The fate of nitrogen in the chemical recovery process in a kraft pulp mill. Part II: Ammonia formation in green liquor

The smelt nitrogen is known to convert gradually to ammonia (NH3) in the liquor cycle. In this paper, the rate of the the NH3 formation reaction was studied and the unknown nitrogen species responsible for the NH3 formation was identified. The formation of NH3 in the studied mill green liquors was slow, taking about 44 h at 90 degreesC to reach the maximum conversion, here about 90% of the liquor nitrogen. It was found to be a first-order reaction with respect to the NH3 forming compound the reaction rate constant (k value) being about 8.4 x 10(8) e((-10100/T)) min(-1), and the activation energy about 84 kJ/mol. No significant differences in the rates between the two studied kraft green liquors were observed. The green liquor dregs had no effect on the rate. Two independent identification methods indicated cyanate (OCN-) to be a strong candidate for the NH3- forming nitrogen species in green liquors.