Experiences in Using Super Pressure Groundwood (PGW-S) in Myllykoski SC-Paper Mill, 1991 International Mechanical Pulping Conference Proceedings

Myllykoski is a producer of mechanical printing papers with at total capacity of 500,000 t/a, 140,000 t/a of which is LWC and 360,000 t/a is SC papers.

The capacity of the groundwood mill is 900 t/d - four hundred tons/day (400 t/d) is pressure groundwood and 500 t/d is stone groundwood (GW). The pressure groundwood mill consists of six pressure groundwood (PGW) grinders and two super pressure groundwood (PGW-S) grinders.

Super pressure groundwood is a new mechanical pulp which is produced at a higher pressure (max. 5 bar) and temperature (shower water temperature max. 120 degrees Celsius) compared to conventional pressure groundwood (PGW). The two PGW-S grinders were started up in November 1989.

In this study, samples were taken from PGW-S grinders operating at different temperatures and pressures to determine the effects of the varied conditions of groundwood pulp properties and bleaching characteristics.

The results show that the PGW-S pulps have considerably better strength properties compared to PGW with no significant change in specific energy consumption. This increase in strength is partly achieved at the cost of lower optical properties. The loss can however be compensated for by incorporating shower water bleaching. The shive content of PGW-S remains at the same level.

The effects on SC gravure paper quality associated with the utilization of PGW-S pulp has proven to be a good alternative as a mechanical pulp for printing papers allowing a decrease in the amount of kraft pulp and improving paper quality.