WO2015044878A1

WO2015044878A1 - An induction furnace and a method of operating it

Info

Publication number: WO2015044878A1
Application number: PCT/IB2014/064801
Authority: WO
Inventors: Louis Johannes Fourie
Original assignee: Louis Johannes Fourie
Priority date: 2013-09-25
Filing date: 2014-09-24
Publication date: 2015-04-02

Abstract

The invention relates to a double loop channel type induction furnace (1) of which its throat (7) includes a central passage (8), two side passages (9, 10) and a floor passage (16); with the floor passage (16) extending and providing fluid communication between the base of the furnace floor (2) and the two side passages (9, 10); the central passage (8) serving as an inlet to the induction heater (6) and the two side passages (9, 10) extending on opposite sides of the central passage (8) as outlets from the induction heater (6), the central passage (8) and two side passages (9, 10) being complimentary shaped and configured to channels in the induction heater (6) and each of the central passage (8) and two side passages (9, 10) being in fluid communication with a complimentary channel of the induction heater (6); and the furnace floor (2) having a base (11) on a first side of the hearth (5) and a floor formation extending from the floor (2) to terminate in a plateau (13) above the throat (7) at a location distal from the first side.

Description

AN INDUCTION FURNACE AND A METHOD OF OPERATING IT

FIELD OF THE INVENTION

This invention relates to an induction furnace used in the melting or smelting of metals and particularly to induction furnaces used in smelting particulate materials floating on the surface of the metal and slag. BACKGROUND TO THE INVENTION

Conventional channel induction furnaces fed with particulate material floating on the surface are designed with relatively deep metal baths. This is so because particulate material floating on the slag layer on top of a bath of molten metal is a poor heat sink which leads to higher metal temperatures and recirculation of heated metal back into the channel heater. This results in overheating of the molten metal and damage to the refractory material lining, if the furnace is designed to operate with a shallow metal bath. Conventionally, operating an induction heated furnace with a shallow bath also results in relatively cold areas where the melting rate is relatively much slower than in the area directly above the channel heater. Conventional channel induction furnaces thus have to be operated with a deep metal bath.

On the other hand operating a channel induction furnace with a deep metal bath has the disadvantage that more metal must be kept in the furnace, leading to greater heat losses compared to using a shallow metal bath and an unnecessary high process inventory. Metal losses, damage to equipment and danger to personnel in the event of a metal leak are also increased when using a deep metal bath.

Further, in an induction furnace with a deep metal bath strong convection currents are set up in the furnace during operation thereof. This results in unstable rapid melting of particulate materials in some areas while in other areas no melting occurs. It has been found that melting particulate materials in a deep metal bath operatively leads to areas of melting migration, in other words the areas where melting occurs move around in the furnace, resulting in unstable flow and melting conditions. A previous attempt to overcome this problem, as set out in SA patent 2002/10025, was successful but the cost and start-up problems rendered this solution difficult to apply in practice. An improvement on the prior art was reflected in PCT patent application PCT/IB2012/050938 by the same applicant and inventor. This improvement related to a double loop induction heated furnace which includes central and side passages for metal flow. The furnace is provided with a plateau above the passages through which heated metal flows upwards to be distributed along it before joining the liquid metal bath. The furnace is provided with a passage between the floor of the furnace and the central passage, from which metal is drawn into the induction heater. This improved furnace may experience, especially in the hands of unskilled labour, a practical problem during start-up of the furnace and during operation thereof. The furnace includes a deep throat with a separate intake throat passage and its two flanking passages for heated liquid metal. The passages extend upwards from the double-loop of the induction heater, and provide fluid communication between the induction heater and the bath. When the furnace is constructed the double-loop of the induction heater is made from metal and the throat passages are hollow passages.

With start-up the induction heater is energised and the metal of double loop melts. With additional energy input this remains liquid and its temperature increases. At this point the liquid metal level is well below the floor level and still below the passages of the furnace so no circulation through the furnace occurs. There is however circulation in the double loop of the induction heater.

Since the aim of the process is to fill the furnace with metal, more metal is added to the furnace. This can be molten metal (from a separate melting pot) or pieces of solid metal that are loaded into a passage to drop into the pool of liquid metal therein.

By balancing the power input with the heating requirement of the added metal (be it molten or solid), the additional metal can be melted (or kept molten in the case where molten metal is added), and the level of metal in the furnace can be increased, first up into the passages and then eventually over the floor level. This process is repeated until a liquid bath is formed.

However, the bath level rises in concert with the liquid level in the side passages, and therefore flow of liquid metal through the side passages into the bath will only start when the bath level is above that of the exit point of the throat passages. Poor preheating of the refractory material can result in half filled passages, which may lead to an air gap between solidified segments of a passage and molten segments of such a passage. This in turn prevents circulation through the passage and the metal in the induction heater channels may then be heated to very high temperatures. This may lead to metal penetration into the refractory of the induction heater and eventually metal leaking from the induction heater.

During operation, the design of this furnace requires the bath level to remain above the plateau to ensure circulation. If the bath level is inadvertently allowed to drop to below the plateau, metal in the induction heater will overheat if power is not reduced to low rates. Such low rates may not be sufficient to keep the metal bath liquid, and the furnace may freeze over or burn through as a result.

OBJECT OF THE INVENTION

It is an object of the invention to provide an induction heated furnace which at least partly overcomes the abovementioned problems.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a double loop channel type induction furnace comprising a shell lined with refractory material, and having a floor and a wall extending from the floor to form a hearth, at least one induction heater associated with the furnace and communicating with the hearth by means of a throat in the floor;

the throat including a central passage, two side passages and a floor passage; with the floor passage extending and providing fluid communication between the base of the furnace floor and the two side passages; the central passage serving as an inlet to the induction heater and the two side passages extending on opposite sides of the central passage as outlets from the induction heater, the central passage and two side passages being complimentary shaped and configured to channels in the induction heater and each of the central passage and two side passages being in fluid communication with a complimentary channel of the induction heater;

the furnace floor having a base on a first side of the hearth and a floor formation extending from the floor to terminate in a plateau above the throat at a location distal from the first side, with the floor formation and plateau extending at least partly between opposing end walls of the furnace and the plateau including a trench which extends at least partly between opposing ends of the plateau, and the side passages terminating in the trench and the bottom of the trench operatively being located in a plane higher than the plane in which the furnace floor is located. There is also provided for the floor passage to be in communication with both side passages by means of a branch in the floor passage, alternatively for the furnace to include two floor passages extending from the base of the floor, one to each of the side passages through the floor formation below the trench. There is further provided for the floor passage to be orientated substantially horizontally.

There is still further provided for the floor to be provided with a recessed portion forming a well in the floor at the top of the central passage and for the floor passage to extend between the recessed portion and the side passages.

There is further provided for the central passage to be located adjacent the floor formation and for the side passages to extend from the induction heater adjacent the central passage through the floor formation to the trench, preferably for the side passages to be angled away from the central passage to extend adjacent each other through the floor formation to the trench, and further preferably for the side passages to be angled towards each other.

There is still further provided for the furnace to include an inspection porthole in the roof above the central passage. There is further provided for the induction heater and the plateau to be located at a second side opposite the first side of the furnace.

There is further provided for the hearth to have an operating depth which corresponds with a liquid metal meniscus level that operatively is located high enough to cover the plateau with liquid metal.

There is further provided for the furnace to include at least one tapping hole, preferably located above the height of the plateau.

There is still further provided for the floor formation to comprise a ramp. According to a further feature of the invention there is provided a method of starting up a furnace as defined above, including applying electrical power to the induction heater to melt the preformed metal channel former of the induction heater, adding discrete metal particles to the central passage whilst controlling the electrical power input to the induction heater to ensure the metal in the passage remains liquid without overheating it, when heated liquid metal flows from the side passages through the floor passage to the hearth increasing the rate of addition of metal particles to the furnace and increasing the electrical power input to the induction heater commensurately, until the furnace is at operational level. There is further provided for the method to include using visual inspection through the inspection porthole to provide visual feedback for controlling the electrical power input to the induction heater to ensure the metal in the passage remains liquid without overheating it.

These and other features of the invention are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a semi-transparent rear perspective view of part of a furnace according to the invention;

Figure 2 is a semi-transparent side elevation view of part of the furnace of Figure 1 ;

and

Figure 3 is a semi-transparent side perspective view of part of the furnace of Figure 1 . DETAILED DESCRIPTION OF THE INVENTION

The drawings show part of a channel type induction furnace (1 ) according to the invention. As shown in the drawings, the furnace (1 ) has a generally rectangular shape and includes a floor (2) with end walls (not shown for the sake of clarity) and side walls (3, 4) extending from it which forms a hearth (5). A double loop induction heater (6) (of which only the channels are shown) is secured to the base of the furnace (1 ) and communicates with the hearth (5) through a throat (7) in the furnace floor (2). The throat (7) includes a central passage (8) which serves as an inlet into the induction heater (6). The throat (7) also includes two side passages (9, 10) on opposite sides of the central passage (8) which serve as outlets from the induction heater. The furnace floor (2) includes a base (1 1 ) proximate a first side of the hearth (5) adjacent the first side wall (3), and a floor formation in the form of a ramp (12) which rises from the base (1 1 ) to terminate in a plateau (13) proximate a second side of the hearth (5). The second side of the hearth (5) is located at the opposing side of the furnace (1 ) adjacent the second side wall (4).

The ramp (12) and plateau (13) extend between opposing end walls (not shown) of the furnace (1 ). The plateau (13) includes a trench (14) which extends between the end walls (not shown), within which a side passage outlet block (21 ) is centrally located. The trench (14) is in fluid communication with the two side passages (9, 10). The bottom of the trench (14) is located higher in the hearth (5) than the base (1 1 ) of the furnace floor (2).

The base (1 1 ) of the furnace floor (2) is in fluid communication with the central passage (8), which opens into a recess (15) in the base (1 1 ), located adjacent the ramp (12). A floor passage (16) extends from within the recess (15) to the two side passages (9, 10). The floor passage (16) extends as a single passage from the recess (15) and then branches to reach both side passages (9, 10).

The central passage (8) is located underneath the recess (15), which is located adjacent the foot of the ramp (12). The central passage (8) and the side passages (9, 10) extend upwards from the channels of the induction heater (6). After a short distance the side passages (9, 10) are angled (17) away from the central passage (8), towards the ramp (12). The side passages (9, 10) are also angled (18) towards each other, as can be seen in Figure 1 in this part of the throat (7).The side passages (9, 10) continue upwards at this angle (17) to the point where they are located underneath the trench (14).

From here the side passages (9, 10) continue directly upwards to the trench (14) in which they open out of a central outlet block (21 ) constructed in the trench (14), each of these (9, 10) then being directed towards (19) one of the opposing ends of the trench (14). The general operation of the furnace of patent application PCT/IB2012/050938 was described in detail therein and apart from the differences that relate to the improvements herein, the description of PCT/IB2012/050938 is included hereby with reference. It will be apparent that the central passage (8) is located underneath the recess (15) which is adjacent the ramp (12), and the side passages (9, 10) are angled away (17) from the central passage (8) and towards (18) each other.

When the furnace is started up for the first time, the metallic channel former of the induction heater (solid metal that forms the channels of the induction heater), is heated by the application of electrical power to the induction heater (6). This melts the metal of the double loop channel former. With additional energy input this remains liquid and its temperature increases. At this point the liquid metal level is well below the floor (2) level and still below the passages (8, 9, 10, and 16) of the furnace (1 ) so no circulation through the hearth (5) occurs. There is however circulation in the double loop of the induction heater (6).

More metal is then added to the furnace (1 ), in the form particles of solid metal or metal rods that are loaded into the central passage (8) to drop into the pool of liquid metal in it. The furnace (1 ) is provided with a loading door (20) above the plateau (13) for this purpose. Alternatively the metal is charged through a hole in the roof (not shown).

By balancing the power input with the heating requirement of the added metal, the additional metal can be melted, and the level of metal in the furnace (1 ) is increased, first up into the passages (8, 9, 10, 16) and then eventually into the hearth (5) up to the level of the plateau (13).

As the level of metal rises in the central and side passages (8, 9, 10), they do so in concert. When the level has risen high enough for liquid metal to flow into the floor passage (16), it will flow into the metal which by then will also be present in the recess (15). At this point circulation will commence through the central passage (8), the side passages (9, 10) and the floor passage (16). This allows circulation to be established before the hearth (5) itself is filled with liquid metal.

With the addition of more metal into the bath the level will rise further. Circulation will continue through the central passage (8), the side throat passages (9, 10) and the floor passage (16). With the addition of enough metal into the furnace (1 ) the level will rise to the plateau (13). At the same time the liquid metal will also rise in the portions of the side passages (9, 10) above the floor passage (16), although there will not yet be active circulation in them. Once the bath level rises above the plateau (13), the flow of heated liquid metal will commence though the side passages (9, 10) into the trench (14) and over the plateau (13). At this point normal circulation is thus established in the furnace (1 ).

In the event that an operator inadvertently taps the furnace (1 ) to a level below that of the plateau (13), circulation will continue through the floor passage (16), and the furnace (1 ) can continue to be operated. New raw material may be charged to the furnace (1 ) to raise the level to above the plateau (13) level, at which point circulation over the plateau (13) will continue again.

The circulation will preferentially follow the path upwards through the side passages (8, 9) into the trench (14) and over the plateau (13), which forms the "upper flow path", instead of through the floor passage (16), which forms the "lower flow path". The reason for this is that the upper flow path presents a clear and even flow path. The lower flow path includes a right angle turn (22), from the side passages (9, 10) into the floor passage (16) which from a preservation of momentum perspective is less efficient. The metal in the side passages (9, 10) is at higher temperature than the metal in the hearth (5) (since it is flowing out of the induction heater (6)), which results in convective flow through the side passages (9, 10). Due to the jet effect, a small amount of metal can be "sucked" from the hearth (5) through the floor passage (16) to join the metal coming from the induction heater (6).

Thus, when the upper flow path is available it will preferentially be followed. Only if this is not available, such as when the furnace (1 ) is busy with start-up or as a result of too much metal having been tapped from the furnace (1 ), will the lower flow path be followed. Switching between the lower flow path and the upper flow path and vice versa happens automatically, as the above circumstances dictate. In this way the furnace (1 ) is protected against mistakes during start-up and normal operation.

It will be appreciated that the embodiment described above is given by way of example only and is not intended to limit the scope of the invention. Changes to the embodiment are possible without departing from the essence of the invention. It is for example possible to utilise a trench without a side passage outlet block (21 ). In such an instance the side passages (9, 10) may simply exit into the floor of the trench (14).

It should be kept in mind that the throat (7), passages () and the conduits (8, 9, 10, and 16) and the channels (6) of the induction heater are all formed as conduits in the surrounding refractory material (which, for the most, is not shown for the sake of clarity). Where the side passages (9, 10) thus exit into the trench (14), it is merely the conduit through the refractory material that terminates in the relevant face of the refractory than a "pipe" that opens into the trench (14). The appearance of a "pipe" is created by the shape of the conduit formed within the relevant refractory portion.

Claims

1 . A double loop channel type induction furnace comprising a shell lined with refractory material, and having a floor and a wall extending from the floor to form a hearth, at least one induction heater associated with the furnace and communicating with the hearth by means of a throat in the floor;

the throat including a central passage, two side passages and a floor passage; with the floor passage extending and providing fluid communication between the base of the furnace floor and the two side passages; the central passage serving as an inlet to the induction heater and the two side passages extending on opposite sides of the central passage as outlets from the induction heater; the central passage and two side passages being complimentary shaped and configured to channels in the induction heater and each of the central passage and two side passages being in fluid communication with a complimentary channel of the induction heater;

the furnace floor having a base on a first side of the hearth and a floor formation extending from the floor to terminate in a plateau above the throat at a location distal from the first side of the hearth, with the floor formation and plateau extending at least partly between opposing end walls of the furnace and the plateau including a trench which extends at least partly between opposing ends of the plateau, and the side passages terminating in the trench and the bottom of the trench operatively being located in a plane higher than the plane in which the furnace floor is located.

2. A furnace as claimed in claim 1 in which the floor passage is in communication with both side passages by means of a branch in the floor passage.

3. A furnace as claimed in claim 1 which includes two floor passages extending from the base of the floor, one to each of the side passages.

4. A furnace as claimed in claim 2 or 3 in which the, or each, floor passage is orientated substantially horizontally.

5. A furnace as claimed in any one of claims 1 to 4 in which the floor is provided with a recessed portion forming a well in the floor at the top of the central passage and the floor passage extends between the recessed portion and the side passages.

6. A furnace as claimed in any one of claims 1 to 5 in which the central passage is located adjacent the floor formation and the side passages extend from the induction heater adjacent the central passage through the floor formation to the trench.

7. A furnace as claimed in claim 6 in which the side passages are angled away from the central passage and angled towards each other to extend adjacent each other through the floor formation to the trench.

8. A furnace as claimed in any one of the preceding claims which includes an inspection porthole in the roof above the central passage.

9. A furnace as claimed in any one of the preceding claims in which the induction heater and the plateau are located at a second side opposite the first side of the furnace.

10. A furnace as claimed in any one of the preceding claims in which the hearth has an operating depth which corresponds with a liquid metal meniscus level that is located high enough operatively to cover the plateau with liquid metal.

1 1 . A furnace as claimed in any one of the preceding claims in which the floor formation comprises a ramp.

12. A furnace as claimed in any one of the preceding claims which includes at least one tapping hole located above the height of the plateau.

13. A method of starting up a furnace as claimed in any one of claims 1 to 12 which includes applying electrical power to the induction heater to melt the preformed metal channel former of the induction heater, adding discrete metal particles to the central passage whilst controlling the electrical power input to the induction heater to ensure the metal in the passage remains liquid without overheating it, and increasing the rate of addition of metal particles to the furnace and increasing the electrical power input to the induction heater commensurately when heated liquid metal flows from the side passages through the floor passage to the hearth, until the furnace is at operational level.

14. A method as claimed in claim 13 which includes using visual inspection through the inspection porthole to provide visual feedback for controlling the electrical power input and feeding rate to the induction heater to ensure the metal in the passage remains liquid without overheating it.