DEVELOPMENT OF WOOD GRINDING 5. FINES CONTENT–TO–SHIVES RATIO

Data by Riissanen were largely applied to learn more about wood grinding and, particularly, possible effects of the moisture content in spruce wood, varying from 65 to 15%, which in terms of moisture ratio covers a range from 1.9 to 0.2 kg water per kg oven dry wood. Tensile index (TI) and light scattering coefficient (LSC) of the pulp sheets were tested corresponding to various wood moisture contents, when the wood samples were ground to pulp by application of 20 and 30 m/s stone surface speeds and 0.7, 1.0 and 1.3 mm/s wood feed rates, respectively. The fines quantity of the screened pulp was related to the shives quantity that was screened off before testing, and accordingly these data were imitating some relevant grinding conditions. Fines were considered indicative of the fibrillation, as shives were indicative of the average fibre length. The respective TI and LSC values were correlated to the fines content–to–shives ratio for some useful results. The highest TI were obtained with fully water impregnated spruce wood (65% MC) by application of 30 m/s stone surface speed, and the lowest with air-dry wood (15% MC) and 30 m/s as well. Fresh spruce wood (58% MC) resulted in a similar trend as obtained with fully water impregnated wood, but on a lower level. Slightly dried spruce wood (44% MC) showed a further lower level, but surprisingly 30 m/s resulted in lower TI values than 20 m/s stone speed. Spruce wood with water saturated fiber walls (28% MC) appeared to react strongly to 20 m/s, but not to 30 m/s stone surface speed. The LSC values appeared quite contrary to the TI values, i.e. the highest LSC values were obtained by grinding the air-dry spruce wood (15% MC), while the fully water impregnated wood (65% MC) again gave the lowest LSC, when grinding at 20 m/s stone surface speed.