Effect of different impact events in fine grinding mills on the development of the physical properties of dried Norway spruce (Picea abies) wood in pulverisation

The effect of different impact events, which occur in fine grinding mills, on the development of the physical properties of dry Norway spruce wood in pulverisation was studied. Dry Norway spruce wood with a median size d50 = 199 μm (± 6 μm) was pulverised in an opposed jet mill and an oscillatory ball mill down to a d50 of approximately 20 μm. In addition, knowledge from an earlier study was used to obtain information about high-speed rotor impact milling. The development of physical properties, such as aspect ratio, width of its distribution and cellulose crystallinity, as a function of particle size was significantly different using oscillatory ball milling compared to using jet milling. When pulverising to median sizes of approximately 20 μm, oscillatory ball milling resulted in lower cellulose crystallinity and rounder, more uniform particle shapes than did jet and rotor impact milling. It is proposed that media mills are capable of imposing higher energies and, consequently, cause stronger physical changes for dry Norway spruce wood particles compared to impact-based non-media mills.