US20040168783A1

US20040168783A1 - Method for recycling pulp rejects

Info

Publication number: US20040168783A1
Application number: US10/476,620
Authority: US
Inventors: Dieter Munchow
Original assignee: ALPHA CALCIT
Current assignee: ALPHA CALCIT
Priority date: 2001-05-08
Filing date: 2002-01-24
Publication date: 2004-09-02
Also published as: CZ20033036A3; EP1313917A1; SK286317B6; ATE266768T1; HUP0400018A3; DE10122331A1; CA2443065A1; ES2201949T1; WO2002090651A1; KR100778426B1; ZA200307798B; PL206622B1; EP1313917B1; JP2004534911A; BG108226A; TR200401126T4; CN1238600C; CN1524145A; SI1313917T1; DE10122331B4

Abstract

The present invention relates to a process for recycling cleaner rejects from the preparation of paper, paperboard and cardboard and to their use for the preparation of paper in the paper stock or as a coating slurry for the paper industry.

Description

In the preparation of paper, the raw material, i.e., wood pulp, wood, fine straw pulp or rag pulp, is also admixed with repulped half-stock, fillers and pigments in order to achieve a closed surface and thus to improve the properties of the paper, especially the whiteness, opacity and printability.

Almost all papers are admixed with fillers which confer a uniform formation, improved softness, whiteness and touch especially to printing and writing papers. These fillers, mostly called “ashes” since they remain as ashes in the combustion analysis, are either added to the fiber suspension or applied in the coating step.

Uncoated papers contain up to 35% by weight of fillers, coated papers contain from 25 to 50% by weight thereof. The amount of fillers employed is highly dependent on the intended use of the paper. Highly filled papers have a lower strength and poorer sizing properties.

The filler content in the paper stock is usually between 5 and 35% by weight and consists of primary pigments or recycled coating pigments which may be derived from coating residuals or from coated rejects. In addition to the whiteness of the filler which is important for whitened papers, its grain size plays an important role since it has a strong influence on the filler efficiency and the physical properties of the paper, in particular porosity. The proportion of filler remaining in the paper is between 20 and 80% of the amount added to the fiber suspension.

As judged by their consumption, the following products have rather great importance today as fillers and coating pigments: china clay, calcium carbonate, precipitated calcium carbonate (PCC), artificial aluminum silicates and oxide hydrates, titanium dioxide, satin white, talcum and calcium silicate.

Subsequently to their coating with coating slurries, papers are often smoothed, after drying, with a blade. The so-called cleaner rejects obtained therein, also referred to as coated rejects, mainly consist of minerals and fibers and may comprise from 1 to 4% of the total volume of the paper produced. In most cases, they are disposed of in dumps. Thus, for example, it has been proposed to compress and compact the cleaner rejects by screw extruders in order to reduce their volume. However, the total amount of the cleaner rejects obtained and passed to disposal is not reduced.

In the past, another possibility of treating the cleaner rejects was to partly process them using centrifugal cleaners in order to separate the fibers, on the one hand, and/or the coating pigments and fillers, on the other hand, and reuse them at least in part.

It is the object of the present invention to provide a new environmentally friendly concept for treating the cleaner rejects, especially from centrifugal cleaners, and to enable them to be recycled as completely as possible.

In particular, the object of the invention is to provide a process for the recycling of cleaner rejects while energy costs and costs of raw materials, shipping costs as well as costs for dumping are saved.

According to the invention, the above object is achieved by a process for recycling cleaner rejects, especially from centrifugal cleaners, from the preparation of paper, paperboard and cardboard, characterized in that cleaner rejects, which contain fibers, coating pigments and/or fillers, are milled to the desired grain size distribution, optionally with the addition of water, coating pigment, fresh filler and/or filler slurry, and employed as a raw material for the paper stock and/or coating slurry.

The cleaner rejects, which are usually solid, contain valuable raw materials which can be recovered by means of the present invention and, after milling, recycled to various processes of papermaking. Especially the cleaner rejects from paper mills which prepare coated paper or cardboard grades contain large quantities of coating pigment flakes. The thicker the coating layer, the more difficult is the recovery of the flakes by the prior art technologies. Double-coated grades or coated cardboard grades may be mentioned here as particular examples. Another group of paper industries are the SC-paper producing plants, which lose large amounts of fillers through cleaner rejects.

Now, by means of the present invention, the cleaner rejects are milled and optionally admixed with water, coating pigment, filler and/or filler slurry. The cleaner reject slurry or the milled powder is then recycled to the papermaking process for the paper stock and/or as a coating slurry.

On a closer inspection of the cleaner rejects, fractions of fillers, coating pigment flakes, fiber suspension residuals, such as fiber lumps or splinters, as well as foreign matter, for example, sand, can be discovered.

After the milling of the cleaner rejects, the distribution of the particle sizes of the cleaner rejects is essentially the same as the original distribution in the coating pigments, fillers or filler/pigment slurries. Thus, at the same time, by means of the present invention, more than 95% of the total cleaner rejects, or even the full amount thereof, can be recovered and recycled to production.

Then, the cleaner rejects processed according to the invention serves, for example, as a filler, whereby the consumption of fresh filler or pigment can be reduced consistently.

According to the invention, the milling of the cleaner rejects is preferably installed in a centrifugal cleaner system downstream from the last of usually several stages of a centrifugal cleaner. The accepted stock from the additional cleaner system is recycled to the previous stage, and the residual amount of the cleaner rejects forms the final reject.

In an existing cleaner system, the simplest application area is intended to comprise installing the milling after the last but one cleaner stage and employing the last cleaner process as a final stage.

When the process according to the invention was performed, it was found that the distribution of particle sizes of the coating pigment flakes had been refined and that the original distribution of the pigment particles had been achieved. This could be seen from the quantity of finer particles, especially in the differential distribution. The fiber knots were extracted and predominantly refined.

In papermaking, it is usual to employ the fillers and coating pigments either as powders or in the form of concentrated slurries with a solids content of preferably from 30 to 85% by weight for the paper stock or for the preparation of coating slurries. The fresh fillers and pigments are usually supplied by the manufacturers with the desired whiteness and grain size distribution as a slurry or powder.

In the processing of cleaner rejects, it is of course required to separate and discard the coarse dirt contents consisting of splinters, sand grains and other impurities. For this purpose, multistage centrifugal cleaners are usually employed. The screenings thus obtained consist of fibers, fillers, pigments, fine sand, black particles and agglomerates of fillers and pigments, or pigments, fibers and fillers. “Filler” usually means the fine particles employed in the paper stock; “pigment” means the fine particles employed in the coat.

Known methods which suggest themselves for the separation of the undesirable contents of the cleaner rejects are flocculation and sedimentation, filtering, screening and/or centrifuging. In this case, a mixture of different pigments is usually present which often contains china clay, calcium carbonate and talcum. Agglomerates frequently form during the separation processes due to flocculation and charge reversal; they can now be milled according to the invention.

According to the invention, the milling into powders or slurries can be performed continuously or discontinuously in usual dry mills or wet mills, especially agitator ball mills, for example, having a content of from 700 to 5000 l or more. Milling media, preferably milling balls, especially having a diameter of from 1 to 4 mm, are used.

Screens, preferably sieve bends, for separating impurities (ball crushings, separating materials, rust etc.) are usually used for processing. Laser measuring instruments serve to determine and control the milling fineness during the milling process and for the computer-based control of the agitator ball mill system.

Therefore, for the preparation of new coating slurries or also, optionally, for use in the paper stock, it may be required to enhance the whiteness by per se known methods. The disruption of agglomerates which adversely affect the flowing properties of a coating slurry at the blade by forming doctor streaks and adversely affect the properties of the coat is particularly preferred. Dispersing aids, fillers and pigments additionally employed in the milling process reduce the overall consumption of these materials.

In the cleaner rejects, the ratio of fillers and/or pigments to fibers can vary widely. It is particularly preferred according to the invention to employ cleaner rejects having an optionally enriched concentration of fillers and/or coating pigments within a range of from 5 to 90% by weight, especially from 30 to 70% by weight, based on the solids content. Thus, the fiber content, on the one hand, or the content of filler and/or coating pigment may vary, for example, from 1 to 99% by weight, or from 99 to 1% by weight. “Enrichment” within the meaning of the present invention especially comprises the addition of fresh filler and/or fresh pigment.

Therefore, according to the present invention, china clay, natural or precipitated calcium carbonates, artificial or natural aluminum silicates and oxide hydrates, titanium dioxide, satin white, dolomite, mica, metal flakes, especially aluminum flakes, bentonite, rutile, magnesium hydroxide, gypsum, sheet silicates, talcum, calcium silicate and other rocks and earths are preferably used as the fresh pigment and/or fresh filler before, after and/or during the milling.

The fresh pigment or fresh filler is preferably employed as a powder, aqueous fresh-pigment containing and/or fresh-filler containing slurries in the presence of the cleaner rejects and optionally usual milling aids and/or dispersing aids to give a slurry with a solids content of, for example, from 30 to 85% by weight, especially from 40 to 75% by weight.

Preferably, the added amount of fibers, coating pigments and/or fillers (recycled or fresh) is from 1 to 100% by weight, based on the cleaner rejects.

Particularly preferred is the addition of water in order to employ the cleaner rejects milled according to the invention in a diluted slurry for the stock flow. Slurries having a solids content of from 5 to 50% by weight, especially from 10 to 20% by weight, are preferably employed. Accordingly, in a preferred embodiment according to the invention, the milling is performed in the presence of water in an amount of from 1 to 1000% by weight, especially from 100 to 1000% by weight, based on the solids content.

The cleaner rejects are preferably milled to a slurry or powder having a grain size distribution of from 10 to 99% by weight of particles <1 μm, especially from 10 to 95% by weight of particles <1 μm, respectively based on the equivalent diameter.

From EP 0 625 611 A1, grain size distributions for coating pigments are known which are also preferably adjusted by means of the present invention. Thus, it is particularly preferred according to the present invention for the pigments to have the following grain size distribution:

a) from 95 to 100% by weight of particles <10 μm;

b) from 50 to 100% by weight of particles <2 μm, especially from 50 to 95% by weight of particles <2 μm;

c) from 27 to 95% by weight of particles <1 μm, especially from 27 to 75% by weight of particles <1 μm; and

d) from 0.1 to 55% by weight of particles <0.2 μm, especially from 0.1 to 35% by weight of particles <0.2 μm;

respectively based on the equivalent diameter of the particles.

In addition, according to the invention, a broad variation of the whiteness and grain size distributions is possible, which can be controlled, in particular, by the manner and duration of milling. Thus, it is possible to mix a relatively coarse fresh filler and water in situ with a large amount of cleaner rejects for introducing this slurry into the paper stock after milling. In the same way, it is possible to use a smaller amount of cleaner rejects and to perform a finer milling with fresh pigment in situ which is then used as a coating pigment, especially in precoating.

Even though per se known wetting agents, stabilizers, milling aids and dispersing aids may be employed according to the invention during the mixing and milling of the cleaner rejects, as known, for example, from EP 0 625 611 A1, the quantity thereof required is clearly reduced according to the invention as compared to the prior art. The cleaner rejects already contain some amount of the mentioned agents which are recycled to the papermaking process according to the invention.

The powders or slurries obtainable according to the present invention may be employed to particular advantage in the paper industry, especially for the preparation of a coat for paper coating or in the paper stock. When the fiber content is low, the cleaner rejects milled according to the invention is preferably employed in the coating slurry, and when the fiber content is high, it is preferably employed in the paper stock. According to the invention, a good retention on the screen is obtained. Particularly preferred is the use of the cleaner rejects milled according to the invention for the preparation of offset paper. In addition, the slurries according to the invention are also suitable for the preparation of a coating slurry for light-weight coated papers, especially with high coating speeds, and for the preparation of rotary offset papers, especially for the preparation of light-weight coated rotary offset papers, the coating of cardboard and special papers, such as labels, wallpapers, silicone base paper, self-copying paper, and for admixture with intaglio printing paper. Thus, the coating pigment slurries obtainable according to the invention may be employed, in particular, in sheet-fed offset papers, especially for sheet-fed offset single coating, sheet-fed offset double coating: sheet-fed offset precoating and sheet-fed offset top coating; in rotary offset papers, especially for LWC rotary offset single coating, rotary offset double coating: rotary offset precoating and rotary offset top coating; in intaglio printing, especially for LWC intaglio single coating, intaglio double coating: intaglio precoating and intaglio top coating; in cardboard, especially for cardboard double coating: cardboard precoating and cardboard top coating; and for special papers, especially for labels and flexible packings.

The process offers the opportunity to employ the pigment slurries prepared according to the invention without a loss in quality in the base papers, coatings and especially final qualities prepared therewith.

In the following, some coating formulations which can be obtained according to the present invention are given for illustrative purposes (all figures converted to weight parts of solids (atro/active ingredient)). The amounts of fillers and/or pigment slurries can be partly or wholly replaced by corresponding amounts of cleaner rejects milled according to the invention.

1. Sheet-Fed Offset Paper

1.1 Sheet-Fed Offset Single Coating

70 parts by weight of commercially available CaCO ₃(type 90)

30 parts by weight of commercially available clay (fine, e.g., U.S. No. 1)

11 parts by weight of commercially available latex (acrylate)

0.6 parts by weight of commercially available carboxymethylcellulose (CMC)

0.8 parts by weight of commercially available hardener (urea-formaldehyde, melamine-formaldehyde, epoxy resin)

0.5 parts by weight of commercially available brightener (opt.)

0.5 parts by weight of commercially available Ca stearate



	solids content:	64%
	Brookfield viscosity (100/min):	1,200 mPa · s
	pH value:	8.5

1.2 Sheet-Fed Offset Double Coating

1.2.1 Sheet-Fed Offset Precoating

100 parts by weight of commercially available CaCO ₃(type 60 or 75)

10 parts by weight of commercially available latex

4 parts by weight of commercially available starch (native, oxidized, corn or potato starch)

0.5 parts by weight of commercially available brightener (opt.)



	solids content:	66%
	Brookfield viscosity (100/min):	1,100 mPa · s
	pH value:	9.0

1.2.2 Sheet-Fed Offset Top Coating

70 parts by weight of commercially available CaCO ₃(type 90)

30 parts by weight of commercially available clay (fine, e.g.; U.S. No. 1)

10 parts by weight of commercially available latex (acrylate)

0.6 parts by weight of commercially available CMC

0.5 parts by weight of commercially available brightener (opt.)

0.7 parts by weight of commercially available Ca stearate

2. Rotary Offset Paper

2.1 LWC Rotary Offset Single Coating

50 parts by weight of commercially available CaCO ₃(type 90)

50 parts by weight of commercially available clay (fine, Engl. clay)

2 parts by weight of commercially available starch (native, oxidized, corn or potato starch)

12 parts by weight of commercially available latex (XSB)

0.7 parts by weight of commercially available brightener (opt.)

0.5 parts by weight of commercially available Ca stearate



	solids content:	62%
	Brookfield viscosity (100/min):	1,400 mPa · s
	pH value:	8.5

2.2 Rotary Offset Double Coating

2.2.1 Rotary Offset Precoating

100 parts by weight of commercially available CaCO ₃(type 60 or 75)

12 parts by weight of commercially available latex (XSB)

0.5 parts by weight of commercially available brightener (opt.)



	solids content:	66%
	Brookfield viscosity (100/min):	1,200 mPa · s
	pH value:	9.0

2.2.2 Rotary Offset Ton Coating

60 parts by weight of commercially available CaCO ₃(type 95)

40 parts by weight of commercially available clay (fine, Engl. clay)

10 parts by weight of commercially available latex (XSB)

0.6 parts by weight of commercially available CMC

0.5 parts by weight of commercially available brightener (opt.)

0.5 parts by weight of commercially available Ca stearate

3. Intaglio Printing Paper

3.1 LWC Intaglio Single Coating

70 parts by weight of commercially available clay (normal, Engl. clay)

30 parts by weight of commercially available talcum

5.0 parts by weight of commercially available latex (acrylate sole binder)

0.2 parts by weight of commercially available thickener (synthetic)

1.0 parts by weight of commercially available Ca stearate



	solids content:	58%
	Brookfield viscosity (100/min):	1,200 mPa · s
	pH value:	8.5

3.2 Intaglio Double Coating

3.2.1 Intaglio Precoating

100 parts by weight of commercially available CaCO ₃(type 75)

6.0 parts by weight of commercially available latex (acrylate sole binder)

0.3 parts by weight of commercially available thickener (synthetic)

0.5 parts by weight of commercially available Ca stearate

3.2.2 Intaglio Top Coating

85 parts by weight of commercially available clay (Engl. clay)

15 parts by weight of commercially available clay (calcined clay)

5.0 parts by weight of commercially available latex (acrylate sole binder)

0.2 parts by weight of commercially available thickener (synthetic)

0.8 parts by weight of commercially available Ca stearate



	solids content:	57%
	Brookfield viscosity (100/min):	1,300 mPa · s
	pH value:	8.5

4. Cardboard

4.1 Cardboard Double Coating

4.1.1 Cardboard Precoating

100 parts by weight of commercially available CaCO ₃(type 75)

3 parts by weight of commercially available starch (native, oxidized, corn or potato starch)

14 parts by weight of commercially available latex (XSB)

0.5 parts by weight of commercially available brightener (opt.)



	solids content:	66%
	Brookfield viscosity (100/min):	1,000 mPa · s
	pH value:	9.0

4.1.2 Cardboard Top Coating

50 parts by weight of commercially available CaCO ₃(type 90)

50 parts by weight of commercially available clay (fine/Engl. clay)

13 parts by weight of commercially available latex (acrylate)

2 parts by weight of commercially available co-binder (acrylate)

0.6 parts by weight of commercially available Ca stearate



	solids content:	60%
	Brookfield viscosity (100/min):	1,200 mPa · s
	pH value:	8.5

5. Special Papers

5.1. Labels

70 parts by weight of commercially available clay (normal/Engl. clay)

10 parts by weight of commercially available TiO ₂(rutile)

20 parts by weight of commercially available CaCO ₃(type 90)

16 parts by weight of commercially available latex (XSB)

0.5 parts by weight of commercially available hardener (EH) (urea-formaldehyde, melamine-formaldehyde, epoxy resin)

0.6 parts by weight of commercially available Ca stearate

5.2 Flexible Packing

80 parts by weight of commercially available clay (normal, Engl. clay)

20 parts by weight of commercially available CaCO ₃(type 90)

14 parts by weight of commercially available latex (acrylate)

0.8 parts by weight of commercially available CMC

0.5 parts by weight of commercially available hardener (urea-formaldehyde, melamine-formaldehyde, epoxy resin)

0.6 parts by weight of commercially available brightener (opt.)

1.0 parts by weight of commercially available Ca stearate

EXAMPLE

Cleaner rejects from the process of a commercially available papermaking machine of the prior art were milled in a 15% by weight suspension to a slurry having a grain size of <10 μm and added to the stock flow of the paper stock. [0141]
Shortly after the connecting of the milling installation with the cleaner rejects milled according to the invention, an increase of the ashes content in the paper could be established. [0142]
The ashes content increased, and the dosage of filler could be consequently reduced. From the milling installation, about 16 to 18 l/min of pigment slurry consisting of milled cleaner rejects was supplied to the pulp chest. To maintain the ashes content on the predetermined constant level, the usual dosage of fresh filler could be reduced from 10 l/h to 6.0 l/h. [0143]
In the covered period, the milling installation ran without trouble. The ashes content of the stock flow varied to the usual extent. The retention slightly decreased initially, but increased to the usual values in the course of the experiment. The amount saved was about 240 l/h of fresh pigment slurry. [0144]
The test run over about 10 hours showed that the processed cleaner rejects can be employed again as a filler in the stock by the process according to the invention without retention loss. [0145]
Statements relating to the composition of the cleaner reject slurries: [0146]

solids content: about 24.0%

pigment proportion: about 85.0%, based on the solids content

fiber proportion: about 15.0%, based on the solids content

Claims

1. A process for recycling solid cleaner rejects from the preparation of paper, paperboard and cardboard, especially from centrifugal cleaners from the preparation of paper, paperboard and cardboard, characterized in that cleaner rejects, which contain fibers, coating pigments and/or fillers, are milled to the desired grain size distribution, optionally with the addition of water, coating pigment, fresh filler and/or filler slurry, and employed as a raw material for the paper stock and/or coating slurry.

2. The process according to claim 1, characterized in that the separation of the cleaner rejects is effected by flocculation and sedimentation, filtering, screening and/or centrifuging.

3. The process according to claim 1, characterized in that cleaner rejects having a concentration of fillers and/or coating pigments in an amount of from 1 to 99% by weight, especially from 30 to 70% by weight, based on the solids content, are employed.

4. The process according to claim 3, characterized in that cleaner rejects having a concentration of fibers in an amount of from 1 to 99% by weight, especially from 30 to 70% by weight, based on the solids content, are employed.

5. The process according to one or more of claims 1 to 4, characterized in that said cleaner rejects are milled in the presence of from 1 to 1000% by weight, especially from 100 to 1000% by weight, of water, based on the solids content.

6. The process according to one or more of claims 1 to 4, characterized in that usual milling aids and/or dispersing aids are added to a slurry having a solids content of from 30 to 85% by weight, especially from 40 to 75% by weight.

7. The process according to any of claims 1 to 6, characterized in that usual milling aids and/or dispersing aids are added in an amount of from 1 to 100% by weight, based on the cleaner rejects.

8. The process according to one or more of claims 1 to 7, characterized in that cleaner rejects are milled to a slurry or powder having a grain size distribution of

from 10 to 99% by weight of particles <1 μm, especially

from 10 to 95% by weight of particles <1 μm,

respectively based on the equivalent diameter.

9. The process according to one or more of claims 1 to 8, characterized in that cleaner rejects are milled to a powder and/or slurry having a grain size distribution of

a) from 95 to 100% by weight of particles <10 μm;

respectively based on the equivalent diameter of the particles.

10. The process according to one or more of claims 1 to 9, characterized in that the milling of the cleaner rejects is performed in a dry mill or wet mill, especially in a vertical agitator ball mill.

11. Use of a cleaner reject slurry according to one or more of the preceding claims for the preparation of a precoating slurry for the paper industry, especially of coating slurries for various segments, such as sheet-fed offset, rotary offset, intaglio printing, cardboard and special papers.

12. Use of cleaner rejects as a slurry or powder according to any of claims 1 to 10 to be employed in the stock in papermaking.