Effect of sodium metaborate on the boiling point rise of slash pine black liquor

Berate can be used as either a total or partial autocausticizing agent for conversion of Na2CO3 to NaOH. For every male of Na2CO3 converted, a mole of NaBO2 is generated and remains in the black liquor. The objective of this study is to determine the effect of this additional NaBO2 on the boiling pa int rise of the resulting black liquor. Commercial black liquor was obtaine d, and berate was added at levels equivalent to 30, 60, and 120% autocausti cizing. With berate present, the boiling point rise observed during evapora tion was greater than that observed during dilution. This phenomenon was no t seen for black liquor without berate added nor for black liquor with only Na2CO3 added. It is proposed that this difference is due to the formation of a complex between kraft lignin and berate. Berate was also found to be t otally soluble in black liquor and to increase the solubility of the other components. The addition of borate had little effect on the boiling point r ise of the black liquor. The largest effect of berate was observed at an ad dition level equivalent to 120% autocausticizing and at over 70% solids, wh ere berate increased the boiling point rise by about 5 K.