Added soap contact time with recycled pulp improves flotation deinking efficiency and reduces cost

Abstract

Optimization of the fatty acid soap addition points increased soap contact time with recycled pulp before flotation. This, in turn, resulted in improved ink removal efficiencies for three of four types of commercial flotation cells. Mill A saved 25% in fatty acid flotation collector dosage while maintaining the same brightness gain and ink removal efficiency. In mill B, increasing the soap contact time improved the aeration of one line of flotation cells, which in turn led to a 1-point brightness improvement of deinked pulp and savings in bleaching chemicals. Both mills showed 30%–54% lower carryover of saturated fatty acids from the flotation collector to newsprint made from deinked pulp and thermomechanical pulp. Flotation cells with longer residence times and aeration injectors in good order did not show any improvement in aeration or flotation efficiency. Soap contact times with pulp, whether before or during flotation, should be on the order of 9–20 min to assure maximal flotation efficiency. Increasing soap contact time before flotation to 45-60 min did not further improve flotation efficiency.