WO2009000345A1 - Method of forming fillers, especially calcium carbonate, in a suspension - Google Patents

Abstract

The method is used to form filler, especially calcium carbonate, in a predominantly aqueous liquid, optionally also in a pulp suspension. Calcium hydroxide is combined with gaseous carbon dioxide, forming calcium carbonate in a chemical reaction. In the course of this reaction, crystallization nuclei (1) in the form of fines, or anionic substances, such as, in particular, salts, polyamides, retention agents or starch particles, or a mixture of these substances, promote the formation of relatively large particles.

Description

 VOITH PATENT GmbH RPL13219 WO

Process for forming fillers, especially calcium carbonate in one

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 chemical reaction and the starting materials provided therefor can be adjusted to form a high quality filler, e.g. in terms of its whiteness and opacity can meet particularly high requirements. The crystals formed are usually relatively fine. The fillers produced in this way can then be added to the fiber raw material in the paper mill.

Another development is to initiate the process of precipitation in the presence of the pulp, that is to say in a mixture in which the fibers destined for papermaking are already contained. Such methods are called fiber loading methods. 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 at the same time subjected to a shear mixing process to produce a fiber material with a substantial amount of calcium carbonate on the fiber surface, in the hollow interior and within the fiber walls of the cellulose fibers.

This loading of the fibers with fillers 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). Another advantage of using this technology is that it can be easily processed 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.

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%.

It is the object of the invention to provide a process for the production of fillers in which relatively large filler crystals are formed. It should be particularly economical and, in specific embodiments, reduce the problems of pulp problems of paper production.

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

With the method it is possible to also produce such calcium carbonate particles, which are larger than usual methods. With the help of the crystallization nuclei, a crystallization is triggered in which accumulates calcium carbonate around the crystallization nuclei. (The calcium carbonate can form clusters). If the process is carried out favorably, the effect of this calcium carbonate addition is driven so far that a complete envelopment of the crystallization nuclei takes place. The crystallization nuclei which can be used here are not necessarily ultrafine particles, as they are usually used for triggering and controlling crystal growth. Rather, it is advantageous that the size of the crystallization nuclei can be adjusted very easily by suitable measures. Since these are added substances, they can first be adjusted to the desired size or the desired size spectrum by selection of the raw materials and then by appropriate treatment, for example milling.

The crystallization nuclei suitable for the process can be procured relatively inexpensively, preferably they are negatively charged fines (e.g., dead-milled fiber fragments) or fine contaminants, such as are produced in the stock preparation in front of the paper or board machine anyway. But it can also be e.g. fine wood particles, polyamides, retention aids, anionic starches or anionic salts. In many stock preparation plants, especially when waste paper is processed, there are anyway enough fine-particle-like residues, which often have a disturbing influence on the production or quality of the paper produced. Such particles are e.g. in wash filtrates, rejects on cleaners or sorters, or the filtrates of reject thickening. These can then instead of being disposed of, which in turn would cause costs, are used as crystallization nuclei in the course of the process according to the invention. As a result of the sheathing by calcium carbonate, they are not only harmless, but contribute because of their filler character even to the quality of the paper later produced from the fibers significantly.

The following describes typical process steps and parameters for the process according to the invention: The starting material used for the process is either a fiber-free water-containing liquid with 0.1 to 30% solids or a pulp suspension if the carbonate is to be produced in the presence of fibers. The liquid is introduced into a reactor together with the crystallization seeds and calcium hydroxide. 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 may also be mixed into the reactor.

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.

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 specific application of the method.

In Fig. 1, a favorable embodiment of the method according to the invention is shown in a plant scheme. Water-containing liquid (water W) is provided in a container 2 and mixed with crystallization seeds 1. In addition, calcium hydroxide is liquid or dry, for example, as here in the form of milk of lime (MOL), added to the container 2. To produce the milk of lime MOL Kalklöscheinrichtung 3 is provided. If necessary, the milk of lime can be ground in a grinding device 14, for example a ball or agitator ball mill. The liquid 4 with water, crystallization nuclei and calcium hydroxide is dissolved in pumped a reactor 5.

In the reactor 5 gaseous carbon dioxide CO _{2 is} added. The gas 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 7. Due to the movements of the contents, the reactants can easily come into contact, so that the calcium carbonate-containing filler suspension 6 is formed.

The process steps for the chemical reaction of calcium carbonate formation are shown here only by way of example. There are also other reactor types, other admixing methods (e.g., static mixers), and other reactant addition sites.

After the reactor 5, the filler suspension 6 enters a finishing unit, e.g. a storage tank 13. In this it can be mixed with an aqueous suspended pulp 12, which comes from the stock preparation plant. The mixture 15 thus formed is then used in another part of the plant not shown here for paper or board production.

FIG. 2 shows an application of the method in which the crystallization nuclei 1 originate from the wash filtrate of the stock preparation plant for the fiber suspension S1. The latter is shown only in part, namely a hydrocyclone 8, a sorter 9 and a washing device 10. In the washing apparatus 10, a fine-containing filtrate 11 and a washed-out fibrous material 12 ^'are formed in a known manner. The filtrate 11 can be fed directly into the container 2, since it contains fines, which can serve as crystallization nuclei in carrying out the method according to the invention. Addition of calcium hydroxide and processing in the reactor 5 then takes place, for example, similarly as already described and shown in FIG.

The pulp 12 ^' formed in the washing device 10 can be mixed with the filler suspension 6 produced in the reactor 5, for example in the storage tank 13. In others Cases it is further processed before, for example ground or dispersed; in still other cases, it is used without mixing with the filler suspension 6 on.

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 a predominantly aqueous liquid and this mixture thus formed mixture (4) in at least one reactor (5) with carbon dioxide, in particular with gaseous carbon dioxide is, wherein filler, in particular calcium carbonate, is formed by chemical reaction with calcium hydroxide, characterized in that the mixture (4) in the chemical reaction contains crystallization nuclei (1) in the form of fines or anionic substances, 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 total proportion of the crystallization nuclei at least 5 wt.%, Preferably at least 10 wt.%, Which is brought to the chemical reaction amount of calcium hydroxide. 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 aqueous liquid. 4. The method according to claim 3, characterized in that the milk of lime (MOL) is ground before addition to the aqueous liquid so that the average particle size of the calcium hydroxide particles is at most 100 microns. 5. The method of claim 1, 2, 3 or 4, characterized in that at least 30 wt.%, Preferably 10 wt.%, The Crystallization nuclei are residues from paper production. 6. The method according to claim 5, characterized in that for the supply of crystallization seeds (1) return water from the paper production, in particular from the thickening in the stock preparation or from the white water of the paper machine is used. 7. The method according to any one of the preceding claims, characterized in that for the supply of crystallization seeds (1) filtrates from the Paper production can be used with a solids content below 0.2%, preferably below 0.1%. 8. The method according to any one of the preceding claims, characterized in that for the supply of crystallization seeds (1) fine particulate impurities from the stock preparation, in particular rejects of the separation devices are used. 9. 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 . 10. The method according to claim 9, characterized in that the pulp suspension in the reactor (5) is stirred by a mechanically driven rotor. 11. The method according to claim 9 or 10, characterized the reactor (5) is operated at a pressure which corresponds to the ambient pressure with a maximum tolerance of 10% deviation. 12. The method according to claim 9 or 10, characterized in that the reactor (5) is operated at a pressure which is between 1 and 6 bar, preferably 1 to 4 bar. 13. 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. 14. The method according to any one of the preceding claims, characterized in that the filler suspension (6) after the calcium carbonate-forming reaction is added to a pulp intended for paper production. 15. The method according to any one of the preceding claims, characterized in that as the predominantly aqueous liquid, a pulp suspension is used with a consistency of between 1% and 30%, to which the crystallization nuclei (1) are added. 16. The method according to claim 15, 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 the crystallization nuclei in this dissolution process or subsequently. 17. The method according to claim 14, 15 or 16, characterized in that the pulp suspension (1) is formed from fresh pulp.