DEVELOPMENT OF WOOD GRINDING 4. EVALUATION OF THE THEORETICAL GRINDING MODEL

Spruce wood was ground under certain fixed conditions to achieve data for interpretation of the grinding mechanism. For this purpose, wood moisture content, wood feeding rate and grindstone peripheral speed were given five, three and two different levels, respectively. The data obtained by application of a laboratory grinder were accordingly most reliable, because the grinder was run under well controlled conditions, and the wood samples represented one and the same wood quality, but at five moisture content levels. The grinding zone temperature was measured at the outlet from the grinding zone, and accordingly it would indicate the average grinding zone temperature. The grinding model suggests that the energy specific fibre production (Gw/Pt) evaluated at various power friction levels (Pc/Pt) would help understanding the wood grinding mechanism. The energy specific fibre production showed significant dispersion, when evaluated as a function of the power friction. However, detailed analysis of the data revealed that there could be two linears, one representing moisture saturated wood, and one representing fresh wood that also contains free water in the lumina. Surprisingly, this linear also included data representing air-dry wood. Increasing wood moisture ratio from 0.2 kg to 1.4 kg per kg o.d. wood decreased the average grinding zone temperature significantly from about 98 degrees to 88 degrees C. Further, there was roughly a difference of 8-12 degrees C in grinding zone temperature due to the wood moisture, when evaluated as a function of the force friction coefficient (Ft/Fc). The two highest moisture ratios resulted in a low-level linear, as the other moisture ratios produced a high-level linear. The wood moisture content seems to affect the grinding process in different ways due to the moisture content level. Since moisture is absorbed in the fibre wall material and evidently linked to various lignin and cellulose components by hydrogen bonds, it will affect the grinding in a certain mode. However, the air-dry wood sample seemed to act as the fresh wood samples, which cannot be explained properly unless also considering fibrillation and fibre properties.