Note: Descriptions are shown in the official language in which they were submitted.
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FILTER MEDIUM IN T~E FORM OF A STABLE POROVS BODY
The invention relates to a filter medium in the
form of a stable porous body of granules of a fire-resistant
material bonded together.
From U.S. Patent Specification 3,524,548 there is
known a rigid porous filter for filtration of molten aluminium,
which consists of a fired granulate-like fire-resistant ma-
terial, which is not attacked by molten aluminium, and which
has as binder a glass-like material, which contains not more
than 10% silicates.
As granulate there is mentioned "fused alumina" or
"tubular alumina". With "fused" or "tubular alumina" one is
dealing with fused corundum broken into pieces. This material
produces a Eilter with relatively slight permeability and
porosity. The filter effectiveness and filtering capacity
is restricted by the internal structure. For this reason in
practice bundles o~ filter tubes are normally installed, in
order to achieve the desired amounts of flow.
It is also known from German OS 22 27 029 that such
kinds of rigid filter elements, for example in the form of
tubes, are very fragile.
It can be assumed that this fragility has its basis
at least partially in the fact that in the firing process un-
avoidable stresses and consequentia] points of fracture arise.
Additionally disadvantageous is the high wei~ht of the filter
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elements according to U.S. Patent Specification 3,524,54~ and
the long preparatory heating up time thereby caused, before
the molten aluminium can be directed through the filter element.
For the start of the filtration and also during the
filtration relatively large pressure differences must prevail,
in order to drive the molten aluminum through the filter
elment.
By means of a filter element of an entirely differ-
ent kind an attempt has been made to eliminate the disad-
vantages, such as great pressure differences during filtration
and restricted filtering capacity. In Swiss Patent Specif-
ication 6~2,230 a filter element is described, which is manu-
factured by impregnation of a polyurethane foam with a cer-
amic suspension, pressing out of the excess suspension, drying
and firing. According to this method one obtains an exact
replica of the original organic foam in rigid ceramic form.
Filter elements of this kind have a high filtering capacity
and high rates of through flow, and thus enable themselves
to be employed in the form of simple filter plates. There
is inherent in these filter elements the disadvantage that
they are expensive in manufacture.
These filter elements are relatively poorly wetted
internally by the metal and for that reason operate in the
majority of cases as surface filters.
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The object of the invention is to overcome the dis-
advantages mentioned and to provide a filter medium, which
can be manufactured easily and in consistent quality, that
has a good filtering efficiency, is well wetted by the ma-
terial to be filtered, and possesses a high filtering capacity.
According to the invention this is attained with
a filter medium in the form of a stable porous body of granules
of fire-resistant material bonded together is characterized
by a porosity of 5 to 45% by volume, a permeability of 2 to
200 ~Pm and a mean diameter of 0.1 to 30 mm.
- The porosity serves to express~ how large
are the spaces which can ~e flowed thLough, be-tween the spheres,
reckoned on the total volume of a filter body. As a space
is designated only the space delimited by the curves of the
grains but however possible cavities in ~he interiors of the
grains. According to the invention the porosity amounts to
5 to 45~ by volume, suitably 20 to 40% by volume and prefer-
ably 20 to 35~ by volume.
The permeability required according to the invention
is measured according to DIN standard 51058, and in the pre-
sent case is expressed in microperm ~ Pm)~
In the present invention the values for permeability
amount to 2 to 200 ~Pm, suitably 2 to 50 ~Pm and preferably
10 to 30 ~Pm.
The granules of fire-resistant material have a
spherical shape or an approximately spherical shape. However,
lens-shaped or drop-shaped granules can also be used within
the meaning of the invention. According to the method of man-
ufacture for such granules mixtures of different external
shapes can also
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be obtained. The granules can exist in solid or preferably
hollow form, where the fire-resistant material in this case
constitutes only an ou~er shell, but structures can also be
employed built up of concentric shells or from a plurality
of cells individually open or closed, which are bonded ex-
ternally by a shellO The shells are not obliged to be con-
tinuously impervious, porosities or points oE fracture, in
the shells, even at random, by subsequent breaking down of
granules provided in spherical form can be employed within
the scope of the invention. These different kinds of granule,
that is to say solid spherical, hollow spherical or broken
granules can be mixed in any proportions. As fire-resistant
material, the ceramic materials familiar to the expert can
be employed. Selection is governed in the first place ac-
cording to the requirements, which the material to be filtered
imposes on the filter as regards chemical stability, heat re-
sistan~e, rigidity, durability, formability and wettability.
Among the materials suitable for employment there
are metallic oxides, such as aluminium oxides, for example
as coxundum, boehmite, hydrargillite~ bauxite, SiO2, e.g.
perlite, silicates, such as mullite, aeromullite, silimanite
or chamotte, then magnesium oxides and magnesium silicates,
such as steatite, forsterite, enstazite and cordierite, as
well as dolomite and mixtures of the oxides mentioned.
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As further metallic oxides there are zirconium oxide,
stabilised or uns~abilised in monoclinic, tetragonal and/or
cubic form; tin oxide with or without doping; aluminium
titanate, calcium silicates, calcium-magnesium-silicates,
magnesium-aluminium-silicates, zirconium silicates, calcium
aluminates, iron-chromium-oxides, aluminium hydroxides, high
melting point glasses, boron carbide, titanium carbide,
titanium diboride and zirconium diboride, silicon carbide,
silicon nitride and its mixed crystals, and also all spinels
and perowskites. To be reckoned with the fire-resistant ma-
terials there are in the present case also carbon, especially
in the form of graphite, coke or pitch and also their mixtures.
Suitably the granules of fire~resistant materials
include aluminium oxides, preferably as corundum, bauxite,
zirconium oxide or spinels.
Mixtures of various individual components in differ-
ing proportions can also be used.
Spherical-shaped fire-resistant material is manu-
factured in a manner known per se. As a rule one obtains
spherical granules by roll granulation, spray granulation or
by atomising and sintering thereafter.
The manufacture of hollow spheres is also known.
One can blow a stream of material to be cast, for
example of liquid corundum, by means of compressed air or
steam. In so doing one obtains hollow spheres of up to 5 mm
diameter.
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One can however also by means of a gas phase op-
eration subject a blowable slip, which for example contains
very finely divided high melting point oxides and either sub-
stances yielding carbon dioxide, or hydrogen peroxide, as
hlowing agent, to a mechanical dispersion, suitably by drip-
ping and/or blowing, and drying and firing the resulting drops.
In similar manner, one can manufacture spherical
granules by the known sol-gel method.
The granules of spherical shape have a mean diameter
of 0.1 to 30 mm. The minimum granule size should amount to
0.08 mm, the maximum granule sîze to 36 mm.
The suitable mean granule diameter amounts to 0.5
to 8 mm, with a minimum granule size of 0.~ mm and a maximum
granule size of 9 mm.
Preferably hollow spherical granules are employed
with a mean diameter of 0.5 to 5 mm.
The granules are so bonded together that a point
of connection between two granules requires 0.1 to 15%, suit-
ably 0.1 to 5~, preferably 0.5 to 1.5~ of the surface of that
sphere. For spherical-like granules such as lens-shaped or
drop-shaped granules, the same percentage amount of the outer
surface similarly applies. In all cases the data applies to
the calculated surface, which xesults from the mean radii of
the granules and not from a special microsurface which can
result from the internal structure of the fire-resistant ma-
terial.
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The connection of the granules together can take
place in various ways~ The granules can be bonded by a
different phase, which has a chemical character, where one
can employ phosphates, such as aluminium orthophosphate,
phosphoric acid, magnesium orthoborate, aluminium hydro~y-
chloride and/or silica gel.
Furthermore they can be ceramically bonded by
glasses, for example silicate or boron glasses and/or by the
employment of glass-forming substances or by very finely
divided material applied to the surface, which corresponds
in its composition to the heat-resistant material in question.
An example for the last embodiment would be corundum spheres,
which are coated or mixed with a very finely divided amorphous
aluminium oxide powder in the angstrom range. The very finely
divided powder sinters at low temperatures into coarsely
granuled powder and is thereby able to form a body which is
homogeneous as to material, rigid and high refractory.
By suitable choice of granules and choice of a
ceramic binder, a body which is homogeneous as to material
can also be achieved ~n that the binder and the fire-resistant
material enter into mutual reaction and by formation of a
new highly refractory material produce a highly refractory
bonding.
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It is also possible to bond the granules together
without addition of a different phase. The granules are simply
sintered together into mutual bonding.
The filter media can be modified according to their
purposes of use.
sy coating of the free granule surface within the
filter medium with activated aluminium oxide, with the
activated aluminium oxide amounting to 3 to 40% by weight
of the total filter medium, a BET surface of at least 10 m /g
can be achieved.
For this purpose the filter medium is suitably
coated with a slip of activated ~ or~ - alumina, preferably
~ - alumina as raw material, and a small quantity of binder,
for example colloidal silicic acid, and then activated.
The filter medium can be coated with carbon, with
the carbon amounting to 3 to 40% by weight of the total filter
medium. By carbon can be understood also coke, pitch and
graphite.
A further possibility lies in coating the free
granule surfaces of the filter medium alone or in addition
to other treatments with 0.5 to 10~ by weight, reckoned on
the total weight of the filter medium, with a flux for metals.
Salts such as chlorides or fluorides serve as fluxes
for metals. For example for aluminium Na3AlF6, NaCl, KCl,
CaF2, AlC13 LiF or their mixtures are employed.
A further advantageous embodiment lies in that
ceramic fibres are contained in the fire-resistant material
or on the
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granule surface in quantities of 0.01 to 10% by weight reckon-
ed on the quantity of fire-resisting material, and the ceramic
fibres extend out beyond the granule surface with at least
one of their fibre ends.
As ceramic fibres there can be used fibres of
aluminium oxides, aluminium silicates, zirconium oxides, boron,
silicon carhide or carbon. Within the scope of the present
invention, there also lie all naturally occurring mineral fibres.
The filter structure can be arranged in different
ways. It is possible to maintain a homogeneous distribution
of granules through an entire filter element. One can adjust
the granule distribution according to requirement, purpose
and desired geometry of the filter element.
Thus the filter medium, either in the direction of
filtration, or perpendicular to it, can have a progression
of the mean diameter of the granules from fine to coarse or
from coarse to fine.
Also progressions of the mean diameter o the
granules from fine via coarse to fine or from coarse via fine
to coarse can be freely selected.
By fine is to be understood a mean diameter of the
granules of 0.1 to 3 mm, by coarse from 3 to 30 mm.
The filter media according to the invention are man-
ufactured in that one selects the spherical granules homo-
geneously or in mixture of solid spherical, hollow spherical
and/or broken granules with reference to their diameter and
if necessary mixes them.
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By distribution of granules one can define the
porosity, that is to say the proportion of space, which is
available for the material to be filtered, and thus also the
permeability.
The granules or the granule mixture, as the case
may be, are mixed with the inorganic binder and a medium which
binds and pre-hardens on heating, to produce a sufficient
green strength. Preferably one pre mixes the chemical or
ceramic binder and the pre-hardener medium, and only then
mixes in the fire-resisting material.
As pre-hardener medium there come into question or-
ganic compounds, such as carboxymethyl cellulose, poly~inyl
alcohols, dextrine, sulphite waste liquors, etc. and inorganic
compounds, such as mono aluminium phosphate, calcium aluminate,
alone or mixed together. As a rule the pre-hardener medium
works in aqueous solution.
The pre-hardener medium has the purpose to
impart binding or adhesive properties to the individual
granules at the beginning, and to produce from the granule
mixture a formable mass up to the final firing. As a rule
the mixture of granules, if necessary, the binder and the pre-
hardener medium is mixed with water in a known manner, such
as by milling or stirring.
The shaping of the mixed mass can take place by
various methods such as stamping, jigging or casting in a
mould~ uniaxial or isostatic pressing or by extrusion. A
drying process is carried out in dependence on the kind and
composition of the
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medium, as a rule at 80 to 100C, and produces a good green
strength of the shaped body after at the latest 24 hours.
The ceramic firing takes place in a gas or electric
oven at temperatures which are dependent on the kind of binder,
and also in dependence on the composition of the fire-
resistant material. For filter media whose granule mixture
is bonded chemically, temperatures around 1000C are suf-
ficient, for granule mixtures which are bonded by glass,
temperatures between 700 and 1600C must be maintained. ~or
the case in which a sel~-bonding by sintering is aimed at,
the calcining temperature is adjusted according to the in-
di~idually known sintering ranges o~ -the fire-resistant ma-
terial, but reaches a maximum of 2000C.
According to the method of the invention the cycle
cold-to-cold amounts as a rule to less than 48 hours. By the
cold-to-cold cycle is understood the period in which the green
body is heated from room temperature to the maximum firing
temperature and is cooled down again to room temperature.
This short baking period is explainable in that the
spherical granules within the granule mixture create no heat
stresses or only very slight ones, and thus lead to very
strong ~ired bodies. The binder and pre-hardener medium
vaporises or burns away completely without residue, at the
latest during the baking process.
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The construction of the filter medium according to
the invention already corresponds in the green condition to
a closest packing of the spheres. In this way is attained
a minimisation of the contraction usually occurring in the
sintering of refractory materials because of transpositions
and diffusion processes.
Filter media manufactured according to the invention
are employed for filtration of molten metals. In a preferable
embodiment he filter media according to the invention are
employed for filtering of molten aluminium or iron.
The filtration of molten copper, copper alloys, grey
iron, titanium, etc. is likewise possible.
According to the melting point and the filtration
temperature of the metal, the choice must be taken of the heat
resistant material and of the inorganic binder.
Filter elements can be manufactured in almost any
desired shape and size. With the emplo~ment of hollow
spherical granules relative low specific gravities are however
attained, so that even large filter elements are selE~suppor-t-
ing and resistant to thermal change. A preferred embodimentis that the filter medium has the form of a plate with bevel-
led edge surfaces. Such a plate can for example be installed
in place of a filter plate such as is described in Swiss Patent
Specification 622 230.
Besides filter plates, also filter tubes, filter
pots and filter blocks can easily be manufactured.
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Example
75 kg hollow spherical corundum of granule si~e
1.6 - 2.0 mm were intensively mixed in an intensive mixer
with a mixture of 15 kg glaze raw mixture and 10 1 carboxy-
methyl cellulose solution for 2 min. The glaze raw mixture
consisted of 30~ SiO2, 30% potash feldspar, 15% calcium car
bonate, 5% calcium silicate, 17% kaolin and 2.5% alumina, in
a grain size of smaller than ~0 micron. The bulk density of
this glaze raw mixture amounted to 1.5 ~/1. The mixture of
hollow spherical corundum, raw glaze and carboxymethyl
cellulose had a dry consistency. A part of this mixture was
jigged into prepared metal frames of size 30 x 30 x 5 cm with
bevelled walls and smoothed on the surface with a metal roller.
The metal frames together with the ceramic material, were
thereupon placed in an electric drying oven and dried for 24
hours at 80 - 100C. ~ter the drying the ceramic material
could be removed, and had a self-supporting consistency with
good strength at the edges.
Thereupon the raw filters were placed in an electric
oven and fired to a maximum of 1280C. The holding time amount-
ed to 10 minutes, the heating up and cooling rate amounted
to about 100C per hour - the linear shrinkage amounted to
0~ .
The fired filters exhibit the following character
istics:
Colour: white
Volume
Weight 3.0 kg
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Bulk density 0.7 kg/l
Permeability, measured according
to DIN 51 058: 14 - 15 Microperm
sending strength, measured on
15 test bars of 25 x 25 x 100 mm
with support radius 14 mm,
support spacing 50 mm, determined
according to the 3 point method: 230 ~ 50 N/cm2
Cold compression strength 410 + 50 N/cm2
10 Edge strength: good
A filter manufactured in the described manner was
installed in a prepared filter trough, as is described in Swiss
Patent Specification 622 230, and preheated with direct gas
flame to about 400C. An aluminium alloy with the identi-
fication AlMg 0.4 Si 1~2 was now supplied at a rate of flow
oE 75 kg/min. The metal temperature amounted to 700C. The
depth of metal above the filter plate amounted to 400 mm,
the pressure difference of inlet and outlet at the beginning
of casting 20, at the end 27 mm. The depth of metal above
a filter according to U.S. Patent Specification 3 524 548 in
a comparative experiment amounted to 600 mm, the pressure
difference at the beginning 30 mm, at the end 40 mm.
In total 12 t of metal were cast in rolling bars
in the format 318 x 1250 x 3100 mm. This occurred by three
pourings through a filter plate according to the invention.
Between the pourings the filter was held at its temperature
by flame heating.
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At the end of casting the filter loaded with metal
was removed and after cooling was cut up and examined metallo-
graphically. Then it appeared that the impurities in the
form of magnesium-aluminium oxides were deposited throughout
the entire filter especially in the zones between the abutting
spheres or in the interior of the hollow spheres, as well as
in the uppermost zone of the filter plate. The titanium di-
boride added as grain refining means could be identified as
accumulated on the surface of the spheres. The space occupied
by the aluminium in this filter was determined, in order to
obtain a measure for the homogeneous penetration of the filter
by the metal. The space occupied by the aluminium after
correction for the volume component taken up by the filter
material itself, but without regard to the portion in hollow
spheres not accessible to the aluminium, was determined as
82%. In contrast to this the degree of space occupation in
a filter according to Swiss Patent Specification 622 230 with
an analogous pore size 40 ppi (pores per inch) was determined
at 55~.
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