Method and arrangement for producing pcc

Abstract

The present invention relates to a method and arrangement for producing precipitated calcium carbonate (PCC) from ground calcium carbonate (GCC). The method comprises the following steps: a) an aqueous process fluid (1), carbon dioxide CO2 (3) and GCC (2) are mixed together and a pressure is set, b) the carbon dioxide dissolves, whereupon the pH falls, c) in the said conditions, the GCC or a part of the GCC dissolves: CaCO3 (s) + 2 H+ (aq) --> Ca2+(aq) + CO2 (g) + H2O(I), d) pressure is reduced, e) the carbon dioxide is released as gas, whereupon the pH rises and Ca2+ and CO2 are crystallized as PCC, f) degassing is carried out, CO2 is led through the recycling line back to process step a), g) the precipitated calcium carbonate (PCC) or its derivatives, such as PCC agglomerates, GCC+PCC clusters or the PCC / GCC mixture, are led forward.

Description

Method and arrangement for producing PCCField of the invention

[0001] The present invention relates to a method and arrangement for producing calcium carbonate CaCCh crystals, that is, PCC, in a process fluid, or suspension, most preferably directly in the process fluid flow itself. More specifically, the invention relates to the method according to the preamble of claim 1 .

[0002] The invention further relates to an arrangement according to the preamble of claim 15.Technical background

[0003] Calcium carbonate is commonly used in papermaking processes as both filler and coating material due to, among others, the high brightness and low cost of carbonate. Calcium carbonate can be produced by grinding from chalk, marble or limestone, which is then called ground calcium carbonate, or calcite, commonly referred to and abbreviated as GCC (Ground Calcium Carbonate). Another method of producing calcium carbonate is the chemical method, in which e.g. carbonate ions, formed when the calcium ions, the other constituent of calcium hydroxide, and carbon dioxide are dissolved in water, are allowed to react, whereby the formed calcium carbonate is precipitated from the solution as crystals the shape and size of which depends on e.g. the reaction conditions. The end product of this production method is called PCC, which is an abbreviation of the words Precipitated Calcium Carbonate (i.e. crystalline CaCOs). The chemical composition of PCC and GCC is the same, but they differ in crystal form and purity. GCC is an irregular, fine, ground, crushed product, whereas PCC crystals have a uniform and regular shape. This invention concentrates on the production of PCC and on an arrangement for producing it in the process apparatus as an in-line process.

[0004] T raditionally, the production of PCC has taken place separate from the actual production process of the main product, e.g. paper manufacture. So far, PCC has been produced either at a dedicated plant located near the paper mill, from which the PCC slurry is directed by pumping along pipelines to paper production, or at a corresponding plant from which the PCC slurry is transported by tank trucks to paper mills located farther away. PCC produced by this method requires the use of retention materials in papermaking in order to have the PCC fastened to the fibers, regardless of whether the fibers are produced chemically or mechanically. The use of retention materials naturally causes additional costs to papermaking in the form of acquiring the chemical itself and as precipitation or recyclability problems possibly caused by the chemical. The traditional method of producing PCC briefly described above brings about problems in addition to the problems relating to the use of retention materials. Tank transportation of PCC to the paper mill from the production site causes transport costs and requires the use of dispersing agents and biocides. The use of the additives affects the properties of PCC while still increasing the acquiring and processing costs of the chemicals.

[0005] There have been numerous suggestions for producing PCC directly at the paper mill for reducing the production costs, whereby at least the transport costs of PCC are eliminated from the cost structure of paper. Additionally, in-line production of PCC in the presence of fiber suspension leads to better fastening of PCC crystals to the fibers, whereby the need for retention materials is at least reduced and in some cases their use can be totally eliminated. In this context, in-line production means producing PCC directly to a suspension used in the production of a fibrous web, fiber product or the like so that PCC or the suspension is not kept in intermediate storage but it is directly used in the production of the fibrous web, fiber product or the like.

[0006] One industrially applicable in-line method for producing PCC is discussed in the patent application WO-A2-2009 / 103854. This disclosure teaches production of PCC from carbon dioxide and lime milk so that the carbon dioxide and lime milk are mixed very effectively, preferably by using injection mixers, directly into the pulp in the flow pipe transporting the pulp to the headbox of the paper machine. Thereby, due to the efficient mixing, the carbonate ions and the calcium ions are located close to each other and the formation of crystals is very fast.

[0007] There are also numerous other uses for PCC, where the ease of PCC production in connection with the actual product process, minimization of the amount of extra additives and the simplicity of the method would be an advantage.Description of the invention

[0008] The aim of the invention is to provide a method that works efficiently and economically as well as reliably to produce PCC in different processes. An aim of the invention is also to provide an arrangement by means of which the said method can be implemented. The aims of the invention are mainly achieved as disclosed in greater detail in independent claims 1 and 15 as well as in the other claims.

[0009] An additional aim of the invention is to provide a method by means of which precipitated calcium carbonate (PCC) can be produced simply and efficiently from ground calcium carbonate (GCC). The method generally comprises the following steps: a) an aqueous process fluid (1), carbon dioxide CO2 (3) and GCC (2) are mixed together and the pressure is set, b) the carbon dioxide dissolves, whereupon the pH falls, c) the GCC or a part of the GCC dissolves:CaCO3(s) + 2 H+(aq) Ca2+(aq) + CO2(g) + H2O(I), d) pressure is reduced,e) the carbon dioxide is released as gas, whereupon the pH rises and Ca2+and CO2 are crystallized as PCC, f) degassing is carried out, CO2 is led through the recycling line back to process step a) and / or for other use, g) the process fluid and the precipitated calcium carbonate (PCC) or its derivatives, such as PCC agglomerates, fiber+PCC composites, GCC+PCC clusters or the PCC / GCC mixture, are led forward.

[0010] The method for producing precipitated calcium carbonate (PCC) from ground calcium carbonate (GCC) can generally be implemented under the following indicative conditions, which are partly dependent on each other and especially on the values at the starting point: a) an aqueous process fluid at a temperature in the range of 5-50°C and with a pH in the range of 6-9, carbon dioxide CO2 and GCC are mixed together and the pressure is set at the level of 3-15, preferably at approximately 10 bar, b) the carbon dioxide dissolves, whereupon the pH falls to a value of 3-4 or 5- 6.5, depending on the order of mixing, c) in the said conditions, all of the GCC or a part of the GCC dissolves:CaCO3(s) + 2 H+(aq) Ca2+(aq) + CO2(g) + H2O(I), d) pressure is reduced, whereupon the pressure of the mixture is lowered to the range of atmospheric pressure (atm) - 5 bar, e) the carbon dioxide is released as gas, whereupon the pH rises to the range of 6-8 and Ca2+and CO2 are crystallized as PCC, f) degassing is carried out, the pressure is reduced depending on step d) to a sufficiently low level, whereupon the carbon dioxide released as bubbles is removed from the process fluid and led through the recycling line back to process step a) and / or for other use, g) the process fluid and the precipitated calcium carbonate (PCC) or its derivatives, such as PCC agglomerates, fiber+PCC composites, GCC+PCC clusters or the PCC / GCC mixture, are led forward.

[0011] Thus, a basic idea of the method is that substances in suitable conditions are mixed together into a solution, the pH falls particularly due to the effectof the dissolved carbon dioxide, whereupon the GCC also dissolves. Carbon dioxide cannot remain dissolved unless the pressure is sufficiently high, whereupon by reducing pressure, the carbon dioxide can be released, which causes the pH to rise, whereupon the dissolved GCC is in turn precipitated as PCC. After degassing, the PCC can be used for the desired purpose and the carbon dioxide can be recycled back into the process. In this way, an almost closed cycle can be achieved regarding the carbon dioxide balance in the method; only the carbon dioxide discharged with the PCC and the process fluid must be genuinely added to the process. On the other hand, since the carbon dioxide can be recycled back into the process, it is not necessary to recycle the same carbon dioxide into this particular process. The carbon dioxide released or a part of it can be diverted to other use or used elsewhere.

[0012] Method step a) describes in general terms the mixing of the aqueous process fluid, carbon dioxide and GCC, but the order in which they are mixed has a significant effect on the course of the process. Carbon dioxide can be mixed in the method as a fluid or gas, when it is taken into account that carbon dioxide exists in liquid form only under pressure (over 5.2 bar, temperature -56.6- +30.6°C). Under atmospheric pressure, carbon dioxide does not exist in liquid form. Similarly, GCC can be introduced into the method as an aqueous slurry, which is probably the easiest method of implementation in terms of mixing technique, but possibly also as an aerosol with air or carbon dioxide acting as a carrier gas. The different mixing order alternatives are described in greater detail in the following.

[0013] Alternative 1): step a) is carried out in the following order:- the pressure of the process fluid is set at a level of 3-15 bar,- carbon dioxide is mixed into the process fluid, whereupon the pH falls to a level of <5, such as 3-4,- GCC is mixed into the process fluid.In this case, when the carbon dioxide is mixed into the process water, the pH will fall relatively easily to a level of 3-4. After this, the GCC dissolves well and quickly in the said fluid.

[0014] Alternative 2): step a) is carried out in the following order:- GCC is mixed into the process fluid,- the pressure of the process fluid is set at a level of 3-15 bar,- carbon dioxide is fed / dissolves in the process fluid, whereupon the pH falls to a level of < 5-6.5.In this alternative, when the GCC is mixed in the process fluid before the carbon dioxide, the GCC brings about a buffer effect, that is, forms a buffer solution with water, the pH value of which does not change significantly when acid (or alkali) is added or the solution is diluted. Therefore, after the addition of carbon dioxide, the pH remains approximately at < 6.5, whereas with the order according to the previous alternative 1) it fell to a level of 3-4. The GCC is preferably fed into the process fluid at such pressure and with such means that the GCC mixes evenly in the process fluid. In this way, the reaction gives the best yield.

[0015] Alternative 3): step a) is carried out in the following order:- the pressure is set at a level of 3-15 bar,- carbon dioxide and the GCC slurry are mixed,- the carbon dioxide dissolves, whereupon the pH falls to a level of <6.5, whereupon the GCC dissolves,- the dissolved GCC is mixed into a process fluid which is at an approximately 3- 5 bar lower pressure.Also in this alternative, the GCC slurry acts as a buffer solution affecting the change in pH.

[0016] Alternative 4): step a) is carried out in the following order:- the pressure is set at a level of 3-5 bar,- carbon dioxide and pure water are mixed,- the carbon dioxide dissolves, whereupon the pH falls to a level of 3-4,- GCC is added, whereupon the GCC dissolves,- the mixture is mixed into a process fluid which is at an approximately 3-5 bar lower pressure.In this alternative, the first reduction of pressure in the method, that is, step d),can be carried out already as the last point under step a) when the dissolved GCC is mixed into the process fluid. A sufficient pressure difference of the GCC when mixing into the process fluid is of the order of 3 bar, which results in a high degree of mixing and the reduction in pressure becomes such that the carbon dioxide begins to release as gas. Furthermore, in this alternative, pure water refers to water with a pH of 7 ± 2 and / or which does not contain a significant amount of the substance that makes up the process fluid suspension or other substance that reacts with carbon dioxide.

[0017] According to one embodiment, in the method, carbon dioxide CO2 is fed in an amount which dissolves, in each of the said conditions, in the fluid to be mixed undersaturated I saturated. The aim of this feature is to achieve an adequate fall in pH for the dissolution of GCC with a reasonable amount of carbon dioxide. The amount of carbon dioxide that dissolves at any given time depends on the temperature and pressure.

[0018] According to one embodiment, in steps d) - f) of the method, pressure is reduced in two or more stages to separate the dissolved carbon dioxide from the process fluid. One factor affecting the separation is that during step d), the process fluid becomes supersaturated with carbon dioxide in the said conditions.

[0019] According to some embodiments of alternatives 2) or 3), more carbon dioxide is fed into the process fluid after the pressure reduction under step d). The purpose of this step is to ensure that the pH is sufficiently low in all respects.

[0020] One embodiment of the invention is an arrangement for producing precipitated calcium carbonate (PCC) from ground calcium carbonate (GCC). The arrangement comprises the following means:- an apparatus for mixing together an aqueous process fluid, carbon dioxide CO2 and GCC and for setting the pressure at a level of approximately 3-15 bar,- a reactor for dissolving the GCC or a part of the GCC: CaCO3(s) + 2 H+(aq) Ca2+(aq) + CO2(g) + H2O(I),- a device, such as a choke, valve or the like, for reducing the pressure to a rangeof 2-5 bar,- a reactor for releasing carbon dioxide as gas and for crystallizing Ca2+and CO2 as PCC,- a gas separator, where the pressure of the process fluid can be further reduced, whereupon the carbon dioxide released as bubbles can be separated from the process fluid and led through the recycling line back to the beginning of the process and / or for other use,- a pipe for leading the process fluid and the precipitated calcium carbonate produced (PCC) or its derivatives, such as PCC agglomerates, fiber+PCC composites, GCC+PCC clusters or the PCC / GCC mixture, forward.

[0021] The apparatus for mixing the aqueous process fluid, carbon dioxide and GCC together and for setting the pressure at the specified level may in practice be, for example, the applicant’s TrumpJet ® mixer connected to the process pipeline or other similar apparatus intended for mixing fluids or gases. For setting the pressure at the desired level can be used pumps, flow resistors or the like, or containers into which the substances to be mixed are fed.

[0022] In connection with this invention, the reactor refers to a space within which the reaction is to take place. It may be, for example, a pipe, container or tank, always depending on the case. One characteristic of the reactor is the retention time; the substance being processed must remain in the reactor for as long as necessary for each reaction.

[0023] According to one embodiment of the invention, a gas separator is arranged, wherein the pressure of the process fluid can be further reduced, whereupon the carbon dioxide released as bubbles can be separated from the process fluid and led through the recycling line back to the beginning of the process. In this case, the process fluid is degassed in step f) of the method, whereupon the carbon dioxide released as bubbles is removed by means of a centrifuge, vacuum tank or similar gas separator, or other gas separation device from the process fluid and led through the recycling line back to method step a). The gas separator may reduce the pressure of the fluid processed to the level ofatmospheric pressure (atm), or even to underpressure, with the aim of making the carbon dioxide form bubbles which can be separated from the fluid. However, the gas separator is preferably selected so that the separation method used therein does not cause significant breakage of the previously formed PCC crystals.

[0024] According to one embodiment of the invention, the arrangement is implemented as an in-line reactor in a process using PCC, such as in a process using pulp fiber, a paint industry process or pharmaceutical industry process. Other additional features characteristic of the invention appear from the accompanying claims and the following description of the embodiments in the figures.List of drawings

[0025] The invention and its operation are described in the following, with reference to the accompanying diagrammatic drawings, in whichFig. 1 shows the solubility of carbon dioxide at different pressures and temperaturesFig. 2 shows diagrammatically an embodiment of the method according to the invention,Fig. 3 shows diagrammatically an embodiment of the method according to the invention,Fig. 4 shows diagrammatically an embodiment of the method according to the invention,Fig. 5 shows diagrammatically an embodiment of the method according to the invention.Description of the drawings

[0026] In Fig. 1 B can be observed that the solubility of carbon dioxide is the greater the closer the solution is to the temperature of 0°C. In addition to the temperature, the pressure levels 1 , 5, 10 and 15 bar shown in Fig. 1 B have a significant effect on the solubility of carbon dioxide. On the basis of the factors affecting the solubility of these two substances and other factors affecting the management of the production process, a process fluid at the temperature range of 5-50°C is estimated to be best suited to the method now in question because the required sufficient solubility of carbon dioxide is easier to ensure by means of pressure.

[0027] Fig. 2 shows an embodiment of the arrangement according to the invention for producing precipitated calcium carbonate (PCC) from ground calcium carbonate (GCC). The arrangement comprises the following means:- an apparatus 2, 3, 7 for mixing together an aqueous process fluid 1 , carbon dioxide CO2 3 and GCC 2 and for setting the pressure 7 at a level of approximately 3-15 bar, preferably 10 bar,- a reactor 4 for dissolving the GCC or a part of the GCC:CaCO3(s) + 2 H+(aq) Ca2+(aq) + CO2(g) + H2O(I),- a device 71 such as a choke, valve or the like for reducing the pressure to the range of 2-5 bar,- a reactor 6 for releasing carbon dioxide as gas and for crystallizing and Ca2+and CO2 as PCC,- a gas separator 8, where the pressure of the process fluid 1 can be further reduced, where the gas separator 8 may comprise a pressure reducer 72, whereupon the carbon dioxide released as bubbles can be separated from the process fluid and led through the recycling line 81 back to the beginning of the process, if necessary, new carbon dioxide being supplied to the recycling line 81 through an additional carbon dioxide feed 31 ,- a pipe 10 for leading the process fluid and precipitated calcium carbonate (PCC)produced or its derivatives, such as PCC agglomerates, fiber+PCC composites, GCC+PCC clusters or the PCC / GCC mixture, forward. To illustrate the method steps, in Fig. 2 and the other figures, the locations of method steps a), b), c), d), e), f) and g) in the diagrammatic process are shown as broken-line boxes or sections.

[0028] Fig. 2, as well as Figs. 3, 4 and 5 show how the method is implemented in the different embodiments. According to the invention, precipitated calcium carbonate (PCC) is produced from ground calcium carbonate (GCC). The method comprises the following steps: a) an aqueous process fluid at a temperature in the range of 5-50°C and with a pH in the range of 6-9, preferably 6.7-8.2, carbon dioxide CO2 and GCC are mixed together and the pressure is set at a level of 3-15 bar, preferably approximately 10 bar, b) carbon dioxide dissolves, whereupon the pH falls to a value of 3-4 or 5-6.5, depending on the order of mixing, c) in the said conditions, the GCC or a part of the GCC dissolves:CaCO3(s) + 2 H+(aq) Ca2+(aq) + CO2(g) + H2O(I), d) pressure is reduced, whereupon the pressure of the mixture is reduced to the range of atmospheric pressure (atm)-5 bar, e) the carbon dioxide is released as gas, whereupon the pH rises to the range of 6-8 and Ca2+and CO2 are crystallized as PCC, f) degassing is carried out, the pressure is reduced depending on step d) to a sufficiently low level, whereupon the carbon dioxide released as bubbles is removed from the process fluid and led through the recycling line 81 back to process step a), g) the process fluid and the precipitated calcium carbonate (PCC) produced or its derivatives, such as PCC agglomerates, fiber+PCC composites, GCC+PCC clusters or the PCC / GCC mixture, are led forward.

[0029] Fig. 2 shows an embodiment where step a) is carried out in the following order:- the pressure of the process fluid 1 is set at a level of 3-15 bar (by means of a pump 7),- carbon dioxide is fed into (mixer 3) I dissolves (reactor 5) in the process fluid, whereupon the pH falls to a level <5, such as 3-4,- GCC is mixed in the process fluid (mixer 2).

[0030] Fig. 3 shows an embodiment where step a) is carried out in the following order:- GCC is mixed in the process fluid 1 (mixer 2)- the pressure of the process fluid 1 is set at a level of 3-15 bar (pump 7),- carbon dioxide is fed into (mixer 3) I dissolves in the process fluid 1 during step c), whereupon the pH falls to a level of < 5-6.5.After this, the method corresponds to what was disclosed in connection with Fig. 2.

[0031] Fig. 4 shows an embodiment where step a) is carried out in the following order:- pressure is set at a level of 3-15 bar,- carbon dioxide 3 and the GCC slurry 2 are mixed,- the carbon dioxide dissolves, whereupon the pH falls to a level of < 6.5, whereupon the GCC dissolves,- the dissolved GCC I CO2 mixture 23 is mixed into a process fluid 1 which is at an approximately 3-5 bar lower pressure, which causes / brings about method step d).After this, the process continues as in the previous embodiments, PCC crystallizes in the reactor 6, carbon dioxide is released, and the remaining degassing is completed by means of the pressure reducer 72 and the gas separator 8 before leading the end product forward along the pipe 10.

[0032] Fig. 5 shows an embodiment where step a) is carried out in the following order:- pressure is set at a level of 3-15 bar,- carbon dioxide 3 and pure water 11 are mixed,- carbon dioxide dissolves, whereupon the pH falls to a level of 3—4,- GCC 2 is added, whereupon the GCC dissolves,- the dissolved GCC 23 is mixed with the process fluid 1 which is at an approximately 3-5 bar lower pressure.

[0033] It should be noted that only a few of the most advantageous embodiments of the invention are disclosed above. It is, therefore, obvious that the invention is not limited to the foregoing embodiments but can be applied in many ways within the scope defined by the accompanying claims. The features presented in connection with the different embodiments may also be used in connection with other embodiments within the basic idea of the invention and / or the features presented may be combined into various entities if so desired and if the technical possibilities for doing so exist.List of reference numerals used:1 Process fluid pipe / aqueous process fluid11 Pure water2 GCC feed (mixer)3 Carbon dioxide feed (mixer)31 Additional carbon dioxide feed23 GCC / CO2 mixture4 Reactor (for dissolving GCC)5 Reactor (for dissolving carbon dioxide)6 PCC reactor7 Pressure adjuster (pump)71 Pressure reducer72 Pressure reducer8 Gas separator81 Carbon dioxide recycling10 Pipe

Claims

Claims1 . A method for producing precipitated calcium carbonate (PCC) from ground calcium carbonate (GCC), characterized in that the method comprises the following steps carried out as an in-line process directly in the process fluid flow: a) an aqueous process fluid (1) at a temperature in the range of 5-50°C and with a pH in the range of 5-9, carbon dioxide CO2 (3) and GCC (2) are mixed together and the pressure is set at a level of 3-15 bar, b) the carbon dioxide dissolves, whereupon the pH falls to a value of 3-4 or 5- 6.5, depending on the order of mixing, c) in the said conditions, all of the GCC or a part of the GCC dissolves:CaCO3(s) + 2 H+(aq) Ca2+(aq) + CO2(g) + H2O(I), d) pressure is reduced, e) the carbon dioxide is released as gas, whereupon the pH rises to the range of 6-8 and Ca2+and CO2 are crystallized as PCC, f) degassing is carried out, the pressure is reduced depending on step d) to a sufficiently low level, whereupon the carbon dioxide (3) released as bubbles is removed from the process fluid (1) and led through the recycling line back to process step a) and / or for other use, g) the process fluid and the precipitated calcium carbonate (PCC) produced or its derivatives, such as PCC agglomerates, fiber+PCC composites, GCC+PCC clusters or the PCC / GCC mixture, are led forward.

2. A method according to claim 1 , characterized in that step a) is carried out in the following order:- the pressure of the process fluid (1) is set at a level of 3-15 bar,- carbon dioxide (3) is mixed into the process fluid, whereupon the pH falls to a level of <5, such as 3-4,- GCC (2) is mixed into the process fluid.

3. A method according to claim 1 , characterized in that step a) is carried out in the following order:- GCC (2) is mixed into the process fluid,- the pressure of the process fluid (1) is set at a level of 3-15 bar,- carbon dioxide (3) is fed / dissolves in the process fluid (1), whereupon the pH falls to a level of < 5 - 6.5.

4. A method according to claim 1 , characterized in that step a) is carried out in the following order:- the pressure is set at a level of 3-15 bar,- carbon dioxide (3) and the GCC slurry (2) are mixed,- the carbon dioxide dissolves, whereupon the pH falls to a level of < 6.5, whereupon the GCC dissolves,- the dissolved GCC (23) is mixed with the process fluid (1) which is at an approximately 3-5 bar lower pressure.

5. A method according to claim 1 , characterized in that step a) is carried out in the following order:- the pressure is set at a level of 3-15 bar,- carbon dioxide (3) and pure water (11) are mixed,- the carbon dioxide dissolves, whereupon the pH falls to a level of 3-4,- GCC (2) is added, whereupon the GCC dissolves,- the mixture (23) is mixed with the process fluid (1) which is at a lower pressure.

6. A method according to any of the preceding claims, characterized in that, in the method, carbon dioxide CO2 is fed in an amount which dissolves, in each of the said conditions, in the fluid to be mixed undersaturated I saturated.

7. A method according to any of the preceding claims, characterized in that, in the method, in steps d) - f), pressure is reduced in two or more stages to separate the dissolved carbon dioxide from the process fluid.

8. A method according to any of the preceding claims, characterized in that, in method step f), the process fluid (1) is degassed, whereupon the carbon dioxide (3) released as bubbles is removed by means of a centrifuge, vacuum tank or similar gas separator, or other gas separation device from the process fluid and led through the recycling line back to method step a).

9. A method according to claim 3, characterized in that the GCC (2) is fed in the process fluid (1) so that the GCC (2) mixes evenly in the process fluid (1).

10. A method according to claim 5, characterized in that pure water (11) refers to water with a pH of 7 ± 2 and which does not contain a significant amount of the substance that makes up the process fluid (1) suspension or other substance that reacts with carbon dioxide.

11. A method according to claim 2 or 3, characterized in that more carbon dioxide (3) is fed into the process fluid (1) after the pressure reduction in step d).

12. A method according to any of the preceding claims, characterized in that during step d) the process fluid (1) becomes supersaturated with carbon dioxide in the said conditions.

13. A method according to any of the preceding claims, characterized in that the temperature of the process fluid (1) is most preferably 5-50°C.

14. An arrangement for producing precipitated calcium carbonate (PCC) from ground calcium carbonate (GCC), characterized in that the arrangement comprises the following means for producing PCC in the process apparatus as an in-line process:- an apparatus for mixing together an aqueous process fluid (1), carbon dioxide CO2 (3) and GCC (2) and for setting the pressure at a level of approximately 3- 15 bar,- a reactor (4) for dissolving the GCC or a part of the GCC: CaCO3(s) + 2 H+(aq) Ca2+(aq) + CO2(g) + H2O(I),- a device (71 , 72), such as a choke, valve or the like, for reducing the pressure to a range of 2-5 bar, - a reactor (6) for releasing the carbon dioxide as gas and for crystallizing Ca2+and CO2as PCC,- a gas separator (8), where the pressure of the process fluid (1) can be further reduced, whereupon the carbon dioxide (3) released as bubbles can be separated from the process fluid (1) and led through the recycling line (81) back to the beginning of the process and / or for other use,- a pipe (10) for leading the process fluid and the precipitated calcium carbonate (PCC) produced or its derivatives, such as PCC agglomerates, fiber+PCC composites, GCC+PCC clusters or the PCC / GCC mixture, forward.

15. An arrangement according to claim 14, characterized in that the arrangement is implemented as an in-line reactor in a process using PCC, such as in a process using pulp fiber, in a paint industry process or pharmaceutical industry process.

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