1. Field of the Invention
The present invention relates to a method for loading fibers contained in a pulp suspension, and more particularly to a method of loading cellulose fibers for producing paper or board.
2. Description of the Related Art
In the manufacture of paper, fillers, in particular precipitated calcium carbonate (PCC) or crushed or ground calcium carbonate (GCC), are standard substances which are used to reduce the fiber content and to improve the optical properties of the paper.
Commercially available PCC or GCC fillers are products produced in bulk in special manufacturing facilities which can be associated with a paper mill in the form of a satellite plant. The online production of PCC has, however, never been, nor is it now, considered in the paper industry because of the special process necessary to produce PCC. Instead, PCC or GCC is typically provided to the paper mills as bulk material or in the form of a suspension.
The loading with an additive, such as a filler can be effected, by way of a chemical precipitation reaction, in particular by what is called a “Fiber Loading™” process, as described, inter alia, in U.S. Pat. No. 5,223,090. In a “Fiber Loading™” process at least one additive, in particular a filler, is incorporated onto the wetted surfaces of the fibrous material. In this process the fibers can be loaded with calcium carbonate, calcium oxide and/or calcium hydroxide are added to the wet, disintegrated fibrous material in such a way that at least part thereof associates with the water which is present in the fibrous material. Carbon dioxide is then added to the treated fibrous material. Upon addition of the medium containing calcium oxide and/or calcium, which hydroxide (preferably in liquid form (milk of lime)) to the pulp suspension, a chemical reaction with exothermic properties takes place. This means that the water incorporated in or attached to the fibrous material of the pulp suspension is not absolutely necessary for starting and running the chemical reaction.
An additional method for loading additive or filler is known from FR 2831565 in which a suspension of mechanically produced, bleached wood pulp fibers (“pate a papier mecanique blanchie”) is loaded with calcium carbonate. Therein, milk of lime is added to form the fiber suspension and a consistency of above 10% is set. After diluting the suspension, the crystallization of calcium carbonate is triggered by the addition of gaseous carbon dioxide.
An additional method for loading with filler is disclosed by WO 03/066962. Therein, the result is further improved by a special grinding operation.
Upon loading of the fibers with filler, calcium carbonate (CaCO3) can be incorporated onto the wetted fiber surfaces by adding calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)2) to the moist fibrous material, at least a portion thereof being able to associate with the water of the mass of fibrous material. Carbon dioxide (CO2) can then be applied to the fibrous material thus treated.
The term “wetted fiber surfaces” may cover all the wetted surfaces of the individual fibers. This also includes the case in which the fibers are loaded both on their outer surfaces and in their interiors (lumen) with calcium carbonate or any other desired precipitation product.
Accordingly, the fibers may be loaded with the filler calcium carbonate, the addition to the wetted fiber surfaces taking place by way of what is called a “Fiber Loading™” process, which is described as such in U.S. Pat. No. 5,223,090. In this “Fiber Loading™” process, the carbon dioxide reacts with the calcium hydroxide (in liquid or dry form) to form water and calcium carbonate.
What is needed in the art is an economic method of processing fibers for the production of a material web.
It is the object of the present invention to provide a method for loading fibers conformed in a pulp suspension, which is more economic than the known methods.
Suitable starting materials with which the method can be performed are in principle any cellulose fibers suitable for producing paper or board. In one embodiment of the present invention, fiber loading can be carried out with chemically produced pulp or with de-inked waste paper. The latter material, termed “dip”, similar to chemical pulp or waste paper, can be supplied to the paper mill in an air-dry state and then be pulped and loaded in accordance with the present invention. With integrated dip production, printed waste paper is pulped, de-inked and loaded according to the present invention (see
The present invention affords the possibility of adding the calcium oxide (CaO) and/or calcium hydroxide (CaOH2) to the fibrous material in a state in which it is of relatively high consistency, e.g. air-dry or moistened. Before introduction into the pulping apparatus and therein, the pulp is relatively absorbent, therefore, chemicals added therein reach the wetted surface of the fibers more easily and quickly and penetration into the cavities of the fibers takes place correspondingly more quickly and more favorably. Since liquid, in particular water, is added in the pulping apparatus, typically such as a pulper or a pulping drum, this process step still remains simple and clear. It is generally necessary to meter in the calcium oxide or hydroxide. It is also possible to design the pulping apparatus such that the precipitation of calcium carbonate can already be triggered therein by adding at least a portion of the carbon dioxide to the apparatus. It may, for example, be dissolved in water. Alternatively, the pulping apparatus is temporarily closed and supplied with gaseous carbon dioxide. In such cases, a steeping section before the pulping apparatus is used in which the calcium hydroxide (milk of lime) is sprayed on or is added in a steeping drum.
A further advantage of the present invention is that under certain circumstances it is possible to economize by omitting a machine for thickening, such as the disintegrated pulp during the puling operation is diluted only to such an extent that the subsequent loading can be carried out. In other cases, in particular should greater demands in terms of cleaning of the stock be necessary (waste paper) or should separation of the fibers not be adequately achieved otherwise, it is also possible that thickening, such as in the form of a screw press, may be necessary. The filtrate from such thickening is returned directly into the pulping apparatus in a short circuit, to serve for dilution therein. This has the decisive advantage that the calcium hydroxide or calcium oxide is used optimally, as not only the water, but also the chemicals can be recirculated.
In many cases, the pulping operation is carried out not only with water, but also with the addition of pulping chemicals, in particular sodium hydroxide solution, in order to provide a basic medium. Here too, the present invention has advantages since the sodium hydroxide solution can be replaced by calcium hydroxide, which is required anyway for the precipitation reaction resulting in calcium carbonate which is carried out later.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrates one embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring now to the drawings, and more particularly to
Pulp suspension S′ thus processed can then be used for producing paper 4, which generally requires a number of additional steps, not shown here. In paper production stage 4, white water is produced, which is suited to being used again, in part, as returned water 9′ in pulping operation 1. Thus, not only is the water used again, but also any remaining calcium hydroxide contained therein.
Now, additionally referring to
Now, additionally referring to
The loaded pulp suspension is collected in a storage chest 26, possibly diluted with water 27 and is then available as a pulp suspension 28 for a papermaking machine 29.
The part of the plant shown in
In that case, at least one of the following apparatus and/or components may be used for the online process: HC cleaner, static mixer, lime slaking apparatus, press, (in particular screw press or belt press), compensating reactor, crystallizer, a further static mixer, CO2 supply device or additional CO2 recovery device, optional CO2 heater, optional addition of chemical bleaching agent, and a press-water tank.
The formation of crystalline precipitation particles results in the advantage that higher gloss values can be achieved for the end product.
According to one embodiment of the present invention, the press filtrate 9 is used, at least partially, as dilution water in crystallizer 20.
Mixing device 25 may be a static mixer and may be used in particular for fine adjustment of the pH value of the pulp suspension, preferably within a range of between 6 and 10, or between 6 and 8, or even between 6.5 and 8.5.
A further embodiment of the present invention is that at least a portion of the required CO2 is provided by a CO2 recovery system. Thus, for example, it can be recovered from flue gas from boilers or flue gas from power stations.
Precipitation product particles of rhombohedral form, with a respective cube size within a range from about 0.05 to about 2 μm may be produced. In certain cases it is also advantageous to produce precipitation particles of scalenohedral form with a respective length within a range from about 0.05 to about 2 μm and a respective diameter within a range from about 0.01 to about 0.05 μm.
According to one embodiment of the present invention the solids concentration of the pulp suspension provided for precipitation is set in a range from about 5 to about 60% and preferably in a range from about 10 to about 35%.
According to one practical embodiment of the loading method, the carbon dioxide of the pulp suspension is added at a temperature in a range from about −15 to about 120° C. and preferably in a range from about 20 to about 90° C.
The paper manufactured may, therefore, contain fillers of a size of about 0.05 to about 5 μm, which increases the optical properties of the end product. The filler may be calcium carbonate, which occurs in nature in the form of calcite or calcspar, aragonite and, in the rarer form, vaterite. The filler may consist mainly of the form calcite, of which over 300 different crystal forms are said to exist. The form of the filler particles used may be, for example, rhombohedral with a respective cube size in a range from about 0.05 to about 2 μm or scalenohedral with a respective length in a range from about 0.05 to about 2 μm and a respective diameter in a range from about 0.01 to about 0.05 μm, depending on the type of paper to be manufactured in each case.
The filler is well distributed on and throughout the fibers, which means that no agglomeration of crystals in bundles is encountered. The respective filler particle, namely the crystal, is provided individually spaced apart or separated on the fiber. The filler particle covers the fiber by attaching to the fiber, which improves the optical properties of the end product. The particle size is therefore essential for achieving optimal opacity. A high opacity is achieved when the colour spectrum of visible light is well scattered. If the colour spectrum is absorbed, the colour black is produced. If the size of the filler particles drops below 0.2 to 0.5 μm, there is a tendency to transparency and higher gloss.
To achieve good loading results, the process for producing the filler crystals may be designed as follows and have the following variables:
The pulp suspension mixed beforehand with Ca(OH)2 is passed to crystallizer 20, such as in a fluffer, refiner, disperger or the like at a consistency or solids concentration in a range from about 5 to about 60%, preferably in a range from about 10 to about 35%. CO2 is then supplied to the pulp suspension. The CO2 can be added at temperatures in a range between about −15 and about 120° C., preferably at temperatures in a range between about 20 and about 90° C.
The pulp suspension passes into the gas zone of the crystallizer 20, where each individual fiber is exposed to a gas atmosphere, followed by the precipitation reaction, which directly yields the CaCO3. The form of the CaCO3 crystals can be rhombohedral, scalenohedral or spherical. The amount of crystals is dependent on the selected temperature range for the pulp suspension and on the CO2 and Ca(OH)2 content in the pulp suspension. Once the pulp suspension with the formed crystals has passed through the gas zone, the PCC formed or the pulp suspension with the crystals in the lumen, on the fiber and between the fibers is passed through a rotor and a stator, where the distribution of the crystals in the pulp suspension is concluded by mixing with low shearing.
While the pulp/crystal suspension passes through the rotor, a shear distribution occurs which brings about a size distribution of the crystals of about 0.05 to about 0.5 μm, preferably from about 0.3 to about 2.5 μm.
The form of the filler particles used is rhombohedral with a respective cube size in a range from about 0.05 to about 2 μm or scalenohedral with a respective length in a range from about 0.05 to about 2 μm and a respective diameter in a range from about 0.01 to about 0.5 μm, depending on the type of paper to be manufactured.
The further the pulp suspension has to meet the rotor disc, the less is the shearing, depending on the H2O added for dilution. The concentration of the pulp suspension passing through the rotor disc is about 0.1 to about 50%, preferably about 35 to about 50%.
The pressure acting on the CO2 feed line is in a range from about 0.1 to about 6 bar, preferably in a range from about 0.5 to about 3 bar, in order to ensure a constant CO2 supply to the gas ring for the desired chemical reaction. As with supplying water via a garden hose, if the demand for water is high the pressure should be increased to convey more through the hose. Since the CO2 is a compressible gas, the necessary amount can also be increased to ensure a complete reaction. The CO2 supply, and hence the precipitation reaction producing the CaCO3 can be controlled by open and/or closed-loop control by way of the pH value.
For example, pH values in a range from 6.0 to about 10.0 pH, preferably in a range from about 7.0 to about 8.5 pH, can be considered for the final reaction of the CaCO3 crystals. The energy used for this process can lie in a range between about 0.3 and about 8 kWh/t, preferably in a range between about 0.5 and about 4 kWh/t. Dilution water can be added and mixed with the pulp suspension in order to obtain a final dilution in which the pulp suspension with filler which is produced has a consistency or solids concentration in a range from for example about 0.1 to about 16%, preferably in a range from about 2 to about 6%. The pulp suspension is then exposed to the atmosphere in a machine, a tank or the next process machine.
The speed of rotation of the rotor disc can be in a range from about 20 to 100 m/s, preferably in a range from about 40 to about 60 m/s at the external diameter. The gap between rotor and stator is, for example, about 0.5 to about 100 mm, preferably about 25 to about 75 mm. The diameter of the rotor and of the stator can be in a range from about 5 to about 2 m.
The reaction time is preferably in a range from about 0.001 to 1 min, preferably in a range from about 0.1 to about 10 sec.
The method described above permits the production of individual particles which are spaced apart equally from each other and are attached to the fibers, covering the fibers in the necessary manner to meet the requirements for the desired high degree on white or gloss paper. The particle size is preferably in a range from about 0.05 to about 5 μm, the preferred size for the rhombohedral form of a cube being in a range from about 0.05 to about 2 μm or for a scalenohedral form with respect to the length in a range from about 0.05 to about 2 μm and with respect to the diameter in a range from about 0.01 to about 0.5 μm. For high-gloss applications, the particle size should expediently be below 0.2 to 0.5 μm.
While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
Number | Date | Country | Kind |
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10 2005 032 383.9 | Jul 2005 | DE | national |
This is a continuation of PCT application No. PCT/EP2006/004922, entitled “METHOD FOR LOADING FIBERS CONTAINED IN A PULP SUSPENSION”, filed May 24, 2006, which is incorporated herein by reference.
Number | Date | Country | |
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Parent | PCT/EP2006/004922 | May 2006 | US |
Child | 11970219 | US |