Corrosion Investigations Due to Increased Sulfidity in Digester House of Paper Mill

Increased demand of paper, pollution control and better energy management are the driving forces for the development of the paper industry aspiring to enter 21st century in a big way. High sulfidity pulping of wood is one of the approaches in this direction, which attacks lignin in a more selective way thereby increasing pulp yield and makes it possible to bleach pulp to higher degree of brightness with less pollution load. Higher sulfidity has been found to affect corrosion of the digester house and related machinery and through this a question mark on the credibility of materials of construction. Citing example of a typical paper mill, which increased sulfidity in their pulping procedure, attempts have been made to analyze the increase in corrosivity due to changed concentrations of sodium sulfide, polysulfide and thiosulfates in the cooking liquor of the mill in question. E-pH diagram, popularly known as the Pourbaix diagram, have been constructed for S-H2O and Fe-S-H2O system with relevant concentrations of the different type of sulfur species and have been used in this endeavor