RECOVERY OF NON-FERROUS METALS FROM DROSS

Abstract: The invention relates to a method for recovering non-ferrous metal from a dross starting material containing this metal, optionally together with non-ferrous metal scrap, and/or a starting material comprised of such scrap. The method comprises introducing the starting material into a rotary furnace or rotatable converter provided with a refractory lining, heating the starting material to a temperature above the melting point of the metal while rotating the furnace either continuously or intermittently, and removing the resultant molten metal from beneath the overlying dross residues. The method is characterized by heating the starting material in the absence of a protective salt layer with the aid of a so-called oxy-fuel-burner which is supplied with a fossil fuel and with a gas that contains at least 80 % oxygen, and by rotating the furnace at a speed of 5 r.p.m or lower.

Description: Recovery of Non-Ferrous Metals from Dross. The present invention relates to a method of recover¬ ing a non-ferrous metal from a dross starting material which contains said metal, possibly together with non-fer¬ rous metal scrap, and/or a starting material comprised of such scrap, said method comprising the steps of feeding the starting material to a rotary furnace or a rotatable con¬ verter provided with a refractory lining, heating the starting material to a temperature above the melting point of the metal while rotating the furnace continuously or intermittently, and removing the resultant molten metal from remaining dross residues and/or from resultant solid dross residues. Methods for recovering non-ferrous metals from dross are known to the art. For example, the Canadian Patent Specification 1,255,914 teaches a method for recovering non-ferrous metal from non-ferrous-metal containing dross, comprising the steps of feeding the dross into a rotary furnace which has a refractory lining, directing a plasma burner into the furnace so as to heat the dross to a tem¬ perature above the melting point of the metal, rotating the furnace either continuously or intermittently, and removing the molten, separated metal from the solid dross residue. It is also known to recover non-ferrous metals from dross and metal waste with the aid of oxy-fuel-burners while using a protective layer of salts. This layer may in¬ clude sodium chloride (NaCl) , potassium chloride (KC1) in roughly equal quantities with respect to weight, and may also contain a flux, for instance cryolite. The effect of the salt layer on the coalescence of aluminum droplets when treating aluminum dross is described, for instance, in the Second International Symposium; Recycling of metals and engineered materials, The minerals, metals and materials Society, 1990, pages 69-84, by Ray D. Petersen, entitled "Effect of salt flux additives on aluminum droplet coales¬ cence". In an article entitled "Interfacial phenomena in molten aluminum and salt systems" published in the same reference, pages 85-103, Francis K. Ho and Yogesh Sahai describe the remelting of aluminum waste beneath a salt layer. An object of the present invention is to recover, or win, non-ferrous metal from a dross starting material which contains said metal, optionally together with non-ferrous metal waste, and/or a starting material containing such waste, with the aid of an oxy-fuel-burner and in the ab¬ sence of a protective salt layer. Another object is to reduce the formation of corrosive salt vapors that are obtained when using protective salt layers, and to obtain an oxidic rest product which is free from water-soluble salts, thereby benefitting the environ¬ ment. These objects are achieved with a method for recover- ing, or winning, a non-ferrous metal from a dross starting material which contains said metal, optionally together with non-ferrous metal waste, and/or from a starting mate¬ rial which contains such waste. The method comprises the steps of introducing the starting material into a rotary furnace or a rotatable converter provided with a refractory lining; heating the starting material to a temperature above the melting point of the metal, while rotating the furnace either continuously or intermittently; and removing the resultant molten metal from the overlying dross resi- due. The method is particularly suited for the recovery of aluminum from aluminum-containing dross and from aluminum scrap. The inventive method is characterized by heating the starting material with a so-called oxy-fuel-burner in the absence of a protective salt layer, said burner being sup¬ plied with a fossil fuel and„with a gas that contains at least 80% oxygen, and wherein the furnace is rotated at a speed of 5 r.p.m or lower. Advantageous embodiments of the invention are set forth in the claims appendant on the main claim. The invention will now be described in more detail with reference to the accompanying drawing, the single Figure of which is a vertical section view of the rotatable converter and associated equipment. The Figure illustrates a converter 1 whose center axis 2 defines an acute angle with the horizontal plane, for example an angle of 10-25°. The converter 1 is rotatable about its center axis 2 and is pivotal about a horizontal axis 3 which extends perpendicularly to and passes through the center axis 3 of the converter 1. The upwardly directed end of the converter 1 narrows into an opening 4, which has mounted on its upstream side 9 a protective hood 9 which merges with a curved, tubular structure 5. One end, or a first end, of the hood 9 is positioned as close as possible to the converter opening 4 without actually coming into contact with the rotatable converter 1. The tubular member 5 is curved and its distal end faces upwards. The tubular member 5 is held in place by means of devices not shown and can be easily removed from the converter opening 4, so as to enable the converter 1 to be filled and emptied. A so- called oxy-fuel-burner 6 is inserted through an opening provided in the curved part, or knee-bend of the tubular member 5. The nozzle orifice extends slightly into the converter 1, when the first end of the tubular member 5 is located immediately in front of the converter opening 1. The requisite fuel, oxygen and cooling water connections are not shown. A lance 7 is inserted through another open¬ ing in the knee-bend of the tubular member. When carrying the inventive method into effect, the starting material 8, which may have the form of aluminum dross and/or aluminum scrap for instance, is introduced into the converter 1, with the tubular member 5, the burner 6 and the lance 7 moved to one side. The tubular member 5, and therewith the burner 6 and lance 7, is then placed tightly in front of the converter opening. The converter is then rotated at a speed of up to 5 rpm, and the burner is ignited for heating the starting material 8. The burner is normally operated with a fossil fuel, for instance a hydro¬ carbon, such a propane, and with an oxygen-containing gas having an oxygen content of at least 80%, and preferably also, containing small amounts of nitrogen. The oxygen-con¬ taining gas used may be comprised of oxygen having a con¬ centration of about 93% oxygen from a PSA-plant, the re- mainder of the gas consisting generally of argon, or may be comprised of oxygen taken from an air separation plant. The burner flame is preferably directed onto a point on the bottom of the converter spaced from the converter axis 2. Because the amount of oxygen supplied to the burner 6 is less than the stoichiometric amount, or possibly equal to the stoichiometric amount, i.e. an amount which corresponds to from 80 to 100% of the oxygen required for complete combustion, oxidation of metal in the converter 6 is mini¬ mized. When the starting material, for instance aluminum-con¬ taining dross, has reached a temperature above the melting point of aluminum, the aluminum will begin to melt and rotation of the converter will cause the spherical bodies which encapsulate an aluminum core and have a surrounding protective aluminum-oxide shell, are deformed and the pro¬ tective shell disintegrated, whereupon the aluminum, which is now in a liquid state, begins to run down and coalesce with molten aluminum that exits from other spheres. The molten aluminum is collected in the lowermost region of the converter. When the maximum amount of aluminum possible has melted and collected beneath the overlying dross resi¬ dues, the protective hood 5 is removed, so as to enable the molten metal to be tapped from the converter 1. According to one preferred embodiment of the inven- tion, when the metal begins to melt, the rotational speed of the converter is raised to 25 r.p.m while simultaneously reducing burner power to 50-10% of the original power. Heating of the metal is terminated when the temperature in the melt has reached a level of 750-950°C, whereafter the molten metal is tapped from the converter in a conventional manner. According to another embodiment of the invention. power is supplied intermittently to the converter at the higher rotational speed of up to 25 rpm. In order to pre¬ vent ingress of atmospheric oxygen into the converter, and therewith oxidize any aluminum which may not be protected by a layer of dross or some other rest product, an inert shielding gas is introduced into the converter, so as to maintain an overpressure therein. This inert gas is prefer¬ ably argon. As illustrated in the drawing, the shielding gas is introduced through a lance which is connected to a source of shielding gas, or alternatively through the burner, for instance through its oxygen nozzle, which may be switched alternately between connection with an oxygen source and connection with a shielding gas source. Although not shown, the burner is also connected to a fossil-fuel delivery line. Example Aluminum-containing dross obtained from the manufac¬ ture of an aluminum alloy, series 3000, was introduced into a converter whose longitudinal axis defined an angle of about 17° with the horizontal plane. 1240 kg of pure dross were charged to the converter. The dross comprised 66% by weight metallic aluminum, 30% by weight aluminum trioxide, and the remainder oxides of silicon, iron and calcium. Subsequent to introducing the dross, in the absence of protective salt layers, the protective hood, with an oxy- fuel-burner fitted thereto, was placed in front of the con¬ verter opening, at a short distance therefrom. The burner had a power output of l MW and was operated with propane and pure oxygen, i.e. an oxygen-containing gas which com- prised at least 99.5% oxygen. The converter was rotated at a speed of about 1 r.p.m, with the burner operating at full power, until the dross had heated to the point at which the aluminum melts, the temperature of the material in the converter being about 750°C. The power output of the burner was then reduced to about 200 kW and the converter was rotated continuously at a speed of 8 rpm. Subsequent to having heated the material in the converter at this reduced power over a period of 20 minutes, the metallic aluminum was found to have separated from the dross, which now lay in a covering layer over the molten metal. 711 kg of pure aluminum was obtained, corresponding to a yield of 57% and a process efficiency of 88% (based on the amount of metallic aluminum charged and the amount of pure aluminum obtained) . The consumption of fossil fuel corresponded to 252 kWh for each tonne of dross introduced, while the oxygen consumption corresponded to 44 normal cubic meters for each tonne of dross introduced. The total process time was 32 minutes. The resultant rest product comprised of pure oxides of the aforesaid metals and alumi¬ num. ' - Highly satisfactory results have been achieved when the starting material charged to the converter was com¬ prised partially of scrap containing the same metal as that contained by the dross. When solely scrap is charged to the converter, and then particularly solely aluminum scrap, it is necessary for the process of combustion required to generate thermal energy takes place under sub-stoichiometric conditions, due to the tendency of this metal to readily oxidize. When the supply of hot combustion gases is interrupted periodically, it is particularly necessary to take measures which will prevent atmospheric oxygen from penetrating to the metal. As before mentioned, an overpressure is preferably main¬ tained in the converter, by supplying a shielding gas, such as argon, to the converter.

Claims: CLAIMS 1. A method of recovering non-ferrous metal from a dross starting material which contains said non-ferrous metal, optionally together with non-ferrous metal waste, and/or from a starting material comprised of such waste, said method comprising the steps of introducing the starting material into a rotary furnace or a rotatable converter having a refractory lining, heating the starting material to a temperature above the melting point of the metal while rotating the furnace continuously or intermittently, and removing the resultant molten metal from the overlying residue, characterized by the further steps of heating the starting material in the absence of a protective salt layer with a so-called oxy-fuel-burner which is operated with a fossil fuel and with a gas that contains at least 80% oxygen, and by rotating the furnace at a speed of 5, or lower. 2. A method according to Claim 1, characterized by lowering the power output of the burner to 50-90% and in- creasing the rotational speed of the furnace to 5-25 rpm and continuing the supply of energy until the metal has melted, subsequent to having reached a temperature above the melting point of said metal. 3. A method according to Claim 2, characterized by sup- plying heat at said reduced power intermittently during the continued rotation of the furnace at a speed of 5-25 rpm; and by introducing an inert gas into the furnace, either through the burner or through a lance, during a period of interrupted heat supply. 4. A method according to Claim 3, characterized in that the inert gas is argon. 5. A method according to one or more of Claims 1-4, characterized by supplying the oxy-fuel-burner with fuel and oxygen in stoichiometric relationships, or in a rela- tionship in which oxygen is present in an amount which at most 20% below the stoichiometric amount, when delivering heat to the furnace burden. 6. A method according to one or more of Claims 1-5, characterized by maintaining an overpressure of combustion gases in the rotary furnace or converter. 7. A method according to one or more of Claims 1-6, charac- terized by terminating the heating process and tapping metal rom the furnace or converter when.the temperature of the metal lies within the range of 750-950°C. 8. A method according to one or more of Claims 1-6, characterized by terminating the heating process and tap- ping metal from the furnace or the converter after a time lapse of 5-60 minutes from the time of increasing the rotational speed of said furnace or converter. AMENDED CLAIMS [received by International Bureau on 18 June 1993 (18.06.93); original claim 1 amended;ot er claims unchanged(2 pages)] 1. A method of recovering non-ferrous metal from a dross starting material which contains said non-ferrous metal, said method comprising the steps of introducing the start- 5 ing material into a rotary furnace or a rotatable converter having a refractory lining, heating the starting material to a temperature above the melting point of the metal while rotating the furnace continuously or intermittently, and removing the resultant molten metal from the overlying res- 0 idue, characterized by the further steps of heating the starting material in the absence of a protective salt layer with an oxy-fuel-burner which is operated with a fossil fuel and with a gas that contains at least 80% oxygen, and by rotating the furnace at a speed of 5, or lower. 5 2. A method according to Claim 1, characterized by low¬ ering the power output of the burner to 50-90% and increas¬ ing the rotational speed of the furnace to 5-25 rpm and continuing the supply of energy until the metal has melted, subsequent to having reached a temperature above the melt- 0 ing point of said metal. 3. A method according to Claim 2, characterized by sup¬ plying heat at said reduced power intermittently during the continued rotation of the furnace at a speed of 5-25 rpm; and by introducing an inert gas into the furnace, either 5 through the burner or through a lance, during a period of interrupted heat supply. 4. A method according to Claim 3, characterized in that the inert gas is argon. 5. A method according to one or more of Claims 1-4, char- 0 acterized by supplying the oxy-fuel-burner with fuel and oxygen in stoichiometric relationships, or in a relation¬ ship in which oxygen is present in an amount which at most 20% below the stoichiometric amount, when delivering heat to the furnace burden. 5 6. A method according to one or more of Claims 1-5, char¬ acterized by maintaining an overpressure of combustion gas¬ es in the rotary furnace or converter. 7. A method according to one or more of Claims 1-6, charac¬ terized by terminating the heating process and tapping met¬ al from the furnace or converter when the temperature of the metal lies within the range of 750-950°C. 8. A method according to one or more of Claims 1-6, char¬ acterized by terminating the heating process and tapping metal from the furnace or the converter after a time lapse of 5-60 minutes from the time of increasing the rotational speed of said furnace or converter.

Inventors: GRAEB HANS-WALTER, GRIPENBERG HENRIK, MUELLERTHANN MICHAEL, GRAEB, HANS-WALTER, GRIPENBERG, HENRIK, MUELLERTHANN, MICHAEL
Applicants: AGA AB, HOOGOVENS ALUMINIUM HUETTENWER, AGA AKTIEBOLAG, HOOGOVENS ALUMINIUM HUETTENWERK
Application year: 1992
Main classification: C22B21
IPC codes: C22B7, C22B9, C22B21, F27B7, F27B14, F27D23