Low consistency (LC) refining of thermomechanical pulp (TMP) has been reported to be an energy saving technology, but its application in the pulp and paper industry is limited by fibre cutting. A pilot scale trial of alkaline peroxide treatments of primary refined TMP prior to LC refining was conducted in order to investigate the effects of the treatments on pulp quality, LC refinabilty and electrical energy consumption. A primary softwood TMP, refined at high consistency, was treated with various peroxide and alkali charges prior to LC refining over a range of specific energies at intensities of 90 kWh/t or 150 kWhit per pass through the refiner. The changes in fibre and handsheet properties of the resulting pulps were assessed and compared. It was found that alkaline peroxide treatments increased fibre resistance to cutting during low intensity LC refining over the whole range of specific energies studied, up to 630 kWh/t. However, for the highly alkaline peroxide treatment, protection of the fibre from cutting during high intensity LC refining was limited to specific energies below 300 kWh/t. The increased fibre resistance to cutting caused by alkaline peroxide treatments was due to increased fibre flexibility. Treatment with highly alkaline peroxide increased tensile strength index by up to 10 Nm/g, providing the potential of lowering the total electrical energy required to achieve a strong pulp. The enhancement of tensile strength caused by highly alkaline peroxide resulted from increased bonding caused by acid group generation rather than promotion of further fibrillation during LC refining. A higher charge of alkali at the same peroxide charge produced higher acid group contents in the pulp but led to lower brightness gains. The increased generation of acids groups with higher alkaline charge resulted mainly from hydrolysis of ester groups in pectins and xylans on the fibre surface and in the fines.
