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Modelling latency removal in mechanical pulping processes Gao, Jiyang


Latency removal is an essential step in the mechanical pulping process. It occurs in a continuous stirred-tank reactor (CSTR) and non-ideal mixing lowers the performance. In order to optimize the latency removal process and reduce the energy consumption in the operation, a kinetic study was carried out. In this work, latency removal was studied at both the individual fibre and the pulp suspension frames of reference. In the first study, the removal of latency of individual TMP fibres was studied using optical microscopy. The fibre deflection under the influence of the heat and water absorption was measured as a function of time. At the pulp suspension level, latency removal was characterized by the change of different pulp properties and the dependency of each property on treatment conditions was determined. Kinetic models of latency removal for secondary refiner TMP and BCTMP pulps were developed, which were based on the rate of latency elimination characterized by freeness. The kinetic study reveals that a potential energy reduction in industrial operation of latency removal can be achieved by properly increasing the power intensity to get better mixing. These results were then complemented in a third study of a more direct measure of latency, i.e. curl index. The change in curl index of TMP pulp was examined and its dependence on temperature and other treatment conditions was determined. The development of tensile and tear strengths of TMP pulp was explored in terms of different treatment conditions and the results were analyzed in terms of fibre straightening and fibre deflocculation. Linear correlations between strength properties, curl index and freeness have been found. In the final portion of the work an industrial case study was performed, where the latency removal of primary BCTMP pulp was examined for the purpose of optimizing an industrial latency removal process. The result of the laboratory test and the onsite measurement in the mill shows latency removal of primary BCTMP pulp is a much faster process in comparison with the secondary BCTMP pulp, and the latency removal process in the pulp mill can be optimized using an existing smaller sized mixing chest.

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