IE74130B1 - Production of aluminium oxides from aluminum-bearing material - Google Patents

Description

PRODUCTION OF ALUMINIUM OXIDES FROM ALUMINUM-BEARING MATERIAL This invention relates to the production of aluminium oxides from non-metallic aluminium compounds.

When a body of aluminium is melted in a furnace for purposes of casting or the like, dross forms on the surface of the molten aluminium which must be periodically removed, for example by skimming or similar operation. The removed dross contains substantial amounts of free aluminium as well as aluminium oxides, such as bauxite, and certain other metals and metal salts, such as magnesium, manganese and lithium, depending on the nature of the aluminium or aluminium alloy being treated. The dross may also include some nitrides and chlorides, possibly because of the manner in which the dross is treated.

It is recognized in the industry that for economical reasons it is critical to recover in usable form the aluminium and other by-product metals from the dross. It * is also recognized, however, that the recovery of these materials from dross is difficult because of, amongst other things, the nature of the dross and the reactivity of aluminium. In a typical recovery process the dross is normally melted at high temperatures in a furnace.

* However, at elevated temperatures the dross, particularly 5 the free aluminium in the dross, is easily susceptible to oxidation and, moreover, commonly tends to ignite and burn in the presence of air. The burning of the aluminium can decrease substantially the amount of aluminium recovered.

To solve the problems associated with treating dross and improve the efficiency of aluminium recovery, it has been proposed to heat the dross in an inductive furnace in the present of a salt flux, as disclosed in US-A-3 676 105.

The use of a salt flux, which tends to agglomerate the free aluminium, is not desirable because of high costs and because the salt, which tends to be water-leachable, must be separated from the aluminium, leading to cost and environmental problems.

It has been suggested in the prior art to recover aluminium metal in the gaseous state by breaking down aluminium salts at temperatures of at least 2500 to 5500°C. The heating to the essential high temperature, as disclosed in US-A-3 938 988 can be carried out using plasma energy from a plasma arc torch. According to the disclosed process, a liquid coolant is utilized to flashcool a mixture of aluminium and other elemental gases to effect separation. This procedure is not conducive to the treatment of dross, and further is not practical for dross treatment either from the standpoint of cost or the ecology.

The use of a plasma jet generator has also been suggested 741 30 for reducing various metal oxides, including aluminium oxides. See, for example, US-A-4 177 060 where carbon and alumina are supplied to a molten reactor bed maintained in a reaction chamber with the carbothermal reduction occurring through application of plasma energy ·> from a plasma torch. The use of carbon in the reduction of alumina, i.e., a chemical reaction, is substantially different from the treatment of dross where separation of materials only is desired.

Accordingly, there is a substantial need in the industry for an improved process for recovering aluminium in the form of aluminium oxides from aluminium dross and other solid material. It is particularly desirable that such process be cost effective and ecologically safe.

The present invention provides a process for converting a solid material containing a mixture of non-metal aluminium compounds, including aluminium nitride or aluminium chloride, to aluminium oxides, comprising heating the material with a plasma arc torch using as the arc gas, oxygen, air, a mixture of air and oxygen, or oxygen mixed with carbon dioxide or argon and, thereafter, recovering the aluminium oxides.

As an added advantage of the present invention, it has been found that in the use of a rotary furnace heated with plasma energy, aluminium oxides — either initially present in the dross or formed during the dross treatment — build up on the walls of the furnace to line the furnace. The free aluminium, which melts at a lower temperature than the oxides, agglomerates within the * interior of the built-up lining, from where it can easily be removed from the furnace. The built-up aluminium . oxide can be periodically removed, for example after each run or after two or three runs, from the walls of the furnace.

The advantageous results of the present invention are obtained when using oxygen or a variety of oxygenous mixtures as the arc gas for the plasma generator, including air.

Specifically, for one megawatt electrical power input and 100 SCFM (standard cubit foot per minute) air plasma gas input, the calculations are as follows: 100 SCFM Air x 0.2 (0?) x 60 Min = 3.166 #-Mole 379 SCF/#-Mole “HR HR 3.166 #-Mole x 32# = 101.34# of 02 #-Mole 2 Al + 3/2 O2 * 1 AI2O3 3.166 #-Mole of O2 consumes 4.221 #-Mole of Al 4.221 #-Mole Al = 27# x 4.221 #-Mole = 113.97# of #-Mole aluminium which is burned to AI2O3.

For a 2.5 ton batch of dross at 50% aluminium content which is melted in one hour, only 4.6% of the aluminium is oxidized, i.e., 113.97# Al Oxidized = 4.6% of Al 2500# Al available The 4.6% of oxidized aluminium is fixed for a constant torch enthalpy of 10 kW.

SFM a constant »h«H nium content of 50% and a constant system heat efficiency, resulting in 2500 pounds of aluminium melted in one hour at a one megawatt input electrical rate. A range of oxidization percentages can be generated from independent variables of enthalpy, heat efficiency, and aluminium content.

Proof that there is an added 40% ± heat input is apparent from heat formation data as follows: 2.110 #-Mole AI2O3 x 399.09 kCal x 1800 (#-Mole) (BTU1 = #-Mole (#-Mole) (kCal) 1,515,743 BTU/hr = 444 kW/hr 3413 BTU/hr/kW/hr Air, therefore, results in a total heat release of 1.444 megawatts at one megawatt of electrical output to provide 40% enhanced heat input.

Proof of the cost effectiveness is shown by the following: N2 Cost = $30/hr f100 SCFM x 60 Min x _£5_) ( 1000 1000) Ar Cost $150/hr (100 SCFM x 60 Min ( 1000 x £25 ) 1000) Air Cost $8/hr (at power cost of $.06/kW/hr).

When treating aluminium alloys containing more active metals than aluminium, such as magnesium, lithium, etc., magnesium and lithium will be oxidized first and result in consumption of these metals first, resulting in less aluminium loss and similar heat output advantages.

This invention has been disclosed in GB 89 27768.5 (GB-A2 228 014) from which this application has been divided.

A presently preferred embodiment will be described.

The present invention will now be described by way of example with reference to the accompanying drawings wherein: FIGURE 1 is a flow diagram of the process of the parent application; FIGURE 2 is a schematic drawing of a rotary furnace, plasma arc torch, and supply system used in the process of this invention; FIGURE 3 is a side elevation of the furnace and 5 plasma torch shown in FIGURE 2; FIGURE 4 is a schematic cross-section of the plasma arc torch used in the present invention, and FIGURE 5 is a comparative temperature profile using air and nitrogen as the arc gas.

Referring to FIGURE 1, in the process of the parent invention, dross is weighed and charged into a furnace 10. After charging the dross to the furnace, a plasma arc torch 30 is brought into position in the furnace and the dross heated to the molten state. The molten free aluminium is recovered. The dust recovered from furnace, which is about 99% aluminium oxide, is passed to a bag house. The slag or residue which forms on the furnace walls is scraped from the furnace and is preferably recharged to the furnace with additional dross, or is further treated with a plasma torch, as will be discussed below, to provide usefulnon-metallic products (NMPs).

The preferred furnace, as shown in FIGURES 2 and 3, is a tilting rotary furnace. Thus, the furnace comprises a rotary drum 12 on frame 14 which is driven on rails 15 by belt 16 and pulley 18 with an electric motor (not shown). As is also shown in Figures 2 and 3, the drum, carrying torch 30, tilts about pivot point 20, preferably actuated by an air cylinder 22, to permit convenient recovery of the free molten aluminium. Accordingly, the supply lines to the plasma torch must be flexible.

Plasma torch 30 is removably positioned in cover 26 of furnace 10. The torch on frame 14 is moved vertically into and out of position by an air cylinder 34. Once in position in the furnace, the torch can be swung back and forth within the furnace in order to cover the entire furnace area around pivot point 36 by activation of air cylinder 38. The independent positioning of the torch also provides conveniently for a safety feature of the furnace system. As designed, the furnace system allows for the ejection of the plasma torch from the furnace whenever the arc in the torch is lost for whatever reason, such as loss of power to the torch, water leaks, etc. According to the furnace design, a door on the furnace chamber is opened simultaneously with the ejection of the torch. The opening of the door prevents a build-up of pressure and any possibility of explosion. Moreover, according to the furnace system, a supervisory control system is utilized wherein the system is computer controlled based on various inputs to the furnace system.

Plasma torches which are operable in the process of the invention are of the transferred-arc and non-transferredarc type commercially available from Plasma Energy Corporation. Suitable torches are also described in USA-4 383 820 and 4 559 439. A simplified cross-section of a suitable transferred-arc torch is shown in FIGURE 4.

As illustrated, the torch includes an electrode 40, a collimator 42, a vortex generator 44, water inlet 46 for cooling the torch mechanism, and a water outlet 48. Gas input means 43 feeds gas to the vortex generator 44 between electrode 40 and collimator 42. In the plasma generator system the furnace base and the dross being heated function as the ground for receiving the transferred arc from electrode 40. As shown in FIGURE 2, the water/gas manifold and the electrical power supply for the torch are supplied to a power/water junction box and then fed to the torch. The arc gas, preferably air, is ionized between the vortex generator and the collimator.

It has also been found that better performance is obtainable when using a non-transferred arc plasma torch wherein the cathode is the front electrode and the anode is the rear electrode. Torches of this type are manufactured by the Plasma Energy Corporation, for example under the designation PT250N. It is believed that the improved result is obtainable because the plasma arc from this type of torch contains a large number of active ions. If the cathode is the rear electrode, these ions would recombine before leaving the torch and reaching the work surface. The front cathode provides, therefore, a more-active ion species. Non-transferred arc, as the term is used herein, is used in the traditional sense, meaning that both the anode and cathode are within the torch. In contradistinction, in a transferred-arc torch, one of the electrodes is, or is at, the work surface.

The invention will be more specifically described by the following examples of which Examples 1 and 2 are of the invention claimed in GB-A-2 228 014 for recovering aluminium: Example 1 2.5 tons (5000 pounds) of aluminium alloy dross containing approximately 50% aluminium were charged into rotary furnace 10. A PT250N non-transferred arc plasma torch 30 was lowered into position and directed by air cylinder 38 to contact substantially the centre of the bottom of furnace drum 12. Electrical power, coolant water,, and air arc gas were supplied to torch 30. With rotation of the furnace drum 12, the charge was heated to the molten condition, and thereafter the heating was continued with torch 30 directed towards the wall of the furnace for a period of one hour. The torch was then withdrawn and the molten aluminium discharged by tilting the furnace drum. The 5000-pound charge produced 2375 pounds of pure aluminium alloy. The slag was scraped from the bottom of the drum to provide 2740 pounds of aluminium oxide. Additionally, 100 pounds of aluminium oxide was recovered from the bag house. The increase in total weight is because of the oxygen present in the form of oxides.

In this example the recovery of pure aluminium alloy was 47.5% of the original charge, with the residue being primarily usable aluminium oxide and stable mixed metal oxides by-product. This contrasts with the usual recovery of approximately 35% free aluminium attainable with a conventional rotary furnace using salt as a flux. For example, it has been found that if the dross contains 50% free aluminium, the recovery using a salt flux will be approximately 35% aluminium, with the remaining 65% being aluminium oxide and other aluminium by-products in mixture with up to about 15% aluminium. This relatively high percentage of free aluminium must either be separately recovered or converted to aluminium oxide since the free aluminium content is too high to permit use of the by-product as an oxide. In contrast, where the dross contains 50% aluminium, the recovered free aluminium is approximately 47% to 50%, with the balance being aluminium oxide and stable mixed metal oxides with less than about 3% free aluminium. The oxides can thus be used effectively as aluminium oxide products without further processing since the low aluminium content is not detrimental.

Example 2 The process of Example 1 was repeated. However, in this instance the arc gas was nitrogen alone. For a two-hour reaction time, i.e., twice the time used with air as the arc gas, the recovery was as follows: 2200 pounds of pure aluminium alloy; 2740 pounds of slag, and 50 pounds of dust.

The advantages of using air as the arc gas is shown by the comparative temperature profile set forth in FIGURE . As seen from FIGURE 5, when starting with a cold furnace and nitrogen as the arc gas, the heating cycle required 178 minutes, with the maximum exhaust temperature approaching 1200eC. In contradistinction, when operating with air, starting with a cold furnace, the maximum temperature of approximately 850°C was reached in approximately 80 minutes. This is s-jnri Tar to the temperature profile obtained with nitrogen on a hot furnace.

In the above examples, the advantage of using air as the arc gas in the aluminium recovery is demonstrated. However, although air is the preferred oxidizing arc gas, it is possible and advantageous to use other oxidizing arc gases, including mixtures of oxygen and air, or mixtures of air and nitrogen, and still realize substantial advantage over prior art methods where a salt flux is used. This is particularly possible since, following the processing of aluminium and aluminium alloy drosses in a rotary furnace configuration using either a plasma torch or fossil fuels, the residual refractorylike product which collects on the furnace wall and which is composed of a mixed metal oxides and/or aluminium nitride, as well as minor amounts of aluminium chloride, magnesium nitrides and trapped aluminium, can be subjected to a controlled plasma oxidation. Thus, the residual refractory-like product is treated in the plasma-furnace system wherein the plasma torch is used to introduce oxygen, air-oxygen enriched blends (greater than 50% oxygen), CO2-oxygen enriched blends (greater than 40% oxygen), or oxygen-argon blends (greater than 40% oxygen) as the arc gas to oxidize the residue and achieve nitride, chloride, and metal contents of less than about 1%. With some aluminium alloys, it may be desirable to introduce a fluxing agent to assist or accelerate the process. Where the flux is less than 5% of the charge, the resultant by-products, which are substantially all oxides, are subsequently used in known applications as refractories and the like.

Example A (Of the Present Invention) The 2740 pounds of slag and 50 pounds of dust recovered in Example 2 above, comprised of 30% aluminium nitride, 3% free molten aluminium trapped in the residue and the balance a mixture of metal oxides, were heated in a rotary furnace with plasma torch 30 to a temperature of 1460°C, using a plasma torch operated with a controlled flow of 150 SCFM of oxygen at a plasma power level of 1 MW. After forty-five minutes of operation, the furnace was shut down and the charge removed. The charge was substantially pure metal oxides (Al, Mg, Al-Mg oxides/spinels).

Accordingly, the disadvantage normally associated with nitriding, or in having chlorides and other by-products present, is eliminated.

Various modifications can be made within the scope of the invention. For example, furnaces other than rotary furnaces can be utilized with suitable modification. Moreover, although the specification is directed primarily to the treatment of dross, aluminium oxides can also be recovered from aluminium scrap according to the invention.

Claims (9)

1. A process for converting a solid material containing a mixture of non-metallic aluminium compounds, 5 including aluminium nitride or aluminium chloride, to aluminium oxides, comprising heating the material with a plasma arc torch using as the arc gas oxygen air, a mixture of air and oxygen or oxygen mixed with carbon dioxide or argon, and, thereafter, 10 recovering the aluminium oxides.

2. A process according to claim 1, wherein the solid material is residue obtained from the removal of free aluminium from aluminium dross.

3. A process according to claim 1 or 2, wherein the aluminium oxides contain less than about 1% of nitride and chloride.

4. A process as claimed in any preceding claim, wherein the arc gas is oxygen.

5. A process as claimed in any of claims 1 to 3, wherein the arc gas is a mixture of carbon dioxide 25 and more than 40% oxygen.

6. A process as claimed in any of claims 1 to 3, wherein the arc gas is a mixture of argon and more than 40% oxygen.

7. A process for recovering aluminium oxides from aluminium dross or aluminium scrap, comprising charging aluminium dross or aluminium scrap to a furnace equipped with a plasma arc torch for heating 35 the charge; heating the charge by providing plasma energy to it by feeding air, or air to which oxygen or nitrogen has been added, to the torch for ionization; continuing the heating until the charge is molten; removing free aluminium in the molten state from the furnace to leave a residue containing non-metal aluminium components; heating the residue with a plasma arc torch using a mixture of air and oxygen as the arc gas, and recovering the aluminium oxides so formed.

8. A process for recovering free metal oxides substantially as described in Example A.

9. Free aluminium oxide recovered by the process claimed in any preceding claim.