CN1639420A - Improvements to processes for manufacturing paper products by improving the physico-chemical behaviour of the paper stock - Google Patents

Abstract

The subject of this invention is a method for producing paper from cellulose fibers dispersed in an aqueous medium, wherein carbon dioxide is injected directly into the pulp or via an aqueous carrier before the addition of chemical additives, in order to control the ion demand and zeta potential downstream, particularly in the additive addition step. Specifically, the additives are added to the mixing tank (2), and carbon dioxide is injected into the white water at the output of the waste pulper (15) or into the short system (9).

Description

Method for improving paper product production process by improving physical and chemical properties of pulp

The invention relates to an improved method of paper production process.

More particularly, it relates to a method for producing paper products from pulp comprising cellulosic fibers dispersed in an aqueous medium, said pulp being able to come from different sources.

The terms "paper" and "paper product" are used in this specification without distinction. Without limitation, they may be: paper for painting, wrapping paper, household paper, special papers of all kinds, and cardboard, etc., also intermediate products of dry cellulose pulp type.

The production of paper products generally consists of several successive steps: First, pulp is produced from different fibrous materials comprising cellulose (wood and/or annuals), water and chemicals. The pulp (suspension of fibers in water) is the starting product for further processing steps. In order to obtain the desired paper properties of the final product, the fibers dispersed in water are processed, in particular they are bleached, refined, washed and interwoven. Then, they are dehydrated and dried. During the dewatering process, the fibers have the property of naturally adhering to each other, thus forming a fibrous layer.

During production, it is also possible to attach different non-fibrous materials to the fibers, such as fillers, dyes, starches and other similar products. These various additives can be added to the fiber suspension before dewatering so that they are present in the fiber layer; they can also be added after the dewatering step by depositing them on the surface of the paper web. The purpose of adding chemical additives to the fibrous layer and/or paper web is to endow the final product with specific properties for specific uses; the above-mentioned additives can be sizing agents, dewatering and retention aids, starches, fillers, dyes, Defoamer, wet strength agent or WS agent, etc.

The invention is widely applicable to different processes for the production of paper products, especially to those in which one or more aqueous carriers are fully or partially recovered in a paper mill, said aqueous carriers being circulated in a closed or semi-closed system of a paper mill, Also applies to those methods in which broke is mixed.

The term "broken paper" refers to waste paper originating from the papermaking process, especially recovered paper from web forming processes, such as scraps from ends or web trimmings; paper from different sources, from one Coated or uncoated paper from one or more paper machines, and waste paper from different origins found therein. Before being blended into the papermaking system, the broken paper is processed in a special system called the broke system.

The term "semi-closed (or semi-open) system" refers to any system in which part of the circulating aqueous carrier is recovered; the term "closed system" refers to any system in which the entire flowing aqueous carrier is recovered and only added to compensate for evaporated water. system.

The term "aqueous carrier" itself means any aqueous medium, especially water of dilution, when its moisture content is such that it can act as a fluid (fibrous consistency of about 15% by weight or less) of cellulose pulp, from the papermaking process waste water, or any other aqueous fluid that circulates in the system.

In paper mills there are many systems for producing paper products which recover all or part of the aqueous carrier circulated therein. Among these systems, the above-mentioned systems may be: 1) a short system or first system, in which the pulp is diluted and purified before entering the paper machine for dewatering and then drying; 2) white water is collected and a second system for distributing white water to dilution operations. This is because especially white water, ie recycled water from pulp dewatering from the first part of the paper machine (the so-called wet end) is used for dilution. By convention, said white water is at least 80%, preferably at least 90%, of the volume of the dilution water; the balance essentially consists of water from other processes and/or fresh water. The terms "white water" and "dilution water" described in this specification are used without distinction; when a distinction is really needed, those skilled in the art can understand the meaning of the term according to the context without any ambiguity .

It is well known that paper machines, whose operating speeds have increased significantly in recent years, are prone to disturbances. These disturbances may be of mechanical, hydraulic or chemical origin; they lead to malfunctions within the paper machine, or to paper breaks which shut down the paper machine, and generally lead to excessive consumption of additives. This requires a substantial increase in normal handling and maintenance costs, especially fouling of exchangers, forming wires, felts, vacuum pumps and other components of the paper machine.

As noted above, some of these problems are of chemical origin. They arise notably from mixtures of many pulps from different sources produced during paper production: virgin pulp and produced broke, chemical pulp and mechanical pulp, especially virgin pulp and recycled pulp. Pulps of these different origins have different properties, especially the nature and amount of additives, such as those pulps mentioned above, which have their own physicochemical properties (especially pH and ionization potential).

Currently, the recirculation of aqueous carriers in semi-closed or closed systems of paper mills, which involves the presence of large amounts of dissolved ions, is becoming more and more important. In order to significantly reduce water consumption, the dilution water is usually recovered, which leads to an enrichment of dissolved substances and additive colloids in the aqueous carrier. The variety of cellulosic pulps used in any one process further exacerbates this problem.

It is well known that the properties of the different components of pulp depend inter alia on the charges carried by the substances present in the medium, ie charged substances and fibers present in the medium. Excessive presence of these charged species can interfere with the proper operation of the papermaking process, and it is therefore necessary to control the state of all these charges in order to improve the operation of the papermaking process, especially the retention of additives and the formation of the paper web.

Therefore even in order to eliminate or at least reduce web breaks, machine failures and excessive consumption of additives, seeking improved web properties on the paper machine and improved additive retention, the zeta potential of the fiber or charge ( zeta potential) or the ionic demand of the medium on the properties of the paper web on the paper machine.

The zeta potential, expressed in millivolts (mV), is an electrical property of solid particles, fibers or fillers present in the pulp water; it is related to the charge distribution on the surface of the particle in question. The zeta potential of the fibers changes due to the presence of various ion-generating chemicals added during paper production which affect the distribution of charges present on the fiber surface.

While the zeta potential undoubtedly affects retention and drainage mechanisms as well as web formation, another parameter is also important, namely ion demand. The latter characterizes the charge on the particles present in solution. It is measured by titration using an electrolyte. A specific amount of filtrate is neutralized with electrolyte; at the point of neutralization, the amount of electrolyte used represents the demand for ions. For example, if 5 milliliters (5 ml) of cationic polyelectrolyte is required to substantially neutralize the anionic demand due to a particular amount of filtrate anionic colloids (better known to those skilled in the art than the term "anionic trash") , then the cation requirement is -5ml.

Denaud, Olsson, Berger, Bley and Burkey, "New approach for chemicals control from stock prep to wet end", presented at the ATIP Conference in Grenoble, 9-11 October 2001, describe the combined use of ionic requirements The amount and zeta potential to control the way additives are added in the papermaking process. In particular, the measurement is carried out on-line so that the amount of the additive corresponds to what is actually required at different points, thus preventing excess from compromising the correct operation of the rest of the process.

It is known from WO 99/54741 to continuously monitor the formation of the paper web by measuring the zeta potential of the fibers of the fiber suspension and adjusting the dosage of chemical additives based on the measured values obtained. More specifically, the operating parameters of the paper machine are adjusted in a first stage in order to obtain a paper with the desired properties. The zeta potential is then measured at various points in the process. The zeta potential distribution thus determined is referred to as an optimum zeta potential distribution. The aim in a continuous production process is therefore to maintain this optimum zeta potential distribution. When the electromotive force distribution deviates from the optimal zeta potential distribution, the operating parameters and especially the amounts of chemical additives are modified so as to approach the determined optimal distribution.

However, the prior art methods described above involve interventions in order to adapt the amount of additive added to a given measured value of the parameter. Thus, in the case of patent WO99/54741, the aim is to reset the predetermined optimum zeta potential distribution, which leads to the further addition of certain additives. Combined with more and more recycled white water, further additives add to the excess ions already in the system and exacerbate the problems that occur. In the case of the Mutek/ATIP publication, the addition of additives is measured in relation to, and although this would prevent overdosing, it is not possible to reduce the amount of additive required.

The object of the invention is to function by pre-treating the aqueous medium with carbon dioxide at the feed point present in the pulp, upstream of the additive feed point.

It is therefore an object of the present invention to 1) reduce the amount of additives initially added and 2) provide a method of acting on the zeta potential other than post-addition of chemical additives. Instead of remedying the decrease in the zeta potential of the fibers by adding additives in large quantities, the present invention consists in acting prophylactically on the disturbing elements of the entire electrochemical system formed by the aqueous medium and the fibers, in such a way that the additives are attached to the fibers and not Interacts with ionic compounds present in aqueous media.

In order to ensure that disturbing elements have no or less influence on the effect of the additive on the fiber, the invention also includes controlling the ionic demand of the medium.

In order to determine the necessary conditions for the effective action of additives in pulp, whatever the composition of the pulp, the object of the present invention was to provide a stock stream with carbon dioxide and to do this before adding the additives. The supply of carbon dioxide can be achieved by direct injection into the slurry stream or indirect injection via an aqueous carrier added to the slurry stream. Thus, the added carbon dioxide improves the physico-chemical properties of the fiber-filler/aqueous medium system. It was surprisingly found that especially when injecting carbon dioxide into the pulp upstream of the additive addition point, it caused a change in ion demand relative to the absence of carbon dioxide. This change in properties manifests itself especially through the stabilization of this requirement for all aqueous carriers treated with carbon dioxide, whether directly or indirectly. For example, properties of the media such as the amount of anionic waste present are reduced, thereby allowing cationic additives to preferentially attach to the fibers during incorporation.

High retention is extremely important when producing paper. If there is too much charge in the white water, the retention rate decreases a lot. The charge of the colloid stabilizes the medium, but too much charge can impair flocculation, resulting in low retention or excess retention aid.

According to an essential feature of the present invention, the process of the present invention is a process for the production of paper products from pulp comprising cellulose fibers dispersed in an aqueous medium, said process comprising:

- at least one step of adding chemical additives;

- the step of forming a paper product from pulp by mechanically separating fibers from an aqueous medium to form a fibrous layer;

- at least one step of injecting carbon dioxide into the pulp,

It is characterized in that at least carbon dioxide is injected into the pulp upstream of at least one chemical additive addition step in order to control the ion demand of the aqueous medium downstream of the carbon dioxide injection point.

When additives, usually cationic additives, are added to papermaking pulp where dissolved ions are present, especially anionic trash, they do not attach to the fibers or do so in small amounts because the anionic The waste prevents their action, ie there is competition between the anionic sites of the fibers and those of the medium. Carbon dioxide injected upstream of the point of addition of additives acts on the anionic waste not only at the point of carbon dioxide injection but also in all delivery systems. This is evidenced by the lower anion and more stable cation requirements in the system when the two cases are compared with and without carbon dioxide.

Preferably, the injection of carbon dioxide upstream of the chemical additive addition step makes it possible to stabilize the zeta potential of the fibers during the chemical additive addition step. Thus, when carbon dioxide was injected in an amount sufficient to control the cation demand, little change in the measured zeta potential was observed until the ions in solution reacted. When the anionic waste reacts, the additive affects the fiber and the zeta potential changes.

Experiments performed by monitoring the cation demand and zeta potential of the system showed that the addition of carbon dioxide to the aqueous carrier resulted in a rapid reduction in the concentration of anion waste associated with the system's cation demand. The action of carbon dioxide on the anionic waste therefore makes better use of the cationic builders which essentially form the builders used. The use of adjuvants is therefore optimized, especially in the case of cationic starches. The roll of cationic starch is twice that, which reduces the demand for cations and improves the physical properties of the paper. Carbon dioxide added upstream will either increase the effectiveness of the starch for the same amount, or reduce the amount of starch required for the same paper properties, and in this case reduce the cost.

Carbon dioxide can be injected at various points in the papermaking system, most preferably when the pulp mix is produced. Advantageously, therefore, the stock stream flowing in the paper production system is at least partly supplied with an aqueous carrier circulating in a closed or semi-closed system forming part of the paper production system, and carbon dioxide is injected into at least one of the aqueous streams which feed at least one of the stock streams. in the carrier. When carbon dioxide is injected at different points, it is also observed that the stability of the cation demand and the stability of the zeta potential is greater, thus allowing better control of the process.

Preferably, the method comprises at least one step of diluting the pulp with dilution water comprising white water removed from the fiber layer during the paper product forming step, and injecting carbon dioxide into the dilution water upstream of the dilution step.

Advantageously, chemical additives are added to the pulp in a mix chest upstream of the short system and carbon dioxide is injected into at least one of the pulp flows into the mix chest.

It should be noted that the processes of injecting carbon dioxide and adding additives do not have to be performed consecutively, nor even closed to each other, or the same aqueous carrier involved in the different steps of the process. All that has to be done, in order to carry out the process according to the invention, is to treat a part of the slurry stream with carbon dioxide directly or via dilution water before the additive is added.

Preferably, the pulp obtained from broke goes into the pulp composition and the carbon dioxide is injected into the broke system.

More preferably, carbon dioxide is injected into the dilution water at the output of the broke pulper prior to refining.

It is even more advantageous to inject at least part of the carbon dioxide into the dilution water used for the dilution of the pulp in the short system of the paper mill and/or directly into the dilution pulp upstream of the paper machine. In this way, the pulp entering the paper machine is treated just like the white water leaving the paper machine and being remixed into the pulp stream.

So by injecting carbon dioxide one or more times appropriately into the short system, it is possible to maximize the effect of those additives added to the stock stream diluted with treated white water or subsequently added to the paper machine.

Advantageously, at least part of the carbon dioxide is injected into the waste water of the paper mill to be treated before treatment and after treatment at least part of the waste water is reintroduced into the production system of the paper mill. The treated part of the wastewater that is reintroduced into the paper mill is generally in the form of dilution water, but it may also contain fibers recovered during wastewater treatment and fibers remixed into the stock stream.

According to a preferred embodiment of the present invention, the method is implemented by combining carbon dioxide injection with the measurement of the zeta potential value and/or the ion demand value. The measurement is preferably carried out continuously on diluted pulp.

Carbon dioxide can be injected in liquid form.

Especially in the case of short carbon dioxide injection systems, the carbon dioxide can be injected in gaseous form.

Carbon dioxide can be injected partly in liquid form and partly in gaseous form.

The injection amount of carbon dioxide is 0.5-15 kg of carbon dioxide per metric ton of paper products, preferably 0.5-3 kg of carbon dioxide per metric ton of paper products.

The use of the method of the present invention brings many advantages, especially due to the reduction of the amount of additives that must be used to impart the desired properties to the paper; therefore, among other advantages, it should be noted that the present invention makes it possible to:

- Limit fouling of felts and forming wires;

- limit clogging and rusting of vacuum pumps, exchangers, etc.;

- reduction of different deposits present in paper products;

- increase the amount of charge attached to the paper;

- Reduce the corrosion of production equipment;

- Reduce the number of machine damage;

- increase the productivity of the paper machine; and

- Reduce COD (Chemical Oxygen Demand) of the system

A better understanding of the present invention can be obtained from the following illustrative examples in conjunction with FIG. 1 , which are given without any limitation.

Figure 1 shows some of the necessary operations of the process for producing a paper web.

The pulp is then refined in the refiner 1 and subsequently introduced into the mix chest 2, where different additional raw materials including cationic additives and broke can also be introduced at 3 if required.

Pulp from the mixing chest, via 4, is stored in the pulp chest 5 before being introduced into the short system. Typically, but not necessarily, short systems include a decontamination step using one of the following, or equivalent:

- Cyclone purification device 6, generally a cyclone washing machine;

- a device 7 for removing gases contained in the diluted pulp in order to improve the homogeneity of the pulp introduced into the headbox of the paper machine without affecting the formation of the paper web; and

- A device 8 for an additional purification step in order to remove the last particles. This device is generally referred to as a "screen".

These three types of installations are part of the short system together with the white water supply line 9 (component of the second system, but considered here by extension to belong to the short system), where it is pre-prepared and stored in the stock chest The pulp is both cleaned and properly diluted before entering the paper machine 10.

The wire front section 11 of the paper machine, the so-called headbox, conveys a layer of fiber suspension as wide as the forming wire. The free water and fibers are then separated mechanically, the pulp is dewatered, and the fibers are then deposited to form a paper web, thus forming a fibrous layer. White water is formed by free water drained from the forming wire by gravity and optionally by means of a vacuum pump. White water is used especially in the short system and broke system, but it is also conveyed to other semi-open systems of the paper mill through the lines of the second system (not shown in the figure).

On leaving the headbox, the web passes through a section 12 of the paper machine, called the press section, where the web is dried to about 40% dryness. The web then leaves the wet end of the paper machine into the dryer section 13 of the paper machine where any remaining water in the web is removed by evaporation. At this point, the dryness of the web is greater than 90%, preferably greater than 95%.

The surface of the paper web is then smoothed and calendered. Next, in order to improve the surface of the final paper web, different ingredients, especially compositions comprising fine pigments and binders, so-called coating slips, are deposited on the paper web. Of course, the coating operation is only performed for certain paper applications such as for the production of writing or printing paper. Coating on the paper machine itself is not necessary, coating can be done off the paper machine, after the finishing operation.

When broken paper is recovered, it is necessary to treat it before it can be re-mixed into the papermaking system; the broken paper is then pulped in a pulper 15, where it undergoes various purification operations (not shown in the figure) before it is stored. shown) and white water is added at different stages of the process for dilution and then re-blended into the pulp, especially in the blend chest 2.

The following examples of practice of the invention illustrate the advantages of the invention; they can be produced in a factory.

Example 1:

Trials were carried out on bleached virgin pulp containing long and short fibers. A filler consisting of natural calcium carbonate is added to increase the whiteness and opacity of the paper.

Carbon dioxide is injected into the white water system 9 of the short system through the delivery pump of the white water recovery tank (not shown in Fig. 1); the amount of carbon dioxide added is based on 2.5 kg of CO ₂ per 1 metric ton of paper produced.

The cation demand (measured on the filtrate using a "PCD" type Mutek device) was measured at different points under the following conditions when no carbon dioxide was added and when carbon dioxide was added, and the results are expressed in milliliters in the table below.

The results are listed in the table below.

Table 1 Results without _CO2 addition Result of adding _CO2 Sewage pond (upstream of the production process - not shown in the figure) Cation requirement (ml) -8 -9 Mixed stock tank 2 Cation requirement (ml) -6 -8 (of a paper machine) headbox Cation requirement (ml) -4 -8

It can be seen that when high molecular weight cationic polymers are added:

- In the absence of carbon dioxide, the cation demand is easy to change;

- In case of carbon dioxide injection into short systems, said demand remains constant. The ionic demand of the media remains constant and is no longer affected by the addition of additives. The effect of the latter is shifted to the fibers in the system rather than to the ions.

Example 2:

The following experiments correspond to raw stock consisting of a mixture of long and short fibers in a 50/50 ratio, into which broke of the WS (wet strength agent) type was mixed. Carbon dioxide is injected into the broke flow before the mix chest 2 and then likewise before the headbox 11 into the diluted stock in the short system.

Broken stock flow accounts for approximately 20% of the total stock flow.

The amounts of carbon dioxide added were 5 kg and 3 kg per metric ton of dry pulp, respectively. The cationic polymer and starch of WS dosage form are added in the mixing tank and mixing pump located before the headbox.

Measure the following data:

- measurement of the cation demand without and with the addition of carbon dioxide under the conditions described above (the filtrate is measured with a device of the "PCD" type and the results are expressed in ml);

- the zeta potential of the cellulose fiber (measured by a "zetameter" measuring instrument, and the result is expressed in mV).

The measured results are shown in Table 2 below:

Table 2 parameter Results without _CO2 addition Result of adding _CO2 Broken pulper Fiber zeta potential (mV) cation demand (ml) +22-1 -10+5 Mixed stock tank Fiber zeta potential (mV) Filtrate cationic demand (ml) +150+100 +50+40 short system Fiber zeta potential (mV) Filtrate cationic demand (ml) +100+50 +40+38

It can be seen that the addition of carbon dioxide to the different pulp streams limits the variation in ion demand. The change in zeta potential is also smaller. This reduction in fluctuation is important to the papermaker, who has a more reliable system and better ability to formulate additives while controlling the process. This will result in real savings in additives.

Depending on the type of pulp used and the type of paper produced, cationic starches, dyes and sizing-type auxiliaries can be saved by approximately 15-20%. The adhesion of fillers added to the fiber pulp was improved by 5%.

Better color fastness of the paper was also observed. This is because when the zeta potential is positive, the pigment adheres poorly, and since the zeta potential is moderated by the infusion of carbon dioxide, the pigment adheres better.

Reducing the frequency of web breaks on a paper machine reduces the number of paper machine interruptions by about 25%, thus significantly improving the productivity of the paper machine.

Claims (14)

1. A method of producing a paper product from pulp comprising cellulose fibers dispersed in an aqueous medium, the method comprising: - at least one step of adding chemical additives; - the step of forming a paper product from pulp by mechanically separating fibers from an aqueous medium to form a fibrous layer; and - at least one step of injecting carbon dioxide into the pulp, It is characterized in that carbon dioxide is injected into the pulp at least at one point upstream of at least one chemical additive addition step to control the ion demand of the aqueous medium downstream of the point of carbon dioxide injection. 2. The method according to claim 1, characterized in that the injection of carbon dioxide upstream of the chemical additive addition step also enables stabilization of the zeta potential of the fiber during the chemical additive addition step. 3. A method according to claim 1 or 2, characterized in that the stock stream flowing in the paper production system is at least partly supplied by an aqueous carrier circulating in a closed or semi-closed system forming part of the paper production system, and wherein carbon dioxide is injected At least one flows into the aqueous carrier of at least one slurry stream. 4. A method according to any one of claims 1 to 3, characterized in that said method comprises at least one step of diluting the pulp with dilution water containing white water which is removed from the fibrous layer during the forming step of the paper product , and inject carbon dioxide into the dilution water upstream of the dilution step. 5. A method according to any one of claims 1 to 4, characterized in that chemical additives are added to the pulp in the mixing chest (2) upstream of the short system, and carbon dioxide is injected into at least one stream into the mixing chest (2) in the slurry flow. 6. A method according to any one of claims 1 to 5, characterized in that pulp obtained from broke is added to the pulp composition and carbon dioxide is injected into the broke system. 7. A method according to claim 6, characterized in that carbon dioxide is injected into the dilution water after the broke pulper (15) and before refining. 8. A method according to any one of claims 4 to 7, characterized in that at least part of the carbon dioxide is injected into the dilution water used to dilute the pulp in the paper mill short system and/or directly into the diluted pulp upstream of the paper machine. 9. A method according to any one of claims 1 to 8, characterized in that before treatment at least part of the carbon dioxide is injected into the waste water of the paper mill to be treated and after treatment at least part of the waste water is reintroduced into the paper making system. 10. A method according to any one of claims 1 to 9, characterized in that the method is carried out by combining carbon dioxide injection with the measurement of zeta potential values and/or ion demand values. 11. A method according to any one of claims 1 to 10, characterized in that the carbon dioxide is injected in liquid form. 12. A method according to any one of claims 1 to 10, characterized in that the carbon dioxide is injected in gaseous form. 13. A method according to any one of claims 1 to 10, characterized in that the carbon dioxide is injected partly in liquid form and partly in gaseous form. 14. A method according to any one of claims 1 to 13, characterized in that 0.5-15 kg of carbon dioxide is injected per metric ton of paper product, and preferably 0.5-3 kg of carbon dioxide is injected per metric ton of paper product.

Images