Reduction of refining energy during mechanical pulping

The effects of pressurised compressive chip pre-treatment and low dosage sulphite pre-treatment were evaluated for production of thermomechanical pulp in mill scale trials using Norway spruce (Picea abies) at the Braviken paper mill (Holmen Paper AB, Sweden). The general aim of the study was to improve the energy efficiency during the production of mechanical pulps suitable for news and improved news grade papers. The pressurised compressive chip pre-treatment performed in an Impressafiner, resulted in a reduced acetone extractive content for first stage blow line pulp by up to 24%. Furthermore, pulp produced from mechanically pre-treated chips had higher tensile- and tear indices, elongation and light scattering and lower freeness compared to pulps from untreated chips produced with equal total specific energy consumption. The total specific energy consumption was reduced by 120 kWh/bone dry ton (6%) at equal tensile index, when pulps were produced together with the Impressafiner pre-treatment.Sulphite pre-treatment increased tensile index, elongation, density and brightness and reduced light scattering and shive content compared to pulps produced with only mechanical pre-treatment at equal specific energy consumption. The increase in tensile index and reduction in light scattering followed linear relations to the dosage of sodium sulphite in the measured dosage range (0-1.2% Na2SO3). The addition of ~1.2% sodium sulphite gave a sulphur content in pulp of ~0.67% (as Na2SO3) and reduced the specific energy consumption by 210-320 kWh/bdt (12-15%) when compared at equal tensile index. However, light scattering was not retained for this energy reduction. Further analyses showed that sulphite pre-treatment did not significantly affect the distribution of the Bauer-McNett fractions or the fibre length for pulps refined with equal specific energy consumption but did increase fibre delamination/internal fibrillation as measured by Fernando and Daniel’s (2010) version of Simons’ staining.The specific energy consumption for pulps produced with sulphite pre-treatment (1.2% Na2SO3) and double disc refining were 650 kWh/bdt (30%) lower than for pulps produced in a two stage single disc refiner line (no pre-treatment), when compared at a similar tensile index and light scattering coefficient.