Process to prepare a surface-reacted calcium carbonate implementing a weak acid, resulting product and uses thereof
The present application relates to a process for preparing a surface-reacted calcium carbonate in an aqueous environment. The process according to the present invention aims at avoiding the mandatory implementation of medium-strong to strong acids. Another aspect of the present invention is directed to an aqueous suspension of surface-reacted calcium carbonate which is obtainable by the inventive process.
1. A process for preparing a surface-reacted calcium carbonate in an aqueous environment comprising the following steps:
a) providing at least one ground natural calcium carbonate (GNCC);
b) providing at least one water-soluble acid in the form of an aqueous solution having an acid concentration, determined as the equivalent weight of pure acid on the weight of the total solution, corresponding to from 25 to 75%;
c) providing gaseous CO 2 ;
d) contacting the GNCC of step a) with the acid of step b) and with the CO 2 of step c); wherein:
(i) the at least one acid of step b) has a pK a of greater than 2.5 and less than or equal to 7, when measured at 20° C., and is associated with the ionisation of the first available hydrogen, and a corresponding anion formed on loss of the first available hydrogen that is capable of forming water-soluble calcium salts;
(ii) following contacting the at least one acid with the GNCC, at least one water-soluble salt is additionally added, wherein the water-soluble salt is a hydrogen-containing salt having a pK a of greater than 7, when measured at 20° C., and is associated with the ionisation of the first available hydrogen, and a salt anion is capable of forming water-insoluble calcium salts.
2. The process according to claim 1 , wherein the GNCC comprises marble, chalk, calcite, dolomite, limestone, or any mixture thereof.
3. The process according to claim 1 , wherein the GNCC of step a) has a weight median diameter of 0.01 to 10 μm.
4. The process according to claim 1 , wherein the GNCC of step a) has a weight median diameter of 0.5 to 2 μm.
5. The process according to claim 1 , wherein the GNCC is provided in the form of an aqueous GNCC suspension.
6. The process according to claim 5 , wherein the suspension has a pH of less than 11.
7. The process according to claim 5 , wherein the suspension has a pH of less than 10.5.
8. The process according to claim 5 , wherein the suspension has a solids content of greater than or equal to 10 wt.-%, based on the weight of the suspension.
9. The process according to claim 5 , wherein the suspension has a solids content of 10 wt.-% and 80 wt.-%, based on the weight of the suspension.
10. The process according to claim 5 , wherein the suspension has a solids content of 16 wt.-% and 60 wt.-%, based on the weight of the suspension.
11. The process according to claim 5 , wherein the suspension has a solids content of 16 wt.-% and 40 wt.-%, based on the weight of the suspension.
12. The process according to claim 5 , wherein the suspension is stabilized by the addition of dispersants.
13. The process according to claim 1 , wherein the at least one water-soluble acid of step b) has a pKa of between 2.6 and 5.
14. The process according to claim 1 , wherein the at least one water-soluble acid is acetic acid, formic acid, propanoic acid or any mixture thereof.
15. The process according to claim 1 , wherein the at least one water-soluble acid is acetic acid, formic acid, or any mixture thereof.
16. The process according to claim 1 , wherein the at least one water-soluble acid is acetic acid.
17. The process according to claim 1 , wherein the at least one water-soluble acid is dosed in a total amount corresponding to at least 1.5×10 −4 mol of hydrogen atoms in the acid/m 2 GNCC provided in step a).
18. The process according to claim 1 , wherein the at least one water-soluble acid is dosed in a total amount corresponding to from 2×10 −4 to 12×10 −4 mol of hydrogen atoms in the acid/m 2 GNCC provided in step a).
19. The process according to claim 1 , wherein the at least one water-soluble acid is dosed in a total amount corresponding to from 3×10 −4 to 10×10 −4 mol of hydrogen atoms in the acid/m 2 GNCC provided in step a).
20. The process according to claim 1 , wherein the at least one water-soluble acid of step b) is dosed in a total amount corresponding to 5 to 40 wt.-% equivalent pure acid based on the dry weight of GNCC provided in step a).
21. The process according to claim 1 , wherein the at least one water-soluble acid of step b) is dosed in a total amount corresponding to 10 to 30 wt.-% equivalent pure acid based on the dry weight of GNCC provided in step a).
22. The process according to claim 1 , wherein the at least one water-soluble acid of step b) is dosed in a total amount corresponding to 15 to 25 wt.-% equivalent pure acid based on the dry weight of GNCC provided in step a).
23. The process according to claim 1 , wherein the at least one water-soluble acid of step b) is provided in the form of an aqueous solution having an acid concentration, determined as the equivalent weight of pure acid on the weight of the total solution, corresponding to from 40 to 60%.
24. The process according to claim 1 , wherein during step d), the at least one acid of step b) is added, in one or more steps, to the GNCC.
25. The process according to claim 1 , wherein the cation of the at least one water-soluble salt is selected from the group consisting of potassium, sodium, lithium, and any mixture thereof.
26. The process according to claim 1 , wherein the cation of the at least one water-soluble salt is sodium.
27. The process according to claim 1 , wherein the anion of the at least one water-soluble salt is selected from the group consisting of phosphate, dihydrogen phosphate, monohydrogen phosphate, oxalate, silicate, hydrates thereof, and any mixture thereof.
28. The process according to claim 1 , wherein the anion of the at least one water-soluble salt is selected from the group consisting of phosphate, dihydrogen phosphate, monohydrogen phosphate, hydrates thereof, and any mixture thereof.
29. The process according to claim 1 , wherein the anion of the at least one water-soluble salt is dosed in a total amount corresponding to at least 5×10 −5 mol of anion/m 2 GNCC provided in step a).
30. The process according to claim 1 , wherein the anion of the at least one water-soluble salt is dosed in a total amount corresponding to 5×10 −5 to 50×10 −5 mol of anion/m 2 GNCC provided in step a).
31. The process according to claim 1 , wherein the anion of the at least one water-soluble salt is dosed in a total amount corresponding to 10×10 −5 to 30×10 −5 mol of anion/m 2 GNCC provided in step a).
32. The process according to claim 1 , wherein the at least one water soluble salt is added in one step.
33. The process according to claim 1 , wherein in step d) the addition of the at least one water-soluble salt take place in a stirred reactor under stirring conditions such as to develop an essentially laminar flow.
34. The process according to claim 1 , wherein in step d) the addition of the at least one water-soluble salt takes place in an aqueous environment having a temperature of above 50° C.
35. The process according to claim 1 , wherein in step d) the addition of the at least one water-soluble salt takes place in an aqueous environment having a temperature of above 60° C.
36. The process according to claim 1 , wherein the aqueous suspension is concentrated followingt step d).