EP0698141B1 - Method of monitoring the deposition of "stickies" from paper-pulp 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

Field of the Invention

The invention relates to a method for controlling the settling of sticky contaminants from pulp suspensions in papermaking.

State of the art

Already when paper was invented in the 2nd century, the use of waste material, i.e. the technique of - at least partially - recycling plays a certain role. In today's world, recycling technology is becoming increasingly important as ecological awareness increases. With an ever increasing production of paper, questions of raw material supply and waste prevention are becoming increasingly important.

By using secondary pulp by recycling waste paper, savings can be made today in terms of raw materials, landfill space and the energy required for paper production achieve. However, this technology is associated with specific difficulties.

In the field of processing waste paper, sticky contaminants, commonly referred to as stickies , can significantly disrupt the production process and negatively affect the quality of the paper produced. The stickies get into the paper manufacturing process if the used paper contains glue spots, adhesive tapes or finished products such as coated, laminated or coated papers or cardboards. In addition, however, the resin of the wood and its interaction with paper auxiliaries can form adhesive contaminations.

If the stickies are in a compact form, they can be removed relatively easily mechanically with the help of sorting machines. As a rule, the stickies are not only available in a compact form, they are also contained in dispersed form in the pulp and are very difficult to remove in this form. Recently, therefore, the increasing use of waste paper in paper production and the narrowing of the water cycles have led to an increasing increase in the proportion of stickies in the circulating water.

Stickies not only cause a number of problems or faults in paper production, but also in paper processing. Because of their stickiness, deposits form on machine parts, pipe walls, screens, wet felts, dry felts, drying cylinders, smoothing rollers, calender rollers and, moreover, also in the finished paper, which causes web breaks in the paper machine and a deterioration in paper quality due to holes, stains, markings (see HL Baumgarten, Das Papier, 1984, 38 , Issue 10A, pp. V121 - V125 ). HL Baumgarten notes that sticky contaminants in industry and institute publications has been called the biggest problem of waste paper recycling for years. Even minimal amounts of glue can still tear on paper and printing machines, making cleaning stoppages necessary. Baumgarten explains: "If 2 g of adhesive is applied to a well-chosen point in the paper machine, several 100 kg of paper can be rejected" (op. Cit., Page V122, right column).

The origin of stickies is not uniform. Essentially, they come from the resin of wood, aids in paper production, binders for painting paper and cardboard, adhesives for paper processing, printing ink binders and materials for paper processing. Of particular importance in the context of the task of the present invention are those adhesive contaminants that come from the resin of the wood and the adhesives used in paper processing.

Depending on the type of wood, the resins present in the pulp and wood pulp contain about 1 - 5% by weight of so-called harmful resins. These can be in a colloidal unbound form or adhere to the paper fibers. As J.Weigl et. al. The difficulties caused by resin deposits in the manufacture and processing of paper have increased continuously in recent years for various reasons (see J. Weigl et al. Das Papier, 1986 , pp. V52 - V 62; there P. V53, left column).

The adhesives used in paper processing can be divided into three groups: the pressure sensitive adhesives, the dispersion adhesives and the hot melt adhesives.

The pressure sensitive adhesives are permanently adhesive and permanently adhesive products. Adhesion is achieved by pressing the surfaces of the parts to be bonded. A large number of base materials in combination with corresponding additives, e.g. tackifying resins, plasticizers or antioxidants. Typical base polymers include Natural rubber, butyl rubber, styrene-butadiene copolymers (SBR rubber), acrylonitrile copolymers, polychloroprene, polyisobutylene, polyvinyl ether, acrylates, polyesters, polyurethanes, silicones.

In the case of dispersion adhesives, the polymers which are suitable for forming the adhesive layer are present as solid particles in an aqueous dispersion medium. During manufacture, the base monomers are first emulsified in an aqueous phase and then polymerized therein, a technique known as emulsion polymerization. The polymer is then in the form of small particles with different particle sizes, which can vary in the range from molecularly disperse to coarsely disperse. As a rule, an aggregation and an associated sedimentation of the polymer particles are counteracted by adding protective colloids or emulsifiers to the system.

The so-called hot melt adhesives, also called "hot melts", belong to the group of thermoplastics. These substances have the property of softening when heated, which makes them flowable. When they cool down, they solidify again. Examples of polymers which are used as hot-melt adhesives include polyamides, copolyamides, polyaminoamines, saturated polyesters and ethylene-vinyl acetate copolymers.

With the stickies a distinction is made between primary and secondary stickies. Primary stickies are understood to mean those sticky contaminants, which due to their high resistance to wet grinding not be dispersed. They are therefore in a compact form and are easy to excrete.

The existence of the secondary stickies stems from the fact that the adhesive impurities are subject to a change in their particle size in the course of the waste paper processing, which is caused by thermal, chemical and mechanical influences. This means that even impurities that are still in a rather coarse form at the beginning of the reprocessing can be reduced to a greater or lesser extent in the reprocessing of waste paper. In particular, the processes in the hot shredder of waste paper processing result in dispersion of sticky contaminants. For example, stickies with a low melting point are liquefied and then finely dispersed. Even crumbly or brittle stickies break down into very small particles. The particle size of the dispersed stickies then ranges from coarse to colloidal to molecular.

In other words: many adhesive contaminants have good dispersibility, with the result that they are present in finely divided form after dissolution and are not detected by the sorting. These substances run the risk of forming agglomerates in the paper machine due to thermal, mechanical or chemical effects, which are referred to as secondary stickies. It is these secondary stickies that cause problems in the course of further paper processing. They are transported, for example, with the paper webs, travel through the paper machine and thus reach a wide variety of locations, where they lead to undesirable deposits, in particular on press felts, dryer fabrics, drying cylinders, smoothing rollers. In addition, they can of course also be found in the finished paper, the quality of which is affected.

It is therefore clear from the relationships described that, in principle, all parameters which favor the agglomeration of particles entail the risk of secondary stickies being formed. Two very important parameters in this context are the pH value and the presence of certain aids in paper production. In detail:

Small solid particles that touch each other or between which there is a very small distance attract each other due to molecular interactions, the so-called Van der Waals forces. The Van der Waals forces working on an agglomeration come in an alkaline environment - i.e. the environment typical for waste paper processing - but generally not to the advantage, since the particles are surrounded by an electrical double layer, which is responsible for the mutual repulsion of the particles charged in the same direction. On the other hand, the paper machine is usually used in a neutral or slightly acidic environment, which reduces the repulsive negative forces.

The drainability of the pulp suspensions that were made using waste paper is generally poor. In practice, aids are therefore often used, which are referred to as drainage or retention aids. The person skilled in the art understands retention agents as substances that bind fine fibers and fillers to the long paper fibers (long fibers). This binding of the short fibers and the fillers to the long fibers prevents a kind of fleece formation of the fine materials, which makes dewatering of the paper pulp suspension difficult. In this way, retention agents improve the drainability by binding the fine substances to the long fibers.

The retention aids can be divided into three groups. A distinction is made between inorganic products such as aluminum sulfate or sodium aluminate, synthetic products such as polyethyleneimines, polyamines or polyacrylamides and modified natural products such as cationic starch.

The mode of action of the retention agents is based on the deposition of fine substances and fillers on the paper fibers. An important mechanism is that polyelectrolytes with sufficient chain length can bridge the distance between two particles and thus cause agglomerate formation. For example, JLHemmes et al. Report that cationic polyelectrolytes, for example cationic starch, are suitable as scavengers for anionic contaminants ( Wochenblatt für Papierfabrikation 1993, pages 163-170 ).

In summary, it can be stated that, according to the general state of the art, on the one hand a neutral or acidic environment, and on the other hand the use of cationic auxiliaries for shifting drainage and retention are conditions which favor agglomeration of particles. With regard to the sticky problem mentioned, this means that the person skilled in the art logically sees these conditions as favorable for the formation of stickies.

Another key role in controlling stickies is temperature. The reason for this is that many adhesives belong to the thermoplastic (hotmelts), the stickiness of which increases with temperature.

In addition, it should be noted that the manifestation of the undesirable properties of adhesive contaminants for the process of paper production or processing depends on a large number of parameters, the details of which are not yet sufficiently known (see HL Baumgarten, loc. Cit. P. V122, left) Column). It is even possible that normally harmless contaminants are caused by the interaction of mechanical, chemical and thermal influences during the production process can be converted into adhesive contaminants (see B.Brattka, Wochenblatt für Papierfabrikation 1990 , pp. 310-313 ).

Various methods are now known from the prior art which attempt to counteract the manifestation of the negative properties of adhesive contaminants for the paper production process. The approach of stopping stickies by means of an additive so that the disturbances caused by the adhesive properties are reduced to a technically acceptable level is of particular importance to experts.

A method is known from US Pat. No. 4,923,566 in which the stickies are checked with the aid of urea . According to the teaching of the American patent US 3,081,219 , the control of the stickies in the pulp of the sulfite pulping of wood is achieved with the aid of N-vinyl-2-pyrrolidone . Attempts have also been made to control the stickies by adding bentonites, diatomaceous earth and the like . This approach, which has been known for a long time, is based on the idea of introducing fine particles which are able to bind adhesive contaminants to their surface (cf. US 3,081,219, column 1, lines 40-44). Another approach is based on the addition of sequestering agents, for example polyphosphates (see US Pat. No. 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, but this entails disadvantages in the case of neutral pH values and leads to unfavorable interactions with cationic auxiliaries (cf. US 3,081,219, column 1, lines 51-58). The American patent application US 4,744,865 describes a process in which the coagulation of adhesive contaminants by polymers containing methoxy groups is to be reduced.

Finally, G. Galland and F. Julien Saint Amand describe that primary acrylate stickies can be removed by flotation in an alkaline environment and in the presence of soap (see EUR. Comn. Eur. Communities 14011, 1992 , pp. 235 - 243 ) . As essential for the performance of their method, the authors state that the alkalinity and soap must be entered in the pulper or immediately after the pulper. They also state that reducing the bubble size increases the effectiveness of primary sticky removal, but at the expense of increased fiber loss. The method of Galland and Saust Amand, by its very nature, cannot help solve the problem of secondary stickies.

Description of the invention

Overall, the state of the art in the field mentioned here is very heterogeneous and a method for checking stickies that is satisfactory in every respect has not yet been implemented. This applies in particular to the secondary stickies mentioned, especially since the proportion of finely dispersed adhesive contaminants which are the cause of the formation of secondary stickies in the circulating water is increasing increasingly.

HL Baumgarten's statement still applies: "A look at the problem chain" Adhesive contaminants "in waste paper ... makes it clear that in addition to the manufacturers of waste paper preparation systems, in particular the manufacturers of the mostly plastic-containing paper finishing and paper processing aids as well as the chemical industry as their raw material supplier are asked to help the paper industry intensively "(Das Papier, 1984 , Issue 10A, page V124). There is therefore a constant need for new or alternative solutions to control stickies in paper production.

The object of the present invention was therefore to develop a method for controlling the settling of adhesive contaminants which appear negatively in the paper machine as secondary stickies. This method should generally be applicable to a wide variety of types of adhesive contaminants, but especially to pressure sensitive adhesives, dispersion adhesives and hot melt adhesives. In addition, it had to be demanded that the sticky control not be at the expense of increased losses of fibers and fillers.

The solution of this object is achieved according to the invention by a method for controlling the deposition of stickies from paper stock suspensions in papermaking, with the paper stock suspension immediately before the first and / or further flotation stages, based on otro paper stock, 0.2 to 3.0% by weight of water glass and / or 0.05 to 1.0% by weight. -% of a fatty acid with essentially 12 to 22 carbon atoms or its salt with mono- to trivalent cations added. Here, “otro paper stock” is understood to mean, as is customary in paper technology, oven-dry paper stock.

The present invention therefore relates to a process for controlling the settling of adhesive contaminants from paper pulp suspensions during paper manufacture, which is characterized in that the paper pulp suspension is - immediately before the first and / or further flotation stages - based on otro pulp - 0 , 2 to 3.0% by weight of water glass and / or 0.05 to 1.0% by weight of a fatty acid with essentially 12 to 22 carbon atoms or its salt with mono- to trivalent cations.

It should be expressly stated that the time at which components i) and / or ii) are added is a critical feature for the success of the process according to the invention. The fact that components i) and / or ii) are to be added immediately before the first and / or further flotation stages implies that they get into the flotation without being subjected to strong shear forces beforehand. Such shear forces occur at various points in the course of papermaking. Examples of this are the waste paper pulp, the so-called pulper, or the sorting devices connected to it.

Compliance with the above-mentioned parameters essential to the invention guarantees that the adhesive contaminants which are present in finely divided form in the aqueous paper pulp suspensions used and which can lead to secondary stickies largely result from the System are removed. Another advantage of the process according to the invention is that the reduction in the content of adhesive contaminants does not come at the expense of an increased loss of fibers and fillers. Rather, the process according to the invention achieves an additional advantage in reducing the loss of fibers and fillers.

In a preferred embodiment of the present invention, the paper stock suspensions used are made from waste paper or paper products which contain waste paper components.

The type of water glass is not critical in itself. However, soda and / or potassium water glass are preferred.

In a further preferred embodiment of the present invention, a mixture of components i) and ii) is used. The weight ratio of the two components is not critical per se, but it is preferred to set a weight ratio of 0.5: 1 to 10: 1. Again, it is preferred to use component i) in excess compared to component ii); a weight ratio of components i) and ii) of 3: 1 to 5: 1 is very particularly preferred.

In a further preferred embodiment of the present invention, the two components i) and ii) are used in combination with an alkali metal hydroxide. The alkali hydroxide, preferably sodium and / or potassium hydroxide, is preferably used in an amount of 0.05 to 2.0% by weight, based on the dry paper stock. The presence of alkali hydroxide further reduces the loss of fibers and fillers, which is of great importance for the economics of the process.

With regard to the action of component ii), it is crucial that it is present, at least in part, in the respective flotation cell in the form of a relatively poorly soluble soap. This is usually achieved by using a fatty acid with 12 to 22 carbon atoms or its soluble salts with 1- to 3-valent cations, which then form the corresponding poorly soluble calcium soaps in situ with the water hardness in the system. If the water hardness is not sufficient, the calcium soaps of fatty acids with 12 to 22 carbon atoms can also be used directly.

It has been found that in some cases the effect of the process according to the invention can be improved further if the flotation is carried out in the presence of an additional cationic flocculant or retention agent. Such means are e.g. B. cationic polymers such as polyacrylamides, polyethyleneimines, polyamide amines or cationic starches and inorganic compounds such as aluminum sulfate.

The method according to the invention is generally applicable to a wide variety of types of adhesive contaminants. However, it is particularly suitable for solving the problems caused by pressure-sensitive adhesives, dispersion adhesives and hotmelt adhesives.

The method according to the invention is suitable in principle for checking the setting and sticking of stickies of different types and thus also of different chemical and physico-chemical nature. However, the advantages of the method according to the invention are particularly evident in the case of stickies based on pressure-sensitive adhesives and hotmelt adhesives.

The following examples serve to illustrate the invention and are not to be understood as restrictive.

Examples 1. Substances and materials used

Sodium hydroxide solution: 50% aqueous NaOH solution. Water glass: "Water glass 37/40" (Na ₂ SiO ₃ ), Henkel / Düsseldorf. 042: Oleic acid mixture; "Olinor 042", Henkel / Düsseldorf.

2. Determination of the dichloromethane extract 2.1. Principle of the method

The dichloromethane extract served as an indirect measure for determining the proportion of adhesive contaminants in paper suspensions. This is obtained by filtering a sample of the paper stock suspension to be investigated, drying the residue and determining the constituents soluble in dichloromethane, which are essentially adhesive contaminants, by extraction.

2.2. Analytical tools

• a) Round filters: Before use, the round filters were dried in a heating cabinet at a temperature of (103 ± 2 ° C) according to the method according to DIN 54359 until they reached a constant mass and weighed in a desiccator after cooling.
• b) Standing flask: The standing flask used was dried to constant weight in a drying cabinet at (105 ± 2) ° C using the method according to DIN 54 354 and weighed after cooling in a desiccator.

2.3. execution

A 500 ml sample was taken from a well-mixed paper stock suspension and passed through a filtering device consisting of a 15 cm diameter Büchner funnel, a large suction bottle and a circular paper filter. filtered. After the filtration, it was checked visually in each case whether the filtrate had no turbidity. If turbidity could still be seen, the filtrate was filtered again using the same filter. The round filter was dried together with the filtered cake in the warming cabinet and then weighed.

The dried fabric cake was then transferred together with the round filter into a Soxhlet extractor with a ground-in cooler and a connected 500 ml standing flask with a ground neck. After adding 400 ml of dichloromethane, the mixture was extracted with heating for 6 hours. After completion of the extraction, the extract solution was concentrated by distillation until it was just liquid. The flask was then dried in a drying cabinet at a temperature of (105 ± 2) ° C to constant weight and then as in 2.2. described weighed. From the values found, the dichloromethane extract DCM was calculated in%, based on the dry weight of the sample weight, as follows: $$\text{DCM =}\frac{{\text{m}}_{\text{4th}}{\text{-m}}_{\text{3rd}}}{{\text{m}}_{\text{2nd}}{\text{-m}}_{\text{1}}}\text{· 100}$$

m₁ =

Masse des Rundfilters in g

m₂ =

Masse des Rundfilters mit Stoffkuchen in g

m₃ =

Masse des leeren Kolbens in g

m₄ =

Masse des Kolbens mit Rückstand in g

In this formula:

m ₁ =

Mass of the round filter in g

m ₂ =

Mass of the round filter with fabric cake in g

m ₃ =

Mass of the empty piston in g

m ₄ =

Mass of the piston with residue in g

3. Carrying out the experiments

In a test facility in which a CF flotation cell from Sulzer-Escher Wyss was integrated, waste paper was pulpered at a consistency of 12 otro with 1% water glass, 0.5% sodium hydroxide solution, 0.5% hydrogen peroxide and 0. 33% Olinor 042 - all percentages related to otro paper stock - opened. After the subsequent rough sorting, a volume corresponding to 200 kg of dry paper was pumped into a chest and, after determining the consistency, diluted to 1.3% consistency with circulating water.

The additives to be investigated were then metered in, a fabric sample was taken and the flotation was then carried out for 30 minutes. The pulp suspension circulated from the chest over the flotation cell to a subsequent second chest back to the first chest. After the end of the flotation, the entire amount of material was pumped back into the first chest and another sample was taken. From the samples obtained before and after the flotation, the dichloromethane extract is determined, as indicated above, and the total loss of filler and fibers is determined via the consistency and the level of the laid paper.

The values obtained for the reduction of the dichloromethane extract (DCM extract) and the loss of fibers and filler (total loss) are shown in Table 1.

Table 1 added additive ¹⁾ Reduction of DCM extract (%) Total loss ³⁾ (%) without addition 33 18th 1% water glass ² ) 39 17th 1% water glass 57 14 1% water glass + 0.07% 042 65 11 1% water glass + 0.5% NaOH + 0.07% 042 64 8th ¹⁾ The addition took place immediately before the flotation as described above. ²⁾ The addition was made here for comparison purposes in the pulper ³⁾ Loss of fibers and filler

2.3. discussion of the results

It is clear from Table 1 that the process according to the invention achieves a considerable reduction in the DCM extract with a simultaneously significantly reduced total loss of fibers and filler. From the comparison of the addition of 1% water glass in the pulper or immediately before the flotation, it becomes clear that the desired reduction in the DCM extract is achieved only in the latter (inventive) procedure. In addition, Table 1 shows that the total loss of fibers and filler is further reduced in the presence of additional alkali metal hydroxide - with otherwise approximately the same value for the DCM extract.

Claims (6)

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.