Recovery of wood extractives by flotation

Wood extractives from softwood species include resin acids, fatty acids and triglycerides. Their water repellency, adhesive properties and biological activity make them useful in manufacturing, agriculture and medicine with broader applications expected to be developed in the future with a shift toward renewable resources. These wood extractives can be problematic in paper manufacture as they cause deposits on machinery and in the paper products, which requires higher levels of maintenance, limits water recycling, increases wastewater toxicity and can reduce product quality. Current methods for addressing these problems in thermo-mechanical pulp (TMP) and paper mills include treatment of wood to reduce extractives content prior to pulping, tighter control of processes to prevent formation of deposits, and the use of additives to fix extractives into the paper product. None of these methods allow for recovery of the valuable wood extractives. This thesis presents flotation as a viable method for recovery of wood extractives from TMP process water in pulp and paper mills. While flotation of wood extractives has been studied previously, it is not well understood in terms of the mechanism of flotation or its dependence on different variables. Furthermore, most published research relates to extractives from Norway spruce (Picea abies) which is a common plantation species in the northern hemisphere. By contrast, there are relatively few studies relating to Radiata pine (Pinus radiata) which is common in the southern hemisphere and contains a different composition of extractives. The studies presented here investigate flotation of extractives from Pinus radiata TMP process water. Optimal flotation of wood extractives was achieved using 80 mg/L of added cationic surfactant dodecyltrimethylammonium bromide (DTAB) at pH 5 and 60 °C. This treatment reduced the extractives concentration by 95% and returned 90% of the water volume for recirculation. The effects of different variables on flotation were studied and the results used to identify possible mechanisms of flotation. The most influential variables were the type of surfactant added, pH, and ionic strength. Flotation mechanisms proposed to explain the results included colloidal flotation, flotation of dissolved extractives as either surfactants or as complexes with DTAB, and entrainment of fibres. Colloidal flotation appeared to depend on electrostatic attraction between air bubbles and wood extractive colloids which occurred either at low pH or with addition of a cationic surfactant. The efficiency of colloidal flotation was reduced by increased ionic strength, explained by compression of the electrical double layers around air bubbles and colloids by the electrolyte. Dissolved wood polymers also reduced colloidal flotation efficiency, presumably by inhibition of bubble-colloid attachment by steric hindrance. Flotation of dissolved extractives was proposed to occur at higher pH levels due to adsorption of resin and fatty acid anions on bubble surfaces by either surfactant adsorption or complex formation with DTAB. Fibre-bound extractives could be floated by physical entrainment of fibres in the froth. Methods for recovery of extractives and DTAB from froth generated by the flotation treatment were also investigated. Maximum recovery of extractives was achieved by solvent extraction with a 60:40 mixture of ethyl acetate and heptane at pH 6 which recovered 88% of resin acids, 75% of fatty acids and 10% of triglycerides. Evidence of complex formation between wood extractives and DTAB was found which could explain the limited recovery of both extractives and DTAB from the froth. Flotation is shown to be an effective laboratory method for removal and recovery of wood extractives from TMP process waters, that could be implemented in industry using existing machinery. The most effective surfactant additives have been identified as well as the most important process parameters and conditions that affect flotation efficiency. The influence of these variables on flotation has provided insight into the mechanisms of flotation to enable better control of flotation processes. Implementation of a flotation process for wood extractives recovery would reduce problems related to wood extractives and provide a valuable by-product. These benefits may offset the costs to make implementation feasible.