Silicate Scale Management Strategy at Millar Western; Whitecourt and Meadow Lake Mills, 1996 Pulping Conference Proceedings

Gerard Orlowski and Dr. Mark Hoddenbagb

Millar Western Pulp

Dr. Gerard Le Fevre

National Silicates Ltd.

Sodium silicate is widely used for hydrogen peroxide brightening of mechanical pulps. It acts as a peroxide stabilizer and as a buffer during the bleaching reaction. The use of silicate can also cause scaling problems on process equipment. The Millar Western Pulp mill in Whitecourt, AB experienced silicate scaling problems since the start-up of the mill in 1988, predominately on the softwood pulp line. An extensive monitoring program of the magnesium (Mg), calcium (Ca) and silica (SiO2 ) concentrations in the process scale and whitewater was initiated in the spring of 1993. It was determined that by increasing the magnesium sulphate to silicate ratio the scaling problem on the pulp dewatering equipment could be significantly reduced. The result was the formation of easily handled magnesium -rich silicate scale (Mg/Ca molarratio of > 2.0) instead of the previously formed hard, tenacious, calcium- rich scale. Scaling problems in the bleach liquor make-up tanks and lines were addressed by relocating peroxide and caustic addition points. The monitoring of the Mg, Ca, and SiO2 concentrations in the whitewater streams was seen to be a quick, predictive tool for determining the propensity of silicate scale formation on process equipment. The Meadow Lake mill recognized that the management of the scaling issue would be critical due to the zero effluent technology being employed. This led to the optimization of the magnesium sulphate to silicate ratio resulting in the avoidance of calcium silicate scale.