An oligomethylene urea product usable as a fertilizer can be more efficiently prepared in several steps by:a) introducing at least urea and an aqueous solution of formaldehyde in the overall molar ratio of urea to formaldehyde of the step 1:1-2:1 into a reactor with a retention time of at least 0.5 h, a temperature maintained in the range of 40 to 95° C. and a pH maintained in the range of 6.5 to 11, in order to produce a methylol urea composition,b) conducting the methylol urea composition to a second reactor with a temperature maintained in the range of 5 to 35° C. and a pH maintained in the range of 1.8 to 3.5, in order to produce a first oligomethylene urea slurry, and optionally,c) conducting the methylol urea composition or the first oligomethylene urea slurry to a third reactor with a temperature maintained in the range of 5 to 35° C. and a pH maintained in the range of 1.8 to 3.5, to produce a second oligomethylene urea slurry,d) conducting the oligomethylene urea slurry, selected from the first and the second oligomethylene urea slurry, to a filter and filtering it to form an oligomethylene urea precipitate and a filtrate,e) recycling the filtrate to the reactor, selected from the second and the third reactor, andf) neutralizing and drying the oligomethylene precipitate and recovering it as said oligomethylene urea product.
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