Use of Borate in Semi-Chemical Pulping, 2005 Engineering, Pulping & Environmental Conference

The original neutral sulfite semichemical (NSSC) pulping process used sodium sulfite as the principle cooking chemical, but had the disadvantage of sulfur emission problems. Considerable research was conducted to investigate the use of other chemicals including sodium hydroxide, but was not completely satisfactory because of high chemical costs. Several mills adapted a sodium carbonate based process but experienced slower pulping and reduced physical properties.

The objective of this research is to study the use of borate as a pulping chemical in sodium carbonate semi-chemical pulping. The experimental plan included digester cooks, one with pure sodium carbonate, and the other with partially borate autocausticized sodium carbonate.

It was found that without heating the borate and sodium carbonate, the effect of borate was similar to that of sodium carbonate. Mixtures of sodium borate and sodium carbonate were then heated up to 850 oC, which is below the melting point of these salts. This was done to simulate the processes occurring in a fluidized bed reactor (a common recovery system in sodium carbonate semichemical pulping). At 850 oC, there was a slight reaction between sodium carbonate and sodium borate. This reaction resulted in the production of sodium hydroxide, which tended to increase the pulping rates and the resulting paper strength properties.