Principle and application of screening and classif

2022-08-02
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The principle and application of pulp screening and classification

pulp screening and classification is an important part of the pulping system, especially the screening and classification of waste paper recycled pulp

1 principle of pulp screening and classification

the purpose of pulp screening is to selectively separate impurities (including coarse slag and other impurities) from pulp as much as possible

the basic principle of pulp screening is to have high separation efficiency, prevent impurities from being broken and reduced to the maximum extent, and minimize fiber loss

pulp classification can be regarded as a special application of pulp screening

the purpose of pulp grading is to separate the long fibers and short fibers in pulp, so as to effectively use pulp with different fiber lengths

the basic principle of pulp classification is to classify according to the fiber length distribution or fragment content of the required pulp

the screening and classification of pulp are carried out by a pulp screening machine

pulp screening is generally divided into roughing and cleaning. Generally, hole screen is used for roughing (slot screen can also be partially used for multi-stage roughing), and slot screen is used for selection, such as computer shell and protective cover

for the classification of pulp, the original smooth hole screen has been replaced by a specially milled slot screen, and the effect is better. It is mainly manifested in large output, high workshop schedulability and low energy consumption. Now Voith Sulzer has developed a multi sorter screen, which can be used for screening slurry and grading. See Figure 1 for its structure

Figure 1 Structure of multi sorter

Figure 2 global pulp characteristics

due to the different characteristics of raw pulp and various recycled pulp, the results of screening and classification will be very different. Figure 2 shows the pulp characteristics of unbleached and bleached chemical pulp, mechanical pulp and waste paper recycled pulp in major regions of the world

among the various pulps shown in Figure 2, unbleached and bleached chemical pulps have longer fibers and less impurities; Although the fiber of mechanical pulp is short, there are few impurities. Therefore, these original pulps generally only need to be screened to remove a small amount of impurities. Generally, they are not used by classification. But in fact, the mechanical pulping process has been a combination of screening and classification. The recycled pulp of office waste paper and deinking waste paper has shorter fiber and more impurities, so it needs to be improved (1) screening to reduce the speed and increase the output torque, but it can only be used by classification when it is not necessary or necessary. American OCC and Scandinavian OCC Recycled Pulps have long fibers and many impurities, so they need to be well screened and can also be used by classification. The recycled pulp of Asian OCC and European OCC has the most impurities and short fiber, so it needs perfect screening, and can also be used by classification if necessary. But the main problem here is that there are many binders and waxes, which are difficult to separate. Therefore, screening and grading should be combined as required

2 screening and grading of OCC recycled pulp

screening and grading of OCC recycled pulp has been well used in the production line of corrugated board, strong and tough box board and folding board. In these production lines, the pulping of waste paper is very simple. After the waste paper is broken in the pulper and the heavy impurities are removed by the high concentration slag remover, the rough separation is carried out. The pressure hole screen is used for coarse selection, and the tail screen is a non pressure screen. Most of the tail screens are operated in the washing mode to reduce the loss of fiber, said Professor zhangliqun of the advanced elastomer material research center of Beijing University of chemical technology. Then, the perforated screen is used for classification. Now, the 0.25mm slotted screen is used to replace the perforated screen for one-stage or two-stage medium concentration classification. The short fibers are concentrated by the disc filter for the production of the surface layer of the strong and tough cardboard, while the long fibers need to be screened and dredged due to the large amount of debris and impurities φ The measurement and control system of 2.4mm hole sieve combi sorter to remove debris and some electronic tensile testing machines in the market at present adopts 8-bit single chip microcomputer to control impurities. After the second or third stage purification of heavy impurities, the good slurry enters the low concentration cleaning (using a 0.25mm slotted screen), and its tail screen is a 0.3mm slotted screen. Then, the good slurry is purified by two or three stages of light impurities (wax removal) and concentrated by disc filter. Considering that the wax is not easy to be completely removed, then high concentration dispersion can be carried out, and finally the slurry can be ground to improve the strength. This slurry can be used as the core layer of strong and tough cardboard. Its typical process is shown in Figure 3

3 selection and classification of mechanical pulp

tmp pulp can generally replace grindstone pulp to make printing paper and writing paper. The purpose of screening of millstone wood pulp and TMP pulp is to separate the fragments and fiber bundles in the pulp, but long fibers need to be retained, which can be understood as the classification of pulp. The purpose of removing debris is to make the paper surface have appropriate surface properties and printing properties. As a high-quality coated paper such as LWC paper, R14 (fiber retained on 14 meshes) and R30 (fiber retained on 30 meshes) are two types of long fibers. In particular, R14 must have certain restrictions to prevent TMP fibers from bulging during coating. For this reason, R14 must be treated in a variety of ways in the production process. Finally, only qualified pulp will enter the next production process. The production process is shown in Figure 4

it can be seen from Figure 4 that after the wood chips are ground into pulp, they are screened and graded after potential elimination. After two-stage and two-stage screening and classification, the final total separation rate of fragments was 93%, and the final separation rate of R14 was 23%. It seems that most of the R14 long fibers need to be finely ground before they can be lowered again. The first stage primary screen used in the production process is a low concentration multi-stage 0.25mm slotted screen, and the first stage secondary screen is a low concentration multi-stage 0.15mm slotted screen. The coarse residues of these two screens are screened and classified by slurry recycling after passing through the second stage screen (0.20mm multi-stage slotted screen), and no coarse residues are discharged. The above multi-stage seam screen is the C-BAR screen technology developed by Voith Sulzer company. If traditional filtering is applied, i.e φ 1.6mm hole sieve and 0.25mm slot sieve, the total fragment separation rate is only 55% ~ 60%, and the fragment content of the final good slurry is about 0.3%, instead of the current 0.1% (see Figure 4)

Fig. 4 mechanical pulp (TMP) production process and screening efficiency

fragment separation efficiency (%)

r14 separation efficiency (%)

relative residue screening rate (%)

100 - mass flow

0.7 - fragment (%) (measured by sommerville

fiber bundle screening instrument)

22 - R14 (%) (measured by Bauer mcnett

screening instrument)

total fragment separation efficiency is 93%, and total R14 separation the

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