CA2162668A1 - A process for controlling the sedimentation of sticky impurities from paper stock suspensions - Google Patents

Abstract

The invention concerns a method of monitoring the deposition of stickies from paper-pulp suspensions in the manufacture of paper.
In this method, the deposition of secondary stickies is monitored by adding to the paper-pulp suspension, immediately before the first and/or other flotation stages, calculated with respect to the dry pulp, 0.2 to 3.0 % by wt. of waterglass and/or 0.05 to 1.0 % by wt. of a fatty acid with essentially 12 to 22 C-atoms or a salt thereof with a mono- to trivalent cation.

Description

A process for controlling the sedimentation of sticky impurities from paper stock suspensions Field of the Invention This invention relates to a process for controlling the sedimentation of sticky impurities from paper stock suspensions in paper manufacture.

Prior Art Even when paper was invented in the second century, the use of waste material, i.e. the technique of at least partial recycling, played a certain role. Nowadays, considerable significance is attributed to recycling technology through increasing ecological awareness. In view of the increasing production of paper, therefore, the supply of raw materials and the avoidance of waste are acquiring increasing significance.
By using secondary fiber stock from the recycling of wastepaper, savings can now be made in regard to raw materials, waste-disposal space and the energy required for paper manufacture. Unfortunately, this technology still involves specific difficulties.
Thus, in the processing of wastepaper, sticky impurities, normally known as stickies, can seriously disrupt the production process and adversely affect the quality of the paper produced. Stickies enter the papermaking process when the wastepaper used contains adhesive bonds, adhesive tapes or refined products, such as coated or laminated papers and paperboards. In addition, however, sticky impurities can also be formed by the resin in wood and through its interaction with paper auxiliaries.
Where the stickies are present in compact form, they can be mechanically removed relatively easily by means of sorting machines. In general, however, the stickies are present not only in compact form, but also in dispersed form in the pulp stock and are very difficult to remove in this form. Recently, therefore, the increasing use of wastepaper in paper manufacture and the restriction of the water circuits has increasingly resulted in a higher percentage of stickies in the circuit water.
Stickies cause a number of problems and disruptions not only in the papermaking process, but also in the processing of paper. On account of their stickiness, deposits are formed on machine parts, tube walls, sieves, wet felts, dry felts, drying cylinders, smoothing rol-lers, calender rollers and, in addition, even on the paper itself, resulting in web tears in the papermaking machine and in a deterioration in paper quality through holes, stains and marks (cf. H.L. Baumgarten, Da~ Papier, 1984, 38, No. 10A, pages V121-V125). According to H.L.
Baumgarten, stickies in industrial and institutional publications have for years been the biggest problem in the recycling of wastepaper. Even minimum quantities of adhesive can cause tears in papermaking and printing machines, so that the machines have to be stopped for - cleaning purposes. Baumgarten states: "2 g of adhesive at the right place in the papermaking machine can turn several hundred kg of paper into waste" (loc. cit., page V122, right-hand column).
Stickies have various origins. Essentially, they emanate from the resin in wood, from auxiliaries involved in paper manufacture, from binders for the coating of paper and cardboard, from adhesives for the processing of paper, from printing ink binders and from materials involved in the processing of paper. Stickies emanating from the resin in wood and from the adhesives used in the processing of paper are particularly important in the context of the problem addressed by the present inven-Wo 94/26973 3 PCT/EP94/01389 tion.
The resins present in chemical wood pulp and mechan-ical wood pulp contain around 1 to 5% by weight of so-called harmful resins, depending on the type of wood.
These resins may be present in colloidal, unbound form or may adhere to the paper fibers. According to J. Weigl et al., the difficulties caused by resin deposits in the manufacture and processing of paper have steadily in-creased in recent years for various reasons (cf. J. Weigl 10et al., Das Papier, 1986, pages V52-V62, more particular-ly page V53, left-hand column).
The adhesives used in the processing of paper may be divided into three groups, namely: contact adhesives, dispersion-based adhesives and hotmelt adhesives.
15Contact adhesives are permanently tacky and per-manently bondable products. Adhesion is achieved by application of pressure to the surfaces of the parts to be bonded. The basic polymers may be any of various key chemicals in combination with corresponding additives, for example tackifying resins, plasticizers or antioxi-dants. Typical basic polymers are inter alia natural rubber, butyl rubber, styrene/butadiene copolymers (SBR
-- rubber), acrylonitrile copolymers, polychloroprene, polyisobutylene, polyvinyl ether, acrylates, polyesters, polyurethanes, silicones.
In dispersion-based adhesives, the polymers involved in formation of the adhesive layer are present as solid particles in an aqueous dispersant. In the production process, the basic monomers are first emulsified in an aqueous phase and then polymerized therein - a technique known as emulsion polymerization. The polymer is then present in the form of small particles with different particle sizes which can vary from molecularly disperse to coarsely disperse. In general, agglomeration and hence sedimentation of the monomer particles is counter-acted by adding protective colloids or emulsifiers to thesystem.
The so-called hotmelt adhesives, also known as hotmelts, belong to the group of thermoplastics. These materials have the property of softening on heating, so that they become fluid. On cooling, they solidify again.
Examples of polymers used as hotmelt adhesives include polyamides, copolyamides, polyaminoamines, saturated polyesters and ethylene/vinyl acetate copolymers.
Stickies are divided into primary and secondary stickies. Primary stickies are those sticky impurities which, on account of their high resistance, are not dispersed during wet size reduction. Accordingly, they are present in compact form and are easy to remove.
The existence of secondary stickies emanates from the fact that, during the recycling of wastepaper, the sticky impurities undergo a change in their particle size brought about by thermal, chemical and mechanical influ-ences. This means that even impurities which are still present in extremely coarse form at the beginning of recycling can undergo more or less considerable size reduction in the recycling process. In particular, - stickies are dispersed by the processes taking place in the hot kneading machines used in the recycling of wastepaper. For example, stickies with a low melting point are liquefied and then very finely dispersed.
Crumbly or fragile stickies also disintegrate into very small particles. The particle size of the dispersed stickies thus ranges from coarsely disperse through colloidally disperse to molecularly disperse.
In other words, many stickies are readily disper-sible with the result that, after the pulping step, they are present in finely divided form and are not picked up at the sorting stage. These substances are in danger of forming agglomerates - also known as secondary stickies -WO 94/26973 5 PCT/EP94~01389 in the papermaking machine under thermal, mechanical orchemical influences. It is precisely these secondary stickies which cause problems in the further processing of paper. For example, they are transported by the paper webs, pass through the papermaking machine and thus arrive at the various places where they lead to unwanted deposits, more particularly at press felts, dry sieves, drying cylinders, smoothing rollers. In addition, they are of course also present in the paper itself, thus adversely affecting its quality.
Accordingly, it is clear from the situation outlined in the foregoing that, basically, any parameters which promote the agglomeration of particles bring with them the danger of formation of secondary stickies. The pH
value and the presence of certain papermaking auxiliaries are mentioned as two very important parameters in this regard. More specifically:
Small solid particles, which touch one another or which are separated from one another by a very narrow gap, attract one another through molecular interactions, so-called Van-der-Waals forces. However, the agglomera-tion-promoting Van-der-Waals forces are generally not - developed in alkaline medium, i.e. the medium typical of the recycling of wastepaper, because the particles are surrounded by an electrical double layer which is respon-sible for the mutual repulsion of particles carrying the same charge. By contrast, papermaking machines are normally operated in a neutral or mildly acidic medium, so that the repelling negative forces are reduced.
The drainability of the paper stock suspensions prepared using wastepaper is generally poor. According-ly, auxiliaries known as drainage or retention aids are often used in practice. Retention aids are understood by the expert to be substances which bind fine fibers and fillers to the long paper stock fibers (long fibers).

This binding of short fibers and fillers to the long fibers prevents the fine fibers from forming a kind of fleece which makes the paper stock suspension difficult to drain. In this way, retention aids improve drainage by binding fine fibers to the long fibers.
Retention aids can be divided into three groups, namely: inorganic products, such as aluminium sulfate or sodium aluminate; synthetic products, such as polyethy-lene imines, polyamines or polyacrylamides; and modified natural products, such as cationic starch.
The way in which retention aids work is based on the attachment of fine fibers and fillers to the paper fibers. An important mechanism in this regard is that polyelectrolytes of adequate chain length can bridge the gap between two particles and, in this way, promote the formation of agglomerates. Thus, J.L. Hemmes et al.
report that cationic polyelectrolytes, for example cationic starch, are suitable as scavengers for anionic impurities (Wochenblatt fur Papierfabrikation 1993, pages 163-170).
To summarize, it may be said that, according to the present state of general specialist knowledge, a neutral - or acidic medium on the one hand and the use of cationic auxiliaries to improve drainage and retention on the other hand represent conditions which promote the agglom-eration of particles. With regard to the problem of stickies discussed in the foregoing, this means that the expert logically regards these conditions as favorable to the formation of stickies.
Another key role in the control of stickies is that played by temperature. The reason for this is that many adhesives belong to the thermoplastics (hotmelts) of which the tackiness increases with temperature.
In addition, it is pointed out that the manifesta-tion of the undesirable properties of sticky impurities WO 94/26973 2 ~ 626~8 PCT/EP94/01389 for the manufacture and processing of paper depends upon a number of parameters which are not yet that well known in every respect (cf. H.L. Baumgarten, loc. cit., page V122, left-hand column). Normally harmless impurities can even be converted into sticky impurities through the cooperation of mechanical, chemical and thermal influen-ces during the production process (cf. B. Brattka, Wochenblatt fur Papierfabrikation 1990, pages 310-313).
Now, there are various known methods which seek to counteract the manifestation of the negative properties of sticky impurities for the process of paper manufac-ture. In this connection, particular significance is attributed among experts to the approach whereby the sedimentation of stickies is suppressed by an auxiliary so that the problems caused by the adhesive properties are reduced to a technically acceptable level.
Thus, US 4,923,S66 describes a process in which stickies are controlled with urea. According to the teaching of US 3,081,219, stickies in the sulfite pulping of wood are controlled with the aid of N-vin~1-2-Pyrroli-done. Attempts have also been made to control stickies by the addition of bentonites, diatomaceous earth and the - like. This well-known approach is based on the idea of introducing fine particles which are capable of binding sticky impurities at their surface (cf. US 3,081,219, column 1, lines 40-44). Another approach is based on the addition of sequesterinq aqents, for example polyphos-phates (cf. US 3,081,219, column 1, lines 45-50).
Finally, attempts have also been made to use various dispersants, for example the sodium salts of sulfonated formaldehyde/naphthalene condensates, although this leads to disadvantages at neutral pH values and to unfavorable interactions with cationic auxiliaries (cf. US 3,081,219, column 1, lines 51-58). US 4,744,865 describes a process in which the coagulation of sticky impurities is said to 2 ~ 62668 be reduced by polymers containing methoxy groups.
Finally, according to G. Galland and F. Julien Saint Amand, primary acrylate stickies can be removed by flotation in alkaline medium in the presence of soap (cf.
EUR. Co Eur. Communities 14011, 1992 pages 235-243).
According to the authors, it is crucially important to the effectiveness of their method that the alkalinity and the soap be introduced either in the pulper or immediate-ly after the pulper. They also state that the effective-ness of the removal of primary stickies is increased by the reduction in the size of the bubbles, but only at the expense of increased fiber losses. By its very nature, however, the Galland/Saint Amand process cannot contri-bute anything towards solving the problem of secondary stickies.

Description of the Invention In overall terms, the prior art in the field in question is extremely heterogeneous, a completely satis-factory process for controlling stickies having still tobe developed. This applies most particularly to the secondary stickies mentioned above, especially since the percentage content of finely dispersed sticky impurities responsible for the formation of secondary stickies in the circuit water is steadily increasing.
H.L. Baumgarten's observation is still relevant today: "A glance at the problem of "sticky impurities"
in wastepaper ..... shows that not only manufacturers of wastepaper recycling plants but also, and in particular, manufacturers of - mostly polymer-containing - paper refining and paper processing auxiliaries and also the chemical industry as the supplier of raw materials have a responsibility to provide close support to the paper industry." (Das Paper, 1984, No. lOA, page V124).
3S Accordingly, there is a constant need for new and alter-2 1 6266~

native solutions to the problem of controlling stickiesin paper manufacture.
Accordingly, the problem addressed by the present invention was to provide a process for controlling the sedimentation of sticky impurities which appear negative-ly as secondary stickies in the paper machine. This process would be generally applicable to various types of sticky impurities, but especially to contact adhesives, dispersion-based adhesives and hotmelt adhesives (hot-melts). In addition, the control of stickies would notbe achieved at the expense of increased losses of fibers and fillers.
According to the invention, the problem stated above has been solved by a process for controlling the sedimen-tation of stickies from paper stock suspensions in paper-making, in which - based on oven-dry paper stock - 0.2 to 3.0% by weight of waterglass and/or 0.05 to 1.0% by weight of a fatty acid essentially containing 12 to 22 carbon atoms or a salt thereof with monovalent to tri-valent cations is/are added to the paper stock suspensionimmediately before the first and/or subsequent flotation stages.
- Accordingly, the present invention relates to a process for controlling the sedimentation of sticky impurities (stickies) from paper stock suspensions in papermaking, characterized in that - based on oven-dry paper stock - 0.2 to 3.0% by weight of waterglass and/or 0.05 to 1.0% by weight of a fatty acid essentially containing 12 to 22 carbon atoms or a salt thereof with monovalent to trivalent cations is/are added to the paper stock suspension immediately before the first and/or subsequent flotation stages.
It is specifically pointed out that the time at which components i) and/or ii) are added is a critical feature so far as the success of the process according to , the invention is concerned. The fact that components i) and/or ii) are added immediatelY before the first and/or subsequent flotation stages implies that they enter the flotation process without being exposed to intensive shear forces beforehand. Corresponding shear forces occur at various places in the papermaking process.
Examples include the wastepaper pulper and following sorting units.
Application of the above-mentioned parameters crucial to the invention ensures that the sticky im-purities which are present in finely divided form in the aqueous paper stock suspensions used and which can lead to secondary stickies are removed from the system to a large extent. Another advantage of the process according to the invention is that the content of sticky impurities is not reduced at the expense of an increased loss of fibers and fillers. On the contrary, a reduction in the loss of fibers and fillers is another advantage achieved by the process according to the invention.
In one preferred embodiment of the present inven-tion, the paper stock suspensions used are prepared from wastepaper or from paper products containing wastepaper -- constituents.
Basically, the type of waterglass used is not critical. However, soda waterglass and/or potash water-glass are preferred.
In another preferred embodiment of the present invention, a mixture of components i) and ii) is used.
Basically, the ratio by weight between the two components is not critical, although it is preferred to establish a ratio by weight of 0.5:1 to 10:1. Component i) is preferably used in an excess by comparison with component ii), a ratio by weight of component i) to component ii) of 3:1 to 5:1 being most particularly preferred.
In another preferred embodiment of the present 2 ~ 626b8 invention, components i) and ii) are used in combination with an alkali metal hydroxide. The alkali metal hydrox-ide, preferably sodium and/or potassium hydroxide, is preferably used in a quantity of 0.05 to 2.0% by weight, based on oven-dry paper stock. The presence of alkali metal hydroxide provides for a further reduction in the loss of fibers and fillers which is extremely important to the economy of the process.
So far as the effect of component ii) is concerned, it is crucial that it should be at least partly present in the form of a relatively poorly soluble soap in the particular flotation cell. This is generally achieved by using a fatty acid containing 12 to 22 carbon atoms or soluble salts thereof with monovalent to trivalent cations which then form the corresponding poorly soluble calcium soaps in situ with the water hardness present in the system. However, if the water hardness is not suffi-cient, the calcium soaps of fatty acids containing 12 to 22 carbon atoms may even be directly used.
It has been found that, in some cases, the effect of the process according to the invention can be further improved by carrying out the flotation process in the - presence of an additional cationic flocculant or reten-tion agent. Corresponding flocculants or retention agents are, for example, cationic polymers, such as polyacrylamides, polyethyleneimines, polyamidoamines, or cationic starches and inorganic compounds, such as aluminium sulfate.
The process according to the invention is generally applicable to the various types of stickies. However, it is most particularly suitable for solving the problems caused by contact adhesives, dispersion-based adhesives and hotmelt adhesives (hotmelts).
In principle, the process according to the invention is suitable for controlling the sedimentation and adhe-2~ 62668 Wo 94/26973 12 PCT/EP94/01389 sion of stickies of various kinds, i.e. differing intheir chemical and physicochemical nature. However, the advantages of the process according to the invention are particularly applicable to stickies based on contact 5 adhesives and hotmelt adhesives (hotmelts).
The following Examples are intended to illustrate the invention without limiting it in any way.

E x a m p 1 e s 1. Substances and materials used Sodium hydroxide: 50% aqueous NaOH solution.
Waterglass: "Wasserglas 37/40" (Na2SiO3), a product of Henkel/Dusseldorf.
042: Oleic acid mixture "Olinor 042", a product of Henkel/Dusseldorf.

2. Determination of the dichloromethane extract 2.1. Principle of the method The dichloromethane extract was used as an indirect measure for determining the percentage content of sticky impurities in paper suspensions. The dichloromethane extract is obtained by filtering a sample of the paper stock suspension to be tested, drying the residue and determining the dichloromethane-soluble constituents present therein - essentially sticky impurities - by extraction.

2.2. Analytical aids a) Round filters: Before use, the round filters used were dried to constant weight in a heating cabinet at a temperature of (103 + 2)C by the method according to DIN
54359 and were weighed after cooling in an exsiccator.

35 b) Flat-bottomed flask: Before use, the flat-bottomed 2 ~ 6266~

flask used was dried to constant weight in a drying cabinet at (105 + 2)C by the method according to DIN
54354 and was weighed after cooling in exsiccator.

2.3. Procedure A 500 ml sample of a thoroughly mixed paper stock suspension was removed and filtered through a filter consisting of a 15 cm diameter Buchner funnel, a large suction bottle and a round paper filter. After filtra-tion, the filtrate was visually examined for clouding.If any clouding was discernible, the filtrate was refil-tered through the same filter. The round filter was dried together with the stock cake filtered off in a heating cabinet and then weighed.
The dried stock cake was then transferred together with the round filter to a Soxhlet extractor with a ground-in condenser and a connected 500 ml flat-bottomed flask with a ground neck. After addition of 400 ml of dichloromethane, the stock cake was extracted with heat-ing for 6 hours. After extraction, the extract solution was concentrated by distillation to such an extent that it was still just liquid. The cake was then dried to -- constant weight in a drying cabinet at a temperature of (105 + 2)C and then weighed as described in 2.2. The dichloromethane extract DCM in ~ (based on the dry weight of the sample) was calculated as follows from the results obtained:

m4-m3 DCM = 100 m2-m where m1 = weight of the round filter in g m2 = weight of the round filter with stock cake in g m3 = weight of the empty flask in g m4 = weight of the flask with residue in g.

3. Test procedure In a test arrangement in which a Sulzer-Escher Wyss type CF flotation cell was integrated, wastepaper was disintegrated with 1% of waterglass, 0.5% of sodium hydroxide, 0.5% of hydrogen peroxide and 0.33% of Olinor 042 (all percentages based on oven-dry paper stock) in a pulper at a pulp consistency of 12 "otro" (= oven-dry).
After coarse sorting, a volume corresponding to 200 kg of oven-dry paper stock was pumped into a vat and, after determination of the stock consistency, was diluted with circuit water to a stock consistency of 1.3%.
The additives to be tested were then added, a sample of the stock was removed and was then floated for 30 minutes during which the paper stock suspension circu-lated from the vat through the flotation cell to an adjoining second vat and then back again to the first vat. On completion of the flotation process, all the Z0 stock was pumped back into the first vat and another sample was taken. The dichloromethane extract of the stock samples obtained was determined before and after -- flotation, as described above, in addition to which the total losses of filler and fibers was determined from the pulp consistency and the vat level.
The values obtained for the reduction of the dichlo-romethane extract (DCM extract) and the loss of fibers and filler (total loss) are set out in Table 1.

Table 1 Additive introduced ) Reduction of Total loss3) DCM extract (%) (%) None 33 18 1% Waterglass2)39 17 1% Waterglass 57 14 1% Waterglass + 0.07% 042 65 11 1% Waterglass + 0.5% NaOH
+ 0.07% 042 64 8 The addition was made immediately before flotation as described above 2) In this case, the addition was made in the pulper for comparison purposes 3) Loss of fibers and filler 2.3 Discussion of the results -- It is clear from Table 1 that a considerable reduc-tion in the DCM extract and, at the same time, a distinct reduction in the total loss of fibers and filler are achieved by the process according to the invention. Com-parison of the additions of 1% of waterglass respectively in the pulper and immediately before flotation clearly shows that the required reduction in the DCM extract is only achieved where the addition is made immediately before flotation (process according to the invention).
In addition, Table 1 shows that the total loss of fibers and filler is further reduced in the presence of addi-tional alkali metal hydroxide for otherwise substantially the same DCM extract value.

Claims (6)

16

1. A process for controlling the sedimentation of sticky impurities from paper stock suspensions in paper manufacture, characterized in that - based on oven-dry paper stock -i) 0.2 to 3.0% by weight of waterglass and/or ii) 0.05 to 1.0% by weight of a fatty acid essentially containing 12 to 22 carbon atoms or a salt thereof with monovalent to trivalent cations is/are added to the paper stock suspension immediately before the first and/or subsequent flotation stages.

2. A process as claimed in claim 1, in which the paper stock suspensions used were prepared from wastepaper or from paper products containing wastepaper constituents.

3. A process as claimed in claim 1 or 2, in which both components i) and ii) are used.

4. A process as claimed in claim 3, in which components i) and ii) are used in a ratio by weight of 0.5:1 to 10:1.

5. A process as claimed in claim 3 or 4, in which components i) and ii) are used in combination with an alkali metal hydroxide.

6. A process as claimed in any of claims 1 to 5, in which the process is additionally carried out in the presence of a cationic flocculant or retention agent.