GB2105634A - Improvements relating to the manufacture of refractory cementitious articles - Google Patents

Abstract

For moulding a refractory valve plate for a sliding gate valve used in controlling metal teeming, refractory concrete is placed in a mould (12) and consolidated by vibratory means (31). Simultaneously, air is sucked out of the mould (12) through planar and tubular mould members (15, 16) respectively responsible for defining the sliding surface and the flow passage of the plate; the suction means comprises a suction box (19) constituting the base (11) of the moulding apparatus and a suction pump connected thereto. The mould members (15, 16) are made from an air-permeable, water-impermeable plastics material. As soon as the casting has attained a self-supporting shape it can be removed from the mould and cured in the manner appropriate to the concrete formulation. <IMAGE>

Description

SPECIFICATION Improvements relating to the manufacture of refractory cementitious articles The present invention relates to improvements in the manufacture of refractory articles with which molten metal comes into contact when pouring the metal from one vessel to another.

Exemplary refractory articles encountered in metal pouring include orificed well blocks and inner nozzles therefor, which are installed in metal pouring vessels in registry with the teeming openings of such vessels. Control of metal flow from a teeming vessel is now commonly accomplished by sliding gate valves.

Known sliding gate valves include two and three plate types wherein the plates, which are orificed and are made of refractory materials such as high alumina, are pressed into firm face-to-face contact. Relative sliding movement between the plates affords the means to open and close the valves to molten metal flow.

For the valves to operate properly and safety, the sliding faces of the plates must be smooth, parallel and free from pockets, otherwise molten metal may penetrate between the confronting surfaces, freeze and give rise to various problems including the danger of leakage.

The plate orifices should also be free from pockets in their walls, and likewise other orificed members which the molten metal may contact, or else solidification and ultimately orifice blockage can occur particularly during times when pouring is interrupted.

The previous practice has been to press and fire the required components, followed by machining of their orifices (and of the sliding surfaces of valve plates). The machining operations, which may involve grinding, are time-consuming, hence costly and productive of fine, air-borne pollution in the form of dusts.

Lately, interest in the art has turned to a radical change from the established practice of producing refractory components by pressing and hightemperature firing. It has been proposed, instead, to cast such parts from concrete. Use of concrete is energy efficient since it may cure at room temperature, if hydraulically bonded, or at a modestly elevated temperature, if chemically bonded. Moreover, concrete is in theory an excellent candidate for manufacturing valve plates reinforced by metal trays or cans, because (i) the latter may serve as self moulds and (ii) high precision is attainable which may eliminate trueing operations or at least keep them to the minimum. It may be noted that canned valve plates made in the conventional way involve securing the refractory plates in their individual cans by a bed of cement.This method is not well adapted to precision manufacture and hence substantial trueing or levelling is necessary after the cement has cured.

We have found, unfortunately, that complete contact between the concrete and mould surfaces is difficult to obtain. Superficial voids or pockets may result, therefore, despite every care being taken to fill moulds fully. If mould filling is performed under vacuum, pockets may be avoided but then the quality of the concrete may change due to dewatering.

The present invention aims to overcome the aforementioned problems and to provide an improved method of making refractory articles, whether or not they possess orifices, from refractory concretes.

According to the present invention, there is provided a method of making a refractory article suitable for use in contact with molten metal, wherein a refractory concrete mix is introduced into a mould and the latter is vibrated to compact the concrete therein, and wherein at least a portion of the mould is formed from an airpermeable, water-impermeable or hydrophobic material and reduced pressure is exerted externally of the said mould portion to draw air therethrough while the mould is being vibrated and to minimise extraction of water from the concrete.

The invention also provides a moulding apparatus for use in making refractory concrete castings comprising a mould made from at least two coacting mould members, at least one of the mould members being or including an element made from air-permeable, water-impermeable or hydrophobic material, and suction means for establishing a reduced pressure on the said element to draw air therethrough from the mould space defined by the mould members, whereby extraction of water from the concrete is minimised.

For producing an article having a void-free level surface, the mould is provided with a smooth planar surface reproducing the said level surface and constituted by the said material which may be a plastics material.

For producing an orificed article, the mould includes a hollow core formed partly or entirely from a tube of the said material.

Should it be desired to produce a moulded article all the external surfaces of which are substantially void-free, the mould will be composed of coacting mould members all of which are made from the said materials.

The concrete can be hydraulically bonded or resin or chemically-bonded; shaped articles made by the method will be cured at a temperature appropriate to the bonding mechanism. Room temperature will suffice for curing hydraulically bonded concretes, although modestly elevated temperatures e.g. 1 0-1 500C may be used to ensure no free waters left within the cured mass.

Refractory articles for which the present method is applicable include, inter alia, well blocks and nozzle bodies for teeming vessels; orificed and non-orificed valve plates for sliding gate valves, and discharge nozzles or collector nozzles for orificed valve plates of such valves. Since the aforesaid articles are prone to wear, erosion and chemical attack, replacement insert members for those areas most likely to degrade in use, as well as replacement nozzle tips, can be made advantageously by the present method.

Ordinarily, sliding valve plates have planar surfaces for sliding, and such surfaces are readily produced at the interface between the mould filling and a flat sheet of the said material.

The method lends itself well to in-situ moulding of refractory material into pre-formed metal cans, to avoid the drawbacks of cementing fired refractory members into reinforcing cans. In-situ moulding is performed in a mould defined by a pre-formed can and the said sheet.

It may be advantageous so to perform the method when making an in-situ valve plate moulding that the sliding surface of the finished refractory stands proud of an adjacent rim of the can.

The invention will now be described in more detail by way of example only with reference to the accompanying drawings in which: Fig. 1 diagrammatically illustrates equipment used in the method according to the invention for making a valve plate and integral nozzle for a sliding gate valve; Fig. 2 diagrammatically illustrates apparatus used for making a well block suitable for a ladle; Fig. 3 diagrammatically illustrates apparatus used for making a wear resistant insert for a sliding gate valve plate; and Fig. 4 diagrammatically illustrates apparatus used for making a nozzle e.g. suitable for use in an outlet of a ladle.

The present method to be described concerns an improvement in the moulding of refractory articles from concretes. Moulds in which the articles are shaped are vibrated to consolidate the concrete introduced into them, and suction is also applied through selected members forming components of the moulds. Suction applied aids consolidation and ensures the concrete faithfully replicates the surfaces of those mould members through which the suction is applied.At least the concrete which interfaced with the said members is found to be void-free and -- depending on such parameters as the size of the moulding the surface area of the mould constituted by members through which the suction is applied, the consistency of the concrete, the effectiveness of vibration and the length of the moulding cycle it may be possible to obtain castings having substantially no internal voids.

The moulds are made from two or more coacting mould members and one or more or all of them may be such that suction can be applied therethrough. To produce the suction, the selected mould members alons can be subjected to a reduced external pressure. Alternatively, the mould members alone can be subjected to a chamber.

The selected mould members are made from an air-permeable, water-impermeable or hydrophobic material. Such a material may be metallic, ceramic or plastic and exemplary plastics materials are usually high density, open pore sintered products and can be polyolefinic e.g. polythene or PVC based. Suitable materials are commercially obtainable and two examples are Fibroplast (RTM) available from Schumacher Filters Limited and Vyon (RTM) from Porvair Limited. Mould members can readily be fabricated from these plastics by welding and machining.

Specific applications of the method will now be described with reference to the drawings.

Fig. 1 illustrates the apparatus 10 used for making a canned bottom plate for a sliding gate valve, this particular plate having an integral discharge nozzle projecting therefrom.

The apparatus 10 comprises a base 11 on which a hollow mould 12 is stood, the mould being a negative counterpart of the valve component to be moulded. The refractory is to have a plate portion and an integral nozzle portion which respectively will be defined by regions P and N of the mould 12. The mould is in two parts, namely a preformed metal can 14 and a smooth, flat plate 1 5 made from the air-permeable plastics material. The flat plastics plate 1 5, which forms one wall of the mould 12, is supported by the base 11.

In order to mould a valve refractory having a flow passage the mould 12 has a core. The core can be in two parts, as shown. Tubular part 16, which is sealed across its upper end, provides for the actual flow passage and is essential. Part 1 7 encircles part 1 6 and is optional. When used, part 1 7 is responsible for establishing an insertaccommodating recess in the moulded valve refractory. Part 1 6 is made from the aforementioned plastics material and it will be noted that its interior registers with a hole 1 8 in the plate 1 5.

The base 11 forms a suction chamber 19, its top on which plate 1 5 sits either being open or formed by a perforated wall. The base 11 is ported at 24 to allow the chamber 1 9 to be evacuated by a vacuum pump (not shown) coupled to the port by line 26.

The apparatus 10 further includes vibrator means (not shown) for vibrating or shaking the mould 1 2 to assure complete mould filling and consolidation of the refractory material. Suitable vibrators are readily available commercially, and vibration is transmitted by rods 31 affixed to the base 11. Additionally, or alternatively, vibrators could be coupled to the can 14, and the arrangement may be such that during moulding, the can 14 and plate 1 5 are moved bodily towards one another to assure complete filling of the mould 12.

In operation, the plate 1 5 and parts 1 6 and 17 may first be coated with a suitable release agent.

The necessary can 14 is then inverted and placed over the plate 1 5 and the mould space 12 filled with the concrete castable mix via a funnel or hopper 28. If preferred, the can 14 may be prefilled by hand before assembling it with the plate 1 5. Thereafter, the vibrators are actuated to consolidate the concrete in the mould space 12 and reduced pressure is applied externally of the mould by the suction chamber 1 9 and the associated suction means. Air which may become trapped between the concrete and either the plate 1 5 or tube 1 6 is sucked therethrough out of the mould space. Thanks to the plastics material being impermeable to water or hydrophobic, the moisture content of the mix is unlikeiy to be affected significantly by the suction exerted.

As consolidation is proceeding, the mould filling may be topped up as necessary via the hopper 28.

When the mould is completely filled, the concrete is caused to set or cure in the can 15, which becomes an integral part of the moulding.

If the concrete is a high alumina mix e.g.

8095% by weight alumina bonded hydraulically by alumina cement, curing can take place at room temperature.

Refractory e.g. high alumina concretes relying on other bonding agents may be preferred for some applications. Such bonding agents include, inter alia, phosphate binders and ligninsulphonate binders. Curing of such concretes will depend on exposing them to modestly elevated temperatures ranging for instance from 1 50 to 3000C or so.

The refractory article, whether it be a valve plate or some other shape, may be subjected to firing before putting it into service.

It will be appreciated that the same technique can be used to make valve plates, e.g. top plates, which do not possess integral nozzle extensions.

Even then a core equivalent to the tubular part 1 6 will advantageously be employed to exhaust air which may become trapped between it and the concrete to ensure the orifice wall is void free.

The centre plate of a three-plate valve has each of its opposite surfaces in sliding contact with a companion valve plate. Its opposite surfaces should therefore be smooth and free from defects.

For moulding such a plate from concrete, opposing mould members should comprise plates equivalent to plate 1 5 and they should each have associated suction means.

It is known from experience that the entrance to the orifice area of a valve plate is subject to extreme wear, erosion and chemical attack and that degradation thereof is particularly common when throttling. Hence it is often desirable to design plates such that the orifice area is defined by a replaceable insert. Mould member 1 7 is used for providing a recess equivalent in shape to a common type of insert. Erosion resistant inserts can be made of such materials as zircon, zirconia, magnesia and alumina. Concrete inserts could also be used to save material costs, although replacement will be more frequent.

Manufacture of a concrete orifice insert can be accomplished by the present method, utilising the apparatus shown in Fig. 3. The apparatus is substantially the same as in Fig. 1 , except that open-topped mould member 30 corresponds in shape with mould part 17 of Fig. 1. The other mould members are plate 1 5 and core 16, the latter being closed at its upper end. Before introducing the concrete into the mould space 12, at least mold member 30 will normally be coated with a release agent, and mould parts 15, 16 likewise if necessary.

Note that mould member 30 does not form an integral part of the finished insert casting, the insert not being metal jacketed.

It will be understood that, similarly, a valve plate may not always be made integral with the can 1 5. When making unjacketed or naked valve plates, the can 1 5 will be a permanent mould member and will usually be release-coated before casting to ensure the concrete shape will separate readily therefrom.

For manufacturing a well block e.g. for a ladle, the equipment of Fig. 2 may be used. The mould members comprise a suitably shaped base 31, a peripheral mould wall 32, a core 33 and a frustoconically-shaped member 34 made from the aforementioned plastics material. The plastics member 34 is backed by a suction chamber 35.

The mould is placed on a vibratory table 36. Using this apparatus, the well block entrance to the flow passage -- which is normally lined by a nozzle tube - will possess a smooth, void-free surface. If desired, the core 33 or part thereof may be constituted by a tube of the aforementioned plastics material. Moreover, if it is desired to minimise or prevent the presence of internal voids, all the mould members 31 to 34 may be fabricated from the aforementioned plastics material.

To form a nozzle tube, either for installation in a well block or for attachment to a valve plate as a discharge spout of a sliding gate valve, apparatus diagrammatically shown in Fig. 4 may be used.

The mould members are a base 40, to be placed on a vibratory table, an outer tube 41 and a boreforming core tube 42. The latter is closed at its upper end, is connected to suction means, not shown, by ducting 43, and is made from the aforementioned plastics material.

For making a nozzle for use in conjunction with a sliding gate valve, the outer tube 41 can be left integral with the casting.

Apparatus based on Fig. 4 can be used to make replaceable nozzle liners, e.g. for collector nozzles, and replaceable discharge tips for attachment to the downstream ends of such nozzles.

The method disclosed hereinbefore can be adapted to produce a composite refractory article e.g. a valve plate which is of laminated form. The sliding surface of the plate and desirably the orifice or bore area too will be produced from one material, e.g. a high duty refractory concrete which is resistant to sliding wear, and this material will be backed by a second refractory material. The physical, mechanical and chemical property requirements of the second material need not be as stringent as the first, face material.

Manufacture of such a composite plate will clearly involve a two-stage mould filling technique. It is expected that the composite plate will be particularly cost effective. One such technique is disclosed in our pending U.K. patent application No. 7943236, Serial No. 2,065,278A, to which reference is directed.

If desired, the composite plate can be made "self-lubricating" by inclusion of e.g. colloidal graphite in the first, face refractory material.

Other lubricating materials could be used as the addressee will recognise. Self-lubrication is not restricted solely to composite plates but could be adopted for monolithic plates produced in accordance with Fig. 1.

If naked refractory valve members are moulded for subsequent cementing into cans, some grinding may be needed for trueing. It is anticipated little or no grinding will be necessary if the members are directly moulded into their cans.

The invention can also be used to produce injection lances for blowing gas or gas/powder mixes into molten metal. Currently, such lances are made by pressing or casting wet refractory around a central steel tube. According to the invention, a dry mix is vibrated, dry into a vertical annulus formed between a central pipe and thin outer steel casing. Suction is applied either through an insert or inserts or the aforesaid material in the inner pipe or externally through such an insert(s) in the outer case. Suction could be applied via both the inner pipe and the outer casing. Lances made in this way may be up to 5 m long and of round or square section typically 200 mm across. After forming the lance will be heated e.g. to 150--4000C, and, depending on the materials used and expected service conditions, maybe to a higher temperature, say up to 600/7000 C.

Claims (14)

1. A method of making a refractory article suitable for use in contact with molten metal, wherein a refractory concrete mix is introduced into a mould and the latter is vibrated to compact the concrete therein, and wherein at least a portion of the mould is formed from an airpermeable, water-impermeable or hydrophobic material and reduced pressure is exerted externally of the said mould portion to draw air therethrough while the mould is being vibrated.

2. A method according to claim 1 for producing an article having a void-free level surface, comprising filling a mould with concrete wherein the mould is provided with a smooth planar surface reproducing the said level surface and constituted by the said material.

3. A method according to claim 1 or claim 2 for producing an orificed article, comprising filling the mould with concrete wherein the mould includes a hollow core formed partly or entirely from a tube of the said material.

4. A method according to claim 1,2 or 3, comprising filling the mould with concrete wherein the mould is composed of co-acting mould members all made from the said material, and air is drawn from substantially the whole of the filling interfaces with the mould members.

5. A method according to claim 2 or claim 3, for making a metal-canned refractory article, wherein the concrete is introduced into a mould comprising a space defined by a metal can or tray on the one hand and a confronting mould member made from the said material.

6. A method according to any of claims 1 to 5, wherein the mould is overfilled with the concrete and cooperating mould members are moved relatively towards one another to aid complete void-free filling of the mould.

7. A method according to any of claims 1 to 6, wherein the mould is filled with hydraulicallybonded refractory concrete, and the shaped article is thereafter allowed to cure at room temperature.

8. A method according to any of claims 1 to 6, wherein the mould is filled with chemicallybonded refractory concrete, and the shaped article is thereafter cured at an elevated temperature of no more than about 3000 Celsius.

9. A method according to claim 1 and substantially as herein described by way of example.

10. A refractory article when made by the method claimed in any of the preceding claims.

11. An article according to claim 10, which comprises a well block or nozzle body for a molten metal teeming vessel, the article having an orifice or bore whose surface is substantially free of voids.

12. An article according to claim 10, which comprises a valve plate for a sliding gate valve having a slide surface which is substantially free of voids.

13. An article according to claim 10, which comprises a discharge or collector nozzle for a sliding gate valve, or a valve plate according to claim 12, having such nozzle integral therewith, wherein said nozzle has a bore whose surface is substantially free of voids.

14. A moulding apparatus for use in making refractory concrete castings comprising a mould made from at least two coacting mould members, at least one of the mould members being or including an element made from air-permeable, water-impermeable or hydrophobic material, and suction means for establishing a reduced pressure on the said element to draw air therethrough from the mould space defined by the mould members.

1 5. Moulding apparatus according to claim 14 and substantially as herein described with reference to any of the accompanying Figs. 1 to 4 of the drawings.