Method for preparing fibers contained in a pulp suspension

Abstract

A method for one of preparing fibers contained in a pulp suspension and preparing coating color for coated papers including the steps of providing fibers in a suspension form having a predefined solids concentration; loading the fibers with a precipitation product; grinding the fibers with the precipitation product to thereby produce precipitation product particles having a dimension. The dimension being from approximately 0.05 μm to 5 μm. With an additional step of producing crystalline precipitation product particles using the precipitation product particles in an online process directly in a stock preparation line.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of PCT application No. PCT/EP03/01019, entitled “METHOD FOR PREPARING FIBRES CONTAINED IN A PULP SUSPENSION”, filed Feb. 3, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing fibers contained in a pulp suspension and/or for preparing coating color for coated papers.

2. Description of the Related Art

During papermaking fillers, such as, precipitated calcium carbonate (PCC) or comminuted or ground calcium carbonate (GCC) are usual substances which are used for the purpose of reducing the fiber content and of improving the optical properties of the paper.

The commercially available PCC or GCC fillers are mass-produced products which are produced in specific manufacturing operations, which can be associated with a paper mill as a satellite plant. However, online production of PCC has never been or is never considered in the paper industry, which can be attributed to the special process properties which are necessary for the production of PCC. Instead, PCC or GCC is transported to the paper mills as a bulk material or in the form of a suspension.

Moreover, PCC and GCC fillers are employed as coating pigments in sizes of 0.3 μm and above. Since the small particles of GCC fillers do not bring with them the necessary optical properties, TiO₂ is added. During coating, the necessary optical properties can be achieved by the use of TiO₂, but this is a very expensive and abrasive pigment, which can be up to 10 times as expensive as the PCC or GCC pigments. Since the optical properties of the GCC and PCC pigments, which are common, at present, are limited as a result of the production methods, hitherto TiO₂ has been used in order to improve these properties.

Loading with an additive, for example, a filler can be carried out, by way of a chemical precipitation reaction, that is to say in particular by way of what is known as a “Fiber Loading™” process, as described inter alia in U.S. Pat. No. 5,223,090. In such a “Fiber Loading™” process, at least one additive, in particular a filler, is deposited on the wetted fiber surfaces of the fiber material. In the process, the fillers can for example be loaded with calcium carbonate. For this purpose, calcium oxide and/or calcium hydroxide are added to the moist, disintegrated fiber material such that at least part thereof associates with the water present in the fiber material. The fiber material treated in this way is subsequently treated with carbon dioxide. During the addition of the calcium oxide and/or of the medium containing calcium hydroxide to the pulp suspension, a chemical reaction with an exothermic property proceeds. The calcium hydroxide is preferably added in liquid form, known as milk of lime. This means that the water, possibly deposited in or on the pulps of the pulp suspension, is not absolutely necessary to cause the chemical reaction to start and proceed.

SUMMARY OF THE INVENTION

The present invention includes a method for preparing fillers contained in a pulp suspension and/or for preparing coating color for coated papers consisting of the following steps:

• • providing fibers in the form of a suspension with a predefined solids concentration,
  • loading the fibers with a precipitation product,
  • grinding the fibers loaded with the precipitation product to produce precipitation product particles with maximum dimensions in a range from about 0.05 to about 5 μm,
  • crystalline precipitation product particles being produced and the production of the crystalline precipitation product particles being carried out in an online process directly in the stock preparation line.

BRIEF DESCRIPTION OF THE DRAWINGS

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 an embodiment of the invention taken in conjunction with the accompanying drawing, wherein:

The FIGURE depicts a fiber loading system used in an embodiment of the present invention

The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

According to one embodiment of the present invention at least one of the following devices can be used: a cleaning device, in particular HC cleaner, a mixing device, in particular a static mixer, a lime slaking device, a press, in particular a screw press or belt press, a balancing reactor, a crystallizer, a further mixing device, in particular a static mixer, a CO₂ supply device or additional CO₂ recovery device, an optional CO₂ heater, an optional chemical bleaching agent addition and/or a press water tank.

The formation of crystalline precipitation product particles is associated, inter alia, with the advantage that, if required, relatively high gloss values for the end product can be achieved. It is to be noted that, as a rule, only loaded fibers are ground. The coating color is not ground as a rule but can be ground. In general, this depends on the respective definition, but also on the respective crystallization operation. If CaCO₃ crystals are produced in the coating kitchen, then there are no fibers in the suspension, which means that the pump crystallizer operates only as a highly efficient chemical reactor or mixer. Of course, a grinding component could also be provided in the mixing and reaction process, specifically by the friction of the particles in the suspension, assisted by the rotor and the stator.

According to one embodiment of the present invention, the press water is used as dilution water on the crystallizer side.

The further mixing device can be used, in particular, for the fine adjustment of the pH of the pulp suspension, preferably in a range of between 6 and 8. The first mixing device is used for mixing the milk of lime into the pulp suspension.

According to another embodiment of the present invention, the cleaning device is used to prevent contamination, occurring during the process, by heavier materials such as sand, stones and pieces of metal.

Advantageously, at least some of the CO₂ needed is provided by a CO₂ recovery system. Thus, it can be recovered, for example, from the flue gas of boilers or the flue gas of power plants.

According to at least one embodiment of the present invention, the precipitation product is calcium carbonate.

During the addition of the calcium oxide, and/or of the medium containing calcium hydroxide, to the pulp suspension, a chemical reaction with an exothermic property proceeds. The calcium hydroxide being added is in a liquid form, known as milk of lime. This means that the water, possibly incorporated in or on the pulps of the pulp suspension, is not absolutely necessary to cause the chemical reaction to start and proceed.

It is possible, for example, to produce precipitation product particles of rhombohedral form with a respective cube size in a range from about 0.05 to about 2 μm. In specific cases, it is also advantageous to produce precipitation particles of a scalenohedral form with a respective length in a range from about 0.05 to about 2 μm and a respective diameter in a range from of about 0.01 to about 0.05 μm.

According to one embodiment of the present invention, the solids concentration of the pulp suspension provided is chosen to be in a range from about 5% to about 60% and preferably in a range from about 10% to about 35%.

It is particularly advantageous if, in order to load the fibers with calcium carbonate, calcium oxide and/or calcium hydroxide is/are added to the pulp suspension and the precipitation is initiated by treating the pulp suspension with carbon dioxide.

In the case of, for example, loading the fibers with filler, it is possible for calcium carbonate (CaCO₃) to be deposited on the wetted fiber surfaces by calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)₂) being added to the wet fiber material, and that at least a part thereof to associate with the water of the quantity of pulp. The fiber material treated in this way can then be treated with carbon dioxide (CO₂).

The term “wetted fiber surfaces” applies to all the wetted surfaces of the individual fibers. This also covers the case in which the fibers are loaded with calcium carbonate or with any other desired precipitation product both on their outer surface and in their interior, also known as lumen.

Accordingly, for example, the fibers can be loaded with the filler calcium carbonate, the deposition on the wetted fiber surfaces being carried out by what is known as a “Fiber Loading™” process, as described as such in U.S. Pat. No. 5,223,090. In this “Fiber Loading™” process the carbon dioxide reacts with the calcium hydroxide to form water and calcium carbonate. The calcium hydroxide can be supplied to the pulp suspension in liquid form or in dry form.

According to one embodiment of the present invention, the carbon dioxide is added to the pulp suspension at a temperature in a range of from about −15° to about 120° C. and preferably in a range from about 20° to about 90° C.

The paper produced can contain fillers of the order of magnitude of about 0.05 to about 5 μm, which means the optical properties of the end product are enhanced. The filler can be, in particular, calcium carbonate, which occurs in nature, for example, as calcite or calc-spar, aragonite and in the rarer form vaterite. The filler can be composed mainly of the form calcite, of which over 300 different crystal forms are supposed to exist. The shape of the filler particles used can be, for example, rhombohedral with a respective cube size range from about 0.05 μm to about 2 μm or, for example, scalenohedral with a respective length in a range from about 0.05 μm to about 2 μm and a respective diameter in a range from about 0.01 μm to about 0.05 μm, depending on the grade of paper respectively to be produced.

The filler is distributed uniformly on, around and within the fibers, which means that no agglomeration of crystals in bundles is to be encountered. The respective filler particle, namely the crystal, is provided on the fiber, spaced apart individually or separated. The filler particle covers the fiber as a result of deposition on the fiber, by which the optical properties of the end product are improved. The particle size is important in order to achieve an optimum opacity. A high opacity is achieved when the color spectrum of visible light is scattered well. If the color spectrum is absorbed, then the result is the color black. If the size of the filler particles falls below 0.2 μm to 0.5 μm, the result is a tendency to transparency and higher gloss.

In order to achieve the aforementioned results, the relevant production process for producing the filler crystals can be configured as follows, for example, and can have the following variables:

• • moist, that is to say not yet dried, pulp or stock
  • calcium hydroxide in liquid or dry form
  • CO₂
  • gas zone
  • rotor
  • stator
  • production of crystals in a gas atmosphere without the introduction of mixing energy
  • mixing with low shear
  • no pressure container

The pulp suspension previously mixed with Ca(OH)₂ is put into a fluffer, a refiner, a disperger or the like at a consistency, or solids concentration, in the range of from about 5% to about 60%, preferably in a range from about 10% to about 35%. The Ca(OH)₂ can be added in liquid or dry form. The pulp suspension is treated with CO₂. The CO₂ can be added, for example, at temperatures in a range of between about −15% and about 120° C. and preferably at temperatures in a range between about 20° and about 90° C.

The pulp suspension passes into the gas zone, where each individual fiber is subjected to a gas atmosphere, followed by the precipitation reaction, with which the CaCO₃ results directly. The form of the CaCO₃ crystals can be, for example, rhombohedral, scalenohedral or spherical. The quantity of crystals depend in particular, on the selected temperature range for the pulp suspension and on the CO₂ content and the Ca(OH)₂ content in the pulp suspension. After the pulp suspension, with the crystals formed, has passed through the gas zone, and the PCC formed or the pulp suspension with the crystals in the lumen, on the fiber and between the fibers, is led through a rotor and a stator, where the distribution of the crystals in the pulp suspension is completed by a low shear mixing.

While the pulp/crystal suspension is passing the rotor, a shear distribution occurs which brings about a size distribution of the crystals from about 0.05 μm to about 0.5 μm and preferably from about 0.3 μm to about 2.5 μm.

The shape of the filler particles used is, for example, rhombohedral with a respective cube size in a range from about 0.05 μm to about 2 μm, or scalenohedral with a respective length in a range from about 0.05 μm to about 2 μm and a respective diameter in a range from about 0.01 μm to about 0.5 μm, depending on the grade of paper to be produced.

The further the pulp suspension strikes the rotor disk, the lower is the shear, depending on the H₂O added for the purpose of dilution. The concentration of the pulp suspension passing the rotor disk is about 0.1% to about 50% and preferably about 35% to about 50%.

The pressure acting on the CO₂ feed line is in a range from about 0.1 bar to about 6 bar, and preferably in a range from about 0.5 bar to about 3 bar, in order to ensure a constant CO₂ supply to the gas ring for the desired chemical reaction. Just like a water supply via a garden hose, the pressure has to be increased when there is a high demand for water, in order to deliver more through the hose. Since CO₂ is a compressible gas, the quantity required can also be increased in order to ensure a complete reaction. The CO₂ supply and therefore the precipitation reaction bringing forth the CaCO₃ can be controlled and/or regulated by way of controlling the pH.

For instance, it is possible to consider pH values in a range from 6.0 to about 10.0, preferably a range from about pH 7.0 to about 8.5, for the final reaction of the CaCO₃ crystals. The energy used for this process lies in a range between about 0.3 kWh/t and about 8 kWh/t and preferably in a range between about 0.5 kWh/t and about 4 kWh/t. Dilution water can be added and mixed with the pulp suspension in order to obtain a final dilution at which the pulp suspension, with filler produced, has a consistency, or solids concentration, in a range from about 0.1% to about 16% and preferably in a range from about 2% to about 6%. The pulp suspension is then exposed to the atmosphere in a machine, in a container or the next process machine.

The rotational speed, at the external diameter of the rotor disk, can lie in a range from about 20 m/s to 100 m/s and preferably in a range from about 40 m/s to about 60 m/s.

The gap between the rotor and the stator is, about 0.5 mm to about 100 mm and preferably about 25 to about 75 mL The diameter of the rotor and of the stator is in a range from about 0.5 m to about 2 m.

The reaction time is in a range from about 0.001 min. to 1 min., and preferably in a range from about 0.1 sec to about 10 sec.

The method described above permits the production of individual particles, which are spaced apart equally from one another and are deposited onto the fibers, and covering the fibers in the required manner, in order to satisfy the requirements for the desired level of white or glossy paper. The particle size lies in a range from about 0.05 μm to about 5 μm, the preferred size for the rhombohedral form of a cube lies in a range from about 0.05 μm to about 2 μm or, for a scalenohedral form, in a range from about 0.05 μm to about 2 μm with respect to the length and a range from about 0.01 μm to about 0.5 μm with respect to the diameter. For high gloss applications, the particle size should expediently lie below 0.2 μm to 0.5 μm.

In particular, an online process for the production of filler particles consisting of precipitated calcium carbonate directly in the stock preparation line is specified.

The advantages of the filler particles obtained consist, inter alia, in the following:

• • It is now possible to distribute the requisite filler particles uniformly over the fiber surface, which means that the best optical properties are achieved online in the stock preparation, it being possible for the filler level achieved to be below or above 40%.
  • Since filler particles are also embedded within the fiber lumen, the tendency to blackening as a result of calendering is considerably reduced.
  • A new way of incorporating pigments is created, in order to achieve the desired optical properties and the desired printability, in and on the paper sheet directly during the paper production and not during the coating process. In one of the present embodiments, the coating process can be provided only for the fine adjustment of the properties of the paper surface. Alternatively exerting a corresponding influence in the coating process is also possible.
  • Since the filler particles are incorporated in the fibers, they can no longer be washed out in the wire section or Fourdrinier section of a papermaking machine. As such, it is not necessary to deal with these particles in the same way as in connection with the GCC or PCC particles normally used by way of a coating process, which means that coating particles can be saved. This leads to higher machine speeds, since a lower amount of coating color has to be applied.
  • Since the filler particles are deposited on the fibers in an online process, that is to say are crystallized in the pulp preparation system, economic advantages can be achieved. Some of the economic advantages include savings in retention aids, reductions of fibers and sludge, a reduction of the white water contamination and the saving of energy and raw material.
  • The production of high gloss paper with the filler particles formed is possible.
  • Since the precipitated filler particles are scouring or abrasive to a low extent, a longer lifetime of the coating equipment and of the paper machine felts and fabrics result.
  • The use of TiO₂ can be reduced, since a higher whiteness and better optical properties is achieved.

The method according to the invention can be used in particular for coating color for coated papers as well. The PCC production can be part of the coating process, it being possible to form the aforementioned crystal forms.

In this case, it is in particular possible to influence an online coating machine between the predrying and afterdrying, as well as the use of a coating device or converting machine, independently of the papermaking machine in the following manner:

• • Less TiO₂ has to be used.
  • The paper surface is improved by small crystals.
  • Less coating slurry is needed.
  • The result is better printability, since the fibers are covered uniformly with crystals.
  • Since the fibers are covered uniformly with crystals, the water absorption and the oil absorption are also reduced.
  • In addition, the wear in the coating equipment and in the papermaking machine is reduced if online coating is carried out.

Consequently, the method according to one embodiment of the present invention is also applied in combined form in a coating machine and a papermaking machine. In principle, both offline and online operations are possible.

Distinct from the conventional PCC fillers, according to the present invention, special crystal forms are produced which, inter alia, can be changed in the desired manner, for example even during the coating process.

The possible paper grades include, amongst others:

Printing and writing papers:

• • These can be produced from newsprint.
  • Woody or wood free coated printing and writing papers.
  • Uncoated woody or wood free printing and writing papers.
    Paper grades determined by ground wood or chemical pulp:
  • Known as woody paper grades with ground wood or chemical pulp in a range from 25% to 100%. Chemical pulp is added in order to increase the strength and the runnability of coating machines and papermaking machines, etc.
    Newsprint grades:
  • Can contain up to 100% recycled fibers or up to 100% ground wood or chemical pulp, which can be either mechanically ground wood, thermo-mechanical pulp (TMP), pressure ground wood pulp (PGB), or CTMP (chemithermomechanical pulp). The use of chemical pulp reaches as far as 30%. The use of recycled fibers (RCF) raises the filler content.
    SC papers:
  • These are paper grades which are determined by the use of chemical pulp and have a filler content of up to 30%.
    Coated paper grades:
  • These paper grades are determined by mechanical pulp, that is to say mechanical ground wood or chemical pulp, up to 100%.
    Chemical pulp grades:
  • These contain up to 10% mechanical pulp. Both hardwood and softwood chemical pulps are used.
    Copy paper:
  • This is composed of up to 90% or even up to 100% new chemical pulp fibers but can contain up to 100% recycled fibers; a filler content up to about 30% can be provided.
    Printing and writing papers:
  • These can be produced from newsprint.
  • Woody or non-woody coated printing and writing papers.
  • Woody and non-woody uncoated printing and writing papers.
    Board grades:
  • These contain a top layer made from a mixture of bleached hardwood (up to 90%) and bleached softwood (up to 30%), the top layer or the bottom layer being coated. It can also be employed in the underlayer, which can contain mixtures of deinked pulp, OCC and computer printouts. The middle layer contains, for example, a mixture of waste and production broke, while the base layer can contain unbleached softwood and production broke as well as OCC.

Referring now to the FIGURE there is shown an apparatus utilizing the method, according to the present invention, implemented, for example, as a “Fiber Loadingm” system.

According to the FIGURE, at least one of the following devices can be used for the online process: cleaning device 10, in particular a High Consistency (HC) cleaner 10, a mixing device 12, in particular a static mixer 12, a lime slaking device 14, a press 16, in particular a screw press 16 or a belt press 16, a balancing reactor 18, a crystallizer 20, a further mixing device 22, in a particular static mixer 22, a CO₂ supply device 24 or an additional CO₂ recovery device, an optional CO₂ heater 26, an optional chemical bleaching agent additions and a press water tank 28.

Cleaning device 10, or an equivalent device, is equipped with at least one mechanism which carries out a protective function.

Mixing device 10, and further mixing device 22 are constructed in accordance with the apparatus disclosed in German laid-open specification DE 41 25 513 A1 for mixing suspended pulp. An apparatus of this type include an introduction line for a suspended pulp (“thick stock”), which opens into the wall of a section of a pipe, in particular a curved section of a pipe, which carries thin stock. The speed at which the thick stock flows out of the introduction line is preferably at least three times the speed of the thin stock flowing in the opening area. Furthermore, the introduction line opens in the central area of the section of the pipe.

In a further refinement, mixing device 10 and/or further mixing device 22 are equipped with or without a known buffer chest.

Control valve 28 is provided in a line to cleaning device 10; a lime pump 30 is provided between lime slaking device 14 and first mixing device 12; a press water pump 32 is provided between press water container 28 and crystallizer 20, a mixing container 34 and also a CO₂ pump 36 are provided between CO₂ supply 24 and CO₂ heater 26.

While this invention has been described as having a preferred design, 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.

List of Designations

• 10 Cleaning device
• 12 First mixing device
• 14 Lime slaking device
• 16 Press
• 18 Balancing reactor
• 20 Crystallizer
• 22 Further mixing device
• 24 CO2 supply device or additional CO2 recovery device
• 26 Optional CO2 heater
• 28 Press water tank
• 30 Lime pump
• 32 Press water pump
• 34 Mixing container
• 36 CO2 pump

Claims

1. A method for one of preparing fibers contained in a pulp suspension and preparing coating color for coated papers, comprising the steps of:

providing fibers in a suspension form having a predefined solids concentration;

loading said fibers with a precipitation product;

grinding said fibers with said precipitation product to thereby produce precipitation product particles having a maximum dimension from between approximately 0.05 μm to 5 μm; and

producing crystalline precipitation product particles using said precipitation product particles in an online process directly in a stock preparation line.

2. The method of claim 1, wherein said online process utilizes at least one of a cleaning device, a HC cleaner, a first mixing device, a first static mixer, a lime slaking device a press, a press, a screw press, a belt press, a balancing reactor, a crystallizer, a further mixing device, a further static mixer, a CO2 supply device, an additional CO2 recovery device, a CO2 heater, a chemical bleaching agent addition, and a press water tank.

3. The method of claim 2, wherein press water is used as dilution water on a crystallizer side of a papermaking machine.

4. The method of claim 2, wherein said online process utilizes said further mixing device that is used for a fine adjustment of the pH of the pulp suspension, said fine adjustment of the pH being in a range from between 6 and 8.

5. The method of claim 2, wherein said first mixing device is used for mixing milk of lime into the pulp suspension.

6. The method of claim 2, wherein said online process utilizes said cleaning device to prevent contamination that occurs during the method, said contamination being heavy materials including sand, stones and pieces of metal.

7. The method of claim 2, wherein said online process utilizes said CO2 recovery system to supply at least some of the CO2 needed by the method.

8. The method of claim 1, wherein said precipitation product is calcium carbonate.

9. The method of claim 1, wherein said precipitation product particles are of a rhombohedral form with a cube size range from about 0.05 μm to about 2 μm.

10. The method of claim 1, wherein said precipitation product particles are of a scalenohedral form with a length in a range from approximately 0.05 μm to 2 μm and a respective diameter in a range from approximately 0.01 μm to 0.5 μm.

11. The method of claim 1, wherein said solids concentration of the pulp suspension is in a range from approximately 5% to 60%.

12. The method of claim 11, wherein said range is from approximately 10% to 35%.

13. The method of claim 1, wherein said loading step includes adding at least one of calcium carbonate, calcium oxide and calcium hydroxide to the pulp suspension, a precipitation being initiated by treating the pulp suspension with carbon dioxide.

14. The method of claim 13, wherein said calcium hydroxide is added to the pulp suspension in liquid form.

15. The method of claim 13, wherein said calcium hydroxide is added to the pulp suspension in dry form.

16. The method of claim 13, wherein said carbon dioxide is added to the pulp suspension at a temperature in a range from approximately −15° C. to 120° C.

17. The method of claim 16, wherein said temperature is in a range from 20° C. to 90° C.