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Density
The PFI refining increased the handsheet density by about 93%.Removing the fines (P200) resulted in a 12% drop in sheet density when compared to the refined pulp (R28+R48), implying that the contribution of fines to the sheet density was relatively small despite their remarkable influence on freeness. Further removal of R200 only slightly reduced the sheet density, suggesting the fibrous elements were stiff.
Tensile index
The PFI refining increased the tensile index by 144% (TABLE III), yielding a tensile index of 50 N*m/g, which is remarkable for a softwood TMP. It would be interesting to see where did the improvement come from. Progressive elimination of fines (P200) and short fibres (R200) reveals that a loss of 30% in tensile index of the refined pulp could be attributed to the fines and about 7% to the short fibres. Hence, one could assume that approximately 60% of the improvement by PFI refining was accounted for by the longer fibres, that is R28, R48 and R100 combined. This means that the bonding potential of the longer fibres could be ameliorated by the PFI refining. Noteworthily, the improvement in fibre bonding of the long fibres might be due to the fact that these fibres had already absorbed a lot of mechanical energy during the primary and secondary refining and, hence, they were more amendable when subjected to further mechanical treatment in a PFI mill.
Tear index
As noted in TABLE III, the PFI refining had a serious negative effect on fibre length by decreasing the tear index by about 16%. However, eliminating the P200 and R200 fractions marginally improved the tearing resistance, which is probably due to the increase in average fibre length. At the conclusion of the trials, the mill elected to remove the two 26-in. double disc refiners and use only the new conical refiner for production. This reduced the process refiners from two to one and reduced total connected energy by 450 hp, equating to energy savings of approximately $76,000/yr. In addition to energy reduction, the mill realized savings by reduction in maintenance and number of fillings purchased annually.
In August 1999, one of the new triple-cone refiners was installed in an upper mid- western U.S. linerboard mill. At the time, the mill was operating one 34-in. double disc refiner with an 800-hp motor processing 150 to 180 tpd of OCC for the top sheet of its two-ply liner. The mill had determined that a second 34-in. double disc refiner operating in series with the existing refiner was required to achieve the new benchmarks that had been set for the final product. However, upon investigation of the new conical refiner and the results at the Brazilian beta site, the mill project team decided to test a conical refiner to see if it could provide the performance required.
The existing 34-in. double disc refiner was removed. The new triple-cone refiner was installed in its place and connected to the existing 800-hp motor. The total motor output was limited to 600-hp, keeping in line with the design criteria of the new refiner. At startup, the new refiner achieved the necessary paper tests on ts own but used only 375 gross horsepower. After a three-month trial, the refiner control was switched over to the mill DCS, and the gross refining power increased to 425 to 450 hp, depending on grade and tonnage. According to the mill, the new refiner is the most stable in DCS control and requires significantly less operator and maintenance attention compared with its double disc refiners. Overall energy savings from the installation is approximately $146,000/yr.
In addition to reducing the energy required for refining, the mill also saved the expense associated with purchasing and maintaining two refiners and motors. An added benefit is tackle life, since the first set of tackle lasted 8 months compared with 60 days for the previous disc refiner. This mill is now evaluating installing two more of the triple-cone refiners in place of the three double disc refiners operating on the base sheet. It is expected that the overall sheet properties will improve when this takes place. The fillings in the refiners can then be optimized to provide fibrillation in the primary refiner for overall strength properties. The tickler refiner can be used for slight cutting to improve ring crush and ply bond. Overall energy savings are projected to be $438,900/yr.