Causes and Countermeasures of anionic impurities i

2022-08-11
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Causes and Countermeasures of anionic impurities in the wet end of the paper machine

with the use of coating damaged paper, deinked waste paper, high yield pulp and the improvement of white water circulation and sealing degree of the paper machine, the concentration of interfering anionic impurities in the wet end of the paper machine is higher and higher, which is more and more harmful to the wet end operation of the paper machine, and makes the effect of some cationic additives worse. Especially with the conversion of papermaking to alkaline system, when aluminum sulfate is no longer added to the size as a sizing and fixing agent, the problem of anionic impurities becomes more prominent. Therefore, it is very necessary to understand the source, harm and Countermeasures of anionic impurities

the source and harm of anionic impurities

1.1 the source of anionic impurities

anionic impurities refer to all dissolved anionic oligomers or polymers and non-ionic hydrocolloid substances in the wet end of the paper machine. These anionic impurities have different properties and sources:

(1) from pulp, such as lignin derivatives, hemicellulose, fatty acids, etc

(2) from anionic additives, such as starch, CMC, organic acids, dyes, fungicides, etc

(3) from filler dispersants, such as polyphosphate, polyacrylate, etc

(4) from clean water, such as humic acid, fungicide, etc

most of the harmful anionic impurities come from pulp, so here we only introduce the accumulation of anionic impurities from pulp. Every link from material preparation to paper making involves the accumulation of anionic impurities. In stock preparation, experience shows that long-term storage of wood has adverse effects on fiber strength and bleaching process, but shortening storage time does not separate volatile substances such as terpenes; Make viscous substances, such as esterified fatty acids, unable to oxidize and saturate. As a result, the storage time is shortened and the enhanced content of anionic impurities in pulp to recycled plastics is increased

in straw pulping, if new straw is used for pulping due to the shortage of raw materials, the content of anionic impurities in straw pulp will also be increased because pectin, starch, etc. in straw are not removed by natural fermentation. In the pulping process, the pulping of wood and grass raw materials largely depends on the dissolution of lignin, hemicellulose, extracts, etc. These dissolved substances constitute the main organic anion impurities in the papermaking system. The amount of these organic anionic impurities is closely related to the pulping and bleaching process conditions, process control, and the cleaning degree of bleached pulp. Generally speaking, paper mills using high yield pulp and waste pulp contain the largest amount of anionic impurities. Anionic impurities from various mechanical pulp and semi chemical pulp are mainly soluble substances in the form of lignosulfonate, humic acid and anionic rubber

anionic impurities from waste paper pulp are mainly a large number of lignin derivatives and other anionic impurities related to waste paper species. Among them, the paper mills that use coated paper or waste paper have the highest amount and most complex anionic impurities. Coated paper or waste paper, especially coated paper or waste paper using calcium carbonate and titanium dioxide, The coating contains a large amount of anionic dispersant (when using starch as binder, more dispersants are needed than when using protein as binder, so that the concentration of anionic impurities increases rapidly.

as for ordinary chemical bleached pulp, straw pulp contains more anionic impurities than wood pulp. No matter what kind of pulp, the low cleanliness of pulp is the most direct reason for the concentration of anionic impurities in the pulp. In the papermaking of paper, when using calcium carbonate as filler and coating pigments, that is, in neutral or alkaline strips When making paper under the piece, anionic impurities cannot be neutralized because aluminum sulfate cannot be used as a fixing agent; In addition, anionic impurities will ionize more anionic groups at alkaline pH; Under alkaline conditions, low molecular weight hemicellulose may also be dissolved from the fiber cell wall (such substances include xylose and arabinose glycans); The development strategy of this energy environment enclosed in fibrous cells under acidic conditions some colloidal substances may also be released under alkaline conditions due to the swelling of cell walls. Therefore, papermaking under alkaline conditions will make the concentration of soluble anionic impurities higher

in recent years, due to the increasing degree of closure of the white water system of the paper machine, and the anionic impurities are not adsorbed on the pulp, these anionic impurities accumulate more and more with the white water circulation. The problem of anionic impurities caused by the closed circulating white water is far more serious than the pulp itself

1.2 the influence of anionic impurities on paper making

anionic impurities can affect the paper making in many ways, such as:

(1) affect the operation of the paper machine: the formed agglomerates (resins, white resins, adhesives) reduce the operation performance of the paper machine and increase the number of broken ends

(2) affect the efficiency of additives: it has adverse effects on the effect of sizing agents, dry strength agents, wet strength agents, retention and drainage aids, dyes, etc

(3) affect paper quality: reduce paper uniformity, reduce paper opacity and brightness, cause small holes and dark spots, and reduce paper strength

in a word, anionic impurities affect papermaking by two mechanisms. One is that its high electronegativity makes the positive charge demand of pulp very high. Adding cationic additives, such as retention and drainage aids, enhancers, etc., anionic impurities first neutralize these polymers, consuming a large amount of cationic additives (generally cationic additives are not effective charge neutralizers), which may make cationic application technology The production process additives are completely ineffective, and its high negative electricity is 5 The drivability after constant temperature also affects the flocculation of fine fibers in the pulp, thus affecting the retention and drainage of the pulp. Therefore, the anion impurity content is usually expressed by the positive charge demand of the slurry filtrate. Then, with the continuous accumulation of anionic impurities, they combine with other substances or themselves to form agglomerates or complexes, which are deposited from the aqueous solution. If they are deposited on fillers, fine fibers and fibers, the extension between fibers will be reduced to avoid the destruction of hydrogen bonding due to human overload, thus reducing the bonding strength between fibers and the brightness of paper; If the paper is left in the section, pipe and blanket in the paper making, it will block the paper making and blanket, cause roller sticking in the pressing section, increase the number of paper breaks, and appear dark spots on the paper, which will also interfere with the sizing and dyeing of the paper

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