Problem No. 2: High biuret level at low plant loads

Biuret is formed from urea according to the following overall reaction: 2 urea <=> biuret + ammonia. In fact this reaction proceeds in three steps: When heating a concentrated urea solution in water or a urea melt, urea is present in two forms, a keto and an enol form. The enol form of urea primarily decomposes into NH₃ and HNCO (cyanic acid). Cyanic acid reacts with urea to form biuret.

All these reactions are equilibrium reactions. Biuret formation is favoured by higher residence times, higher temperatures and lower ammonia partial pressures. Biuret is considered an impurity in urea when used as a fertilizer as it has been identified as injurious to citrus foliage and to certain germinating seeds, but the level at which biuret is damaging to plants is disputable. The earliest urea plant designs produced urea with up to 5 wt-% biuret. From 1955 to present, many urea plants were designed to produce low-biuret product with not more than 0.3 wt-% biuret by means of a crystallisation section. The lower biuret concentration was found to be safe for these special applications which amount to about two percent of total urea fertilizer consumption.