Evaluation of a Novel Hydrocyclone Design for Pulp Fractionation

Abstract The fractionation efficiency and operating conditions of a novel hydrocyclone design were evaluated. The novel design comprised a modified inlet section, where the pulp suspension had to pass a narrow ring-shaped opening, and a very compact fractionation zone. The effect of feed concentration and fine fraction mass ratio was studied. The experiments were performed with fully bleached, unrefined and never-dried chemical softwood pulp. Fractionation efficiency was evaluated in terms of change in surface roughness of handsheets. Feed concentration had a large influence on the fractionation results, with a lower feed concentration resulting in improved fractionation efficiency. Furthermore, the surface roughness improved considerably for very low fine fraction mass ratios. Surprisingly, these ratios were much lower compared to the share of earlywood fibres in the pulp. The novel hydrocyclone design showed a lower thickening of the coarse fraction when compared with previous experiences with conventional hydrocyclones. Based on these results a hypothesis on the separation mechanism in a hydrocyclone was proposed. It suggests the existence of a radial variation in suspension composition in the fractionation zone, and most likely also a layer of unfractionated feed pulp along the outer wall. The existence of the gradient and the layer could explain that, especially at higher feed concentrations, a significant improvement in surface roughness was only obtained when the fine fraction ratio was significantly below the earlywood fibre share in the pulp.