The main reactions in causticizing are as follows:

CaO+H₂O ? Ca(OH)₂ + heat

Ca(OH)₂ + Na₂CO₃ ? 2NaOH+ CaCO₃

The second reaction is an equilibrium one, with a typical conversion of about 80%; this varies with TTA concentration and sulphidity of the liquor. This reaction can be driven to the right by adding more lime. However, this will create problems for the performance of the equipment (i.e., pressure filters) due to the presence of free lime. A lot of mills with overloaded kilns would like to increase their causticizing capacity and reduce the amount of purchased lime, without capital investment. The option here is to use partial borate autocaustcizing (AC). In this method, sodium tetra-borate pentahydrate (Na₂B₄O₇.5H₂O) is added to the liquor cycle. The borate AC reaction occurs in the recovery boiler, and caustic forms in the smelt dissolving tank:

Na₂CO₃ + NaBO₂ ? Na₃BO₃ + CO₂

Na₃BO₃ + H₂O ? NaBO₂+ 2 NaOH

Laboratory studies* have shown that sodium borates can readily react with molten Na₂CO₃, at any Na/B molar ratio larger than 3, to form tri-sodium borate. Tri-sodium borate (Na₃BO₃) hydrolyzes to form NaOH, and regenerates NaBO₂. For every mole of NaOH produced, half a mole of NaBO₂ is required. This means a small amount of borate can partially causticize Na₂CO₃ in the recovery boiler. The causticizing reaction can then be completed in the recaust plant, with a reduced amount of lime. Borates are water soluble and remain dissolved in the alkaline liquors, repeating their AC function continuously. The concentration of borate in the mill liquors is then determined by the rate of borate addition and loss (about 2.5%).

Mill trials have shown that the presence of borate in the mill liquors has little effect on the mill operation and the quality of the pulp produced. There are indications that the yield may be enhanced due to the possible retardation of cellulose peeling reactions. Partial borate AC is now used in several mills in Europe, the U.S., Brazil and Indonesia.

*Tran, H. et al, Partial autocausticizing of kraft smelt with sodium borates; Part 1: Effects on recovery boiler performance, Tappi, Vol. 1, No.1, September 2001.