Production of high purity precipitated calcium carbonate

Abstract

The present invention relates to a process for the preparation of precipitated calcium carbonate comprising the steps of a) providing and calcining calcium carbonate comprising material; b) slaking the reaction product obtained from step a) with an aqueous ammonium chloride solution; c) separating insoluble components from the calcium chloride solution obtained from step b); d) carbonating the calcium chloride solution obtained from step c); e) separating the precipitated calcium carbonate obtained from step d); the precipitated calcium carbonate obtained by this process, as well as uses thereof.

Claims

1. A product obtained by the process comprising the steps of: a) providing and calcining a calcium carbonate comprising material to obtain a reaction product comprising calcium oxide; b) slaking the reaction product obtained from step a) with an aqueous ammonium chloride solution; c) separating insoluble components from the calcium chloride solution obtained from step b); d) carbonating the calcium chloride solution obtained from step c) to form precipitated calcium carbonate; e) separating the precipitated calcium carbonate obtained from step d), and f) washing the precipitated calcium carbonate from step e) to obtain a product, wherein aragonitic seed crystals are added to the calcium chloride solution obtained from step c) before precipitation, and wherein the product obtained in step f) contains>95% aragonitic calcium carbonate, contains less than 0.5 wt % impurities, and has a TAPPI brightness of 95 to 99, a luminous reflectance factor R.sub.y of 96 to 99, a yellow index of from 0.5 to 3, and a BET specific surface area of 3.5 to 12.5 m.sup.2/g.

2. The product according to claim 1, wherein the calcium carbonate comprising material of step a) is selected from the group consisting of precipitated calcium carbonate, a natural calcium carbonate mineral, marble, limestone, chalk, a mixed alkaline earth carbonate mineral comprising calcium carbonate, and dolomite.

3. The product according to claim 1, wherein the calcium carbonate comprising material of step a) is natural calcium carbonate mineral obtained from one or more of marble, limestone, chalk or mixture thereof.

4. The product according to claim 1, wherein the calcium carbonate comprising material has a minimum calcium carbonate content of at least 50 wt %.

5. The product according to claim 1, wherein the calcium carbonate comprising material has a minimum calcium carbonate content of at least 75 wt %.

6. The product according to claim 1, wherein the calcium carbonate comprising material has a minimum calcium carbonate content of at least 90 wt %.

7. The product according to claim 1, wherein the calcium carbonate comprising material has a minimum calcium carbonate content of at least 98 wt %.

8. The product according to claim 1, wherein the molar ratio of ammonium chloride to calcium oxide in step b) is from 1:1 to 8:1.

9. The product according to claim 1, wherein the molar ratio of ammonium chloride to calcium oxide in step b) is from 1.5:1 to 4:1.

10. The product according to claim 1, wherein the molar ratio of ammonium chloride to calcium oxide in step b) is from 2:1 to 3:1.

11. The product according to claim 1, wherein in step c), separation of insoluble components is carried out by screening, sedimentation and decanting, and/or filtration.

12. The product according to claim 1, wherein the insoluble components may be flocculated before their removal by addition of flocculants selected from the group consisting of high molecular flocculants, cationic flocculants, anionic flocculants, non-ionic flocculants, and copolymers based on polyacrylic acid.

13. The product according to claim 12, wherein in step c), the flocculant is added in an amount of from 1 to 50 ppm based on dry CaO.

14. The product according to claim 12, wherein in step c), the flocculant is added in an amount of from 5 to 15 ppm based on dry CaO.

15. The product according to claim 1, wherein in step d), the carbonation is carried out by feeding pure gaseous carbon dioxide or technical gases containing at least 10 vol.-% of carbon dioxide into the alkaline calcium chloride solution.

16. The product according to claim 1, wherein in step e), precipitated calcium carbonate obtained from step d) is separated from a mother liquor by filtration.

17. The product according to claim 1, wherein in step f), the precipitated calcium carbonate is washed with water.

18. The product according to claim 1, wherein after step f), the product is upconcentrated or dried.

19. The product according to claim 1, containing less than 0.3 wt % impurities.

20. The product according to claim 1, containing less than 0.1 wt % impurities.

21. The product according to claim 1, containing less than 0.05 wt % impurities.

22. The product according to claim 1, having a yellow index of from 0.5 to 2.

23. The product according to claim 1, having a yellow index of from 0.7 to 3.

24. The product according to claim 1, having a yellow index of from 1 to 2.

25. The product according to claim 1, having a weight median diameter d.sub.50 of not more than 5 μm.

26. The product according to claim 1, having a weight median diameter d.sub.50 of not more than 2 μm.

27. The product according to claim 1, having a weight median diameter d.sub.50 of not more than 1 μm.

28. Paint, plastic or paper comprising the product according to claim 1.

29. The paint, plastic or paper according to claim 28, wherein the product is used as a filler and/or pigment.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a principal flow sheet of a setup for the process of the present invention.

EXAMPLES

(2) In the following examples the process according to the present invention is exemplified, and product properties of the precipitated calcium carbonates obtained by this process are described:

(3) 1. Basic Setup for Carrying Out the Present Invention

(4) In FIG. 1, a schematic illustration of an advantageous embodiment of how the process of the present invention may be conducted in principle is given by a flow sheet, also comprising optional steps such as recycling ammonia into the process including scrubbing and washing in order to purify exhaust gases and the precipitated calcium carbonate product.

(5) For the following examples low grade qualities of chalk and limestone from different quarries were chosen as feed material, which, usually, are not suitable for the preparation of common PCC qualities. Among these materials, a waste material from a screening step of a marble feed stone (so called “Riesel”) was selected.

(6) While chalk and limestone feed materials was crushed to obtain a suitable feed size of 1 to 8 mm for the process of the present invention, the Riesel material was screened at 1 mm to remove clay minerals.

(7) 2. Preparation and Characterization of Products Obtained by the Process of the Present Invention.

(8) For determining the purity of the product obtained by the process and thus of the efficiency of the process of the present invention, the precipitated calcium carbonate was prepared as follows and its chemical composition, as well as the ones of the feed material and the reject was determined and summarized in the following table.

(9) 2.1. Preparation of High Purity PCC from Different Materials and Characterization with Respect to Brightness and Yellow Index

(10) 2.1.1. Preparation

(11) For the following lab trials, several different feed materials were used in the process of the present invention and subsequently analysed with respect to their optical properties:

(12) Feed Material

(13) Sample No. 1: Riesel 1-4 mm Gummern (Austria)

(14) Sample No. 2: Chalk Harmignies (Belgium)

(15) Sample No. 3: Chalk ordinaire Omey (France)

(16) Sample No. 4: Chalk Mjelnik (Poland)

(17) Sample No. 5: Riesel 1-4 mm Gummern (Austria)

(18) Sample No. 6: Fe-rich marble Gummern (Austria)

(19) Sample No. 7: Limestone Burgberg (Germany)

(20) Sample No. 8: Limestone Vollmer (Germany) a) 5000 g of the respective feed material was calcined in a lab furnace at a temperature of 1000° C. for 2 hours. b) 400 g of the burnt limestone obtained from the calcining step was fed into an 8 l reactor, which was previously fed with 800 g ammonium chloride dissolved in 4 l water. The reaction mixture was slaked under stirring at room temperature for 30 minutes, while the temperature of the solution decreased. c) Subsequently, insoluble components were separated from the resulting calcium chloride solution by screening at 100 μm. The separated reject was analysed, as mentioned below. d) The calcium chloride solution obtained was fed into a precipitation reactor, into which pure gaseous carbon dioxide was fed from a storage tank under stirring at a starting temperature of 40° C. When the reaction was completed, which was determined by pH control, the precipitated calcium carbonate was separated by filtration, the filtrate was washed with water, again filtered, and finally dried in a drying cabinet at 105° C.
2.1.2. Characterization

(21) Sample No. 1 was compared with precipitated calcium carbonate produced by a process of the prior art, meaning the slaking of the burnt lime was done in water instead of an ammonium chloride solution resulting in the formation of calcium hydroxide.

(22) The results are summarized in following table 1, wherein the brightness of the feed material was measured for a particle size of d.sub.50=5 μm, while the final products are measured as received.

(23) TABLE-US-00001 TABLE 1 Tappi Sample Brightness Rx Ry Rz Index Sample 1 feed material 65.8 80.1 72.2 65.4 19.1 PCC (prior art) 91.7 95.6 94.8 91.6  4.3 Sample 1 final product 96-98 96-98.8 96-98.7 96-98.1 0.6-1.5 (invention)

(24) Looking at these results, it is evident that the precipitated calcium carbonate produced according to the process of the present invention from a low quality natural calcium carbonate material has excellent brightness, which is even better than the precipitated calcium carbonate produced according to a process of the prior art.

(25) These results were confirmed by further tests with samples 2 to 8, which were processed as described above and compared with the obtained precipitated calcium carbonate as summarized in the following table 2.

(26) TABLE-US-00002 TABLE 2 Tappi Sample Brightness Rx Ry Rz Index Sample No. 2 (feed) 81.9 89.3 87.8 81.7 8.8 Sample No. 2 (final product), 97.5 98.6 98.5 97.5 1.2 d.sub.50 = 4.8 μm Sample No. 3 (feed) 80.4 86.6 85.2 80.1 7.6 Sample No. 3 (final product), 96.7 98 97.8 96.6 1.4 d.sub.50 = 2.2 μm Sample No. 4 (feed) 70.7 79.4 77.6 70.4 11.7 Sample No. 4 (final product), 97.8 96.5 98.4 97.7 0.9 d.sub.50 = 4.8 μm Sample No. 5 (feed) 86.9 91.6 90.7 86.8 5.3 Sample No. 5 (final product), 96.4 98.2 97.9 96.3 2 d.sub.50 = 6 μm Sample No. 6 (feed) 85.9 93 91.5 85.7 8 Sample No. 6 (final product), 95.7 97.7 97.3 95.6 2.2 d.sub.50 = 14 μm Sample No. 7 (feed) 75.2 86.1 83.8 74.9 13.4 Sample No. 7 (final product), 94.5 95.5 95.4 94.4 1.2 d.sub.50 = 12.4 μm Sample No. 8 (feed) 79.9 88.7 87 79.6 10.4 Sample No. 8 (final product), 97.8 98.5 98.4 97.7 0.8 d.sub.50 = 2 μm
2.1.3. Chemical Analysis

(27) The elemental analysis of the dried product, as well as of the feed material and the reject of sample 3 was analysed by means of X-ray fluorescence (XRF ARL-9400, from Thermo-ARL). For the determination of loss on ignition and the specific surface area (measured using nitrogen and the BET method according to ISO 9277) standard lab methods were used (cf. table 3).

(28) TABLE-US-00003 TABLE 3 Feed material Product Reject [wt %] [wt %] [wt %] SiO.sub.2 1.05 <0.1 14.14 Al.sub.2O.sub.3 0.47 <0.1 5.94 Fe.sub.2O.sub.3 0.13 <0.04 1.76 MgO 0.31 <0.1 4.34 CaO 96.42 55.45 37.87 Na.sub.2O <0.1 <0.1 <0.1 K.sub.2O 0.05 <0.01 0.04 TiO.sub.2 0.02 <0.01 0.21 P.sub.2O.sub.5 0.15 <0.01 2.00 Loss on ignition 1.29 44.63 24.56 ppm: Y 16 7 140 Ba 24 23 64 Sr 1096 112 5615 Pb 7 5 23 Ni 22 19 53 Cr 20 8 146 Mn 92 <10 3090

(29) From the above table it can be seen that the product obtained from the process according to the invention has a high chemical purity with respect to its calcium carbonate content (CaO+loss on ignition) of 99 to 100 wt %.

(30) Particularly, comparing the SiO.sub.2, Al.sub.2O.sub.3, Fe.sub.2O.sub.3, MgO, P.sub.2O.sub.5 contents of the raw material, the product and the reject, it can be found that the separation of the corresponding impurities can be achieved almost completely.

(31) Also the amount of heavy metals like Y, Sr, Mn, and Cr can be significantly reduced in the product.

(32) 2.2. Preparation and Characterization of High Purity PCC Using Seed Crystals

(33) 2.2.1. Preparation

(34) In 7 technically scaled trials the method of the present invention was evaluated with respect to different feed materials and process parameters as mentioned in the below table.

(35) Feed Material

(36) Samples No. 9-13: Washing-Riesel 1-4 mm (marble) Gummern (Austria)

(37) Samples No. 14-15: Chalk ordinaire Omey (France) a) 2 Tons of the respective samples were calcined in a rotary kiln at a temperature of 1000° C. for 2 hours, to obtain comparable feed material for the slaking process. b) 180 kg of the burnt lime obtained from the calcining step was fed into a slaking reactor (volume: 2.3 m.sup.3; diameter: 1.2 m; height: 2 m), which was previously fed with 360 kg ammonium chloride and 1800 kg of water. The reaction mixture was slaked under stirring at a temperature of 40° C. for half an hour. The reaction was completed, when the burnt lime was dissolved to a clear solution. c) Subsequently, 15 ppm (based on CaO) of an anionic flocculant (Superfloc A-130 from Kemira) was added to the resulting reaction mixture in order to improve the separation of the insoluble components from the resulting solution. The separation was finally carried out by sedimentation and decanting the clear solution. d) 600 l of resulting calcium chloride solution was fed into an Ultramill (volume: 700 l), and a defined amount of aragonitic seed crystals, as given in the table below was added thereto.

(38) Subsequently, the calcium chloride solution was heated to the corresponding starting temperature, and a technical gas containing 20 vol.% of carbon dioxide, was fed from a storage tank under defined stirring power as mentioned in the table below and a flow rate of 100 m.sup.3/h. When the reaction was completed, which was determined by a final pH of 7, the precipitated calcium carbonate slurry was separated by filtration on a vacuum drum filter from Metso, the filter cake was washed with water and finally upconcentrated by a centrifuge to a final slurry.

(39) The solids content in the precipitated calcium carbonate slurry is related to the recovery, whereas the solids content in the filter cake indicates the fineness of the final product.

(40) The above procedure yielded PCCs having a BET surface of between about 4 and about 12 and having excellent optical properties as mentioned in the below table. R457 Tappi brightness and yellow index were determined by Datacolor measurements as mentioned above.

(41) A comparison indicates that best results regarding fineness and brightness could be achieved with 2 wt % of aragonitic seeds, low precipitation starting temperature and high stirrer power.

(42) It can be concluded, that too low concentrations of seed crystals enlarge the resulting size of precipitated crystals, while too high concentrations reduce viscosity and disturb gas transport in the reactor. Also higher stirring power and lower starting temperatures promote the formation of more and finer particles.

(43) TABLE-US-00004 TABLE 4 Trial No. 9 10 11 12 13 14 15 Feed Riesel Riesel Riesel Riesel Riesel Chalk Chalk material Seed 0 1 2 2 5 0 5 crystals [wt % (based on CaO)] Starting 55 55 40 53 55 55 55 temper- ature [° C.] Stirrer [% 75 75 75 50 70 75 70 Power] Solids 7.1 8.1 8.9 6.0 12.4 9.3 9.2 content CaCO.sub.3 slurry (before filtration) [wt %] Solids 60.6 57.0 53.0 52.4 47.0 72.0 50.2 content CaCO.sub.3 filter cake [wt %] BET- 4.2 4.8 11.8 6.4 6.5 3.5 12.5 surface [m.sup.2/g] Datacolor 92.3/ 95.5/ 96.4/ 93.3/ 95.9/ 92.9/ 81.2/ R457/ 3.6 1.7 1.1 3.0 1.2 3.1 9.5 Index

(44) The brightness (R457) of sample 15 was rather pure. This, however was due to an incomplete flocculation of the impurities in this sample. The brightness of this sample could be significantly improved by the addition of some more flocculant. In this case, however, sample 15 was used for the following experiments, in order to verify the efficiency of a subsequent microfiltration, which provided very good results, as well.

(45) 2.3. Preparation of High Purity PCC from Different Materials and Characterization with Respect to Improved Filtration

(46) In order to evaluate the influence of an improved filtration step after slaking the burnt lime, samples from above mentioned trials 13 and 15 were filtered again by use of a microfiltration unit with 0.2 μm PP membranes from Microdyn-Nadir.

(47) Subsequently, the filtrate was precipitated in a lab unit as mentioned above with respect to example 1.

(48) By separation of residual organic molecules and colloidal iron hydroxide impurities a further brightness increase and index reduction could be verified (cf. table 5).

(49) By applying the micro filtration step also high pure aragonitic products could be gained. The purity was controlled by X-ray diffraction using a D8 XRD from Bruker in combination with Rietveld software Topas (cf. table 5).

(50) TABLE-US-00005 TABLE 5 Brightness after microfiltration XRD Tappi Aragonit R457 Ry Index [%] Sample No. 13 97.6 97.9 0.5 99.4 Sample No. 15 97.5 97.9 0.6 99.5