UBC Theses and Dissertations
The role of suction boxes on forming section retention and filler migration Montgomery, James
An apparatus was constructed to observe fibre mat formation under applied vacuum pressure comparable to that experienced in a suction box. The main focus of study was to determine the effects of system parameters on overall retention, filler distribution, and filler migration by analysing samples through weighing, ashing and EDX analysis. It was found that increasing suction pressure slightly decreased the overall retention, although the effect was greater at higher filler loadings. The forming fabric selection was statistically significant in determining the overall retention, and the significance of forming fabric selection grows with increasing filler loading. The most important factor in determining overall retention was the presence of retention aids. Measurement of the instantaneous permeability of the forming fabric and fibre mat during the forming process showed differences in the permeability curves between forming fabrics, particularly during the initial forming process. The permeability variations are correlated with differences in retention. Tests performed under gravity drainage, to provide a correlation between fabric properties and overall retention results, showed that air permeability had the highest correlation. Sheet splitting and ashing hand sheets formed using cationic PCC filler showed that applying a vacuum during the drainage process decreased the average filler content. The filler was preferentially removed from the wire side of the sheet. Similar tests performed with anionic filler showed that filler charge affects the distribution in a hand sheet. Anionic filler also showed little change in distribution when vacuum was applied during forming. Cationic and anionic filler have different retention mechanisms; attachment and filtration, respectively. Applied vacuum was found to affect the filler distribution of particles retained by attachment with the highest changes at the wire side. Applied vacuum was found to have only a small affect on particles retained by filtration. The reason for both is the compaction of the fibre mat, which occurs to the greatest extent at the wire side. The compaction reduces the size of openings in the mat which will further trap large particles retained by filtration but increase the drag on small particles, possibly exceeding the attractive forces responsible for attachment to the pulp fibres.
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