Aggregation studies of pinus radiata wood extractives under increased system closure

Within the pulp and paper industry, the recycling of process water to reduce water consumption leads to accumulation of colloidal Material in this water and greater risk of deposition. A major factor in the colloidal stability of these substances, which arise from the wood extractives, is the presence of natural polymers originating from the wood as well as salts that accumulate in the process water as a result of increased system closure. This work explores the factors that affect the stability of wood extractive colloids under varying conditions of ionic strength, ionic valency, shear, pH, mixtures of cations and wood polymers released from pinus radiata thermomechanical pulp. Coagulation of a colloidal wood extractive solution by a single salt was found to follow the Schultz-Hardy rule, with the critical salt coagulation concentration (CCC) strongly influenced by salt valency (z). Changes to both pH and shear experienced by the colloid, were observed to affect the concentration of salt required to destabilise the colloid. However, on addition of a second salt to the solution, the CCC decreased for calcium + sodium in comparison to when only a single salt was present. Addition of wood polymers to an aqueous dispersion of wood extractives caused two stages of destabilization of the wood extractive colloids, which were separated by an apparently stable region. The behaviour was typical of aggregation by polymers in which polymer bridging at low polymer additions caused firstly colloid destabilization, followed by steric stabilisation of the colloids at medium concentration of the polymer, then depletion flocculation followed finally by depletion stabilization at higher polymer concentrations.