Manufacture of symmetrical dialkylureas

A process for the manufacture of a symmetrical dialkylurea by reacting a monoalkylamine with CO.sub.2 at an elevated temperature and under superatmospheric pressure, in which the unconverted starting

• materials, water and the by-products formed in the reaction zone, as well as alkylamine alkylcarbamate, after thermal decomposition into monoalkylamine, CO.sub.2 and water, are separated from the dialkylurea formed, and thereafter a part of the unconverted starting materials
• plus reaction by-products, with or without water, may or may not be recycled to the reaction zone, before isolating an aqueous solution, containing a monoalkylamine, CO.sub.2 and other reaction by-products, which solution is treated with alkali metal hydroxide solution in a column, the monoalkylamine, after separation from CO.sub.2 and water, being recycled to the reaction zone, in which process the aqueous solution, before treatment with alkali metal hydroxide solution, is stripped, in a subsidiary column, with an inert gas to which CO.sub.2 or monoalkylamine may or may not be admixed, and at least a part of the reaction by-products formed is taken off at the top of the column, the liquid discharged from the bottom of the subsidiary column is fed, via a collecting vessel, to the alkali treatment column, and the stream of gas issuing at the top of the treatment column is compressed without further purification and is liquefied by cooling.