This invention is written together with patent 2933526 which describes a process which consists of a vacuum evaporation stage, a cooling phase with air until nearly the crystallization temperature and that a further cooling against urea fines leading to a free flowing product with a crushing strength of 30 kg/cm2, a biuret figure lower than 2 wt% and a low moisture content of 0.1-0.2 wt%.
A further improvement to reduce the biuret content is described in this patent, which is accomplished by maintaining a gaseous phase of ammonia in the vacuum evaporation  stage and cooling phase. The biuret figure reduces then to 1 wt%.