Brightness reduction of mechanical pulp in the wet end of a paper machine: The effect of pH, pulp washing and shear rate

Brightness reduction of mechanical pulp in paper machine water circulations is a well-known phenomenon. However, the reasons behind the darkening are not fully known. The present paper is part of larger study aiming at finding out mechanisms and phenomena causing the brightness reduction of mechanical pulp and fillers in the wet end of a paper machine. The main focus of this paper is on the effect of pH, pulp washing and shear rate on the brightness reduction of mechanical pulp fines in the short circulation. The effect of pH and pulp washing was studied in a device that simulates the short circulation of a paper machine. The brightness of the fines fraction was lower at higher pH (7.3 compared to 4.8) due to the lower light scattering of the fines whereas pH did not have a notable effect on the light absorption. The effect of pH on the optical properties of the fiber fraction was negligible in the tested pH range. Pulp washing clearly reduced the tendency to darkening of the fines and fiber fraction. This was probably due to light absorbing substances whose amount in the pulp suspension typically decreases during the washing. The effect of shear rate was studied by agitating peroxide bleached TMP at 60 degrees C using different agitation speeds. The increase in shear rate increased the rate of darkening. At the same shear rate levels, the rate of darkening was higher for the fines fraction than for the fiber fraction.