WO2008128595A1

WO2008128595A1 - Method for forming filler materials, particularly calcium carbonate in a fibrous material suspension

Info

Publication number: WO2008128595A1
Application number: PCT/EP2008/001389
Authority: WO
Inventors: Uwe Brand; Christian Naydowski; Stefan Scholl; Johannes Trummer; Andreas Arnhold; Ingo Kauer; Christian Steinmassl
Original assignee: Voith Patent Gmbh
Priority date: 2007-04-20
Filing date: 2008-02-22
Publication date: 2008-10-30
Also published as: DE102007018726A1

Abstract

The invention relates to a method for forming filler materials, particularly calcium carbonate in a fibrous material suspension. In a special embodiment, calcium hydroxide is brought into contact with an aqueously coated fibrous material and gaseous carbon dioxide is added, whereby the filler material, particularly calcium carbonate, is formed by a chemical reaction. In at least one reactor (5) used for said reaction, the consistency of the fibrous material suspension is set to between 0.1 and 6%. In particularly advantageous embodiments of the invention, a plurality of reactors operated under different conditions is used.

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).

It is the object of the invention to provide a process for loading a pulp suspension in which the advantages of the loading process are increased, in particular by increasing the amount of fillers fixed in and / or on the fiber.

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

With the aid of the new process, the production of calcium carbonate in special crystal forms can be realized, i. So be optimally matched to the intended use of the paper or cardboard produced from it later. In particular, due to the relatively low consistency between 0.1 and 6%, preferably between 1 and 5%, a large amount of water is provided, which is mixed with carbon dioxide. The calcium carbonate produced under these conditions tends to crystallize largely as a skaleoneder, thus has the form of elongated crystals. Another advantage of the relatively low consistency is the easier penetration of the reactants into the fiber interior (lumen). As a result, more calcium carbonate can also be produced in the fibers (lumen loading).

A particularly advantageous embodiment of the method is the combination of several reactors, each of which is operated with different consistency. If, for example, a reactor operated at a lower consistency is preceded by a further one in which a higher consistency, for example 6% to 30%, is used, the reaction can proceed in such a way that preferably crystals which have the form of rhombohedra form, ie rather are compact. After passing through both reactors is then a particularly effectively loaded pulp for the paper or Cartonboard production. The order mentioned, ie first higher then lower consistency, has procedural advantages, since less has to be thickened. But it may also be that, for reasons related to the optimal formation of fillers, the reverse is true. In other cases, it may be useful to divide the pulp stream and process the streams with different consistency, ie, not the two reactors but one after the other to switch in parallel.

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 wetted pulp material, in particular pulp material with 0.1 to 30% consistency. This is registered in at least one reactor. The addition of gaseous or dissolved carbon dioxide forms calcium carbonate. In particular embodiments, especially where multiple reactors are used, calcium hydroxide or calcium oxide in aqueous and / or solid form may also be mixed into the reactor, e.g. be sprayed as lime milk.

It is also possible that the substance added to the first reactor already contains calcium hydroxide or calcium oxide, which was added in the material dissolving system, a receiver chest and / or in a static mixer, in particular a pH in the range between 5 and 12 was set.

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, eg 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 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 ,

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 process with two reactors, connected in series; Fig. 3 system diagram for a method with two reactors, connected in parallel.

The following elements are used in the system diagram of FIG. 1: A pulper 10 for the unloaded pulp suspension 1, mixing device, in particular feed tank _2, lime leaching device 3, reactor 5, CO ₂ supply device 16, optional CO ₂ heater 17, press water tank 8.

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, which then passes into a feed tank 2 in which it is mixed with calcium hydroxide liquid or dry, for example, as here in the form of lime (MOL). It may also be advantageous to at least partially add the required milk of lime MOL to the pulper 10. The pulp suspension is then passed with a consistency between 0.1 and 6% in a reactor 5, in the 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 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.

In the system diagram of FIG. 2, the following elements are used:

A pulper 10, in particular HC pulper (high consistency pulper) for the unloaded pulp suspension 1, mixing device, in particular feed tank 2, lime lapping device 3, press 4, in particular screw press or screen press, a reactor 5, a reactor 6, CO ₂ supply device 16 , optional CO ₂ heater 17, press water tank 8.

From the raw materials 9 a pulp suspension 1 is formed in the pulper 10, which passes here via a feed tank 2 in the press 4, in which the consistency is set to a value, for example, is up to 30%. The squeezed filtrate 14 flows into the press water tank 8. The dewatered pulp is then passed with a consistency of between 6% and 30% in a reactor 6 and mixed there with calcium hydroxide, here in the form of lime milk (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, 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. It may be advantageous to add some of the required calcium hydroxide to the fibers before the reactor 6, for which purpose lime milk MOL is added to the feed tank 2. Additionally or alternatively, the milk of lime MOL can also be added to the pulper 10.

By adding, in particular spraying lime milk (MOL) into the reactor 6, the loading in the reactor 6 proceeds more efficiently. Both the carbon dioxide and the supplied lime milk MOL can impinge on already formed calcium carbonate, so that a further crystallization occurs. The process has the particular advantage that relatively much time is available for crystallization. It may initially form nuclei, which are then built up to larger crystals. In addition, a reaction to free PCC can additionally take place between milk of lime droplets and carbon dioxide. The reactions are e.g. controlled by temperature, residence time, pH and concentration of the reactants.

In cases where the reactor 6 is operated at a pressure other than the ambient pressure, e.g. between absolutely 1, 1 and 5 bar, a seal at the inlet 18 and outlet 23 is usually required, e.g. in the form of rotary valves 24. Of course, there are other possibilities, e.g. a plug formation with the help of a screw plug. The seal at the outlet 23 may be omitted if the subsequent reactor 5 is operated at the same pressure.

The loaded pulp then passes into another reactor 5, which has already been described in connection with FIG. In this reactor 5, which works with much lower consistency, for example 0.1 to 6%, differently shaped calcium carbonate crystals can be formed. Thus, the combination of these two reactors 5 and 6 enables optimum loading of the fibers. These then pass into a storage tank 13. The filtrate 14 originating from the press 4 can be added here for dilution by means of filtrate pump 12. 3 shows a variant of the method in which two reactors operated in different consistency ranges are connected in parallel, namely a reactor 5 with 0.1 to 6% and a reactor 7 with 6 to 30%.

In many cases, it is advantageous to carry out a grinding of the loaded fibers. This can be done at high consistency (8% to 25%) or at low consistency (2% to 8%). It serves e.g. to even out fibers and / or fillers.

Claims

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, 6, 7) is mixed with carbon dioxide, in particular with gaseous carbon dioxide , wherein filler, in particular calcium carbonate, is formed by chemical reaction, characterized in that in at least one reactor (5) the consistency of the pulp suspension between 0.1% and 6%, preferably 1% to 5%, is set.

2. The method according to claim 1, characterized in that the pulp suspension is mixed with calcium hydroxide in liquid form and then introduced into the reactor (5).

3. The method according to claim 1 or 2, characterized in that the pulp suspension is carried out by a dissolution process with admixing of water and that the addition of calcium oxide or calcium hydroxide takes place in this dissolution process.

4. The method according to any one of the preceding claims, characterized in that the reactor (5) is a closed vessel is used, is introduced into the gaseous carbon dioxide.

5. The method according to claim 4, characterized in that in the closed vessel, a pressure is set, which is above the ambient pressure and preferably at most 5 bar.

6. Method according to one of the preceding claims, characterized in that the pulp suspension in the reactor (5) is agitated by a mechanically driven rotor.

7. The method according to any one of the preceding claims, characterized in that the operated with a consistency between 0.1 and 6% reactor (5) operated with a higher consistency reactor (6) upstream or downstream, in which a chemical reaction for the formation of fillers, in particular calcium carbonate, is triggered, wherein the fibrous material contained therein a

Consistency between 6 and 30%, preferably 10% to 25%.

8. The method according to any one of the preceding claims, characterized in that operated parallel to the one with a consistency between 0.1 and 6%

Reactor (5) a reactor (7) is used with a higher consistency in which a chemical reaction to form fillers, in particular calcium carbonate, is triggered, wherein the pulp contained therein has a consistency between 6 and 30%, preferably 10% to 25% , having.

9. The method according to claim 7 or 8, characterized in that in the reactor operated at a higher consistency (6, 7) the thickened pulp, a calcium hydroxide-containing liquid, preferably milk of lime (MOL), is supplied.

10. The method according to any one of the preceding claims, characterized in that at least 20% and at most 50% of the calcium oxide or calcium hydroxide of the pulp suspension (1) already before the reactor (5) or the reactors (5, 6, 7) are added.

11. The method according to any one of claims 1 to 9, characterized in that at least 90%, preferably 100%, of the required calcium oxide or calcium hydroxide in the reactor (6) or the reactors (6, 7) are given, operated with a higher consistency become.

12. The method according to any one of claims 7 to 11, characterized in that the calcium hydroxide-containing liquid in the interior of the higher

Consistency operated reactor (6, 7) is sprayed.

13. The method according to claim 12, characterized in that the calcium hydroxide-containing liquid at several points within the

Reactor (6, 7) is sprayed.

14. The method according to any one of claims 7 to 13, characterized in that a closed screw conveyor is used as the reactor operated with a higher consistency (6, 7), 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, 6, 7) 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, 6, 7) is operated at a pressure, the 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, 6, 7) 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, 6, 7) 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 ground.

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