Examination of variables influencing the selective froth flotation of wood extractives from Pinus radiata TMP process water

摘要

Effective separation of wood extractives from thermo-mechanical pulp (TMP) process water by froth flotation involves optimisation of many variables that influence the removal efficiency. To this end, the effects of surfactant concentration, flotation time, pH, temperature and conductivity have been examined. Flotation was improved by addition of the cationic surfactant dodecyltrimethylammonium bromide (DoTAB) which increased the removal efficiency of colloidal extractives by increasing attachment of negatively charged colloids to air bubbles. Longer flotation time increased the removal efficiency, but the benefit was offset by the accompanying water loss. The pH was found to strongly influence flotation of both colloidal and dissolved extractives. Changes to the surface charge on both air bubbles and colloids with pH resulted in optimal flotation of the colloidal component at pH 3-5 with added DoTAB, and at pH 2.5 or below without added surfactant. Flotation of dissolved extractives increased with rising pH based on the increasing dissolution of resin and fatty acids. Temperature had some influence on flotation while conductivity had minimal influence on removal efficiency over the conductivity range studied. Maximum extractive removal was achieved by flotation with 80 ppm of added surfactant DoTAB at pH 5.5 and 50°C for 20 min. Under these conditions the removal efficiency was 96% for resin acids and 94% for triglycerides and relied on flotation of both dissolved and colloidal components. Process water treated in this way would be suitable for recirculation and should reduce the problems caused by accumulation of wood extractives.