In-process detection of fiber cutting in low consistency refining based on measurement of forces on refiner bars

A major drawback of low consistency refining is the degradation of mechanical properties due to fiber cutting at high refining energies. Conventional strategies to avoid fiber cutting are based on post-refining measurement of pulp properties and, typically, this does not enable rapid adjustment of refiner operation in response to the onset of fiber cutting. The objective of this study is to detect the onset of fiber cutting by using custom-designed piezoelectric force sensors that measure shear and normal forces applied to pulp fibers by the refiner bars. Trials are performed in an AIKAWA/Advanced Fiber Technologies Inc. 16-in. single-disc low consistency refiner. The trials are run using mechanical softwood SPF (spruce, pine, and fir) pulp with 378 ml CSF at 2.5% and 3.5% consistency at rotational speeds of 800 rpm, 1000 rpm, and 1200 rpm. Distributions of the peak normal and shear forces and peak coefficient of friction are determined for each operating condition, and a two-parameter Weibull function is fit to each of these distributions. The scale parameter, which is one of the key parameters of the Weibull function, is calculated, and length-weighted fiber length is plotted as a function of this parameter. The results show that the onset of fiber cutting consistently corresponds to a distinct transition in the plot of length-weighted fiber length versus scale parameter. This transition is believed to be caused by a fundamental transition in the fiber-bar interaction. Moreover, frequency analysis of the sensor data shows that the magnitude of the dominant frequency remains relatively constant while the plate gap is reduced, up to a threshold value, which corresponds to the onset of fiber cutting. These results suggest that these sensors have potential to be used for in-process detection of the onset of fiber cutting.