PRODUCTION OF PCC

Abstract

The present invention is directed to a process for producing an aqueous suspension of precipitated calcium carbonate, wherein a milk of lime is prepared by mixing water, a calcium oxide containing material, at least one water-soluble polymer having a molecular weight M.sub.w in the range from 200 to 6500 g/mol, and at least one slaking additive, and subsequently, the milk of lime is carbonated to form an aqueous suspension of precipitated calcium carbonate.

Claims

1. An aqueous suspension of precipitated calcium carbonate obtained by a process comprising the steps of: i) providing a calcium oxide containing material, ii) providing at least one water-soluble polymer having a molecular weight M.sub.w in the range from 200 to 6500 g/mol, wherein the at least one polymer has the chemical structure of formula (I): ##STR00021## wherein n, m, and p are integers and at least one of n, m, or p is greater than zero and n+m+p is less than or equal to 70, R.sub.1 is H or CH.sub.3, R.sub.2 is H or CH.sub.3, R.sub.3 is C(?O)OR.sub.4 or C(?O)NHR.sub.4, wherein R.sub.4 is a C.sub.1 to C.sub.20 alkyl group, a C.sub.3 to C.sub.20 cycloalkyl group and/or a C.sub.6 to C.sub.30 aryl group, being optionally substituted with one or more sulfonate groups, and wherein the cycloalkyl group and/or the aryl group comprises one ring or several rings, which are linked to each other, R.sub.5 is H or CH.sub.3, R.sub.6 is H or CH.sub.3, and X is H and/or M, wherein M is Na, K, Li, Mg, and/or Ca, and wherein the structural units ##STR00022## are arranged randomly, regularly and/or in blocks, iii) providing at least one slaking additive, wherein the at least one slaking additive is selected from the group consisting of organic acids, organic acid salts, sugar alcohols, monosaccharides, disaccharides, polysaccharides, gluconates, phosphonates, lignosulfonates, and mixtures thereof, iv) preparing a milk of lime by mixing water, the calcium oxide containing material of step i), the at least one polymer of step ii), and the at least one slaking additive of step iii), wherein the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:12, and v) carbonating the milk of lime obtained from step iv) to form an aqueous suspension of precipitated calcium carbonate.

2. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the at least one polymer has the chemical structure of formula (II): ##STR00023## wherein n, m, and p are integers and at least one of n, m, or p is greater than zero and n+m+p is less than or equal to 70, R.sub.1 is H or CH.sub.3, R.sub.2 is H or CH.sub.3; R.sub.3 is C(?O)OR.sub.4 or C(?O)NHR.sub.4, wherein R.sub.4 is a C.sub.1 to C.sub.20 alkyl group, a C.sub.3 to C.sub.20 cycloalkyl group and/or a C.sub.6 to C.sub.30 aryl group, being optionally substituted with one or more sulfonate groups, and wherein the cycloalkyl group and/or the aryl group comprises one ring or several rings, which are linked to each other, and X is H and/or M, wherein M is Na, K, Li, Mg, and/or Ca, and wherein the structural units ##STR00024## are arranged randomly, regularly and/or in blocks.

3. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein R.sub.1 is H and R.sub.5 is H.

4. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein in step iv) the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:9.

5. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein in step iv) the calcium oxide containing material and the water are mixed in a weight ratio from 1:3 to 1:5.

6. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the at least one slaking additive is selected from the group consisting of sodium citrate, potassium citrate, calcium citrate, magnesium citrate, monosaccharides, disaccharides, polysaccharides, sucrose, sugar alcohols, meritol, citric acid, sorbitol, sodium salt of diethylene triamine pentaacetic acid, gluconates, phosphonates, sodium tartrate, sodium lignosulfonate, calcium lignosulfonate, and any mixture thereof.

7. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the at least one slaking additive is sodium citrate and/or saccharose.

8. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the milk of lime of step iv) has a Brookfield viscosity from 1 to 1000 mPa.Math.s at 25? C.

9. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the suspension of PCC of step v) has a Brookfield viscosity of less than or equal to 1000 mPa.Math.s at 25? C.

10. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the suspension of PCC of step v) has a Brookfield viscosity of less than or equal to 800 mPa.Math.s at 25? C.

11. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the suspension of PCC of step v) has a Brookfield viscosity of less than or equal to 600 mPa.Math.s at 25? C.

12. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the suspension of PCC of step v) has a solids content of at least 20 wt.-%, based on the total weight of the suspension.

13. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the suspension of PCC of step v) has a solids content of from 20 to 50 wt.-%, based on the total weight of the suspension.

14. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the suspension of PCC of step v) has a solids content of from 30 to 40 wt.-%, based on the total weight of the suspension.

15. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the at least one slaking additive is added in an amount from 0.01 to 2 wt.-%, based on the total amount of calcium oxide containing material.

16. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the at least one slaking additive is added in an amount from 0.05 to 1 wt.-%, based on the total amount of calcium oxide containing material.

17. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the at least one slaking additive is added in an amount from 0.06 to 0.8 wt.-%, based on the total amount of calcium oxide containing material.

18. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the at least one slaking additive is added in an amount from 0.07 to 0.5 wt.-%, based on the total amount of calcium oxide containing material.

19. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the water used in mixing step iv) has a temperature that is adjusted to be in the range from more than 0? C. and less than 100? C.

20. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the water used in mixing step iv) has a temperature that is adjusted to be in the range from 1? C. to 70? C.

21. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the milk of lime obtained from step iv), which is employed in step v), has a temperature that is adjusted to be in the range from 20? C. to 60? C.

22. The aqueous suspension of precipitated calcium carbonate of claim 1, wherein the milk of lime is screened after step iv) and before step v).

23. Precipitated calcium carbonate obtained by a process comprising the steps of: i) providing a calcium oxide containing material, ii) providing at least one water-soluble polymer having a molecular weight M.sub.w in the range from 200 to 6500 g/mol, wherein the at least one polymer has the chemical structure of formula (I): ##STR00025## wherein n, m, and p are integers and at least one of n, m, or p is greater than zero and n+m+p is less than or equal to 70, R.sub.1 is H or CH.sub.3, R.sub.2 is H or CH.sub.3, R.sub.3 is C(?O)OR.sub.4 or C(?O)NHR.sub.4, wherein R.sub.4 is a C.sub.1 to C.sub.20 alkyl group, a C.sub.3 to C.sub.20 cycloalkyl group and/or a C.sub.6 to C.sub.30 aryl group, being optionally substituted with one or more sulfonate groups, and wherein the cycloalkyl group and/or the aryl group comprises one ring or several rings, which are linked to each other, R.sub.5 is H or CH.sub.3, R.sub.6 is H or CH.sub.3, and X is H and/or M, wherein M is Na, K, Li, Mg, and/or Ca, and wherein the structural units ##STR00026## are arranged randomly, regularly and/or in blocks, iii) providing at least one slaking additive, wherein the at least one slaking additive is selected from the group consisting of organic acids, organic acid salts, sugar alcohols, monosaccharides, disaccharides, polysaccharides, gluconates, phosphonates, lignosulfonates, and mixtures thereof, iv) preparing a milk of lime by mixing water, the calcium oxide containing material of step i), the at least one polymer of step ii), and the at least one slaking additive of step iii), wherein the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:12, v) carbonating the milk of lime obtained from step iv) to form an aqueous suspension of precipitated calcium carbonate, vi) separating the precipitated calcium carbonate from the aqueous suspension obtained from step v), and vii) optionally drying the separated precipitated calcium carbonate obtained from step vi).

24. The precipitated calcium carbonate of claim 23, wherein step vii) is performed to obtain dried precipitated calcium carbonate in the form of a powder.

25. A product comprising the aqueous suspension of precipitated calcium carbonate of claim 1.

26. A product comprising the precipitated calcium carbonate of claim 23.

27. A product comprising the precipitated calcium carbonate of claim 24.

28. The product of claim 26, which is a paper, a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product or a pharmaceutical product.

29. The product of claim 27, which is a paper, a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product or a pharmaceutical product.

30. The product of claim 27, which is a plastic or a polymer composition.

Description

DESCRIPTION OF THE FIGURE

[0236] FIG. 1 is a sketch of a continuous slaking process.

EXAMPLES

1. Measurement Methods

[0237] In the following, measurement methods implemented in the examples are described.

Brookfield Viscosity

[0238] 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.

pH Value

[0239] 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).

Particle Size Distribution

[0240] The particle size distribution of the prepared PCC particles was measured using a Sedigraph 5100 from the company Micromeritics, USA. 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.

Solids Content of an Aqueous Suspension

[0241] 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.

Specific Surface Area (SSA)

[0242] 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. Prior to such measurements, the sample is filtered within a Bichner 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.

Specific Carbonation Time

[0243] The monitoring of the conductivity, which slowly decreases during the carbonation reaction and rapidly decreases to a minimal level, thereby indicating the end of the reaction, was used to assess the time needed to perform the complete precipitation. The specific carbonation time (min/kg Ca(OH).sub.2) was determined by the following formula:

[00001]$\mathrm{Specific}.Math..Math.\mathrm{carbonation}.Math..Math.\mathrm{time}=\frac{{10}^5.Math.\mathrm{Tf}}{M.Math.{\mathrm{SC}}_{\mathrm{Mol}}}$

wherein: [0244] Tf (min) is the time needed to complete the carbonation of the milk of lime, as determined by monitoring the conductivity, [0245] M (g) is the weight of the milk of lime introduced into the carbonation reactor, and [0246] SC.sub.MoL (%) is the weight solids content of the milk of lime.

Molecular Weight M.SUB.w

[0247] The molecular weight of the polymers was determined by Gel Permeation Chromatography (GPC), wherein a liquid chromatography device equipped with a refractometric concentration detector, was used (Waters Corporation, USA).

[0248] Said liquid chromatography equipment was fitted with a steric exclusion column appropriately chosen in order to separate the different molecular weights of the analysed polymers. The liquid elution phase was an aqueous phase, which was adjusted to pH 9.00 using 1 N sodium hydroxide and contained 0.05 M of NaHCO.sub.3, 0.1 M of NaNO.sub.3, 0.02 M of triethanolamine, and 0.03% of NaN.sub.3.

[0249] In a first step, the polymerisation solution was diluted to a concentration of 0.9 wt.-% in the GPC solubilisation solvent, which corresponds to the GPC's liquid elution phase, to which 0.04% of dimethylformamide was added as a flow marker or internal standard. A 0.2 m filter was then applied, and subsequently 100 ?l were injected into the chromatography device (eluent: an aqueous phase, which was adjusted to pH 9.00 with 1 N sodium hydroxide and contained 0.05 M of NaHCO.sub.3, 0.1 M of NaNO.sub.3, 0.02 M of triethanolamine, and 0.03% of NaN.sub.3).

[0250] The liquid chromatography device contained an isocratic pump (Waters 515 HPLC pump, Waters Corporation, USA), the flow rate of which was set to 0.8 ml/min. The chromatography device also included an oven, which itself included the following system of columns, in series: a pre-column (guard column ultrahydrogel, Waters Corporation, USA) having a length of 6 cm and an internal diameter of 40 mm, and a linear column (ultrahydrogel, Waters Corporation, USA), having a length of 30 cm, and an internal diameter of 7.8 mm. The detection system, in turn, consisted of a refractometric detector (Waters 410 ri refractometric detector, Waters Corporation, USA). The oven was heated to a temperature of 60? C. and the refractometer was heated to a temperature of 45? C.

[0251] The chromatography device was calibrated by standards of powdered sodium polyacrylate of different molecular weights, certified for the supplier: Polymer Standard Service or American Polymer Standards Corporation.

Polydispersity Index (DPI)

[0252] The polydispersity index of a polymer is the ratio of the mass-average molecular weight in weight M.sub.w to the number-average molecular weight M.sub.n. Both M.sub.w and M.sub.n were determined by gel permeation chromatography.

2. Polymers and Slaking Additives

[0253] The following polymers were used in the processes for producing PCC described in examples 1 to 6:

##STR00020## [0254] wherein R.sub.5?H and R.sub.6?H.

TABLE-US-00001 TABLE 1 Characteristics of polymers used in Examples 1 to 8 (comp: comparative example). M.sub.w Polymer m n p R.sub.1 R.sub.2 R.sub.3 X (g/mol) PDI P1 45 0 0 H Na 4270 2.3 P2 106 0 0 H Na 10 000 3.0 (comp) P3 106 38 0 H Na 16 000 3.5 (comp) P4 0 4 0 50 mol-% H/ 600 1.2 50 mol-% Na P5 106 0 12 H H C(?O)NHC(CH.sub.3).sub.2CH.sub.2SO.sub.3X 54 mol-% H/ 12500 3.0 (comp) 46 mol-% Na P6 branched polyethylene glycol, which does not fall under formula (I) (comp) P7 5 10 0 H H Na 3090 2.1 P8 15 0 1 H H C(?O)OCH.sub.2CH.sub.3 Na 2230 1.9 P9 23 0 7 H H C(?O)OCH.sub.2CH.sub.3 Na 7015 3.0 (comp) P10 29 0 0 H Na 2695 2.0

[0255] The following slaking additives were used in the processes for producing PCC described in examples 1 to 8: [0256] A1: Sodium citrate (commercially available from Sigma-Aldrich, Germany), [0257] A2: Natural sugar (commercially available from any consumer market), [0258] A3: Sodium gluconate (commercially available from Roquette Corp., France), [0259] A4: Sodium diethylene triamine pentaacetic acid (commercially available from Akzo Nobel, Netherlands), [0260] A5: Calcium lignosulfonate (commercially available from Burgo Group spa, Italy), [0261] A6: Sodium lignosulfonate (commercially available from Burgo Group spa, Italy), [0262] A7: Disodium tartrate dihydrate (commercially available from Dr. Paul Lohmann GmbH, Germany).

3. Examples

Example 1

[0263] A milk of lime was prepared by mixing under mechanical stirring water with dry sodium citrate (A1) as slaking additive (if present) and polymer P1 (if present) at an initial temperature between 40 and 41? C. (the amounts of slaking additives and polymer are indicated in Table 2 below). Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 ?m screen.

[0264] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (26 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0265] The characteristics of the prepared milks of lime and aqueous PCC suspensions are described in Tables 2 and 3 below.

TABLE-US-00002 TABLE 2 Characteristics of produced milks of lime of Example 1 (comp: comparative example). P1 polymer Sodium citrate Solids Brookfield amount (A1) amount content viscosity Sample [wt.-%/wt. CaO] [wt.-%/wt. CaO] [wt.-%] [mPa .Math. s] 1 0.2 29.1 1220 (comp) 2 0.2 28.8 269 (comp) 3 0.1 0.1 28.9 300

TABLE-US-00003 TABLE 3 Characteristics of the obtained aqueous PCC suspensions of Example 1 (comp: comparative example). All samples had a calcite structure with the indicated aragonite content. Carbon- Brook- ation Arago- field time Solids nite SSA viscos- Sam- [min/kg content content d.sub.50 [m.sup.2/ ity ple Ca(OH).sub.2] [wt.-%] [wt.-%] [?m] g] pH [mPa .Math. s] 1 carbonation was not carried out since viscosity of (comp) the milk of lime was too high 2 62.0 36.6 33 1.16 5.8 8.0 2680 (comp) 3 46.0 36.6 <2 1.37 5.2 7.4 560

[0266] The results compiled in Table 2 show that the use of a slaking additive alone leads to a milk of lime having a high Brookfield viscosity (comparative sample 1). On the other hand, the use of a polymer alone results in a PCC suspension having a very high Brookfield viscosity (comparative sample 2, Table 3). Furthermore, the carbonation time of comparative sample 2 is longer compared to the inventive sample 3.

[0267] In contrast, inventive sample 3 confirms that the kinetic of carbonation and the crystallographic structure of the prepared PCC is not changed by using the inventive process, compared to a process involving the use of a slaking additive alone. Furthermore, by using the combination of the polymer and the slaking additive, the viscosity of the obtained PCC suspension is significantly reduced.

Example 2

[0268] A milk of lime was prepared by mixing under mechanical stirring water with dry sodium citrate (A1) as slaking additive and a polymer at an initial temperature between 40 and 41? C. (the amounts of slaking additives and polymer as well as the used polymer types are indicated in Table 4 below). Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 m screen.

[0269] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (26 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0270] The characteristics of the prepared milks of lime and aqueous PCC suspensions are described in Tables 4 and 5 below.

TABLE-US-00004 TABLE 4 Characteristics of produced milks of lime of Example 2 (comp: comparative example). Sodium Polymer citrate amount (A1) amount Solids Brookfield [wt.-%/wt. [wt.-%/wt. content viscosity Sample Polymer CaO] CaO] [wt.-%] [mPa .Math. s] 4 P2 0.15 0.05 28.0 too high (comp) 5 P3 0.15 0.05 28.0 too high (comp) 6 P4 0.20 0.10 28.2 418 7 P5 0.20 0.10 28.2 too high (comp) 8 P6 0.20 0.10 28.2 foam (comp) build-up

TABLE-US-00005 TABLE 5 Characteristics of the obtained aqueous PCC suspensions of Example 2 (comp: comparative example). Carbonation time Solids Brookfield [min/kg content viscosity Sample Ca(OH).sub.2] [wt.-%] pH [mPa .Math. s] 4 carbonation was not carried out since viscosity of the (comp) milk of lime was too high 5 carbonation was not carried out since viscosity of the (comp) milk of lime was too high 6 51.0 37.4 7.4 560 7 carbonation was not carried out since viscosity of the (comp) milk of lime was too high 8 not measurable since lime slaking resulted in severe foam (comp) build-up

[0271] The results given in Table 5 show that the use of the comparative polymers P2, P3, and P5, which had a M.sub.w of more than 6500 g/mol, resulted in a milk of lime having such a high Brookfield viscosities (above 1 000 mPa.Math.s at 25? C.?1? C. at 100 rpm) that a further processing of the samples was impossible. Furthermore, the use of comparative polymer P6, which had a structure different to that of formula (I), led to a severe foam build-up during the slaking step.

Example 3

[0272] A milk of lime was prepared by mixing under mechanical stirring water with dry sodium citrate (A1) as slaking additive (if present) and a polymer (if present) at an initial temperature between 40 and 41? C. (the amounts of slaking additives and polymer as well as the used polymer types are indicated in Table 6 below). Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 m screen.

[0273] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (26 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0274] The characteristics of the prepared milks of lime and aqueous PCC suspensions are described in Tables 6 and 7 below.

TABLE-US-00006 TABLE 6 Characteristics of produced milks of lime of Example 3 (comp: comparative example). Sodium Polymer citrate amount (A1) amount Solids Brookfield [wt.-%/wt. [wt.-%/wt. content viscosity Sample Polymer CaO] CaO] [wt.-%] [mPa .Math. s] 9 P7 0.2 28.7 545 (comp) 10 P7 0.1 0.1 28.0 376 11 P8 0.2 28.6 203 (comp) 12 P8 0.1 0.1 29.0 197 13 P9 0.2 28.1 349 (comp) 14 P9 0.1 0.1 28.4 420 (comp)

TABLE-US-00007 TABLE 7 Characteristics of the obtained aqueous PCC suspensions of Example 3 (comp: comparative example). All samples had a calcite structure with the indicated aragonite content. Carbon- Brook- ation Arago- field time Solids nite SSA viscos- Sam- [min/kg content content d.sub.50 [m.sup.2/ ity ple Ca(OH).sub.2] [wt.-%] [wt.-%] [?m] g] pH [mPa .Math. s] 9 64 36.4 24 1.09 5.5 8.2 3310 (comp) 10 46 35.4 <2 1.47 4.3 7.3 433 11 63 36.3 24 1.12 6.1 8.5 2740 (comp) 12 46 35.9 <2 1.44 4.4 7.5 424 13 68 36.3 27 1.05 6.1 7.9 5370 (comp) 14 46 35.7 <2 1.43 4.4 7.3 1622 (comp)

[0275] Using the inventive process (samples 10 and 12), it was possible to produce both a milk of lime and a PCC suspension with a high solids content and an acceptable viscosity (see Tables 6 and 7). As can be gathered from Table 7, the viscosity of the PCC suspension is much lower if a combination of polymer and slaking additive is used. Furthermore, the results compiled on Table 7 show that the specific carbonation time and the crystallographic structure was not significantly changed by using the inventive process, which means that the carbonation reaction is not significantly affected by the addition of the polymer during the slaking of the lime. In contrast, the comparative samples 9 and 11 show that the addition of a polymer alone, during the slaking step increases the specific carbonation time significantly. Table 7 also shows that the use of comparative polymer P9 having a M.sub.w of more than 6500 g/mol yielded a PCC suspension having an unacceptable high viscosity (comparative samples 13 and 14).

Example 4 (Comparative Example)

[0276] A comparative sample was prepared using the following sodium polyacrylates:

[0277] Sample 15: Dispex AA 4140 (commercially available from BASF SE, Germany, also known under trade name Dispex N40, in EP 0844213 A1).

[0278] A milk of lime was prepared by mixing water with 0.20 wt.-%, based on the total weight of the calcium oxide, of the respective polymer under mechanical stirring at an initial temperature between 40 and 41? C. Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 m screen.

[0279] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (26 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0280] The characteristics of the prepared milk of lime and aqueous PCC suspension are described in Table 8 below.

TABLE-US-00008 TABLE 8 Characteristics of produced milk of lime (MoL) and the obtained aqueous PCC suspension (PCC) of Example 4 (comp: comparative example). MoL Carbonation PCC PCC MoL solids Brookfield time solids Brookfield content viscosity [min/kg content viscosity Sample [wt.-%] [mPa .Math. s] Ca(OH).sub.2] [wt.-%] [mPa .Math. s] 15 27.9 238 67 36.0 4250 (comp)

[0281] It can be gathered from Table 8 that the use of the above-mentioned sodium polyacrylate polymer alone during the slaking step, without slaking additives, yielded a PCC suspension having an unacceptably high viscosity.

Example 5

[0282] A milk of lime was prepared by mixing under mechanical stirring water with 0.05 wt.-%, based on the total weight of calcium oxide, dry sodium citrate (A1) (if used) and 0.2 wt.-%, based on the total weight of calcium oxide, polymer P10 (if used), as indicated in Table 9 below, at an initial temperature between 40 and 41? C.

[0283] Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 300 m screen.

[0284] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (26 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0285] The employed amounts of polymers and the characteristics of the obtained milks of lime and aqueous PCC suspensions are described in Tables 9 and 10 below.

TABLE-US-00009 TABLE 9 Characteristics of produced milks of lime of Example 5 (comp: comparative example). Sample 16 Sample 18 (comp) Sample 17 (comp) Polymer P10 P10 Slaking additive A1 A1 T.sub.max slaking [? C.] 74 100 99.8 Solids content [wt.-%] 16.5 30.1 31.0 Brookfield viscosity [mPa .Math. s] 31 386 550

TABLE-US-00010 TABLE 10 Characteristics of the obtained aqueous PCC suspensions of Example 5 (comp: comparative example). Sample 16 Sample 18 (comp) Sample 17 (comp) Specific carbonation time 45 47 46 [min/kg Ca(OH).sub.2] Solids content [wt.-%] 20.6 39.1 38.4 Particle size d.sub.50 [?m] 1.49 1.40 1.23 pH 7.6 8.6 8.9 pH after 8 days 10.3 10.6 11.1 Brookfield viscosity 25 328 1080 [mPa .Math. s]

[0286] The results given in Table 10 clearly show that the process of the invention allows an increase of the solids content of the PCC slurries obtained without imparting the specific carbonation time, as well as the features of the so-obtained PCC particles.

Example 6

[0287] A milk of lime was prepared by mixing under mechanical stirring water 0.2 wt.-%, based on the total amount of the calcium oxide, of the polymer P1 (if present) with 0.15 wt.-%, based on the total amount of the calcium oxide, of a slaking additive at an initial temperature between 40 and 41? C. (the amounts of slaking additives and polymer as well as the used slaking additives are indicated in Table 11 below). Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 m screen.

[0288] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (26 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0289] The characteristics of the prepared milks of lime and aqueous PCC suspensions are described in Tables 11 and 12 below.

TABLE-US-00011 TABLE 11 Characteristics of produced milks of lime of Example 6 (comp: comparative example). Solids Brookfield Slaking content viscosity Sample additive Polymer [wt.-%] [mPa .Math. s] 19 (comp) A1 13.6 32 20 A1 P1 26.0 275 21 A2 P1 26.5 220 22 A3 P1 27.1 371 23 A4 P1 25.6 327 24 A5 P1 25.9 266 25 A6 P1 26.4 393 26 A7 P1 25.0 273

TABLE-US-00012 TABLE 12 Characteristics of the obtained aqueous PCC suspensions of Example 6 (comp: comparative example). Carbonation Solids Brookfield time content d.sub.50 SSA viscosity Sample [min/kg Ca(OH).sub.2] [wt.-%] [?m] [m.sup.2/g] pH [mPa .Math. s] 19 50 18.5 1.82 4.7 7.6 34 (comp) 20 52 32.7 1.26 8.8 7.6 637 21 47 32.9 1.45 6.3 8.2 383 22 50 33.3 1.43 5.2 8.4 341 23 49 31.8 1.35 6.7 8.6 336 24 44 31.0 1.52 6.9 7.4 228 25 48 32.4 1.36 6.7 7.6 406 26 60 33.5 1.25 7.6 7.5 685

[0290] The results compiled in Table 12 confirm that the kinetic of carbonation is not changed by using the inventive process or even can be slightly speeded up (see sample 24). Both the milk of lime and the PCC suspension of all inventive samples (samples 20 to 26) revealed a low viscosity at a high solids content.

Example 7

[0291] A milk of lime was prepared by mixing under mechanical stirring 1800 l water with natural sugar (A2) as slaking additive and 0.15 wt.-%, based on the total weight of the calcium oxide, polymer P1 at an initial temperature of about 40? C. (the amount of slaking additive is indicated in Table 13 below). Subsequently, 370 kg calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 30 min at 50 rpm. Care was taken not to exceed a slaking temperature of 80? C.

[0292] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to a temperature between 31 and 35? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (200 Nm.sup.3/h and 11 vol.-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 200 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0293] The obtained aqueous suspension of precipitated calcium carbonate was sieved through a 45 ?m screen in order to separate the PCC. The characteristics of the prepared aqueous PCC suspensions and the obtained PCC are described in Table 13 below.

TABLE-US-00013 TABLE 13 Characteristics of the obtained aqueous PCC suspensions and PCC of Example 7. Sample 27 28 Amount slaking additive [wt.-%/wt. CaO] 0.25 0.20 Starting temperature carbonation [? C.] 35 31 Reaction time carbonation [min] 635 452 Solids content 28.6 27.5 d.sub.50 [?m] 0.97 1.03 SSA [m.sup.2/g] 13.6 17.0 pH 7.6 7.6 Brookfield viscosity [mPa .Math. s] 400 230

[0294] The results compiled in Table 13 confirm that a PCC suspension with a low viscosity at a high solids content can be obtained by the inventive process.

Example 8

[0295] A milk of lime was prepared by mixing under mechanical stirring 1800 l water with natural sugar (A2) as slaking additive and 0.15 wt.-%, based on the total weight of the calcium oxide, polymer P1 at an initial temperature of about 40? C. (the amount of slaking additive is indicated in Table 14 below). Subsequently, 370 kg calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 30 min at 50 rpm. Care was taken not to exceed a slaking temperature of 80? C.

[0296] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to a temperature between 31 and 35? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (200 Nm.sup.3/h and 11 vol.-% CO.sub.2).

[0297] During the carbonation step, the reaction mixture was stirred with a speed of 200 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0298] The obtained aqueous suspension of precipitated calcium carbonate was sieved through a 45 ?m screen in order to separate the PCC. The characteristics of the prepared aqueous PCC suspensions and the obtained PCC are described in Table 14 below.

TABLE-US-00014 TABLE 14 Characteristics of the obtained aqueous PCC suspensions and PCC of Example 8. Sample 29 30 31 Amount slaking additive [wt.-%/wt. CaO] 0.25 0.20 0.20 Starting temperature carbonation [? C.] 35 31 35 Reaction time carbonation [min] 522 457 491 Solids content 26.6 27.7 27.8 d.sub.50 [?m] 1.14 1.20 1.27 SSA [m.sup.2/g] 13.6 11.0 9.3 pH 8.4 8.2 8.1 Brookfield viscosity [mPa .Math. s] 450 286 220

[0299] The results compiled in Table 14 confirm that a PCC suspension with a low viscosity at a high solids content can be obtained by the inventive process.