| 摘要 |
Process for the preparation of urea, wherein in a first step ammonia and carbon dioxide react in suitable conditions so as to form a first mixture containing a quantity of urea, wherein in one or more subsequent steps ammonia and carbon dioxide are removed from the first mixture and the obtained urea is concentrated until a urea concentration has been reached suitable to be granulated in a gas stream, whereby apart from urea granules a second mixture containing air, gaseous ammonia and urea dust is formed which subsequently is washed, and wherein before or during the washing an amount of formaldehyde is added to the second mixture of air, ammonia and urea under circumstances wherein the ammonia can react with the formaldehyde in order to form hexamethylenetetramine (HMTA) whereupon the so formed HMTA is separated from the gas stream and is returned into the process before the granulation step.
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