Flotation as a separation technology for recovering pulp fines and sustainable nanocellulose production

Abstract

• Fines from refined pulp were separated and enriched by froth flotation. • Flotation was controlled by pulp suspension rheology and fluid flow hydrodynamics. • Fines separation was quantified by fines enrichment ratio and fines recovery. • Fines recovery and fines enrichment ratio balance enables process optimisation. • Quality of fines and fines sheets was comparable to nanocellulose. Nanocellulose has great potential in the pulp and paper industry, but current nanocellulose production methods require high energy. One way to achieve energy efficient production of nanocellulose is to develop a large-scale process of separating small fibers (fines) from large fibers. In this study, a laboratory mill was used to produce moderately refined (30.8% fines) and highly refined (81.8% fines) Bleached Eucalyptus Kraft (BEK) pulp. Refined samples were subjected to flotation separation, and separation efficiency was quantified by fines recovery and fines enrichment ratio. At high pH (11.9), with a surfactant concentration of 0.15 mM, the surface tension of the air–water interface was 44 mN/m and the Sauter mean diameter of the bubbles in the froth was 0.948 mm. These conditions provide the required froth stability and low bubble size to achieve selective separation of fines. Maximum recoveries of 80.4% and 43.0% were achieved at pH 11.9 and 1.0 wt% for moderately refined and highly refined pulp, respectively. Maximum enrichment ratios of 4.33 and 4.37 were observed for moderately refined and heavily refined pulp at pH 11.9 and 1.5 wt%, respectively. The aspect ratio of float fibers increased by 14% and 18% for moderately and highly refined BEK, respectively. Flotation was shown to be controlled by the viscosity and surface charge of the pulp suspension. Viscosity and zeta potential values were maximised at pH 11.9 and 1.0–1.5 wt% pulp solids, promoting agglomeration of long fibers and enhanced fines separation. Experimental results were found to be statistically significant via ANOVA. Fines were shown to have analogous properties to commercial nanocellulose. This study demonstrates that size-selective fiber flotation is governed by pulp suspension rheology and fluid flow hydrodynamics, and that flotation represents a promising technology for fines separation and production of a sustainable nanocellulose substitute.