Beating and Refining

 

Introduction. After chips have been converted to pulp it can either be bleached or processed directly. The pulp processing section has shown how stuff is moved around. What we need to look at now is how the pulp is prepared to give the final properties necessary for good paper and/or board production.

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General. Beating and refining basically mean the same thing, the only difference is what kind of equipment is used. Older technology relied on equipment called beaters which is basically a drum with bars along them. Upon spinning the drum, the bars come in close contact with a solid surface. With pulp present, the fibers are exposed to the mechanical action of being forced between two hard objects. Most mills have converted to refiners which basically perform the same function, and you should refer to the section on mechanical pulping to see what they look like.

 

Mechanism of Refining. The mechanism can be envisioned with the schematic shown in your handout. Basically the pulp gathers at a breaker bar and dewaters. Shear stresses result first in more water removal followed by cutting and fibrillation (wall disruption). Cutting is not truly desired as this can result in the production of fines which can reduce freeness and slow drainage on the paper machine. Stiffer fibers suffer more cutting and thus kraft fibers are more susceptible to cutting than sulfite. After passage by the breaker bar the pulp reabsorbs water and disperses.

 

Effects of Treatment. To a first approximation, beating and refining will remove the primary wall exposing the cellulose microfibrils. This increases pulp surface area for hydration and the pulps will swell in response to the mechanical action. The fibrillation leads to more surface area for fiber-fiber contact which will eventually allow for a stronger sheet of paper. Too much mechanical energy will result in significant fiber shortening and the formation of fines. The is undesirable but somewhat unavoidable. All mechanical energy, whether fibrillating the pulp fibers or creating fines, will result in a decrease in freeness (CSF).

 

Refining and Pulp Properties. As mentioned previously, kraft fibers are more difficult to refine than sulfite due to their stiffness. High lignin contents also result in poorer refining as the higher the lignin content, the stiffer the fiber. due to lignins hydrophobic nature, high lignin contents also lead to less hydration. Pulps with high hemicellulose contents typically refine well, as these polysaccharides have good swelling and gelation characteristics. With respect to tear, burst and tensile strength properties, the effects of refining can be envisioned as being a reflection of fiber length and hydrogen bonding. Tear strength typically decreases upon extended refining due to the loss in average fiber length. Tensile and burst strengths increase upon refining due to higher surface areas and therefore, increased hydrogen bonding between fibers in a paper sheet.

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