WO2015001182A1

WO2015001182A1 - Production of cpcc with different crystal forms

Info

Publication number: WO2015001182A1
Application number: PCT/FI2014/050536
Authority: WO
Inventors: Päivi SOLISMAA
Original assignee: Upm-Kymmene Corporation
Priority date: 2013-07-01
Filing date: 2014-06-27
Publication date: 2015-01-08
Also published as: EP3017113A4; EP3017113A1

Abstract

The invention relates to a method for precipitating calcium carbonate (PCC) in a non-rhombohedral crystal form, said method comprising: introducing a source of calcium ions and a source of carbonate ions into process water of a mechanical pulp mill; and arranging the carbonate ions to react with the calcium ions to form precipitated calcium carbonate (PCC) in a non-rhombohedral crystal form. This invention further relates to a method for preparing printing paper product comprising PCC made by said novel method. This invention further relates to said paper product.

Description

Production of cPCC with different crystal forms

Field of the Invention The invention relates to a method for precipitating calcium carbonate (PCC) in a non-rhombohedral crystal form. The invention also relates to a method and a system for preparing printing paper product comprising said PCC.

Background of the Invention

Printing paper is paper intended to be printed by printing machines and manufactured, for example, for magazines and newspapers. The printing paper may be coated, such as LWC (light weight coated) paper, or uncoated, such as SC (super calendered) paper or newsprint. The main raw material for printing paper is usually cellulose based natural fibre, for example mechanically pulped natural fibre, chemically pulped natural fibre, and/or recycled fibre. In prior art, there are several established printing paper grades with their own established requirements for the properties of the paper. In some paper products, calcium carbonate is used both as a filler and a coating material, due to factors such as the high brightness of carbonate. One way of producing calcium carbonate is to react calcium ions, for example milk of lime, and carbonate ions, for example carbon dioxide, whereby the resulting calcium carbonate is precipitated from the solution as crystals. The final product of this production method is referred to by the name of PCC, which is an abbreviation of Precipitated Calcium Carbonate. Calcium carbonate can be categorized into three different forms according to their crystal structures: the calcite form, the aragonite form and the vaterite form. Calcite form is the most abundant and the most stable in nature, and it exists in several different shapes, such as scalenohedral shape, rhombo- hedral shape, prismatic shape, spherical shape and plate-like shape.

For making paper coated or filled with PCC, PCC has been conventionally produced in its own separate plant, from where the PCC slurry is either conveyed by pumping trough pipelines or transported by tank trucks to paper mills. The PCC is made in the plant in a batch process using clean water, and the reaction takes several hours. PCC used as either fillers or in coating can have significant effects on the optical performance of paper, for example different crystal types of PCC filler exhibit a different response to colour, and the optical performance of the paper can be invaluable in product development. Therefore, each type of PCC has its particular economic value, for example the scalenohedral form gives very good light scattering properties, and thus PCC in various forms is very interesting. There are many methods to prepare different PCC crystal shapes in the batch process, for example to perform the precipitation under various conditions such as calcium hydroxide concentration, carbon dioxide concentration, temperature, and/or the to use chemical additives.

Summary of the Invention The invention relates to a novel method for precipitating calcium carbonate (PCC) in a non-rhombohedral crystal form. This invention further relates to a method for preparing printing paper product comprising PCC made by said novel method. This invention further relates to said paper product. The method for precipitating PCC in a non-rhombohedral crystal form is primarily characterized in what will be presented in claim 1 . The method for preparing printing paper product comprising said PCC is primarily characterized in what will be presented in claim 9. The system for preparing said paper product is primarily characterized in what will be presented in claim 12.

The method relates especially to continuous process for preparing precipitated calcium carbonate (cPCC).

The method makes it possible to prepare PCC in a non- rhombohedral crys- tal form and to purify the process water that is from paper or pulp industry, such as a mechanical pulp mill, a chemical pulp mill, or a paper machine .

In one aspect, the method for precipitating calcium carbonate in a non- rhombohedral crystal form comprises:

- introducing a source of calcium ions and a source of carbonate ions into the process water that is from paper or pulp industry ; and - arranging the carbonate ions to react with the calcium ions to form precipitated calcium carbonate (PCC) in a non-rhombohedral crystal form.

Advantageously, the process water is from a mechanical pulp mill, a chemical pulp mill, or a paper machine. More advantageously, the process water is from a mechanical pulp.

The process waters of mechanical pulp mills are normally waters that are circulated for reuse in the same process stage or in a different process stage in the mill. These process waters have been in contact with the wood material or with the recycled fibers, and they contain impurities because of the process. These process waters can be filtered to remove solids, which are normally fibers and other solid matter, so that they can be reused in the plant. In this way PCC is made in the process water which is free of solids. Alternatively, process waters can also be non-filtered. In that case calcium carbonate is precipitated on the solids as a coating. This is advantageous because the coating can passivate the impurities. For example impurities are no longer sticky. Advantageously, the method comprises

- after precipitation, collecting and separating the PCC from the process water, and

- obtaining the process water that is substantially purified. Preferably, PCC is precipitated in a continuous process. More preferably, PCC is precipitated by introducing the source of calcium ions and the source of carbonate ions continuously in a flow of the process water. Most preferably, the method is a continuous method for precipitating calcium carbonate (cPCC method). In this continuous method, the process water flows in a pipe and source of the calcium ions and the source of carbonate ions are introduced in the process water during its flow in the pipe. The reaction is instantaneous. Immediately after the second source has been introduced, PCC is formed.

Advantageously, the process water is shower water, circulation water, filtrates such as washing filtrate, pressates or similar from chip or pulp handling , in general any process water from a process of a mechanical pulp mill . The process water can be also waters from a chemical pulp mill, or a paper machine. Advantageously, said mechanical pulp is groundwood, pressure groundwood, super pressure groundwood, or thermomechanical pulp. Advantageously, the source of carbonate ions is carbon dioxide, and the source of calcium ions is milk of lime. The carbon dioxide and milk of lime are injected into the flow of process water. The injection order can be either way. Advantageously, the precipitation is conducted at a starting temperature of 30-80°C. However, adjusting the temperature is not necessary to perform the method. The process waters are normally at this temperature range, because they have absorbed heat in the preceding process.

The process waters usually flow at low pressure. The pressure of carbon dioxide, which is injected in the form of gas, need not be high. Flue gases, which are mixture of carbon dioxide and inert gases, can also be used as the source of carbonate ions.

In another aspect, the method for preparing printing paper product comprises:

- feeding cellulose based material to a system,

- feeding PCC in a non-rhombohedral crystal form to the system, said PCC being made by reacting a source of calcium ions and a source of carbonate ions in process water from paper or pulp industry, such as a mechanical pulp mill, a chemical pulp mill, or a paper machine, and

- manufacturing a printing paper product comprising at least said cellulose based material and said PCC in a non-rhombohedral crystal form as filler.

Alternatively or additionally, the method comprises

- introducing and admixing a source of calcium ions and a source of carbonate ions into the process water,

- precipitating calcium carbonate in a non-rhombohedral crystal form in the process water,

- collecting and separate the precipitated calcium carbonate in a non- rhombohedral crystal form from the process water,

- mixing the cellulose based material and the precipitated calcium carbonate in a non-rhombohedral crystal form. Alternatively or in addition, the cellulose based material is supplied in the form of at least two separate fibre flows to a paper machine, for manufacturing a printing paper product having a layered structure. The PCC is admixed to one flow or more than one flow.

Description of the Drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Figs. 1A-D show SEM images of PCC, wherein PCC produced by a conventional way in continuous process (Fig. 1A) and PCC produced by a method according to one embodiment (Figs. 1 B-1 D) are presented, and Fig. 2 shows the schematic view of a method according to one embodiment.

Fig. 3 shows the general principle of the method.

Detailed Description of the Invention

Pulping is a method for converting logs, wood chips or other plant fiber source into so-called paper pulp, which can be then conveyed to a paper mill for further papermaking process. Pulp can be manufactured by mechanical pulping in a mechanical pulp mill or by using semi-chemical or fully chemical methods, for example kraft or sulfite processes, in a chemical pulp mill . Mechanical pulps including, but not limited to, stone groundwood, pressure groundwood, super pressure groundwood, thermo groundwood, refiner mechanical pulp, pressure refiner mechanical pulp, thermomechanical pulp and so on.

Mechanical pulp mills are, with some exceptions, integrated with a paper mill. Therefore, there is a certain overlapping between mechanical pulping and stock preparation on the one hand and papermaking and associated processes on the other.

In the mechanical pulping process, plenty of water is used in various process points and some of the water streams are recycled into the system. For example, a flow of shower water is always supplied during grinding to control the temperature as well as to carry the pulp away from the surface of the grinder. The shower water together with the pulp is then subject to screening and the screened pulp is partially separated from the water, which then eventually enters the circulation water tank. Besides shower water, the recycle stream candidates are various filtrates such as washing filtrate, pressates or similar from chip or pulp handling and so on. Said streams are fed to specific uses according to their availability and suitability and are referred to herein as "process water", to distinguish from "waste water", which is not applicable before subject to special waste water treatments in a waste water plant.

Many factors influence on the suitable degree of water recycling in the pulping. Some compounds generated by chip pretreatment, refining or bleaching are released into process water in the process points, and dilution is needed to effectively carry away the undesirable compounds from the wood material or pulp, or otherwise they are carried to the succeeding process stages with adverse effects. For example, wood extractives, the main compounds thereof identified included sterols, sterol esters, fatty acids, resin acids, steroid ketones, hydrocarbons and triglycerides, are natural components of wood and are liberated from wood into process water in the production of mechanical pulp. When water circulations are closed, these compounds are accumulated in the process water leading to problems in process operation.

In the present method, where the PCC is made with process water of a mechanical pulp mill, impurities such as wood extractives, carbohydrates and metals can be efficiently reduced in the process water. Preferably, 30% of wood extractives can be reduced in the process water, more preferably 50%, even more preferably 75%. Also, 20% of carbohydrates can be reduced, preferably 40%, more preferably 65%. In the present method, the process water is substantially purified in such a manner that the chemical oxygen demand (COD) in the water can decrease by 70%, preferably 85%, more preferably 95 %.

In another aspect of the present method, PCC can be produced in a non- rhombohedral crystal form. The term "non-rhombohedral crystal form" refers to the crystal shapes that belong to calcite form family, which is the most stable crystal form of calcium carbonate, but these crystal forms of calcite are not rhombohedral. The examples of non-rhombohedral crystal form are scalenohedral shape, prismatic shape and plate-like shape.

Still another aspect of the method is to manufacture paper products comprising PCC in a non-rhombohedral crystal form, said PCC is produced by precipitating calcium carbonate in process water from paper or pulp industry, such as a mechanical pulp mill, a chemical pulp mill, or a paper machine.

Preferably, the printing paper product comprises wood fibres as the main raw material. The content of cellulose based material in the printing paper product, calculated as dry content, is advantageously not higher than 80 w-% and more advantageously not higher than 70 w-%, but preferably at least 50 w-%. Advantageously, at least 30 wt.% or at least 40 wt.%, more preferably at least 50 wt.% or at least 60 wt. %, and most preferably at least 80 wt. % or at least 90 wt.% of the cellulose based material is wood based. The printing paper product may advantageously comprise mechanical pulp, i.e. mechanically pulped cellulose based particles and/or fibres. The mechanical pulp is preferably selected from thermomechanical pulp (TMP), chemi- thermomechanical pulp (CTMP), bleached chemithermomechanical pulp (BCTMP), pressurized ground wood (PGW), ground wood (GW), refiner mechanical pulp (RMP), recovered fibre (RCF), or mixtures of these. The main raw material is preferably softwood, such as pine, Southern pine, and/or spruce. Advantageously, the main raw material of natural fibre is spruce. Advantageously, the amount of the mechanical pulp is at least 30 wt.% or at least 40 wt.%, more preferably at least 50 wt.% or at least 60 wt. %, and most preferably at least 70 wt. % or at least 80 wt.% of the cellulose based material. Alternatively or in addition to the mechanical pulp, the printing paper product may also comprise chemical pulp, i.e. chemically pulped cellulose based material. In an example, the chemical pulp comprises pine, spruce, birch, eucalyptus, and/or acacia. The printing paper product is paper inten- ded to be printed by printing machines and manufactured, for example, for magazines and newspapers. The printing paper product may be coated, such as LWC paper, or uncoated, such as SC paper or newsprint. In the method, the filler in the printing paper product comprises precipitated calcium carbonate in a non-rhombohedral crystal form, for example prismatic, scalenohedral or transitional form between cubic and prismatic or scale- nodehral form. The amount of the PCC in the non-rhombohedral crystal form could be 1 -100 dry w-% and is preferably between 10 and 100 dry w-%, more advantageously between 15 and 50 dry w-% and most advantageously between 20 and 40 dry w-% calculated of the total amount of inorganic fillers in the printing paper product. If the printing paper product is the SC paper, the amount of the PCC in the non-rhombohedral crystal form could be 1 -100 dry w-% and is preferably between 10 and 50 dry w-%, more advantageously between 15 and 40 dry w-% and most advantageously between 20 and 30 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

In this application, reference is made to Figs. 1 to 3, in which the following reference numerals are used:

7 point for precipitating calcium carbonate,

8 process in a mechanical pulp mill,

9 precipitated calcium carbonate in a non-rhombohedral crystal form,

10 source of carbonate ions, and

1 1 source of calcium ions.

Fig. 2 shows the general principle of the method. A source of carbonate ions 10, for example carbon dioxide, and a source of calcium ions, for example milk of lime, are introduced into a flow of process water that is from paper or pulp industry, such as a mechanical pulp mill, a chemical pulp mill, or a paper machine, and precipitated calcium carbonate in a non-rhombohedral crystal form 9 is obtained. The order of the source of carbonate ions 10 and the source of calcium ions 1 1 can be either way.

Fig. 3 shows an embodiment of the method. The process water coming from a process 8 flows through the pipe and the source of calcium ions and the source of carbonate ions are injected into the flow at point 7 where PCC is formed. The PCC 9 is separated from the flow, and the purified process water is recycled back to the process 8 to the same stage or to a different stage. Fig. 4 shows another embodiment of the method. After precipitating, the water is not recycled back to process 8, but instead to some other steps, for example the feed into the paper machine, together with impurities coated and passivated by PCC. In this case, PCC is not separated from the flow.

The present method can be used in any location in a mechanical pulp mill where process water is available. Existing pipes can be used for precipitating calcium carbonate continuously by adding injection devices for injecting the source of calcium ions and the source of carbonate ions.

Examples

PCC trials with non-rhombohedral PCC were implemented, wherein the effect of the precipitation on the purifying process water of a mechanical pulp mill was examined.

Water samples taken from clean water (sample 1 ), clear filtrate of a paper machine (sample 2), washing filtrate of a groundwood mill (sample 3) and circulation water of a groundwood mill (sample 4) were used. Samples were introduced into a system at a flow rate of 2 l/s, and a commercially available feeding device TrumpJet^(R) was used to feed CO₂ and milk of lime into the sample flow by using pressure 3 bar at mixing water flow rate of 0,5 l/s. Table 1 shows the conditions of the test.

The scanning electron microscope (SEM) images of the resulting PCC are shown in Figs. 1A-1 D. As shown in Fig. 1A, PCC made by a conventional method in continuous process, wherein clean water was used, had cubic, rhombohedral form, which is known in the prior art. As a comparison, PCC made with clear filtrate from a paper machine had also cubic shape as shown in Fig. 1 B, but not so sharp as with clean water. Surprisingly, PCC produced with process water of a mechanical pulp mill did not contain any cubic particles, instead it was more like prismatic and scalenohedral in shape, as shown in Fig. 1 C, or showed the tendency to prismatic and/or scalenohedral shape, as shown in Fig. 1 D.

The effect of the precipitation on the purifying process water of a mechanical pulp mill was examined. Table 2 shows the result of examining the contents in the water phase of the samples before and after the precipitation. Detailed analysis on the change of carbohydrates in the water phase is shown in Chart 1 .

Content of aluminium, iron and manganese decreased notably after precipitation. Content of phosphor decreased a lot with samples 3 and 4. That means in samples that had crystal form very clearly different from rhombo- hedral.

As can be seen in the table, the amounts of the extractives, carbohydrates, metals and other elements decreased notably after precipitation in sample 3 and sample 4. Also, as shown in Chart 2, chemical oxygen demand (COD) decreased a lot in sample 3 (column (a)) and sample 4 (column (b)), and this further proves that the amount of organic compounds in water has decreased. Chart 3 also shows the amount of lignin in water phase decreased after precipitation in sample 3 and sample 4.

The contents in PCC were also examined. As shown in Chart 4, a lot of galacturonic acid (pectin) was found in PCC. PCC from washing filtrate from a groundwood mill contained 2 % of it, from circulation water from a groundwood mill 1 % and from paper machine clear filtrate 0,25 %.

It should be noted that the method is not limited solely to the examples presented in Figs. 1 -2 and in the above description, but it can be modified. For example, the source of the process water is not limited to that from a mechanical pulp mill, and any process water from paper or pulp industry, such as a mechanical pulp mill, a chemical pulp mill, or a paper machine, may apply.

Claims

Claims:

1 . A method for precipitating calcium carbonate in a non-rhombohedral crystal form, comprising:

- introducing a source of calcium ions and a source of carbonate ions into process water that is from paper or pulp industry; and

- arranging the carbonate ions to react with the calcium ions to form precipitated calcium carbonate (PCC) in a non-rhombohedral crystal form.

2. The method according to claim 1 , wherein the process water is from a mechanical pulp mill or a chemical pulp mill.

3. The method according to claim 1 , wherein the process water is from a paper machine.

4. The method according to any of preceding claims , wherein PCC is precipitated in a continuous process.

5. The method according to claim 4, wherein PCC is precipitated by introducing the source of calcium ions and the source of carbonate ions continuously in a flow of the process water.

6. The method according to any of the preceding claims, wherein the method comprises

- obtaining the process water that is substantially purified.

7. The method according to any of the preceding claims, wherein the process water is shower water, circulation water, filtrates such as washing filtrate, pressates or similar from chip or pulp handling, or the like in a mechanical pulp mill.

8. The method according to any of the preceding claims , wherein said mechanical pulp is groundwood, pressure groundwood, super pressure groundwood, or thermomechanical pulp.

9. The method according to any of the preceding claims, wherein the source of carbonate ions is carbon dioxide.

10. The method according to any of the preceding claims, wherein the source of calcium ions is aqueous suspension of calcium hydroxide, such as milk of lime.

1 1 . A method for preparing printing paper product, comprising:

- feeding cellulose based material to a system,

- feeding PCC in a non-rhombohedral crystal form to the system, said PCC is made by reacting a source of calcium ions and a source of carbonate ions in process water that is from paper or pulp industry, such as a mechanical pulp mill, a chemical pulp mill, or a paper machine, and

12. The method according to claim 1 1 , comprising,

-mixing the cellulose based material and the precipitated calcium carbonate in a non-rhombohedral crystal form.

13. The method according claim 1 1 or 12, wherein the cellulose based material is supplied in the form of at least two separate fibre flows to a paper machine, for manufacturing a printing paper product having a layered structure.

14. A printing paper product comprising precipitated calcium carbonate in the non-rhombohedral crystal form obtained by a method according to any of claims 1 -13.