Method for granulating urea

Abstract

Precipitation of an ammonium salt is prevented while suppressing an increase in water content of the aqueous urea solution which is to be supplied to a urea granulation step, when recovering and using urea and NH3 in a gas which contains urea dust and NH3 and which arises from a urea granulation step. Provided is a method for granulating urea including a granulation step of producing granular solid urea from a feed aqueous urea solution by using air; a recovery step of recovering urea dust and NH3 from air discharged from the granulation step, by use of an aqueous solution containing an acid, so as to obtain a recovered solution containing urea and an ammonium salt; a salt concentration adjustment step of mixing the recovered solution with an aqueous urea solution having a lower ammonium salt concentration than the recovered solution, so as to adjust the ammonium salt concentration of the recovered solution; a concentration step of concentrating the solution obtained from the salt concentration adjustment step by vaporizing water contained in the solution obtained from the salt concentration adjustment step, so as to obtain a concentrated recovered solution; and a mix step of mixing the concentrated recovered solution with the feed aqueous urea solution, wherein the ammonium salt concentration of the concentrated recovered solution is 7 mass % or less.

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