GB2493493A

GB2493493A - A reverbatory furnace with a dry hearth for preheating scrap metals and a barrier to prevent waste gases entering the main chamber

Info

Publication number: GB2493493A
Application number: GB1110835.4A
Authority: GB
Inventors: Clive Hall
Original assignee: MELTING SOLUTIONS Ltd
Current assignee: MELTING SOLUTIONS Ltd
Priority date: 2011-06-27
Filing date: 2011-06-27
Publication date: 2013-02-13
Also published as: GB201110835D0

Abstract

A reverberatory furnace 1 for melting aluminium or other metals has a main chamber 5 and a dry hearth 3 with a barrier 6 between them. The dry hearth has burners 4 to heat contaminated scrap metal 22 and an outlet to allow waste gas and particles to be extracted, it may vent to outside the furnace, or to an afterburner chamber 8 where the waste is combusted further; an outlet 15 from the afterburner vents into the main chamber to add heat. The barrier may be a movable refractory lined wall or a fluid flow such as an air knife; when closed it prevents gases and particles produced from entering the main chamber, it may be sealed or configured to allow molten metal to flow under; when open the preheated scrap is pushed toward the main chamber as the dry hearth is charged with further material.

Description

Title: A furnace This invention relates to a furnace.

Standard gas fired reverberatory furnaces need to melt down a wide range of contaminated scrap, as well as melting normal charges of ingot and clean feed stocks.

The reverberatory furnace principle, whereby a burner flame, or flames, are fired into an enclosed refractory lined chamber is used in examples of the invention. This design uses predominantly radiated heat to melt the solid charge. This is the standard work horse furnace of the aluminium industry.

However the industry needs to recycle more and more scrap, as well as the more traditional ingot charges.

If the scrap aluminium is thin with a high surface area to weight ratio, then melting by radiation as in a standard reverberatory furnace results in high metal losses as the aluminium overheats and turns to oxide effectively rendering the reverberatory design ineffective.

Scrap will most likely include paint, lacquers, oils and other combustible elements. The combustible elements of the charge will burn in an uncontrolled way in a standard furnace, giving rise to pollutants in the form of unburnt compounds and dioxins as well as high oxides and generation of drosses.

Contaminated scrap therefore needs to be melted by liquation', i.e. dissolving of the metal in a bath of liquid aluminum. In this way, the heat transfer to the metal to be heated is by convection and conduction thus avoiding excessive loss of material through oxidation.

Before liquation can be truly effective, substantially all moisture content and combustible elements should be removed. The scrap may be contaminated with moisture such as water, snow or ice which should be removed before the scrap is introduced to the main chamber. Only then can the scrap can be safely and properly melted in a liquid bath. Alternatively, contaminated scrap can be melted under a salt based flux using a rotary furnace. However this gives rise to considerable amounts of waste salt slag, which has a significant environmental impact.

Legislation in the form of EU guidance (BREF NOTES) for the aluminium industry do not consider rotary slat flux melting as BAT (Best Available Technology) and therefore there is a need for energy efficient, flexible, salt free melting furnaces which can achieve the highest metal recovery.

Some salt free non-rotary melting furnaces utilise a twin chamber design.

These comprise a furnace divided into two zones by a fixed dividing wall. On one side is the heating chamber zone, where heat is transferred to the surface of the bath by one or more burners. The bath is in effect the heat sink. On the other side of the fixed wall is the closed charging zone which has a ramp where the scrap is loaded. The fixed wall has submerged ports and a pump is used to circulate the metal around the furnace between the two zones.

The scrap is pushed into the metal stream in the closed well thereby melting the scrap by liquation. Often the scrap is heated on the ramp or dry hearth in the closed well to burn off volatiles and moisture so good levels of metal recovery are achieved.

There are disadvantages to this design which include: -very large furnace capacities are needed relative to the melt rate as the heating chamber needs a large surface area to transfer the heat for melting.

Typically 10 tonnes of capacity is needed for one tonne per hour melt rate.

Thus a 5t hour melt rate needs a 50t furnace.

-a considerable metal heal is needed and the twin chamber furnace cannot easily change alloys or be quickly emptied.

-this design is not flexible.

An object of the invention is to provide improved energy efficiency and metal recovery by using a new regenerative design.

Modifying a standard gas fired reverberatory furnace or producing a furnace in accordance with examples of the invention enables the furnace to melt down a wide range of contaminated scrap, as well as being able to melt down normal charges of ingot and clean feed stocks.

One aspect of the present invention provides a melting furnace for melting down material, the furnace comprising: a main melting chamber for melting material; a dry hearth for receiving potentially contaminated material; a barrier between the main chamber and the dry hearth to isolate the main melting chamber from gaseous and particulate matter produced from material heated in the dry hearth when the barrier is in a closed configuration.

A further aspect of the present invention provides a method of operating a melting furnace for melting down material, the furnace comprising: a main melting chamber for melting material; a dry hearth for receiving potentially contaminated material; a barrier between the main chamber and the dry hearth to isolate the main melting chamber from gaseous and particulate matter produced from the material heated in the dry hearth when the barrier is in a closed configuration, the method comprising: pre-heating material in the dry hearth with the barrier in the closed configuration; placing the barrier in an open configuration; and receiving the pre-heated material from the dry hearth into the main chamber.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the following drawings, in which: Figure 1 is a cross section view through a furnace embodying the present invention in a first configuration; Figure 2 is a cross section view through the furnace of Figure 1 in a second configuration; Figures 3 to 6 are a series of cross section views through the furnace of Figure 1 illustrating different stages in a mode of operation of the furnace; Figure 7 is a cross section view through the furnace of Figure 1 in the second configuration illustrating another mode of operation of the furnace; Figures 8 to 11 are a series of cross section views through another furnace embodying the present invention illustrating different stages in a mode of operation of the furnace; and Figure 12 is a cross section view through the furnace of Figures 8 to 11 illustrating another mode of operation of the furnace.

Description of Invention

The invention can use the reverberatory furnace principle, whereby a burner flame, or flames, are fired into an enclosed refractory lined chamber. Such a design uses predominantly radiant heat to melt the solid charge.

Reverberatory furnaces are the standard workhorse furnaces of the aluminium industry. The invention can also be used in non-reverberatory furnaces.

Referring to figures 1 and 2, a furnace 1 embodying the present invention comprises a refractory lined chamber 2 comprising a metallurgical melting furnace for melting down contaminated scrap as well as traditional ingots and charges.

The furnace 1 comprises a dry hearth 3 for receiving contaminated scrap and a dry hearth burner 4 so that the metal on the dry hearth 3 is fully preheated and all combustible elements and volatiles are burnt off. The dry hearth 3 comprises a distinct zone in the furnace 1.

The dry hearth 3 is separated from the main chamber 5 of the furnace 1 by a movable refractory lined dividing wall 6 sleeved in a division wall housing 7.

The main chamber 5 comprises another distinct zone of the furnace 1. The dry hearth 3 has a door 3A to afford access to the dry hearth.

A loading machine 20 with a horizontal thrust ram powered by a hydraulic cylinder is operable to charge the dry hearth 3 with metal, comprising contaminated scrap or otherwise, whilst the dry hearth door 3A is open. The stroke of the loading machine is sufficiently long to allow material to be pushed into the dry hearth 3 and also through the dry hearth, past the division wall 6 and into the main chamber 5. The main chamber 5 has melting burners 16 to heat the furnace and radiate heat to the metal in a melting bath 17 with an electromagnetic stirrer 18 or metal pump which is used to stir the metal.

Access from the dry hearth 3 into the main chamber 5 is via a ramp 21 sloping downwardly from the dry hearth 3 toward the melting bath 17. The division wall 6 is positioned toward the base of the ramp 21. The division wall 6 has an open position as shown in figure 1 in which it is preferably raised above the dry hearth 3 at or toward the base of the ramp 21 and retracted within the division wall housing 7.

The division wall 6 has a closed position in which the door 6 sits on and preferably seals with the floor of the dry hearth 3 at or toward the base of the ramp 21 as shown in figure 2.

Preferably, the division wall 6 sits on a dry part of the dry hearth 3 and does not make contact with or sit in the melting bath 17 in the main chamber 5.

Conveniently, the division wall 6 translates between open and closed configurations in a substantially vertical direction but the door may also operate to slide or roll horizontally between these configurations. A hinged door or rolling door is also a possible variation on the division wall 6 illustrated in the figures. The division wall 6 serves a purpose which is to isolate the dry hearth 3 contents from the main chamber 5 contents thus preventing the main chamber contents from being contaminated by unprocessed material from the dry hearth 3. A high pressure air knife wall or other form of non-solid wall created by a fluid flow may also serve the same purpose of isolating the dry hearth contents from the main chamber contents. If the material in the dry hearth 3 has been appropriately processed either in the dry hearth 3 itself or in an afterburner chamber, then the dry hearth material can be introduced into the main chambers.

In some embodiments, the division wall 6 does not form a seal with the dry hearth 3 to allow any molten metal to pass under or through the division wall 6 into the melting bath. If the metal in the dry hearth is at sufficient temperature to be molten, then it should not contain contaminants, volatile elements or combustibles.

The furnace 1 also has a separate afterburner chamber 8 which can be mounted above the dry hearth 3 and main chamber 5 or elsewhere to suit.

The afterburner chamber 8 has an open inlet flue 8A to receive gases and particulates from the dry hearth 3. In the afterburner chamber there are sited one or more afterburner chamber burners 9 and then, above these, at the head ot the afterburner chamber 8, the afterburner control elements are located: including an oxygen sensor 10, a thermocouple 11 and a pressure sensor 12. The outlet flue 13 from the afterburner chamber 8 is controllably gated by a pressure control damper 14 and vents through to the main chamber 5 via a melting chamber inlet flue and damper 15.

When the division wall 6 is in the closed configuration, there is controllable gaseous communication between the dry hearth 3 and the main chamber 5 of gaseous and particulate matter carried in the gaseous flow originating from material in the dry hearth 3, via the afterburner chamber 8, controlled by the dampers 14,15 and, into the main chambers. When the division wall 6 is in an open configuration, there is solids/liquid communication of the material in the dry hearth 3 from the dry hearth 3 into the melting bath 17, past the division wall 6 -i.e. the material in the dry hearth 3 can be mechanically moved from the dry hearth 3, past the division wall 6 and into the melting bath 17 or at least into the main chamber 5 for melting down into the melting bath 17. Preferably, the loading machine 20 pushes the material from the dry hearth 3, past the division wall 6 into the main chamber 5.

A mode of operation for this example of the present invention is now described in relation to figures 3 to 6.

In figure 3, the dry hearth door 3A is open and contaminated scrap material 22 is loaded onto the dry hearth 3 using the conventional charging machine 20 with a horizontal thrust ram powered by a hydraulic cylinder. At the start of the melt cycle, the movable refractory lined dividing wall 6 is in the closed position, fully down as shown in figure 3. In figure 4, the scrap material 22 is heated using the dry hearth burner 4 with sufficient air that the charge is dried out and heated releasing any volatile gasses in a controlled manner. Additionally, a series of nozzles or vortexes inject an additional air/oxygen supply to assist the combustion of any un-burnt combustibles, particulates and gasses.

Referring to figure 4, un-burnt gasses are convected or are drawn into the separate afterburner chamber 8 mounted in this example above the dry hearth zone 3 and the main chamber zone 5. The afterburner 8 is controlled at a temperature of approximately 850°C to burn any un-burnt carbonaceous compounds, contaminants, combustibles, particulate matter and smoke. A separate burner 9 for the afterburner chamber provides additional heat and an oxygen monitor together with pressure sensors control the gas flow into and through the afterburner chamber 8. The retention time in the afterburner chamber 8 is typically two seconds at an oxygen content of approximately 6%.

This has been found to adequately process the particulate and gaseous material that scrap material produces from the dry hearth zone 3. Much if not all of the contaminants, particulates, compounds are burnt off thereby adequately processing the scrap material in the dry hearth 3.

The outlet gasses from the afterburner are controllably ducted into the main chamber/melting chamber 5 of the furnace 1 on the opposite side of the movable dividing wall 6. This process takes heat generated in the afterburner chamber 8 and carried with the gas exhausted from the afterburner chamber 8 into the main chamber to be used directly to heat the melting bath 17 in the main chamber 5. The main chamber 5 is also heated by the one or more main chamber burners 16.

Staying with figure 4, under the melting bath 17, the electromagnetic stirrer 18 or metal pump stirs the molten metal 22 in the bath 17. After approximately 30 -45 minutes the metal on the dry hearth (3) is fully preheated: much if not all of the contaminants, particulates, compounds are burnt off thereby adequately pre-processing the scrap material in the dry hearth 3.

Fre-processing has been achieved in a controlled way without the burners 16 in the main chamber 5 directly heating or overheating the charge and causing excessive oxidization of material on the dry hearth 3. At this stage, the scrap material is at 350-400°C which is significantly pre-heated.

Referring now to figure 5, the movable wall 6 is raised up to open a path for non-gaseous material from the dry hearth to the main chamber and exposing the dry hearth 3 to direct heat from the main chamber burners for the first time in the cycle. The division wall 6 is enclosed in a division wall housing which comprises an insulated cover to retain heat and form a gas tight seal.

The pre-heated and pre-processed clean scrap material 23 is pushed into the metal bath 17 by the ram on the charger 20. In some embodiments, the loading machine is arranged such that the ram at the same time also introduces the next charge of material for the dry hearth 3 through the open charging door 3A.

As shown in figure 6, immediately after charging the dry hearth 3 with fresh scrap material 22, the division wall 6 is closed to divide the furnace into two separate zones: the dry hearth zone 3 and the main chamber zone 5. The scrap material 22 in the dry hearth 3 is not subject to direct heat from the main chamber burners 16 and the heat from the dry chamber 3 and the afterburner chamber 8, particularly the heat from the afterburner exhaust gases contributes to the heat in the main chamber 5 as well as pie-processing and pre-heating the scrap material 22. The pre-processing and pre-heating cycle has begun again.

In the melting bath 17 there is vigorous stirring using the electromagnetic stirrer 18 or other circulation device. The stirred metal is shown with reference numeral 24 in figure 2. The metal 23 introduced into the bath 17 is clean and free from contaminants, combustibles and volatiles and is already at around 400°C from the pie-processing and pie-heating steps performed on the material whilst in the dry hearth 3. The radiant heat transfer from the melting hearth burners 16 is used effectively on the pie-heated and pre-processed material so melting is very fast and efficient minimising metal loss and oxidation that without pre-heating and stirring would otherwise generate high metal losses and render scrap melting impossible. The heat recovered from the afterburner chamber 8 and the dry hearth 3 also contributes to the heat in the main chamber 5 thereby promoting more rapid melting in the bath 17.

This cycle (as shown in figures 3 to 6) represents one mode of operation for pre-processing and pre-heating scrap metal. The furnace design is flexible and has another mode of operation as shown in figure 7.

In this additional mode of operation, only clean scrap and clean ingot are to be melted -the furnace is used as a traditional reverberatory furnace by simply not using the dry hearth 3 and closing the dividing wall 6. The afterburner chamber 8 is closed off using the melting chamber inlet flue damper 15.

The melted metal 23 in the melting bath 17 is poured out of the main chamber through the opened melting hearth door 5A or drained out from the bath.

The furnace 1, or just the main chamber 5, is operable to be rotated to pour the melted metal out of the bath 17.

In another example of the invention shown in figures 8 to 12, the main chamber 5 is modified so that there is no main chamber door and the only material 22,23 access for the main chamber 5 is via the opening afforded by the barrier/division wall 6 when in the open configuration. Access to the main chamber 5 is via the opened division wall 6 and to the dry hearth 3 via the dry hearth door 3A.

The same cycle illustrated in figures 3 to 6 is performed by this furnace example of the invention and is illustrated in figures 8 to 11.

Turning to figure 12: this is an analog of the figure 7 example furnace. If only clean scrap and clean ingot are to be melted, then this example of the furnace is used as a traditional reverberatory furnace by simply closing the dividing wall 6 and using the dry hearth 3 as a conventional opening to the main chamber 5. The afterburner chamber B is closed off using the melting chamber inlet flue damper 15.

In this description, reference numeral 22 denotes material which is contaminated or is undergoing pre-heating and/or pre-processing to remove contaminants, combustibles, volatiles, gaseous by-product and/or particulate matter. The material may be scrap material but may also be clean. Reference numeral 23 denotes cleaned material which has been pre-heated and/or pre-processed and which can advance to the main melting chamber 5.

Contaminated scrap material includes scrap having a content of paint, lacquers, oils and other combustible or volatile elements. The combustible elements of the charge will burn in an uncontrolled way in a standard furnace, giving rise to pollutants in the form of un-burnt compounds and dioxins as well as high oxides and generation of drosses. The scrap may also be contaminated with moisture such as water, snow or ice which should be removed before the scrap is introduced to the main chamber.

The invention is not limited to metallurgical melting furnaces and the principle can be utilised in other furnaces such as lower temperature processing furnaces which are used for processing metallic ores and minerals. The invention is not limited to pre-heating aluminium or scrap aluminium.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, 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

<claim-text>CLAIMS: 1. A melting furnace for melting down material, the furnace comprising: a main melting chamber for melting material; a dry hearth for receiving potentially contaminated material; a barrier between the main chamber and the dry hearth to isolate the main melting chamber from gaseous and particulate matter produced from material heated in the dry hearth when the barrier is in a closed configuration.</claim-text> <claim-text>2. A furnace according to claim 1, further comprising an outlet from the dry hearth to pass gaseous and particulate matter produced from the material heated in the dry hearth.</claim-text> <claim-text>3. A furnace according to claim 1 or 2, wherein the barrier is a movable refractory lined dividing wall.</claim-text> <claim-text>4. A furnace according to any preceding claim, wherein the barrier is movable between an open configuration and the closed configuration.</claim-text> <claim-text>5. A furnace according to any preceding claim, wherein the barrier allows the passage of molten material under or through the barrier from the dry hearth to the main chamber.</claim-text> <claim-text>6. A furnace according to any preceding claim, further comprising one or more burners for heating the material in the dry hearth and/or the material in the main hearth.</claim-text> <claim-text>7. A furnace according to any preceding claim, wherein an afterburner chamber is downstream of the outlet from the dry hearth to heat further the gaseous and particulate matter produced from the heated scrap.</claim-text> <claim-text>8. A furnace according to claim 7 comprising a further burner for heating the material in the afterburner chamber.</claim-text> <claim-text>9. A furnace according to claim 7 or 8, wherein an outlet of the afterburner chamber vents into the main chamber to introduce further heat into the main chamber from the dry hearth and the afterburner.</claim-text> <claim-text>10. A furnace according to any one of claims 2 to 7, wherein the outlet from the dry hearth vents to outside of the main chamber.</claim-text> <claim-text>11. A furnace according to any preceding claim, wherein the furnace is a reverberatory furnace.</claim-text> <claim-text>12. A furnace according to any preceding claim, wherein the furnace is a metallurgical melting furnace and the material is metal and/or scrap metal and/or aluminium.</claim-text> <claim-text>13. A furnace according to any preceding claim, wherein there is a heat transfer path between the main chamber and the dry hearth via the barrier when the barrier is in a closed configuration and, when the barrier is in the open configuration, the barrier does not hinder the heat transfer path from the main chamber to the dry hearth.</claim-text> <claim-text>14. A method of operating a melting furnace for melting down material, the furnace comprising: a main melting chamber for melting material; a dry hearth for receiving potentially contaminated material; a barrier between the main chamber and the dry hearth to isolate the main melting chamber from gaseous and particulate matter produced from the material heated in the dry hearth when the barrier is in a closed configuration, the method comprising: pre-heating material in the dry hearth with the barrier in the closed configuration; placing the barrier in an open configuration; and receiving the pre-heated material from the dry hearth into the main chamber.</claim-text> <claim-text>15. The method of claim 14 further comprising, subsequent to receiving the pre-heated material from the dry hearth into the main chamber, placing the barrier in the closed configuration and repeating the pre-heating, placing and receiving actions.</claim-text> <claim-text>16. The method of claim 14 or 15, further comprising removing the gaseous and particulate matter produced from the material heated in the dry hearth from the dry hearth.</claim-text> <claim-text>17. The method of claim 16, further comprising processing the gaseous and particulate matter produced from the material heated in the dry hearth in an afterburner.</claim-text> <claim-text>18. The method of claim 17, further comprising introducing the processed gaseous and particulate matter from the afterburner into the main chamber.</claim-text> <claim-text>19. The method of any one of claims 14 to 18, wherein charging the dry hearth with further material moves pre-heated material from the dry hearth toward the main chamber.21. A furnace substantially as hereinbefore described and/or illustrated in the accompanying figures.22. A method of operating a furnace substantially as hereinbefore described and/or illustrated in the accompanying figures.23. Any novel feature or combination of features disclosed herein.</claim-text>