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due to their high surface area which reduces perme- ability and slows the dewatering rate. Therefore, pulp flexibility must be promoted in moderation to ensure compressibility while reducing the risk of sheet seal- ing. Bleached hardwood pulp observed some of the highest dewatering rates. It had the highest WRV of 1.56 g/g which indicates high compressibility. The low fines content of 25.0% in the pulp also prevented the sealing phenomenon, consequently resulting in accelerated dewatering when compared to its recycled and mechanical counterparts. Recycled pulp followed with a slightly higher fines content of 46.3% and a rel- atively lower WRV of 1.39 g/g. Mechanical pulp had the poorest dewatering performance due to its low flexibility which was signified by low water retention value of 1.16 g/g. This was further aggravated b y a high fines content of 74.0%, which reduced the per- meability o fmedia during dewater ing. Figures 10 and 11 graphically demonstrate the results discussed. Conclusion and recommendations Vacuum pulses were achieved using a robust labora- tory suction box design. Appropriate vacuum con- ditions were maintained due to sufficient vacuum reservation. Formation of hand sheets was achieved through a hand sheet former thus avoiding the need for a large capacity of removed filtrate storage which could have led to excessive air leakage as was the case in the study done by Räisänen et al. ( 1995). The newly developed laboratory suction box adequately replicated high vacuum dewatering by suction boxes in the forming section of paper machines. This was further proven by the pressure profiles obtained dur- ing the dewatering of hand sheets. Multiple suction pulses were successfully generated, and the results observed followed established trends reported pre- viously. Larger vacuum pressures result in better dewatering of pulps which eventually plateau after a certain dwell time as shown by the maximum pulp concentration constant which is essentially a repre- sentation of the point at which pulps reach a limiting solids content for a given pressure at long dewatering times. The dewatering time constant shows the rate at which filtrate is removed. It was found to be much quicker for bleached hardwood and recycled pulp. Mechanical pulp had the slowest rate of dewatering as it was extremely externally fibrillated, which means
that it requires more vacuum energy to achieve pulp concentration like its bleached counterpart. Free- ness or drainability successfully corroborated high vacuum dewatering trends of pulps where pulps with the same rate of drainability achieved similar dryness levels or pulp concentration. Therefore, fibre char- acteristics may be used as a preliminary measure o f pulps’ expected vacuum dewatering behaviour. All pulps were dewatered at the same vacuum pressures; however, exploring specific vacuum regions for each pulp type would be beneficial to determine optimum energy-saving operating conditions in the high vac- uum zone of paper machines. Author contribution MM: Contributed to design and test- ing of new equipment, undertook all dewatering and pulp char- acterisation testing, analysis and interpretation of data, drafted the manuscript. TL: Contributed to design of new equipment, design of experiments and interpretation of data, co-super- vision of researcher (MM), revised manuscript critically for important intellectual content, approved the final manuscript to be published. JG: Conception of the project, contributed to design of new equipment, design of experiments and interpreta- tion of data, co-supervision of researcher (MM), reviewed man- uscript critically for important intellectual content, approved the revised manuscript to be published. GS: Detailed design of new equipment and testing of hardware, reviewed manu- script critically for important intellectual content, approved the revised manuscript to be published. MG: Detailed design of new equipment and testing of hardware, reviewed manu- script critically for important intellectual content, approved the revised manuscript to be published. JK: Detailed design of new equipment and testing of hardware, reviewed manuscript criti- cally for important intellectual content, approved the revised manuscript to be published. WW: Contributed to design of new equipment, design of experiments and interpretation of data, reviewed manuscript critically for important intellectual content, approved the revised manuscript to be published. LT: Contributed to design of new equipment, design of experi- ments and interpretation of data, co-supervision of researcher (MM), reviewed manuscript critically for important intellectual content, approved the revised manuscript to be published. All of the authors have consented to publication of the manuscript in its present form. Funding Open access funding provided by Stellenbosch University. This work was financially supported by the Paper Manufacturers Association of South Africa (PAMSA), Grant No. PRU/126. Data Availability Data sets generated during the current study are available from the corresponding author on reason- able request. Declaration
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