Autocausticizing of Smelt with Sodium Borates, 1998 International Chemical Recovery Conference Proceedings

The autocausticizing technology involving the use of borate to "causticize" sodium carbonate (Na2CO3) in the soda or kraft smelt directly in the recovery boilers was reviewed by re-examining the basic reactions between Na2CO3 and sodium borates (NaBO2 and Na2B4O7) at high temperatures. The results show that in air, both NaBO2 and NaB4O7 react readily with molten Na2CO3 at any Na:B molar ratio, and the reaction product is likely to be trisodium borate (Na3BO3), not disodium borate (Na4B2O5). The reaction kinetics depends strongly on the sample size, temperature, and the CO2 concentration in the gas. The formation of Na3BO3 as a reaction product means that for each mole of NaOH to be produced in the liquor system, only 0.5 mole of NaBO2 is needed, not 1 mole as was previously suggested. This 50% decrease in borate requirement substantially reduces concerns over high black liquor viscosity and large deadload in the liquor cycle associated with borate autocausticizing. Partial borate-autocausticizing appears to be technically feasible, since proportionally requires less borate.