PCC with reduced portlandite content

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, and a precipitation enhancer, 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 having a low portlandite content, obtained by a process comprising the steps of: i) providing a calcium oxide containing material having a calcium oxide content of at least 90 wt.-%, ii) providing a water-soluble calcium salt as a precipitation enhancer wherein the water-soluble calcium salt is an anhydrous salt or hydrate salt, iii) preparing a milk of lime by mixing, in any order, water, the calcium oxide containing material of step i), and the precipitation enhancer of step ii), wherein the precipitation enhancer is included in an amount from 0.01 to 25 wt.-%, based on the total weight of the calcium oxide containing material, and iv) carbonating the milk of lime obtained from step iii) until an aqueous suspension of precipitated calcium carbonate having a portlandite content of less than 1 wt.-%, based on the total dry weight of the precipitated calcium carbonate, is obtained, wherein the obtained aqueous suspension of precipitated calcium carbonate has a solids content of from 10 to 50 wt.-%, based on the total weight of the suspension.

2. The aqueous suspension of claim 1, wherein step iii) comprises the steps of: a1) mixing the calcium oxide containing material of step i) with water, and a2) adding the precipitation enhancer of step ii) to the mixture of step a1); or b1) mixing the precipitation enhancer of step ii) with water, and b2) adding the calcium oxide containing material of step i) to the mixture of step b1).

3. The aqueous suspension of claim 1, wherein the obtained precipitated calcium carbonate has a portlandite content of less than 0.1 wt.-%, based on the total dry weight of the precipitated calcium carbonate.

4. The aqueous suspension of claim 1, wherein the milk of lime is screened after step iii) and before step iv), with a screen having a sieve size from 100 to 300 p.m.

5. The aqueous suspension of claim 1, wherein the obtained suspension of precipitated calcium carbonate has a solids content of at least 5 wt.-%, based on the total weight of the suspension.

6. The aqueous suspension of claim 1, wherein the water-soluble calcium salt is selected from the group consisting of calcium nitrate, calcium sulfate, calcium acetate, calcium benzoate, calcium bicarbonate, calcium bromate, calcium bromide, calcium chlorate, calcium chloride, calcium iodite, calcium nitrite, calcium perchlorate, calcium permanganate, hydrates thereof, and any mixture thereof.

7. The aqueous suspension of claim 1, wherein the water-soluble calcium salt is selected from the group consisting of calcium nitrate, calcium sulfate, calcium acetate, calcium benzoate, calcium bicarbonate, calcium bromate, calcium bromide, calcium chlorate, calcium chloride, calcium iodite, calcium nitrite, calcium perchlorate, calcium permanganate, calcium nitrate tetrahydrate, calcium chloride dihydrate, and any mixture thereof.

8. The aqueous suspension of claim 1, wherein the water-soluble calcium salt is selected from the group consisting of calcium nitrate, calcium nitrate tetra hydrate, calcium chloride, calcium chloride dihydrate, and any mixture thereof.

9. The aqueous suspension of claim 1, wherein the precipitation enhancer of step ii) is added in an amount from 0.1 to 20 wt.-%, based on the total weight of the calcium oxide containing material.

10. The aqueous suspension of claim 1, wherein the precipitation enhancer of step ii) is added in an amount from 1 to 15 wt.-%, based on the total weight of the calcium oxide containing material.

11. The aqueous suspension of claim 1, wherein the precipitation enhancer of step ii) is added in an amount from 5 to 10 wt.-%, based on the total weight of the calcium oxide containing material.

12. The aqueous suspension of claim 1, wherein a slaking additive is added before, during or after step iii).

13. The aqueous suspension of claim 12, wherein the slaking additive is added in an amount from 0.01 to 2 wt.-%, based on the total amount of calcium oxide containing material.

14. An aqueous suspension of precipitated calcium carbonate having a low portlandite content, obtained by a process comprising the steps of: i) providing a calcium oxide containing material having a calcium oxide content of at least 90 wt.-%, ii) providing a precipitation enhancer comprising calcium carbonate nanoparticles, the calcium carbonate nanoparticles having a number based median particle size d.sub.50 of from 30 to 70 nm, and a portlandite content of less than 1 wt.-%, based on the total dry weight of the calcium carbonate nanoparticles, iii) preparing a milk of lime by mixing, in any order, water, the calcium oxide containing material of step i), and the precipitation enhancer of step ii), wherein the precipitation enhancer is included in an amount from 0.01 to 25 wt.-%, based on the total weight of the calcium oxide containing material, and iv) carbonating the milk of lime obtained from step iii) until an aqueous suspension of precipitated calcium carbonate having a clustered scalenohedral crystal structure and a portlandite content of less than 1 wt.-%, based on the total dry weight of the precipitated calcium carbonate, and a solids content of from 10 to 50 wt.-%, based on the total weight of the suspension, is obtained.

15. The aqueous suspension of claim 14, wherein the precipitation enhancer further comprises a water-soluble calcium salt, and wherein the calcium carbonate nanoparticles and the water-soluble calcium salt are in a mass ratio of from 1:10 to 10:1.

16. The aqueous suspension of claim 15, wherein the water-soluble calcium salt is an anhydrous salt or hydrate salt.

17. An aqueous suspension of precipitated calcium carbonate having a low portlandite content, obtained by a process comprising the steps of: i) a providing a calcium oxide containing material having a calcium oxide content of at least 90 wt.-%, ii) providing a water-soluble calcium salt as a precipitation enhancer wherein the water-soluble calcium salt is an anhydrous salt or hydrate salt, iii) preparing a milk of lime by mixing, in any order, water, the calcium oxide containing material of step i), and the precipitation enhancer of step ii), wherein the precipitation enhancer is included in an amount from 0.01 to 25 wt.-%, based on the total weight of the calcium oxide containing material, and iv) carbonating the milk of lime obtained from step iii) until an aqueous suspension of precipitated calcium carbonate having a portlandite content of less than 1 wt.-%, based on the total dry weight of the precipitated calcium carbonate, is obtained, wherein a slaking additive is added before, during or after step iii), the slaking additive being selected from the group consisting of an organic acid, an organic acid salt, a sugar alcohol, a monosaccharide, a disaccharide, a polysaccharide, a gluconate, a phosphonate, a lignosulfonate, and any mixture thereof.

Description

EXAMPLES

1. Measurement Methods

(1) In the following, measurement methods implemented in the examples are described.

(2) Particle Size Distribution of Precipitated Calcium Carbonate (PCC)

(3) The particle size distribution of the prepared PCC particles was measured using a Sedigraph 5120 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.

(4) Particle Size Distribution of Calcium Carbonate Nanoparticles

(5) The number based particle size distribution of the calcium carbonate nanoparticles was determined by the use of a Malvern Zetasizer Nano ZS.

(6) The samples slurry was diluted with a 0.1 wt.-% solution of Na.sub.4P.sub.2O.sub.7 until 100 g of a slurry with a solids content of 0.5 wt.-% was attained. 1 g of Polysalz (BASF, Germany), was added and the slurry was mixed at high shear for 5 minutes. After mixing, the sample was treated in a sonication bath for 15 to 20 minutes.

(7) The particle size distribution was determined by adding the obtained slurry to a standard 1 cm×1 cm cuvette. The cuvette was placed in the instrument, wherein the particle size was determined using dynamic light scattering. The values are reported in the number based distribution. This means that a d.sub.50, by number, is defined as 50% of the number of particles have a diameter of less than d.sub.50 in all three dimensions.

(8) Solids Content of an Aqueous Suspension

(9) 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.

(10) Specific Surface Area (SSA)

(11) 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 Buchner 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.

(12) X-Ray Diffraction

(13) The purity of the PCC samples was analysed with a D8 Advance powder diffractometer (Bruker Corporation, USA) obeying Bragg's law. This diffractometer consisted of a 2.2 kW X-ray tube (Cu), a sample holder, a ϑ-ϑ goniometer, and a VÅNTEC-1 detector. Nickel-filtered Cu K.sub.α radiation was employed in all experiments (λK.sub.α-Cu=1.5406 Å). The profiles were chart recorded automatically using a scan speed of 0.7° per minute in 2ϑ (XRD GV_7600). The measurement was carried out at angles from 5 to 70°.

(14) 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). 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.

2. Materials

(15) A1: Precipitated calcium carbonate nanoparticles (d.sub.50: 0.04 μm; d.sub.98: 0.10 μm, portlandite content: <LOD).

(16) A2: Precipitated calcium carbonate nanoparticles (d.sub.50: 0.07 μm; d.sub.98: 0.16 μm, portlandite content: <LOD).

(17) A3: Calcium nitrate tetrahydrate, commercially available from Riedel de Haën, Germany.

(18) A4: Ground calcium carbonate nanoparticles (d.sub.50: 0.13 μm; d.sub.98: 0.33 μm) produced from undispersed marble obtained from Omya SPA, Carrara, Italy.

(19) A5: Ground calcium carbonate (d.sub.50: 3.5 μm; d.sub.98: 10.6 μm), commercially available from Omya SAS, Orgon, France.

(20) LOD: limit of detection.

(21) Preparation of PCC Nanoparticles A1 and A2:

(22) PCC Nanoparticles A1:

(23) A milk of lime was prepared by mixing under mechanical stirring 5 liters water with 1000 g calcium oxide (quicklime raw material from Austria) at an initial temperature of 50° C. The obtained mixture was stirred for 30 min, wherein additional 4 liters of water were added. Subsequently, the mixture was sieved through a 100 μm screen.

(24) 6.5 L of the obtained milk of lime were transferred into a stainless steel reactor. After the addition of 5 wt.-% of sucrose and 2.5 wt.-% of strontium hydroxide octahydrate, based on the total weight of calcium hydroxide, the milk of lime was heated to 60° C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (first 15 minutes: CO.sub.2=1 L/min and air=14 L/min, rest of experiment: CO.sub.2=3.6 L/min and air=11.4 L/min). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The reaction was monitored by online pH and conductivity measurements.

(25) The precipitated calcium carbonate nanoparticles were obtained by filtering the suspension and rinsing the residue with water.

(26) PCC Nanoparticles A2:

(27) A milk of lime was prepared by mixing under mechanical stirring 5 liters water with 800 g calcium oxide (quicklime raw material from Austria) and 0.1 wt.-%, based on the total weight of calcium oxide, dry sodium citrate as slaking additive at an initial temperature of 40° C. The obtained mixture was stirred for 30 min, wherein additional 4 liters of water were added. Subsequently, the mixture was sieved through a 100 μm screen.

(28) 8 L of the obtained milk of lime were transferred into a stainless steel reactor. After the addition of 5 wt.-% of sucrose, based on the total weight of calcium hydroxide, the milk of lime was cooled down to 10° C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (CO.sub.2=3 L/min and air=12 L/min). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The reaction was monitored by online pH and conductivity measurements.

(29) The precipitated calcium carbonate nanoparticles were obtained by filtering the suspension and rinsing the residue with water.

3. Example

(30) A milk of lime was prepared by mixing under mechanical stirring 5 liters water with 1000 g calcium oxide (quicklime raw material from Austria) and 0.1 wt.-%, based on the total weight of calcium oxide, dry sodium citrate as slaking additive at an initial temperature of 40° C. The obtained mixture was stirred for 30 min, wherein additional 4 liters of water were added. Subsequently, the mixture was sieved through a 100 μm screen.

(31) The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50° C. Subsequently, the precipitation enhancer (if present) was added to the milk of lime (the employed precipitation enhancers are indicated in Table 1 below). Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (20 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The reaction was monitored by online pH and conductivity measurements.

(32) The precipitated calcium carbonate was separated from the obtained suspension by filtration, rinsed with ethanol and dried in a drying cabinet at 90° C. The purity of the obtained precipitated calcium carbonate was controlled by X-ray diffraction using the method described above.

(33) The characteristics of the prepared milks of lime and aqueous PCC suspensions are described in Tables 1 and 2 below.

(34) TABLE-US-00001 TABLE 1 Composition of the produced milks of lime. Amount of precipitation enhancer Precipitation [wt.-%, based on total weight Sample enhancer of calcium oxide]  1 (inventive) A2 1  2 (inventive) A2 5  3 (inventive) A1 5  4 (inventive) A3 5  5 (inventive) A3 10  6 (inventive) A1 5 A3 5  7 (inventive) A2 5 A3 5  8 (inventive) A4 5  9 (comparative) A5 5 10 (comparative) — —

(35) TABLE-US-00002 TABLE 2 Characteristics of the obtained precipitated calcium carbonates (LOD: limit of detection). SSA d.sub.50 d.sub.98 Calcite Portlandite Sample [m.sup.2/g] [μm] [μm] [wt.-%] [wt.-%]  1 (inventive) 9.30 1.30 2.40 99.40 0.6  2 (inventive) 11.40 1.40 3.00 100.00 < LOD  3 (inventive) 7.30 3.87 5.81 99.3 0.7  4 (inventive) 5.02 1.32 3.65 99.3 0.7  5 (inventive) 4.00 1.33 4.13 99.5 0.5  6 (inventive) 6.90 1.67 3.78 100.00 <LOD  7 (inventive) 11.80 1.30 3.20 100.00 < LOD  8 (inventive) 4.70 1.90 2.50 99.50 0.5  9 (comparative) 4.90 3.80 8.10 97.70 2.3 10 (comparative) 4.72 1.62 4.07 97.20 2.9

(36) The results compiled in Table 2 show that by the use of the inventive precipitation enhancer (samples 1 to 8), a PCC with a significantly reduced portlandite content can be obtained. Furthermore, no portlandite at all was detectable in samples 6 and 7, which was produced by using a combination of calcium carbonate nanoparticles and water-soluble calcium salt as precipitation enhancer.