GB2165628A

GB2165628A - Metal-holding furnace

Info

Publication number: GB2165628A
Application number: GB08420652A
Authority: GB
Inventors: Ermenegildo Botta
Original assignee: MORGANITE CRUCIBLE Ltd
Current assignee: MORGANITE CRUCIBLE Ltd
Priority date: 1984-08-14
Filing date: 1984-08-14
Publication date: 1986-04-16
Also published as: GB2165628B; GB8420652D0

Abstract

A metal-holding furnace comprises a crucible (1) surrounded by an array of malleable rod electric resistance heating elements (3) supported by an imperforate refractory cup (5). The cup (5) is intended to hold any metal leaking from the crucible, and is insulated from and carried in a lift-out metal shell (7) for removal, when required, of the whole crucible assembly from an insulated furnace chamber (8). <IMAGE>

Description

SPECIFICATION Metal-holding furnace This invention relates to metal-holding furnaces and provides a furnace especially for holding non-ferrous metal in a molten state ready for baling out and use in a foundry. The furnace is particularly suitable for aluminium.

It is known to use as a bale-out metal bath, for holding molten metal in a furnace, a crucible which is an open-top vessel of a refractory material, such as silicon carbide or clay graphite, resistant to chemicals used in metal treatment or processes.

Economical heating of a bale-out crucible presents a problem in keeping as short as possible the heat transmission path to the metal. Electrical radiant heaters directed downwardly on to the molten metal surface are known but only a limited depth of metal can be economically heated in this way. Heating panels, with embedded electric resistance heaters, surrounding the crucible are also known but their heating effect is indirect, with a relatively long transmission path, and they are costly to make and replace.

Crucibles of chemically-resistant refractory material are fragile and the result of a crucible failure is not only that any surrounding heating elements may be damaged, by leaking molten metal, but also the thermal insulation of the furnace may be damaged and this is very expensive to replace.

There is a requirement for a furnace from which a crucible can be lifted as a whole but the molten metal charge of a crucible which can safely be lifted is limited by the crucible wall strength.

The present invention provides a metal-holding crucible furnace with an efficient heating system and provision for limiting damage from crucible failure as well as safter crucible lifting when required.

According to the invention, in a metal-holding furnace having a crucible surrounded by electrical resistance heating members, the heating members are malleable rod elements formed as an array closely to surround the crucible wall and supported by an imperforate cup of refractory material, capable of retaining molten metal in the event of crucible leakage, the cup being held in a thermally insulating lining of a metal shell which rests in and is upwardly removable from a furnace chamber.

The malleable rod elements can be formed closely to espouse the shape of the crucible wall, and even contact the crucible without causing damage, so that the metal can be efficiently heated, throughout its depth, by conduction through a transmission path little greater than the thickness of the crucible wall.

The advantage of the metal-retaining supporting cup will be apparent and the metal shell not only forms a lifting carrier, when needed, for the whole assembly of the crucible, heating elements and metal-retaining cup, but protects the expensive thermal insulation of the furnace chamber from damage by leaking metal.

In a preferred furnace construction, both the rim of the mouth of the crucible and the surrounding rims of the metal-retaining cup and metal shell are sealed against molten metal spilling over.

Also, preferably, a furnace chamber cover includes a pair of sliding lids, on either side of a baffle which depends into the crucible and divides the molten metal surface area into a filling and replenishing compartment and a bale-out compartment respectively.

Other features of the invention are included in the following description, given by way of example, of a crucible metal-holding furnace according to the invention as shown by the drawings, in which: Figure 1 is a vertical sectional view, Figure 2 is a half-plan, corresponding to Fig.

1, and Figure 3 is a somewhat diagrammatic plan of the lay-out of the electrical resistance heating elements.

The furnace illustrated has a cylindrical crucible 1, of a silicon carbide or clay graphite material, which rests on a plinth 2, of thermal refractory insulating material, such as a lightweight castable material, and is very closely surrounded by an array of malleable metalsheathed, electrical resistance, heating elements 3. The actual space between the elements 3 and the wall of the crucible 1 may be as small as 20 mm or even less.

As shown by Fig. 3, the heating elements 3 are wired in groups to terminal blocks 4 so that, by selection of the groups energised, the heating of the crucible can be controlled.

The heating elements 3 are supported by an imperforate cup 5 of a refractory material, preferably a low iron content castable material, which will retain any molten metal which might leak from the crucible 1.

The cup 5 is surrounded by highly thermal insulating material 6, preferably a microporous silica refractory material in the form of boards, fitted into a rectangular metal shell 7. The rim of the metal shell 7 has lifting attachment holes or other means, not shown, by which the shell and all its contents, comprising the crucible 1, plinth 2, heating elements 3, cup 5 and insulation 6, can be lifted bodily from and lowered into the furnace chamber generally indicated as 8.

The furnace chamber 8 has an outer fabricated metal case 9 and a thick thermal insulating lining, of vermiculite slabs 10 with a wrapping 11 of mineral wool insulating blanket, held in place, inwardly spaced from the case 9, by upper peripheral capping slabs 12 of vermiculite.

The thermal insulating material outside the metal shell 7 is expensive and easily damaged by molten metal. The cup 5 however retains molten metal which might leak from the crucible and the metal shell enables the whole crucible assembly to be lifted out, for repair or replacement, without damaging the furnace chamber lining.

A pyrometer 13 is provided as an attachment depending into the crucible 1.

The furnace chamber has a removable upper frame 14 with a thermal insulating lining of refractory material comprising microporous silica boards 15 and a thick inner facing 16, of a hot face castable material, which rests, with interposed refractory sealing gaskets 17 and 18, on the rim of the crucible 1 and the periphery of the shell 7 and the assembly removable therewith.

Suspended from the upper frame 14 is a refractory baffle 19 which depends into the mouth of the crucible and divides the metal bath into a bale-out compartment 20 and a filling compartment 21. These two compartments each have a sliding cover, 22 and 23 respectively, each comprising a metal top 24, mounted to run on rails 25, and an insulating refractory lining of micro-porous silica boards 26 with a thick filling of mineral wool 27.

Claims

1. A metal-holding furnace having a crucible surrounded by electrical resistance heating members, the heating members being malleable rod elements formed as an array closely to surround the crucible wall and supported by an imperforate cup of refractory material, capable of retaining molten metal in the event of crucible leakage, and the cup being held in a thermally insulating lining of a metal shell which rests in and is upwardly removable from a furnace chamber.

2. A furnace according to claim 1, in which the metal shell contains, as a lift-out assembly, the crucible, the array of heating elements, the cup and thermal insulation between the cup and metal shell.

3. A furnace according to claim 2, in which the furnace chamber comprises an outer case and a thick thermal insulating lining in which the metal shell rests.

4. A furnace according to any of claims 1 to 3, in which the furnace chamber has an upper frame from which a baffle depends into the crucible, to divide the metal bath into bale-out and filling compartments, and separate covers are provided on the upper frame for these compartments.

5. A metal-holding furnace substantially as described and illustrated by the drawings.