Pulp type processes

Shotcreting Using a modification of normal shotcreting techniques, it has been found possible to produce steel and polypropylene fibre shotcretes, for use particularly for lining of tunnels, and for stabilization of rock slopes. With this method, too, relatively high volumes of fibres can be added to the mix. Pulp type processes For asbestos cement replacements (cellulose or other fibres are used as a replacement for the asbestos), the fibres are dispersed in a cement slurry, which is then dewatered to produce thin sheet materials. These can be built up to the required thickness by layering. This process yields fibre contents of typically from 9% to over 20% by volume. 

Table 21.3 presents an overview of wood pulping types by the method of fiber separation, resultant fiber quality, and percent of 1998 U.S. pulp production.1112 Many mills perform multiple pulping processes at the same site, most frequently nondeink secondary fiber pulping and paper-grade kraft... 

The pulping (cooking) process traditionally was performed on a batch basis in a large pressure vessel called a digester. Conditions vary depending upon the type of wood being pulped and the quality of end product desired. Typical conditions for kraft cooking are listed in Table 28.3. 

Once the pulp production process is well understood, the first step is to characterize the methodological hardware and software elements that will be part of the integrated EPC/SPC. Within this work, the term hardware is used to describe the more physical elements, such as the mathematical model, the controllers and the control charts. On the other hand, the software is related to the rather intangible elements, namely the intervention criteria, the type of disturbances (for testing purposes) and the performance measures. The intervention criteria will constrain the rule to be applied by the controller and the control chart, as well as the way they interact in the integration. Altogether, the software elements allow the evaluation and comparison of different integration... 

Fruit juice clarification by pectinases and cellulases is another interesting application. In the conventional process after the enzymatic reaction in the pulp treatment step takes place, filtration over diatomaceous earth follows. This filtration-type process produces a lot of solid waste, and results in costly enzyme loss. MBR are appropriate for such application either for enzyme recovery and recycle or in the form of a more compact CMR type system, with the biocatalyst immobilized on the membrane itself [4.58]. 

The wood species and the type of pulp manufacturing process relate to the reactivity of the cellulose in acetylation. For example, a highly purified acetate-grade wood pulp from hardwoods manufactured by the prehydrolyzed kraft (PHK) process has about the highest reactivity. Within the category of softwood pulps, there are different levels of reactivity depending on the wood species and the extent of purification. The importance of cellulose... 

Pulps have since been made by various chemical processes, covering the full pH range from acid to neutral to alkaline. Each pulping method produces characteristic pulp properties, depending on the type of wood used, and each pulp type lends itself to a different end use. The most widely used chemical cooking process in North America today is the alkaline kraft process . The kraft process was invented in 1884, and the first U.S. kraft mill started production in 1909. As will be detailed later, semi-chemical, chemi-mechanical, and non-suUur cooking processes are also used. These methods are often considered more environmentally friendly than some of the older processes. 

Fiber Analysis. Paper may be composed of one or several types of fibers, eg, animal, vegetable, mineral, and synthetic (see Eibers). Paper is generally composed of woody vegetable fibers obtained from coniferous (softwood) and deciduous (hardwood) trees. QuaUtative and quantitative methods have been developed to determine the fibrous constituents in a sheet of paper (see TAPPI T401). However, the proliferation in the number and types of pulping processes used have made the analysis of paper a much more complex problem. Comprehensive reviews of the methods are given in References 20 and 23. 

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. 

A high yield chemical pulp, eg, 52—53% bleached yield from softwoods, can be obtained, but strength properties ate inferior to those obtained from the kraft process. If a protector, eg, potassium iodide, is added, an additional 2—3% yield is obtained, as is an improvement in all strength properties. The gas penetration problem can be minimized if ftbetization is accompHshed before treatment with oxygen. Oxygen treatment of virtually all types of semichemical and mechanical pulps has been explored (55). Caustic, sodium bicarbonate, and sodium carbonate have been used as the source of base (56,57). In all cases, the replacement of the kraft by these other processes has not been justified over the alternative of pollution abatement procedures. 

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