US3881709A - Diffuser castings - Google Patents

Abstract

A method of making a cast refractory diffuser for feeding gas into molten aluminium or other fluid material at high temperatures, said diffuser having a tubular inlet stem integral with a hollow, gas-permeable outlet head which extends laterally from the stem and has an unbroken wall around a cavity extending from the bore of an inlet stem, in which method there is used at least for the outlet head a compressible, fugitive casting core that maintains its shape during casting of the refractory from an aqueous mix but yields to shrinkage of the cast during drying and commencement of firing and, before completion of firing, is lost to leave at most only an insignificant ash or other residue in the hollow head.

Description

United States Patent [1 1 Steen [451 May 6,1975

[ DIFFUSER CASTINGS [75] Inventor: William John Steen, Wirral, England [73] Assignee: Morgan Refractories Limited,

Wirral, England [22] Filed: Mar. 12, I973 [21] Appl. No.: 340,199

[30] Foreign Application Priority Data Mar. 13, 1972 United Kingdom 11590/72 [52] U.S. Cl. 266/34 T; 266/34 PP; 164/259; 239/145; 239/590.3

[51] Int. Cl C22b 21/06 [58] Field of Search 239/57.6, 553.3, 590.3, 239/DIG. 19, 602, 145; 164/259, 55, 56, 66;

266/34 PP, 34 PT, 34 T; 261/87, 122;

FOREIGN PATENTS OR APPLICATIONS 221,071 9/1924 United Kingdom 181/61 341,949 12/1959 Switzerland 266/34 PP 446,329 4/1936 United Kingdom 164/66 Primary Examiner-M. Henson Wood, Jr. Assistant ExaminerAndres Kashnikow Attorney, Agent, or FirmLarson, Taylor & Hinds [57] ABSTRACT A method of making a cast refractory diffuser for feeding gas into molten aluminium or other fluid material at high temperatures, said diffuser having a tubular inlet stem integral with a hollow, gas-permeable outlet head which extends laterally from the stem and has an unbroken wall around a cavity extending from the bore of an inlet stem, in which method there is used at least for the outlet head a compressible, fugitive casting core that maintains its shape during casting of the refractory from an aqueous mix but yields to shrinkage of the cast during drying and commencement of firing and, before completion of firing, is lost to leave at most only an insignificant ash or other residue in the hollow head.

6 Claims, 3 Drawing Figures PATENIEUMAY' ems SHEET- 2 OF 2 DIFFUSER CASTINGS This invention relates to refractory articles and provides, inter alia, a new construction for a tubular refractory unit for introducing gas into a high temperature region.

In particular, but not exclusively, such a unit is suitable for feeding nitrogen into molten aluminium in a crucible, or other molten-metal holding furnace vessel, during treatment of the aluminium to remove gas and impurities. (Ref. Fumeless in-line de-gasing and cleaning of liquid aluminium M. V. Brant, D. C. Bone and E. F. Emley of British Aluminium Co.).

Such treatment is carried out in a crucible divided into an inlet and an outlet side by a vertical partition extending nearby to the bottom. Molten aluminium flows in through a layer of flux, down to a layer of fluxcoated alumina balls, across the bottom of the crucible through a layer of uncoated balls and up the outlet side through more uncoated balls. The nitrogen is introduced above the balls on the inlet side of the crucible and bubbles up the inlet side to escape through the layer of flux.

The two essential structural features of the refractory unit that introduces the nitrogen are a tubular inlet stem, made gas-tight such as by a liner, and a hollow, gas-permeable, outlet head which branches from the stem to extend across the stream of molten metal and diffuse the gas into it. The stem and head together may have, for example, an L-shaped, an inverted T-shape or mushroom shape or the head may be a spider.

It is natural to contemplate making the stern and head separately, to suit their respective requirements, and to join them together. Such units have been used, but the difficulties of joining refractory components by a refractory joint that will withstand molten metal are very great and the life of the units is short.

It would therefore be desirable to make the stem and head as an integral unit by casting and subsequent firing of suitable refractory material, but the difficulty is to cast hollow units of the requisite large size.

The junction between the stem and the head of a cast unit involves a change in direction of the gas passage, between the bore of the tubular stern and the interior of the hollow head, through which a casting core cannot be withdrawn. The obvious solution is to use two withdrawable cores, meeting at the junction, and to plug the hole through which the head core is withdrawn. Such a plug however entails a refractory joint and this, like the junction between a separate stem and head, has proved liable to early failure when immersed in molten metal.

Recourse has therefore been had to the use of straight tubes, or tubes bent as green casts into a shallow curve at the bottom, but the gas distribution achieved is very poor. Straight tubes inserted through the side of the crucible have even been proposed, to give the required gas distribution, but their replacement means the shutdown of what is designed as a continuously operating plant, and in any case the seal with the crucible is liable to failure. This expedient too has proved unworkable.

The present invention solves the problem by casting, as an integral branch from a refractory tubular stem, a laterally extending hollow head with an unbroken wall. Use is made, at least and preferably only for the head, of a fugitive or expendable casting core, in particular of combustible material, that retains its shape for easting of the refractory material from an aqueous mix, yields to shrinkage of the cast material during drying and commencement of firing so that the casts do not crack and, before completion of firing, becomes consumed or reduced to an insignificant ash or other residue in the hollow head.

A particularly useful combustible core material is a plastics foam or cellular material of the rigid or semirigid type, such as a polyurethane, which can be cast to foamed or cellular form by known simple mixing and pouring techniques. Such materials can be made finepored with good self-sustaining strength characteristics but yieldable to cast refractory shrinkage and combustible at a suitable temperature, such as 750C, with a small residue.

A fugitive core for the head is preferably used in conjunction with a withdrawable core, such as a metal tube, for the stem and this can serve to locate the fugitive core by engagement therewith at the junction of the gas passage between the stem and the head, or a short distance up the stem.

For best results, especially in the case of a relatively long head, such as of T-shaped or L-shaped unit, the or each terminal end of the fugitive core is temporarily supported, until shortly before casting is completed, by a supporting pin withdrawable through the wall of the plaster or other outer former for the casting. Possible location difficulties with for example the slightly flexible polyurethane cores that allow for casting shrinkage are thus avoided. After withdrawal of the pin, the fugitive core being by then well located and supported by cast material, the small hole left by the pin is filled by flow of the cast material.

The present invention includes the casting method described above, using a fugitive core preferably in combination with a withdrawable core, as well as the particular cast product.

More specifically, the product can be defined as a diffuser, for feeding gas into fluid material at high temperature, especially molten aluminium, comprising a cast integral unit of refractory material having a tubular inlet stem and a hollow, gas-permeable, outlet head which branches laterally from the stem and has an unbroken wall around a cavity which extends laterally from the bore of the stem.

Suitable materials for such a diffuser are silicon carbide bonded with carbon, alumina or clay and especially a zirconia-mullite bonded silicon carbide such as is described in our U.K. Pat. Application No. 35977/71 (Steen US. application Ser. No. 274,656 filed July 24, 1972).

These materials are made from a particular mix comprising zircon, alumina and silicon carbide and optionally a clay or other inorganic or organic plasticiser, the proportions and particle sizes of the zircon and alumina being such that they react on firing to give a bonding and protective matrix for silicon carbide.

The casting method and two examples of a diffuser in accordance with the invention are illustrated in the accompanying drawings, in which:-

FIGS. 1 and 2 are axial sections respectively of an inverted T-shaped diffuser and an L-shaped diffuser, and

FIG. 3 is an exploded perspective view of a split plaster mould and the cores for casting of refractory material to form a diffuser as shown by FIG. 1.

The diffuser shown by FIG. 1 comprises a tubular stem 1, open at its upper end for supply of gas, and an inverted T-shaped hollow head 2 branching from the lower end of the stem.

Before use, the stem 1 is lined with a metal tube (not shown) so that its bore 3 forms a gas-tight passage to lead gas to the interior 4 of the head 2 through the wall of which gas can permeate to diffuse into the molten metal under treatment.

The diffuser shown by HO. 2 has a stem 5 and a head 6 forming an L-shape but is otherwise similar to that of FIG. 1.

Each diffuser is cast integrally from a pourable composition of a refractory material, preferably a silicon carbide composition as mentioned above, which after drying and firing has the required refractory and porous characteristics, as known in the arts of refractory production and metal treatment.

As shown by H6. 3, a mould for casting, head uppermost, a diffuser of inverted T-shape, comprises a plaster case in two parts 7 and 8 which each have a longitudinal half-cavity, 9 and 10 respectively, which conjointly define the external shape of the diffuser when the two case parts are closed.

A mould base 11 has a socket for the root of a withdrawable metal core 12, to form the bore 3 of the stem 1 of the diffuser, and at the top of the core 12 a dowel 13 is provided to locate, as a spigot in a socket 14, a combustible core 15 to form the interior 4 of the diffuser head 2.

To ensure location and prevent any sagging of the core 15, each end thereof is bored to receive a withdrawable pin 16 withdrawable through a respective hole 17 in the mould case.

Specifically, the production of a diffuser may be carried out as follows:

1. Place the metal core upright in the base and position the combustible core on the dowel;

2. Carefully place the half mould side sections into the base, ensuring that all mating surfaces are clean;

3. Fasten the sides together;

4. Insert the centralising pins;

5. Fill the mould;

6. After the material has partly set, remove the centralising pins;

7. Fill the recesses in the casting left by the pins, by placing more mix and levelling off the filler face;

8. Invert the mould, filler face down, onto a board and remove the base section;

9. When the casting has reached the leather-hard stage withdraw the stem core.

10. Lay the mould flat and remove one side;

1 1. When the casting is sufficiently strong to handle remove it from the mould, dry it and fire it.

I claim:

1. A diffuser for feeding gas into molten aluminum or other fluid material at high temperatures, comprising a cast refractory having a tubular inlet stem integral with a hollow, gas-permeable outlet head which extends sharply laterally from the stem and has an unbroken wall around a cavity extending from the bore of the inlet stem, said gas-permeable outlet head being an integrally cast branch of said tubular inlet stem, and said unbroken wall being constituted by as-cast refractory surfaces devoid of joints or joined refractory components or like discontinuities.

2. A diffuser as claimed in claim 1 wherein said outlet head and its cavity extend laterally at about relative to said bore of the inlet stem.

3. A cast refractory diffuser as claimed in claim 1 made by casting pourable refractory material for the entire diffuser about suitably shaped core members, the core member for the cavity in the outlet head comprising a compressible fugitive casting core that maintains its shape during casting of the pourable refractory mix but yields to shrinkage of the casting during drying and commencement of firing and, before completion of firing, is lost to leave at most only an insignificant residue in the hollow head, and drying and firing the casting.

4. A cast refractory diffuser as claimed in claim 3 in which said fugitive core is located by a withdrawable stem core removed after the cast has become selfsupporting.

5. A cast refractory diffuser as claimed in claim 3 in which the or each end of the fugitive core away from the stem of the diffuser is temporarily supporting during casting by a pin which is withdrawn through the wall of the casting mold once the wet cast is firm enough to support the core, the hole in the cast left by withdrawal of the pin being filled by flow of the cast material.

6. A cast refractory diffuser as claimed in claim 3 wherein said fugitive core is of foamed or cellular polyurethane.

Claims (6)

1. A diffuser for feeding gas into molten aluminum or other fluid material at high temperatures, comprising a cast refractory having a tubular inlet stem integral with a hollow, gas-permeable outlet head which extends sharply laterally from the stem and has an unbroken wall around a cavity extending from the bore of the inlet stem, said gas-permeable outlet head being an integrally cast branch of said tubular inlet stem, and said unbroken wall being constituted by as-cast refractory surfaces devoid of joints or joined refractory components or like discontinuities.

2. A diffuser as claimed in claim 1 wherein said outlet head and its cavity extend laterally at about 90* relative to said bore of the inlet stem.

3. A cast refractory diffuser as claimed in claim 1 made by casting pourable refractory material for the entire diffuser about suitably shaped core members, the core member for the cavity in the outlet head comprising a compressible fugitive casting core that maintains its shape during casting of the pourable refractory mix but yields to shrinkage of the castiNg during drying and commencement of firing and, before completion of firing, is lost to leave at most only an insignificant residue in the hollow head, and drying and firing the casting.

4. A cast refractory diffuser as claimed in claim 3 in which said fugitive core is located by a withdrawable stem core removed after the cast has become self-supporting.

5. A cast refractory diffuser as claimed in claim 3 in which the or each end of the fugitive core away from the stem of the diffuser is temporarily supporting during casting by a pin which is withdrawn through the wall of the casting mold once the wet cast is firm enough to support the core, the hole in the cast left by withdrawal of the pin being filled by flow of the cast material.

6. A cast refractory diffuser as claimed in claim 3 wherein said fugitive core is of foamed or cellular polyurethane.

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