Mechanisms of pulp loss in flotation deinking

The objective of this study was to investigate the effects of pulp type, consistency and bubble size on the flotation of pulp fibres and fines. Laboratory-scale batch flotation experiments were conducted using bleached kraft and mechanical pulp in the presence of sodium dodecyl sulphate. Pulp losses were linearly dependent on the recovery of water in the foam. These pulp losses were generally greater with small bubbles (∼ 100-400 μm) and with the mechanical pulp. With dilute pulp suspensions, the consistencies of the foam and the feed were similar. For more concentrated pulp suspensions, the consistencies of the foams were less than those of the feed, and pulp fines tended to be concentrated in the foam. The primary pulp loss mechanism was entrainment, in which pulp fibres and fines were transported hydraulically into the foam with the water. Two regimes of entrainment are possible, depending on the extent of flocculation. In the absence of flocculation, the individual loose fibres and fines from the bulk pulp suspension are carried into the foam. When the pulp is flocculated, the bubbles are diverted around the floes, and entrain the smaller fibres and fines from the more dilute, inter-floc regions.