Abstract:
The present invention serves for loading a pulp suspension (S) with calcium carbonate. Liquid calcium hydroxide as milk of lime ( 7 ) is added during or before the pulp operation ( 1 ) carried out to form the pulp. By introducing gaseous carbon dioxide ( 8 ) into the pulp suspension thus treated, a chemical reaction is triggered therein, in which the finely divided calcium carbonate is precipitated. The present invention is particularly economic and effective due to the early addition of calcium oxide or calcium hydroxide.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    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. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    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. 
         [0004]    2. Description of the Related Art 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    An additional method for loading with filler is disclosed by WO 03/066962. Therein, the result is further improved by a special grinding operation. 
         [0010]    Upon loading of the fibers with filler, calcium carbonate (CaCO 3 ) 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 (CO 2 ) can then be applied to the fibrous material thus treated. 
         [0011]    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. 
         [0012]    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. 
         [0013]    What is needed in the art is an economic method of processing fibers for the production of a material web. 
       SUMMARY OF THE INVENTION 
       [0014]    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. 
         [0015]    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  FIG. 2 ). 
         [0016]    The present invention affords the possibility of adding the calcium oxide (CaO) and/or calcium hydroxide (CaOH 2 ) 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. 
         [0017]    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. 
         [0018]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    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: 
           [0020]      FIG. 1  is the method according to the invention in diagrammatic form; 
           [0021]      FIG. 2  a special embodiment of the method for processing printed waste paper; and 
           [0022]      FIG. 3  a simplified plant diagram for performing the method according to the invention. 
       
    
    
       [0023]    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. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Referring now to the drawings, and more particularly to  FIG. 1 , which illustrates an embodiment of the present invention, the pulp P, which may be waste paper or alternatively, fresh chemical pulp, is processed in pulping operation  1 . Therein, a pulp suspension S is produced by admixing water W with the aid of a mechanical working. The calcium hydroxide in this example is added as milk of lime  7  during the pulping operation, and is provided by slaking lime in the milk-of-lime production stage  5 . Optionally, the pulp suspension S may be passed through a cleaning system, not shown here, in which heavy particles and plastics or other trash are removed. Whether this is necessary will depend predominantly on the quality of the raw material. A thickening operation  2  may then take place, but is not always necessary and, therefore, is shown in broken lines. Filtrate  9  from thickening operation  2  is used for pulping operation  1 . The precipitation  3  of fine matter, in particular calcium carbonate, is triggered by addition of gaseous carbon dioxide  8 , which comes from a carbon dioxide production stage  6 . 
         [0025]    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. 
         [0026]    Now, additionally referring to  FIG. 2 , a pulp suspension S obtained by pulping  1  printed waste paper is treated by a flotation process  31  such that the ink particles and other fine anionic trash are removed as rejects  32 . It is possible during the pulping operation  1  to replace the sodium hydroxide, which was hitherto conventionally used with calcium hydroxide in the pulping operation. 
         [0027]    Now, additionally referring to  FIG. 3  there is illustrated an example of an installation with the most important process steps and devices. In this installation, the pulp P, together with water W, is introduced into a pulper  10 . This pulper  10  may be of conventional construction, i.e. contain a pulper screen  11  in its bottom region, and also a pulper rotor  12  which circulates the pulp in the pulper and keeps the pulper screen  11  free from clogging. The pulp is supplied to a plurality of cleaning devices which are represented symbolically as a pulp cleaner  14  and a pulp screener  15 . There is also a screw press  16 , in order to increase the consistency of the pulp suspension. The press filtrate  18  from screw press  16  can be returned for pulping in pulper  10 . The thickened pulp  19  then passes into a crystallizer  20 , which serves, with the aid of supplied carbon dioxide gas  24 , to trigger the desired precipitation of calcium carbonate in the form of finely-divided filler. The carbon dioxide gas is provided by a carbon-dioxide supply device  21  and can be brought to the desired temperature in a heat exchanger  22  with the aid of a cooling or heating device  23 . There are, however, also other possible methods of temperature control, such as the direct addition of steam if it is expedient to increase the temperature. If necessary, a portion of the carbon dioxide gas  24 ′ may also be added after crystallizer  20  using a mixing device  25  in order to complete the loading operation. 
         [0028]    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 . 
         [0029]    The part of the plant shown in  FIG. 3  for loading with fillers, in particular calcium carbonate, should be regarded only as a simplified example. In many cases, a bleaching operation may also be integrated therein. The present invention allows for the loading process then to be operated as an online process in the paper mill. 
         [0030]    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, CO 2  supply device or additional CO 2  recovery device, optional CO 2  heater, optional addition of chemical bleaching agent, and a press-water tank. 
         [0031]    The formation of crystalline precipitation particles results in the advantage that higher gloss values can be achieved for the end product. 
         [0032]    According to one embodiment of the present invention, the press filtrate  9  is used, at least partially, as dilution water in crystallizer  20 . 
         [0033]    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. 
         [0034]    A further embodiment of the present invention is that at least a portion of the required CO 2  is provided by a CO 2  recovery system. Thus, for example, it can be recovered from flue gas from boilers or flue gas from power stations. 
         [0035]    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. 
         [0036]    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%. 
         [0037]    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. 
         [0038]    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. 
         [0039]    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. 
         [0040]    To achieve good loading results, the process for producing the filler crystals may be designed as follows and have the following variables:
       moist pulp or pulp being disintegrated   calcium hydroxide in liquid or dry form   CO 2      gas zone   rotor (crystallizer)   stator (crystallizer)   production of crystals in a gas atmosphere without introduction of mixing energy   mixing with low shearing   no pressure vessel       
 
         [0050]    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%. CO 2  is then supplied to the pulp suspension. The CO 2  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. 
         [0051]    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 CaCO 3 . The form of the CaCO 3  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 CO 2  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. 
         [0052]    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. 
         [0053]    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. 
         [0054]    The further the pulp suspension has to meet the rotor disc, the less is the shearing, depending on the H 2 O 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%. 
         [0055]    The pressure acting on the CO 2  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 CO 2  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 CO 2  is a compressible gas, the necessary amount can also be increased to ensure a complete reaction. The CO 2  supply, and hence the precipitation reaction producing the CaCO 3  can be controlled by open and/or closed-loop control by way of the pH value. 
         [0056]    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 CaCO 3  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. 
         [0057]    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. 
         [0058]    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. 
         [0059]    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. 
         [0060]    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.