Note: Descriptions are shown in the official language in which they were submitted.
3~
This invention relates to Mineral tubular
filter elements and more par-ticularly to filters formed
from at least one thin coating of at least one metailic oxide
and/or at least one metallic fluoride deposited on a
, . . . .
ceramic or metallic support~
Accordin~ to a known process, filter elements
... .
; of this kind are generally tubular and are made by moulding
; pulverulent inorganic material and heat treatment to
:~`
make the assembly mechanically strong, in accordance
with techniques which are now conventional ~n the ceramics
and powder metallurgy industries. ~hese elements may be
made from various metals and alloys such as steel, bronæe
and nickel or from oxides such as alumina, magnesia and
silicates, or compounds which are substantially refractory,
such as the fluorides, carbides, nitrides and borides.
To produce tubes, these pulverulent materials
are mixed with an organic binder and extruded under pressure
through an annular no~zle. The tubes are then dried
and heat treated to eliminate the binders and ensure
cohesion oE the powder particlesO
- This technique is very suitable for the
production of relatively thick tubes, for example of a
thickness of several millimetres.
. -- . .
; ' - ~
'~ .
~ 3~3~
These porous supports or filters, however, must
in most cases have a permeability which is as high as
possible to permit maximum flow and very srnall pressure
` drop in the fluids passing through them.
; 5 In view of these requirement~, therefore, an
attempt has been made to make such tubeswith ~ery small
thicknesses, but their industrial production is difficult
and complex and these tubes have a very low mechanical
strength,. Filter elements are thereore conventionally
made with an appreciable flow, i.e. relatively hi~h pore
dimensions, i~e. using powders ha~ing coaxse particles~
It is9 however, freqtlently necessary, particularly ~or the
separatlon of gaseous isotopes or for checking very fine
particles, that the said elements should have a very fine
`~ 15 teX-ture with very small pore radii. Such a requirement
~ is not compatible with the requirement of hiqh permeability,
`~ which is a prime requirement for the filter ele~ents in
.
question.
,.......................................................... .
To produce fiIter elements which satisfy these
requirements, it is now con~entional practice to make
multi-layer materials, i.e. materials which in respect
; of thickness are formed from a high-permeability layer
and large pore radii, and low-permeability layers with
~ery fine pore radii.
.
~ --3
; , . .
.:
... .
. . ' ' , ~ .
:', .
.. ;- .
,3~8
In actual fact~ and in the most usual cases 9
these filter elements are formed from a high-permeability
, .
tubular support providing the mechanical strength of the
,:.
;~ assembly, on which a very thin coating is deposited,
. 5 generally inside the tube and which imparts its own
'.~ characteristics as to flow and given pore radii. The .
radius of the tubular support pores is usually between
1 and 20 microns~ ' Of course the coating must be as
thin as possible in order not to reduce the permeability
of the assembly excessively.
Since the inside diameter o* the high-
.: permeability support is oftensmall t it is not possible to
~.
deposit the fine coating by spraying a suspended powder,
whether electro-statically or otherwise, as is conventional
; 15 practice in painting. Various coating processes have
therefore been proposed, for example: spraying dry powder
. inside a porous tube previously impregnated with a volatile
liquid. Because of the difficulty of feeding a nozzle
with a dry powder, this pL-OCeSS results in het~rogeneous
20 deposits of non-uniorm thickness~ said deposits particularly
.. belng very fragile. In practi.ce ~t is impossible to
: handle these elements after coating without damaging them
and this very slow process is not an industrial process.
In another coating method, a suspension.of the
mineral powder for deposition in a volatile liquid ~Jhich
, ' ' .
.~' ' .
' -..
.. ~ .
~' ', . I
3~8
is inert ~lith respect to said powder is filtered through
.
the porou~s supportin~ tube.
.- The thickness of.the coating can be adjusted
given an equal suspension concentration, by the filtering
time or by measuring the volume of filtered liquid. As
a result of turbulence phenomena during filtering, it is
not possible with this method to obtain very thin coatings
such as are required ~or the production of el.emenets
havin~ a high rate of flowO Also~ this process uses
heavy and complex equipment and very appreciable quantities
of suspension~
, Other methods which are still more complex
may be used, fbr example deposition by centrifugation;
- the tube filled with a suspension of the powder in a liquid
is r o tated about its axis so that the powder is deposited
. on the inner wall of the support; alternatively there is
-. coating by b~ushing of a paste formed from the powder
.1. and a solvent, and so on. All these techniquesrequire
.,~, .. . .
relat~vely complex apparatus which does not readily lend
; . 20 itself to large scale production of composite porous
elements and none o these techniques results in the
required characteristics, i~eO a very thin deposited
coating and an absence of faults in said coating.. The
i~ present invention provides considerable improvements in
the construction of these separator elements~
... : 5_
... .. .
.
~ ~ .
,
.. ,. ,~,. . .
...... .
... .
... . .
.;. .
~ 3
~'`'"
. The invention relates to a met~od o:E producing
a tubular filter element ha~ing both high permeability.
and very small pores, from a rigid metal or ceramic support
and at least one thin mineral. porous coat.ing, the or each
porous coa~ing having pores smaller than those o~ the
said support, said method comprislng the steps of bringing
the inner and/or outer wall of the support in-to contact
, with a liquid containing in suspension a mineral powder
which is required to form the porous coa~ing, said liquid
ensuring transport of the mineral powder and its adhesion
I . to the support, draining the support, drying it.,~compressin~
.`~ it and heating itL The resulting coating may be compressed
on its support by ~eans o a resilient diaphragm to compact
it and give it greater mechanical strength and, if required,
reduce the pore size which is between 0.001 and 1 micron.
It may also be heat-treated so as to be mechanically
~; consolidated~ The suspension of mineral powder in a
liquid adapted to ensure deposition on the porous suppo.rt
~ay contain a small proportion of one or more orgaI~ic
products which can act as a temporary binder to ensure
coheslon of the particles of mineral powder after elimina-
tion of the liquid, or as an agent to facilitate dispersion
or defloculation o said mineral powder or alternatively
- which can favourably modify the rheological characteristics
of the suspension.
. . .
.' ' .
~ -6- ~
... . . .
. ,:
,
",~,
, . .
3~)~
In this wa~ the nature, concentration of the
organic binder used~ and the pH of the suspension determine
the viscosity o said suspension and its yield value~
which have a great influence on the regulation of the
thick~ess of -the deposit and on the homogeneity of the
latter along the support. Using suspensions having
a Ne~nian flow, during draining o-~ the liquid the top
part of the support is stripped while the base is loaded ?
and this results in appreciable heterogeneity of the
deposit~ By selecting the constituents of the suspension
and, in particular~ the organic binder so that it has a
yield value or minimum stren~th point, the deposit can
on the other hand be perfec~ly homogeneous~ In this
connection see "Ceramique Génerale - Notions de Physico-
Chimie", Vol~ II, pa~es 137 to 144, by C. A. Jouenne~
According to one simple process, the filter
elements according to the invention may be constructed .
by filling the support or supports with the suspension,
by leaving them in contact therewith for a given time,
emptying out the suspension and allowing the element
coated in this way to drain and dryO This filter is
then subjected to a heat treatment adapted to eliminate
the organic binder or binders or volatile or chemical
. . . .
products and consolidate the deposited mineral film.
The invention also relates to apparatus for
. performing the method according to the invention9 said
... . .
. ~ -7_ .
. .
'.` ., ' '~ . .
-^ i .
~;
., .
33~)~
apparatus comprising a closed container formed with an
orifice f~or the supply of mineral powder suspension, a
venting conduit and a conduit for connecting the interlor
of the container to pressure, a supportinq tube imrnersed
5 to near the bottom of the container and adapted to support
the tubular element, and a control unit for the level of
the suspension at a tube surmounting the tubular element.
According to a specific embodiment of this
apparatus, the ends o the tubular elements are provided
10 with sealing-tight gasketsg the top part comrnunicating
with atube which, as a result of its transparency, allows
the suspension level to be monitored, and which is
el-~tr.'c~l
provided with a photoelectric cell or~probe system which
. ensures that coating stops when the level of the suspension
15 reaches said transparent part~ The bo-ttom end of each
tu~ular element communicates with a supporting tube
immersed in. he.suspension in a sealed tank to which
excess ~ir pressure is appliedO This excess pressure
. causes the suspension to rise in the support and results
20 in its coating~
One embodiment of the apparatus will now be
. ~ described by way.of example of the invention with reference
. to the accornpanying ~awlngs, in which the single figure
. is a.sectional ~iew of said apparatus~
. . . . .
. "~
, -8-
", .
. ~, .
-' ,.i~, . . .
:`:
, ,~ ' .
.`.: '
... .
33~
A tubular filter element 1, the ends of ~Jhich
are provided with two sealing-tight gaskets 2 and 3, ls
kept in communication with a glass tube 4 at the top and
with a metal supporting tube 5 at the bottom, the tube 5
. 5 being connected to a tank 6. The latter is provided with
.-~. a T-tube 7, the limbs.of which are provided with a
valve 8 and a val~e 9 which r.especti~ely allow the
ad~ission o~ a compressed gas and connection to atmospheric
pressureg The supp~ting tube 3 is immersed al~ost to
~0 the bottom of the tank which is filled with liquid
.: suspension 10, fillin~ being via the sealed orifice 11
~ The suspension level is mon.itored by the photoelectric
- cell 12, its receiver 13 and a control unit 14. Coating
is effected by placing the filter element 1 between the
: 15 tu~es ~ and 5, closing the atmospheric pressure venting
:;. valve 9 and opening the pressure gas admission valve 8.
., j .
. This pressure, which is applied to the liquid suspension 10,
~: causes the latter to rise in the supporting tube. When
` ~ the suspension level reaches the detection zone of the
photoelectric cell 12 and 13, the latter closes the
,: . valve 8 and opens the valve 9 via the control unit 14.
.~ The suspen~ion falls back into the tank 6. After
.~ d~aining, the support is disconnected from the machine and
: .. . .
`~: the operation can be re-commenced with a new support.
.; . 25 Of course, instead of using a pressure on the
. ~ .
~.~ liquid suspension, a negative pressure could be produced
~,.. . .
_g_
, ' ' ' ' '
, '
... . .
.~ . . .
~' ~
. .
.: .
. . .
33~
via the tube 4 so as to cause the suspension to rise
in the same way ln the support9 and this adaptation
is within the scope of the invention~
The invention is illustrated with the aid of
the following examples, which are given by way of example
only.
Exam~le 1: A tubular refractory ceramic support
having an outside diameter o-E 20 mm, a thickness of 2 mm,
a length of 500 mm and pores having radii of 5 microns,
~s filled with the suspension prepared as below:-
300g of electrically melted alumina powder having a
grain size very close to 5 microns, are suspended in
a well-homogenised solution of:
15g of carboY~ymethyl cellulose
lS 30g o glycerine
500cc of ethyl alcohol
... .
;~ 500cc of water~
This suspension is introduced into the support
~; ~ by means of the apparatus as described. After draining
and drying at ambient temperature and then in an oven the
element obtained in this way is provided with two inner
,
and outer polymer diaphragms and subjected to an isostatic
compression of 1500 bars in a water-filled press pot~
.
The ob;ect of this compression is to make the deposited
layer very compact and anchor said layer in the support,
: ', ' ' .
:, ' .
-10--
: . ~
. ~ ' '' . ' ` `~ ~
` .
~ ~t7~
After this operation the layer is subjected to heat
treatment at 1800C ~or one hour. The resulting filter
has a very high permeability and the radius of the pores
of the deposited layer or coating is 1 micron~
Example 2: A tubular support identical to that in
Example 1 is coated by means of the following suspension:
300q of a calcined alumina powder of a grain siz~
between 1 and 15 microns and a specific s~rface of
8m per gram are added to X litre of an aqueous gel
containing 0.2% of ethyl methyl cellulose. The
suspension is agitated and then left for two days
, and finally screened on a 40 micron cloth. The
- coating vbtained has a thickness of 20 microns and
` ~ is compressed, as in Example 1, between two diaphragms
- 15 at a pressure of 800 bars. After heat treatment in
. .
air at 1500c,~the resulting element has pores of a
;;` radius of 0.~ micron on average~
~ ~ : A sintered nickel tube having an outside
~ ,.
` ~ diameter of 15 mm, a thickness of 0.5 mm and a length of
500 mm is coated with a suspension formed rom the
~"' follol,Jing:
, . .
;'~ 250g of calcium fluoride of a grain size of the
order of 3 microns
2 g of polyvinyl aicohol
~ 25 1 litre of water.
,''~.'~ ,
,,, ' -11-
:.. ' . , ' ' ` ~ .
;- .
1~'7~33~?~3
A-Eter deposition, dryin~, comp~resion at 500
bars and heat treatment at 550C, the resulting element
has a ~ery high permeability and a pore radius of
approximately 0O3 microns~
' , '
. ' ' .
. . ~
.
. ' 1' , ' ' .
.'~' '.' .
.. . .
,. . . .
," .:
,, j .
.,~ ' .
'.~ .
--~2--
~'' '
-
:` .
