Process for the production of precipitated calcium carbonate

Abstract

The present invention relates to a process for the preparation of precipitated calcium carbonate allowing for a more sustainable and cost-efficient use of milk of lime. The obtainable products show good optical properties as well as fine particle size distributions.

Claims

1. A process for the preparation of precipitated calcium carbonate, the process comprising the steps of: (a) providing milk of lime; (b) separating grit from the milk of lime provided in step (a), the grit corresponding to an oversize fraction retained by a screen having an aperture size of 400 m or finer, to obtain: (i) grit reduced milk of lime, and (ii) a grit fraction; (c) milling the grit fraction of step (b) in at least one milling unit to obtain fine grit; and (d) subjecting to a carbonation step either: (i) the fine grit of step (c), or (ii) a mixture comprising the grit reduced milk of lime of step (b) and the fine grit of step (c); wherein said carbonation step comprises the introduction of carbon dioxide to obtain precipitated calcium carbonate.

2. The process according to claim 1, wherein the milk of lime provided in step (a) has a solids content of from 1.0 to 40.0 wt.-%, based on the total weight of said milk of lime.

3. The process according to claim 1, wherein the milk of lime provided in step (a) has a solids content of from 5.0 to 35.0 wt.-%, based on the total weight of said milk of lime.

4. The process according to claim 1, wherein step (b) comprises a screening step by use of a screen having an aperture size of 400 m to 150 m.

5. The process according to claim 1, wherein step (b) comprises a screening step by use of a screen having an aperture size of 350 to 180 m.

6. The process according to claim 1, wherein step (b) comprises a screening step by use of a screen having an aperture size of 250 to 200 m.

7. The process according to claim 1, wherein the at least one milling unit of step (c) comprises a sand mill, a roller mill, a ball mill, or a hammer mill.

8. The process according to claim 1, wherein the at least one milling unit of step (c) comprises a sand mill.

9. The process according to claim 1, wherein the solids content of the grit fractions during milling is in the range of from 20.0 to 80.0 wt. -%, based on the total weight the grit fractions.

10. The process according to claim 1, wherein the solids content of the grit fractions during milling is in the range of from 25.0 to 50.0 wt.-%, based on the total weight the grit fractions.

11. The process according to claim 1, wherein step (c) further comprises a step of screening the grit fraction subsequently to the milling of the fraction, by use of a screen and removing the oversize fraction retained by the screen.

12. The process according to claim 11, wherein the screen has an aperture size of 300 m or finer.

13. The process according to claim 11, wherein the screen has an aperture size of 300 m to 45 m.

14. The process according to claim 11, wherein the screen has an aperture size of 250 m to 50 m.

15. The process according to claim 1, wherein the fine grit of step (c) has a d.sub.50 ranging from 0.1 to 10.0 m.

16. The process according to claim 1, wherein the fine grit of step (c) has a d.sub.50 ranging from 0.2 to 5.0 m.

17. The process according to claim 1, wherein the fine grit of step (c) has a d.sub.50 ranging from 0.3 to 3.0 m.

18. The process according to claim 1, wherein the fine grit of step (c) has a d.sub.98 ranging from 2.0 to 20.0 m.

19. The process according to claim 1, wherein the fine grit of step (c) has a d.sub.98 ranging from 6.0 to 12.0 m.

20. The process according to claim 1, wherein the starting temperature in step (d) ranges from 5 to 80 C.

21. The process according to claim 1, wherein the process further comprises a step of adding at least one additive selected from monosaccharides, disaccharides, polyacrylic acids in their neutralized or partly neutralized forms, citric acid, or sodium citrate to the milk of lime provided in step (a) and/ or the grit reduced milk of lime of step (b) and/or the grit fraction of step (b) and/or the fine grit of step (c) in a total amount of from 0.005 to 1.0 wt.-%, based on the total dry weights.

22. The process according to claim 1, wherein the process further comprises a step of adding at least one additive selected from monosaccharides, disaccharides, polyacrylic acids in their neutralized or partly neutralized forms, citric acid, or sodium citrate to the milk of lime provided in step (a) and/or the grit reduced milk of lime of step (b) and/or the grit fraction of step (b) and/or the fine grit of step (c) in a total amount of from 0.01 to 0.5 wt.-%, based on the total dry weights.

23. The process according to claim 1, wherein step (d) consists of subjecting to a carbonation step the fine grit of step (c).

24. The process according to claim 1, wherein step (d) consists of subjecting to a carbonation step the mixture comprising the grit reduced milk of lime of step (b) and the fine grit of step (c), and wherein the fine grit is added to the grit reduced milk of lime in one or more portions.

25. The process according to claim 24, wherein the total amount of fine grit added to the grit reduced milk of lime ranges from 0.1 to 15.0 wt.-%, based on the total dry weights.

26. The process according to claim 24, wherein the total amount of fine grit added to the grit reduced milk of lime ranges from 0.2 to 10.0 wt.-%, based on the total dry weights.

27. The process according to claim 24, wherein the total amount of fine grit added to the grit reduced milk of lime ranges from 0.5 to 5.0 wt.-%, based on the total dry weights.

28. The process according to claim 24, wherein the fine grit is added to the grit reduced milk of lime at a time corresponding to from 0 to 100%, conversion of calcium hydroxide into precipitated calcium carbonate.

29. The process according to claim 24, wherein the fine grit is added to the grit reduced milk of lime at a time corresponding to from 35% to 95%, conversion of calcium hydroxide into precipitated calcium carbonate.

30. The process according to claim 24, wherein the fine grit is added to the grit reduced milk of lime at a time corresponding to from 60% to 80%, conversion of calcium hydroxide into precipitated calcium carbonate.

Description

EXAMPLES

(1) The scope and interest of the invention may be better understood on basis of the following examples which are intended to illustrate embodiments of the present invention. However, they are not to be construed to limit the scope of the claims in any manner whatsoever.

Example 1

Grit Separation

(2) In a first trial, calcium oxide was slaked with 5 parts per weight of water at 230 rpm for 25 min during which the temperature was kept at 40 C. The obtained milk of lime was screened through a screen having an aperture size of 200 m to obtain grit reduced milk of lime and a grit fraction.

(3) In a second trial, 200 kg of calcium oxide were slaked with 1,700 kg of water at 40 C. The obtained milk of lime was screened through a screen having an aperture size of 200 m to obtain grit reduced milk of lime and a grit fraction.

Example 2

Milling Trials, Production of Fine Grit

(4) A grit fraction (d.sub.50=1.1 mm) obtained from screening milk of lime through a screen having an aperture size of 200 m was used in the following milling trials to produce fine grit. For this purpose, the grit fraction was charged into an octagonal sand mill (volume: 1.8 m.sup.3; number of baffles: 4; beads: 1,200 kg of Bitossi Microbits 1.5 to 2.5 mm). Milling was then carried out in a continuous process to produce fine grit using a 0.4 mm grid as the outlet of the mill which was installed at 34% of the overall vertical height of 1,400 mm and equipped with a pump. A dual net sieve having aperture sizes of 100 and 45 m, respectively, was installed downstream of said pump. Varying amounts of sucrose, based on the dry weight of the milk of lime, were added in Trials 2 to 5 and 7 in order to reduce the viscosity of the grinding stocks. The milling trials are listed in Table 1 below.

(5) TABLE-US-00001 TABLE 1 Results of grit milling (n/d = not determined), dt refers to dry tonnes. Trial 1 2 3 4 5 7 Mill inlet Flow rate [dt/h] 0.50 0.53 0.66 0.79 0.79 1.35 Sucrose [kg/h] 0.00 3.00 3.30 4.00 11.9 3.8 Solids content [wt.-%] 25.3 39.4 42.6 46.8 53.8 45.3 Mill outlet d.sub.50 [m] 2.1 1.5 1.7 1.9 n/d 1.46 d.sub.98 [m] 20 17 16 19 n/d 11.5 pH 12.7 12.7 12.7 12.6 n/d 12.6 Viscosity [mPa .Math. s] 85 600 639 875 2388 448 Brightness (R457) [%] 79.3 80.1 79.7 79.5 n/d 82.9 Grinding energy [kWh/t] 241 205 160 133 n/d 122

(6) In Trial 5, the pump at the vertical outlet was not able to pump the slurry due to the high viscosity. However, the sand mill was still able to perform properly at this high solids content and no blocking of the 0.4 mm grid was observed.

Example 3

Carbonation of Fine Grit

(7) Fine grit having a d.sub.50 of 1.9 m, a d.sub.98 of 19 m and a brightness (R457) of 79.5% obtained from grit milling (Example 2, Trial 4) was charged into a glass beaker and adjusted to approx. 15 wt.-% solids content and 100% carbon dioxide was introduced until a pH of below 7 was reached. The obtained product showed a d.sub.98 of 10 m and the brightness (R457) was 82.8% as compared to 79.5% of the starting material.

Example 4

Combined Carbonation of Fine Grit and Grit Reduced Milk of Lime Using Different Amounts of Fine Grit

(8) As a reference sample (Trial 1), 200 kg of calcium oxide were slaked with 1,700 kg of water at 40 C. The obtained milk of lime was screened through a screen having an aperture size of 200 m. The obtained grit reduced milk of lime was heated to 50 C. and 0.04 wt.-% of sucrose, based on the dry weight of the milk of lime, were added. Carbonation was then conducted by injecting 20% carbon dioxide at 200 Nm.sup.3/h and 240 rpm until 5 min after conductivity minimum.

(9) Further carbonation trials were carried out under identical conditions, wherein different amounts of fine grit having a d.sub.50 of 1.9 m, a d.sub.98 of 19 m and a brightness (R457) of 79.5% obtained from grit milling (Example 2, Trial 4) and also different amounts of sucrose, both based on the dry weight of the milk of lime, were added prior to carbonation (Trials 2 to 5). The carbonation product was screened at 45 m aperture size prior to determination of the brightness (R457) and particle size distribution of the screened product as well as the percentage of retained material (see Table 2). The retained oversize material was subjected to XRD analysis using a Bruker AXS D8 Advanced XRD system (5 to 100 2theta Bragg diffraction; Cu K- radiation; automated divergence slits; linear position-sensitive detector; tube current and voltage: 50 mA, 35 kV; step size: 0.02 2theta; counting time: 0.5 s per step; for results see Table 3).

(10) TABLE-US-00002 TABLE 2 Results of combined carbonation. Trial 1 2 3 4 5 Sucrose [wt.-%] 0.04 0.06 0.06 0.06 0.00 Fine grit [wt.-%] 0.00 1.35 4.05 8.10 8.10 Brightness (R457) [%] 94.7 94.4 94.2 93.6 93.4 Residues >45 m [wt.-%] 0.38 0.30 0.30 0.28 0.26 d.sub.50 [m] 2.8 2.2 2.0 1.6 1.7 d.sub.98 [m] 5.3 4.5 4.0 4.0 4.1

(11) TABLE-US-00003 TABLE 3 XRD analysis of 45 m screen residues. Trial [wt.-%] 1 2 3 4 5 Aragonite 1.83 1.95 0.00 0.00 0.88 Calcite 93.00 92.44 94.19 93.25 90.97 Cristobalite 0.21 0.23 0.26 0.41 0.39 Dolomite 0.00 0.50 0.07 0.32 0.07 Gehlenite 0.88 1.53 2.27 1.80 2.55 Larnite 1.68 1.54 1.10 1.62 1.40 Plagioclase 0.12 0.00 0.00 0.00 0.00 Quartz 2.28 0.00 2.19 2.60 3.74 White mica 0.00 0.18 0.00 0.00 0.00

Example 5

Combined Carbonation of Fine Grit and Grit Reduced Milk of Lime with Time Dependent Addition of Fine Grit

(12) As a reference sample (Trial 1), calcium oxide was slaked with 5 parts per weight of water at 230 rpm for 25 min during which the temperature was kept at 40 C. The obtained milk of lime was screened through a screen having an aperture size of 200 m. The obtained grit reduced milk of lime was heated to 50 C. and 0.08 wt.-% of sucrose, based on the dry weight of the milk of lime, were added. Carbonation was then conducted by injecting 20% carbon dioxide at 15 l/min and 750 rpm until 5 min after conductivity minimum.

(13) Further carbonation trials were carried out under identical conditions, wherein fine grit having a d.sub.50 of 1.9 m and a brightness (R457) of 81.0% was added at different times corresponding to a conversion of calcium hydroxide as set out below in Table 4 (Trials 2 to 6).

(14) All carbonation products were screened at 45 m aperture size prior to their characterization.

(15) TABLE-US-00004 TABLE 4 Results of time dependent grit addition (n/a = not applicable). Trial 1 2 3 4 5 6 Sucrose [wt.-%] 0.08 0.06 0.06 0.06 0.06 0.04 Fine grit [wt.-%] 0.00 4.00 4.00 4.00 4.00 8.00 Conversion at n/a 27 41 66 95 68 grit addition [%] Brightness (R457) [%] 94.7 93.7 94.3 95.1 93.0 94.2 d.sub.50 [m] 2.2 2.2 2.5 2.3 2.4 2.3 Specific surface 7.8 6.6 6.9 6.8 6.1 6.7 area [m.sup.2/g]

(16) All grit addition Trials 2 to 6 yielded good brightness values (R457) of 93.0% or higher. In Trial 4, the brightness was even higher than in comparative Trial 1.

Example 6

Combined Carbonation and Grit Addition in Industrial Scale

(17) Calcium oxide (CaO) was slaked in the presence of water at 40 C. and with a total citric acid dosing ratio of 0.107 wt.-%, based on the CaO, to obtain 30 m.sup.3 milk of lime having a density of 1086 kg/m.sup.3 and a solids content of 16.5 wt.-%. The obtained milk of lime was screened through a screen having an aperture size of 200 m.

(18) 27 m.sup.3 of the obtained grit reduced milk of lime was introduced a 45 m.sup.3 stainless steel PCC reactor. The starting temperature of the milk of lime was 44 C. This milk of lime was used for the reference sample (Trial 8) as well as for the inventive sample (Trial 9).

(19) Reference Sample (Trial 8)

(20) Carbonation was then conducted by injecting 20 Vol.-% carbon dioxide at 4770 Nm.sup.3/h and stirring until 5 min after conductivity minimum.

(21) Inventive Sample (Trial 9)

(22) This carbonation trial was carried out under identical conditions, with the following exceptions:

(23) The carbonation was conducted for the first 100 minutes. Then a slurry of fine grit having the following properties: a d.sub.50 of 1.46 m, a d.sub.98 of 11.5 m, a brightness (R457) of 82.9% (Example 2, Trial 7), a grit solids content of 44 wt.-%, based on the dry weight of the grit, a grit slurry sucrose content of 0.14 wt.-%, based on the dry weight of the milk of lime, a grit slurry sucrose content of 0.31 wt.-%, based on the dry weight of the grit, and a grit slurry density of 1378 kg/m.sup.3, was added to the reaction mixture in the PCC reactor. This corresponds to about 60% of the carbonation time.

(24) The carbonation was continued until 5 min after conductivity minimum.

(25) The properties of the products obtained from Trials 8 and 9 can be derived from the following Table 5.

(26) TABLE-US-00005 TABLE 5 The properties of the products obtained from industrial scale trials. Trial 8 9 Solids content [wt.-%] 21.3 21.1 d.sub.50 [m] 2.36 2.47 pH 7.8 7.7 Brightness (R457) [%] 94.9 93.8

Example 7

Use of Precipitated Calcium Carbonate in Handsheets

(27) Calcium oxide was slaked with 5 parts per weight of water at 230 rpm for 25 min during which the temperature was kept at 40 C. One part per weight of the obtained milk of lime was mixed with one part per weight of the PCC obtained according to Trial 6 of Example 5 (8.00 wt.-% grit added at 68% conversion) prior to carbonation which was subsequently conducted by injecting 20% carbon dioxide at 15 l/min and 750 rpm until 5 min after conductivity minimum.

(28) For the handsheet study, eucalyptus pulp (FPI TSI-Lab FP standard fibres) refined to 30 SR was used. In Trial 1 of the present study, an aqueous suspension was prepared by diluting 80 g (dry) pulp and 17 g PCC as described above to a total volume of 10 dm.sup.3. The obtained suspension was stirred for 30 min. Subsequently, 450 ml of the obtained suspension were mixed with 0.06 wt.-% (based on dry weight) of polyacrylamide (Polymin 1530, commercially available from BASF, Ludwigshafen, Germany) as retention aid. Then, handsheets of 75 g/m.sup.2 were formed using a Rapid-Kothen hand sheet former. Each handsheet was pressed for 1 min at 0.42 MPa in a wet press using a chromo board and two conditioned felts (pre-treatment with tap water for 2 to 3 min). The sheets were dried for 6 min at 105 C. The filler content of the handsheets was controlled and adjusted, if necessary.

(29) Further handsheets were produced under identical conditions by use of the PCC of Trials 2 and 4 of Example 5 as well as of commercial PCC (Omya Syncarb 270), see Trials B-D in Table 6 below.

(30) TABLE-US-00006 TABLE 6 Handsheet studies. Trial A B C D Filler type Example 5 Example 5 Example 5 Omya Trial 6 + Trial 2 Trial 4 Syncarb Milk of 270 lime (1:1) Filler content [wt.-%] 19.4 19.4 19.5 19.3 Bulk [cm.sup.3/g] 1.75 1.73 1.74 1.69 Opacity [%] 90.4 90.8 90.7 90.1

(31) The obtained handsheets show an increased bulk density as well as an increased opacity at equal filler contents.

(32) The filler content of handsheets was determined after constant weight was reached upon rapid incineration of a quarter handsheet sample at 570 C. in a muffle furnace. After burning was completed, the residue was transferred in a desiccator and allowed to cool down. After room temperature was reached, the weight of the residue was measured and the mass was correlated with the initial weight of the quarter hand sheet. To determine the grammage, handsheets were kept at 23 C. and 50% relative humidity for 24 hours. Opacity was determined according to DIN 53146.