Effect of Sodium Metaborate on the Boiling Point Rise of Slash Pine Black Liquor

Borate can be used as either a total or partial autocausticizing agent for conversion of Na 2 CO 3 to NaOH. For every mole of Na 2 CO 3 converted, a mole of NaBO 2 is generated and remains in the black liquor. The objective of this study is to determine the effect of this additional NaBO 2 on the boiling point rise of the resulting black liquor. Commercial black liquor was obtained, and borate was added at levels equivalent to 30, 60, and 120% autocausticizing. With borate present, the boiling point rise observed during evaporation was greater than that observed during dilution. This phenomenon was not seen for black liquor without borate added nor for black liquor with only Na 2 CO 3 added. It is proposed that this difference is due to the formation of a complex between kraft lignin and borate. Borate was also found to be totally soluble in black liquor and to increase the solubility of the other components. The addition of borate had little effect on the boiling point rise of the black liquor. The largest effect of borate was observed at an addition level equivalent to 120% autocausticizing and at over 70% solids, where borate increased the boiling point rise by about 5 K.