WO2008154970A1 - Method for forming filler material, particulary calcium carbonate in a fibrous suspension - Google Patents

Abstract

The invention relates to a method for forming filler materials, particularly calcium carbonate, in a fibrous suspension. Calcium hydroxide is brought into contact with an aqueously coated fibrous material and said mixture is ground. Gaseous carbon dioxide is then added and calcium carbonate is formed in a chemical reaction.

Description

 Process for forming fillers, especially calcium carbonate in one

Fibrous suspension

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

The use of such mineral fillers in paper and board production has long been known. They serve 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 process 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 simultaneously subjected to a shear mixing process to form a fibrous material having a considerable 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 fibrous 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 used in a paper machine.

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 this paper is compared with conventionally produced papers, the same and / or higher filler contents show higher and / or equal strengths, porosity and specific volume (which may also be targeted) can be set lower), opacity and printability. Depending on the degree of loading of the pulp, this type of fiber loading can increase the productivity of the paper machine and / or make its production more cost-effective (through, for example, raw material energy cost reduction).

The following describes typical process steps and parameters for loading a pulp suspension:

The starting material for the loading process is an aqueous wetted pulp material, in particular pulp material with 0.1 to 30% consistency.

This is introduced together with calcium hydroxide in at least one reactor. The addition of gaseous or dissolved carbon dioxide forms calcium carbonate. In particular embodiments, additional

Calcium hydroxide or calcium oxide in aqueous and / or solid form are also mixed into the reactor.

Preferably, the process temperature is between generation of the calcium carbonate + 15 ⁰ C and 130 ° C ₁ 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.

For example, crystals of a rhombohedral shape having a grain size fraction in a range of about 0.02 to about 5 μm may be produced. In certain cases, it is also advantageous to produce crystals of a scalenohedral shape or agglomerates having a respective length in a range of about 0.02 to about 5 μm and a respective diameter in a range of about 0.02 to about 5 μm , It is the object of the invention to provide a method for loading a pulp suspension, in which the advantages of the loading process at least remain intact and which is even more economical, in which in particular the energy requirement decreases.

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

It is known that fibrous materials - especially when it comes to fresh pulp - only obtained by a grinding process, the properties that are required in order to produce paper or cardboard with the required quality can. It must be taken into account for grinding a considerable energy consumption, which increases the cost of paper production. With the aid of the method according to the invention, the grinding is carried out after the addition of calcium hydroxide, preferably in the form of lime milk. This makes it easily possible to carry out the grinding in the alkaline range, i. with a pH above 7, which both improves the effect of milling and reduces the required energy input. In addition, the calcium hydroxide particles can be crushed in the same grinding process, creating better conditions for the subsequent loading of the pulp. The pure specific refining work, that is the work done, after deducting the idling power of the grinding machine, based on the pulp (otro), may advantageously be higher than 50 kWh / t in order to achieve the desired fiber changes (e.g., increase in strength). If the raw material is unmilled fresh pulp, much higher values are possible. Just then, the energy savings of the inventive method significantly.

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 variant of the method shown in Fig. 1. In Fig. 1, a favorable embodiment of the method according to the invention is shown in a plant scheme. From the raw materials 9, eg fresh pulp, a pulp suspension 1 is formed in the pulper 10, which then passes into a Ableerbütte 2. In the pulper and / or in the Ableerbütte 2 calcium hydroxide is liquid or dry, for example as here in the form of lime (MOL) added. To produce the milk of lime MOL Kalklöscheinrichtung 3 is provided. If necessary, the milk of lime can be ground in a grinding device 20, for example a ball or agitator ball mill.

The pulp suspension mixed with calcium hydroxide is then milled at a consistency of between 2 and 8% in a refiner 7, preferably in an alkaline medium. After this grinding, the pH is lowered by adding, for example, sulfuric acid. For this purpose, a chemical dosage 11 is present. Thereafter, the suspension is fed into a reactor 5. In these gaseous carbon dioxide CO _{2 is} added. The gas is provided here in a CO ₂ supply device 16, whereby it is also possible 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. It can then be advantageously processed without further or only with weak grinding ("Egalisiermahlung" with pure specific grinding below 40 kWh / t) to paper or cardboard.

The plant scheme of Fig. 2 shows a variant of the method. The formation of the pulp suspension and its pretreatment takes place as described in the example of FIG. Thereafter, it does not immediately pass into a reactor, but first in a 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 the Pressing a part of the calcium hydroxide is in the filtrate 14, it is advantageous to supply this upstream in the process, for example in the pulper 10 again. 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, we added gaseous carbon dioxide CO ₂ . It is provided here in a CO ₂ supply device 16, whereby 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 in which the reactor 6 is operated at a pressure which deviates from the ambient pressure, 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. The seal at the outlet 19 may be omitted if the subsequent apparatus, such as another reactor, is operated at the same pressure. In the example shown in FIG. 2, another reactor 5 ^' , which is similar to the reactor 5 shown in FIG. 1, follows. In many cases, however, a single reactor is sufficient.

Claims

1. A process for the formation of filler, in particular calcium carbonate, wherein calcium oxide or calcium hydroxide is introduced in liquid or dry form into an aqueous wetted pulp and the pulp in at least one reactor (5, 5 ^' , 6) mixed with carbon dioxide, in particular with gaseous carbon dioxide is, wherein filler, in particular calcium carbonate, is formed by chemical reaction, characterized in that the pulp is milled together with the calcium hydroxide before adding the carbon dioxide. 2. The method according to claim i, characterized in that during the grinding a pure specific grinding of at least 50 kWh / t, preferably at least 100 kWh / t, is transmitted. 3. The method according to claim 1 or 2, characterized in that the calcium hydroxide in the form of lime milk (MOL) is added to the pulp. 4. The method according to claim 3, characterized in that the milk of lime (MOL) is ground before addition to the pulp so that the average particle size (d 50) of the calcium hydroxide particles is at most 5 microns, preferably at most 1 micron. 5. The method of claim 1, 2, 3 or 4, characterized in that the grinding is carried out at a pH between 8 and 12. 6. The method according to any one of the preceding claims, characterized that the milling of the pulp is carried out at a consistency of between 3% and 8% in a refiner (7). 7. The method according to any one of claims 1 to 5, characterized in that the grinding of the pulp is carried out at a consistency between 8% and 30% in a high consistency refiner or in a disperser. 8. The method according to any one of the preceding claims, characterized in that after the grinding, a reduction of the pH is carried out. 9. The method according to claim 8, characterized in that the pH is lowered by the addition of sulfuric acid. 10. The method according to any one of the preceding claims, characterized in that the pulp suspension (1) by a dissolution process in a pulper (10) is formed with admixing of water and that the addition of calcium oxide or calcium hydroxide in this dissolution process and the grinding of the pulp suspension then takes place. 11. The method according to any one of the preceding claims, characterized in that as a reactor (5, 5 ^' ) for the formation of calcium carbonate, a closed vessel is used, is introduced into the gaseous carbon dioxide CO ₂ . 12. The method according to claim 11, characterized in that the pulp suspension in the reactor (5, 5 ') by a mechanical is driven driven rotor. 13. The method according to any one of the preceding claims, characterized in that a reactor operated with a higher consistency (6) is used, in which the chemical reaction for the formation of calcium carbonate, is triggered, wherein the pulp therein is a consistency between 6 and 30 %, preferably 10% to 25%. 14. The method according to claim 13, characterized in that a closed screw conveyor is used as a reactor (6), in which the moist solid occupies at most 80%, preferably at most 50% of the volume. 15. The method according to any one of the preceding claims, characterized in that at least one reactor (5, 5 ^' , 6) is operated at a pressure corresponding to the ambient pressure with a tolerance of a maximum of 10% deviation. 16. The method according to any one of the preceding claims, characterized in that at least one reactor (5, 5 ^' , 6) is operated at a pressure which is between 1 and 6 bar, preferably 1 to 4 bar. 17. The method according to any one of the preceding claims, characterized in that the reactor (5, 5 ^' , 6) with a temperature between 15 ° and 130 ° C, preferably 20 ° to 60 ° C, operated. 18. The method according to any one of the preceding claims, characterized in that the average residence time of the pulp suspension (1) in the reactor (5, 5 ^' , 6) to 1 - 15 min, preferably 5 - 10 min, is set. 19. The method according to any one of the preceding claims, characterized in that the loaded pulp contains a maximum of 50% calcium carbonate. 20. The method according to any one of the preceding claims, characterized in that the loaded fibers are not ground or only with a pure specific grinding work of at most 40 kWh / t. 21. The method according to any one of the preceding claims, characterized in that the aqueous pulp suspension (1) is formed from fresh pulp.