A range of surfactants have been compared for use as foaming agents in Flotation Equipment for removal of wood extractives from thermo mechanical pulp (TMP) process water. The surfactants studied were dodecyltrimethylammonium bromide (DoTAB), hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and (1,1,3,3- tetramethlbutyl)phenyl-polyethylene glycol (Triton X-100). The surfactants were evaluated on the basis of their selectivity for removal of wood extractives. The cationic surfactants DoTAB and CTAB were found to be the most effective for removal of extractives, concentrating the solution by a factor of 6 at pH 6.5. Triton X-100 concentrated the extractives by a factor of 3.5, while addition of SDS did not provide any improvement over the inherent surfactant properties of the process water. The selectivity of extractives removal with these cationic surfactants was found to be dependent on pH with higher selectivity achieved at pH 6.5 over pH 5.0. This can be explained by ionic interactions with resin and fatty acid anions in the colloidal phase. In all cases, the resin acids were concentrated to a higher degree than the fatty acids or triglycerides. This effect was most pronounced in flotation with DoTAC, CTAB and Triton X-100, in which the concentration factor for resin acids was more than twice that for fatty acids or triglycerides.

It is unclear whether this is due to selective flotation of fibre particles which are high in resin acids or due to removal of resin acids from the fibre or colloid surfaces. Further work is required to determine this. It is also expected that further optimisation of parameters including temperature, pH, surfactant concentration and airflow will result in more selective flotation of extractives. Wood extractives liberated from Pinus radiata during thermo mechanical pulping can cause extensive problems for paper manufacture. The extractives include fatty acids, resin acids, triglycerides, sterols and sterol esters, of which resin acids and triglycerides are the major components. These extractives are lipophilic and form a colloidal suspension in process water. Stability of the colloids is dependent on many factors including pH, temperature, shear stress, ionic strength and dissolved organic matter, all of which can change during paper processing. Hence, wood extractives are prone to aggregate and deposit as pitch on machinery and in the paper causing production losses due to downtime for machinery maintenance, and reduction in paper quality. These problems are exacerbated by efforts to reduce water usage by closure of process water circuits as this allows the wood extractives to build up in the process water. Froth flotation is a possible cost effective method for removal of wood extractives from process water. Flotation techniques are used extensively in the mineral processing industry and in water treatment. Methods vary considerably depending on the substances to be separated.

In the paper industry, dissolved air flotation (DAF) is currently used for ink removal from recycled fibre but has also been investigated by Richardson and Grubb for removal of Pinus radiata wood extractives. Induced air flotation methods also known as froth flotation, have been investigated by Zasadowski et al for the removal of extractives from Picea abies spruce TMP process water. Effective flotation requires generation of a froth that is concentrated in extractives and is stable enough to be removed from the liquid. Extractives removal relies on three factors: particle capture, particle attachment, and stability of the bubble- particle aggregate. Hydrophobic particles attach to air bubbles through surface interactions. Surfactants alter the surface properties of the air- water interface and the particle surface influencing foam stability and bubble-particle interactions. Therefore they are capable of affecting the efficiency and selectivity of flotation.

Theory for the upper limit of coarse particle Flotation Machine suggests that a quiescent flow field is necessary to prevent the particles from becoming detached from the bubbles. A liquid-fluidized bed provides a suitable environment. The flotation feed is introduced into the fluidized bed, and air bubbles are dispersed in the fluidizing water. Coarse particles attach to the bubbles rising through the bed and are lifted into the froth layer that is maintained on top of the cell in the usual way. Particles of galena up to 1 mm in diameter have been recovered in such a bed, while for particles of lower density such as quartz and coal, the upper limit for flotation has been extended to at least 2 mm and 5.6 mm respectively. The fluidized bed technology provides major advantages beyond the ability to recover coarse particles currently lost in existing operations. Thus, if the upper flotation limit can be extended, the top size for grinding can be raised, with significant reductions in energy costs. Liberation of the values is the key limitation. Also, a fluidized bed flotation cell can accept a feed with much higher percent solids, leading to significant reductions in water requirements.

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