Vaterite and production method

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106465, 423430, 423432, C09C 102

Patent

active

052903532

DESCRIPTION:

BRIEF SUMMARY
In the process according to the invention, calcium oxide or hydroxyde is treated in a reaction medium containing an acid salt of an orgnic amine so as to obtain a medium having a pH of treatment higher than 10 and a calcium content form 0.35 to 0.65 mole/litre, CO.sub.2 is introduced under stirring up to the obtention of a pH equal to about the pH of treatment and CO.sub.2 is introduced in the medium at a rate lower to 50 g CO.sub.2 /hour/litre.
The invention relates also to a vaterite having a spherical shape with two diametrically opposite hollows and a grain size distribution factor lower than 0.5.


STATE OF THE ART

FR-A-2298511 discloses a process for the preparation of vaterite In this known process, calcium oxide is treated in a reaction medium containing nitrates or chlorides and an amine, insoluble compounds are removed from the medium and CO.sub.2 are introduced in the medium so as to precipitate calcium carbonate in the form of aggregates of vaterite particles. The so obtained vaterite is not stable and does not allow the preparation of calcium derivates of very high purity, for example having a purity higher than 99,99%. Moreover, it is only possible to obtain by means of this process a mixture of vaterite, the grain size distribution factor of which is higher than 1.
A process allowing the preparation of calcium carbonate of high purity (higher than 99%) is known by U.S. Pat. No. 4,871,519. In this process, decarbonated dolomitic lime is treated in a solution containing an acid salt of an organic amine so as to convert calcium into soluble salt and after filtration, the solution is treated by means of CO.sub.2 so as to precipitate calcium carbonate.
All the examples of this document do not allow to obtain stable vaterite, in particular a vaterite having a grain size distribution factor less than 1.
Grain size distribution factor is a parameter allowing to determine the size repartition of a mixture of particles. An important size distribution factor means that the mixture contains an important amount of particles the size of which differs from the mean size.
The grain size distribution factor of a mixture may be calculated by the following formula: ##EQU1## in which d.sub.80% is the upper diameter of the particles of the fraction of the mixture passing at 80% mixture passing at 20% than a determined value, this fraction corresponding to x% by weight of the mixture.


BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a process for the preparation of substantially spherical vaterite.
Another subject matter of the present invention is a vaterite in the form of particles, the grain size distribution factor of which is less than 1, preferably than 0.5.
In particular, the invention relates to a vaterite having a size lower than 20.mu., advantageously than 10 microns. In a preferred form, the vaterite according to the invention has a mean diameter (mean by weight) lower than 5.mu., advantageously than 3.mu. and especially lower than 1.mu..
The process according to the invention, especially the preparation process for obtaining a substantially spherical vaterite, not in the form of aggregates of particles,
for example a vaterite in the form of particles having a grain size distribution factor less than 1, advantageously than 0.5, comprises the following steps: and/or hydroxide is treated in a reaction medium containing an acid salt of an organic amine so as to convert the calcium into a soluble salt and so as to obtain a medium, the pH of which, called pH of treatment, has a value higher than 10; the pH of the medium reaches a pH called pH of saturation, the value of which is about 0.83.times.pH of treatment (so as to have a CO.sub.2 saturation of the medium), and lower than 50 g CO.sub.2 /hour/liter so as to form substantially spherical vaterite especially having a predetermined size.
According to a characteristic of this process CO.sub.2 is introduced in the medium when the CO.sub.2 content of the medium is comprised between 0.3 and 1, advantageously between 0.35 and 0.65, prefer

REFERENCES:
patent: 3304154 (1967-02-01), Kiouzes-Pezas
patent: 4871519 (1989-10-01), Zikmund et al.
patent: 5232678 (1993-08-01), Bleakley et al.
Chemical Abstract for "Manufacture of Hexagonal Plate-Shaped Calcium Carbonate Powder", Tanaka et al. JP 62-113,718 May 25, 1987.
Chemical Abstract for "Production of Cubic Calcium Carbonate", Hideo et al. Sep. 30, 1986 JP 61-219,716.

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