Drainage and fine particle retention in forming fibrous mats

Drainage and fine particle retention phenomena during papermat formation over a wire screen are investigated. Drainage of a pulp suspension under gravity is modeled taking the formation of a compressible fiber mat into account. The drainage process is idealized as a sequence of constant pressure filtrations with the gravity head of the suspension determining the driving pressure. Compressibility effects are taken into account and theoretical predictions are obtained. Pressure drop, concentration and specific filtration resistance profiles are presented. It is found that the pressure inside the pulp mat undergoes a relaxation with time as mat build up occurs. Concentration profiles within the forming fiber mat are found to reflect this behavior as do the specific filtration resistance distributions. Comparison of drainage rate and filtrate volume data with experimental results for a variety of pulps indicates good agreement.; Fine particle (titanium dioxide) retention is studied using elements of depth filtration theory. Concentration profile, of fine particles, inside the forming mat as a function of time is determined under different electrokinetic conditions. A new procedure is developed to measure the fine particle concentration profile in a loosely bonded wet sheet. It is found that the cause for the well-known fine particle distribution in handsheets with a maximum on the wire-side is due to the age of the fiber mat layers. The layer closest to the wire captures fine particles for the maximum time as compared to layers near the top of the sheet that are among the later ones to form. Theoretical predictions are found to be in good agreement with the experimental data.; Drainage data from the gravity filtration of a suspension forming a compressible mat is shown to be useful in determining the specific surface area, specific volume and compressibility constants for papermaking pulps. An error minimization procedure is used to search for optimal parameters characterizing the pulp drainage. This procedure is applied to drainage data for variety of pulps to obtain the respective permeability and compressibility characteristics. These characteristics agree with earlier data for these pulps as well as with the data obtained by independent testing using the Pulmac permeability tester.; Gravity filtration theory is also applied to analyze freeness and drainage in a Tappi standard handsheet mold apparatus. The freeness of unrefined pulps is predicted quite accurately. Upon refining, fines are generated some of which are lost during the drainage process in a handsheet mold and freeness tester. Thus, experimental freeness values show deviation from theoretical predictions. When fines retention is taken into account, the predictions of freeness and drainage time agree with experimental measurements.