WO2011141632A1

WO2011141632A1 - Gypsum -fibre composite product

Info

Publication number: WO2011141632A1
Application number: PCT/FI2011/050426
Authority: WO
Inventors: Tarja Turkki
Original assignee: Kemira Oyj
Priority date: 2010-05-10
Filing date: 2011-05-09
Publication date: 2011-11-17
Also published as: FI20105502L; FI20105502A0

Abstract

The invention relates to a gypsum-fibre composite product, wherein the gypsum appears as crystals on the surface of the fibre and wherein the gypsum crystals are obtained by mixing a water soluble calcium compound and a water soluble sulphate compound with an aqueous fibre suspension such that the calcium compound and the sulphate compound react to form gypsum crystals. The invention also relates to a process for the preparation of the gypsum-fibre composite product. The composite product can be used as a filler or coating pigment in the production of paper.

Description

GYPSUM -FIBRE COMPOSITE PRODUCT

Technical field

The invention relates to gypsum-fibre composite product. The gypsum-fibre com- posite product can be used as a coating pigment or a filler pigment in the production of paper. The invention also relates to a process for the preparation of the gypsum-fibre composite product. In the production of paper improved tensile strength and brightness and homogenous filler distribution can be obtained. Background of the invention

Gypsum or calcium sulphate dihydrate CaSO -2H₂O is suitable as material for both coating pigment and filler, especially in paper products. Especially good coating pigment and filler is obtained if the particular gypsum has high brightness, gloss and opacity. The gloss is high when the particles are sufficiently small, flat and broad (platy). The opacity is high when the particles are refractive, small and of equal size (narrow particle size distribution).

The morphology of the gypsum product particles can be established by examining scanning electron micrographs. Useful micrographs are obtained e.g. with a scanning electron microscope of the type Philips FEI XL 30 FEG. The size of the gypsum product particles is expressed as the weight average diameter D₅₀ of the particles contained therein. More precisely, D₅₀ is the diameter of the presumably round particle, smaller than which particles constitute 50% of the total particle weight. D₅₀ can be measured with an appropriate particle size analyzer, such as Sedigraph 5100. The flatness of a crystal means that it is thin. The form of flat crystals is suitably expressed by means of the shape ratio SR. The SR is the ratio of the crystal length (the longest measure) to the crystal thickness (the shortest transverse measure). By the SR of the claimed gypsum product is meant the average SR of its individual crystals. The platyness of a crystal means that it is broad. Platyness is suitable expressed by means of the aspect ratio AR. The AR is the ratio between the crystal length (the longest measure) and the crystal width (the longest transverse measure). By the AR of the claimed gypsum product is meant the average AR of its individual crystals.

Both the SR and the AR of the gypsum product can be estimated by examining its scanning electron micrographs. A suitable scanning electron microscope is the above mentioned Philips FEI XL 30 FEG.

Equal crystal particle size means that the crystal particle size distribution is narrow. The width is expressed as the gravimetric weight distribution WPSD and it is expressed as (D₇5-D25) D₅o wherein D₇₅, D₂5 and D₅o are the diameters of the presumably round particles, smaller than which particles constitute 75, 25 and 50%, respectively, of the total weight of the particles. The width of the particle distribution is obtained with a suitable particle size analyzer such as the above mentioned type Sedigraph 5100.

Gypsum occurs as a natural mineral or it is formed as a by-product of chemical processes, e.g. as phosphogypsum or flue gas gypsum. In order to refine the gyp- sum further by crystallizing it into coating pigment or filler, it must first be calcined into calcium sulphate hemihydrate (CaSO₄ ½H₂O), after which it may be hydrated back by dissolving the hemihydrate in water and precipitating to give pure gypsum. Calcium sulphate may also occur in the form of anhydrite lacking crystal water (CaSO₄). Depending on the calcination conditions of the gypsum raw material, the calcium sulphate hemihydrate may occur in two forms; as a- and β-hemihydrate. The β- form is obtained by heat-treating the gypsum raw material at atmospheric pressure while the a-form is obtained by treating the gypsum raw material at a steam pressure which is higher than atmospheric pressure or by means of chemical wet cal- cination from salt or acid solutions at 45^°C.

WO 88/05423 discloses a process for the preparation of gypsum by hydrating calcium sulphate hemihydrate in an aqueous slurry thereof, the dry matter content of which is between 20 and 25% by weight. Gypsum is obtained, the largest measure of which is from 100 to 450 μιτι and the second larges measure of which is from 10 to 40 pm.

AU620857 (EP0334292 A1 ) discloses a process for the preparation of gypsum from a slurry containing not more than 33,33% by weight of ground hemihydrate, thereby yielding needle-like crystals having an average size of between 2 and 200 μηη and an aspect ratio between 5 and 50. See page 15, lines 5 to 1 1 , and the examples of this document.

US 2004/0241082 describes a process for the preparation of small needle-like gypsum crystals (length from 5 to 35 μιτι, width from 1 to 5 μιτι) from an aqueous slurry of hemihydrate having a dry matter content of between 5 and 25% by weight. The idea in this US document is to reduce the water solubility of the gypsum by means of an additive in order to prevent the crystals from dissolving during paper manufacture.

DE 32 23 178 C1 discloses a process for producing organic fibres coated with one or more mineral substances. One embodiment comprises mixing cellulose fibres, gypsum and water. The mixture is compacted to give a plastic mass which subsequently is dried and mechanically comminuted to give fine particles. The obtained product can be used as an additive or filler e.g. in bitumen masses or putties.

WO 2007/003697 discloses a method for coating cellulose particles, produced from dissolved cellulose by precipitation, by contacting the cellulose particles with a light scattering material, followed by the attachment of the light scattering material on the surface of the cellulose particles. The size of the cellulose particles is between 0.05 and 10 μιτι. The light scattering material comprises silica, silicate, PCC, gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sul- phate or zinc oxide. The coated cellulose particles may be used as a filler or coating pigment of paper or board.

WO 2008/092990 discloses a gypsum product consisting of intact crystals having a size from 0.1 to 2.0 μιτι. The crystals have a shape ratio SR of at least 2.0, preferably between 2.0 and 50, and an aspect ratio AR between 1 .0 and 10, preferably between 1 .0 and below 5.0.

WO 2008/092991 discloses a process for the preparation of a gypsum product wherein calcium sulphate hemihydrate and/or calcium sulphate anhydrite and water are contacted so that the calcium sulphate hemihydrate and/or calcium sulphate anhydrite and the water react with each other and form a crystalline gypsum product. The formed reaction mixture has a dry matter content of between 34 and 84% by weight.

WO 2010/018301 discloses a gypsum-fibre composite product, wherein the gypsum appears as crystals on the surface of the fibre and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sul- phate anhydrite and an aqueous fibre suspension. The composite product can be used as a filler pigment or coating pigment in the production of paper.

Summary of the invention The aim of the invention is to provide a gypsum-fibre composite product wherein the gypsum is crystallized on the surface of the fibre and attached fairly strongly to the fibre, and wherein alternative raw materials for the production of gypsum in situ are used. According to the present invention it was surprisingly found that calcium sulphate dihydrate can be produced by simply mixing a water soluble calcium compound and a water soluble sulphate compound together with an aqueous fibre suspension in the presence of a small amount of free water originating from the aqueous fibre suspension. The composite product can be used as a filler pigment or coating pigment in the production of paper. In the production of paper improved tensile strength and brightness and homogenous filler distribution can be obtained. Also higher filler load can be obtained.

Brief description of the drawings

Fig. 1 shows scanning electron microscope (SEM) micrograph of calcium sulphate dihydrate -fiber composite product of Example 1 after 6 months storage representing the present invention, and

Fig. 2 shows scanning electron microscope (SEM) micrograph of prior art calcium sulphate dihydrate -fiber composite product of Reference example 1 .

Detailed description of the invention Thus, according to a first aspect of the invention there is provided a gypsum-fibre composite product, wherein the gypsum appears as crystals on the surface of the fibre and wherein the gypsum crystals are obtained by mixing a water soluble calcium compound and a water soluble sulphate compound with an aqueous fibre suspension such that the calcium compound and the sulphate compound reacts to form gypsum crystals.

The present invention is based on the surprising discovery that gypsum can be produced by simply mixing a water soluble calcium compound and a water soluble sulphate compound without the addition of water. This discovery can be applied on the crystallization of gypsum on cellulosic fibres. Although the cellulosic fibre suspension contains water, the cellulosic stock, however, efficiently absorbs and retains water, thereby minimising the amount of free water available for crystallizing gypsum. As the reaction starts and proceeds the crystal water of the calcium com- pound and/or the sulphate compound is released and the compounds dissolve and react with each other to form a slurry of gypsum and another salt.

Due to the small amount of free water, the size of the crystallized gypsum will be small.

The gypsum crystals may have a size of at most 5 μιτι, preferably between 0.1 and 5 μιτι, more preferably between 0.1 and 4 μιτι and most preferably between 0.2 and 4 μιτι. The size of the particles may also be between 0.1 and 2 μιτι, preferably between 0.2 and 2 μιτι. The crystals are rod shaped or rhombohedral. During storage the crystal size is slightly increased.

Preferably at least one of the calcium and sulphate compounds is in the form of a hydrate. In a preferred embodiment both the calcium and sulphate compounds are in the form of a hydrate.

Preferably the water solubility of the calcium and sulphate compounds is at least 100 g/l, preferably at least 200 g/l, determined at a temperature of 25°C.

Suitable calcium compounds include organic salts, such as calcium acetate and hydrate forms thereof, and inorganic salts, such as calcium bromide, calcium chloride, calcium chromate, calcium iodide, calcium nitrate and hydrate forms thereof.

Especially suitable calcium compounds include (the solubility in water at 25°C is in brackets) calcium acetate dihydrate Ca(CH₃COO)2'2H₂O (340 g/l), calcium bromide hexahydrate CaBr₂ 6H₂O (1530 g/l), calcium chloride hexahydrate CaCI₂-6H₂O (830 g/l), calcium chromate dihydrate CaCrO₄ 2H₂O (170 g/l), calcium iodide Cal₂ (2100 g/l ) and calcium nitrate tetrahydrate Ca(NO₃)₂-4H₂O (1380 g/l).

Suitable sulphate compounds include inorganic salts, such as ammonium sulphate, magnesium sulphate, potassium sulphate, sodium sulphate and hydrate forms thereof. Especially suitable sulphate compounds include (the solubility in water at 25°C is in brackets) ammonium sulphate (760 g/l), magnesium sulphate heptahydrate (NH₄)₂SO₄-7H₂O (364 g/l), potassium sulphate K₂SO₄ (120 g/l), sodium sulphate Na₂SO₄, sodium sulphate heptahydrate Na₂SO₄ 7H₂O and sodium sulphate deca- hydrate Na₂SO₄ OH₂O (280 g/l). The solubility of sodium sulphate and sodium sulphate heptahydrate are probably the same as of the sodium sulphate decahy- drate. Preferred combinations of calcium and sulphate salts include calcium nitrate or calcium chloride in hydrate form, such as calcium nitrate tetrahydrate or calcium chloride hexahydrate and sodium sulphate or sodium sulphate in hydrate form, such as sodium sulphate heptahydrate or sodium sulphate decahydrate.

The molar ratio of calcium to sulphate in the calcium and sulphate compounds is typically from 0.7 to 1 .3:1 , preferably from 0.8 to 1 .2:1 , more preferably from 0.9 to 1 .1 :1 , and most preferably the compounds are introduced in stoichiometric amounts.

The fibre of the gypsum-fibre composite product preferably comprises a cellulosic fibre such as a chemical, mechanical, chemi-mechanical or deinked pulp fibre or a synthetic fibre, such as a polyolefine, e.g. polypropene. Chemical pulps include kraft pulp and sulphite pulp. Mechanical pulps include stone groundwood pulp (SGW), refiner mechanical pulp (RMP), pressure groundwood (PGW), thermome- chanical pulp (TMP), and also chemically treated high-yield pulps such as chemi- thermomechanical pulp (CTMP). Deinked pulp can be made using mixed office waste (MOW), newsprint (ONP), magazines (OMG) etc. Also mixtures of different pulps can be used.

The average length of the fibre is preferably between 0.5 and 5 mm. Cellulosic fibres derived from softwood typically have an average length of between 1 and 5 mm, preferably between 2 and 4 mm. Cellulosic fibres derived from hardwood typ- ically have an average length of between 0.5 and 3 mm, preferably between 1 and 2 mm.

Preferably the weight ratio of gypsum to fibre on dry basis is in the range from 95:5 to 50:50, preferably from 75:25 to 50:50.

According to the invention the gypsum-fibre composite product may additionally comprise additional substances such as a natural or synthetic polymer binder and/or an optical brightener and/or a rheology modifier. According to the invention the crystallized gypsum is preferably rod shaped or needle shaped or rhomboedric. By using the crystallized gypsum, a gypsum product is obtained giving high brightness.

As was stated before, the gypsum product of the invention is typically a coating or filler pigment. In addition to use as a paper additive, it can also be used as plastics filler, and as a raw material in glass industry, cosmetics, printing inks, building materials and paints.

According to a second aspect of the invention there is provided a process for the preparation of a gypsum-fibre composite product comprising mixing a water solu- ble calcium compound and a water soluble sulphate compound with an aqueous fibre suspension such that the calcium compound and the sulphate compound reacts to form gypsum crystals on the surface of the fibre.

In a preferred embodiment the calcium compound and the sulphate compound are added to the aqueous fibre suspension in the form of a powder. The calcium com- pound and the sulphate compound may be added simultaneously or sequentially.

The obtained gypsum-fibre composite product and the calcium compound and the sulphate compound are as defined above.

Preferably the weight ratio of the calcium compound and the sulphate compound to water in the crystallization is in the range from 0.5 to 2:1 , preferably from 0.6 to 1 .5:1 .

The dry fiber content in the crystallization is preferably from 1 to 20% by weight, more preferably from 1 to 10% by weight.

Preferably the content of the calcium compound and the sulphate compound in the crystallization is from 10 to 57% by weight. The obtained gypsum-fibre composite product may be washed to remove the other salt compound formed in the reaction between the calcium compound and the sulphate compound.

The process of the invention may additionally comprise the steps of drying and comminuting the obtained product to form a gypsum-fibre composite product in the form of particles. According to the invention a fixative can be introduced into the crystallization. The fixative can be selected from the group consisting of poly aluminum chloride, poly diallyldimethylammonium chloride (poly DADMAC), anionic and cationic polyacry- lates. The temperature of the reaction mixture can be anything between 0 and 100^°C. Preferably, the temperature is between 0 and 80^°C, more preferably between 0 and 50^°C, even more preferably between 0 and 40^°C, most preferably between 0 and 25^°C.

As the calcium compound, the sulphate compound, aqueous fibre suspension and optionally additives have been contacted, they are allowed to react into calcium sulphate dihydrate i.e. gypsum. The reaction takes place by mixing, preferably by mixing strongly, said substances together for a sufficient period of time, which can easily be determined experimentally. At high dry matter contents strong mixing is necessary because, the slurry is thick and the reagents do not easily come into contact with each other. The initial pH is typically between 3.5 and 9.0, most preferably between 4.0 and 7.5. If necessary, the pH is regulated by means of an aqueous solution of NaOH and/or H₂SO₄, typically a 10% solution of NaOH and/or H₂SO₄.

Because gypsum has a lower solubility in water than the water soluble starting calcium and sulphate compounds, the gypsum formed by the reaction of the calcium compound and the sulphate compound immediately tends to crystallize from the aqueous medium.

The gypsum-fibre composite product of the present invention can also be treated with other additives. A typical additive is a biocide which prevents the activity of microorganisms when storing and using the product.

According to a third aspect of the invention there is provided a paper product comprising the gypsum-fibre composite product of the invention as a filler pigment or coating pigment.

Preferably the amount of the gypsum-fibre composite product in the paper product is from 20 to 100% by weight on dry basis. Other preferred ranges are from 20 to 90% and 20 to 80% and 20 to 70% and 20 to 60% and 20 to 50% by weight on dry basis. Additional preferred ranges are from 30 to 100% and from 40 to 100% and from 50 to 100% and 60 to 100% by weight on dry basis. The balance of the paper product comprises a cellulosic fibre such as a chemical, mechanical, chemi-mechanical or deinked pulp fibre as defined above.

Additionally the invention relates to the use of the gypsum-fibre composite product of the invention as a filler pigment or coating pigment in the production of paper.

EXAMPLES

In the following the invention will be illustrated in more detail by means of examples. The purpose of the examples is not to restrict the scope of the claims. In this specification the percentages refer to % by weight unless otherwise specified. The samples were imaged using SEM microscope FEI XL 30 FEG.

Reference example 1

A reference gypsum-fiber composite was prepared from calcium sulphate hemi- hydrate and 4% eucalyptus kraft pulp in line with the process described in WO 2010/018301 . The obtained gypsum-fiber composite is shown in Fig. 2. Example 1

800g of 4% eucalyptus kraft pulp slurry was placed in Hobart N50CE mixer. First 1 .38 mols of sodium sulphate decahydrate Na₂SO₄ OH₂O (444g) was added under mixing and then 1 .38 mols calcium nitrate tetrahydrate Ca(NO3)2'4H₂O (326g) was added. Mixing was continued till the reaction was completed. The obtained readymade gypsum-fiber composite was stored for 6 months. The structure is shown in Fig. 1 . The particle size of the gypsum grew slightly during the storage period.

The particle shape is more rhombohedral in Example 1 than in Reference example 1 possible due to the high salt content of the liquid phase. Example 2

Application tests were carried out by using the gypsum-fiber composite of Example 1 and Reference example 1 as filler. Hand sheets were made using Rapid- Kothen laboratory sheet former, eucalyptus kraft pulp (SR about 30), Fennopol 3400R (cationic polyacrylamide) as retention aid and the tested fillers. Target ba- sis weight was 60 g/m² and filler level was 25% (refers to the gypsum level). The tensile strength was measured using L&W tensile tester and ISO standard 1924-3. The gypsum-fiber composite filler of Example 1 gave the tensile strength index of 28.4 kNm/kg and the gypsum-fiber composite filler of Reference example 1 gave the tensile strength index of 21 .3 kNm/kg. Thus, the application test results show that the filler of Example 1 gave a substantially higher tensile strength than the filler of Reference example 1 .

Example 3

The brightness of the gypsum-fiber composite fillers of Example 1 and Reference example 1 were measured. The filler of Example 1 gave the ISO brightness of 91 .4% and the filler of Reference example 1 gave the ISO brightness of 90.4%. Thus, the application test results show that the filler of Example 1 gave a higher brightness of about 1 % unit than the filler of Reference example 1 .

Claims

1 . A gypsum-fibre composite product, wherein the gypsum appears as crystals on the surface of the fibre and wherein the gypsum crystals are obtained by mixing a water soluble calcium compound and a water soluble sulphate compound with an aqueous fibre suspension such that the calcium compound and the sulphate compound reacts to form gypsum crystals.

2. The gypsum-fibre composite product according to claim 1 , wherein the gypsum crystals have a size of at most 5 μιτι, preferably between 0.1 and 5 μιτι, more preferably between 0.1 and 4 μιτι, and most preferably between 0.2 and 4 μιτι.

3. The gypsum-fibre composite product according to claim 1 or 2, wherein at least one of the calcium and sulphate compounds is in the form of a hydrate or both the calcium and sulphate compounds are in the form of a hydrate.

4. The gypsum-fibre composite product according to any of claims 1 to 3, wherein the calcium compound comprises an organic salt, such as calcium ace- tate or a hydrate form thereof, preferably calcium acetate dihydrate, or an inorganic salt, such as calcium bromide, calcium chloride, calcium chromate, calcium iodide, calcium nitrate or a hydrate form thereof, preferably calcium bromide hexahydrate, calcium chloride hexahydrate, calcium chromate dihydrate, calcium iodide or calcium nitrate tetrahydrate.

5. The gypsum-fibre composite product according to any of claims 1 to 4, wherein the sulphate compound comprises an inorganic salt, such as ammonium sulphate, magnesium sulphate, potassium sulphate, sodium sulphate or a hydrate form thereof, preferably ammonium sulphate, magnesium sulphate heptahydrate, potassium sulphate, sodium sulphate, sodium sulphate heptahydrate or sodium sulphate decahydrate.

6. The gypsum-fibre composite product according to any of claims 1 to 5, wherein the calcium compound is calcium nitrate or calcium chloride in hydrate form, such as calcium nitrate tetrahydrate or calcium chloride hexahydrate, and the sulphate compound is sodium sulphate or sodium sulphate in hydrate form, such as sodium sulphate heptahydrate or decahydrate.

7. The gypsum-fibre composite product according to any of claims 1 to 6, wherein the fibre comprises a cellulosic fibre such as a chemical pulp fibre, e.g. kraft pulp fibre, or a mechanical pulp fibre including chemi-mechanical pulp fibre, or a deinked pulp fibre, or a synthetic fibre.

8. The gypsum-fibre composite product according to any of claims 1 to 7, wherein the weight ratio of gypsum to fibre on dry basis is in the range from 95:5 to 50:50, preferably from 75:25 to 50:50.

9. A process for the preparation of a gypsum-fibre composite product comprising mixing a water soluble calcium compound and a water soluble sulphate compound with an aqueous fibre suspension such that the calcium compound and the sulphate compound reacts to form gypsum crystals on the surface of the fibre.

10. The process according to claim 9, wherein the calcium compound and the sulphate compound are added to the aqueous fibre suspension in the form of a powder.

1 1 . The process according to claim 9 or 10, wherein the gypsum crystals have a size of at most 5 μιτι, preferably between 0.1 and 5 μιτι, more preferably between 0.1 and 4 μιτι, and most preferably between 0.2 and 4 μιτι.

12. The process according to any of claims 9 to 1 1 , wherein at least one of the calcium and sulphate compounds is in the form of a hydrate or both the calcium and sulphate compounds are in the form of a hydrate.

13. The process according to any of claims 9 to 12, wherein the calcium com- pound comprises an organic salt, such as calcium acetate or a hydrate form thereof, preferably calcium acetate dihydrate, or an inorganic salt, such as calcium bromide, calcium chloride, calcium chromate, calcium iodide, calcium nitrate or a hydrate form thereof, preferably calcium bromide hexahydrate, calcium chloride hexahydrate, calcium chromate dihydrate, calcium iodide or calcium nitrate tetra- hydrate.

14. The process according to any of claims 9 to 13, wherein the sulphate compound comprises an inorganic salt, such as ammonium sulphate, magnesium sulphate, potassium sulphate, sodium sulphate or a hydrate form thereof, preferably ammonium sulphate, magnesium sulphate heptahydrate, potassium sulphate, so- dium sulphate, sodium sulphate heptahydrate or sodium sulphate decahydrate.

15. The process according to any of claims 9 to 14, wherein the calcium compound is calcium nitrate or calcium chloride in hydrate form, such as calcium ni- trate tetrahydrate or calcium chloride hexahydrate, and the sulphate compound is sodium sulphate or sodium sulphate in hydrate form, such as sodium sulphate heptahydrate or decahydrate.

16. The process according to any of claims 9 to 15, wherein the fibre comprises a cellulosic fibre such as a chemical pulp fibre, e.g. kraft pulp fibre, or a mechanical pulp fibre including chemi-mechanical pulp fibre, or a deinked pulp fibre, or a synthetic fibre.

17. The process according to any of claims 9 to 16, wherein the weight ratio of gypsum to fibre on dry basis is in the range from 95:5 to 50:50, preferably from 75:25 to 50:50.

18. The process according to any of claims 9 to 17, wherein the weigh ratio of the calcium compound and the sulphate compound to water in the crystallization is in the range from 0.5 to 2:1 , preferably from 0.6 to 1 .5:1 .

19. The process according to any of claims 9 to 18, wherein the dry fibre content in the crystallization is from 1 to 20% by weight, preferably from 1 to 10% by weight.

20. The process according to any of claims 9 to 19, wherein the content of the calcium compound and the sulphate compound in the crystallization is from 10 to 57% by weight.

21 . The process according to any of claims 9 to 20, wherein the process additionally comprises drying and comminuting the obtained product to form a gypsum-fibre composite product in the form of particles.

22. Use of a gypsum-fibre composite product as defined is any of claims 1 to 8 as a filler pigment or coating pigment in the production of paper.

23. A paper product comprising a gypsum-fibre composite product as defined in any of claims 1 to 8 as a filler pigment or coating pigment.

24. The paper product according to claim 23, wherein the amount of the gypsum-fibre composite product is from 20 to 100% by weight on dry basis.