Fiber trapping in low-consistency refining: new parameters to describe the refining process

Fiber trapping in refining has been defined by the fraction f of the bar edges that trap fibers as they cross, and by the number of fibers i trapped under each section of bar. From these parameters, equations were derived to calculate the number of impacts that a fiber undergoes during refining and the maximum force experienced during each impact. Refiner power versus gap was measured for a conical laboratory refiner at maximum peripheral speeds ranging from 4 m/s to 27 m/s and consistencies ranging from 1% to 6% for one softwood kraft pulp. The data were used to calculate relative changes in i and f as a function of consistency and speed. It was found that f was extremely sensitive both to an increase in refiner rotational speed and to a reduction in pulp consistency. A reduction in consistency from 4% to 2% at 20 m/s (3000 rpm) led to a decrease in the trapping fraction, f, by approximately 80%. The number of fibers trapped under each section of bar also decreased, although to a lesser extent. The reduction in fiber trapping greatly increased the forces on the fibers, leading to enhanced fiber shorten-ing and reduced refining efficiency.