Ink Detachment, Fragmentation and Alkali Darkening During Pulping of ONP/OMG Part II: Effects of Sodium Sulphite, 2007 TAPPI 8th Research Forum on Recycling

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In the first part of this series, we looked at the effects of alkalinity and silicate on ink detachment, ink fragmentation, and ISO brightness during pulping of ONP/OMG. To differentiate the effects caused by hydroxyl ions from the ones caused by silicate ions, we pulped ONP/OMG mixtures in buffered solutions. To differentiate the effects of sodium sulphite from that of hydroxyl ions, buffers were again used. The results show that, in a batch helical pulper operated at 10% consistency, in the absence of surfactant, the presence of sodium sulphite was clearly beneficial; it reduced the extent of alkali darkening and provided a bleaching benefit. Gains will depend of the pH in the repulper, and also of the pulps’ final ERIC. At a pH of about 8.0 and without any ink, the brightness gain provided by 2% sodium sulphite was about 4% ISO brightness. For a 2% dosage and normal ERIC values after flotation (between 100 and 200 ppm), typical gains should be about 3.0±0.5%. Sodium sulphite also slightly reduced ink fragmentation and improved ink detachment. Thus, to realize the full bleaching benefit of sodium sulphite, it is important to optimize ink detachment in repulping while minimizing ink redeposition.