GB2253686A - Metal melting, smelting or recovery - Google Patents

Abstract

Apparatus for melting or smelting metal, typically for aluminium cans scrap recovery, has a muffle chamber (14) through which a flow of hot gases is provided for melting there being a heat retaining matrix (18) made up from a body or bodies of ceramic or other heat stable material. e.g. a layer of balls (20), defining random interstices through the matrix so that the hot gases, and the metal when molten can flow therethrough to promote effective heat exchange between the gas and the body or bodies, and between the gas and body or bodies and the molten or other metal in surface contact therewith. <IMAGE>

Description

METAL PROCESSING OR RECOVERY This invention relates to the processing or recovery of metallic or metal containing material by melting or smelting. While the invention is particularly applicable to the recovery of metal scrap, notably aluminium can scrap, other applications in the metallurgical field are contemplated, for example the smelting of metalliferous ores, metal refining, compounding metal alloys, or remelting metal for further processing such as casting.

any of the long established methods and apparatus for melting or smelting said materials require substantial installations which are costly to build.

maintain and run: have low thermal efficiency: may cause unacceptable atmospheric pollution; and,in some cases, are only suited to the handling of higher melting point metals such as iron and steel.

More recently apparatus such as electric induction melting furnaces and vortex melting devices have been developed for lighter and lower melting point metals such as aluminium scrap but again such installations will only operate effectively on a large scale involving substantial capital cost and inability to process small batches of material.

In the case of aluminium can scrap the empty cans are normally first passed through a delacquering process and shredded, and these shreds are then fed into an induction or vortex melter which has to hold a substantial volume of already molten metal because the incoming scrap is only heated to melting point by heat directly conducted to it from the molten bath. If too great a quantity of unmelted material enters the bath it may cool the latter so that it solidifies, thus a large volume of molted metal has to be kept heated. Moreover the shredded scrap tends to float on the surface of the bath so that it fails to melt. To counteract this agitation has to be provided e.g. either by the action of the induction heating current and/or by provision of mechanical agitation as with the impeller of vortex melting plant.

Furthermore excessive oxidation of the melt has to be prevented so that the whole process has to be carried out in a closed system with consequent problems of air locks or other sealing for the entry of the material and the tapping of the melt.

The heat requirements and heat losses are substantial if only because a large bath of metal has to be maintained in a molten state continuously prior to start up and throughout the operation.

The use of disposable aluminium cans, notably for beverages, is becoming more widespread particularly as brands are increasingly promoted nationwide and internationally e.g. on the European market necessitating transport and distribution throughout wide areas. There is also increasing emphasis on the collection and recycling of can scrap both on economic and environmental grounds. hitherto can scrap recovery plants have had to be large scale operations with substantial throughput for the reasons stated above so that optimum operation only becomes possible if the scrap is collected from a wide catchment area with consequent transport costs. Small scale plant, e.g. such as might be operated by a local scrap merchant, have not hitherto been practicable.

The same considerations apply to the recycling or recovery of similar scrap aluminum or other metal material, e.g. swarf and chippings or shredded or pulverised scrap from other sources.

The object of the invention is to provide metal melting or smelting apparatus, particularly for aluminium or other scrap recovery, which is particularly efficient and economical in use, can be provided in a wide range of sizes for large or small throughputs and the handling of small or large batches of scrap, which is of low initial cost and easy to install and maintain, and which is environmentally acceptable both in scale and in its manner of operation.

According to the invention there is provided apparatus for melting or smelting metallic or metal-containing material including a heat retaining matrix, means for feeding the material in particulate, shredded or granular form onto the matrix, heating means providing a flow of hot gases for raising the temperature of the matrix to at least the melting point of metal content of the material in use, and collecting means for receiving molten metal from the matrix: characterized in that the matrix comprises a body or assembly of bodies of heat stable material defining a plurality of random or other interstices through the matrix throughout its effective area for passage of the hot gases and molten metal therethrough, and for promoting effective heat exchange between the gasses and the body or bodies, and between the body or bodies and the material in operative surface contact therewith.

The heating means may be arranged to provide said flow of gases counter to the direction of flow of the molten metal through the matrix in use.

Conveniently the matrix will consist of a plurality of spherical bodies heaped together to form a layer or layers of uniform or non---uniform thickness.

Conveniently the body or bodies are formed of a ceramic material.

It is contemplated that the matrix might also be a mat or grid made up of fibrous or rod like bodies disposed regularly or at random or, for some applications, the matrix might consist of one or more preformed bodies e.g. grille or honeycomb structures moulded from ceramic material, having the interstices defined therein as a plurality of through apertures or passages.

An example of the invention is now more particularly described with reference to the accompanying drawing being a diagram of an aluminium can scrap recovery plant.

The plant incorporates melting apparatus 10 of the invention and ancillary equipment referred to hereafter.

Apparatus 10 comprises a vertical cylindrical muffle 12, in this example 2.7m in diameter. Muffle 12 defines a chamber which is divided into upper and lower regions 14, 16 by a heat retaining matrix 18 extending generally horizontally across said chamber and which consists of a layer of heat stable ceramic balls 20 each some 50mum in diameter, the layer in this example being about 1m in overall thickness and resting on a supporting grille or the like to maintain it at the desired level. The balls constituting the top of matrix 18 may be levelled as indicated generally in the drawing or may be arranged so that the layer is somewhat thicker towards the centre, or they may possibly be arranged in a dish formation so that the layer is thicker at its outer margin.

The bottom of muffle 12 consists of a sump 22 for collecting molten metal 24 in operation, this sump need not have a large capacity as it plays no part in the melting process as such. An extension of the sump laterally beyond the muffle 12 provides the facility for drawing off or tapping the molten metal as required while providing a liquid air lock preventing passage of gases or ingress of air from or to the muffle chamber.

Apparatus 10 further includes heating means in the form of a substantially closed hot gas circuit including an inlet duct 26 communicating with the chamber lower region 16 above the level of the molten metal 24: an outlet duct 28 at the top of the muffle and provided with a regulating damper valve 30; a recirculation fan 32 in a top duct 34 connected between outlet duct 28 and the inlet to a gas fuel after burner 36, the output from the latter passing into inlet duct 26.

Material feed means of the apparatus includes a hopper 38 delivering delacquered and shredded can scrap into a scrap pre-heater 40, the hopper incorporating or feeding through an air lock (not shown). Material output from pre-heater 40 passes through a feed passage 42 opening into upper region 14 of the muffle chamber, the material dropping or being distributed onto the upper face of the layer of balls constituting matrix 18.

A proportion of the hot gases in chamber upper region 14 are diverted to pass through the scrap pre-heater 40 in contra-flow to the passage of the material therethrough and to exhaust from pre-heater 40 through a branch extension of the top duct 34 to merge with the flow to fan 32 so that this portion of the hot gasses is utilized to pre-heat the incoming material, e.g. to about 3200C.

To avoid oxidation, the oxygen content of the circulating gases must be kept below a predetermined level and this is controlled, inter alia , by an oxygen sensing device 44 acting on the incoming flow of gases in inlet duct 26 as they pass into the muffle 12, output from said sensor regulating the operation of a control valve 46 of an air inlet 4S communicating with top duct 34 upstream of fan 32. Surplus gases are bled off under the control of a stack valve 50 opening into an exhaust stack 52 which may include suitable purifying equipment for discharge to atmosphere. The effective temperature of the gases passing through muffle 12 is further regulated automatically, inter alia, by a terperature sensor 54 acting in inlet duct 26, and the flow rate is regulated by adjusting fan 32, and/or the setting of the damper valve 30.

In operation the flow of hot gases passes upwardly through matrix 18 by flowing through the interstices between the balls 20, the large overall surface area exposed to the flow ensuring that the assembly of bodies consisting of the balls is thoroughly heated by direct heat transfer. Typically the temperature of the incoming gases as they enter the bottom of the layer of balls will be about 8500C and the temperature after they have passed through the layer to exit into chamber upper region 14 will be about 7600C during operation. They will pass through fan 32 at about 6200C prior to reheating and the gas throughput at afterburner 36 may be around 30000 to 31000 Kg. per hour.

At the same time the incoming shredded scrap.

preferably preheated to a temperature of about 3200C drops onto the top of matrix ls where it will not only be subjected to heating by the through gas flow but is also in intimate contact with the surfaces of the hot balls 20 so that the material is heated by direct conduction from the latter, again facilitated by the substantiat surface area of the balls exposed for contact with the material. As the material melts the heating effect is enhanced as a film or films of liquid metal forms on the ball surfaces. The melted metal runs down through the interstices in contra flow to the gases to drop from the bottom of matrix 18 into the collecting sump 22, the operating temperature of the melted metal in the latter being around 7500C.

The gas flow rate through the muffle will be regulated for most effective and economical heating depending to some extent on the throughput of material.

It will be noted that the velocity of the gasses will be somewhat slowed by their passage through the matrix and the latter may also have a desirable effect in distributing and evening out said flow by the time it exits from matrix 18 to upper region 14 so reducing any tendency for the gases to blow the comparatively light scrap shreds too far upwards in the muffle chamber.

However, it is anticipated that it will not matter if a small proportion of the shreds are lifted or blown off the face of the matrix and/or only blown a relatively short distance from it as they will still be subjected to the heating effect of the gases and will then either melt and tend to coalesce into (larger) drops which will fall back onto and pass through the matrix, or will fall back onto it at random still in solid state to be melted thereon The action of the matrix may be regarded to some extent as analogous to the action of the matrix of slats or the like carrying a film of cooling water flowing in opposition to the incoming gases for effective heat exchange in a cooling tower.

As a least a major part of the melting effect results from conduction of heat from the surfaces of the solid balls to the material in contact therewith the problems of attempting to transfer heat from a molten bath of the metal, referred to above, are wholly avoided.

There is no large volume of metal needing to be maintained in molten state to effect the process, and the apparatus will operate effectively even at small rates of throughput. It will also handle small batches of material as the start-up time and wastage of heat on close down are minimal compared with most conventional melting apparatus.

The melt output from sump 22 can be directly cast into blocks without remelting.

It is calculated that the plant will operate extremely efficiently with overall heating or fuel requirements for aluminium can scrap as low as 2.1 to 1.4 megajoules per kilogram of metal melted, the efficiency of heat transfer being extremely high and the heat losses or wastage being extremely low.

It is contemplated that other forms of matrix could be employed but the ceramic bails are preferred as being readily available, simple to replace as necessary, and easy to service, e.g. if they should become clogged they can readily be cleared by simple agitation in situ. Also their quantity and arrangement (e.g. heaped, dished or flat) is readily varied to obtain optimum operation.

While the contraflow of gases through the matrix against the direction of flow of the melted metal is believed to be preferable, it is also contemplated that other gas flow arrangements might be employed, for example gas flow in the same direction as melt flow so that, possibly, the incoming material is blown onto the face of the matrix, and/or introduction of gas flow from or through the sides of the matrix. In some applications it is contemplated that gas fuel, or a gas/air mixture may be fed to the matrix for combustion therein to provide the melting heat and gas flow.

Claims (10)

1. Apparatus for melting or smelting metallic or metal- containing material including a heat retaining matrix, means for feeding the material in particulate, shredded or granular form onto the matrix, heating means providing a flow of hot gases for raising the temperature of the matrix to at least the melting point of metal content of the material in use, and collecting means for receiving molten metal from the matrix; characterized in that the matrix comprises a body or assembly of bodies of heat stable material defining a plurality of random interstices through the matrix throughout its effective area for passage of the hot gases and molten metal therethrough, and for promoting effective heat exchange between the gasses and the body or bodies, and between the body or bodies and the material in operative surface contact therewith.

2. Apparatus as in Claim 1 wherein the heating means is arranged to provide said flow of gases counter to the direction of flow of the molten metal through the matrix in use.

3. Apparatus as in Claim 1 or Claim 2 wherein the body or bodies are formed of a ceramic material.

4. Apparatus as in Claim 1, 2 or 3 wherein the natrix consists of a plurality of spherical bodies heaped together to form a layer or layers of uniform or nonuniform thickness.

5. Apparatus as in Claim 1,2 or 3 wherein the matrix is a mat or grid made up of fibrous or rod like bodies disposed at random.

Apparatus as in Claim 1,2 or 3 wherein the matrix consists of one or more preformed bodies having random interstices defined therein as a plurality of through apertures or passages.

7. Apparatus as in Claim 4 wherein the spherical bodies are ceramic balls each having a diameter of approximately 50mm in a layer having a thickness of about lm.

8. Apparatus as in any preceding claim wherein the heating means includes provision for re-circulation of the hot gases in a closed circuit.

9. Apparatus as in Claim 8 including means for regulating the oxygen content of the circulating gases.

10. Metal melting or smelting apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.