Over the last few decades, the continuing decline in mechanical pulp-based grades has led the pulp producers to modify operation and implement measures to reduce production cost in order to stay competitive. In spite of a considerable effort to reduce energy consumption, the latter is still a major portion of the production cost in the process of bleached chemi-thermomechanical pulping (BCTMP). In this study, we evaluated the impact of inter-stage screening fractionation (ISSF) and low consistency refining for producing BCTMP with the objective of reducing refining energy consumption while maintaining or improving bulk and strength properties. We obtained softwood CTMP pulp samples after the primary refining stage from a Canadian BCTMP mill for pilot plant investigation. The collected primary pulp was fractionated in a pilot plant Hooper PSV-2100 pressure screen where the accept stream contained mainly fine materials. We used several smooth profile round-hole screen baskets ranging from 0.060" to 0.085" in opening diameter. The reject stream was collected and refined in both high (HC) and low (LC) consistency refiners. Low consistency refining of the reject stream was conducted in two different Specific Edge Loads (SEL): 0.4 J/m and 0.6 J/m. In the first step and to establish a baseline for mill's existing configuration, the collected primary refined pulp was further refined in a HC refiner to target freeness levels. The accept and refined reject streams from the inter-stage screening fractionation stage were recombined, and their properties were compared to those of the control case. The results showed that the highest fractionation efficiency was achieved with a 0.070" opening diameter basket at a mass reject rate of about 60%. The accept stream from the 0.070" basket contained a higher fraction of fines as compared to that from the 0.085" basket resulting in a lower freeness. Inter-stage screening fractionation with 0.070" basket demonstrated a higher saving in energy to reach the same target Canadian Standard Freeness (CSF). At a given freeness of 400 mL and compared to the control case (without fractionation), the inter-stage screening fractionation followed by reject refining led to about 25% energy saving in the 2nd stage HC refining. Handsheets properties showed that utilization of Inter-stage screening fractionation could produce CTMP pulp with higher bulk, similar average fibre length and tear index. However, a slight reduction in tensile strength was also observed. Similar refining specific energy consumption between the control configuration and Inter-stage screening fractionation using an 0.085" basket was observed. In the second set of trials, we used low-consistency refiner as the second stage of refining and evaluated its impact on pulp quality and energy reduction. We refined both primary pulp and the rejects from the inter-stage fractionation using 0.070" basket. The results showed that, at the same freeness of 400 mL and compared to refined primary pulp, the inter-stage fractionation saved about 26% in net LC refining energy. At a SEL of 0.4 J/m, the produced pulp had similar bulk and strength properties compared to those of the control sample. A higher SEL of 0.6 J/m in LC refining of the primary and rejects of the inter-stage fractionated pulps led to a decrease in net refining energy consumption, however, it also led to reduction in fibre length, bulk and strength properties.
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