Some Properties of Black Liquor from Kraft and Kraft-Borate Pulping, 2003 Fall Technical Conference

Implementation of the partial borate autocausticizing requires the study of the operation of both pulping and recovery line in the kraft process. The recovery line, especially its first evaporation phase, is governed by the black liquor properties, such as the boiling point rise (BPR). The BPR measurements should be included in the analyses of the black liquor, which resulted from the kraft process with the partial borate autocausticizing since an additional chemical, sodium metaborate, is present in this process. Also, there is a concern that sodium metaborate might affect the pulping chemistry. Studies of lignin dissolved in the black liquor (kraft lignin) during kraft and kraftborate pulping can provide insights into this question. In this paper, the boiling point rise of black liquor resulting from kraft and kraft-borate (35 % autocausticizing level) pulping of spruce, birch, and maple was studied. Corresponding kraft lignins were isolated and compared regarding the free phenolic hydroxyl group content and the characteristics of FTIR spectra. The effect of sodium metaborate at the level of 35 % autocausticizing on the boiling point rise of black liquor and the kraft lignin properties are discussed.