École Nationale Supérieure de
Techniques Avancées

Résumé

The present research project aims at documenting the thickening behavior of reject pulp in pressure screening. Firstly, the effect of the rotor tip speed on reject thickening near the point of plugging, when the rotor speed becomes too low to prevent aperture clogging, was studied. A zone of high reject thickening just before plugging was observed. It represents a zone of low energy consumption and high levels of fibers fractionation. This research also showed that the thickening factor increases more quickly when a slotted cylinder with smaller apertures is used. In a second stage,the prototype of a photoelectric consistency sensor was designed to allow for completing a more time efficient research on reject thickening. The prototype works by emitting light with an infrared and a green LED alternatively, against a transparent container filled with pulp. The light passing through the container is received by photodiodes and the output voltage is acquired on a computer using LabVIEW. The device was tested with pulp solutions with a consistency ranging from 0 % to 5 % and can measure instantaneously the consistency of pulp with a precision of 1 %. However, other tests revealed a lack of precision of the sensor. Comparing the two types of LEDs showed that the use of only the visible LED leads to satisfying results. Increasing the light intensity lead to the improvement of the precision of the sensor prototype. As a future work, the sensor’s precision should be further increased and it should be calibrated. Finally, a final and permanent version of the sensor should be manufactured and mounted on the reject line of the pressure screen used for research on reject thickening.