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UBC Theses and Dissertations

Low consistency refining of mechanical pulps Welch, Lorrie V.S.

Abstract

Mechanical pulping produces a much higher yield than its chemical counterpart. It has lower water and atmospheric effluent loads but requires a high level of energy input. There is a drive towards reducing energy use and costs through modifications to mechanical pulping equipment. The vast majority of research in the field of mechanical pulping focuses on the energy split between the first and second refiners in the production stages of pulping. This thesis examines an alternate process, low consistency refining, to see if there is a potential to improve mechanical pulp properties with this technique. The experimental work included production and examination through fibre and paper test of several groups of mechanical pulps. The results indicate that low consistency refining of mechanical pulp can produce paper quality similar to that of high consistency refining at reduced energy input levels. However, the conditions for low consistency refining, namely number and intensity of refining impacts, must be chosen carefully as too high an impact intensity can damage fibres and reduce paper quality. Individual fibres show similar development for high consistency and low-intensity low consistency refining. Most notably, average fibre length and the number of long fibres are kept at high levels to maintain the network strength of paper. Low consistency refining of latent thermomechanical (TMP) and chemithermomechanical (CTMP) pulps straightens fibres. If fibre curl is taken as a measure of latency removal, low consistency refining can achieve a delatent pulp without the separate step of latency removal. However, latent pulp may be more brittle resulting in lower paper strength. Low consistency refining of latent pulp may offer a usable alternative where a mill is limited by either space or latency removal equipment and lower paper strength is acceptable. With regards to flexibility testing, LC-refined TMP and CMP show changes in flexibility distribution when compared to base pulps. The coarser summerwood fibres seemed to be more affected by refining as measured by changes in flexibility. Much work was done to complete a fracture toughness analysis. The results proved to be strongly correlated to the tensile strength test results. Therefore this test may not offer new information regarding paper runnability and strength.

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