Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- The invention concerns a process for the production of
pure alumina, principally alpha-alumina.
Because of the high present-day demands of technology,
industry processing alumina for the production of oxide-based ceramic
products requires the highest quality basic materials. The most
important basic material of the oxide-based ceramic industry is
alumina.
The quality of alumina is generally judged by its purity,
its particle size and i~s crys~alline form or modification. In the
processing industry a purity of 99.99% or better is expected from
the alumina starting material. In addition to the purity, requirements
are a particle size generally less than one micrometre and the presence
of only the alpha crystalline form, since these factors significantly
influence the quality of the final product as well as the economy of
the processing.
A difficulty in producing alpha-alumina arises from the
stable alpha form being obtainable only by means of heat treatment
taking place at a high temperature, in excess of 1500 C. At such
a temperature the desired stable alpha form is given but the crystals
coalesce into large groups or lumps, agglomerate and shrink. In this
form the alumina is barely suitable for the manufacture of oxide-based
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ceramic products. Consequently, it must be~ Ject for example to
grinding and even to classifying. Ho~ever, in the course oE
comminution most of the alumina crystals are damaged, cracked~ broken
or crumbled and thereby lose some of their strength. The quality and
strength of oxide-based ceramic products made from damaged crystals
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are ~æ~ than the quality and strength of products produced from
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undamaged crystals. To this should be added that during comminution
the high purity alpha-alumina necessarily takes up contaminants
which disadvantageously influence its quality.
Alumina of greater purity than technical grade alumina
is produced from bauxite, but although generally these materials
are produced to satisfy predetermined aims and demands, they do not
satisfy demands for the highest quality materials, where the basic
alumina materials are analysed for 10-15 components with an attendant
purity requirement of 99.99%.
A higher degree of purity can be achieved with processes
wherein, as starting material, a water-soluble aluminium ~alt or
a thermally decomposable aluminium compound is employed. The weak
point of these processes is generally the fact that for a unit amount
of alumina a very large quantity and/or large volume of starting material
is required. A good example of this is aluminium sulphate, which
9 contains 18 molecules o water of crystallization, or ~ aluminium
ammonium sulphate, a frequently employed starting material for
alumina produced by thermal decomposition. Undoubtedly, considering
the ratio of the amounts of starting material and final product, the
most favourable processes are those employing aluminium as starting
material. Thus from 5~0 g metal 1020 g alumina can be produced.
Clearly it is to this that one can attribute the fact that numerous
processes are known for producing alumina from aluminium.
Thus it is known to powder aluminium and ignite to constant
~eight, and also to produce alumina by heating hydrated alumina
produced in water by spark machining of aluminium. Further, numerous
processes are known for producing hydrated alumina in pressurized
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reactors in the presence of mercury or a mercury salt.
' It is also known to reac~ aluminium at normal a~mospheric pressure
in the presence of water vapour, water, and mercury or a mercury salt, as a re-
sult of which hydrated alumina is obtained as long loose flakes or in an agglom-
erated state. In ignition of these loose flakes utili~ation of oven capacity
is extraordinarily poor and the heat transfer between individual flakes is un-
even; consequently undesirably high temperatures are necessary and the quality
of the alumina is uneven.
In another method so-called thermal grinding is used for comminuting
the agglomerated hydrated alumina. In this process, there is placed in an oven
of suitable heat capacity that amount of wet hydrated alumina which can be
heated in a slngle batch within at most two minutes to at least 1100C. Under
the efEect of the vapour generated the material is comminuted, i.e., as it were,
is thermally gound.
The aim of the present invention is to provide a process, simpler
than those known hitherto~ for the economic production of high purity alpha-
alumina, expediently of sub-micronic particle si~e.
Accordingly the invention provides in a process for the preparation
of pure alumina wherein metallic aluminum is activated, the activated aluminum
is reacted in an aqueous solution to obtain an intermediate and then the inter-
mediate is heat treated to obtain an alumina end product, the improvement in
producing said intermediate which comprises employing aluminum having a purity
of at leas~ 99.5% and a specific surface of at least eight cm 2/g, activating
only the surface of the aluminum by contacting it with a member selected from
(1) an aqueous acidic solution selected from NaCl, KCl, NH4Cl, NH~IHCO3, ICI,
and HgC12, and (2) water containing dissolved C0~, S02, CI2 or F2 then washing
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said aluminum, having only its surface activated wi~h an aqueous medium
having a neutral pH; then reacting the washed only surface activated aluminum
in aqueous solution and adjusting the pH of such solution to produce alumina.
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The essence of the invention lies in that the activation
(removal of oxide layer) is performed exclusively in an aqueous
solution of acidic pH value, a subsequent washing is performed
exclusively in an aqueous solution of neutral pH value, and the
reaction (to give hydrated alumina) is performed in an aqueous
solution of alkaline, neutral or acidic pH value. In dependence
on the alkaline, neutral or acidic pH value of the reaction solution,
the hydrated alumina reaction product consists of fine particles
and plate-like cyrstals or is in gel form. It is heat~treated in
a known manner to produce the final p~oduct of alumina of fine,
particulate, plate-like or chip-like form.
As starting material according to the invention-aluminium
metal of at least 8 cm tg specific area is desirable.
In a preferred process according to the invention, the
alumina is produced from metallic aluminium (plate, foil, shavings,
powder) of at least 99.5% purity and at least 8 cm ~g specific area.
In removing the protective oxide layer of the metal in a per se known
manner, i.e. chemically, the aluminium metal is activated in an
of acidic
aqueous solutionlpH value. Subsequent washing is performed in water
of neutral pH value. Then the activated and washed metal is transformed
to hydrated alumina, in the presence of water, at normal atmospheric
pressure and in a temperature range of 10 to 100C. Then after heat
treatment alumina is obtained.
The great advantage of the invention is that by changing
only one technological parameter, alumina may be produced in a most
simple manner and with apparatus which may easily be automated, which
alumina has excellent properties and is, if desired, of fine particle
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size and plate-like crystalline structure.
The following Ex~mples illustrate the process of the
; invention, without limitation.
Example 1
To remove the oxide layer from 0.05 mm thick aluminium
foil of at least 99.99% purity, and to activate its surface~ it
was passed through a bath at a rate of 10 m/min, which bath consis~ed
of twice-distilled water containing 0.1% HgC12 and with its pH
adjusted to 5 to 6 by means of hydrochloric acid. From the bath the
foil was passed at the same rate of 10 m/min to a second bath
containing twice distilled water of neutral p!l (pH 7~ wherein the
salts and acids remaining on the surface of the metal after the
activation were removed. The aluminium foil thus activated was
further processed as follows.
The foil was passed at the rate already given to a bath
containing twice-distilled water and with its pH adjusted to an
alkaline value of 8-9 by means of a 25% aqueous solution of ammonium
hydroxide. The temperature of the reaction solution was 80 to 90 C,
the maintenance of the temperature at a constant value being ensured
by equilibrium between production of heat of reaction and addition
of cold make-up solution replacing reaction solution removed and
passed to a filter. Vigorous exothermic reaction between the
alumlnium metal and the water produced a suspension of hydrated
alumina in a fine, readily sedimenting form. The hydrated alumina
crystals formed did not agglomerate into larger particles and during
~he subsequent processing they retained their sub-micronic particle
si~e. ~Iydrated alumina particles of similar electric polarity or
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charge repel each other and this property ensures good filterability
and the prevention of agglomeration~
The hydrated alumina crystals were continuously removed
from the aqueous solution by filtration, then, by drying, a powder
was produced. This hydrated alumina powder had a crystal size
of 0.005 to 0.09 micrometres. From this hydrated alumina the gamma
form of alumina could be obtained wi~h a specific surface area of
25 to 30 m2/g, by heating at 800 to 1000C. By heating to
1000 to 1200C alumina a mixture of the gamma and alpha forms
with a specific surface area of 15 to 25 m /g could be obtained,
and by heating between 1250 to 1800C alpha-alumina with a
specific surface area of 10 to 15 m /g.
Example 2
As starting mate~ial 99.99% pure aluminium foil was used,
activated and washed as described in Example 1, but with twice-
distilled water of neutral pH value in place of the alkaline
reaction solution. As a result of the chemical reaction taking
place a suspension of hydrated alumina of plate-like ~rystal form
was obtained. From this material the following products could
be obtained by heating: at 800 to 1000 C, gamma alumina powder o
plate-like, crystal form and specific surface area 9 to 15 m /g;
at 1000 to 1200C, mixed gamma and alpha alumina powder of plate-
like crystal form and specific surface area 5 to 9 m2/~; and a~
1250 to laO0 C alpha alumina powder of plate-like crystal form and
specific surface area of 1 to 5 m /g.
Example 3
As starting material aluminium foil of 99.5% purity
was used and the foil, activated and washed as described in Example 1,
was reacted in an aqueous acidic solution of a pH between 5 and 6,
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acidified by hydrochloric acid. The product of the reaction was
a gel of hydrated alumina. This material could not be filtered
and thus was dried in its original condition, giving blocks or
Lumps of cracked structure. ~y heating this ma~erial at 800 to
1000C lumpy ganlma alumina was obtained, specific surface area
10 to 15 m /g; by heating at 1000 to 1200 C lumpy mixed gammalalpha
alumina was obtained, specific surface area 8 to 12 m /g; while
by heating at 1400 to 1800 C alpha alumina with a sub-micronic
lattice was obtained, with at least 3.87g/cm3 solid specific
gravity, a hardness of least 92 to 94 ~a and a flexural strength
of 50 to 70 ~p/mm2, the alumina being in the form of spllntery
sinter bodies.
Example 4
Aluminium shavings were used as starting material.
After removal of the oxide layer and activation of the metal surface
by means of chemicaL salt solutions for destroying the metal oxide
(e-g- NaCl, KCl~ NH4Cl, NH4~!~03~ ICl, HgC12~ or gases dissolved in
water e.g. C02, S02, C12, F2, or metallic mercury), the shavings
were washed in twice-distilled water at neutral pH to remove the
residues of the activation. Finally the shavings were ~
in a vibratory or ultrasonic reactor with the solutions described
in Examples 1, 2 and 3. A material was obtainecl in each case
which conEormed with those described in the Examples.
Example 5
Aluminium foil of at least 99.99% purity and at least
8cm /g specific surface area was passed through a twice-distilled
aqueous solution of acidic p~l value of 5 to 6 and ~hrough a layer of
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mercury at the bottom of the solution. Thereafter the procedure was
as described in Examples 19 2 and 3 and with the same results as
described there.
Example 6
As starting material there was used a granulate of aluminium
of 99.5% purity and 8 cm Ig specific a~ea made by spraying molten metal,
through a protective barrier atmosphere, into the activation solution
described in Example 1. The washing and reaction took place with the
solutions of Examples, 1, 2 and 3 in three serially connected,
continuously operated continuous-transfer tube reactors. The reactions
and heat treatments resulted in the materials already describedq
In the Examples the alumina produced had a purity equal to or
greater than the purity of the starting aluminium.
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