Process for producing precipitated calcium carbonate in the presence of natural ground calcium carbonate
The present invention relates to a process for producing an aqueous suspension of precipitated calcium carbonate, an aqueous suspension of precipitated calcium carbonate obtainable by the process, a precipitated calcium carbonate obtainable by the process, a product comprising the precipitated calcium carbonate as well as the use of the natural ground calcium carbonate (NGCC) in a process for producing an aqueous suspension of precipitated calcium carbonate.
1 . A process for producing an aqueous suspension of precipitated calcium carbonate, the process comprising the steps of:
i) providing a calcium oxide containing material,
ii) providing a natural ground calcium carbonate (NGCC) as precipitation additive, wherein the natural ground calcium carbonate (NGCC) has a weight median diameter d 50 of at least 0.3 μm,
iii) preparing a milk of lime by mixing water, the calcium oxide containing material of step i), and the natural ground calcium carbonate (NGCC) of step ii), wherein the natural ground calcium carbonate (NGCC) is present in an amount ranging from >5 to 25 wt.-%, based on the total weight of the calcium oxide containing material,
iv) carbonating the milk of lime obtained from step iii) to form an aqueous suspension of precipitated calcium carbonate.
2 . The process according to 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 natural ground calcium carbonate (NGCC) of step ii) to the mixture of step a1).
3 . The process according to claim 1 , wherein step iii) comprises the steps of:
a1) mixing the natural ground calcium carbonate (NGCC) of step ii) with water, and
a2) adding the calcium oxide containing material of step i) to the mixture of step a1).
4 . The process according to claim 1 , wherein the natural ground calcium carbonate (NGCC)
a) is selected from marble, limestone, chalk and mixtures thereof, and/or
b) has a content of calcium carbonate of >90 wt.-%, based on the total weight of the natural ground calcium carbonate (NGCC), and/or
c) has a weight median diameter d 50 in the range from 0.3 to 10 μm.
5 . The process according to claim 1 , wherein a slaking additive is added before, during or after step iii).
6 . The process according to claim 5 , wherein the slaking additive is added in an amount of ≤2 wt. %, based on the total amount of calcium oxide containing material.
7 . The process according to 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.
8 . The process according to claim 1 , wherein the milk of lime is screened after step iii) and before step iv).
9 . A process for producing precipitated calcium carbonate comprising the steps i) to iv) of the process according to claim 1 , and further a step v) of separating the precipitated calcium carbonate from the aqueous suspension obtained from step iv).
10 . The process according to claim 9 , wherein the process further comprises a step vi) of drying the separated precipitated calcium carbonate obtained from step v), and optionally a step vii) of contacting at least a part of the surface of the precipitated calcium carbonate with a surface-treatment agent.
11 . The process according to claim 1 , wherein the natural ground calcium carbonate (NGCC) has a content of calcium carbonate of ≥92 wt.-%, based on the total weight of the natural ground calcium carbonate (NGCC).
12 . The process according to claim 1 , wherein the natural ground calcium carbonate (NGCC) has a content of calcium carbonate of ≥96 wt.-%, based on the total weight of the natural ground calcium carbonate (NGCC).
13 . The process according to claim 1 , wherein the natural ground calcium carbonate (NGCC) has a weight median diameter d 50 in the range from 1 to 4 μm.
14 . The process according to claim 5 , wherein the slaking additive is selected from the group consisting of organic acids, organic acid salts, sugar alcohols, monosaccharides, disaccharides, polysaccharides, gluconates, phosphonates, lignosulfonates, and mixtures thereof.
15 . The process according to claim 6 , wherein the slaking additive is added in an amount of 0.1 to 1 wt.-%, based on the total amount of calcium oxide containing material.
16 . The process according to claim 6 , wherein the slaking additive is added in an amount of 0.01 to 0.2 wt.-%, based on the total amount of calcium oxide containing material.
17 . The process according to claim 1 , wherein the obtained suspension of precipitated calcium carbonate has a solids content of from 10 to 45 wt.-%, based on the total weight of the suspension.
18 . The process according to claim 1 , wherein the obtained suspension of precipitated calcium carbonate has a solids content of from 12 to 25 wt.-%, based on the total weight of the suspension.
19 . The process according to claim 8 , 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 μm.