Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Patent
1993-04-16
1995-03-28
Langel, Wayne
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
423636, C01B 1314, C01F 514
Patent
active
054014840
DESCRIPTION:
BRIEF SUMMARY
The present invention pertains to a process for preparing metal hydroxides of low specific surface.
Both the state of the art and the present invention will be described in greater detail below on the basis of magnesium hydroxide. However, this description is intended only as an example. The present invention is also applicable to other metal hydroxides, such as calcium hydroxide, even though magnesium hydroxide is particularly preferred.
It has been known, for example, that various fillers are added during the preparation of paper, plastics, or rubber, in order to improve, e.g., the mechanical properties, especially the strength and the modulus of elasticity. Mainly metal hydroxides, such as aluminum hydroxide or magnesium hydroxide, are used during the preparation of plastics, especially during the preparation of halogen-free, flame-retarded plastics.
For example, recently plastics for cable sheathing contain magnesium hydroxide as a filler. Since the viscosity of the polymer melt is strongly affected by the specific surface of the filler (larger specific surfaces leading to higher viscosity values), one seeks to keep the specific surface of the filler as small as possible in order not to adversely affect the processability by the addition of the filler.
Consequently, the goal of the present invention is to provide a process for preparing metal hydroxides of the smallest specific surface possible. The smallest specific surface possible is defined as the surface that is preferably smaller than that obtained during the processing of a metal hydroxide according to comparable simple processes.
For example, magnesium hydroxide, which is obtained by precipitation from sea water by adding milk of lime or burned dolomite, has a specific surface of about 40 m.sup.2 /g (measured according to the BET method), and is consequently unsuitable for use as a filler for plastics.
The precipitation of magnesium hydroxide from natural or artificial solutions (e.g., brines as generated in the potash industry) also leads to precipitated hydroxides with excessively large specific surface.
It has also been known that magnesium hydroxide can be precipitated from aqueous solutions of magnesium salts by adding substoichiometric amounts of a precipitant, e.g., sodium hydroxide, and the basic salt obtained therefrom, which has the general formula Mg(OH).sub.x (A).sub.y. zH.sub.2 O (in which A is an anion, e.g., Cl.sup.-, SO.sub.4.sup.2-, NO.sub.3.sup.-), can be separated and be subjected to an aftertreatment in an autoclave, while the basic salts will split and a magnesium hydroxide will be formed, which can have a specific surface of less than ca. 15 m.sup.2 /g. Even though the resulting product has a relatively small specific surface, the process is complicated and also has the disadvantage that wastewaters with high salt content must be disposed of.
The present invention is based on the surprising discovery that a metal hydroxide of small specific surface can be obtained in a simple process by using a metal oxide as the starting compound and first hydrating it into metal hydroxide by adding water and subsequently treating it in an autoclave under pressure until the desired specific surfaces below 20 m.sup.2 /g and especially below 10 m.sup.2 /g are reached.
This process makes it possible to use as the starting compound a conventional metal oxide, e.g., caustic burned magnesium oxide or a magnesium oxide as is obtained during the thermal decomposition of magnesium chloride in a spray roasting plant.
Contrary to the above-mentioned prior-art precipitation process, there is no precipitation reaction, and no extraneous substances are added (with the exception of water).
Only the magnesium hydroxide prepared beforehand is treated under pressure at elevated temperature, without other manipulations and additives, even during the subsequent autoclave treatment. It was surprisingly found that the said autoclave treatment alone is sufficient for markedly reducing the specific surface of the metal hydroxide to values even below 10 m.sup.2 /
REFERENCES:
patent: 3044857 (1962-07-01), Sable
patent: 3676079 (1972-07-01), Morgan
patent: 4698379 (1987-10-01), Nakaya et al.
International Publication No. WO92/05113, published Apr. 2, 1992.
Krivanec Heinz
Wurmbauer Dieter
A. Earley John F.
A. Earley III John F.
Chemson Polymer-Additive Gesellschaft m.b.H.
Langel Wayne
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