PROCESS FOR PRODUCING NANO PRECIPITATED CALCIUM CARBONATE

Abstract

The present invention refers to a process for producing nano precipitated calcium carbonate (nano-PCC), as well as to the nano precipitated calcium carbonate (nano-PCC) obtained by such a process and a system for producing the nano precipitated calcium carbonate (nano-PCC) comprising at least one dry mill, preferably a dry sandmill. Furthermore, the present invention refers to the use of the nano precipitated calcium carbonate (nano-PCC) as well as to the use of the inventive system for producing nano precipitated calcium carbonate (nano-PCC).

Claims

1. A process for producing nano precipitated calcium carbonate (nano-PCC) comprising the steps of: a) providing a calcium oxide containing material, b) providing water in an amount of up to 200 wt.-%, based on the total dry weight of the calcium oxide containing material, c) providing a carbon dioxide source, d) preparing nano-PCC by simultaneously or consecutively carrying out the following steps (i) and (ii): (i) simultaneously mixing and milling the calcium oxide containing material of step a) with the water of step b), and (ii) adding under simultaneously mixing and milling the carbon dioxide source of step c) in an amount which corresponds to a mole ratio of the calcium oxide in the calcium oxide containing material of step a) to the carbon dioxide in the carbon dioxide source of step c) of 1:1 to 1:3.5, with the proviso that when steps (i) and (ii) are carried out consecutively, in the first step (i) the calcium oxide containing material of step a) is simultaneously mixed and milled with an amount of the water of step b), which corresponds to a mole ratio of the calcium oxide in the calcium oxide containing material of step a) to water of 1:1 to 1:1.5, and in the second step (ii) the remaining amount of water of step b) is added under simultaneously mixing and milling.

2. The process of claim 1, characterized in that the process further comprises the step of e) separating the nano precipitated calcium carbonate from the mixture obtained from step d).

3. The process of claim 1, characterized in that the process does not involve a drying step and the nano precipitated calcium carbonate obtained in step d) has a residual total moisture content of from 0.1 wt.-% to 10 wt.-%, based on the total dry weight of the nano precipitated calcium carbonate.

4. The process of claim 1, characterized in that the calcium oxide containing material provided in step a) has i) a minimum calcium oxide content of at least 75 wt. % based on the total weight of the calcium oxide containing material and/or ii) has a weight median particle size d.sub.50 of between 1.0 and 300 μm.

5. The process of claim 1, characterized in that the carbon dioxide source of step c) comprises between 4 and 99.8 vol.-% carbon dioxide based on the total volume of the carbon dioxide source.

6. The process of claim 1, characterized in that steps (i) and (ii) are carried out consecutively and in the first step (i) the calcium oxide containing material of step a) is simultaneously mixed and milled with an amount of the water of step b), which corresponds to a mole ratio of the calcium oxide in the calcium oxide containing material of step a) to water of 1:1.01 to 1:1.40.

7. The process of claim 1, characterized in that in step d) the mole ratio of the calcium oxide in the calcium oxide containing material of step a) to the carbon dioxide in the carbon dioxide source of step c) is 1:1 to 1:2.

8. The process of claim 1, characterized in that in step d) the steps i) and ii) are carried out consecutively and the amount of water added in step ii) is from 1 wt.-% to 140 wt.-%, based on the total dry weight the calcium oxide containing material.

9. The process of claim 1, characterized in that in step d) the steps i) and ii) are carried out simultaneously and the amount of water added in step ii) is from 1 wt.-% to 180 wt.-%, based on the total dry weight the calcium oxide containing material.

10. The process of claim 1, characterized in that the nano precipitated calcium carbonate obtained in step d) i) has a specific surface area from 5.0 to 80.0 m.sup.2/g measured using nitrogen and the BET method according to ISO 9277:2010 and/or ii) is in form of particles having a number-based median particle size d.sub.50 of below 900 nm and/or iii) is in form of particles having a number-based top cut particle size d.sub.98 of below 1000 nm and/or iv) has a residual total moisture content of from 0.1 wt.-% to 10 wt.-%, based on the total dry weight of the nano precipitated calcium carbonate.

11. The process of claim 1, characterized in that the process is performed in at least one dry mill.

12. Nano precipitated calcium carbonate, obtained by the process according to claim 1.

13. System for producing a nano precipitated calcium carbonate comprising at least one dry mill, the system comprising: A) at least one inlet suitable for feeding calcium oxide containing material into at least one dry mill, wherein said inlet is arranged such that it does not come into direct contact with the milling beads within the at least one dry mill during milling; B) at least one inlet suitable for feeding water into at least one dry mill; C) at least one inlet suitable for feeding a carbon dioxide source into at least one dry mill, wherein said inlet is arranged such that it is located below the liquid inlet level in the at least one dry mill; and D) at least one outlet for removing the nano precipitated calcium carbonate from at least one dry mill, wherein said outlet comprises a sieve and is arranged such that it is in direct contact with the milling beads within the at least one dry mill during milling and is at least partially located below the liquid inlet level in the at least one dry mill.

14. System according to claim 13, wherein the system comprises one dry mill, which has inlets A), B) and C) and outlet D).

15. System according to claim 13, wherein the system comprises at least two serially arranged dry mills, wherein the first dry mill at least has inlet A) and wherein the second dry mill at least has inlets B) and C) and outlet D) and wherein the at least two serially arranged dry mills are connected to each other for transporting the product of the first dry mill to the second dry mill.

16. System according to claim 15, wherein the first dry mill also has inlet B).

17. Paper, paper products, paper coatings, ink, paint, coating, plastics, polymer compositions, adhesives, building products, sealants, foodstuff, agricultural products, cosmetic products or pharmaceutical products including the nano precipitated calcium carbonate of claim 12.

18. (canceled)

19. The process of claim 1, characterized in that the process does not involve a drying step and the nano precipitated calcium carbonate obtained in step d) has a residual total moisture content of from 0.2 wt.-% to 3 wt.-%, based on the total dry weight of the nano precipitated calcium carbonate.

20. The process of claim 1, characterized in that the calcium oxide containing material provided in step a) has i) a minimum calcium oxide content of at least 95 wt.-% based on the total weight of the calcium oxide containing material and/or ii) has a weight median particle size d.sub.50 of between 6 and 80 μm.

21. The process of claim 1, characterized in that the carbon dioxide source of step c) comprises between 8 and 25 vol.-% carbon dioxide based on the total volume of the carbon dioxide source; and that in step d) the mole ratio of the calcium oxide in the calcium oxide containing material of step a) to the carbon dioxide in the carbon dioxide source of step c) is 1:1.4 to 1:1.6; and that steps (i) and (ii) are carried out consecutively and in the first step (i) the calcium oxide containing material of step a) is simultaneously mixed and milled with an amount of the water of step b), which corresponds to a mole ratio of the calcium oxide in the calcium oxide containing material of step a) to water of 1:1.03 to 1:1.08; or that steps i) and ii) are carried out simultaneously and the amount of water added in step ii) is from 70 wt.-% to 90 wt.-%, based on the total dry weight the calcium oxide containing material.

Description

FIGURES

[0259] FIG. 1 is a SEM photograph of trial 13, showing nano precipitated calcium carbonate

[0260] FIG. 2 is a SEM photograph of trial 15, showing nano precipitated calcium carbonate

[0261] FIG. 3 is a SEM photograph of trial 19, showing nano precipitated calcium carbonate

[0262] FIG. 4 is a photograph of the inventive system for producing nano-PCC comprising one dry sandmill and inlets A), B) C) and outlet D)

[0263] FIG. 5 is a schematic figure of a the inventive system comprising a dry sandmill and inlets A), B) C) and outlet D)

[0264] FIG. 6 is a schematic figure of a the inventive system comprising a dry sandmill and inlets A), B) C) and outlet D) wherein milling beads are present in the dry sandmill as well as educts up to the liquid inlet level

[0265] FIG. 7 is a schematic figure of a the inventive system comprising two dry sandmills and inlets A), B) C) and outlet D) wherein the two dry sandmills are connected via a transporting pipeline

[0266] FIG. 8 is a schematic figure of a the inventive system comprising two dry sandmills and inlets A), B) C) and outlet D) wherein the two dry sandmills are connected via a transporting pipeline

EXAMPLES

1. Measurement Methods

[0267] The following measurement methods are used to evaluate the parameters given in the examples and claims.

BET Specific Surface Area of a Material

[0268] Throughout the present document, the specific surface area (in m.sup.2/g) of the mineral filler is determined using the BET method (using nitrogen as adsorbing gas), which is well known to the skilled man (ISO 9277:2010). The total surface area (in m.sup.2) of the mineral filler is then obtained by multiplication of the specific surface area and the mass (in g) of the mineral filler prior to treatment.

Total Moisture Content Measurement

[0269] The total moisture content of the nano-PCC is measured according to the Karl Fischer coulometric titration method, desorbing the moisture in an oven at 220° C. and passing it continuously into the KF coulometer (Mettler Toledo coulometric KF Titrator C30, combined with Mettler oven DO 0337) using dry N.sub.2 at 100 ml/min for 10 min. A calibration curve using water has to be made and a blind of 10 min gas flow without a sample has to be taken in account.

Particle Size Distribution of Particulate Material Other than Precipitated Calcium Carbonate (PCC)

[0270] The particle size distribution of the particles other than PCC was measured using a Sedigraph™ 5120. The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement was carried out in an aqueous solution comprising 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and supersonics. For the measurement of dispersed samples, no further dispersing agents were added.

Particle Size Distribution of Precipitated Calcium Carbonate (PCC)

[0271] The particle size distribution of the prepared PCC particles was measured using a Malvern Zetasizer Nano ZS. 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 using dynamic light scattering and the Stokes-Einstein relation. The samples were dispersed using a high speed stirrer and supersonics. For the measurement of dispersed samples, no further dispersing agents were added.

Brightness Measurement

[0272] The pigment brightness of the obtained particles were measured using an ELREPHO 450x from the company Datacolor according to ISO 2469 and DIN 6167, respectively.

[0273] The samples were dried in an oven at 105° C. to a residual moisture content of <0.5% by weight and the resulting powder was treated to deagglomerate the powder particles. From 12 g of said powder a tablet was pressed via application of 4 bar pressure for 15 s. The resulting powder tablet with a diameter of 45 mm was then subjected to the measurement.

Detection of the Ca(OH).SUB.2 .Content

[0274] The Ca(OH).sub.2 content is measured by titration with a standard aqueous hydrochloric acid solution with a concentration of about 0.5 M. (The HCl solution was obtained by dissolving an amount of HCl 37% Analar Normapur obtained from VWR in distilled water). Titration was made by adding 2 grams of the sample, 5 grams of sucrose sugar obtained from RAR and 100 mL of water to a beaker. After mixing for 15 minutes, the sample was titrated with the standard hydrochloric acid solution until a pH of 7.5 was reached. Ca(OH).sub.2 was then calculated from the volume of the acid used.

2. Examples

Used Materials and Equipment

Preparation of the Used Calcium Oxide Containing Material

[0275] The calcium oxide containing material is lime obtainable from Lusical under the trade name Cal viva 3-60. The calcium oxide containing material was ground in a Forplex FO pin mill from Poittemill Forplex with a 0.4 mm outlet screen. In some cases, a second grinding of the lime was performed in a ZM200 laboratory mill from Retsch. The obtained product had a d.sub.50 of 48 μm and a d.sub.98 of 552 μm after the first grinding step and a d.sub.50 of 8 μm and a d.sub.98 of 276 μm after the second grinding step and was used in the following experiments.

[0276] The lime with only one grinding step was used in trials 1-7 and the lime with two grinding steps was used in trials 8-10.

[0277] The water that has been used in the following examples was distilled water.

[0278] The carbon dioxide source that has been used in the following examples was Air Liquide industrial CO.sub.2, obtainable from Air Liquide.

[0279] The dry mill that has been used in the following examples is a dry sandmill in the form of a cylinder, comprising an agitator which is located in the median axis of the cylinder. The inside volume of the dry sandmill had 2356 cm.sup.3. The calculated value of the length to diameter ratio of the cylinder was 3 and the calculated value of the diameter ratio of the cylinder to the agitator was 0.4 The agitator comprised 10 arms and the agitator speed was 260 rpm. Inlet A) was in form of a tube and the calcium oxide containing material has been added manually. Inlet B) was in form of a tube and the water has been inserted by a Selecta Percom N-M II peristaltic pump. Inlet C) was in form of a tube and the carbon dioxide source has been inserted by a flowmeter. The outlet D) is represented by the downside diameter of the cylinder and comprises a wedge wire screen of 0.8 μm. The milling balls in the dry sandmill were Bitossi Alubit Leonardo 1.5-2.5 mm alumina beads, obtained from Industrie Bitossi.

Two-Step Process

Process Step i)

[0280] Before process step d) i) has been prepared, the dry sandmill has been started running at 260 rpm until it reached a temperature of about 50° C. The prepared calcium carbonate containing material and water feed were then started at the specified flowrates given in the table below. Calcium carbonate containing material was fed through inlet A) at the top of the mill, while water was fed through inlet B) at a lateral hole in the mill. The obtained calcium hydroxide leaves the mill by the bottom screen (outlet D)) and was collected.

TABLE-US-00001 TABLE 1 Characteristics of step i) if the inventive process Trial CaO Water Product # g/min g/min ° C. % Ca(OH).sub.2 % water d.sub.50 (μm) d.sub.98 (μm) BET (m.sup.2/g) 1-10 9.4 3.18 150 84-99 0.1-0.3 5-16 100-800 19

Process Step ii)

[0281] Before process step d) ii) has been prepared, the dry sandmill has been started running at 260 rpm until it reached a temperature of about 50° C. The collected calcium hydroxide was inserted in the second dry sandmill via a transporting pipeline and fed through the top. The water was fed through inlet B) at a lateral hole in the mill and the carbon dioxide source was fed through inlet C) at a lateral hole in the mill. The obtained precipitated calcium carbonate leaves the mill by the bottom screen (outlet D)) and was collected.

TABLE-US-00002 TABLE 2 Characteristics of step ii) if the inventive process Trial Ca(OH).sub.2 CO.sub.2 water # g/min l/min % excess g/min % of feed 11 9.43 5 50 3.18 34 12 9.43 5 50 6.36# 67 13 9.43 5 50 6.36 67 14 9.43 5 50 3.83 41 15 9.43 5 50 3.83* 41 16 9.43 5 50 4.55* 48 *The calcium hydroxide has been pre-moisturized with 40 wt.-% water before adding CO.sub.2. #In example 12, 7.1 mg/min of sugar was added dissolved in the water. This amount relates to an equivalent of 0.1 wt.-% of the Ca(OH).sub.2, expressed as CaO.

TABLE-US-00003 TABLE 3 Characteristics of the produced nano-PCC Trial Ca(OH).sub.2 Humidity BET Brightness # g/min % m.sup.2/g R457 L b y 11 24 3.0 12 9.4 2.0 13 3.1 14.2 90.2 96.7 1.00 2.03 14 24.5 0.9 15 2.0 9.5 92.7 97.4 0.67 1.30 16 6.6 2.9

[0282] The inventors showed that by the inventive process it is possible to control the particle size of the precipitated calcium carbonate and provide nano precipitated calcium carbonate (nano-PCC) which has defined particle sizes in the nanometer range. This can, for example be seen from the SEM photographs of trials 13 and 15 (see FIGS. 1 and 2). Furthermore from the above experimental data it can be seen that the obtained nano precipitated calcium carbonate has a relative low residual moisture content. Additionally, it can be seen that only a low amount of water is used in the inventive process.

One Step Process

[0283] The prepared calcium carbonate containing material, water and carbon dioxide source were feed to the dry sandmill at the specified flowrates given in the table below. Calcium carbonate containing material was fed through inlet A) at the top of the mill, while water was fed through inlet B) at a lateral hole in the mill and calcium dioxide source was fed through inlet C) at a lateral hole in the mill. The obtained precipitated calcium carbonate leaves the mill by the bottom screen (outlet D)) and was collected.

TABLE-US-00004 TABLE 4 Characteristics of the inventive process Trial CaO CO.sub.2 water # g/min l/min % excess g/min % of feed 17 9.43 6 146 3.18 34 18 9.43 6 146 6.36 67 19 4.40 5 260 5.64 128 20 4.04 5 284 2.75 68 21 3.30 5 347 2.75 83

TABLE-US-00005 TABLE 5 Characteristics of produced nano-PCC Trial Ca(OH).sub.2 Humidity BET Brightness # % % m.sup.2/g R457 L b y 17 53.7 0.4 18 42.4 0.6 13.1 19 16.6 2.7 93.14 97.6 0.6 1.17 20 24.4 140.4 21 19.8 0.3

[0284] The inventors showed that by the inventive process it is possible to control the particle size of the precipitated calcium carbonate and provide nano precipitated calcium carbonate (nano-PCC) which has defined particle sizes in the nanometer range. This can, for example be seen from the SEM photographs of trial 19 (see FIG. 3). Furthermore from the above experimental data it can be seen that the obtained nano precipitated calcium carbonate has a relative low residual moisture content. Additionally, it can be seen that only a low amount of water is used in the inventive process.

Comparative Example

[0285] A milk of lime was prepared by mixing under mechanical stirring water with dry sodium citrate (commercially available from Sigma-Aldrich) as slaking additive at an initial temperature between 40 and 41° C. (the amounts of slaking additive are indicated below). Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 μm screen.

[0286] 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.

[0287] The characteristics of the prepared milks of lime and aqueous PCC suspensions are described in the tables below.

TABLE-US-00006 TABLE 6 Characteristics of produced milks of lime Sodium citrate amount Solids content Brookfield viscosity Sample [wt.-%/wt. CaO] [wt.-%] [mPa .Math. s] 22 0.15 13.6 32

TABLE-US-00007 TABLE 7 Characteristics of the obtained aqueous PCC suspensions Solids Brookfield Carbonation time content d.sub.50 BET viscosity Sample [min/kg Ca(OH).sub.2] [wt.-%] [μm] [m.sup.2/g] pH [mPa .Math. s] 22 50 18.5 1.82 4.7 7.6 34

[0288] As can be seen from Example 22, slaking additives like sodium acids are necessary when preparing the milk of lime. Furthermore, as can be seen from Example 22 the obtained PCC has no nanometer size range since it has a d.sub.50 of 1.82 μm. Additionally, only low BET values, for example 4.7 m.sup.2/g which is below 5 m.sup.2/g can be obtained. Another drawback is that high amounts of water have to be used to prepare the milk of lime and, therefore, the obtained PCC is very wet and has to be dried for further processing.