WO2007067146A1

WO2007067146A1 - A method of producing precipitated calcium carbonate

Info

Publication number: WO2007067146A1
Application number: PCT/SE2006/050561
Authority: WO
Inventors: Philip HÅKANSSON
Original assignee: Stora Enso Ab
Priority date: 2005-12-07
Filing date: 2006-12-07
Publication date: 2007-06-14
Also published as: FI123286B; FI20080427A

Abstract

A method of producing precipitated calcium carbonate (PCC), for use as a filler in paper or paper board production, wherein the carbonating of calcium hydroxide is performed in the presence of starch and/or carboxy methyl cellulose. The precipitated calcium carbonate produced by the method of the invention decreases the dusting tendency and increases the strength of the paper or paper board when used as a filler. Furthermore, the use of the inventive precipitated calcium carbonate enables the paper maker to increase the amount of precipitated calcium carbonate, and thus to increase the amount of fillers, in the paper or paper board. The invention further relates to a precipitated calcium carbonate and a paper or a paper board comprising said precipitated calcium carbonate as a filler.

Description

METHOD OF PRODUCING PRECIPITATED CALCIUM CARBONATE

Field of the invention

The present invention relates to a method for producing a precipitated calcium carbonate for use as a filler in paper making production. Other embodiments are related to a precipitated calcium carbonate, to a paper or paper board and to a method of producing a paper or paper board.

Background

In paper making production, fillers are added to the furnish in order to improve optical and surface properties of the paper. Increasing the filler content of paper can provide the papermaker with numerous benefits, including savings in the cost of raw materials and improved optical properties .

Typically, clay or different forms of calcium carbonate are used as fillers. Calcium carbonate can for example be in the form of chalk, marble or precipitated calcium carbonate (PCC) . In recent years, precipitated calcium carbonate (PCC) has become common. Today, PCC is the most prevailing filler used in the production of fine paper.

Precipitated calcium carbonate (PCC) can be produced by calcining limestone (calcium carbonate) at high temperature to dry off C02, slaking the resulting lime (calcium oxide) by addition of water to form a lime suspension (calcium hydroxide) , and then treating the resulting lime suspension with C02 gas.

There are, however, limits to the amount of filler that can be substituted for papermaking fiber at the paper or paper board production. At high filler contents, the paper or the paper board can suffer losses in stiffness and in strength. Furthermore, the dusting tendency of paper or paper board increases at higher filler contents. High filler content in, e.g. newsprint paper, involves problems at the printing presses due to the increased dusting tendency and the losses in strength.

Efforts to reduce the dusting tendency of fine paper include the treatment of both sides of the paper with starch in the paper machine. This treatment, the so called surface sizing of the paper, is expensive and reduces the productivity of the paper machine.

Other attempts to solve these problems have been done by pretreatment of the filler before being mixed with the celllulosic suspension, e.g. by mixing the filler with certain additives such as polymers. US5827398 describes for example a process wherein PCC is mixed with a cationic polymer. However, the cationic polymer added to the PCC is repelled by the slightly cationic PCC. On the other hand, if PCC is mixed with anionic polymers, the PCC-particles become non-charged whereby their affinity to the fibers is decreased. Moreover, the shear forces in the papermaking process peals the added polymers away from the PCC- surfaces. Thus, these attempts of pretreating the PCC filler have not been satisfying.

Description of the invention

One object of the present invention is to decrease the dusting tendency of paper or paper board.

Another object of the present invention is to allow for the paper maker to increase the filler content of the paper.

Yet another object of the present invention is to allow for the paper maker to increase the filler content of the paper without substantially increasing the dusting tendency or decreasing the strength of the paper or board. Yet another object of the present invention is to produce a fine paper with a low dusting tendency, without surface size the paper.

It has now surprisingly been shown that one or more of the above objects as well as other advantages can be achieved by providing a paper-making process which utilizes a precipitated calcium carbonate filler produced by a method wherein the above described reaction between lime (calcium hydroxide) and carbon dioxide occurs in the presence of starch and/or carboxy methyl cellulose (CMC) .

The present invention relates to a method of producing precipitated calcium carbonate (PCC) , for use as a filler in paper or paper board production, comprising the steps of providing a suspension of calcium hydroxide, carbonating said suspension of calcium hydroxide, wherein said step of carbonating said calcium hydroxide is performed in the presence of starch and/or carboxy methyl cellulose. The step of carbonating said calcium hydroxide is preferably performed by bubbling a gas stream containing carbon dioxide into a slurry of calcium hydroxide. This can be performed at atmospheric pressure or at a pressure above atmospheric pressure.

It has been shown that the use of the precipitated calcium carbonate produced by the method of the invention as a filler in paper or paper board decreases the dusting tendency and increases the strength of the paper or paper board. Furthermore, the use of the inventive precipitated calcium carbonate enables the paper maker to increase the amount of precipitated calcium carbonate, and thus to increase the amount of fillers, in the paper or the paper board without substantially increase the dusting tendency or decrease the strength of the paper.

Although not wishing to be bound by a particular theory, it is believed that the affinity of the PCC particles formed by the method of the invention to the fibers is improved, since the starch and/or the cmc has a binding effect due to its sticky character. When the starch and/or the cmc is present at the carbonation of calcium hydroxide, i.e at the formation of the PCC particles, the starch and/or cmc is incorporated in the PCC particles whereby problems with the starch and/or the cmc being pealed off from the PCC

particles, connected to the prior art solutions, are avoided.

The calcium hydroxide solution can be provided by calcining limestone at a high temperature and slaking the resulting lime by addition of water. In this way, the carbon dioxide liberated by the burning of limestone can be used to carbonate the calcium hydroxide.

In one preferred embodiment of the present invention, the step of carbonating said calcium hydroxide is preformed in the presence of starch. The starch can be nonionic,

anionic, cationic, or amphoteric starch, or a mixture of two or more of said starches. The starch can be unmodified, oxidized, crosslinked, in the form of an ester or an ether or modified in any other way. The starch can be based on any raw material, e.g. potato, maize, wheat, tapioca, rize, corn, waxy maize or waxy corn. Most preferably, the ratio between starch and the precipitated calcium carbonate (PCC) formed is within the interval of 1/100 and 25/100.

In another preferred embodiment of the present invention, the step of carbonating the suspension of calcium hydroxide is performed in the presence of carboxy methyl cellulose. Most preferably, the ratio between said carboxy methyl cellulose and the precipitated calcium carbonate formed is within 1/100 to 15/100. The carboxy methyl cellulose can be nonionic, anionic or cationic carboxy methyl cellulose or a mixture of two or more of said carboxy methyl celluloses. The Carboxy methyl cellulose can be of any substitution grade or have any length of molecular chain.

The degree of substation of the starch and/or the cmc chosen depend on the wet-end chemistry, and especially on the retention agent system, of the paper machine that is used to produce the paper containing the PCC. A cationic starch or carboxy methyl cellulose is preferably used when the z-potential in the headbox is sufficiently negative to prevent fibre flocculation, e.g. at z-potentials of -5 or less. However, if the z-potential is close to 0, e.g.

between -1 and -5, an anionic or cationic starch or carboxy methyl cellulose is preferably used.

The precipitated calcium carbonate of the invention is most preferably not produced in the presence of fibers, since fibers and fines can disturb the incorporation of the starch and/or the carboxy methyl cellulose in the PCC particles. Moreover, the production of precipitated calcium carbonate separated from the paper-making process, i.e. not in-situ during paper-making, makes it easier to control the process .

The method of producing precipitated calcium carbonate according to the invention is performed at normal reaction conditions with regard to temperature, pH and pressure etc., although other conditions are contemplated.

The starch and/or the CMC can be added to the reaction mixture in dry form, as solutions or as slurries.

In one embodiment of the present invention, the starch and/or the CMC is added to the lime suspension before the carbonation .

In one embodiment of the invention, the suspension of calcium hydroxide comprises dissolved starch and/or carboxy methyl cellulose and is provided by continuously mixing said lime, water and the starch and/or the carboxy methyl. In this embodiment, lime, water and starch and/or CMC are continuously conducted to a reaction tank, whereafter the therein produced calcium hydroxide comprising starch and/or carboxy methyl cellulose is further conducted to the carbonating tank for carbonation. In this embodiment, the heat produced by the exothermic reaction when the calcium oxide reacts with water to form calcium hydroxide can be used to dissolve the starch and/or the CMC. One advantage of this embodiment is that the starch and/or the CMC can be added in a dry form and do not have to be dissolved and/or digested in a separate step, whereby the process is simplified and the energy consumption is decreased.

In another embodiment of the present invention, the starch and/or the CMC is added to the lime (calcium oxide) prior to the addition of water and the formation of calcium hydroxide. Similarly to the embodiment described above, the heat produced by the exothermic reaction when the calcium oxide reacts with the water to form calcium hydroxide can be used to dissolve the starch and/or the CMC

In yet another preferred embodiment of the present

invention, the starch and/or the CMC is slurred in water to form a starch and/or CMC slurry, whereupon lime (calcium oxide) is added to said slurry.

The invention further relates to a precipitated calcium carbonate produced by the method of producing precipitated calcium carbonate as described above. Furthermore, the invention relates to a paper or paper board comprising said inventive precipitated calcium carbonate and to a method of producing paper or paper board comprising providing a fiber-containing suspension containing cellulose fibers, adding the precipitated calcium carbonate of the invention as a filler to said suspension and subsequently forming and dewatering the fiber containing suspension

The precipitated calcium carbonate can be added to the fiber containing suspension as a slurry at any stage of the paper making process prior to the head box including, but not limited to on the suction side of the machine chest pump, on the suction side of the fan pump or on the suction side of the mixing pump. The inventive precipitated calcium carbonate can be added to the fiber containing suspension as a pure solution or slurry of precipitated calcium carbonate, or mixed with other additives or fillers. The precipitated calcium carbonate produced by the method of the invention can for example be mixed with clay, talc, calcium sulfate hemihydrate, and/or calcium sulfate

dehydrate prior to its add to the fiber containing

suspension. The fiber containing suspension employed in the present invention can comprise any kind of pulp, including but not limited to chemical pulps, including sulfate and sulfite pulps from both hard and soft woods, thermo- mechanical pulps, mechanical pulps and ground wood pulps. Furthermore, the fiber containing suspension can include one or more conventional paper additives such as

hydrophobing agents, dry strength agents, wet strength agents etc.

The precipitated calcium carbonate of the invention can be used as a filler in the production of any kind of paper or paper board. The invention is of particular value in the production of fine paper, super-calendered paper or

newsprint paper. The use of the precipitated calcium carbonate of the invention in, e.g. newsprint paper, improves the quality of the paper significantly. The increase of filler in the newsprint paper, which the present invention makes possible, increases the ash content whereby the printing quality and the opacity of the paper is improved.

By the method of the invention, the amount of filler in the paper can be increased by at least 5%. The content of precipitated calcium carbonate (PCC) produced according to the present invention in uncoated fine paper can be e.g. 25-35 % by weight based on dry paper. In newsprint paper, the content of precipitated calcium carbonated (PCC) produced according to the present invention can be e.g. 10- 15%.

Detailed description of a preferred embodiment

Below is a detailed description of a preferred embodiment of the present invention. The basic steps of calcination, slaking and carbonation are well known in the art and only presented briefly herein.

Step I: Preparation of Calcium Hydroxide solution

A sample of limestone is calcined in a rotating kiln at high temperature in order to dry off carbon dioxide. The kiln temperature range is from an initial temperature of about 900 ⁰C to a final temperature of about 1000 ⁰C. These temperatures can vary dependent upon the nature of the limestone being calcined. The lime is thereafter slaked in water to give a suspension of hydrated lime of a

concentration of. about 200g/l.

Step II: Mixing with starch

27 g of a standard cationic potato starch supplied by

Emfland Starke with a degree of substitution of 0.045 is added to 1 1 of the slurry of calcium hydroxide produced in step. I. The slurry is stirred at an initial temperature of about 90 ⁰C for about 30 minutes.

Step III: Preparation of Calcium Carbonate

The starch containing calcium hydroxide solution produced in step II is transferred to an atmospheric pressure reactor. A gas stream containing carbon dioxide at about 10-20wt% by volume is bubbled into the reactor at a rate of about 1 L carbon dioxide per minute at atmospheric

pressure. The carbon dioxide is applied to the solution until the pH reaches about 7 to 8.

The produced precipitated calcium carbonate is filtered whereby a PCC slurry of about 20w% is obtained. The PCC slurry is added to a furnish of a mixture of 70% hardwood craft pulp and 30% softwood craft pulp in a pulp chest prior to being supplied to the head box of the paper machine. The PCC is added to the furnish in an amount of about 28wt% of the thereof produced paper. The furnish is thereafter applied to a wire and subsequently formed and dewatered in conventional manners. The paper produced by the method is uncoated surface sized office paper.

In addition to the example embodiments of the invention described herein above, other embodiments, other steps and other elements are also contemplated. Those knowledgeable in the art will appreciate that the present invention lends itself to other papers and other methods for making papers that will be understood based on consideration of the detailed description of example invention embodiments provided herein above.

Claims

Patent claims

1. A method of producing precipitated calcium carbonate

(PCC) , for use as a filler in paper or paper board production, comprising the steps of;

providing a suspension of calcium hydroxide,

carbonating said suspension of calcium hydroxide, wherein said step of carbonating said calcium

hydroxide is performed in the presence of starch and/or carboxy methyl cellulose.

2. The method according to claim 1, wherein the

suspension of calcium hydroxide is provided by the steps of; ^■

calcining limestone at high temperature

slaking the resulting lime by addition of water.

3. The method according to anyone of claims 1-2, wherein the starch and/or the carboxy methyl cellulose is added to the calcium hydroxide suspension before the treatment with carbon dioxide .

4. The method according to claim 2 , wherein the

suspension of calcium hydroxide comprises dissolved starch and/or carboxy methyl cellulose and is provided by continuously mixing said lime, water and the starch and/or the carboxy methyl.

5. The method according to claim 2 , wherein the starch and/or carboxy methyl cellulose is added to the lime prior to -the addition of water.

6. The method according to claim 2 , wherein the

suspension of calcium hydroxide comprises dissolved starch and/or carboxy methyl cellulose and is provided by the steps of;

slurring starch and/or carboxy methyl cellulose in water;

adding lime to said slurry.

7. The method according to anyone of claims 1-6, wherein said step of carbonating said suspension of calcium hydroxide is performed in the presence of starch.

8. The method according to anyone of claims 1-6, wherein said step of carbonating said suspension of calcium hydroxide is performed in the presence of carboxy methyl cellulose .

9. The method according to claim 7 , wherein the starch is present in the calcium hydroxide in an amount so that the ratio between the starch and the precipitated calcium carbonate formed is within 1/100 to 25/100.

10. The method according to claim 8, wherein the

carboxy methyl cellulose is present in the calcium hydroxide in an amount so that the ratio between said carboxy methyl cellulose and the precipitated calcium carbonate formed is within 1/100 to 15/100.

11. A precipitated calcium carbonate produced by the method according to anyone of claims 1 - 10.

12. A method of producing paper or paper board

comprising the steps of;

providing a fiber-containing suspension;

adding the calcium carbonate according to anyone of claims 1 - 10 as a filler to said suspension;

forming and dewatering the fiber containing

suspension.

13. Paper or paper board having as a filler

precipitated calcium carbonate produced by the method of anyone of claims 1 - 10.

14. The paper according to claim 13 , wherein the paper is fine paper and the total filler content of the paper is from 25 - 35% by weight, based on dry paper.

1⁵. The paper according to claim 13 , wherein the paper is newsprint paper and the total filler content of the paper is from 10 - 15% by weight, based on dry paper.