Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 20~699~
A Process for Preparing Fibrous Magnesium Oxide
The present invention relates to a process for preparing
fibrous magnesium oxide, in which a material in form of needle-
shaped particles containing magnesium and water of crys-tallization
is converted by calcining into fibrous magnesium oxide.
Magnesium oxide has a very high melting point und possesses a
good chemical resistance to basic systems and also good electric
insulating properties as well as a reasonable good thermal
conductivity. Due to these properties it is used widely in
different fields, especially as material for refractory bodies and
10 refractory bricks respectively for use in the metallurgical
industry and as a filler for synthetic resins being capable of
improving the thermal conductivi-ty o~ the synthetic resin
material. Magnesium oxide usually consists of particles in
granular form, i.e. particles having approximately the same
15 dimensions in different directions similar to cubes and spheres.
Also known is fibrous magnesium oxide, that means magnesium oxide,
the particles of which have an elongated form, the ra-tio o~ the
length of said particles to the greates-t average transverse
dimension being at least 10 : 1 and -the sectional area of the
20 particles being less than 0,05 mm2 and the width of the particles
being smaller than 0,25 mm (see also ASTM D 3878). Such particles
of fibrous magnesium oxide have a reasonable high strength, so
that a distinctly higher strength of bodies or bricks respectively
consisting of magnesium oxide and used e.g. for the formation of
25 refractory linings, and especially of bodies of plastic materials
containing magnesium oxide as filler may be achieved than in case
of using the usual magnesium oxide in form of granular particles.
The known processes for preparing fibrous magnesium oxide
show a number of disadvantages, such as e.g. a low performance,
30 the production costs at the same time being burdened by a great
expendi-ture of energy, and a relatively complicated procedure
involving intermediates formed by slow reactions, as well as the
additional serious disadvantage that corrosive and toxic gases are
formed in the reaction of said intermedia-tes to give the fibrous
35 magnesium oxide, the disposal of said gases being difficult and
increasing the costs of the process. Such a known process involves
20~992
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the precipitation of magnesium oxide from the vapor phase preceded
by the reaction of magnesium metal vapor with oxygen so as to form
magnesium oxide, and during the precipitation of the magnesium
oxide from the vapor phase fibrous particles of magnesium oxide
are formed. This process has a relatively low performance and
requires a great expenditure of energy. In another known process
needle-shaped basic intermediate magnesium compounds having a
monovalent to -tetravalent anion and containing water of
crystallization are formed, which intermediate products then are
converted by calcination into fibrous magnesium oxide or first
magnesium hydroxide is prepared from such an intermediate produc-t
and then the fibrous magnesium oxide is formed therefrom. The
preparation of said basic magnesium compounds is complicated and
corrosive or toxic gases are liberated in the calcination thereof
rendering the process more complicated due to the inevitable
disposal of said gases.
It is an object of the present invention to provide a process
as mentioned above, which may be carried out as simple as possible
and with low expenditure of costs and which is efficient quantita-
-tively and does not cause grea-ter problems with by-products formed
in the process.
The present invention provides a process for preparing
fibrous magnesium oxide, in which a material in form of needle-
shaped particles containing magnesium and water of crystallization
is converted into fibrous magnesium oxide by calcina-tion, wherein
one or several neutral magnesium carbonates con-taining water of
crystallization and having the formula MgCO3.x~2O, in which
l s x s 5, especially magnesium carbonate trihydrate, are used as
material to be calcined.
The above mentioned object may be achieved by said process.
Neutral magnesium carbonates containing water of crystallization,
especially magnesium carbonate trihydrate, may be obtained
relatively easily, e.g. by precipi-tating solutions of magnesium
salts with ammonia and carbon dioxide, ammonium carbonate and
ammonium hydrogen carbonate respectively or by carboniza-tion of
precipitated magnesium hydroxide. Thus e.g. magnesium carbonate
-trihydrate in form of needle-shaped crystals can be precipitated
simply and rapidly from an aqueous solution, the size of the
2~5~92
-- 3
crystals being influenceable by choice of the temperature of the
reactor. Said temperature is only slightly above ambient
temperature and does not require any constructional special
provisions or energetical expenditure.
The neutral magnesium carbonates containing water of
crystallization can be calcined in a simple manner to give fibrous
magnesium oxide with largely retaining the particle form. In said
calcination process toxically or corrosively acting products are
not formed. The calcination temperature may be selected within
broad ranges. Good results are obtained at calcination tempera-
tures of between 350 and 2000~C. Preferably calcination tempera-
tures of between 800 and 1600C are used. Depending on the device
used for the calcination very short calcination periods of e.g.
few seconds up to long calcination periods of several hours are
employed. Suitably the material to be calcined is dried in vacuo
at a temperature of below 100C or partially dehydra-ted
respectively before calcination.
In the process of the invention preferably the heating up of
the material to be calcined is effected with a rate of less than
10C/minute. This resul-ts in a gentle progress of the calcina-tion,
which favors the conversion of the neutral magnesium carbonates
into fibrous magnesium oxide.
The particles of the fibrous magnesium oxide prepared by the
process of the invention have a high strength. Higher strength
values may be achieved here than in case of particles of fibrous
magnesium oxide prepared in known manner from basic magnesium
compounds. Thus, if fibrous magnesium oxide prepared by the
process of the invention is used as a filler of synthetic resins a
considerable improvement of the strength of the filled plas-tic
rnaterial may be achieved already wi-th relatively small amounts of
filler.
The following examples illustrate the invention further.
Example 1:
500 ml of distilled water were filled into a stirred vessel
provided with thermostatic means and 500 ml of a solution of
magnesium chloride containing 298 g/litre of MgCl2 were added and
the mixture was heated to 35~C. The solution was intensively mixed
by means of a stirrer and 213 g of a 25 % solution of ammonia were
2 ~ 9 2
-- 4 --
dropped in over a period of 30 minutes and 35 litres of C02 were
introduced into the solution. The reaction mixture was maintained
at a temperature of 40C by means of a heat exchanger. After
complete reaction the suspension was filtered, the filter cake was
washed with water and the obtained product was dried at low
temperatures of below 40C in vacuo. The product was identified as
nesquehonite, MgC03.3H20, and had the appearance of needle-shaped
crystals.
Scanning electron micrographs showed -that the needles of
nesquehoni-te had a length of about 150 ~m and an average diameter
of about 5 ~m.
The product thus obtained was calcined at a -temperature of
1100C for 3 hours to give fibrous magnesium oxide. The synthe-
tized fibres similarly to the used startiny fibres of nesquehonite
had a length of about 150 ~m and an average diameter of about
5 ~m. By means of X-ray diffraction the product could be identi-
fied as magnesium oxide.
E mple 2:
The process of example 1 was repeated, however, the filter
cake obtained after complete reaction and filtration of the
suspension and washed wi-th water was dried in vacuo at a
temperature of 70C for 2 hours. Chemical analysis showed that a
neutral magnesium carbonate having the composition MgC03.2,4H20
had been formed. Scanning electron micrographs showed that th
obtained product was composed of fibres having a length of about
150 ~m and an average diame-ter of about 5 ~m.
The produc-t obtained in this manner was heated to a
temperature of 850C with a heating rate of 3C/minute and was
cooled without control after a holding time of 1 hour. A fibrous
substance having fibres with lengths of about 150 ~m and average
diameters of about 5 ~Im was obtained. X-rax diffraction confirmed
that the material was magnesium oxide.
Example 3:
The product of example 1 iden-tified as nesquehonite,
MgC03.3H20, and having the appearance oE needle~shaped crystals
was heated to a temperature of 1600C with a heating ra-te of
5C/minute and was cooled without control after a holding time of
20~6~
-- 5 --
1 hour. By means of X-ray diffraction the fibrous product could be
identified as magnesium oxide. Similarly to the used nesquihonite
the fibre lengths were 50 to 150 ~m and the average fibre
5 diameters 2 to 5 ~m.
Example 4:
The neutral magnesium carbonate of example 2 having the
composition MgCO3.2.4H20 was heated to a temperature of 400C with
lOa heating rate of 10C/minute. ~f-ter a holding time of 3 hours
fibrous magnesium oxide was obtained. The synthetized fibres did
not dif-fer morphologically from the used neutral magnesium
carbonate and showed fibre lengths of up to 150 ~m and average
fibre diameters of about 3 to 5 ~m. X-ra~ diffraction confirmed
15 tha-t the product was ~agnesium oxide.
Example 5: Use of Mg-fibres for improving the strength of
composites
The MgO-fibres obtained in example 1 as well as silicon
carbide whiskers and commercially available magnesium oxide were
20embedded into an epoxy resin in amounts of 4 % by volume and the
flexural strength at break was examined on test specimens. The
results of these tests are summarized in table 1. It results that
the MgO-fibres effect a significan-t improvement of the flexural
strength at break of the composites already in small amounts. If
25 the flexural strength at break of the composites is compared among
one another and with that of the pure epoxy resin, -taking the
mixing standard as a basis and assuming an isotropic distribution
of the fibres in the matrix, an intrinsic strength of the MgO-
fibres of the invention of about 7000 to 8000 MPa will be
30 calcula-ted.
T a b 1 e
Strength of composites containing 4 % by volume of filler
Flexural streng-th
Matrix Reinforcing componentat break
MPa
Epoxy resin - 44
epoxy resin silicon carbide whisker 84
epoxy resin MgO-fibres of the invention 62
epoxy resin commercially available MgO 49
