Corrosivity Of Black Liqors From A Modern Digester To A High Dry Solids Concentrator, 2004 11th International Sympsoium on Corrosion in the Pulp and Paper Industry

The sulfidity at many Nordic mills is close to 45 % and thus the new digesters built are seldom made of carbon steels but of duplex stainless steels. Higher sulfidity means higher amounts of volatile components during cooking and black liquor evaporation, especially at the end of evaporation when the dry solids content gets higher. Some of these vapour components are corrosive to carbon steel. However, the effect of different organic and sulfur-containing components is not yet known. The increase in the dry solids content of the combustion liquor means higher corrosivity of black liquors during storage and black liquor evaporation. Thermal treatments of the black liquor during its cycle at a pulp mill change the corrosivity of the liquor. The degradation of lignin is strongly affected by the temperature (it accelerates at temperatures > 140oC) and by the dry solids content of the black liquor. Lignin degradation mean releases of sulfurous gases from the black liquor and increases the risk of stress corrosion cracking of austenitic stainless steels. The cracking of vapor outlet tubes of the final concentrators in several Nordic pulp mills is most obviously caused by these sulfurous gases.