Causes and countermeasures of anion impurities in the wet end of paper machine
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With the application of coated broke, deinked waste paper, high-yield pulp and the degree of white water circulation closure of the paper machine, the concentration of interfering anionic impurities in the wet end of the paper machine is getting higher and higher, which is harmful to the operation of the wet end of the paper machine. It is getting bigger and bigger, and some cationic additives are getting worse. Especially with the conversion of papermaking to an alkaline system, the problem of anionic impurities becomes more prominent when aluminum sulfate is no longer added as a sizing agent to the slurry. Therefore, it is necessary to understand the source, hazard and solution of anionic impurities.
Sources of anionic impurities and their hazards
1.1 Source of anionic impurities
Anionic impurities are a general term for all dissolved anionic oligomers or polymers and nonionic hydrocolloids present in the wet end of a paper machine. These anionic impurities have different properties and sources:
(1) From pulp, such as lignin derivatives, hemicellulose, fatty acids, and the like.
(2) From anionic auxiliaries such as starch, CMC, organic acids, dyes, bactericides, and the like.
(3) From a filler dispersant, such as polyphosphate, polyacrylate, and the like.
(4) From clear water, such as humic acid, fungicides, etc.
Most of the harmful anionic impurities are from the pulp, so only the accumulation of anionic impurities from the pulp is described here. Every step from preparation to papermaking involves the accumulation of anionic impurities. In the preparation, experience has shown that long-term storage of wood has an adverse effect on fiber strength and bleaching process, but shortening the storage time does not separate volatile substances such as terpenes; making viscous substances such as esterified fatty acids It cannot be oxidized and saturated, and as a result, shortening the storage time increases the content of anionic impurities in the pulp.
In the grass pulping, if the raw material is scarce, the new grass pulp is used, and the anion impurity content in the straw pulp is also increased due to the natural fermentation of the pectin and starch in the grass. In the pulping process, the pulping of wood and grass raw materials is largely determined by the dissolution of lignin, hemicellulose, extracts, etc. in the raw materials, which constitute the most important organic anions in the papermaking system. Impurities. The amount of these organic anionic impurities is closely related to the pulping and bleaching process conditions, process control, and degree of bleaching of the bleached pulp. Paper mills that use high yield pulp and waste pulp generally contain the largest amount of anionic impurities. The anionic impurities from various mechanical and semi-chemical pulps are mainly in the form of lignosulfonates, humic acids and anionic gums. A soluble substance present.
The anionic impurities from the waste paper pulp are mainly a large amount of lignin derivatives and other anionic impurities associated with waste paper grades. Paper mills that use coated broke or waste paper have the highest amount and most complex anionic impurities. Coating damaged 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 (required when using starch as a binder than when using protein as a binder) More dispersants increase the concentration of anionic impurities.
For ordinary chemical bleaching syrups, straw pulp contains more anionic impurities than wood pulp. Regardless of the pulp, the low degree of pulp cleansing is the most direct cause of the concentration of anionic impurities in the pulp. In the papermaking of paper sheets, when calcium carbonate is used as a filler and a coating pigment, that is, when paper is produced under neutral or alkaline conditions, since an aluminum sulfate cannot be used as a fixing agent, anionic impurities are not neutralized; At alkaline pH, anionic impurities can ionize more anionic groups; under alkaline conditions, low molecular weight hemicellulose may also dissolve from the fiber cell wall (such substances include xylose and arabinose-based glycans). Some colloidal substances enclosed in fibroblasts under acidic conditions may also be released under alkaline conditions due to swelling of the cell wall. Therefore, papermaking under alkaline conditions will result in a higher concentration of soluble anion impurities.
In recent years, due to the increasing degree of sealing of the white water system of paper machine, and the anionic impurities are not adsorbed on the pulp, these anionic impurities accumulate more and more with white water > the problem of anionic impurities caused by closed circulating white water The pulp itself is serious.
1.2 Effect of anionic impurities on papermaking
Anionic impurities can affect the sheet making of paper in a variety of ways, such as:
(1) Affecting the operation of the paper machine: The formed agglomerates (resin, white resin, and adherent) reduce the running performance of the paper machine and increase the number of breaks.
(2) Affecting the efficiency of additives: it has an adverse effect on the effects of sizing agents, dry strength agents, wet strength agents, retention and drainage aids, dyes, etc.
(3) Affect the quality of the paper: reduce the sheet formation, reduce the opacity and brightness of the paper, cause small holes and dark spots, and reduce the strength of the paper.
In short, anionic impurities affect the papermaking by two mechanisms. First, its high electrical conductivity makes the positive charge of the pulp very high. Adding cationic additives, such as retention and drainage aids, reinforcing agents, etc., anionic impurities first Neutralization reaction with these polymers, consumption of a large number of cationic auxiliaries (generally cationic auxiliaries are not effective charge neutralizers), it is possible to completely eliminate the cation application technology. The production process auxiliaries completely fail, and its high electrical and electrical properties also affect the pulp. The flocculation of medium and fine fibers affects the retention of pulp and drainage. Therefore, the anion impurity content is usually expressed as the positive charge requirement of the slurry filtrate. Then, as the anionic impurities accumulate, they are deposited from the aqueous solution by combining with other substances or by themselves to form agglomerates or complexes. If deposited on the fillers, fine fibers and fibers, the hydrogen bonding between the fibers is reduced. Reduces the bonding strength between the fibers and the brightness of the paper; if left in the net, pipes and blankets in the paper sheet, it will block the paper net, the felt, cause the sticky roller in the press section, and increase the break of the paper. The number of times, and dark spots appear on the paper, but also interfere with the sizing, dyeing, etc. of the paper.