GB2231646A

GB2231646A - Melting metals

Info

Publication number: GB2231646A
Application number: GB8911420A
Authority: GB
Inventors: Stephen Warr; Colin Thomas
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-05-18
Filing date: 1989-05-18
Publication date: 1990-11-21
Also published as: GB8911420D0

Abstract

Ingots e.g. of aluminium are heated in two stages to melt the ingots. In the first stage, energy which has previously been converted to thermal energy from some other form and then stored in a heat storage mass (18) is transferred to the ingots in a housing (10). The ingots are then transferred to a melting furnace as and when they are required. <IMAGE>

Description

TITLE: method of melting metals and apparatus for use in the method" Description of the Invention The present invention relates to the heating of low melting point metals to melt them. Typical examples of low melting point metals are aluminium and its alloys. It will be noted that the expression "metals" is used herein to embrace metallic elements and alloys.

Many articles of low melting point metals are conveniently formed by casting. The metal is typically supplied as ingots which must be melted, before the metal can be cast. The rate at which the molten metal can be used in the casting process is restricted by restrictions on the rate at which heat is or can be supplied to the metal in a melting furnace. The efficiency of the process is impaired by steps taken to make it possible to release heat in the melting furnace at a higher rate and thereby enable the metal to be used at a higher rate. For example, in a case where the heat is released by burning a fuel in the melting furnace, increasing the rate of combustion of fuel whilst other conditions are substantially unchanged leads to greater loss of heat in the flue gases.

According to a first aspect of the invention, there is provided a method of melting low melting point metals wherein heat is imported to heat storage means to raise the temperature thereof to a first value, heat is subsequently transferred from the heat storage means to the metal to raise the temperature thereof to a second value which is substantially less than the first value and is substantially less than the melting point of the metal, the metal is transferred to a melting vessel and heat is imparted to the metal in the melting vessel to raise the temperature thereof to a value above the melting point of the metal and to maintain the metal in a fused condition.

In a method in accordance with the present invention, some of the heat which must be imparted to the metal, in order to melt same, is imparted to the metal from the heat storage means before the metal is placed in the melting vessel. Accordingly, the rate at which heat must be supplied to the metal in the melting vessel, in order to provide molten metal at a required rate, is lower than is the case when all of the heat necessary to melt the metal is imparted to the metal in the melting vessel. The present invention enables metal to be melted at a required rate in a relatively small melting furnace and therefore contributes to effeciency because the rate of heat loss from a small furnace can be restricted to a value below the rate of heat loss from a larger furnace under similar conditions.

the temperature to which the metal is heated prior to being placed in the melting vessel is preferably well above 1000 C. This ensures that any moisture present on the metal when the metal is at ambient temperature is evaporated before the metal is introduced into the melting vessel. Introducing metal which is at ambient temperature into a melting vessel containing molten metal involves the risk of introducing moisture into the melting vessel. This can cause a violent eruption of molten metal, which is a serious hazard.

Heat may be imparted to the heat storage means at a rate which is substantially lower than the rate at which heat is imparted to the metal in the melting vessel. Accordingly, the peak demand for energy can be moderated by use of the present invention.

Heat is preferably transferred from the heat storage means to the metal in a substantially closed thermal environment, that is to say, the escape of heat from the heat storage means and the metal, considered collectively, is restricted to a rate which is low relative to the rate of transfer of heat from the heat storage means to the metal when the metal is at ambient temperature. It will be understood that the thermal environment must be opened periodically, te facilitate the transfer of metal to the melting vessel and the placing of fresh metal in the substantially closed thermal environment. Alternatively, additional heat may be introduced into the environment of the metal and of the heat storage means during transfer of heat to the metal.In each case, loss of heat from the environment of the metal and the heat storage means can be reduced to a very low level without particular difficulty.

In known procedures for melting low melting point metal, the metal at ambient temperature is usually introduced into a body of molten metal. Heat is then transferred rapidly to the solid metal until it has fused and has attained a temperature approximately equal to that of the initially molten metal. In contrast with this, the rate of transfer of heat from the heat storage means to the metal in a method according to the present invention is severly reduced when the temperature of the metal approaches said second value and whilst the temperature of the heat store is still much higher than that of the metal. The temperature of the metal can then be held at said second value for an indefinite period until the metal is to be fused, in order to replenish the supply of molten metal.Whilst the temperature of the metal has said second value and the metal is in a solid condition, loss of heat from the metal can be reduced to a very low level without particular difficulty so that heat imparted to the metal, in order to raise its temperature to the second value is not wasted, even if the temperature of the metal is maintained at the second value for a long period.

According to a second aspect of the invention, there is provided heating apparatus for use in a method according to the first aspect and comprising a thermally insulating housing which includes supporting means for supporting metal stock inside the housing, wherein there are contained in the housing heat storage means, converting means for converting to thermal energy, energy which is supplied in another form, the converting means being arranged for applying the thermal energy to the heat storage means, and control means for controlling the transfer of heat from the heat storage means to metal on the supporting means.

An example of apparatus embodying the second aspect of the invention and which is used in a method according to the first aspect will now be described, with reference to the accompanying drawings, wherein FIGURE 1 shows a cross-section through the apparatus in a vertical plane, FIGURE 2 shows a cross-section through the apparatus in a further vertical plane which is perpendicular to the plane of Figure 1, FIGURE 3 shows a cross-section in a horizontal plane on the line CC of Figure 1; and FIGURE 4 shows a cross-section in a horizontal plane on the line BB of Figure 1.

The apparatus comprises a housing 10 which includes opposite side walls 11 and 12, a front wall 13, a rear wall 14, a bottom wall 15 and a top wall 16. The front wall 13 includes a lower portion which is fixed with respect to the side walls and an upper portion which is movable relative to the remainder of the housing and constitutes a door 17. The walls 11 to 16 incorporate one or more layers of thermally insulating material and substantially isolate the interior of the housing 10 thermally from the environment of the housing, whilst the door 17 is closed. When the door is closed, a body of air is trapped inside the housing and significant flow of air into or from the housing is prevented.

A lower part of the housing 10 contains heat storage means 18. The heat storage means typically comprises an assembly of ceramic bricks. These may be of a known composition used for the manufacture of heat storage bricks. Between at least some mutually adjacent bricks, there are passages which permit flow of air through the assembly. As shown in Figure 4, there may also be passages between the assembly of bricks and one or more of the walls 11 to 15.

In an upper part of the housing 10, there is a support 19 for a number of ingots of metal. The support may include several shelves and these may be sub-divided by vertical partitions into compartments, each of which is suitable for receiving a single ingot.

The support is adapted to permit flow of air around the ingots and between the upper and lower parts of the housing. The vertical dimension of the support 19 corresponds approximately to the vertical dimension of the door 17 so that each compartment defined by the support is readily accessible, when the door 17 is open.

The apparatus illustrated in the drawings further comprises control means for controlling the transfer of heat from the heat storage means 18 to metal ingots supported by the support 19. The example of apparatus illustrated is arranged for transfer of heat between the heat storage means and the ingots substantially entirely by convection. Accordingly, the control means is arranged for preventing the flow of convectIon currents between the upper and lower parts of the housing. The control means comprises an array of butterfly valves 20 arranged for pivoting about respective mutually parallel, horizontal axes. The valves can be pivoted between open positions illustrated in Figure 1 and closed positions in which the valves are each substantially horizontal and collectively close off the upper part of the housing 10 from the lower part thereof.

The valves 20 are carried on respective spindles which protrude through openings in the side wall 11 of the housing. At the outside of the housing, there is provided a mechanism for transmitting motion from a motor 21 to the spindles. The control means further includes temperature-sensing means inside the housing 10 (but not shown in the drawing) and an electrical control circuit for controlling operation of the motor 21 in accordance with the temperature sensed in the upper part of the housing.

The valves 20 are formed of steel or other material which is opaque to radiant heat energy. It will be understood that, when the temperature of the heat storage means 18 is considerably greater than the temperature of ingots at the bottom of the support 19, there will be some transmission of heat by radiation from the heat storage means to the ingots when the valves 20 are open. When the valves are closed, they prevent the transmission of heat by radiation from the lower part of the housing to the upper part thereof.

There is associated with the heat storage means 18 converting means for converting to thermal energy, energy which is supplied in another form. The particular example of converting means in the accompanying drawing is a set of electrical resistance heaters. These are disposed in the passages of the heat storage means 18 and are connected with electrical circuit components outside the housing 10 by means of suitable connectors which extend through apertures in the side wall 11. The apparatus further comprises electrical control gear for controlling energisation of the heating elements in accordance with the temperature sensed in the lower part of the housing 10. Suitable control systems are well known.The arrangement may be such that any one of several heating elements in the heat storage means can be energised independently of the other heating elements to convert electrical energy at a selected rate to thermal energy within the heat storage means.

The apparatus illustrated in the accompanying drawings is used in conjunction with a melting furnace which may be constructed and arranged in a known manner. The melting furnace may incorporate electrical heating elements or burners for burning a gaseous or liquid fuel in air to release heat within the melting furnace.

Prior to use of the melting furnace for melting a number of ingots, the ingots are placed in the support 19 and the door 17 is closed. The electrical heating elements 22 are energised to supply heat to the heat storage means 18. If the interior of the housing 10 is initially cool, the valves 20 will be open. As the temperature of the heat storage means increases, heat will be transferred by convection of air inside the housing 10 to the ingots in the support 19. As the temperature of the ingots approaches a predetermined value, for example a temperature in the region of 4000 C, the motor 21 will close the valves 20. The transfer of heat from the heat storage means 18 to the ingots is thus terminated.

Whilst energisation of the heating elements 22 is continued, the temperature of the heat storage means 18 continues to rise.

Energisation of the heating elements is terminated when the temperature of the heat storage means attains a predetermined value which is considerably greater than the maximum temperature attained by the ingots. For example, heating of the heat storage means may be terminated when the temperature thereof attains a value of 8000C.

Heating of the heat storage means 18 by energisation of the heating elements 22 can be carried out over a long period, for example a period of up to eight hours, and at a rate which is substantially lower than the rate at which heat is imparted to an ingot during melting of same. For example, heat may be imparted to the heat storage means at a rate within the range 15 to 20 kW per hour. Since the heat is stored in the storage means 18 and, possibly, in ingots in the upper part of the housing 10, it is not necessary for the electrical energy to be converted to thermal energy during melting of an ingot. The conversion from electrical energy to thermal energy can take place several hours before an ingot to which that heat is imparted is melted and can take place during a period when the associated melting furnace is out of use, for example overnight. If electrical energy is available at a low price during a restricted part of each twenty four hour period, low price electrical energy can be used.

When the supply of molten metal in the melting furnace is to be replenished, the door 17 is opened, an ingot is removed from the housing 10 and is introduced into the melting furnace and a fresh ingot is placed in the housing. The door 17 is then closed.

The temperature of the ingot which is transferred from the housing 10 to the melting furnace is considerably above 1000C. Accordingly, no moisture will be carried by the ingot to the melting furnace.

Whilst the door 17 of the housing is closed, the contents of the housing are isolated thermally from the environment of the housing.

The fresh ingot which is introduced into the housing will usually be at ambient temperature. Heat will be imparted to that ingot from other ingots already present in the housing and, as the temperature in the upper part of the housing falls, from the heat storage means 18. Typically, after a fresh ingot has been placed in the housing, the valve 20 will open to permit the transfer of heat from the heat storage means 18 to the ingots by convection of air within the housing. It will be noted that the air which transfers heat from the storage means 18 to the ingots is confined within the housing.

The rate at which heat is transferred to the ingots and the maximum temperature attained by the ingots in the housing 10 can both be controlled by means of the valves 20.

The control means preferably includes means which responds to opening of the door 17 by closing and maintaining closed the valves 20 so that, whilst the door 17 remains open, the lower part of the housing is thermally isolated from the upper part of the housing. ror providing a signal indicating that the door 17 is open, the control means may include a known limit switch.

The apparatus illustrated in the drawing may be modified by the provision of an inlet for admitting ambient air to the housing 10 at or adjacent to the bottom wall 15 and an outlet for permitting air to escape from the interior of the housing to the ambient atmosphere. Preferably, the outlet would be at or adjacent to the top wall 16. In a case where the housing has such an inlet and an outlet, at least one of these would be provided with a valve for controlling flow of air through the inlet or outlet, as the case may be. Preferably, respective valves would be provided for both the inlet and the outlet.Admission of ambient air to the housing at or adjacent to the bottom wall 15 and escape of air from the housing at or adjacent to the top wall 16 facilitates the transfer of heat from the heat storage means 18 to metal ingots on the support 19 at a higher rate than can be achieved under similar conditions with the contents of the housing in a substantially closed thermal environment.

It is envisaged that, in a case where an air inlet and an air outlet are provided, these would be opened only when a particularly high rate of heat transfer is required and would be closed at other times, so that the housing would normally provide a substantially closed thermal environment. For example, when the temperature in the upper part of the housing is at or near to the ambient temperature, both the inlet and the outlet may be open. The inlet and/or the outlet may be closed when the temperature in the upper part of the housing approaches or reaches 4000C.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:

1. A method of melting low melting point metals wherein heat is imparted to heat storage means to raise the temperature thereof to a first value, heat is subsequently transferred from the heat storage means to the metal to raise the temperature thereof to a second value which is substantially less than the first value and is substantially less than the melting point of the metal, the metal is transferred to a melting vessel and heat is imparted to the metal in the melting vessel to raise the temperature thereof to a value above the melting point of the metal and to maintain the metal in a fused condition.

2. A method according to Claim 1 wherein heat is imparted to the heat storage means at a rate which is substantially lower than the rate at which heat is imparted to the metal in the melting vessel.

3. A method according to Claim 1 or Claim 2 wherein heat is transferred from the heat storage means to the metal in a substantially closed thermal environment.

4. A method according to any preceding Claim wherein heat is imparted from the heat storage means to a plurality of bodies of the metal and is stored in those bodies for a substantial period and wherein heat is subsequently transferred to a cool body of metal from both hot bodies of metal and the heat store.

5. A method according to any preceding Claim wherein the rate of transfer of heat from the heat storage means to the metal is severely reduced when the temperature of the metal approaches said second value and whilst the temperature of the heat store is still much higher.

6. A method according to any preceding Claim wherein the transfer of heat from the heat storage means to the metal is effected mainly by convection.

7. A method according to Claim 6 wherein the heat storage means and the metal are together enclosed in a housing which retains in the housing air which carries heat from the heat storage means to the metal.

8. A method according to Claim 7 wherein the flow of air between the heat storage means and the metal is substantially prevented whilst the temperature of the metal has the second value.

9. Heating apparatus for use in a method according to Claim 1 and comprising a thermally insulating housing which includes supporting means for supporting metal stock inside the housing and wherein there are contained in the housing heat storage means, converting means for converting to thermal energy energy which is supplied in another form, the converting means being arranged for applying the thermal energy to the heat storage means, and control means for controlling the transfer of heat from the heat storage means to metal on the supporting means.

10. Apparatus according to Claim 9 wherein the control means is adapted to control the flow of air between the heat storage means and the metal.

11. In combination, heating apparatus according to Claim 9 or Claim 10 and a melting furnace for receiving metal from said heating apparatus and melting the metal by application thereto of energy converted in the melting furnace from another form into thermal energy.

12. A method of melting low melting point metals substantially as herein described.

13. Heating apparatus substantially as herein described with reference to the accompanying drawing.

14. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.