Fate of Nitrogen In the Chemical Recovery Process in a Kraft Pulp Mill, 1998 International Chemical Recovery Conference Proceedings

It is known that green liquor contains significant amounts of nitrogen, in particular ammonia. It has been suggested that this ammonia originates in the recovery boiler smelt /l/. However, little or no reliable information has been available concerning the fate of this nitrogen further in the recovery cycle. The nitrogen and the ammonia content of different liquor samples of the chemical recovery process at a Finnish pulp mill were analyzed, and the nitrogen mass balance of the whole recovery process was performed. Practically no NH3 was found in smelt samples. The smelt nitrogen was found to be in a form which slowly converted to ammonia under the process conditions. The green liquor leaving the dissolver contained clearly less ammonia than the green liquor entering the slaker, about 20 % and 70 % of its Kjeldahl nitrogen, respectively. The nitrogen of weak wash entering the smelt dissolver was mostly in the form of NH3 . Some NH3 had maybe also been formed from the smelt nitrogen in the dissolver and was released in the vent stack gases. The green liquor nitrogen, originating mostly both in smelt and weak wash, corresponded to almost 40 % of the black liquor nitrogen, suggesting that in the recovery boiler, a third of the black liquor nitrogen had to remain in the smelt. The major part , abou t 60 %, of the green liquor nitrogen left the recovery process along with white liquor into the cooking process. A minor part, less than 20 %, continued into weak wash, thus coming back to the smelt dissolver. The rest, about 20 % of the green liquor nitrogen, was assumed to exit the process as NH3 around the slaker and the causticizers. The nitrogen found in white liquor was mostly in the form of NH3 and was released mainly during the first stages of black liquor evaporation, less during the cooking. This NH3 was then condensed into foul and secondary condensates. The stripping of the foul condensates was then found to be the main exit point for ammonia in the recovery process, the amount of this ammonia nitrogen being very significant and corresponding almost to a typical NO, emission of a recovery boiler.