GB2339888A

GB2339888A - Induction furnace

Info

Publication number: GB2339888A
Application number: GB9813110A
Authority: GB
Inventors: Brian Nalborough
Original assignee: RUSTEC Ltd
Current assignee: RUSTEC Ltd
Priority date: 1998-06-17
Filing date: 1998-06-17
Publication date: 2000-02-09
Anticipated expiration: 2018-06-17
Also published as: GB2339888B; GB9813110D0

Description

2339888 INDUCTION FURNACE This invention relates to an induction furnace,

and in particular to an induction furnace which is particularly suitable for the disposal of waste materials by high temperature heat treatment, although it may be used in other applications.

Electrically powered furnaces in which heat is produced by electrical induction are well-known. The basic structure of such furnaces comprises an electrical coil within which is placed a susceptor.

Passage of alternating electrical current through the coil produces heat in the susceptor which is used to heat the furnace. A preferred material for the susceptor is graphite. However, particularly at high temperatures, graphite is attacked by oxygen and thereby eroded in use and therefore is unsuitable for use in a furnace for prolonged use at high temperatures unless oxygen is totally excluded from the furnace. Nevertheless, there are applications of such furnaces where it is either not possible to exclude oxygen or oxygen-releasing materials, or it is advantageous in the application to use oxygen.

Attempts have been made to solve this problem by chemical doping of the graphite or to use materials other than graphite as the susceptor, but these have not been entirely satisfactory.

It has also been known to use various refractory materials for the purposes of heat insulation or heat shielding in induction furnaces.

According to the present invention there is provided an induction furnace wherein graphite as a susceptor is embedded within a refractory material the refractory material forming the lining of the furnace chamber.

It is preferred that the graphite susceptor is in the shape of a cylinder which is embedded within a cylinder of the refractory material, the latter forming the wall of the furnace chamber. It is possible to use two or more graphite susceptors in series in an induction furnace in which case each susceptor would be surrounded by a corresponding coil. For maximum efficiency the induction coil is about 1k times the length of the susceptor and the susceptor is positioned symmetrically within the coil.

The inclusion of the graphite cylinder in the cylinder of refractory material may for example, be accomplished by providing a corresponding slot in the refractory material into which the graphite cylinder can be slid. In this case, after the graphite cylinder has been positioned, any remaining space within the slot can be filled, for example, with a suitable particulate material such as carbon black and the end of the refractory cylinder through which the graphite cylinder has been inserted can be blocked off, for example, by a cylindrical extension to the corresponding end plate of the furnace which can protrude into the cylindrical slot.

An alternative way of embedding the graphite susceptor in the refractory material is to cast the refractory material around the graphite. However, the graphite may be in the form of particulate graphite which is dispersed in the refractory material and is concentrated within an interior volume of the refractory material.

The refractory material to be used in the present invention is preferably chemically resistant, has high thermal shock resistance, a low coefficient of thermal expansion and refractoriness at least up to 17000C.

High purity alumina is particularly suitable although it is envisaged that other suitable materials can be used. When high purity alumina is used it is preferable that its purity is at least 99%, and more preferably at least 99.5%. Particularly preferred types of material for use in the furnaces of the invention are those which are designated SKA 100 NG and Alsint 99.7 as supplied by the firm Haldenwanger. However, other similar materials can be used.

It is preferred that the coil of the furnace is contained within a gas-tight chamber surrounding the cylindrical refractory wall of the furnace. This provides a safety factor in the unlikely event that the wall of the refractory material should crack and release gases from the furnace chamber. In such an event the gases would still be retained within the furnace by the aforesaid gas-tight chamber which is preferably provided with means to fill it with nitrogen or some other inert gas.

The furnace will preferably be arranged to operate at a small angle to the horizontal so that material fed through the furnace at its upper end is assisted by gravity to move to the lower end. To further assist the progress of the material through the furnace means are provided to rotate the cylinder about its major axis. Furthermore, the inner surface of the cylinder of refractory material is preferably formed with one or more protrusions to assist progress through the furnace of the material which is being heated by the furnace, such protrusion or protrusions being particularly preferably in the form of one or more helical flanges.

Particularly in applications such as the disposal of waste, but also in other possible applications of the furnace, it is important that the furnace provides a sealed environment and to this end rolling seals may be provided at each end of the cylinder, such seals being made of a suitable steel, and further that air locks are provided also at each end of the furnace.

For such applications as waste disposal it is also desirable to provide means for injecting air, oxygen, water or steam into the furnace chamber in order to control the chemistry of the particular waste disposal operation which is being performed.

With a view to controlling the furnace it is also desirable to include means for temperature measurement at a plurality of locations within the furnace chamber by detecting and measuring heat radiation from said locations.

Regarding the furnace as a whole it will be is appreciated that, as in all structures involving the use of refractory materials at high temperatures, the whole of the revolving part of the furnace should be very adequately supported in order to prevent undue stresses in the refractory material.

The induction furnace of the invention will now be illustrated by way of example with reference to the accompanying drawings in which:

Figure 1 is a vertical section of the main part of an induction furnace is accord with the present invention; and Figure 2 is a perspective view of part of the structure shown in Figure 1 in disassembled form showing the individual components.

In the furnace exemplified, a cylinder of high purity alumina (1) having a length of 2.357 metres, an internal diameter of 0.9 metre and an external diameter of 1.155 metres is held between two annular end plates (2, 3). The structure is positioned at a small angle to the horizontal so that the end plate (2) can be regarded as a upper end plate and the end plate (3) can be regarded as the lower end plate. The alumina cylinder is held in position between the two end plates by clamping rings (4, 5) which abut respectively against external annular flanges (6, 7) at respective ends of the refractory cylinder. Each clamping ring is formed in two halves to enable its fitment to clamp the refractory cylinder.

The cylinder (1) is formed with a cylindrical slot having an external diameter of one metre and an internal diameter of 0.93 metre, the length of the slot being approximately 1.67 metres from the upper end of the cylinder.

Positioned within the slot and abutting its inner end is a graphite cylinder (9) having a length of 1.2 metres, an internal diameter of 0.93 metre and an external diameter of 0.97 metre.

In this particular embodiment of the invention the slot 8 is closed by an annular projection (10) of the upper end plate. It will be understood therefore that the graphite cylinder is already in position in the refractory cylinder (1) before the end plate 2 is secured to the refractory cylinder (1).

Remaining space in the slot (8) may be filled for example with carbon black (not shown).

Surrounding the refractory cylinder (1) is a conventional electrical induction coil (11) having a length of 1.8 metres and a thickness of about 0.15 metres. The induction coil 11 is encased in the cylindrical stainless steel cover 12 so that the coil occupies a gas-tight space surrounding the refractory cylinder and furnace chamber and means not shown are provided to fill the space with nitrogen or some other inert gas, as required.

To assist the movement of material which is being heat treated through the furnace chamber (13), a helical protrusion (14) is formed integral with the internal surface of the refractory cylinder.

The whole structure is mounted at each end on bearings (not shown) to provide rotation, and rolling seals and airlocks (also not shown) are also fitted at both ends of the furnace. This ancillary equipment, along with the electrical circuitry of the induction heater and also the heat radiation detector means and related control equipment are all of a conventional nature and therefore need not be described in order to enable to skilled person to operate the new furnace structure of the invention.

It will be understood that many variations could be adopted based on the specific structure hereinbefore described without departing from the scope of the invention as defined in the following claims.

7

Claims

1. An induction furnace wherein graphite as a susceptor is embedded within a refractory material, the refractory material forming the lining of the furnace chamber.

2. An induction furnace as claimed in claim 1 wherein the refractory material is cylindrical in shape and the interior surface of the cylinder forms the lining of the furnace chamber.

3. An induction furnace as claimed in claim 1 or claim 2 wherein the graphite is in the form of particulate graphite which is dispersed in the refractory material and is concentrated within an interior volume of the refractory material.

4. An induction furnace as claimed in claim I or claim 2 wherein a solid cylindrical graphite susceptor is embedded within the refractory material.

5. An induction furnace as claimed in claim 5 wherein the graphite cylinder is positioned within a cylindrical slot formed in the cylinder of refractory material.

6. An induction furnace as claimed in any one of the preceding claims wherein the refractory material is chemically resistant, has high thermal shock resistance, a low coefficient of thermal expansion and refractoriness at least up to 17000C.

7. An induction furnace as claimed in any one of the preceding claims wherein the refractory material is a high purity alumina.

8. An induction furnace as claimed in any one of claims 2 to 7 wherein the coil of the furnace is contained within a gas-tight chamber surrounding the cylindrical wall of the furnace.

9. An induction furnace as claimed in claim 8 wherein means are provided to fill the gas-tight chamber with nitrogen or inert gas.

10. An induction furnace as claimed in any one of claims 2 to 9 wherein means are provided to rotate the cylinder about its major axis.

11. An induction furnace as claimed in claim 10 wherein the inner surface of the cylinder of refractory material is formed with one or more protrusions to assist progress through the furnace of the material which is being heated by the furnace.

12. An induction furnace as claimed in claim 11 wherein the protrusion or protrusions are in the form of one or more helical flanges. 25

13. An induction furnace as claimed in any one of claims 10 to 12 wherein the cylinder is provided at each end with a rolling seal.

14. An induction furnace as claimed in any one of the preceding claims which is provided at each end with an air lock.

15. An induction furnace as claimed in any one of the preceding claims which also comprises means for injecting air, oxygen, water or steam into the furnace chamber.

16. An induction furnace as claimed in any one of the preceding claims comprising means for temperature measurement at a plurality of locations within the furnace chamber by detection and measurement of heat radiation from said locations, for the purpose of furnace control. 10

17. An induction furnace as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

18. Use of a furnace as claimed in any one of the preceding claims in the disposal of waste materials.