Fibre floc behaviour of softwood kraft pulp in flowing suspensions

Deformation of floes in flowing suspensions was studied in a flow loop system. A fibre suspension jet from a model headbox was further accelerated by a secondary contraction with a width reduction of 50%. The fibre suspension flocculation was studied using an optical detection method. For the suspension of bleached softwood kraft Pulp at a fibre concentration of 5 g/l, fibre floes were followed manually along the secondary contraction. About 115 of the floes were stretched to breakage in the contraction. The floes were stretched in MD whereas they were even more compressed in CD, so that the total floe area in the MD-CD plane decreased, indicating an increase of fibre concentration in the individual floes. This was confirmed by an increase of gray value of the floes in the light transmission images. Fibre floe orientation in MD increased when passing along the contraction. The manual evaluation confirmed the validity of the power spectrum analysis results. Increasing fibre concentration gave larger fibre floes. The floe area reduction in MD-CD plane during contraction passage increased when increasing fibre concentration, which might be due to an increase of mechanical forces in CD generated by the contracting walls, since less free water existed between the fibre floes at a higher fibre concentration. Increased absolute flow velocity had an insignificant effect on the fibre floes within the investigated range.