PROCESS FOR PREPARING SURFACE-REACTED CALCIUM CARBONATE

Abstract

The present invention relates to a process for producing a surface-reacted calcium carbonate, wherein a calcium carbonate-comprising material is treated with at least one inorganic acid and carbon dioxide in an aqueous medium to form an aqueous suspension of surface-reacted calcium carbonate, wherein the at least one water-soluble, inorganic magnesium salt is added before, during and/or after the treatment step. Furthermore, the present invention relates to a surface-reacted calcium carbonate obtained from said process and its use.

Claims

1. Process for producing a surface-reacted calcium carbonate comprising the steps of: a) providing a calcium carbonate-comprising material, b) providing at least one inorganic acid, c) providing at least one water-soluble, inorganic magnesium salt, and d) treating the calcium carbonate-comprising material of step a) with the at least one inorganic acid of step b) and carbon dioxide in an aqueous medium to form an aqueous suspension of surface-reacted calcium carbonate, wherein the carbon dioxide is formed in situ by the inorganic acid treatment and/or is supplied by an external source, and wherein the at least one water-soluble, inorganic magnesium salt of step c) is added before, during and/or after step d).

2. The process of claim 1, wherein the calcium carbonate-comprising material is a natural ground calcium carbonate and/or a precipitated calcium carbonate, preferably the natural ground calcium carbonate is selected from the group consisting of marble, chalk, limestone, and mixtures thereof, and/or the precipitated calcium carbonate is selected from the group consisting of precipitated calcium carbonates having an aragonitic, vateritic or calcitic crystal form, and mixtures thereof.

3. The process of claim 1, wherein the calcium carbonate-comprising material is in form of particles having a weight median particle size d.sub.50(wt) from 0.05 to 10 μm, preferably from 0.2 to 5.0 μm, more preferably from 0.4 to 3.0 μm, and most preferably from 0.6 to 1.2 μm, and/or a weight top cut particle size d.sub.98(wt) from 0.15 to 55 μm, preferably from 1 to 30 μm, more preferably from 2 to 18 μm, and most preferably from 3 to 7 μm.

4. The process of claim 1, wherein the at least one inorganic acid is selected from the group consisting of hydrochloric acid, sulphuric acid, sulphurous acid, phosphoric acid, an inorganic acid salt thereof, and mixtures thereof, preferably the at least one inorganic acid is selected from the group consisting of hydrochloric acid, sulphuric acid, sulphurous acid, phosphoric acid, H.sub.2PO.sub.4.sup.−, being at least partially neutralised by a cation selected from NH.sub.4.sup.+, Li.sup.+, Na.sup.+ and/or K.sup.+, HPO.sub.4.sup.2−, being at least partially neutralised by a cation selected from NH.sub.4.sup.+, Li.sup.+, Na.sup.+′ and/or K.sup.+, and mixtures thereof, more preferably the at least one inorganic acid is selected from the group consisting of hydrochloric acid, sulphuric acid, sulphurous acid, phosphoric acid, or mixtures thereof, and most preferably the at least one inorganic acid is phosphoric acid.

5. The process of claim 1, wherein the at least one inorganic acid is provided in an amount from 1 to 60 wt.-%, based on the total weight of the calcium carbonate-comprising material, preferably from 5 to 55 wt.-%, more preferably from 7 to 50 wt.-%, and most preferably from 10 to 40 wt.-%.

6. The process of claim 1, wherein the at least one water-soluble, inorganic magnesium salt is selected from the group consisting of magnesium chloride, magnesium nitrate, magnesium sulfate, magnesium hydrogen sulfate, magnesium bromide, magnesium iodide, magnesium chlorate, magnesium iodate, hydrates thereof, and mixtures thereof, preferably the at least one water-soluble, inorganic magnesium salt is selected from the group consisting of magnesium bromide, magnesium nitrate, magnesium sulfate, hydrates thereof, and mixtures thereof, and most preferably the at least on water-soluble, inorganic magnesium salt is magnesium sulfate or a hydrate thereof.

7. The process of claim 1, wherein the at least one water-soluble, inorganic magnesium salt is provided in an amount from 0.3 to 270 mmol Mg.sup.2+/mol Ca.sup.2+ of the calcium carbonate-comprising material, preferably from 0.7 to 200 mmol Mg.sup.2+/mol Ca.sup.2+ of the calcium carbonate-comprising material, more preferably from 2 to 135 mmol Mg.sup.2+/mol Ca.sup.2+ of the calcium carbonate-comprising material, and most preferably from 3 to 70 mmol Mg.sup.2+/mol Ca.sup.2+ of the calcium carbonate-comprising material.

8. The process of claim 1, wherein in step d) the calcium carbonate-comprising material is treated with a solution comprising the at least one inorganic acid of step b) and the at least one water-soluble, inorganic magnesium salt of step c).

9. The process of claim 1, wherein the carbon dioxide is formed in situ by the inorganic acid treatment and/or step d) is carried out at a temperature from 20 to 90° C., preferably from 30 to 85° C., more preferably from 40 to 80° C., even more preferably from 50 to 75° C., and most preferably from 60 to 70° C.

10. The process of claim 1, wherein the calcium carbonate-comprising material is a natural ground calcium carbonate, the at least one inorganic acid is phosphoric acid, the at least one water-soluble, inorganic magnesium salt is selected from the group consisting of magnesium bromide, magnesium nitrate, magnesium sulfate, hydrates thereof, and mixtures thereof, and preferably is magnesium sulfate or a hydrate thereof, and in step d) the calcium carbonate-comprising material is treated with a solution comprising the at least one inorganic acid of step b) and the at least one water-soluble, inorganic magnesium salt of step c).

11. A surface-reacted calcium carbonate obtainable by a process according to claim 1.

12. The surface-reacted calcium carbonate of claim 11, wherein the surface-reacted calcium carbonate has a specific surface area of from 20 m.sup.2/g to 200 m.sup.2/g, preferably from 30 m.sup.2/g to 180 m.sup.2/g, more preferably from 35 m.sup.2/g to 150 m.sup.2/g, even more preferably from 40 m.sup.2/g to 130 m.sup.2/g, and most preferably from 50 m.sup.2/g to 100 m.sup.2/g, measured using nitrogen and the BET method.

13. The surface-reacted calcium carbonate of claim 11, wherein the surface-reacted calcium carbonate has an intra-particle intruded specific pore volume in the range from 0.1 to 2.3 cm.sup.3/g, preferably from 0.2 to 2.0 cm.sup.3/g, more preferably from 0.3 to 1.8 cm.sup.3/g, and most preferably from 0.35 to 1.6 cm.sup.3/g, calculated from mercury porosimetry measurement, and/or an intra-particle pore size in a range of from 0.004 to 1.0 μm, preferably in a range of between 0.005 to 0.8 μm, more preferably from 0.006 to 0.6 μm, and most preferably of 0.007 to 0.4 μm, determined from mercury porosity measurement.

14. A surface-reacted calcium carbonate comprising a calcium carbonate-comprising material, preferably calcite, and at least one water-insoluble calcium salt selected from tricalcium phosphate and/or apatitic calcium phosphate, preferably selected from the group consisting of hydroxylapatite, substituted hydroxylapatite, octacalcium phosphate, and mixtures thereof, more preferably selected from the group consisting of hydroxylapatite, fluoroapatite, carboxyapatite, and mixtures thereof, and most preferably hydroxylapatite, wherein the surface-reacted calcium carbonate comprises (i) a specific surface area of from 20 to 200 m.sup.2/g measured using nitrogen and the BET method according to ISO 9277:2010; (ii) an intra-particle intruded specific pore volume in the range of from 0.1 to 2.3 cm.sup.3/g calculated from mercury porosimetry measurement, and/or an intra-particle pore size in a range of from 0.004 to 1.0 μm, determined from mercury porosity measurement, and (iii) whitlockite in an amount of at least 0.1 wt.-%, based on the total amount of the calcium carbonate and the at least one water-insoluble calcium salt, preferably in an amount of at least at least 0.5 wt.-%, more preferably in an amount of at least 1 wt.-%, and most preferably in an amount of at least 2 wt.-%.

15. The surface-reacted calcium carbonate of claim 14, wherein the mass ratio of calcium carbonate to the at least one water-insoluble calcium salt is in the range of from 1:0.1 to 1:76, preferably in the range from 1:0.2 to 1:10, more preferably from 1:0.5 to 1:6, even more preferably from 1:0.9 to 1:2, and most preferably in a range from 1:0.95 to 1:1.2.

16. Use of a surface-reacted calcium carbonate according to claim 11 in polymer applications, paper coating applications, paper making, paints, coatings, sealants, printing inks, adhesives, food, feed, pharmaceuticals, concrete, cement, cosmetics, water treatment, engineered wood applications, plasterboard applications, packaging applications and/or agricultural applications, preferably in polymer applications and/or food applications.

17. An article comprising a surface-reacted calcium carbonate according to claim 11, wherein the article is selected from paper products, engineered wood products, plasterboard products, polymer products, hygiene products, medical products, healthcare products, food products, feed products, filter products, woven materials, nonwoven materials, geotextile products, agriculture products, horticulture products, clothing, footwear products, baggage products, household products, industrial products, packaging products, building products, and construction products.

18. Use of a surface-reacted calcium carbonate according to claim 14 in polymer applications, paper coating applications, paper making, paints, coatings, sealants, printing inks, adhesives, food, feed, pharmaceuticals, concrete, cement, cosmetics, water treatment, engineered wood applications, plasterboard applications, packaging applications and/or agricultural applications, preferably in polymer applications and/or food applications.

19. An article comprising a surface-reacted calcium carbonate according to claim 14, wherein the article is selected from paper products, engineered wood products, plasterboard products, polymer products, hygiene products, medical products, healthcare products, food products, feed products, filter products, woven materials, nonwoven materials, geotextile products, agriculture products, horticulture products, clothing, footwear products, baggage products, household products, industrial products, packaging products, building products, and construction products.

Description

EXAMPLES

1. Measurement Methods

[0202] In the following, measurement methods implemented in the examples are described.

[0203] Particle Size Distribution

[0204] Volume determined median particle size d.sub.50(vol) and the volume determined top cut particle size d.sub.98(vol) was evaluated using a Malvern Mastersizer 2000 Laser Diffraction System (Malvern Instruments Plc., Great Britain). The d.sub.50(vol) or d.sub.98(vol) value indicates a diameter value such that 50% or 98% by volume, respectively, of the particles have a diameter of less than this value. The raw data obtained by the measurement was analyzed using the Mie theory, with a particle refractive index of 1.57 and an absorption index of 0.005. The methods and instruments are known to the skilled person and are commonly used to determine particle size distributions of fillers and pigments.

[0205] The weight determined median particle size d.sub.50(wt) and the weight determined top cut particle size d.sub.98(wt) was measured by the sedimentation method, which is an analysis of sedimentation behaviour in a gravimetric field. The measurement was made with a Sedigraph™ 5120 of Micromeritics Instrument Corporation, USA. The method and the instrument are known to the skilled person and are commonly used to determine particle size distributions of fillers and pigments. The measurement was carried out in an aqueous solution of 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and sonicated.

[0206] Specific Surface Area (SSA)

[0207] The specific surface area was measured via the BET method according to ISO 9277:2010 using nitrogen, following conditioning of the sample by heating at 250° C. for a period of 30 minutes. Prior to such measurements, the sample was filtered within a Buchner funnel, rinsed with deionised water and dried overnight at 90 to 100° C. in an oven. Subsequently, the dry cake was ground thoroughly in a mortar and the resulting powder was placed in a moisture balance at 130° C. until a constant weight was reached.

[0208] Intra-Particle Intruded Specific Pore Volume (in Cm.sup.3/g)

[0209] The specific pore volume was measured using a mercury intrusion porosimetry measurement using a Micromeritics Autopore V 9620 mercury porosimeter having a maximum applied pressure of mercury 414 MPa (60 000 psi), equivalent to a Laplace throat diameter of 0.004 μm (˜nm). The equilibration time used at each pressure step was 20 seconds. The sample material was sealed in a 5 cm.sup.3 chamber powder penetrometer for analysis. The data were corrected for mercury compression, penetrometer expansion and sample material compression using the software Pore-Comp (Gane, P. A. C., Kettle, J. P., Matthews, G. P. and Ridgway, C. J., “Void Space Structure of Compressible Polymer Spheres and Consolidated Calcium Carbonate Paper-Coating Formulations”, Industrial and Engineering Chemistry Research, 35(5), 1996, p 1753-1764.).

[0210] The total pore volume seen in the cumulative intrusion data can be separated into two regions with the intrusion data from 214 μm down to about 1-4 μm showing the coarse packing of the sample between any agglomerate structures contributing strongly. Below these diameters lies the fine inter-particle packing of the particles themselves. If they also have intra-particle pores, then this region appears bi-modal, and by taking the specific pore volume intruded by mercury into pores finer than the modal turning point, i.e. finer than the bi-modal point of inflection, the specific intra-particle pore volume is defined. The sum of these three regions gives the total overall pore volume of the powder, but depends strongly on the original sample compaction/settling of the powder at the coarse pore end of the distribution.

[0211] By taking the first derivative of the cumulative intrusion curve the pore size distributions based on equivalent Laplace diameter, inevitably including pore-shielding, are revealed. The differential curves clearly show the coarse agglomerate pore structure region, the inter-particle pore region and the intra-particle pore region, if present. Knowing the intra-particle pore diameter range it is possible to subtract the remainder inter-particle and inter-agglomerate pore volume from the total pore volume to deliver the desired pore volume of the internal pores alone in terms of the pore volume per unit mass (specific pore volume). The same principle of subtraction, of course, applies for isolating any of the other pore size regions of interest.

[0212] X-Ray Diffraction (XRD) Analysis

[0213] The prepared samples were analysed with a Bruker D8 Advance powder diffractometer obeying Bragg's law. This diffractometer consisted of a 2.2 kW X-ray tube, a sample holder, a ϑ-ϑ goniometer, and a VANTEC-1 detector. Nickel-filtered Cu Kα radiation was employed in all experiments. The profiles were chart recorded automatically using a scan speed of 0.7° per minute in 2ϑ (XRD GV 7600). 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).

[0214] 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.

[0215] Semi-Quantitative (SQ) calculations to estimate the rough mineral concentrations were carried out with the DIFFRAC.sup.suite software package EVA. The semi-quantitative analysis was performed considering the patterns relative heights and I/I.sub.cor values (I/I.sub.cor: ratio between the intensities of the strongest line in the compound of interest and the strongest line of corundum, both measured from a scan made of a 50-50 by weight mixture).

2. Examples

Example 1 (Comparative Example)

[0216] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0217] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution.

[0218] Whilst mixing the slurry, 2.3 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0219] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 4.1 μm, a volume determined top cut particle size d.sub.98 of 8.0 μm, and a specific surface area SSA of 46.2 m.sup.2g.sup.−1.

Example 2

[0220] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0221] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution. 14 wt.-% Mg(NO.sub.3).sub.2.6H.sub.2O, based on the total weight of the neat phosphoric acid, was then added to this solution and the solution was stirred until it fully dissolved.

[0222] Whilst mixing the slurry, 2.4 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0223] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 3.8 μm, a volume determined top cut particle size d.sub.98 of 7.8 μm, and a specific surface area SSA of 78.2 m.sup.2g.sup.−1.

Example 3

[0224] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total mass of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0225] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution. 28.7 wt.-% Mg(NO.sub.3).sub.2.6H.sub.2O, based on the total weight of the neat phosphoric acid, was then added to this solution and the solution was stirred until it fully dissolved.

[0226] Whilst mixing the slurry, 2.5 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0227] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 3.9 μm, a volume determined top cut particle size d.sub.98 of 8.0 μm, and a specific surface area SSA of 84.2 m.sup.2g.sup.−1.

Example 4

[0228] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0229] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution. 43.0% Mg(NO.sub.3).sub.2.6H.sub.2O, based on the total weight of the neat phosphoric acid, was then added to this solution and the solution was stirred until it fully dissolved.

[0230] Whilst mixing the slurry, 2.6 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0231] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 3.6 μm, a volume determined top cut particle size d.sub.98 of 7.3 μm, and a specific surface area SSA of 79.5 m.sup.2g.sup.−1.

Example 5 (Comparative Example)

[0232] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0233] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution.

[0234] Whilst mixing the slurry, 2.3 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0235] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 3.9 μm, a volume determined top cut particle size d.sub.98 of 8.0 μm, and a specific surface area SSA of 43.9 m.sup.2g.sup.−1.

Example 6

[0236] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0237] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution. 4.5 wt.-% MgSO.sub.4.7H.sub.2O, based on the total weight of the neat phosphoric acid, was then added to this solution and the solution was stirred until it fully dissolved.

[0238] Whilst mixing the slurry, 2.4 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0239] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 4.0 μm, a volume determined top cut particle size d.sub.98 of 9.1 μm, and a specific surface area SSA of 93.1 m.sup.2g.sup.−1.

Example 7

[0240] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0241] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution. 13.8 wt.-% MgSO.sub.4.7H.sub.2O, based on the total weight of the neat phosphoric acid, was then added to this solution and the solution was stirred until it fully dissolved.

[0242] Whilst mixing the slurry, 2.6 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0243] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 3.8 μm, a volume determined top cut particle size d.sub.98 of 7.7 μm, and a specific surface area SSA of 92.7 m.sup.2g.sup.−1.

Example 8

[0244] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0245] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution. 16.4 wt.-% MgBr.sub.2.6H.sub.2O, based on the total weight of the neat phosphoric acid, was then added to this solution and the solution was stirred until it fully dissolved.

[0246] Whilst mixing the slurry, 2.4 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0247] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 4.2 μm, a volume determined top cut particle size d.sub.98 of 9.7 μm, and a specific surface area SSA of 70.9 m.sup.2g.sup.−1.

Example 9

[0248] Surface-reacted calcium carbonate was obtained by preparing 10 litres of an aqueous suspension of ground calcium carbonate in a mixing vessel by adjusting the solids content of a ground marble calcium carbonate from Hustadmarmor Norway such that a solids content of 20 wt.-%, based on the total weight of the aqueous suspension, is obtained. The ground calcium carbonate had a weight based particle size distribution of 90% less than 2 μm, as determined by sedimentation, a d.sub.50(wt) of 0.7 μm, and a d.sub.98(wt) of 3.4 μm.

[0249] In addition, phosphoric acid was diluted such that it contained 30% phosphoric acid, based on the total weight of the solution. 49.3 wt.-% MgBr.sub.2.6H.sub.2O, based on the total weight of the neat phosphoric acid, was then added to this solution and the solution was stirred until it fully dissolved.

[0250] Whilst mixing the slurry, 2.6 kg of the phosphoric acid solution was added over 10 minutes. Throughout the whole experiment the temperature of the suspension was maintained at 70° C.±1° C. Finally, after the addition of the acid, the suspension was stirred for additional 5 minutes before removing it from the vessel and allowing it to cool.

[0251] The obtained surface-reacted calcium carbonate had a volume determined median particle size d.sub.50 of 3.9 μm, a volume determined top cut particle size d.sub.98 of 8.4 μm, and a specific surface area SSA of 72.8 m.sup.2g.sup.−1.

Results

[0252] The prepared surface-reacted calcium carbonate particles were characterized with respect to their particles size distribution, their specific surface area, and their porosity, as described above. The results as well as the employed magnesium salt and its concentration are compiled in Table 1 below. Furthermore, XRD measurements of the surface-reacted calcium carbonate particles prepared according to Examples 1, and 3 to 5 were carried out in order to determine their crystalline structure (see Table 2).

[0253] It can be gathered from Table 1 that the addition of the water-soluble, inorganic magnesium salt led to a noticeable increase of the BET values, compared to comparative examples 1 and 5. It can also be seen from said experimental data that the increase of the BET can be controlled by the selection of a specific magnesium salt.

[0254] Furthermore, Table 1 shows that the intra-particle intruded specific pore volume of the surface-reacted calcium carbonate can be modified in a predetermined way. While a lower concentration of 4.1 mmol Mg.sup.2+/g CaCO.sub.3 lead to an increase in intra-particle intruded specific pore volume, compared to comparative examples 1 and 5, the addition of magnesium salt concentrations of 8.2 mmol Mg.sup.2+/g CaCO.sub.3 or more resulted in an intra-particle intruded specific pore volume being lower than that of the comparative example. It is also to be noted that the particles size distribution of the surface-reacted calcium carbonate particles is not affected significantly, as can be seen from the d.sub.50 and d.sub.98 values in Table 1.

[0255] The XRD analysis of the surface-reacted calcium carbonate particles of Examples 1, and 3 to 5 compiled in Table 2 reveals that the inventive samples (Examples 3 and 4) contain a higher amount of hydroxylapatite, which indicates that in the inventive particles more crystalline calcium phosphate has been formed leaving only little amorphous calcium phosphate remaining. In other words, the inventive surface-reacted calcium carbonate particles exhibit a higher crystallinity. Moreover, the magnesium mineral whitlockite is clearly detectable in the surface-reacted calcium carbonates obtained in inventive Examples 3 to 4.

TABLE-US-00001 TABLE 1 Characteristics of the surface-reacted calcium carbonate particles prepared according to Examples 1 to 9. Amount of Infra-particle Mg.sup.2+ intruded . . . for pore [wt.-% Mg.sup.2+/ specific pore diameter Magnesium total weight volume range SSA d.sub.50 d.sub.98 Example salt CaCO.sub.3] [cm.sup.3g.sup.−1] . . . [μm] [m.sup.2g.sup.−1] [μm] [μm] Example 1 — — 0.88 0.004-0.34 46.2 4.1 8.0 (comparative) Example 2 Mg(NO.sub.3).sub.2•6H.sub.2O 0.4 0.94 0.004-0.31 78.2 3.8 7.8 Example 3 Mg(NO.sub.3).sub.2•6H.sub.2O 0.8 0.75 0.004-0.25 84.2 3.9 8.0 Example 4 Mg(NO.sub.3).sub.2•6H.sub.2O 1.2 0.60 0.004-0.20 79.5 3.6 7.3 Example 5 — — 0.88 0.004-0.38 43.9 3.9 8.0 (comparative) Example 6 MgSO.sub.4•7H.sub.2O 0.4 0.90 0.004-0.24 93.1 4.0 9.1 Example 7 MgSO.sub.4•7H.sub.2O 1.2 0.57 0.004-0.17 92.7 3.8 7.7 Example 8 MgBr.sub.2•6H.sub.2O 0.4 1.05 0.004-0.27 70.9 4.2 9.7 Example 9 MgBr.sub.2•6H.sub.2O 1.2 0.68 0.004-0.38 72.8 3.9 8.4

TABLE-US-00002 TABLE 2 Quantitative Rietvield analysis of surface-reacted calcium carbonate particles prepared according to Examples 1 to 5. Data are normalized to 100% crystalline material and values are expressed in wt.-%. Example 1 Example 5 Mineral [%] Formula (comparative) Example 3 Example 4 (comparative) Calcite CaCO.sub.3 57 49 48 56 Hydroxylapatite Ca.sub.5(PO.sub.4).sub.3(OH) 43 50 48 44 Whitlockite Ca.sub.9Mg(PO.sub.4).sub.6(HPO.sub.4) — 1 4 — Total 100 100 100 100