Solubility in the NaOH-Na2CO3-Na2SO4-Na2SO3-Na2S2O3-Na2S-H2O system, a simulated black liquor recovery boiler smelt

Results are presented for a study on the dissolution behavior of salt mixtures formulated to simulate smelt deposits inside pulp mill recovery boilers. Solubilities of carbonate-rich NaOH-Na2CO3-Na2SO4 and NaOH-Na2CO3-Na2SO4-Na2SO3-Na2S2O3-Na2S mixtures in water at 25 degreesC and 1 bar exhibited an increase in the concentration of Na2CO3(aq) and Na2SO4(aq) with an increasing amount of water added. An ion-interaction solution model explains this behavior as resulting from the suppressive effect of NaOH on the solubility of other salts; dilution of NaOH increases the solubility of Na2SO4 and Na2CO3. In addition, the formation of hydrated salts of Na2CO3 and Na2SO4 reduced the amount of liquid water present and thus had a concentrating effect on NaOH(aq). These results are applied to understanding the role of smelt-water mixtures in corrosion of stainless steel in boiler floors.