This patent from Kaasenbrood is again related to an improved process of crystallization, so the production of low biuret urea product. Typically urea and biuret can be separated by means of their difference in solidification points. Urea with a low biuret content is separated from a mother liquor stream with a high biuret content (8-15 wt%). When recycling this stream to the urea synthesis section it has a negative influence on the equilibrium of the urea synthesis reaction. Kaasenbrood suggests to convert the mother liquor stream into a crystal mass consisting of urea and biuret. Then mix this crystal mass with urea melt, the biuret part will dissolve while urea crystals remain behind. When gaseous ammonia is put into contact with the urea/biuret melt, biuret will be converted into urea.