Preparation of urea with a low biuret content

Claim 1

IN A PROCESS FOR THE PREPARATION OF UREA HAVING A LOW BIURET CONTENT BY PASSING AN AQUEOUS UREA SOLUTION CONTAINING BIURET INTO A FIRST CRYSTALLIZATION ZONE WHEREBY UREA IS CRYSTALLIZED BY EVAPORATION OF WATER FROM SAID SOLUTION AND SEPARATING SAID CRYSTALLIZED UREA FROM THE REMAINING MOTHER LIQUOR OF SAID SOLUTION, THE IMPROVEMENT WHICH COMPRISES EVAPORATING WATER FROM SAID REMAINING MOTHER LIQUOR IN A SECOND CRYSTALLIZATION ZONE TO CONVERT THE LIQUOR TO A SUBSTANTIALLY DRY CRYSTALLINE MASS OF UREA AND BIURET, PROVIDING A UREA-BIURET MELT CONTAINING ABOUT 15-28% BY WEIGHT OF BIURET, MIXING SAID CRYSTALLINE MASS OF UREA AND BIURET WITH SAID MELT AT A TEMPERATURE NEAR THE MELTING POINT OF THE RESULTING MIXTURE WHEREBY BIURET IN SAID CRYSTALLINE MASS OF UREA AND BIURET A DISSOLVED IN SAID MELT, CONTACTING THE SEPARATED UREA-BIURET MELT WITH AMMONIA GAS AT A TEMPERATURE OF 120*-150*C. TO CONVERT PART OF THE BIURET PERSENT IN SAID UREA-BIURET MELT TO UREA WHEREBY THE BIURET CONTENT IN SAID MELT IS ADJUSTED TO ABOUT 15-28% BY WEIGHT AND RECYCLING SAID CONVERTED UREA-BIURET MELT FOR FURTHER MIXING WITH CRYSTALLINE MASS OF UREA AND BIURET.