GB2096291A

GB2096291A - Combined smelting and holding furnace

Info

Publication number: GB2096291A
Application number: GB8208699A
Authority: GB
Original assignee: NEVREN LENNART; OTTOSSON BENGT GORAN
Current assignee: NEVREN LENNART; OTTOSSON BENGT GORAN
Priority date: 1981-04-03
Filing date: 1982-03-24
Publication date: 1982-10-13
Also published as: IT8248161A0; JPS5828977A; DE3211075A1; FR2503342A1; IT1148912B; SE435547B; CA1172035A; DK150982A; SE8102150L

Abstract

Combined smelting and holding furnace, primarily for aluminium alloys, comprises a vat (2) for a melt (11), the furnace having one filling-up part (4), one heat-emitting element part (5) and one discharge part (6). The furnace is designed for continuous operation in such a way that the temperature of the removed, molten metal is not lowered when the furnace is being refilled; also a reduction of oxidation of the metal is achieved. The furnace has the closed heat-emitting element part (5), separated from the filling-up part via a partition wall (22, 23) which has a lower part (22), made from a heat-insulating material and creating, at the bottom of the vat, a passage (21) through the smelt between the filling-up part and the heat-emitting element part. The partition wall also has an upper part (23). constructed from a heat- conducting material to radiate heat from the element part to the filling-up part in order to preheat material entering the furnace. <IMAGE>

Description

SPECIFICATION Device for combined smelting and holding furnace This invention concerns a combined smelting and holding furnace for continuous operation, designed for melting and maintaining the temperature of a metal alloy, for example such an aluminium alloy that is employed for die casting, and designed to prevent the temperature of the molten metal from being affected when the furnace is being refilled with metal for melting.

In casting engineering it is common practice for the metal, which is to be used for casting, to be melted in melting pots and, when the metal has melted, to be taken out to be used for casting.

However, if a smelting furnace, which is designed to be used in this manner, is to be used for continuous operation for long periods of time, problems will arise. In the first place it is impossible to keep the temperature of the metal melt within sufficiently close limits for long periods of time because refills of metal must be given at even intervals, which will seriously affect the temperature level. Furthermore, the refill of metal - but naturally also the removal of molten metal - means that the furnace must be opened, thereby increasing the thermal losses and causing a drop of temperature.Also it is far from safe to pour, for example, scrap metal for melting into a liquid pool of metal ctue to the fact that such scrap metal often contains blowholes, moisture or other substances that are likely to emit gas when heated. This may result in gas being intermittently emitted, causing the liquid metal to be thrown around. Neediess to say, molten metal may cause personal injuries and fires if thrown out in this manner. In addition, the risk of corrosion is great if aluminium melt is thrown around, hitting, for example, the heating elements of the furnace. It is a fact that heating elements of this type are quickly destroyed when getting into contact with molten aluminium.

It is also a well-known method to melt considerably greater amounts of aluminium in batches in a separate smelting furnace, the molten aluminium being taken out when its temperature exceeds that of the finished casting. The molten metal is transported at that temperature to separate holding furnaces where it is kept untii being used. The main drawback of this method is that two smelting furnaces are required to achieve an acceptable level of reliability, each furnace having the capacity to supply the entire requirement of molten metal. Furthermore, the transport of molten metal involves certain elements of risk and, in addition, the temperature of the transported metal may be difficult to control.

Oxidation has also been a problem in furnaces of previous designs, partly by causing metal losses, partly by causing furnace coatings which have a tendency to "climb" the inner surfaces of the furnace, thereby causing damage.

The object of the present invention is to achieve a combined smelting and holding furnace for continuous operation, designed in such a way that the discharge temperature of the molten metal is not affected when the furnace is being refilled with crude metal. In addition, the object of this invention is to achieve a smelting furnace, designed to eliminate the safety and corrosion risks mentioned above.

According to this invention this object is achieved with a device for a combined smelting and holding furnace, equipped with a vat, made from a heat-resistant and heat-insulating material and made to hold a smelt, especially a metal smelt; the furnace having one filling-up part, one discharge part and one heat-emitting element part and being characterized by having the lower part of the filling-up part in the vat and the upper part above the vat, the filling-up part being separated from the other components of the furnace by a partition which reaches down into the smelt, creating a connecting passage in the smelt.

According to this invention the device is also characterized by its partition having one upper part, made from heat-conducting material, adjacent to the upper part of the filling-up part, and one lower part, made from heat-insulating material and reaching down into the upper part of the smelt. By these features of construction is achieved that cold metal, which is supplied to the filling-up part, is pre-heated by heat transferthrough the upper part of the partition to a temperature level that causes drying-up and release of gas from the material, eliminating the risk of having liquid metal thrown around. In addition, the presence of the lower heat-insulating part of the partition means that the temperature level of the rest of the furnace will not be substantially affected when the furnace is refilled with cold metal.

In practice this device is also characterized by the fact that the upper part of the partition is made of sheet metal, installed close to a heat-radiating element of the element part to radiate heat to the filling-up part. By this feature of construction cold material, which is being supplied to the filling-up part, is reliably and effectively pre-heated.

Referring to the drawings enclosed this invention will be described in the following.

In these drawings figure 1 shows a schematic cross section along the section marking B-B of figure 2, figure 2 showing a horizontal cross section mainly along the section marking A-A of figure 1. As can be seen in figure 1 the furnace consists of one upper part 1 and one lower part 2, the lower part constituting a vat with a suitable base for installation on a surface. By means of devices, not shown, the upper part of the furnace is pivoting around a shaft (according to arrow 3), shown with a dashed line in figure 1 as well as in figure 2. The reason for making the upper part of the furnace pivoting is to make its innards easily accessible for cleaning when needed.

Bearing upon the operation and function of the furnace it is divided into three separate parts, viz.

one filling-up part 4, one element part 5 and one discharge part 6. These furnace parts can be considered to reach through the overall height of the furnace and their function and division will be described in detail below.

The lower part 2 of the furnace, or its vat, is constructed as an outer chassis 7, built from heavy sheet-metal as an all-welded box. Inside this box, or chassis 7, is arranged a heat-insulation 8, consisting of disc-shaped insulating material which is placed in the box in layers. The type and the quality of this insulating material do not constitute a part of this invention but can be regarded as conventional. Behind the insulating layer 8 a heat-resistant brick lining 9 is arranged in the lower part of the furnace, serving as a holder and support for an inner lining 10 of a heatresistant ceramic or cement-like material. Nor does the type of this material constitute any part of this invention but can be regarded as wholly conventional within this and similar branches of industry.

As mentioned above the lower part of the furnace 2 can be regarded as a vat, designed to hold a metal melt 1 its top level 12 being suggested in figure 1. A certain, optimum size of the vat 2 is required in order to make the furnace operate properly, that is, among other things it should enable the molten metal, which is taken from the discharge part 6 of the furnace, to maintain a stable temperature level. Experience has shown that the vat should be designed to hold a metal melt of 1000-2000 kilos of aluminium.

Along the upper edge of the lower part 2 of the furnace a protective sheet 13 is arranged, covering both the insulating layer 8 and the brick lining 9 as well as part of lining 1 0. As shown in the drawing the protective sheet 13 has a foldeddown portion, situated at some distance outside chassis 7 and attached to the latter part via spacers 14. The result is that the protective sheet 13 is thermically well separated from chassis 7, preventing any significant heat transfer from the protective sheet 13 to the chassis.

As mentioned above one of the objects of the present invention is to achieve a melting furnace for continuous operation, designed in such a way .that the discharge temperature of the molten metal is not affected when the furnace is being refilled with crude metal. In order to handle such operating conditions certain requirements as to the design of the furnace have to be met. In particular it is important for the filling-up part 4 to be screened from the rest of the furnace in such a manner that a drop of temperature in the former part cannot be propagated to the other parts of the furnace. It also would seem to be important for the furnace to hold a sufficiently large melt, excluding that of the filling-up part.

As can be seen in figure 1 as well as in figure 2 in the drawings the furnace has a discharge outlet 15 to make access to the open surface of the metal smelt. This discharge outlet is separated from the interior of the furnace by a heat insulating partition 16, reaching down a certain distance into the metal melt. Although not evident from the drawings it is advisable to use a heatinsulating lid to cover the discharge outlet when molten metal is not taken out. Thanks to the heatinsulating partition 1 6 cold air is prevented from entering the interior of the furnace; also, the interior of the furnace is prevented from cooling down when, for instance, a cold ladle is used to remove molten metal. Oxidation is also reduced.

As is also evident from the drawings the furnace is equipped with a comparatively big centre part or element part 1 7 which has a suitable number of heat-giving sources, situated in an upper space above the metal smelt 11 and constructed in a suitable way. Since these heat sources do not constitute any part of the present invention they will not be described in detail but it is implied that they are situated in the upper part 1 of the furnace at a safe distance from the top level 12 of the metal melt and, also, that their heatgiving output is suitably adjusted. Furthermore, the element part is sealed to the surroundings in order to prevent an excessive amount of air from entering.

As can be seen in figure 1 the filling-up part 4 of the furnace has, in the first place, an upper part 1 8, defined outwards by the upper part 1 of the furnace, i.e. a part of the latter one being designed as a hinged hatch 19. Hatch 1 9 is sized to cover the entire end wall of the furnace, enabling comparatively bulky pieces of scrap metal to be fed into the furnace. Since hatch 1 9 is relatively heavy it is hinged by hinges 20 to the rest of the upper part 1 of the furnace and, by devices not shown in the drawings, it can be operated to an open position, pivoted at the substantial angle of 6O9O0 in a counter-clockwise direction around the hinge-pins of hinges 20.In the second place, the filling-up part 4 of the furnace has a lower part 20 which is connected with the centre part of the furnace, i.e. element part 17, via a passage 21, located in the metal melt in the lower part of the vat. The upper portion of the lower part 20 of the filling-up part is separated from the adjoining part of element part 1 7 by a screen wall 22, made from a heat-insulating material and reaching down into the metal melt 11 in such a way that said passage 21 is created at the bottom of the vat.

The upper part 18 of filling-up part 4 is separated from element part 1 7 by another screen wall 23 which can be regarded as an upper extension of the screen wall 22 of the metal melt 1 The upper screen wall 23 is made from sheetmetal, i.e. a heat-conducting material. In addition, the quality of this material has to be chosen not only for its ability to resist high temperatures without scaling problems but also to be corrosionresistant as possible to the prevailing environment of the furnace and to splashes of liquid metal from the filling-up part. The upper screen wall 23 is designed to completely separate the upper part 18 of the filling-up part from the element part 1 7 and it should be suitably hinged by hinge device 24 in the upper boundary wall of the upper part 1 of the furnace. In order to prevent the upper screen wall 23 from pivoting inwards towards the elements of element part 1 7 it is supported by suitable stops which prevent it from pivoting inwards beyond the position shown in figure 1. The reason for making the screen wall 23 pivoting is to facilitate cleaning of the furnace. By opening this screen wall the interior of the furnace can easily be reached and any slag that may have accumulated can be removed.

As mentioned above the lower screen wall 22 is made from a heat-insulating material and it reaches down into the smelt 1 Along the upper edge of the lower screen wall this heat-insulating material is held in a metal mount. The upper edge of this metal mount is in close contact with the lower edge of the upper screen wall 23, forming a seal. In addition, the metal mount comprises supporting surfaces 25, resting on the upper rim of lining 10 of the vat, thereby keeping screen wall 22 in the designed, vertical position. The screen wall is kept in the correct, horizontal position thanks to the interaction with the lower edge of the upper screen wall 23, as described above.

As mentioned above the upper screen wall 23 is made from a heat-conducting material. Since screen wall 23 is located comparatively close to one of the elements of element part 1 7 thermal radiation will heat it to a very high temperature. In this way the screen wall will, in its turn, radiate a considerable amount of heat to the upper part 18 of the filling-up part. The object of this thermal radiation is to heat by radiation the material that is supplied to the filling-up part, for example scrap metal, to such a high temperature level that the material is broken up and all gases contained in the material are released or that contaminated material, which may release gas, manages to do so before it sinks down into the metal melt in the lower part 20 of the filling-up part.Pre-heating of the material that is fed into the filling-up part through screen wall 23 is important because water- or moisture-holding pores in the material that is poured into the metal smelt of the lower part 20 of the filling-up part quickly would evaporate, largely in bursts, causing liquid metal to splash around, constituting a moment of risk in several ways. In the first place erupting, liquid aluminium naturally constitutes a personal hazard but such aluminium splashes also involve a severe strain on the metal parts of the furnace, causing them to corrode quickly or to be eaten away by the aluminium melt.

Heat is also supplied from below to the re-fill material of filling-up part 4. This heat is mainly supplied by heat transfer through passage 21 and upwards through the lower part 20. By the fact that the lower screen wall 22 is heat-insulating and reaching downwards to approximately one third of the level of the metal smelt 11, a limited volume of molten metal remains in the lower part 20 of the filling-up part. With the addition of the heat that is conducted through the material in passage 21, the heat contents of this limited volume of molten aluminium is sufficient to melt the pre-heated material as it sinks down into the melt.By careful adjustment of the height of the screen wall 22 and its heat-insulating characteristics, i.e. its thickness, it is possible to adapt the heat transfer to the filling-up part in such a manner that the temperature of melt 11 in the element part of the furnace and, in particular, the temperature of the discharge part is not affected to such an extent that operation is disturbed.

Another important function of screen walls 22 and 23 is to prevent splashes of niolten aluminium from entering the other parts of the furnace and, primarily, from getting into contact with the heat elements of element part 1 7. Such metal splashes may not only be caused by gas or moisture, trapped in the material as mentioned above, but simply also by the material being more or less thrown into the liquid metal of the filling-up part.

Finally, the screen wall also prevents free entrance of air to the element part, thus reducing oxidation.

With the dimensions mentioned in connection with the volume of vat 2, the furnace is designed to have a melting capacity of 100--120 kilos of aluminium per hour, i.e. the full capacity of the furnace will seldom be fully made use of. For that reason the furnace is equipped with a control system for control of the effect and, consequently, also of the temperature. This control system comprises temperature sensors, situated in vat 1 2, as well-as temperature sensors, situated in the open space above melt 11. During the periods when comparatively small amounts of molten metal is needed the supply of effect is reduced to enable only the temperature to be maintained at a suitable pre-arranged level, the effect being unsufficient for melting new material. In accordance with this invention this reduction of effect should be carried out only when all material in the filling-up part has been melted so that the level of the melt of the filling-up part is the same as that of the rest of the furnace.

The invention can be modified within the scope of the following claims.

Claims

1. Device for a combined smelting and holding furnace, equipped with a vat (2), made from a heat-resistant and heat-insuiating material and made to hold a melt (1 1), especially a metal melt, the furnace having one filling-up part (4), one discharge part (6) and one heat-emitting element part (5); characterized by having the lower part (20) of the filling-up part (4) in the vat (2) and the upper part (18) above it, the filling-up part being separated from the other components (1 5, 17) of the furnace by a partition wall (22, 23) which reaches down into the melt (1 1), creating a connecting passage (21) in the melt.

2. Device according to claim 1, characterized by its partition wall having one upper part (23), made from heat-conducting material and adjacent to the upper part (18) of the filling-up part (4), and one lower part (22), made from heat-insulating material and reaching down into the upper part of the melt (11).

3. Device according to claim 2, characterized by the upper part of the partition wall (23) being made of sheet-metal, installed close to a heatradiating element in the element part (17) to radiate heat to the filling-up part (4).

4. Device according to claim 3, characterized by having the lower part of the partition wall (22) arranged to reach down into the melt 1) to approximately one third of the total depth of the latter.

5. Device according to any of the previous claims, characterized by its filling-up part (4) being closed and, in addition to the vat (2) and the partition wall (22, 23), being defined by a pivoting hatch (19), made from a heat-insulating material.

6. A device for a combined smelting and holding furnace substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.