Magnesium oxide in fine powder form and its use

Details Magnesium oxide in fine powder form and its use Magnesium oxide in fine powder form and its use

1990-04-05

1992-04-21

Morris, Theodore

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

423274, 423155, C01F 1102

Patent

active

051066084

DESCRIPTION:

BRIEF SUMMARY
The subject of this invention is a pulverulent magnesium oxide having an MgO content greater than 95 wt. %, preferably greater than 98 wt. %, a particle size less than 15 .mu.m and a BET specific surface area less than 20 m.sup.2 /g as determined from the nitrogen adsorption isotherm.
For attaining high-density ceramics, e.g. in the manufacture of refractory products, it is desirable to fill up the hollow space between the grains of the matrix with very small particles, i.e. with a micro powder. A suitable material, especially as far as refractoriness is concerned, is a micro powder based on high purity magnesium oxide.
It is the object of the invention to provide a magnesium oxide micro powder which can suitably be used to fill up the hollow space in a ceramic matrix and to manufacture ceramics of high density, while the use of such powder shall not be limited to these applications.
According to the invention, the statistical mean of the particle shape factor of the primary particles of a magnesium oxide micro powder of this kind lies between 1 and 1.5 and the particles are coated with a hydrophobic substance.
The particle shape factor K is defined by the expression ##EQU1## where U is the particle circumference and F is the particle surface area, both related to the elevation of such particle as it appears under the microscope. "Primary particles" are the dispersed, non-agglomerated particles. For perfectly spherical particles, the particle shape factor K, a dimension-less figure, equals 1. It is the higher the more the shape of the particle differs from the spherical shape. Thus, a statistical mean in the distribution curve of the particle shape factor in the range between 1 and 1.5 defines primary particles of approximately spherical shape.
Substances which according to the invention may suitably be used for coating the magnesium oxide particles are surface-active materials, in particular unsaturated or saturated fatty acids or derivatives thereof, organo-metallic compounds--such as silicanes, titanates, zirconium aluminates, metal soaps--or silicones or organic polymers. These substances impart to the magnesium oxide powder hydrophobic properties.
These substances reduce the internal friction in the magnesium oxide powder, and the particles--when mixed with the binding agent applied--are given a high degree of mobility; as a result, high densification of the ceramic can be attained during the shaping process.
In general, the magnesium oxide according to the invention contains between 0.1 and 10 wt. % of coating substance.
This coating increases the degree of dispersion in the magnesium oxide micro powder, i.e. apart from individual Primary particles, the micro powder according to the invention contains only relatively small agglomerations of primary particles. Accordingly, the powder has a particle size less than 15 .mu.m, preferably between 0.2 and 15 .mu.m. The size of the primary particles is preferably in the range of 0.2 to 2 .mu.m.
The specific surface area of the micro powder, which can be determined from the nitrogen adsorption isotherm according to BET, should not be too large. For the magnesium oxide according to the invention it is in the range of 2 to 7 m.sup.2 /g.
A magnesium oxide powder especially suited for the purposes of the invention may be prepared by pyrohydrolysis from a magnesium chloride solution, and a spray roaster may be applied for attaining thermal decomposition. The powder produced in this process is then coated with a substance which imparts to it hydrophobic properties.
The following examples illustrate some of the benefits attainable with the invention.


EXAMPLE 1

A magnesium oxide powder A was prepared from a purified magnesium chloride solution in a spray roasting process. A part of the matertial produced was mixed for some time (6 hours) with a metal soap; the product resulting from this was the coated magnesium oxide powder B. The properties of the materials A and B are compared as follows in Table 1:


TABLE 1 ______________________________________ MgO powder

REFERENCES:
patent: 4298379 (1981-11-01), Zambrano

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