THE EFFECTS OF RETENTION AIDS, DRAINAGE CONDITIONS, AND PRETREATMENT OF SLURRY ON HIGH-VACUUM DEWATERING - A LABORATORY STUDY

After the initial drainage by gravity and foils on the paper machine w ire section, water is removed by suction. A certain pressure differenc e across the web is reached, depending on the volume of airflow and th e resistance of the wet-web wire system. Most of the resistance comes from the web. The objective of this study was to determine the importa nce of chemical addition and shearing conditions prior to drainage for the air permeability of the sheet. In addition, more knowledge was so ught on the dynamics of vacuum-assisted water removal. The study was d one with a specially designed laboratory-scale drainage tester called the moving belt drainage tester. Different chemical additions, shear f orces, and shear times prior to forming were used in order to change t he structure of the formed sheet. The tiller content was also varied. After the initial drainage, very short, strong suction pulses were app lied, and the solids content of and airflow through the sheet were mea sured. The effect of different suction pulse rates on water removal wa s examined.