AU598025B2 - Ceramic filter for filtering molten metal - Google Patents

Description

AU-Al-77844187 WELTORGANISAW, N 1 44',GE 645ES 6DENT'L PCT Ill., 'I S Bu' a ]I n .1 de I 0 INTERNATIONALE ANM T CH%-E ELDUNG VEIR1161 F X 4 M 5RTRAG 0BER DIE INTERNATIONALE ZUSAMMENARBEIT AUF DEM GEBIET DES PATENTWESENS (PCT) (51) Internationale Patentklassifikation 4 (11) Internationale Veroffentlichungsnummer: WO 88/ 01532 B01D 39/20, C22B 9/02 Al (43) linternationales C04B 35/10 Veroffentlichungsdatum: 10. M~rz 1988 (10.03.88) (21) Internationales Aktenzelcben: PCTj'CH87/0OlO0 (81) Bestinnnungsstaaten: AT (europAisches Patent), AU, BE (europ~isches Patent), BR, CH (europiiisches Pa- (22) Internationales Anineldedatum: tent), DE (europiiisches Patent), DK, FR (europai- 14. August 1987 (14.08.87) sches Patent), GB (europlisches Patent), IT (europ~isches Patent), JP, LU (europ~isches Patent), NL (eu-, (31)Prioltitakienzechen 901432 ropiiisches Patent), NO, SE (europkiisches Patent), (32) Priorititsdatum: 28. August 1986 (28.08.86) Ver fentlicht Mit internationalem Recherchenibericht.

(33) Prioritaitslnd: us (71) Anmelder: SCHWEIZERISCHE ALUMINIUM AG ECH/CH]; CH-8212 Neuhausen am Rheinfall (CH).

(72) Erfinder: BROCKMEYER, Jerry, W. 3032 Laurel Park Hwy., Hendersonville, NC 28739 DORE, James, E. 1306 Balmont Drive, Hendersonville, NC OL .L JP- 2, 8 AP R 1988 28739 AUBREY, Leonard, S. ;58 Chestnut Ridge Road, Arden, NC 28704

AUSTRALIAN

This document contain,, the 2 4 MAR '988 I amendments made under SSection 49, and is corre ct for PATENT OFFICE (54) Title: CERAMIC FILTER FOR FILTERING MOLTEN METAL (54) Bezeichnung: KERAMISCHER FILTER FOR DIE FILTRATION VON GESCHMOLZENEM METALL (57) Abstract The ceramic filter has a plurality of passageways for the liquid metal and is coated with a thin layer of an inorganic material which can be easily wetted by the molten metal.

(57) Zusammenfassung Gebundener keramischer K6rper mit einer Vielzahl von Fliesswegen for die Durchstrbmung durch flissiges Metall, wobei der Kbrper eine diinne Beschichtung, im wesentlichen durch den ganzen K~3rper hindurch, aufweist und die Beschichtung eim anorganisches Material, welches leicht durch das geschmolzene Metall benetzt wird, darstellt.

r .4 p 4 INTERNATIONAL SEARCH REPORT Intemational Application No PCT/CH 8 7 0010 0 To: The Commissioner of Patents Klaus. .ve.r P18/7/81 PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia i r I -la- Ceramic filter for the filtering of molten metal The invention refers to a ceramic filter for filtering molten metal made from a bonded or sintered ceramic body containing a plurality of passageways for the flowing through by the molten liquid metal.

Generally, molten metal contains penetrated solid materials, which affect detrimentally the finished casting.

These penetrated solid materials appear as inclusions in the finished casting after the molten metal has solidified and cause defects in the finished product. The inclusions may originate from different sources, for example from superficial oxide films and insoluble contaminant like carbides, borides and others. Ceramic filters, as described in the U.S.A. patents 3,893,917, 4,081,371 and 3,962,081, are particularly suitable for the filtering of molten metal. Especially these ceramic foam filters with open cell *structure with a plurality of interconnected hollow spaces, which are surrounded by a network of ceramics, to ensure thereby a winding throughout track, are suitable. These S0.. filters are advantageous due to various reasons, for instance due to their remarkable filtering effectiveness, low cost, easy application and the possibility to use them on a changeover and throw-away basis. The fact, that the 0 ceramic foam filters are especially practical and cheap in their manufacture and can be used on a once-only and throw-away basis, has led in the last years to a further S* spreading of their use in the field of molten metal treatment.

Particular problems have arisen, however, in case of 0000 filtering of molten metals like steel, which has bad flow characteristics. During the filtering of materials of this type it is often difficult to initiate the flow of the metal through the filter. This leads to a building up of a high metal resistance through the filter before the filter is wetted, i.e. before the flow of the metal begins through the filter. Further, it has been found that the molten metal often does not penetrate through the entire filter or that A B i i s 1 L.: i i: -2the filter is not flown through in all its areas.

Furthermore, again in different cases, the filter is not wetted at all and this causes a very high metal accumulation above the filter. This problem mentioned is particularly important for steel, it exists however in case of other metals also, depending on their composition or temperature of casting.

A process to solve this problem was to overheat the alloy and thus achieve better flow characteristics. It has been found, that an overheating improves the capability to flow, which in turn promotes the wetting. But this does not represent an entirely satisfactory solution, as an excess in overheating causes again other problems, depending on the composition. An overheating above the normal, therefore, does not always produce a successful solution.

A coated ceramic filter has also become known from the U.S.A. patent specification 4,302,502, wherein a proportionally large activating coating for the removal of oooo specific impurities from the molten metal is provided.

However, the theory about this filter does not deal with the above mentioned problem of metal accumulation either.

The task of the invention is to eliminate the disadvantage of metal accumulation and accordingly to propose an improved ceramic filter for the filtering of molten metal, which promotes the filtering of metals which do not have a very good ability to flow, without building up an excessive metal accumulation. It is a further feature of this invention at least in preferred forms, to produce an improved filter of the type mentioned before, based on a bonded ceramic body, which can be manufactured cheaply and fast and which has no harmful influence either on the filtering process or on the properties of the finished 5 casting. A further object of this invention is to provide an improved filter, which enables avoidance of the necessity of a strong overheating for the purpose of wetting the filter.

According to the invention there is provided a ceramic filter for the filtering of molten metal, comprising QALL4

CS

-1B k -3a bonded or sintered ceramic body containing a plurality of passageways for the flow-through of molten metal, wherein a thin coating which is easily wetted by the molten metal extends completely throughout the filter, the thin coating being of a material selected from at least one of silica, sodium silicate, sodium chloride, calcium chloride, calcium oxides, magnesium oxides, calcium carbonates and magnesium carbonates, the coating adhering to the ceramic body and 0 having a thickness of between 5 A to 1 micron, and being removable from the ceramic filter on contact with the molten metal.

Accordingly, this present invention describes a high-efficiency ceramic filter for the filtering of molten metal, which overcomes the problem of metals with poor ability to flow, like that of steel, and shows advantageous wetting properties without the excessive building up of metal accumulation and without the necessity of strong S*overheating of the metal.

e The ceramic filter of this present invention encompasses a bonded or sintered ceramic filter with a **plurality of passageways for the flow-through by the molten liquid metal with a thin coating, bonded onto the ceramic body and extended essentially over the entire extent of the inorganic material, which is wetted by the molten liquid metal. The coating is removed by the flowing through molten liquid metal.

The ceramic filter is preferably a ceramic foam filter S with open cell foam structure and a plurality of interconnected hollow spaces, which are surrounded by a network of this ceramics. The ceramic body contains as of s ceramic material preferably aluminium oxide. The preferred thickness of the coating is 10 to 100 A. The thickness of the coating is even and contains preferably silica for the filtering of steel.

As indicated before, it was a technological problem that metals with poor ability to flow, e.g. steel, did not flow cleanly through ceramic filters. The problem is caused by the material of the ceramic filter which is not -4completely wetted by the molten metal, i.e. the molten metal does not flow or does not spread on the surface of the filter material. For instance, in case of a ceramic filter containing aluminium oxide as filter material, this material will not be fully wetted by steel.

The ideal situation of a high filtering efficiency is achieved when the material of the to-be-filtered base metal does not wet the filter properly, but the inclusions, which are to be removed, can wet the fil!er material completely.

For example, an aluminium oxide inclusion in steel wets completely a ceramic filter based on aluminium oxide, while the steel wets the filter only incompletely. Therefore, the inclusions have the tendency to leave the metal flow and adhere to the filter, while the base metal has the tendency not to remain in contact with the filter or the inclusions.

It is also advantageous, that the base metal does not wet the filter and thereby causes a limited attack on the filter gee.

o material. This attitude creates the desirable situation, as soon as the flow starts through the filter, i.e. when the ge filter is already being poured through. It is therefore ge desirable to provide a filter which has these characteristics while it can be easily poured through. It is also desirable to provide a filter which can be poured through easily by the molten liquid metal without disturbing the advantageous properties of the filter or the filtering efficiency.

00 0 The ceramic body of this present invention is a bonded or sintered ceramic body with a plurality of passageways for the flow-through by the molten liquid metal. All known ceramic bodies may be used, for example extruded ceramic g e.

*0o0.. bodies, bodies from sintered ceramic particles or preferably a oo ceramic foam filter with open cell structure, characterized by a plurality of inter-connected hollow spaces which are bound by a network of ceramics and have winding passageways. A particularly suitable ceramic foam filter is described in the German published patent s pecification 34 24 504.

4 AB (I-iill*~ilrl:.Ti~i~1~(~-i]~~I According to the invention a thin coating is applied onto these ceramic bodies, this coating extending essentially throughout the entire body and the coating adheres to the body. The coating is an inorganic material which is completely wetted by the molten metal and is o present with a thickness of 5 A to 1 micron. In a useful 0 execution the thickness of the coating is 10 to 100 A. From this it is apparent, that the coating is very thin. During the use of the filter the coating is removed by the contact with the molten metal, and after the start of the flow of the metal the entire filtering surface necessary for the filtering efficiency is available. The coating is preferably fully removed in less than 5 seconds, be it by dissolving by the molten metal or by the mechanical effect of the molten metal on the ceramic network. Because of this 9:08 a coating is preferred, which is soluble or partly soluble 0ooo in the molten metal. To achieve a suitable and fast removal of the coating at the start of the flow of the metal, preferably a coating less than 100 A is applied, while it is also possible to use coatings up to 1 micron, especially when the mechanical effect to remove the particular coating is also used.

As described before, the coating should extend essentially throughout the entire ceramic body in an even eS coating thickness, while fluctuations in the coating thickness and uncoated sections are still tolerable.

o However, to promote the wetting, the coating should extend S-O over the entire network and be preferably even.

The coating material should be a material which does not affect detrimentally the to-be-filtered molten metal or the final cast product. Therefore, a mechanical separation of the coating from the network is less desirable than a solubility or partial solubility in metal, as the mechanically separated coating particles may appear in the final casting as inclusions. Additionally, the coating material must not have any detrimental effect on the molten metal which could, for example, lead to reduced mechanical properties, increased grain size or undesirable secondary L -6phases.

In a preferred example of embodiment a colloidal silica coating is applied, especially because it has been established that this is particularly advantageous for the filtering of steel and thereby a suitable silica coating on the ceramic body is achievable. With regard to the fact that thin coatings are applied to the ceramic body, the silica coating has no detrimental effect in the quantitites used for the to-be-filtered steel. At the most, only in large quantitites, which are not subject of the invention, could silica have a deleterious effect. The thin, according to the invention, silica coating is completely removed anyway on contact with the molten liquid steel.

In a preferred embodiment an aqueous dispersion of a colloidal silica is used, originating with a 0.5 to concentrated colloidal silica in water, preferably in a concentration of Naturally, additives like wetting agent, dispersing agent or organic materials which support the affinity to coating, etc. may be used in the aqueous dispersion.

The beforesaid dispersion is prepared in agreement with the preferred embodiment and the ceramic body immersed into the dispersion to achieve a thin coating on it, essentially throughout the entire ceramic body. Excess material is removed, for instance shaken off, the wet ceramic body dried, for example at 93 to 149 C, preferably at 121 C, and at 982 to 1038 C and preferably after S fired at 1000°C. The temperature treatment should be high enough to prevent the rehydration of the silica and its adhesion to the ceramic body. The drying and refiring can .J be followed in separate stages or continuously. It is not necessary to maintain the temperature. The thus coated material will be found as particularly progressive for filtering of steel.

Naturally, no coating will be applied which requires such a high firing temperature which may lead to the ^PLI damaging of the concerned ceramic body.

S//It is also possible to use materials other than C' /AB -7silica, provided this material is an inorganic one which is wetted easily by the molten material and is removed in contact with the molten metal. Silica is especially preferred for the steel filtering, as it is particularly well wetted by steel and is dissolved easily by molten metal. Further suitable additional inorganic materials which may be used include, for example, an aqueous silicate disperson like sodium silicate and salts, like sodium chloride or calcium chloride, where the latternamed salts can be simply dried on the network, instead of once again being fired. Equally, one may use oxides like that of calcinm or magnesium, which as salt, for example as carbonates, can be subsequently dried and fired.

The invention's effectiveness of the coating composition according to this invention is the use of an inorganic material as a thin coating, which is easily wetted by the molten metal and is easily removable by the molten o. metal. Due to the fact, that the coating is easily wetted by the metal, the metal passes easily the ceramic body as well as the winding passageways in the ceramic body, when the preferred ceramic foam material is used and the casting time and the metal accumulation can be considerably reduced.

Moreover, it has been established, that not only does the coated composition reduce the casting time. Rather, it 0 has been established, that the coated filters cause a surprisingly faster carrying out of the filtering process, for example the total material flow through the ceramic body in accordance with this invention is generally faster than through an uncoated ceramic body. Surprisingly, filters in accordance with this invention, achieve a higher s 00.0 flow-through rate than uncoated filters. This despite the fact, that the coating is removed on contact with the molten

S

metal. This could be due to the faster pouring through the complete filter and the faster flowing through all pores in the ceramic filter body. This too, repirsents a considerable progress.

Example 1 Given are bonded ceramic filter bodies, 18.7mm thick, L;r_ _y i; Y1 LI -I I -8with an open cell structure, characterized by a plurality of interconnected hollow spaces, surrounded by a network from ceramics, where the filter bodies are based on an aluminium oxide ceramic. The filters used have 10 pores per cm.

Samples of the filter were in agreement with this invention, coated by inserting the filter bodies in an aqueous dispersion, containing 1% colloidal silica, for a short duration, ensuring a complete impregnation, to obtain a thin coating, dried at 121°C and fried at 1000° 0 C. The o thickness of the coating after the firing was approx. 10 A and the coating spread essentially over the entire ceramic body in ball-shaped silica particles. Various samples of the coated filter and one uncoated filter were fitted into the casting pipes. Each of these casting pipes were slowly inserted into a bath containing steel deoxidised with aluminium and held at a temperature of 1621°C. The oeo* too. beginning of the pouring is established by visual foe observation through the centre of the pipe. The result has shown that the coated filter was wetted immediately after i S.

the insertion in the bath up to a thickness of 5.08cm. The uncoated filter was not wetted up to the insertion depth of 5.08cm. The pipe was inserted further, up to a depth of approx. 10.16cm, when the pipe broke due to the heat shock.

A wetting of the filter was not observed on the uncoated filter.

Example 2 Coated and uncoated filters have been provided, essentially according to the composition mentioned in Example 1 and with the coatings prepared essentially according to Example 1.

i1. Coated and uncoated filters 1.86cm thick, were positioned next to each other in a crucible. Molten steel was directly poured into the crucible from an induction oven at a temperature of 1621 0 C. By filling the crucible, the metallostatic pressure on the filter rises until the metal accumulation reaches the height of the overflow. The time LI of the wetting of the filter was determined by visual s bservation of the flow-out side of the filter. The results S B r 1 i 'i)l~ have shown, that the coated filter was wetted 4 to 5 seconds before an uncoated filter. The flow of the molten metal through the coated filter seemed to be very fluid, while the flow through the uncoated filter was in comparison very viscous and slow. At the end of the run-off the total of 76.6kg metal passed the coated filter, while only 11.3kg flowed through the uncoated filter. In addition, the resulting casting was of higher quality. The coating on the filter had no disturbing influence on it. Furthermore, the silica coating was removed essentially by dissolving in the molten metal during the contact with the molten metal.

Example 3 Coated and uncoated filter were manufactured essentailly in the composition of example 1 according to example 1, with the exception that the filter bodies were characterized by 6 pores per cm. Four sand moulds have been prepared, three having a filtering device. All four moulds were cast directly from an induction oven with 204kg of average carbon steel deoxidised with aluminium. The results are shown in the following Table 1.

Table 1 0 *0r* Sr~ 0S00 ar

S

55

S.

0

S.

Casting No.

S S* 5* 0S*O 0 4005r S. @0 5 5

S

Test Conditions coated filter no filter uncoated filter coated Casting Temperatures 1621°C 1599°C 1621°C 1595°C Results filter wetted, clean casting surface great number of slag inclusions on the casting surface filter failed during wetting filter partially wetted C) AB L~?i, The above results show clearly, that casting 1 has a totally inclusion-free casting surface when using a coated filter, compared with casting 2, where no filter was used and where a considerable number of inclusions were present.

Casting 3, which contained an uncoated filter, failed completely during the wetting of the filter. Although the casting process was carried out at the same temperature as at casting 1. Casting 4 used a coated filter, which became partially wetted, although the casting temperature was 26°C lower.

0*00 .e0 0€ w 000 0 *0 0 *4e, 00 00 L ii Z

Claims (5)

1. 2. A ceramic filter according to claim 1, wherein the ceramic body is a foam filter with an open cell form :structure, said ceramic body comprising a plurality of ooo.. interconnected hollow spaces, which are surrounded by a *"00 ceramic network. oe 3. A ceramic filter according to claim 1 or claim 2, o wherein the coating is 10 to 100 A thick.
2. 4. A ceramic filter according to any one of claims 1 to 3, wherein the coating is of substantially uniform thickness. A ceramic filter according to any one of claims 1 to 4, wherein the coating is a silica coating.
3. 6. A ceramic filter according to any one of claims 1 to 4, wherein the ceramic body contains aluminium oxide.
4. 7. A ceramic filter according to claim 1, substantially as herein before described with reference to the Examples. DATED: 22 MARCH, 1990 PHILLIPS ORMONDE FITZPATRICK Attorneys For: SWISS ALUMINIUM LTD. 0 i/7.rE YW ^r B I INTERNATIONAL SEARCH REPORT International Application No PCT /CH 8 7 /0 0100 CLASSIFICATION OF SUBJECT MATTER (if several classification symboll soply, Indicate all) According to International Patent Clasfication (IC) or to both National Cassificatlon and IPC 4 Int.C1. B 01 D 39/20, C 22 B 9/02; C 04 B 35/10 1. P9LDS SEARCHED Minimum Documentatiin Searched 7 ClassificationSystm Classifiction Symbols B -01 D 4 :1 C 2 Int.Cl. 0 Oocumentallon Searched other than Minimum Documentation to the Extent that such Documents are Included In the Fields Searched 9 Il. DOCUMENTS CONSIDERED TO E 1RELEVANT' Category Citation of Documentl with Indication, where appropriate, Of the relvant paies" 12 Relevant to Claim No.'3 X GB, A,.2166758 (BRIDGESTONE CORP.) 14 May 1986, see abstract; page 2, lines 33-64 1-4,6 Y US, A, 4395333 (DANIEL E. GROTEKE) 26 July 1983, see abstract; column 2, lines 1-4,16
5. 48-62 Y Patent Abstracts of Japan, volume 8, No. 156 (C-234) (1593) 19 July 1984, see the whole abstract 1-4,6 JP, A, 5962324 (TDK 09 April 198 A Patent Abstracts of Japan, volume 9, No. 254 (C-308) (1977), 11 October 1985, 1,2,6 JP, A, 60106514 (TOYOTA JIDOSHA K.K.) 12 June 1985 A FR, A, 2439170 (BRIDGESTONE TIRE CO.) 16 May 1980, see page 1, lines 1-9; claims 1,2,6 1-3 Special categories of cited documents: 1 0T" later document published fhtr the international filing date or priority date and not in conflict with the application but document defining: the general state at the art which is not cited to understand the principie or theory underlyi0n the considered to be of particular relevance invention earlier document but published on or after the international document of particular relevance; the claimed invention filing date cannot be considered novel or cannot be considered to IL" document which may throw doubts on priority clalm(a) or Involve an inventive step which Is cited to establish the publication date of anothar document of particular relevance; the claimed Invention citation or other special reason (as specfied) cannot be considered to Involve an Inventive step when the document referring to an oral disclosure, use, exhibition or document is combined with one or more other such docu- Other means menta, such combination being obvious to a person skilled document published prior to the InternatIonal filing dte but In the art. later than the priority date claimed -Al document member of the same paent family IV. CERTIFICATION Date of the Actual Completion of the Internatonal Search Date of Mailing of this international Search Report October 11.987 (30.10.87) 23 November 1987 (30.11487) International SarchirAuthorIty Signature of Authorlitd Officer EUROPEAN PATENT OFFICE Form PCT/it$.210 (second shoot) (January 195) US FR ANNEX TO THE INTERNATIONAL SEARCH REPORT ON INTERNATIONAL APPLICATION NO. PCT/CH 87/00100 (SA 18165) This Annex lists the patent family members relating to the patent documents cited in the above-mentioned international search report. The members are as contained in the European Patent Office EDP file on 04/11/87 The European Patent Office is in no way liable for these particulars which are merely given for the purpose of information. Patent document Publication Patent family Publication cited in search date member(s) date report GB-A- 2166758 14/05/86 JP-A- 61117182 04/06/86 DE-A- 3539522 15/05/86 US-A- 4395333 26/07/83 None FR-A- 2439170 16/05/80 GB-A,B 2034298 04/06/80 DE-A,C 2942042 08/05/80 JP-A- 55056077 24/04/80 US-A- 4258099 24/03/81 CA-A- 1120066 16/03/82 For more details about this annex see Official Journal of the European Patent Office, No. 12/82 i i. i r