WO2009000346A1

WO2009000346A1 - Method of forming calcium carbonate in a pulp suspension

Info

Publication number: WO2009000346A1
Application number: PCT/EP2008/001386
Authority: WO
Inventors: Klaus Doelle; Reinhard Gather
Original assignee: Voith Patent Gmbh
Priority date: 2007-06-27
Filing date: 2008-02-22
Publication date: 2008-12-31
Also published as: DE102007029686A1

Abstract

The method is used to load fibers with filler, more particularly with calcium carbonate. Calcium hydroxide is combined in milk of lime (MOL), for example, with gaseous carbon dioxide (CO2), forming calcium carbonate in a chemical reaction. Prior to this reaction, anionic substances, such as, in particular, salts, polyamides, retention agents or starch particles, or a mixture of these substances, are added to promote the penetration of the fibers with calcium hydroxide.

Description

VOITH PATENT GmbH RPL13216 WO

Process for forming calcium carbonate in a pulp suspension

The invention relates to a method according to the preamble of claim 1.

The use of calcium carbonate as a mineral filler in paper and board production has long been known. He serves u.a. to significantly improve the optical quality of graphic papers. There are already methods in which the calcium carbonate is precipitated in an aqueous liquid by e.g. a reaction between calcium hydroxide and gaseous carbon dioxide is brought about. The fillers produced in this way can then be added to the fiber raw material in the paper mill. A more recent development is to initiate the process of precipitation in the presence of the pulp, that is, in a mixture already containing the fibers destined for paper production. Such methods are called fiber loading methods.

Several methods of fiber loading technology for loading pulp fibers with calcium carbonate are already known. US 5,223,090 discloses a method employing fibrous material having elongated fibers with a cell wall surrounding a cavity, the fibers having a humidity sufficient to form a dewatered pulp slurry. The fibers have a moisture content corresponding to a proportion of 40 to 50% of the weight of the fibers. The water is essentially present inside the fibers and inside the fiber walls. Subsequently, alternatively calcium oxide or calcium hydroxide is added to the pulp so that at least part of the incorporated calcium oxide or calcium hydroxide is associated with the water present in the pulp. Subsequently, the fibrous cellulosic material is combined with carbon dioxide while being subjected to a shear mixing process to obtain a fibrous material having a substantial amount of calcium carbonate on the fiber surface in the hollow interior and within the fiber walls of the cellulose fibers.

From US 5,665,205 a method for loading a waste paper suspension is known which serves in particular to increase the whiteness of the - deinked or non-deinked - waste paper. In this method, a waste paper suspension is preferably introduced at a consistency of between 0.1 and 5% in a gas / liquid reactor. Downstream of this insertion then takes place the supply of a basic salt-containing liquid, in particular of calcium hydroxide, wherein the thus mixed components with a reaction gas, in particular carbon dioxide, are brought into contact, thereby precipitating the filler. The thus loaded waste paper suspension is then used for paper or board production. As a particularly suitable gas / liquid reactor, a mixing container is proposed in which gas bubbles are distributed in a liquid. Apparently, an unusually high degree of filling of the pulp (filler content several hundred percent of the fiber weight) is required to achieve the desired whiteness.

From DE 102 04 254 A1 a further method for loading a pulp suspension is known. The process comprises the following steps: introduction of calcium hydroxide in liquid or dry form or of calcium oxide into the pulp suspension, thickening and heating of the pulp suspension, feeding of gaseous carbon dioxide in a crystallizer, precipitation of calcium carbonate by the carbon dioxide.

Processes of this kind make it possible to produce fiber loaded with calcium carbonate (FLPCC = fiber loaded precipitate calcium carbonate), in particular for pulp production or for pulp use in papermaking. The fiber raw material to be loaded is made, for example, from recycled paper, from DIP (= deinked paper), from secondary pulp, bleached or unbleached Kraft-cell fuel or wood pulp of any kind, bleached or unbleached sulphite / sulphate pulp, finished pulp, linen, cotton and / or or hemp fibers and / or any other paper stock produced in a paper machine Use finds.

The field of application of the invention extends to paper and pulp production and process technology, including the filler produced, and includes fields of application of all types of paper, including those resulting from their production, which have a filler content between 1% and 60%. Preferably, the filler content may be between 5% and 50%.

Compared to conventional processes for the production of a pulp suspension, the same strengths can be achieved energetically more favorably by loading (in lumens, fiber walls and / or fiber surface) in a paper or board web produced therefrom; In particular, grinding energy can be saved in this way.

Loading has a positive effect on the processes in stock preparation, paper machine and / or further processing. Thus, a paper web made of loaded fibers is easier to dewater, for which reason e.g. the machine speed can be increased and / or the press section can be operated with a lower pressing pressure. In addition, it would be possible to drive in the dryer section with less energy input (e.g., steam).

A further advantage of the use of the technology according to the invention in the paper grades listed above is that they can also be further processed without problems in a calender. The use of fiber-loading technology, in which fiber-loading particles are deposited in, around and on the fibers, reduces or completely eliminates blackening.

If one compares this paper with conventionally produced papers, higher and / or the same appear at the same and / or higher filler contents

Strengths, porosity, specific volume (which can optionally also be deliberately set lower), opacity and printability. By this kind Depending on the degree of loading of the pulp, the fiber load can be used to increase the productivity of the paper machine and / or to make its production more cost-effective (by, for example, raw material energy cost reduction).

It is the object of the invention to provide a method for loading a pulp suspension in which the advantages of the loading method are increased, in particular by increasing the amount of fillers fixed in the cavities of the fiber.

According to the invention this object is achieved by the measures mentioned in claim 1.

The new process will increase the calcium hydroxide uptake of the fibers. By the electrical action of the added anionic substances, a stronger connection to the fiber surfaces and / or a better penetration into the cavities can be achieved and also the production of calcium carbonate within the fibers ("lumen loading") can be increased.

Lumen loading can be further intensified if, after addition to the pulp suspension, the anionic substances are removed by a special process step, e.g. Washing / thickening of the outer surfaces of the fibers are removed again, so that the charge setting of lumens and fiber outer surfaces is different.

In the following, typical process steps and parameters for the loading of a pulp suspension according to the invention are described:

The starting material for the loading process is an aqueous pulp suspension, in particular pulp suspension with 0.1 to 30% consistency. This is mixed with calcium oxide or hydroxide in liquid or solid form. The calcium hydroxide is suspended to a large extent in the form of fine solid particles and is brought into contact with the surface of the fibers. Because the Fibers have the form of at least one side open elongated hollow bodies, the calcium hydroxide can also penetrate into the interior of the fibers, the so-called lumen.

In the direct or indirect subsequent chemical reaction to form the calcium carbonate gaseous CO ₂ is preferably initiated, it can be assumed that the distribution of CO ₂ in the aqueous suspension very quickly and easily progresses, so that wherever Calcium hydroxide is located, a Calciumcarbonatausfällung will take place. Depending on the process, it may be necessary to thicken the pulp suspension beforehand.

In particular embodiments, particularly where multiple reactors are used, additional calcium hydroxide or calcium oxide in aqueous and / or solid form may also be mixed into the reactor, e.g. be added as lime milk.

Preferably, the process temperature is between generation of the calcium carbonate + 15 ⁰ C and 130 ° C, in particular between 20 ° and 60 ° C.

The formation of the calcium carbonate is preferably carried out in a pressure range between 0.1 and 6 bar, in particular between 1 and 4 bar. There are also higher pressures, e.g. up to 20 bar possible. The average residence time of the substance in the reactor is between 60 seconds and 15 minutes, in particular between 5 and 10 minutes.

According to a further embodiment of the method according to the invention, the solids concentration of the pulp suspension provided for reaction with carbon dioxide is selected in a range from about 0.1 to about 40% and preferably in a range from about 0.5 to 30%.

For example, crystals of a rhombohedral form may be used Particle size fraction in a range of about 0.02 to about 5 microns are generated. In certain cases, it is also advantageous to use crystals of a scalenohedral form or agglomerates having a respective length in a range from about 0.02 to about

5 microns and a respective diameter in a range of about 0.02 to about 5 microns to produce.

The invention and its advantages will be explained with reference to drawings. Showing:

Fig. 1 is a plant diagram for illustrating the method according to the invention; Fig. 2 system diagram for a varied method.

In Fig. 1, a favorable embodiment of the method according to the invention is illustrated by means of a plant scheme. From the raw materials 9, a pulp suspension 1 is formed in the pulper 10. The dissolution in pulper 10 should be so complete that practically all fibers are separated. Otherwise, it would be advisable to use a de-stuffer downstream of the pulper 10. A mixing vessel 2 with stirrer 28 serves to provide the fiber suspension 1 with calcium oxide or calcium hydroxide, e.g. as here in the form of milk of lime MOL, to mix and also to allow a defined residence time. The lime milk MOL is provided by a lime leaching device 3 and conveyed by a pump 11. The attachment of the added or formed calcium hydroxide to the fibers is favored by the addition of anionic substances, which is indicated with a chemical container 22 and corresponding dosage. The anionic substances are preferably added before the calcium oxide or hydroxide. They are composed in particular of salts, polyamides, retention aids or starch particles.

The thus treated mixture 7 is then here with a consistency between 0.1 and

6% passed into a reactor 5, is added to the gaseous carbon dioxide CO ₂ . It is provided in a CO ₂ supply device 16, wherein the

Possibility exists to heat it in a CO ₂ heater 17. A suitable reactor 5 is for example a closed container provided with a stirrer 27. Due to the movements of the contents, the reactants can easily come into contact.

The loaded pulp then passes into a finishing unit, e.g. a storage tank 13. From there it is pumped for further processing, ultimately for the production of paper or cardboard.

The plant scheme of Fig. 2 shows a variant of the method. The formation of the pulp suspension and its treatment with anionic substances can be carried out similarly as described in FIG. As a possible variant, the addition of at least part of the milk of lime MOL into the pulper 10 is indicated here (dashed line). If the solution is dissolved in batch mode, the lime milk MOL can later be added to pulper 10 as the anionic substances. The mixture 7 enters the press 4, in which the consistency is set to a value, for example, is up to 30%. The pressed filtrate 14 flows into the press water tank 8 and can be conveyed via a filtrate pump 12 to the desired locations. Since a part of the calcium hydroxide also goes into the filtrate 14 during pressing, it is advantageous to feed it back into the process upstream, for example into the pulper 10. The dewatered pulp S is then fed into a reactor 6 at a consistency of between 6% and 30%. Eventually there will be another addition of calcium hydroxide in the form of milk of lime (MOL). A rotating coil 15 transports the substance through the reactor 6. In addition, gaseous carbon dioxide CO _{2 is} added. It is provided here in a CO ₂ supply device 16, wherein it is also possible to heat it in a CO ₂ heater 17. A suitable reactor 6 is, for example, a closed screw conveyor, which is preferably operated so that it is filled to a maximum of 80% - VoI, preferably 50% -vol, with a moist solid, wherein the water volume of the pulp is included. The remaining volume in the reactor 6 is occupied by gas or steam or spray liquid. Due to the movements of the contents, the reactants can easily come into contact.

In cases where the reactor 6 is operated at a pressure from the Ambient pressure deviates, that is, for example, between absolutely 1, 1 and 5 bar, a seal at the inlet 18 and outlet 19 is usually required, for example in the form of known rotary valves. Of course, there are other possibilities, such as a plug formation with the help of a screw plug.

Claims

claims

1. A method for loading fibers with filler, in particular with calcium carbonate, wherein calcium oxide or calcium hydroxide is introduced in liquid or dry form into an aqueous fiber and this mixture thus formed (7) in at least one reactor (5, 6) with carbon dioxide, in particular is mixed with gaseous carbon dioxide, wherein filler, in particular calcium carbonate is formed by chemical reaction with calcium hydroxide, characterized in that anionic substances are added to the pulp, in particular salts, polyamides, retention aids or starch particles or a mixture of these substances.

2. The method according to claim 1, characterized in that the anionic substances are added to the pulp prior to the introduction of the calcium oxide or calcium hydroxide.

A method according to claim 1 or 2, characterized in that the electrical charge in the cavities of the fibers is set more negative than at the outer surfaces of the fibers.

4. The method according to claim 3, characterized in that at least a part of the anionic substances removed from the outer surfaces of the fibers again, in particular washed off.

5. The method according to any one of the preceding claims, characterized in that the calcium hydroxide in the form of lime milk (MOL) is added to the pulp.

6. The method according to claim 5, characterized in that the milk of lime (MOL) is ground before addition to the pulp so that the average grain size (d 50) of the calcium hydroxide particles is at most 5 microns, preferably at most 1 micron.

7. The method according to any one of the preceding claims, characterized in that as a reactor (5) for the formation of calcium carbonate, a closed vessel is used, is introduced into the gaseous carbon dioxide CO ₂ .

8. The method according to claim 7, characterized in that the pulp suspension in the reactor (5) is stirred by a mechanically driven rotor.

9. The method according to claim 7 or 8, characterized in that the reactor (5) is operated at a pressure corresponding to the ambient pressure with a tolerance of a maximum of 10% deviation.

10. The method according to claim 7 or 8, characterized in that the reactor (5) is operated at a pressure which is between 1 and 6 bar, preferably 1 to 4 bar.

11. The method according to any one of the preceding claims, characterized in that the reactor (5) for the formation of calcium carbonate at a temperature between 15 ° and 130 ° C, preferably 20 ° to 60 ° C, operated.

12. The method according to any one of the preceding claims, characterized in that the pulp suspension (1) is formed by a dissolution process in a pulper (10) with admixing of water (W) and that the addition of the anionic substances takes place in this dissolution process.

13. The method according to any one of the preceding claims, characterized in that the pulp suspension (1) is formed from fresh pulp.

14. The method according to any one of the preceding claims, characterized in that the pulp suspension is used after the calcium carbonate forming reaction for paper or board production.