NO842387L - PROCEDURE FOR AA WEEK DRAWING SPEED IN PAPER MAKING. - Google Patents

Abstract

A process for increasing rate of dewatering in the manufacture of paper from an aqueous furnish which comprises at least 40% by weight of mechanical wood pulp, thermomechanical wood pulp, or mixtures thereof, by addition thereto of an aluminum salt, e.g., alum, and a water-soluble copolymer containing from about 2 to about 30 mole percent repeating units derived from 2-acrylamido-2-methylpropane sulfonic acid, from 0 to about 25 mole percent repeating unit derived from acrylic acid, and from about 45 to about 98 mole percent repeating units derived from acrylamide, while maintaining pH of the furnish in the range of from about 3.5 to about 6.5.

Description

This invention generally relates to one more method

to increase the dewatering rate of the pulp in the manufacture of paper, in particular a method in which the fiber

the part in the mass has a high content of mechanical and/or thermomechanical mass.

State of the art. In general papermaking practice, an aqueous pulp suspension of cellulose fibers is produced by comminuting the wood material conveyed hydraulically and mechanically on a metal wire cloth, the wire, which is in motion and on which a wet web of cellulose fibers is formed. The wet fiber web is dewatered on the wire by draining liquid from it, after which the wet web can be processed further, dried, calendered and subjected to further treatments if desired.

In common practice, a number of additives are added to the starting material that is fed to the wire (on which the paper web is formed). These additives may include processing aids to

to improve the operation of the paper machine, as well as chemicals to improve the properties of the final paper product.

Suitable processing aids may include agents such as

increases the retention of added fillers in and on the resulting paper tap, and agents that reduce the loss of pulp fines during the dewatering step, and drainage aids that improve the rate of pulp dewatering on the wire. Other additives may include shaping aids, flocculants, antifoam agents, wet strength and dry strength additives, pitch control agents, slimicides, creep aids and the like, as is well known to those skilled in the art.

Functional additives may include fillers as mentioned, sizing agents, additives to increase strength, and the like. The fillers may include agents that provide optical lightness, agents that provide opacity, as well as pigments. Sizing agents are used to make the paper product resistant to wetting by liquids, such as printing ink, water and the like, and rosin or wax are typically used for this purpose.

To increase the efficiency of the processing and facilitate

to this, it is desirable to add drainage aids to the starting material before the wet path is formed to achieve increased

capacity or processing speed in the papermaking process in systems where dewatering or liquid drainage is the rate-limiting step in the process.

Although it is desirable to increase the drainage rate as much as possible in the papermaking system, the additives that have been used for this purpose until now give low activity levels when used in newsprint starting materials, which are usually produced under strongly acidic conditions and high shear forces. These include conventional drainage aids containing as anionic substituents -COOH groups, as well as copolymer additives containing -SO^H groups.

By "newspaper pulp" is meant in the present a starting material for the production of paper and cardboard, especially newsprint, product qualities to be coated and fine paper qualities, containing fine substances and fillers and produced under acidic conditions, and whose pulp component contains at least 40% by weight of a wood-based pulp selected from the group consisting of mechanical pulp, thermomechanical pulp and mixtures thereof.

There is thus a need for improved dewatering additives for newsprint pulps characterized by stability and high activity.

It is therefore an object of the present invention to provide a method for increasing the deaeration rate for newsprint pulps at the low pH values that characterize such pulps.

The invention. The present invention relates to a method for increasing the dewatering rate in the production of paper from a starting material whose pulp component comprises at least 40% by weight of a wood-based pulp selected from the group consisting of mechanical pulp, thermomechanical pulp and mixtures thereof, comprising: (a) it is added to said starting material before the dewatering of this (1) from approx. 0.5 to approx. 5% by weight, based on the cellulose fibers in the starting material, of an aluminum salt,

and (2) from approx. 0.01 to approx. 0.5% by weight, based on the weight of cellulose fibers in the starting material, of a water-soluble copolymer containing from approx. 2 to approx. 30 mol% repeating units derived from 2-acrylamido-2-methylpropanesulfonic acid, from

0 to approx. 25 mol% repeating units derived from acrylic acid,

and from approx. 45 to approx. 98 mol% repeating units derived from acrylamide, and

(b) the pH of the starting material is maintained during step (a) and during the dewatering in the range from approx. 3.5 to approx. 6.5. Fig. 1 is a graphical representation showing the drainage change, i.e. the change in the amount of drained liquid, in ml, for a starting material containing various drainage additives in relation to a starting material without any additive, as a function of the pH of the starting material, for 3% addition of aluminum sulfate (alum) to the starting material. Fig. 2 is a graph showing the drainage change, ml, as a function of pH, for 1% alum addition. Fig. 3 is a graphical representation showing the drainage change, ml, as a function of the alum addition, at pH = 4.5. Fig. 4 is a graphical representation showing the drainage change, ml, as a function of pH, and which parametrically shows the effect of varying levels of alum addition and of pre-elevated temperature.

In connection with the present invention, it has been surprisingly and unexpectedly discovered that the use of a water-soluble copolymer containing from approx. 2 to approx. 30 mol% repeating units derived from 2-acrylamido-2-methylpropanesulfonic acid (hereinafter referred to as "AMPS"), from 0 to ca. 25 mol% repeating units derived from acrylic acid, and from approx. 4 5 to approx. 98 minor-5 repeating units derived from acrylamide, in combination with the addition of an aluminum salt, such as aluminum sulfate (alum), aluminum chloride or aluminum nitrate, at low pH values of the order of magnitude from approx. 3.5

to approx. 6.5, is remarkably effective in increasing the dewatering rate of a starting material whose pulp component comprises at least 40% by weight of a wood-based pulp selected from the group consisting of mechanical pulp, thermomechanical pulp and mixtures thereof.

The method according to the invention provides a high speed and degree of drainage of newsprint pulp under strongly acidic conditions, where conventional anionic or cationic polymers are not effective. As mentioned, conventional drainage aids containing carboxylic acid groups (and those containing sulphonic acid groups) are ineffective under such acidic conditions, and high molecular weight cationic polymers are not sufficiently effective. Although AMPS polymers and copolymers have been proposed as drainage aids in the past, for example in BRD-off. document 2 248 752, in combination with alum at low pH values for the treatment of hardwood/softwood pulps, it has not been suggested that such additives could be used in starting materials of the type of newsprint pulps as according to the present invention, as experience have shown that dewatering aids that work well in bleached pulps are not effective for pulps containing grinding pulp. In view of the fact that most additives which are satisfactory for improving drainage in neutral or alkaline pulps and kraft pulps are characterized by very poor performance in strongly acidic newsprint pulps,

it is very unexpected that the method according to the invention can be used with advantage and provides extremely satisfactory drainage.

The AMPS copolymer used according to the present invention contains from approx. 2 to approx. 30 mol% repeating units derived from AMPS, from 0 to ca. 2 5 mol% repeating units derived from acrylic acid, and from approx. 4 5 to approx. 98 mol% repeating units derived from acrylamide. As used herein, AMPS denotes 2-acrylamido-2-methylpropanesulfonic acid as well as any suitable salts thereof.

Suitable AMPS copolymers include those containing, for example, from about 2 to approx. 20 mol% repeating units derived from AMPS and from ca. 80 to approx. 98 mol% repeating units derived from acrylamide. In the present context, "acrylamide" is to be understood to denote acrylamide as such as well as acrylamide derivatives, for example substituted acrylamides. Such copolymer materials can be used with particular advantage in masses where an aluminum salt, for example aluminum sulphate, aluminum nitrate or aluminum chloride, is added to the mass in an amount of from approx. 2 to approx. 4% by weight, based on the weight of cellulose fibers in the pulp. With this weight% addition of aluminum salt, the pH of the mass is preferably kept in the range from approx. 4.1 to approx. 6.5 during the addition of the copolymer and further up to and including the dewatering of the mass.

The aluminum salt is used in the present process as a source of polyvalent metal ions to improve the efficiency of the AMPS copolymer, and the dosage of the aluminum salt required in a given system can be easily determined without too much experimentation by simple tests such as determination of Canadian Standard Freeness (CSP) or Britt jar drainage provisions with it. starting material to be processed. The preferred aluminum salt is aluminum sulfate (alum).

In systems where the AMPS/acrylamide copolymer described above is used with additions of the aluminum salt to the mass in an amount from approx. 0.5 to approx. 2% by weight, based on the weight of cellulose fibers in the mass, is satisfactory, it is desirable to maintain the pH in the mass during the copolymer addition and further through the dewatering step in the range from approx. 4.8 to approx. 6.5 to achieve optimal performance of the drainage additives.

Particularly preferred in the general embodiment of the present invention are AMPS copolymers containing from approx. 2 to approx. 30 mol% repeating units derived from AMPS, from ca. 5 to approx. 2 5 mol% repeating units derived from acrylic acid, and from approx. 45 to 93 mol% repeating units derived from acrylamide. This terpolymer system, as explained in more detail below, has been found to provide particularly improved drainage performance when the pulp temperature is maintained in the range of approx. 20 to approx. 60°C during the terpolymer/aluminium salt addition and further through the dewatering step. Most preferably, the temperature is kept in the range 40-60°C as this temperature range has been found to result in particularly improved performance.

The above-mentioned terpolymer material has been found

to be particularly favorable when the pH of the mass is kept in the range from approx.

4 to approx. 6.5 during the terpolymer/aluminium salt addition and further through the dewatering step.

In papermaking systems in which the preferred aluminum salt, aluminum sulfate (alum), is used, and where the amount of alum for optimal drainage improvement in the terpolymer is in the range from approx. 2 to approx. 4% by weight, based on the weight of cellulose fibers in the mass, the pH of the mass is appropriately kept in the range from approx. 4.5 to approx. 6.3 during the terpolymer/alum addition and further through the dewatering step. When lower amounts of alum addition are most effective, for example in the range from approx. 0.5 to approx. 2 weight% addition of alum, based on the weight of cellulose fibers in the pulp, keeps the pH of the pulp appropriate in a range from approx. 4.5 to approx. 5.6 during the terpolymer/alum addition and further through the dewatering steps. These relationships may vary to some extent for different pulps, temperature conditions and the presence or absence of recycling in the papermaking system.

In practice, the optimum pH conditions can be determined precisely by simple tests carried out on the relevant paper machine.

As mentioned, the method according to the invention is particularly useful when it is applied to masses of the newsprint pulp type,

where the pulp component is wood-based mechanical pulp and/or thermomechanical pulp. Special benefit is achieved by applying the method according to the invention to millstone-ground mechanical masses.

The AMPS copolymer or terpolymer preferably has

a molecular weight from approx. two million to approx. twenty million. Particularly preferred copolymers may, for example, have a viscosity (Standard Brookfield), measured in a 0.20% solution at 25°C in 0.33 M NaCl with a No. 1 spindle and at 60 rpm. minute, of 2-10 centipoise.

Although in the preferred embodiment of the present invention alum is used as a source of polyvalent metal cations in the treatment of the mass with AMPS-containing copolymers, it is possible to use other sources of cationic metal (aluminum) sols that are capable of linking with sulfonic acid -groups or carboxylic acid groups, as an alternative to the alum component. Other aluminum salts that can be used in combination with the AMPS copolymer at low pH values include aluminum chloride and aluminum nitrate.

Heating the mass to maintain it at an elevated temperature through the AMPS copolymer/alum addition and dewatering will, as mentioned, further improve the dewatering of the mass, presumably because more of the necessary cationic aluminum oxide complex is formed by olation of aluminum hydroxide groups to a configuration of the type Al<+->0-Al<+>, which is formed at lower pH and favored by higher bulk temperatures.

In the production of newsprint, it is extremely volatile

to improve the drainage or removal of water and reduce as much as possible problems with pitch deposition. Both problems can be greatly reduced by using a suitable drainage aid which causes flocculation of the grinding pulp fines and retention of the pitch particles on the fibers under the strongly acidic conditions that characterize newsprint pulps.

Typically, a mass of newsprint will contain approx. 25% of long-fibre chemical pulp, such as bleached sulphite pulp or bleached kraft pulp, and approx. 75% by weight mechanical high-yield pulp,

such as millstone ground (GW) or a mixture of millstone ground and thermomechanical (TMP) pulp. After the formation of a sheet (wet web) in a fast-running industrial paper machine, much of the fine fibres, which primarily contain the fine fraction of the GW or TMP pulp components, will pass through the paper machine wire, and it is characteristic that the retention at the first passage in such systems is low, of the order of approx. 50-60%. Accordingly, such fines are returned to the wet web forming part of the process system by recycling the desilted water. With these precautions, most of the fines in the original starting material will eventually be retained in the sheet after several recyclings.

High speed paper machines are generally very sensitive to any change in drainage rate, and it is very important to achieve flocculation of fines and pitch particles on long fibres, as such flocculation minimizes the pitch deposition problems and improves the water removal rate. Drainage aids which work satisfactorily in fine paper grades will generally not give noticeable favorable results in pulp-type pulps. This inefficiency may be due to the significant surface area in high-yield pulps (due to the content of fines) and the significantly reduced (inhibited) binding ability of the polymeric additives on the surfaces of lignin-rich fibers in mechanical pulps.

Another factor that prevents the achievement of good drainage

and high retention of fines in newsprint-type pulps, the high hydrodynamic shear in a fast-

continuous paper machine of the type that is conventionally used for the production of newsprint.

In summary, it has thus not been possible to transfer the good performance characteristics of polymeric drainage/retention additives in fine paper pulps (see the above-mentioned BRD official publication 2 248 752) to pulps containing high percentages of high-yield pulps. The present invention thus represents a significant advance in the area in that it provides a pulp treatment (pulp additive) which provides a significant, surprising and completely unexpected improvement in the dewatering rate for pulps that have a significant content of mechanical pulp and/or thermomechanical pulp.

The following examples illustrate specific aspects of the present invention. These examples will further illustrate the invention. In all examples, parts and percentages are by weight unless otherwise stated.

EXAMPLE 1

A laboratory drainage test method was developed for use in the following examples. It is usually very difficult to achieve accurate measurements of drainage changes in the case of masses with a high content of grinding mass, as such masses typically show slow drainage. As mentioned, the composition of a typical industrial newsprint pulp is approx. 75% by weight of sanding compound and 25% by weight of long-fibred chemical compounds. For the measurements carried out in the following examples, the mass consisted of 50% by weight of each fiber component, i.e. grinding mass and long-fibre chemical mass. The long-fibre pulp portion of the starting material consisted of parts of bleached softwood and hardwood pulp which had been ground to approx. 450 CSF. The portion of sandpaper in this experimental starting material represented a typical millstone-ground sandpaper produced for newsprint by Bowaters Paper Company, Calhoun, Tennessee, U.S.A., with a pH of 4.7 and an alum content of about 1.0% by weight, based on the weight of fibers, the alum being added during the production of the grinding stock to reduce pitch deposition in the subsequent papermaking operation.

In each of the experiments described below, the mixture of 50% by weight of long fiber chemical pulp and 50% by weight of abrasive pulp was thinned to 0.5% fiber consistency and treated with added alum, for carrying out the method according to the invention, during which the pH of the starting material was adjusted by adding dilute sodium hydroxide.

To the starting material described above was added a 0.1% solution of the relevant polymer or copolymer drainage additive at a dosage of 0.025% polymer as such, based on the total fiber weight. This starting material was then mixed by transferring it from one container to another six times. A 500 ml sample of the treated starting material was then transferred to a drainage

tube fitted with paper machine wire at the lower end. The starting material was allowed to drain for 15 seconds,

and the amount of filtrate collected during this time period was measured. A large increase in the amount of filtrate during a given experiment compared to the control starting material which did not contain any drainage aid indicates a significantly improved water release or drainage of the web being formed.

In each trial in the examples that follow, a control trial was conducted in which the starting material contained no additive except alum. An increase or decrease in the amount of filtrate collected, compared to the control, indicates an increase or decrease in the drainage rate of the starting material, respectively.

In the evaluation of the method according to the present invention for producing newsprint, a typical cationic and a typical anionic polyacrylamide-based retention/drainage aid were used in separate drainage tests for comparison purposes. These conventional cationic and anionic polyacrylamide additives had molecular weights in the range of 4-15 million.

EXAMPLE 2

Fig. 1 shows, graphically, the drainage change, i.e. the change

in amount of drained liquid, in ml, for a starting material containing different drainage additives, compared to a starting material containing no additive (control),

as a function of the pH of the pulp, for 3% addition of alum to the pulp. In fig. 1 is curve A the drainage curve for the above

for described starting material containing as drainage aid a copolymer containing 15% by weight AMPS and 85% by weight acrylamide (AM). Curve B is the drainage curve for a starting material containing 5 wt% AMPS, 10 wt% acrylic acid (AA) and 85 wt% AM. Curve C represents the drainage performance of a starting material containing the above-mentioned conventional polyacrylamide anionic drainage/retention agent, and curve D is the performance curve for a starting material containing the conventional polyacrylamide cationic drainage/retention agent described earlier.

It will be seen from fig. 1 that changes in pH dramatically affect the performance of all the pulp compositions tested, especially the starting materials that contained highly anionic AMPS copolymers (curves A and B) and those that contained anionic polyacrylamide (curve C). The aforementioned 15/85 AMPS/AM copolymer (curve A) provides the best drainage in the pH range 4.3-5.7 of all the masses tested, while the anionic (carboxyl group-containing) polyacrylamide is relatively unaffected by pH change in this area. The cationic polyacrylamide (curve D) has a moderate effect in this pH range. This pH range and alum dosage (3% by weight) generally correspond to process conditions in many newsprint machines. With both the AMPS copolymer according to curve A and the AMPS terpolymer according to curve B, the alum that is added to the mass should be partially neutralized, i.e. in the form of a polymer of cationic aluminum oxide. Since too strong flocculation may be undesirable in a given application, the best composition, as between a copolymer of the type represented by curve A, and a terpolymer of the type represented by curve B, can be determined by paper machine trials, as pointed out above. As is clear from the graphic representation, both types of AMPS-containing polymer are nevertheless more effective than the cationic polyacrylamide (curve D) which has been used up to now as a conventional drainage/retention aid.

The anionic polyacrylamide according to curve C becomes very active only at high pH where the polymer is more extended structurally. However, high pH values are not attractive in the manufacture of newsprint because of the problems with pitch deposition associated therewith. On the other hand, if the pH is reduced to very low values, of the order of magnitude less than 4.0, the AMPS-containing polymers become significantly less effective, presumably due to the absence of sufficient amounts of cationic polymeric alumina, which likely provides activated binding sites for such polymers.

EXAMPLE 3

Fig. 2 is a graphical representation of drainage change, ml, as a function of pH, for a weight% alum addition to the mass. The different curves correspond to the same pulp compositions and drainage aids as the correspondingly named curves in fig. 1.

As can be seen from the graphic representation, the AMPS-containing copolymers become very effective when the pH of the mass is increased to a point (approx. 5-5.5) where a sufficient amount of cationic polymeric alumina is formed. The observed change in optimum pH, compared to the results in example 2, is probably due to the lower dosage of alum in this case compared to example 2, which reduces the effective concentration of the active cationic alumina. The results shown in fig. 2, indicates that the optimum operating pH is a function of the available cationic alumina (cationic Al ions in the polymer)

in the mass.

EXAMPLE 4

Fig. 3 is a graphical representation of drainage change, ml, as a function of alum addition, at a pH of 4.5. Curve A applies to a mass containing as a drainage aid a 15% by weight/85% by weight AMPS/AM copolymer; curve B refers to a mass containing 5:15:80% by weight AMPS/AA/AMD terpolymer; and curve C applies to a pulp in which an anionic polyacrylamide containing 30% free carboxyl groups is used as a drainage aid.

In this experiment, the dosage of alum was varied from 0.5 to 2.0% by weight based on the weight of fibers in the pulp, and the pH was adjusted to 4.5. The results obtained are in accordance with the results shown in example 3, in that they show that the polymer at low alum additions (for example 0.5-1.0% by weight) can actually delay the drainage. In this pulp system, a minimum of 1.5-2.0% by weight seems to be essential for sufficient activation of the AMPS-containing polymers. The carboxyl group-containing anionic polyacrylamide is unaffected by changes in the alum content of the pulp.

EXAMPLE 5

Fig. 4 is a graphical representation of drainage change, ml, as a function of pH and shows parametrically the effect of varying levels of alum addition and of elevated temperature. Effect of temperature on drainage rate to determine whether heat will affect the chemistry of the alum by favoring the formation of oxolated (polymeric) alumina types at increased temperature.

The parametric alum concentration and temperature conditions are indicated in the figure. The drainage aid used in all experiments was a terpolymer of 5/15/80% by weight AMPS/AA/AM.

In each trial, the mass was regulated to the desired temperature by heating a stainless steel beaker containing the mass in a water bath. When the parametric temperature was reached, the mass was treated with alum and neutralized with an appropriate amount of sodium hydroxide and left for 5 minutes to reach equilibrium.

Claims (10)

1. Process for increasing the dewatering rate in the production of paper from a starting material whose pulp component comprises at least 40% by weight of a wood-based pulp selected from the group consisting of mechanical pulp, thermomechanical pulp and mixtures thereof, characterized in that (a) one adds to the starting material before dewatering this (1) from approx. 0.5 to approx. 5% by weight, based on the weight of cellulose fibers in the starting material, of an aluminum salt, and (2) from approx. 0.01 to approx. 0.5% by weight, based on the weight of cellulose fibers in the starting material, of a water-soluble copolymer containing from approx. 2 to approx. 30 mol% repeating units derived from 2-acrylamido-2-methylpropanesulfonic acid, from 0 to ca. 25 mol% repeating units derived from acrylic acid, and from approx. 45 to approx. 98 mol% repeating units derived from acrylamide; and (b) one maintains the pH in the starting material during step (a) and through the dewatering in the area from approx. 3.5 to approx. 6.5. 2. Method according to claim 1, characterized in that a copolymer is used which contains from approx. 2 to approx. 20 mol% repeating units derived from 2-acrylamido-2-methylpropanesulfonic acid and from ca. 80 to approx. 98 mol% repeating units derived from acrylamide. 3. Method according to claim 2, characterized in that the aluminum salt is added to the starting material in an amount of from approx. 2 to approx. 4% by weight, based on the weight of cellulose fibers in the starting material. 4. Method according to claim 3, characterized in that the pH in the starting material during step (a) and through the dewatering is kept in the range from approx. 4.1 to approx. 6.5. 5. Method according to claim 2, characterized in that the aluminum salt is added to the starting material in an amount of from approx. 0.5 to approx. 2% by weight, based on the weight of cellulose fibers in the starting material. 6. Method according to claim 4, characterized in that the pH in the starting material during step (a) and through the dewatering is kept in the range from approx. 4.8 to approx. 6.5. 7. Method according to claim 1, characterized in that a copolymer is used which contains from approx. 2 to approx. 30 mol% repeating units derived from 2-acrylamido-2-methylpropanesulfonic acid, from ca. 5 to approx. 25 mol% repeating units derived from acrylic acid, and from approx. 45 to approx. 93 mol% repeating units derived from acrylamide. 8. Method according to claim 6, characterized in that the pH in the starting material during step (a) and through the dewatering is kept in the range from approx. 4 to approx. 6.5. 9. Method according to claim 1, characterized in that the temperature in the starting material during step (a) and through the dewatering is kept in the range from approx. 20 to approx. 60°C. 10. Method according to claim 6, characterized in that the temperature in the starting material during step (a) and through the dewatering is kept in the range from approx. 4 0 to approx. 60°C.