Screening, pulp

To prepare a pulp for Klason lignin analysis, a sample equivalent to approximately 10 g oven-dry is disintegrated in a blender or ground to pass a 10- to 20-mesh screen. Pulps which do not contain coarse fibers or shives and which can be dispersed in sulfuric acid readily may be used without prior disintegration. 

The pulp to be treated with cellulolytic enzymes should first be screened in a laboratory flat screen to remove any shives. The screened pulp is beaten either in a laboratory Hollander beater (TAPPI Standard Method T 200 os-70) or in a PFI mill (TAPPI Standard Method T 248cm-85), depending on the amount of pulp to be treated, to a final Canadian Standard Freeness of 200-300 ml. The beating operation helps convert shives into fibers and improves the digestibility of the pulp towards cellulolytic enzymes. The pulp is dewatered to a consistency of approximately 25% and stored in a polyethylene bag after fluffing. 

A series of preliminary experiments were employed to find the feasibility of using laccase as a precursor to kraft pulping. The resulting kraft pulps were analyzed in terms of the screened pulp yield and pulp kappa number. 

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

In the extraction of citms juices it is desirable to have as gende an extraction pressure as possible. There should be minimal contact time between juice and pulp to reduce the amount of bitter substances expressed from the peel into the juice. The amount of suspended soHds in citms juice is controlled in a subsequent separation in a finisher. A screw action is used to force the juice through a perforated screen and separate the larger pulp particles from the juice. The oil level in the juice is adjusted by vaporizing under a vacuum (10). The separated pulp is washed and finished several times to produce a solution which is then either added back to the juice to increase juice yield, or concentrated to produce pulp wash soHds, also called water extract of orange soHds, which can be used as a cloudy beverage base. 

The fine mica fraction is deslimed over 0.875—0.147-mm (80—100-mesh) Trommel screens or hydrocylcones, or is separated with hydrosi2ers. The deslimed pulp (<0.589 mm (—28 mesh)) of mica, feldspar, and quart2 is then fed to a froth flotation circuit where these materials are separated from each other either by floating in an acid circuit with rosin amine and sulfuric acid (2.5—4.0 pH), or an alkaline circuit (7.5—9.0 pH) with tall oil amine, goulac, rosin amine acetate, and caustic soda (see Eig. 2). 

Oversized chips are sometimes sHced to control thickness in a slicing rechipper. The uniform thickness results in more even penetration of pulping liquors and therefore less screen rejects in the resultant pulp. 

Processing Chip Screen Rejects. Overlong chips are usually reprocessed in a smaller version of the disk-type roundwood chipper. Overthick chips are reprocessed in a chip sheer. The product from these reprocessing operations returns to the main chip flow ahead of the screens (see Eig. 10). The fines are sent to a hog boiler as fuel, or else rescreened. Pin chips are metered back into the main chip flow or sent to a fine-particle pulping system. 

Wood Residuals. Chips, sawdust, and other residuals such as planar shavings are used as a primary source of fiber for some pulp mills. Chips are screened and placed in a purchased chip pile. Sawdust and other residuals obtained from wood processing plants must be cooked separately and requite special digesters and handling equipment. These materials may also be burned as hog fuel. 

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