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

iber 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 /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 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 shortening and reduced refining efficiency. Fiber trapping is a hidden variable affecting refining efficiency. This paper describes how to estimate relative changes in fiber trapping and shows how fiber trapping impacts the refining process.