CA2007450A1

CA2007450A1 - Ceramic filter for the filtering of molten metals

Info

Publication number: CA2007450A1
Application number: CA002007450A
Authority: CA
Inventors: Werner Kallisch; Reiner Stotzel; Rolf Rietzscher
Original assignee: Georg Fischer AG
Current assignee: Georg Fischer AG
Priority date: 1989-01-11
Filing date: 1990-01-10
Publication date: 1990-07-11
Also published as: BE1002969A5; NO175874C; GB2227185B; IL92973A0; AU624465B2; FR2641475B1; NO900129D0; IT9019040A0; FI900143A; BR9000098A; YU250189A; GB2227185A; AT396875B; NO900129L; AU4773690A; ES2019014A6; MX173465B; IT1238156B; ZA90198B; CH679746A5

Abstract

Abstract of the Disclosure The invention relates to a ceramic filter with open-celled foam structure based on high-melting ceramic for the filtering of molten metals, having two opposite through-flow faces, running transversely to the direction of through-flow of the molten metal, and at least one side face, extending essentially in the direction of through-flow, an organic foam material being impregnated with a high-viscosity ceramic slip, then, after drying, the foam material removed by heating and the remaining ceramic material calcined, as well as a process for its production. In order to achieve an increased mechanical and thermal stability, it is provided that the side face(s) are provided with a closed layer of refractory material of a depth of 0.5 to 3 mm over at least 50% of their length in circumferential direction essentially symmetrically on opposite sides and preferably entirely, and the free cell membranes of the foam structure at the through-flow faces are closed by a coating of refractory material.

Description

xo~so - s GEORG FISCHER AG - Schaffhausen/Switzerland /January ll, 1989/MS/sm Ceramic filter for the filtering of molten metals The invention relates to a ceramic filter for the filtering of molten metals according to the preamble of claim 1 and to a process for its production.
Such ceramic filters have long been known and are used with good success in foundries to keep impurities such as slag, sand and refractory materials away from the castings to be produced.
Ceramic filters with open-celled foam structure are usually produced by impregnating organic foam, for instance polyurethane foam~ with a low-viscosity ceramic slip or by impregnating with a high-viscosity slip and squeezing out the excess slip by means of pairs of rolls.
If a low-viscosity slip is used, a uniform distribution of the ceramic material over the foam is obtained. If a high-viscosity slip is used, depending on the crushing strength of the foam, the roll setting and the rheology of the slip, accumulations of slip material can occur inside or on the sides running parallel to the direction of conveyance through the rolls.
In order for the filtering effect to be ensured, the filter must have a high degree of reliability in terms of thermal and mechanical properties with respect xo~7~sn to the liquid metal. In the case of known filters, a certain percentage - depending on loading - is affected again and again by fractures and erosion of the c~ramic in the liquid metal. This occurs in particular in the region of the side faces extending essentially in the direction of through-flow of the molten metal, even if there is in their region an increased accumulation of material due to corresponding rolling, and at outwardly open hollow membranes of the foam structure. The latter are caused in impregnation of the foam by some of the slip being removed again during conveyance of the impregnated foams and by the vapor pressure occurring during burning out of the foam leading to more or less slit-shaped openings at the exposed cell membranes.
WO 82/03339 discloses a ceramic filter with open-celled foam structure based on high-melting ceramic which is produced by impregnating an organic foam material with a high-viscosity ceramic slip, drying, heating for removal of the foam material and calcining. Excess slip is removed after the impreqnation of the foam by the impregnated foam being passed through a system of pairs of rolls. In addition, exposed cell membranes lying at the surface of the filter can be protected against breaking off by the dried, impregnated foam additionally being subjected at the surface to a further impregnation with a ceramic slip. As a result, the temperature resistance of the filter is increased at the same time.
However, this afterimpregnation is, on the one hand, ~0~)7~S~

disadvantageous to the extent that, as a result, not only the exposed cell membranes but also regions of the filter lying thereunder are provided with an additional coating of slip, which impairs the permeability of the filter, and on the other hand results in a strength which is not adequate for many applications, to be precise in partic~llar in the case of relatively grea-t drop heights of the molten metal, so that fractures and erosion still occur relatively frequently.
The object of the invention is therefore to provide a ceramic filter according to the preamble of claim 1 which has an increased mechanical and thermal stability with respect to molten metals.
This object is achieved in accordance with the characterizing part of claim 1.
This achieves the effect that the fil~ers have a closed frame in the region of the peripheral side faces.
In particular, the free cell membranes at the thrbugh-flow faces are hereby closed by a coating which does not impair the porosity of the filter.
In the region of the peripheral side faces (in the case of a filter with round or oval base area there is only one peripheral side face) of the foam of the organic foam provided in the dimensions of the filter to be produced, so much material with refractory properties is introduced into the foam that a closed layer having a depth of 0.5 to 3 mm is obtained in their region over the entire length in the circumferential direction, and an ~0~37450 aftertreatment in the form of a coating of the free cell membranes at the through-flow faces of the foam structure is performed with a material with refractory properties.
The coatiny of the free cell membranes preferably takes place in a thickness of 0.1 to 1 mm, the coating material being applied in particular in a quantity of 40 to 400 mg/cmZ.
Just like the coating of the free cell membranes, the closed layer at the side faces is preferably produced from the ceramic slip used for impregnation. Materials -used for this are known per se. For example, substances with a principal component of Al2O3 or other highly refractory substances, in particular containing high proportions of alumina, such as sillimanite, mullite or chamotte, come into consideration. The viscosity of the slip used is advantageously in the range from 104 to 2.104 cps at 20 rpm. If appropriate, instead of the slip used for impregnation, a different slip of refractory material or else an agent setting in air with refractory proper-ties, such as for example water glass, silica sol, resins, aluminum phosphates, zirconium oxide dichloride or ethyl silicate, may also be used for the production of the closed layer andtor the coating of the free cell membranes.
The invention is explained in more detail ~elow with reference to the enclosed figures.
Fig. 1 shows a schematized, enlarged, segmented section through a ceramic filter before firing.

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Figs. 2a and 2b show an embodim~nt of a process for the production of a peripheral closed layer.
Figs. 3 and 4 show two embodiments of a process for the production of two opposite closed outer layers.
Fig. 5 shows a test set-up for ceramic filters.
In Fig. 1, an uncalcined filter before firing is represented in section, on which a foam structure 1 of organic material, such as for instance polyurethane foam or the like, which is for example cuboid with four adjoining peripheral side faces 2 and two mutually opposite parallel through-flow faces 3, has firstly been impregnated -~ith a high-viscosity ceramic slip 4. As set out in detail below, a closed layer 5 of a depth of O.S
to 3 mm of refractory material, in particular of the slip likewise used for impregnation, is produced on all the side faces 2. In addition, free cell membranes 6 at the through-flow faces 3 are provided with a coating 7 of refractory material, in particular of the slip also used for impregnation. ~fter firing, the foam structure is burned out and the slip is solidified.
According to Figs. 2a and 2b, to achieve a peripheral closed layer 5 in the region of the side faces 2, the foam structure 1 impregnated with the slip can be compressed by means of a ram 8, which has the same base area as the foam structure 1, to such an extent that axcess slip arrives in the region of the side faces 2 and partially accumulates on the outside in the manner of a bead, as indicated in Fig. 2a. During relaxing of the X~C37~

foam structure 1 ~y raising of the ram 8, a frame of slip forms, which is closed on al1 sides, since the excess bead of slip at the side faces 2 distributes itself uniformly over the side faces 2 during the relaxing.
Subsequently, the Lmpregnated foam is dried and fired and provided with the coating 7 before or after firing.
As represented in Fig. 3, after drying as well, the foam structure 1 with slip can be passed through an opening pair of belts 9, between which the impregnated foam structure 1 is firstly compressed, as a result of which excess slip is forced out sideways and forms a corresponding bead there. During further passage through the pair of belts 9, the foam structure 1 is relieved again and the bead of slip distributed evenly over the two opposite side faces 2, so that two opposite closed layers 5 develop.
According to Fig. 4, the closed layers 5 are produced by the impregnated and dried foam structures 1 being passed by means of a horizontal conveyor 10 through a vertically upright pair of rolls 11, which applies in each case appropriate refractory material, slip or material setting in air to two opposite side faces 2 and force~ it into the pore structure to the specified depth.
In this process, an even layer thickness of material to be applied is always ensured on the surfaces of the rolls, for instance by means o a doctor knife or a roll frame.

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This method of producing closed layers S may be arranged downstream of those of Fig. 3, in order to provide all four side faces 2 with a closed layer 5.
However, two apparatuses according to Fig. 4, with a statiorl for turning the foam structures 1 through 90, may also be arranged one after the other in order to provide all four side faces 2 with a closed layer 5.
Instead, however, the closed layer 5 may also be produced by a layer of foam material with a correspond-ingly fine number of pores being adhesively attached to the side faces 2 of the foam structure 1 or the said faces being provided with a web of fine plastic fila-ments. During impregnation with the slip, the small pores or intermediate spaces in the peripheral side edge region remain filled with slip, as a result of which a peri-pheral closed layer S develops.
The method according to Fig. 4 may also be used to provide the side faces 2 of an already fired filter with a closed layer S of a material setting in air with refractory properties.
In addition, the method of Fig. 4 may be used in order to apply the coating 7 either after drying and before firing in the form of a slip or after firing in the form of a material setting in air with refractory properties.
If foam-ceramic filters are used for the filtra-tion of molten metals such as cast iron (for instance lamellar graphite cast iron, nodular graphite iron, ~007~0 malleable cast iron Ni resist) in the castin~ system, a the~mal and static loading of the filters suddenly occurs due to the liquid melt flowing onto them. The degree of the thermal loading is more or less dependent on the composition and nature of the fired slip which was used for the production of the ceramic filter.
Other features influencing stability are the supporting surfaces ~supports) of the filter in the mold and the structurally determined own form of the filter.
The latter can be improved distinctly, without adversely influencing the through-flow rate for the liquid material, by the design according to the invention.
Accordingly, in contrast to conventionally produced filters, the filter according to the invention can be exposed to far higher loads (drop heights and pressure levels) in use. This can be demonstrated by the test set-up represented in Fig. 5.
The test set-up represented comprises a reservoir 12 for receiving liquid material, which is closed at the hottom by a plug 13. Underneath the reservoir 12 there is a filter receptacle 14 with a standardized core print 15, which receives a filter 16 to be tested of, for example, a size of 50 mm x 50 mm x 22 mm. Between the reservoir 12 and the filter receiver 14 there is a downpipe 17, which can be extended, for example by sections 17' of pre-determined length.
The filters 16 to be tested are placed in the test print 15 and, after pulling the plug 13, subjected 5~

to a through-flow of a predetermined type and quantity of iron.
Ceramic filters produced according to the inven-tion of the specified size (with peripheral closed layer 5 of a thickness of 2 mm and a coating 7 of a thickness of 0.5 mm) were compared with essentially the same ceramic filters which were produced however in conven-tional way only by Lmpregnation without layer 5 and coating 7 and consequently had the same through-flow resistance, using the test set-up represented in Fig. 5 and lamella graphite cast iron as material. The weight spectrum was essentially the same in the case of both types of filter. In addition, both types of filter were subjected to the same furnace campaign. The result is given in the following table.

Conv. Produced Conv. Produced produced according produced accon~
to the to the invention invention Ca~ting height 450 mm 450 mm 527 mm 527 mm Casting temp.C 144Q - 1440 - 1438 - 1438 -1379 1379 1380 13~0 Number of tested filters 5 5 S 5 of which number fractured 3 0 4 0 of which number intact 2 5 1 5

Claims

1. Ceramic filter with open-celled foam structure based on high-melting ceramic for the filtering of molten metals, having two opposite through-flow faces (3), running transversely to the direction of through-flow of the molten metal, and at least one side face (2), extending essentially in the direction of through-flow, an organic foam material (1) being impregnated with a high-viscosity ceramic slip (4), then, after drying, the foam material (1) removed by heating and the remaining ceramic material (4) fired and, if appropriate, an aftertreatment on the surface with refractory material performed after the drying, characterized in that the side face(s) (2) are provided in circumferential direction entirely with a closed layer (5) of refractory material of a depth of 0.5 to 3 mm.

2. Ceramic filter according to claim 1, characterized in that the free cell membranes (6) of the foam structure at the through-flow faces (3) are closed by a coating (7) of refractory material.

3. Ceramic filter according to claim 1 or 2, charac-terized in that the coating (7) is applied in a thickness of 0.1 to 1 mm.

4. Ceramic filter according to one of claims 1 to 3, characterized in that the layer (5) and/or coating (7) of refractory material are produced from the ceramic slip used for the impregnation.

5. Process for producing a ceramic filter according to one of claims 1 to 4, an organic foam in the dimen-sions of the filter to be produced being impregnated with a high-viscosity slip of high-melting ceramic material, dried, heated for removing the organic foam and calcined, if appropriate an aftertreatment on the surface with a material with refractory properties being performed, characterized in that so much material with refractory properties is intro-duced into the foam in the region of the peripheral side face(s) of the foam that a closed layer having a depth of 0.5 to 3 mm is obtained in its region over the entire length in the circumferential direction.

6. Process according to claim 5, characterized in that the aftertreatment in the form of a coating of the free cell membranes at the through-flow faces of the foam structure is performed with a material with refractory properties.

7. Process according to claim 5 or 6, characterized in that the coating of the free cell membranes is performed in a thickness of 0.1 to 1 mm and 40 to 400 mg/cm2 of coating material.

8. Process according to one of claims 5 to 7, charac-terized in that, after impregnation, the organic foam is compressed by means of a ram which has the same base area as the foam, after which the foam is allowed to relax.

9. Process according to one of claims 5 to 7, charac-terized in that the impregnated organic foam is passed through an opening pair of belts for squeez-ing out excess slip.

10. Process according to one of claims 5 to 7, charac-terized in that the number of pores of the organic foam at the peripheral side face(s) is reduced in such a way that the closed slip layer forms there during impregnation.

11. Process according to claim 10, characterized in that the number of pores is reduced by adhesively attach ing foam material having a finer number of pores, according to the layer thickness of the closed slip layer.

12. Process according to claim 10, characterized in that the number of pores is reduced by applying a web of fine plastic filaments to the peripheral side face(s).

13. Process according to one of claims 5 to 7, charac-terized in that the impregnated, dried and unfired foam is passed with the side faces through at least one vertically upright pair of rolls, by which high-viscosity slip is applied and forced into the foam to the specified depth.

14. Process according to one of claims 5 to 7, charac-terized in that the fired filter is passed with its side faces through at least one vertically upright pair of rolls, which applies an agent setting in air with refractory properties and forces it into the filter to the specified depth.

15. Process according to one of claims 5 to 14, charac-terized in that the impregnated, dried and unfired foam is passed through a vertically upright pair of rolls and the free cell membranes are thereby coated with a high-viscosity slip.

16. Process according to one of claims 5 to 15, charac-terized in that the fired filter is passed through a vertically upright pair of rolls and the free cell membranes are thereby coated with an agent setting in air with refractory properties.