Motor load and freeness control of CMP pulp refining

A simple refiner control strategy aimed at minimizing variations in freeness of a Ultra High Yield Sulphite (UHYS) pulp has been implemented in a newsprint mill. The feeder screw speed is being manipulated to compensate for variations in the bulk density of the chip feed and to maintain a constant motor load at the primary refiners. This ensures a more constant mass flow rate through the primary refiners which translates into a more constant primary specific energy. This has resulted in a reduction in motor load fluctuations of about 60% and a reduction in freeness variations of 40%. The final pulp freeness is regulated by manipulating the plate gap of the secondary refiners using on-line freeness measurements made at the outlet of the latency chest. Control of the secondary plate gap has led to an additional 35% reduction of the variation in freeness. This two-stage refining control strategy has reduced the 2-sigma confidence interval for variation in pulp freeness from 48ml to less than 14ml, a reduction of more than 70%. The complete strategy has reduced the refiner energy consumption by 5% and has had a beneficial impact on production. In addition, the strategy has compensated for changes in cooking conditions and their effect on the refining properties of the chips.