Abstract:
The energy requirement for chemithermomechanical pulping (CTMP) northern hemisphere spruce (Picea abies) to achieve a given level of paper strength decreases with increasing sulphonation. In contrast, the energy demand for CTMP pulping the radiata pine wood used in the southern hemisphere to achieve the same level of paper strength is considerably higher than that of spruce and increases with increasing sulphonation. It is shown that the injection of sulphite liquor into the refiner inlet with the dilution water (dilution water sulphonation (DWS)) lowers the energy demand to produce sulphonated pulps from radiata pine because defibration is initiated before the fibre is sulphonated. The result is a more energy-efficient process alternative to CTMP for radiata pine. The reason for the lower energy demand of the DWS process has been demonstrated by analysing the chemical composition of the long fibre and fines fractions of DWS and CTMP pulps produced in a sequence in which the fines were removed after each of four successive refining stages. The CTMP process lowers the lignin softening temperature before the chips enter the refiner, enabling the fibres to be liberated from the wood matrix with less cell wall damage and fracture. The reduced stiffness of the sulphonated fibre wall makes these relatively undamaged CTMP fibres more elastically compressible than the DWS fibres in the refining operation and as a consequence more refining energy is required to develop their papermaking properties. The alternative hypothesis that the higher energy demand of CTMP was due to the fibres being more heavily coated in lignin and difficult to refine was not supported by the chemical analysis of the fibres and fines fractions. No substantive differences were found between the lignin and cellulose contents of the surfaces of the DWS and CTMP fibres. These findings explain why CTMP is a more energy-efficient process for spruce than radiata pine. The increase in energy demand to a given strength property that occurs when radiata pine chips are sulphonated is due to the combined effects of the stiffer, thicker-walled earlywood fibres of radiata pine and the lower softening temperature of the sulphonated lignin creating fibres which are relatively more resistant to collapse when dried and formed into a paper sheet than the spruce earlywood fibres. The conventional CTMP and DWS pulps were produced in the PAPRO refiner pilot plant using an industrial-size 1.25 MW, 900 mm, Jyllia SD52/36, pressurised single-disc refiner. Wood chips were prepared from top logs obtained from 25 -· 27 year-old radiata pine trees. All pulps and paper handsheets were prepared and evaluated using Appita standard test methods and chemical analyses were in accordance with Tappi and CPP A test methods.