Thermodynamic modeling of the stability and melting properties of sodium borates relevant to black liquor combustion and gasification

The use of borates as an autocausticizing agent in kraft recovery boilers of pulp mills is an interesting concept for decreasing the lime consumption at pulp mills. The main autocausticizing reaction is between NaBO2 and Na2CO3 forming Na3BO3 and CO2 at high temperatures in a liquid phase in the recovery boiler. The thermodynamic and phase equilibrium data of the Na2CO3-NaBO2-Na3BO3 system were evaluated and optimized. The liquid phase was modeled with the modified quasichemical model. The thermodynamic database obtained can be used to calculate the phase equilibrium and thermodynamic properties of multicomponent sodium salt mixtures of importance for the borate autocausticizing concept. The thermodynamic database is a part of an extensive thermodynamic database for alkali salt mixtures of importance for black liquor combustion in the pulp and paper industry.