CA1208903A

CA1208903A - Filter medium in the form of a stable porous body

Info

Publication number: CA1208903A
Application number: CA000412790A
Authority: CA
Inventors: Wolfhart Rieger; Konrad Kampfer; Ludwig Gauckler; Albert Maurer
Original assignee: Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1981-10-06
Filing date: 1982-10-04
Publication date: 1986-08-05
Also published as: AU8888682A; ES8308706A1; ZA827115B; ES516241A0; NO823309L; EP0076781A2; AU560797B2; JPS58131115A; EP0076781A3; DE3140098A1

Abstract

A B S T R A C T

Filter medium in the form of a stable porous body Filter medium in the form of a stable porous body of granules of spherical form bonded together by a different phase or by sintering. Preferably hollow spherical granules of corundum are manufactured into filter media in plate form.
The filter media are employed for filtration of molten metals preferably aluminium.

Description

~Z~ ù^3 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 _4_ 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.

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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~8~3 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.

~%~9~3 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.

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 ~9_ ~ZQ~

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.

~:'' .1 ~10-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 `~ -11-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.

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.

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 ~2~ 3 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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~J~ -15-~2~ 3 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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A filter medium in the form of a stable porous body of granules of a fire-resistant material bonded together, wherein, the filter medium has a through-flow porosity of 5 to 45% by volume and a permeability of 2 to 200 µPm, that the granules of high-resistant material have a spherical shape with a mean diameter of 0.1 to 30 mm and the spherical granules are thermally bonded together in such a way that a point of connection between two granules requires 0.1 to 15%
of the surface of that sphere.

2. A filter medium according to claim 1, wherein, the filter medium has a flow-through porosity of 20 to 40%
by volume, and a permeability of 2 to 50 µPm, that the granules of fire-resistant material have a mean diameter of 0.1 to 10 mm, and that a point of connection of the granules requires 0.1 to 5%, of the surface of that sphere.

3. A filter medium according to claim 2, wherein said flow-through porosity is 20 to 35% by volume, said permeability is 10 to 30 µPm, said mean diameter is 0.5 to 8 mm and said point of connection of the granules requires 0.1 to 1.5% of the surface of that sphere.

4. A filter medium according to claim 1 or 2, wherein the fire-resistant material has a spherical form, is hollow and has a mean diameter of 0.5 to 8.0 mm.

5. A filter medium according to claim 1, 2 or 3, wherein the granules of fire-resistant material contain aluminium oxide, bauxite, zironium oxide or spinels.

6. A filter medium according to claim 1, 2 or 3, wherein said granules of fire-resistant material contain corrundum.

7. A filter medium according to claim 1, 2 or 3, wherein the granules are bonded together by their own material, or are bonded together by an inorganic binder as a different material.

8. A filter medium according to claim 1, 2 or 3, wherein the free granule surface is coated with activated aluminium oxide, with the activated aluminium oxide amount-ing to 3 to 40% by weight of the total filter medium and with the specific surface amounting to at least 10 m2/g.

9. A filter medium according to claim 1, 2 or 3, wherein the free granule surface is coated with a flux for metals, amounting to 0.5 to 10% by weight, based on the total weight of the filter medium.

10. A filter medium according to claim 1, wherein ceramic fibres are contained in the fire resistant material or on the granule surface in a quantity of 0.01 to 10% by weight, based on the quantity of fire-resisting material, and the ceramic fibres extend out beyond the granule sur-face with at least one of their fibre ends.

11. A filter medium according to claim 10, wherein said ceramic fibres are present in a quantity of 0.1 to 5%
by weight.

12. A filter medium according to claim 1, 2 or 3, wherein the free granule surface is coated with carbon, with the carbon amounting to 3 to 40% by weight of the total filter medium.

13. A filter medium according to claim 1, 2 or 3, wherein the filter medium has a progression of the mean diameter of the granules from fine to coarse in the direction of filtration.

14. A filter medium according to claim 1, 2 or 3, wherein the filter medium has a progression of the mean diameter of the granules from coarse to fine in the direction of filtration.

15. A filter medium according to claim 1, 2 or 3, wherein the filter medium has a progression of the mean diameter of the granules from coarse to fine perpendicular to the direction of filtration.

16. A filter medium according to claim 1, 2 or 3, wherein the filter medium has a progression of the mean diameter of the granules from fine to coarse perpendicular to the direction of filtration.

17. A filter medium for filtering molten metal in the form of a stable porous body characterized by a plurality of spherical shaped granules of fire-resistant material having a mean diameter of from about 0.1 mm to 30 mm thermally bonded together such that the surface area of the point of contact between any two granules is from about 0.1% to 15% the surface area of the granules, said filter having a through flow porosity of about 5% to 45% by volume and a permeability of about 2µPm to 200µPm.

18. A filter according to claim 17 wherein said spherical shaped granules have a mean diameter of from about 0.1 mm to 10 mm and a point contact surface area of from about 0.1% to 5%, said filter having a through flow porosity of about 20% to 40% by volume and a permeability of about 2 µPm to 50 µPm.

19. A filter according to claim 17 wherein said spherical shaped granules have a mean diameter of from about 0.5 mm to 8 mm and a point contact surface area of from about 0.1% to 1.5%, said filter having a through flow porosity of about 20%
to 35% by volume and a permeability of about 10 µPm to 30 µPm.

20. A filter according to claim 17 wherein said spherical shaped granules are hollow and have a mean diameter of from 0.5 mm to 8 mm.

21. A filter according to claim 17 wherein said spherical shaped granules are formed in part of a material selected from the group consisting of aluminum oxide, corundum, bauxite, zirconium and spinels.

22. A filter according to claim 17 wherein said spherical shaped granules are bonded together by their own material.

23. A filter according to claim 17, wherein said spherical shaped granules are bonded together by an inorganic binder.

24. A filter according to claim 17 wherein the surface of said spherical shaped granules is coated with active aluminum oxide.

25. A filter according to claim 24 wherein said aluminum oxide is about 3% to 40% by weight of the total filter.

26. A filter according to claim 17 wherein the surface of said spherical shaped granules is coated with a flux material.

27. A filter according to claim 26 wherein said flux material is about 0.5% to 10% by weight of the total filter.

28. A filter according to claim 17 wherein ceramic fibers are contained in the fire-resistant material in quantities of about 0.01% to 10% by weight of the fire-resistant material, said ceramic fibers extend out beyond the surface of the fire-resistant material.

29. A filter according to claim 17 wherein ceramic fibers are contained in the fire-resistant material in quantities of about 0.1% to 5% by weight of the fire-resistant material, said ceramic fibers extend out beyond the surface of the fire-resistant material.

30. A filter according to claim 17 wherein ceramic fibers are contained on the granule surface in quantities of about 0,01% to 10% by weight of the fire-resistant material, said ceramic fibers extend out beyond the surface of the fire-resistant material.

31. A filter according to claim 17 wherein ceramic fibers are contained on the granule surface in quantities of about 0.1% to 5% by weight of the fire-resistant material, said ceramic fibers extend out beyond the surface of the fire-resistant material.

32. A filter according to claim 17 wherein the surface of said spherical shaped granules is coated with carbon.

33. A filter according to claim 24 wherein said carbon is about 3% to 40% by weight of the total filter.

34. A filter according to claim 17 wherein said filter medium has a progression of the mean diameter of the granules from fine to coarse in the direction of filtration.

35. A filter according to claim 17 wherein said filter medium has a progression of the mean diameter of the granules from coarse to fine in the direction of filtration.

36. A filter according to claim 17 wherein said filter medium has a progression of the mean diameter of the granules from coarse to fine perpendicular to the direction of filtration.

37. A filter according to claim 17 wherein said filter medium has a progression of the mean diameter of the granules from fine to coarse perpendicular to the direction of filtration.

38. A method of removing impurities from molten metal comprising filtering said molten metal with a filter medium as defined in claim 1.

39. A method according to claim 38, wherein said molten metal is aluminium.

40. A method according to claim 38, wherein said molten metal is a ferrous metal.