Black Liquor Gasification with Direct Causticization Using Titanates: Equilibrium Calculations

Abstract

Different alternatives to the conventional recovery of spent pulping liquors in the kraft pulping process have been studied by equilibrium calculations. These calculations were performed using FactSage. Three different black liquor gasification systems with addition of TiO2 for direct causticization were studied: (i) MTCI conditions (600 °C and 0.3 MPa), (ii) Chemrec atmospheric booster conditions (950 °C and 0.1 MPa), and (iii) Chemrec pressurized system conditions (950 °C and 3 MPa). The studies of the influence of the molar ratio TiO2/Na2O showed that a TiO2/Na2O molar ratio of 0.5 is needed to capture all of the sodium in the condensed phase as sodium titanates (i.e., achieve complete direct causticization). Furthermore, at these conditions all of the sulfur is released to the gas phase. Different forms of sodium titanates are formed depending on the TiO2/Na2O molar ratio, and conceptually all of them could be used to produce NaOH in the consecutive leaching step.