes via a line 26 to a centrifuge 4 except a portion which may be recycled to tank 3 via a line 25.  The mother liquor from the centrifuge is passed to a prilling apparatus except a portion which may be recycled to tank 1 via line 14.  The separated solid, which may contain 1-2% of water, is passed via a hopper 6 and screw conveyer 7 to a drying kiln 8 fed with hot air wherein the water content may be reduced to 0,2%.  The temperature during drying is preferably not greater than 225 DEG F.  The dried granular product passes from a hopper 38 on to a belt conveyer 9 where it is mixed with dried finely divided product separated in a cyclone 10. Specifications 753,342 and 775,515 are referred to.ALSO:<PICT:0899182/IV (b)/1> Urea is crystallized from solution as fine crystals by rapid cooling, e.g. from 150 DEG F.-212 DEG F., to 135 DEG F.-185 DEG F., the crystal suspension is held at a substantially constant temperature, e.g. 180 DEG F., for a period of 1.5-2 hours to allow some crystals to grow at the expense of others, and the resulting crystals are separated from the remaining solution. 27% of the urea may be crystallized from solution.  The crystals may grow to such a size that not more than 2% are held on a 20 mesh screen.  The urea may be crystallized from a solution obtained by the interaction of ammonia and carbon dioxide at high temperature and pressure and containing, e.g. 0.3%-1.0% of biuret.  The crystallized urea may contain not more than 0.04% of biuret.  The solution may be purified before crystallization by treating with carbon and aerating. Up to 50% of the urea may be recovered in a cyclic process.  As shown, urea solution is passed from a line 11 into a temperature adjustment tank 1 having a cooling or heating coil 12 and stirrer 13. Any recycled solution passes into the tank from a line 14 and evolved ammonia passes off via line 15.  The temperature-adjusted solution passes via a line 16 to a jacketed cooler 2 having scraping blades 2a and an inlet 17 and outlet 18 for a cooling medium.  The resulting suspension of fine crystals passes to a holding tank 3 having a stirrer 20 and a steam-heating coil 39.  The resulting suspension of enlarged crystals passes via a line 26 to a centrifuge 4 except a portion which may be recycled to a tank 3 via a line 25.  The mother liquor from the centrifuge is passed to a prilling apparatus except a portion which may be recycled to tank 1 via line 14.  The separated solid, which may contain 1%-2% of water, is passed via a hopper 6 and screw conveyer 7 to a drying kiln 8 fed with hot air wherein the water content may be reduced to 0.2%.  The temperature during drying is preferably not greater than 225 DEG F.  The dried granular product passes from a hopper 38 on to a belt conveyer 9 where it is mixed with dried finely divided product separated in a cyclone 10. Specifications 753,342 and 775,515 are referred to.