Precipitated calcium carbonate with improved resistance to structural breakdown

Abstract

The present invention is directed to a process for producing precipitated calcium carbonate with improved resistance to structural breakdown, wherein the milk of lime is carbonated in the presence of at least one gas other than carbon dioxide, or the carbonation is carried out in the presence of a static gas bubble comminution unit as well as to precipitated calcium carbonate obtained by such a process.

Claims

1. A process for producing precipitated calcium carbonate comprising the steps of: a) providing a calcium oxide containing material, b) providing an aqueous solution, c) providing a gas comprising carbon dioxide, d) preparing a milk of lime comprising Ca(OH).sub.2 by mixing the aqueous solution of step b) with the calcium oxide containing material of step a), e) carbonating the milk of lime obtained from step d) with the gas of step c) to form an aqueous suspension of precipitated calcium carbonate, wherein: the carbonation is carried out in the presence of a static gas bubble comminution unit that is located in the milk of lime and the gas of step c) is flushed around and/or through the static gas bubble comminution unit.

2. The process of claim 1, wherein the gas of step c) further comprises at least one gas other than carbon dioxide and the gas of step c) is introduced at a rate of 0.06 to 5.00 kg gas/h per kg of dry Ca(OH).sub.2 with the proviso that the carbon dioxide is introduced at a rate of 0.05 to 3.50 kg CO.sub.2/h per kg of dry Ca(OH).sub.2.

3. The process of claim 1, wherein the aqueous solution of step b) consists of water.

4. The process of claim 1, wherein the aqueous solution of step b) comprises one or more further additives selected from the group consisting of water soluble polymers, calcium carbonate nanoparticles, water-soluble calcium salts, and slaking additives.

5. The process of claim 1, wherein the calcium oxide containing material of step a) and the aqueous solution of step b) are mixed in a mass ratio from 1:1 to 1:15.

6. The process of claim 1, wherein the calcium oxide containing material of step a) and the aqueous solution of step b) are mixed in a mass ratio from 1:4 to 1:12.

7. The process of claim 1, wherein the gas of step c) comprises between 4 and 99 vol.-% carbon dioxide, based on the total volume of the gas.

8. The process of claim 1, wherein the gas of step c) comprises between 6 and 40 vol.-% carbon dioxide, based on the total volume of the gas.

9. The process of claim 1, wherein the gas of step c) comprises between 8 and 25 vol.-% carbon dioxide, based on the total volume of the gas.

10. The process of claim 1, wherein the gas in step e) is introduced at a rate of 0.06 to 5.00 kg gas/h per kg of dry Ca(OH).sub.2, and/or the carbon dioxide is introduced at a rate of 0.05 to 3.50 kg CO.sub.2/h per kg of dry Ca(OH).sub.2.

11. The process of claim 1, wherein the gas in step e) is introduced at a rate of 0.09 to 4.00 kg gas/h per kg of dry Ca(OH).sub.2, and/or the carbon dioxide is introduced at a rate of 0.07 to 2.00 kg CO.sub.2/h per kg of dry Ca(OH).sub.2.

12. The process of claim 1, wherein the gas in step e) is introduced at a rate of 0.12 to 3.00 kg gas/h per kg of dry Ca(OH).sub.2, and/or the carbon dioxide is introduced at a rate of 0.10 to 1.50 kg CO.sub.2/h per kg of dry Ca(OH).sub.2.

13. The process of claim 1, wherein the precipitated calcium carbonate obtained in step e) has a specific surface area from 2.0 to 80.0 m.sup.2/g, measured using nitrogen and the BET method according to ISO 9277:1995.

14. The process of claim 1, wherein the precipitated calcium carbonate obtained in step e) has a specific surface area from 2.5 to 13.0 m.sup.2/g, measured using nitrogen and the BET method according to ISO 9277:1995.

15. The process of claim 1, wherein the precipitated calcium carbonate obtained in step e) is in a form of particles having a weight median particle size d.sub.50of between 1.0 and 9.0 μm.

16. The process of claim 1, wherein the precipitated calcium carbonate obtained in step e) is in a form of particles having a weight median particle size d.sub.50 of 1.2 and 3.7 μm; and wherein the carbonation is carried out in the absence of agitators and stirrers.

17. The process of claim 1, wherein the precipitated calcium carbonate obtained in step e) is in a form of particles having a calcitic crystal form.

18. The process of claim 1, wherein the milk of lime is stirred during step e).

19. The process of claim 1, further comprising the steps of: f) separating the precipitated calcium carbonate from the aqueous suspension obtained from step e), and optionally g) drying the separated precipitated calcium carbonate obtained from step f).

20. The process of claim 19, wherein step g) takes place.

21. The process of claim 20, which further comprises a step h) of contacting at least a part of the surface of the precipitated calcium carbonate obtained from step g) with a surface-treatment agent.

Description

EXAMPLES

1. Measurement Methods

(1) In the following, measurement methods implemented in the examples are described.

(2) Friability Value

(3) The aqueous suspension of precipitated calcium carbonate was filtered and the residue was rinsed with water and dried in an oven at 100° C. to obtain the precipitated calcium carbonate. The dried precipitated calcium carbonate was shaken through a 1 mm mesh sieve to reduce larger agglomerates.

(4) The dried and sieved precipitated calcium carbonate was formed into tablets by placing 11.5 g of the precipitated calcium carbonate in a press chamber of the manually operated hydraulic press Herzog TP 40/2D, Herzog Maschinenfabrik GmbH & Co, Osnabruck, Germany. The press chamber was closed by placing a piston/lid on top of the press chamber. The PCC was compacted in the press for 5 minutes at predetermined pressures of 60 MPa, 90 MPa, and 120 MPa, 240 MPa and 300 MPa. After 5 minutes the press chamber was opened and a calcium carbonate tablet with a diameter of 4 cm was obtained.

(5) The pore volume and the pore size distribution was calculated from a mercury intrusion porosimetry measurement using a Micrometrics Autopore V mercury porosimeter. The mercury porosimetry experiment entailed the evacuation of the obtained tablet to remove trapped gases, after which the tablet was surrounded with mercury. The amount of mercury displaced by the tablet allows calculation of the sample's bulk volume, V.sub.bulk. Pressure was then applied to the mercury so that it intruded into the tablet through pores connected to the external surface. The maximum applied pressure of mercury was 414 MPa, equivalent to a Laplace throat diameter of 0.004 μm. The data were corrected using Pore-Comp (Gane et al. “Void Space Structure of Compressible Polymer Spheres and Consolidated Calcium Carbonate Paper-Coating Formulations”, Industrial and Engineering Chemistry Research 1996, 35 (5):1753-1764) for mercury and penetrometer effects, and also for sample compression.

(6) By taking the first derivative of the cumulative intrusion curve the pore size distribution based on equivalent Laplace diameter assuming a mercury-solid surface contact angle of 140° and mercury surface tension of 480 dyn.Math.cm.sup.−1, inevitably including the effect of pore-shielding when present, was revealed. The pore diameter of the sample is defined as the peak maximum of the pore size distribution, i.e. volume modal pore size.

(7) The compaction pressure of the tablet formation, x, was plotted versus said pore diameter, y. The graph was fitted with the logarithmic equation y=a−b.Math.lnx. The fit was performed with the computer program SYSTAT 5.0 for Windows, available from SYSTAT Software Inc., San Jose, US.

(8) The normalized specific pore volume difference was calculated by the formula 100. [(total specific pore volume at 60 MPa−total specific pore volume at the given pressure greater than 60 MPa)/total specific pore volume at 60 MPa] e.g. the normalized specific pore volume difference between measurements at 60 MPa and 120 MPa is given by 100.Math.[(total specific pore volume at 60 MPa−total specific pore volume at 120 MPa)/total specific pore volume at 60 MPa].

(9) The compaction pressure of the tablet formation, x, was plotted versus the normalised specific pore volume difference, y, as calculated by the formula above. The graph was fitted with the logarithmic equation y=c−d.Math.lnx. The fit was performed with the computer program SYSTAT 5.0 for Windows, available from SYSTAT Software Inc., San Jose, US.

(10) The friability value is calculated as the product of b and d.

(11) Furthermore, the coefficient of determination R.sup.2 of the b and d values is calculated by the formula R.sup.2=1−SSE/SSM, wherein SSE is the sum of the squared errors and SSM is the sum of squares about the mean.

(12) Particle Size Distribution of Precipitated Calcium Carbonate (PCC) and Steepness Factor

(13) The particle size distribution of the prepared PCC particles was measured using a Sedigraph™ 5120. The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement was carried out in an aqueous solution comprising 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and supersonics. For the measurement of dispersed samples, no further dispersing agents were added. The “steepness factor” d.sub.75/25 was calculated as the quotient of the d.sub.75 and the d.sub.25 value.

(14) Solids Content of an Aqueous Suspension

(15) The suspension solids content (also known as “dry weight”) was determined using a Moisture Analyser MJ33 from the company Mettler-Toledo, Switzerland, with the following settings: drying temperature of 160° C., automatic switch off if the mass does not change more than 1 mg over a period of 30 sec, standard drying of 5 to 20 g of suspension.

(16) Specific Surface Area (SSA)

(17) The specific surface area was measured via the BET method according to ISO 9277 using nitrogen, following conditioning of the sample by heating at 250° C. for a period of 30 minutes. Measurement was performed with a TriStar II from Micromeritics, US. Prior to such measurements, the sample is filtered within a Buchner funnel, rinsed with deionised water and dried overnight at 90 to 100° C. in an oven. Subsequently the dry cake is ground thoroughly in a mortar and the resulting powder placed in a moisture balance at 130° C. until a constant weight is reached.

(18) X-Ray Diffraction

(19) The purity of the PCC samples was analysed with a D8 Advance powder diffractometer (Bruker Corporation, USA) obeying Bragg's law. This diffractometer consisted of a 2.2 kW X-ray tube (Cu), a sample holder, a ϑ-ϑ goniometer, and a VÁNTEC-1 detector. Nickel-filtered Cu K.sub.α radiation was employed in all experiments (λK.sub.α-Cu=1.5406 Å). The profiles were chart recorded automatically using a scan speed of 0.7° per minute in 2ϑ (XRD GV_7600). The measurement was carried out at angles from 2ε=5° to 70°.

(20) The resulting powder diffraction pattern was classified by mineral content using the DIFFRAC.sup.suite software packages EVA and SEARCH, based on reference patterns of the ICDD PDF 2 database (XRD LTM_7603). Quantitative analysis of the diffraction data, i.e. the determination of amounts of different phases in a multi-phase sample, has been performed using the DIFFRAC.sup.suite software package TOPAS (XRD LTM_7604). This involved modelling the full diffraction pattern (Rietveld approach) such that the calculated pattern(s) duplicated the experimental one.

(21) Brightness Measurement and Yellowness Index

(22) The pigment brightness and yellowness index of the obtained particles were measured using an ELREPHO 450× from the company Datacolor according to ISO 2469 and DIN 6167, respectively.

(23) The samples were dried in an oven at 105° C. to a residual moisture content of <0.5% by weight and the resulting powder was treated to deagglomerate the powder particles. From 12 g of said powder a tablet was pressed via application of 4 bar pressure for 15 s. The resulting powder tablet with a diameter of 45 mm was then subjected to the measurement.

(24) In the present measurement the yellowness index was measured via measuring the reflectance of the obtained precipitated calcium carbonate product, the illuminant used being D 65 and the standard observer function being 10°.

(25) The Yellowness Index according to DIN 6167 is calculated as follows:

(26) $\mathrm{YI}\left(\mathrm{DIN}6167\right)=\frac{a\times X-b\times Z}{Y}\times 100$
where X, Y, and Z are the CIE Tristimulus values and the coefficients depend on the illuminant and the observer function as indicated in the Table below:

(27) TABLE-US-00001 Illuminant D 65 Observer 10° a 1.301 b 1.149

(28) Brookfield Viscosity

(29) The Brookfield viscosity of the liquid coating compositions was measured after one hour of production and after one minute of stirring at 25° C.±1° C. at 100 rpm by the use of a Brookfield viscometer type RVT equipped with an appropriate disc spindle, for example spindle 2 to 5.

(30) pH Value

(31) The pH of a suspension or solution was measured at 25° C. using a Mettler Toledo Seven Easy pH meter and a Mettler Toledo InLab® Expert Pro pH electrode. A three point calibration (according to the segment method) of the instrument was first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20° C. (from Sigma-Aldrich Corp., USA). The reported pH values are the endpoint values detected by the instrument (the endpoint was when the measured signal differed by less than 0.1 mV from the average over the last 6 seconds).

2. Examples

Example 1

Comparative Examples 1A and 1B

(32) A milk of lime was prepared by mixing under mechanical stirring 5.00 kg water with 1.334 kg calcium oxide obtained from the US at an initial temperature of 40° C. The obtained mixture was stirred for 30 min, wherein additional 3.73 kg water was added. Subsequently, the mixture was sieved through a 100 μm screen.

(33) 10 kg of the obtained milk of lime were transferred into a stainless steel reactor and heated to 50° C. The stainless steel reactor contained a propeller stirrer. Then the milk of lime was carbonated by introducing gas consisting only of CO.sub.2 at a rate of 0.472 kg gas/h per kg dry Ca(OH).sub.2. The reaction was monitored by online pH and conductivity measurements.

(34) The precipitated calcium carbonate was obtained by filtering the suspension and rinsing the residue with water and drying the obtained precipitated calcium carbonate in an oven at 100° C. The dried precipitated calcium carbonate was shaken through a 1 mm mesh sieve to reduce larger agglomerates. The purity of the obtained precipitated calcium carbonate was controlled by X-ray diffraction using the method described above.

(35) The reaction parameters and characteristics of the prepared PCCs are listed in Table 1 below and the b and d values with the corresponding R.sup.2 values as well as the friability values are listed in Table 2 below.

(36) TABLE-US-00002 TABLE 1 reaction parameters and characteristics of the prepared PCCs of the comparative Examples 1A and 1B stirrer speed weight median during particle size BET/ steepness Example carbonation d.sub.50/μm m.sup.2 g.sup.−1 d.sub.75/d.sub.25 1A 700 rpm 1B stirrer off 2.21 5.2 1.73

(37) TABLE-US-00003 TABLE 2 friability values of the prepared PCCs of the Comparative Examples 1A and 1B R.sup.2 of R.sup.2 of line line b .Math. d b .Math. d reduction Ex- fit for fit for (abso- (per- of ample b b d d lute) centage) friability 1A 0.1163 0.99 27.4291 1.00 3.19 100 1B 0.1151 1.00 27.4437 0.99 3.16 99.06 0.94

(38) As can be seen from Table 2 the Comparative Example 1B which is not stirred during carbonation has a friability value that is 0.94% lower than the friability value of precipitated calcium carbonate that has been obtained by a similar process, wherein the carbonation is carried out under stirring at 700 rpm.

Example 2

Comparative Examples 2A, 3A and 4A

(39) A milk of lime was prepared by mixing under mechanical stirring 5.00 kg water with 1.334 kg calcium oxide obtained from Austria for Example 2A, from France for Example 3A and from Brazil for Example 4A at an initial temperature of 40° C. for Examples 2A and 3A and at an initial temperature of 50° C. for Example 4A. The obtained mixture was stirred for 30 min, wherein additional 3.73 kg water was added. Subsequently, the mixture was sieved through a 100 μm screen.

(40) 10 kg of the obtained milk of lime were transferred into a stainless steel reactor and heated to 50° C. for Examples 2A and 3A and heated to 55° C. for Example 4A. The stainless steel reactor contained a propeller stirrer. Then the milk of lime was carbonated by introducing gas consisting only of CO.sub.2 at a rate listed in table 3.

(41) During the carbonation step, the reaction mixture was stirred with the speed listed in table 3. The reaction was monitored by online pH and conductivity measurements.

(42) The precipitated calcium carbonate was obtained by filtering the suspension and rinsing the residue with water and drying the obtained precipitated calcium carbonate in an oven at 100° C. The dried precipitated calcium carbonate was shaken through a 1 mm mesh sieve to reduce larger agglomerates. The purity of the obtained precipitated calcium carbonate was controlled by X-ray diffraction using the method described above.

(43) The reaction parameters and characteristics of the prepared PCCs are listed in Table 3 below.

(44) TABLE-US-00004 TABLE 3 reaction parameters and characteristics of the prepared PCCs of the Comparative Examples 2A, 3A and 4A weight kg gas/h median stirrer speed per kg particle during dry size BET/ steepness Ex. carbonation Ca(OH).sub.2 d.sub.50/μm m.sup.2 g.sup.−1 d.sub.75/d.sub.25 2A 700 rpm 0.472 2.22 6.2 1.93 3A 380 rpm 0.124 2.20 3.8 1.84 4A 700 rpm 0.248 2.34 4.8 1.82

Inventive Examples 2B, 3B, 3C and 4B

(45) A milk of lime was prepared by mixing under mechanical stirring 5.00 kg water with 1.334 kg calcium oxide obtained from Austria for Example 2B, from France for Examples 3B and 3C and from Brazil for Example 4B at an initial temperature of 40° C. for Examples 2B, 3B and 3C and at an initial temperature of 50° C. for Example 4B. The obtained mixture was stirred for 30 min, wherein additional 3.73 kg water was added. Subsequently, the mixture was sieved through a 100 μm screen.

(46) 10 kg of the obtained milk of lime were transferred into a stainless steel reactor and heated to 50° C. for Examples 2B, 3B and 3C and heated to 55° C. for Example 4B. The stainless steel reactor contained a static gas bubble comminution unit in the form of a gas permeable perforated plate. In addition to the gas bubble comminution unit the stainless steel reactor further contained a propeller stirrer for Example 3C and the reaction mixture was stirred during the carbonation step with the speed listed in table 4. The milk of lime was carbonated in the presence of the static gas bubble comminution unit that was located in the milk of lime by introducing gas consisting only of CO.sub.2 at a rate listed in table 4. The gas was flushed around and through the static gas bubble comminution unit. The reaction was monitored by online pH and conductivity measurements.

(47) The precipitated calcium carbonate was obtained by filtering the suspension and rinsing the residue with water and drying the obtained precipitated calcium carbonate in an oven at 100° C. The dried precipitated calcium carbonate was shaken through a 1 mm mesh sieve to reduce larger agglomerates. The purity of the obtained precipitated calcium carbonate was controlled by X-ray diffraction using the method described above.

(48) The reaction parameters and characteristics of the prepared PCCs are listed in Table 4 below.

(49) TABLE-US-00005 TABLE 4 reaction parameters and characteristics of the prepared PCCs of the inventive Examples 2B, 3B, 3C and 4B weight kg gas/h median stirrer speed per kg particle during dry size BET/ steepness Ex. carbonation Ca(OH).sub.2 d.sub.50/μm m.sup.2 g.sup.−1 d.sub.75/d.sub.25 2B no stirrer 0.472 2.41 4.8 2.20 3B no stirrer 0.124 2.54 3.6 1.87 3C 380 rpm 0.124 2.14 3.9 1.91 4B no stirrer 0.248 2.29 5.3 1.87

(50) The b and d values with the corresponding R.sup.2 values as well as the friability values of Examples 2A to 4B are listed in Table 5 below. The reduction of friability was calculated for the inventive Examples compared to the corresponding comparative Examples (based on the same gas flow): 1A-2B, 3A-3B/C and 4A-4B.

(51) TABLE-US-00006 TABLE 5 friability values of the prepared PCCs of the comparative Examples 2A, 3A and 4A and the inventive Examples 2B, 3B, 3C and 4B R.sup.2 of R.sup.2 of line line b .Math. d b .Math. d reduction Ex- fit for fit for (abso- (per- of ample b b d d lute) centage) friability 2A 0.1531 0.97 27.7858 1.00 4.25 100 — 2B 0.1442 0.98 27.881 1.00 4.02 94.59 5.41 3A 0.1578 0.99 27.5009 0.99 4.34 100 — 3B 0.1539 0.99 27.5203 0.99 4.24 97.70 2.30 3C 0.1491 0.99 28.1430 0.99 4.20 96.77 3.23 4A 0.1529 0.99 26.3756 1.00 4.03 100 — 4B 0.1006 0.98 26.3010 0.99 2.65 65.76 34.24 

(52) As can be seen from Table 5 the inventive Examples have friability values that are at least 2% lower than the friability value of precipitated calcium carbonate that has been obtained by a similar process, wherein the carbonation is carried out without a static gas bubble comminution unit that is located in the milk of lime. Therefore, it has been shown that by the inventive process according to the present invention precipitated calcium carbonate can be obtained that is more resistant, especially more resistant to compression.

Example 3

Inventive Example 5A

(53) A milk of lime was prepared by mixing under mechanical stirring 5.00 kg water with 1.334 kg calcium oxide obtained from France at an initial temperature of 40° C. The obtained mixture was stirred for 30 min, wherein additional 3.73 kg water was added. Subsequently, the mixture was sieved through a 100 μm screen.

(54) The obtained milk of lime was heated to 50° C. and added to a reactor pipe with a diameter of 100 mm. The reactor pipe contained a static gas bubble comminution unit in the form of a gas permeable iron chromium foam obtained from American Elements, US. Iron chromium foams are metal foams with a high porosity wherein typically 75 to 95% of the volume consist of void spaces. The milk of lime was carbonated in the presence of the static gas bubble comminution unit that was located in the milk of lime by introducing flue gas comprising 20 vol.-% CO.sub.2 at a rate listed in Table 6 for 2 hours. The reaction was monitored by online pH and conductivity measurements.

(55) The precipitated calcium carbonate was obtained by filtering the suspension and rinsing the residue with water and drying the obtained precipitated calcium carbonate in an oven at 100° C. The dried precipitated calcium carbonate was shaken through a 1 mm mesh sieve to reduce larger agglomerates. The purity of the obtained precipitated calcium carbonate was controlled by X-ray diffraction using the method described above.

(56) The reaction parameters and characteristics of the prepared PCCs are listed in Table 6 below.

(57) TABLE-US-00007 TABLE 6 reaction parameters and characteristics of the prepared PCCs of the inventive Example 5A weight kg gas/h kg CO.sub.2/h median per kg per kg particle dry dry size d.sub.50 BET/ steepness Ex. Ca(OH).sub.2 Ca(OH).sub.2 μm m.sup.2 .Math. g.sup.−1 d.sub.75/d.sub.25 5A 1.65 0.33 2.1 9.0 1.7

Example 4

Inventive Examples 6A and 6B

(58) A milk of lime was prepared by mixing under mechanical stirring 5.00 kg water with approximately 1.00 kg calcium oxide obtained from Austria at an initial temperature of 40° C. The contained 0.1 wt.-% (active/on dry calcium oxide) sodium citrate. The obtained mixture was stirred for 30 min, wherein additional 4.00 kg water was added. Subsequently, the mixture was sieved through a 200 μm screen.

(59) 4 litre of the obtained milk of lime were heated to 50° C. and added to a 10 litre plastic bucket. A static gas bubble comminution unit in the form of a gas permeable plastic porous material was used. The gas permeable plastic porous material was a Microdyn® tube (type VA/2, polypropylene, 0.2 μm pore width, inner diameter=5 mm, max pressure ˜30 bar) from the company Microdyn-Nadir. The milk of lime was carbonated in the presence of the static gas bubble comminution unit that was located in the milk of lime by introducing flue gas comprising vol.-% CO.sub.2 for 3 hours for Example 6 A and by introducing 100 vol.-% CO.sub.2 for 30 minutes for Example 6B. The reactions was monitored by online pH, temperature and conductivity measurements.

(60) The precipitated calcium carbonate was obtained by filtering the suspension and rinsing the residue with water and drying the obtained precipitated calcium carbonate in an oven at 100° C.

(61) The characteristics of the prepared PCCs are listed in Table 7 below.

(62) TABLE-US-00008 TABLE 7 characteristics of the prepared PCCs of the inventive Examples 6A and 6B weight median particle size d.sub.50 steepness Ex. μm BET/m.sup.2 .Math. g.sup.−1 d.sub.75/d.sub.25 6A 2.76 7.2 2.6 6B 2.39 5.2 2.0

Example 5

Comparative Example 7A and Inventive Examples 7B and 7C

(63) A milk of lime was prepared by mixing under mechanical stirring water with calcium oxide obtained from USA at an initial temperature of 28° C. The obtained mixture was stirred for 30 min. Subsequently, the mixture was sieved through a 325 μm screen.

(64) All of the obtained milk of lime was transferred into a stainless steel reactor and adjusted to the temperature indicated in Table 8 below. The stainless steel reactor contained no static gas bubble comminution unit (comparative Example 7A), a static gas bubble comminution unit in the form of one gas permeable perforated plate (inventive Example 7B) and a static gas bubble comminution unit in the form of two gas permeable perforated plates (inventive Example 7C). In addition the stainless steel reactors further contained a propeller stirrer, however, the stirrer was turned off. The milk of lime was carbonated in the absence of the static gas bubble comminution unit (Example 7A) or in the presence of the static gas bubble comminution unit that was located in the milk of lime (Examples 7B and 7C) by introducing flue gas comprising 20 vol.-% CO.sub.2 at a rate listed in Table 8. The gas was flushed around and through the static gas bubble comminution unit. The reaction was monitored by online pH and conductivity measurements.

(65) The precipitated calcium carbonate was obtained by filtering the suspension and rinsing the residue with water and drying the obtained precipitated calcium carbonate in an oven at 100° C.

(66) The amount of the compounds used in the reaction as well as the reaction conditions are listed in Table 8 below.

(67) TABLE-US-00009 TABLE 8 Amount of the compounds used in the reaction as well as the reaction conditions for preparation of Examples 7A, 7B and 7C Carbonation reaction Temp. Milk of lime preparation milk of kg gas kg CO.sub.2 Amount Amount Initial lime was per kg per kg of water of CaO temperature adjusted to dry dry Ex. (tonnes) (tonnes) [° C.] [° C.] Ca(OH).sub.2 Ca(OH).sub.2 7A 33.2 7.8 28 36 1.85 0.46 7B 33.0 7.3 28 35 1.98 0.49 7C 33.1 7.5 28 35 1.93 0.49

(68) The characteristics of the prepared PCCs are listed in Table 9 below.

(69) TABLE-US-00010 TABLE 9 characteristics of the prepared PCCs of Examples 7A, 7B and 7C weight median particle size d.sub.50 steepness Ex. μm BET/m.sup.2 .Math. g.sup.−1 d.sub.75/d.sub.25 7A 1.51 8.4 1.76 7B 1.34 9.0 1.75 7C 1.44 8.5 1.77

(70) The b and d values with the corresponding R.sup.2 values as well as the friability values of Example 4 (Inventive Examples 6A and 6B) as well as of Example 5 (Comparative Example 7A and Inventive Examples 7B and 7C) are listed in Table 10 below.

(71) TABLE-US-00011 TABLE 10 friability values of the prepared PCCs of comparative Example 7A, and the inventive Examples 6A, 6B, 7B and 7C R.sup.2 of R.sup.2 of line fit for line fit for b .Math. d Example b b d d (absolute) 7A 0.0948 0.98 30.1307 1.00 2.86 6A 0.1407 1.00 28.1118 1.00 3.96 6B 0.1508 0.97 30.5407 0.99 4.61 7B 0.0932 0.93 31.0234 0.99 2.89 7C 0.0856 0.99 29.9025 1.00 2.56