Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR PREPARING COMPLEX FLUOROALUMINATES, THE
FLUOROALUMINATES PREPARED AND USE OF SPRAY DRYING AND
POLYALKYLENE GLYCOLS FOR CONTROLLING THE STRUCTURE OF
FLUOROALUMINATES
The present invention relates to a process for
preparing a complex fluoroaluminate, with the
fluoroaluminate being obtained from a suspension in
which the fluoroaluminate is present by spray drying
during the course of this process. The present
invention likewise relates to a process in which the
particle structure of this fluoroaluminate is
controlled by means of a structure-influencing
substance.
Fluoroaluminates are used in many areas of
industry. Thus, for example, potassium
tetrafluoroaluminate is used as an additive to
abrasives, in glass production or as flux in industrial
processes.
One way of preparing, for example, potassium
tetrafluoroaluminate is disclosed in JP 08157212. In
the process described there, aluminum hydroxide is
reacted with 20o strength by weight hydrogen fluoride
and the resulting solution, in which
tetrafluoroaluminic acid is present, is neutralized
with KOH.
DE 31 16 469 describes a process in which ~.n
aqueous, HA1F4-containing solution is neutralized with
KOH to form potassium tetrafluoroaluminate.
A ~~roblem which can occur in the preparation r~f
tetrafluoroaluminates involves the particle structure,
in particular the particle size of the fluoroaluminate
obtained. Thus, for example, formation of coarse
particles adversely affects the quality of the
fluoroaluminate requ_Lred for certain applications in
which amorphous structures in the lower ~.un range are
necessary. Moreover, subsequently increasing the
proportion of fines is uneconomical.
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A further problem which can occur in the
preparation of tetrafluoroaluminates is the residual
moisture which remains in the product and can have an
adverse effect on the desired applications.
It is therefore an object of the present
invention to provide a process for preparing
fluoroaluminates which does not have these
disadvantages.
The present invention provides a process for
preparing a complex fluoroaluminate, wherein the
fluoroaluminate is obtained from a suspension in which
the fluoroaluminate is present by spray drying.
In this process, the suspension in which the
fluoroaluminate is present is firstly fed to the spray
dryer. It is preferably fed to the spray drying via a
metering device.
In the process of the invention, there are in
principle no restrictions in respect of the way in
which the suspension in which the fluoroaluminate is
present is sprayed. It is possible to use, for example,
rotary disk atomizers, hydrodynamic introduction of the
feed via single-fluid nozzles or introduction by means
of compressed air via two-fluid nozzles. In one
embodiment of the process of the invention, the
suspension in which the fluoroaluminate is present is,
for example, fed via a controlled spindle pump to a
spray dryer and introduced via a rotary disk atomizer
having a diameter of 150 mm and a rotational speed of
16,000 rpm.
The product can likewise be discharged by all
conceivable means. Examples which may be mentioned are
cyclone discharge or discharge via one or more
dedusting filter units.
The temperatures of the hot air stream employed
in spray drying can be selected essentially freely and
are in principle limited only by the melting point of
the fluoroaluminate and the circumstances of the plant.
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The choice of temperature- allows, inter alia,
the drying capacity to be influenced and the residual
moisture content of the spray-dried material to be
controlled. This makes it possible to match the
moisture content of the fluoroaluminate to the demands
made of the material by the user. In general, the
temperatures are in a range from 100 to 500°C, which,
for example, gives a relatively low residual moisture
content of the fluoroaluminate of < 1o by weight.
It is of course also possible to employ two or
more spray drying steps in the process of the
invention.
It is likewise possible, in a modification of
the process, to follow spray drying by a further drying
step according to the prior art, for example fluidized
bed drying.
A further advantage of the process of the
invention is the influence on the particle structure
of
the fluoroaluminate which can be exercised by spray
drying. The choice of the dispersion device in the
spray dryer makes it possible to influence the part icle
structure.
A further possible way of controlling the
particle structure and in particular the particle size
distribution and also the floury appearance of
fluoroaluminates generally required for practical use
is to add a structure-influencing substance in the
process of the invention at a suitable point in the
preparation of t:he suspension in which the
fluoroaluminate is present.
The present invention therefore also provides a
process as described above, characterized in tha t
a
structure-influencing substance is used in the
preparation of the suspension in which the
fluoroaluminate is present.
A finely structured and floury appearance of
the fluoroaluminate can be obtained when the
fluoroaluminate is obtained from a solution which may
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comprise one or more precursors and the structure-
influencing substance is added to the solution before
the fluoroaluminate is obtained from the solution.
In particular, the structure-influencing
substance is used in a process in which the
fluoroaluminate is obtained as a solid by precipitation
from the solution in which the fluoroaluminate is
present.
The present invention accordingly provides a
process, characterized in that it comprises the steps
(i) to (iv)
(i) preparation of a solution comprising a
precursor of the fluoroaluminate;
(ii) addition of the structure-influencing substance
to the solution from (i);
(iii) precipitation of the fluoroaluminate from the
solution obtained from (ii) to give the
suspension in which the fluoroaluminate is
present;
(iv) spray drying of the suspension obtained from
(iii) to give the fluoroaluminate.
The precipitation is preferably carried out by
addition of aqueous alkalis. Particular preference is
given to precipitating the fluoroaluminate by addition
of an aqueous potassium hydroxide solution.
The concentration of the aqueous potassium
hydroxide solution is relatively uncritical and can
extend from a very low concentration to the highest
possible concentration. It is preferably in the range
from 40 to 50% by weight.
It is likewise possible to use solutions which
comprise not only KOH but also further K+-donating
components. These may be, for example, KZC03 or KCl.
In a preferred embodiment of the process of the
invention, the solution is stirred during the
precipitation of the fluoroaluminate. It is possible,
if required, to optimize the particle structure of the
fluoroaluminate by selection of a suitable stirrer.
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The temperature at which the fluoroaluminate is
precipitated in the process of the invention is
generally in the range from 0 to 100°C, preferably in
the range from 60 to 90°C, particularly preferably in
the range from 65 to 85°C, more particularly preferably
in the range from 65 to 80°C and in particular about
70°C.
In the process of the invention, the pH of the
suspension obtained after precipitation of the
fluoroaluminate is set to a value which is preferably
in the range from 4.5 to 7.0, particularly preferably
in the range from 5.5 to 6.5 and in particular 6.
In the process of the invention, it is in
principle possible to control the particle structure of
all fluoroaluminates which can be prepared by the
above-described process by the use of a structure-
influencing substance.
According to the present invention, particular
preference is given to controlling the particle
structure in the preparation of tetrafluoroaluminates,
in particular potassium tetrafluoroaluminate.
The present invention therefore provides a
process as described above, characterized in that the
fluoroaluminate is potassium tetrafluoroaluminate.
If the fluoroaluminate is prepared by the
above-described process of the invention, the solution
from step (i) can comprise any conceivable precursors
from which this fluoroaluminate can be obtained.
In particular, the solution prepared in step
(i) of the process of the present invention comprises
tetrafluoroaluminic acid as precursor from which the
potassium tetrafluoroaluminate preferably prepared is
obtained.
The present invention therefore also provides a
process as described above which is characterized in
that the precursor of the fluoroaluminate is
tetrafluoroaluminic acid.
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Such a solution comprising tetrafluoroaluminic
acid can, for the purposes of the process of the
invention, be prepared by all methods known from the
prior art.
According to the present invention, this
solution is preferably prepared from hydrated aluminum
oxide and an aqueous solution of hydrogen fluoride. Use
is generally made of commercial hydrated aluminum
oxide. The A1203 content of the hydrated aluminum oxide
is preferably in the region of 65o by weight. It is of
course also possible to use hydrated aluminum oxides
having a lower concentration, for example those
obtained from recycling plants.
The hydrated aluminum oxide and the aqueous
solution of hydrogen fluoride are mixed with one
another in such amounts that the molar ratio of A1:F is
generally in a range from 1:3.9 to 1:4.5, preferably in
a range from 1:4.0 to 1:4.4, particularly preferably in
a range from 1:4.1 to 1:4.3 and in particular about
1:4.2.
The concentration of the resulting solution of
tetrafluoroaluminic acid is, in the process of the
invention, set so that it is generally in a range from
5 to 40o by weight, preferably in a range from 10 to
30o by weight and particularly preferably in a range
from 15 to 20~ by weight. Depending on the
concentration of the hydrated aluminum oxide used and
the aqueous solution of hydrogen fluoride, it may be
necessary to add additional solvent to the
tetrafluoroaluminic acid solution so as to bring the
concentration to within the ranges described.
Here, it is in principle possible to use all
solvents which are suitable for this purpose and which
do not interfere in the later isolation of the
fluoroaluminate. Preference is given to using water as
solvent in the process of the invention.
In step (ii) of the process of the invention,
the structure-influencing substance is added to the
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solution comprising the precursor of the
fluoroaluminate, in particular the tetrafluoroaluminic
acid.
However, it is likewise conceivable to add the
structure-influencing substance either to the hydrogen
fluoride solution or to the solution in which the
hydrated aluminum oxide is present or to both solutions
prior to the preparation of the solution in which the
precursor of the fluoroaluminate is present.
The structure-influencing substance can be
added as a solid or as a liquid, depending on the
physical state of the structure-influencing substance.
In the case of a solid structure-influencing
substance, it is preferably firstly dissolved in a
suitable solvent before the addition.
For the present purposes, the term "suitable
solvent" means that the structure-influencing substance
dissolves in this solvent and the solvent does not
interfere in the later precipitation of the
fluoroaluminate.
It is of course also possible to use a mixture
of two or more suitable solvents. Particular preference
is given to using water as solvent.
It is likewise conceivable t~o suspend a solid
structure-influencing substance in a suitable liquid or
in a suitable liquid mixture and to add the resulting
suspension to the solution comprisi-rlg the precursor of
the fluoroaluminate.
Should the addition of the structure
influencing substance to the solution comprising the
precursor of the fluoroaluminate be exothermic, it may
be necessary to remove all or some of the heat
generated by methods known from the prior art.
The amount of structure-influencing substance
added to the solution obtained from step (i) is, in the
process of the invention, calculated so that, based on
the theoretical yield of fluoroaluminate, the
concentration of the structure-influencing substance in
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the solution is generally in the range from 0.01 to 1~
by weight, preferably in the range from 0.05 to 0.5o by
weight and in particular in the range from 0.1 to 0.20
by weight.
For the purposes of the present invention,
polyalkylene glycols has been found to be a
particularly useful group of substances by means of
which the particle structure of fluoroaluminates can be
controlled.
The present invention accordingly provides a
process as described above which is characterized in
that the structure-influencing substance is a
polyalkylene glycol.
Examples of polyalkylene glycols which may be
mentioned are: palyethylene glycol, polypropylene
glycol, polytetrahydrofurans, polypropylene glycol
ethoxylates or polyethylene glycol propoxylates.
Depending on the desired particle structure,
different polyalkylene glycols can be used. It is
naturally also possible to use a mixture of two or more
thereof .
It is likewise possible to use polyalkylene
glycols having different molar masses. Thus, for
example, it is conceivable to use polyalkylene glycol
which is made up of molecules having a uniform degree
of polymerization. It is naturally also conceivable to
a~>e mixtures consisting of collections of molecules
having different molar masses.
Should it be necessary for the purposes of the
wad- in which the process is carried out ~~nd/or the
desired particle structure of the fluoroalurninate, it
is of course also possible to use mixtures of two or
more different polyalkylene glycols of which each can
be molecularly uniform or polymolecular in the process
of the invention.
The polyalkylene glycols used in the process of
the invention can be prepared by all methods known from
the prior art. An overview of the most important
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preparative methods may be found, for example, in
Ullmanns Encyklopadie der technischen Chemie, Volume
19, 4th Edition, Verlag Chemie, Weinheim, 1980, pp. 31
to 38, the relevant contents of which are hereby fully
incorporated by reference into the present application.
Preference is given to using polyethylene
glycol as structure-influencing substance in the
process of the invention.
The present invention therefore also provides a
process as described above, characterized in that the
polyalkylene glycol is a polyethylene glycol.
In general, the molar mass of the polyethylene
glycol used is in the range from 200 to 40,000 g,
preferably in the range from 400 to 25,000 g and in
particular about 20,000 g.
The present invention accordingly also provides
a process as described above, characterized in that the
polyethylene glycol has a molar mass in the range from
200 to 40,000 g.
The present invention likewise provides a
complex fluoroaluminate which can be prepared by a
process comprising step (I) below:
(I) Obtaining the complex fluoroaluminate from a
suspension in which the fluoroaluminate is present
by spray drying.
In a preferred embodiment of the process of the
invention, complex fluoroaluminates having a particle
diameter which is generally in the range from 1 to 150
Vim, preferably in the range from 1 to 100 ~,m, are
obtained.
Furthermore, the complex fluoroaluminates
prepared according to the invention have a particle
diameter distribution which has a reduced proportion of
oversize particles compared to fluoroaluminates
prepared by processes of the prior art. In general, the
maximum of the particle diameter distribution is in the
range from 5 to 17 ~,m, preferably in the range from 7
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to 15 ~m and more preferably in the range from 9 to 13
~.m .
A further advantage of the process of the
invention is that the reduced proportion of oversize
particles makes it possible to prepare fluoroaluminates
for which further mechanical processing can be
dispensed with.
The process of the invention offers, inter
alia, the advantage that it gives complex
fluoroaluminates which have a lower melting point than
complex fluoroaluminates prepared by a process
according to the prior art. For example, the process of
the invention makes it possible to prepare potassium
tetrafluoroaluminate having a melting point which is in
the range from 540 to 550°C and is thus significantly
below the melting paint of the KA1F4 - K3A1F6 eutectic .
This melting point is far below the previously known
melting points, which are in the range from about 560
to 575°C for commercial products.
The present invention accordingly also provides
a process as described above which is characterized in
that the potassium tetrafluoroaluminate has a melting
point in the range from 540 to 550°C.
Since potassium tetrafluoroaluminate is used
predominantly as flux in hard soldering processes, the
low melting range is of particular economic and
industrial importance.
The present invention accordingly provides for
the use of a complex fluoroaluminate which can be
prepared by a process as described above or a complex
fluoroaluminate which can be prepared by a process
comprising the step (I) as described above in the field
of metallurgy.
The present invention likewise provides for the
use of a complex fl.uoroaluminate as described above,
characterized in that it is used as flux, in particular
in hard soldering processes.
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The present invention further provides for the
use of spray drying for controlling the macroscopic
structure of a complex fluoroaluminate.
The invention likewise provides for the use of
a polyalkylene glycol for controlling the macroscopic
structure of a~complex fluoroaluminate.
Furthermore, the invention also provides for
the use as described above, characterized in that the
polyalkylene glycol used is polyethylene glycol.
The present invention additionally provides for
the use as described above, characterized in that the
complex fluoroaluminate is potassium
tetrafluoroaluminate.
The present invention is illustrated by the
following examples.
Examples
Example
1000 g of technical-grade hydrated aluminum
oxide having an A1203 content of 65o by weight were
reacted at room temperature with 2150 ml of an aqueous
hydrogen fluoride solution having an HF concentration
of 42.60 by weight.
The solution obtained had, after addition of
water, an HA1F4 concentration of l8.Oo by weight.
2.7 g of a polyethylene glycol having a molar
mass of 20,000 g were added to the HAlF4-containing
solution.
1300 ml of a 45o strength by weight aqueous KOH
solution were added to the above solution over a period
of 10 minutes, with the temperature of the solution
being maintained at 70°C.
The pH of the resulting suspension was set to 6
using an electronic pH measurement.
The suspension was subsequently spray dried at
a temperature of 130°C.
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The particle size distribution of the potassium
tetrafluoroaluminate obtained was determined using an
HR 850-B granulometer from Cilas Alcatel (Figure 1a).
In Figures 1a and 1b, the diameter d in ~.un is
plotted on the abscissa and the amount of material D
below this size is plotted in o on the ordinate.
Comparative Example
The comparative example was carried out in the
same way as the above example. The only difference was
that no polyethylene glycol was added.
As in the above example, the particle size
distribution was determined using an HR 850-B
granulometer from Cilas Alcatel (Figure 1b).
Comparison of the particle size distributions
of the example and the comparative example clearly
shows that the proportion of oversize particles was
significantly reduced when the structure-influencing
substance polyethylene glycol was added.
