Recovery of non-ferrous metals from dross
FIELD OF THE INVENTION 
     The present invention relates to a method of recovering a non-ferrous metal from a dross starting material that contains the metal, possibly together with non-ferrous metal scrap, and/or a starting material comprised of such scrap. The method includes the steps of feeding the starting material to 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 continuously or intermittently, and removing the resultant molten metal from remaining dross residues and/or from resultant solid dross residues. 
     BACKGROUND OF THE INVENTION 
     Methods for recovering non-ferrous metals from dross are known to art. For example, the Canadian Patent 1,255,914 teaches a method for recovering non-ferrous metal from non-ferrous-metal containing dross. The method includes the steps of feeding the dross into a rotary furnace that has a refractory lining, directing a plasma burner into the furnace so as to heat the dross to a temperature 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 include sodium chloride (NaCl), potassium chloride (KCl) 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 &#34;Effect of salt flux additives on aluminum droplet coalescence&#34;. In an article entitled &#34;Interfacial phenomena in molten aluminum and salt systems&#34; published in the same reference, pages 85-103, Francis K. Ho and Yogesh Sahai describe the remelting of aluminum waste beneath a salt layer. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to recover, or win, non-ferrous metal from a dross starting material that contains the metal. The dross starting material may also include non-ferrous metal waste, and/or a starting material containing such waste. The non-ferrous metal is recovered, or winned with the aid of an oxy-fuel-burner and in the absence of a protective salt layer. 
     Another object of the present invention is to reduce the formation of corrosive salt vapors that are obtained when using protective salt layers, and to obtain an oxidic rest product that is free from water-soluble salts, thereby benefitting the environment. 
     These objects are achieved with a method for recovering, or winning, a non-ferrous metal from a dross starting material that contains the metal. The dross starting material may also include non-ferrous metal waste, and/or from a starting material that 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 residue. 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. The burner is supplied with a fossil fuel and with a gas that contains at least 80% oxygen. The furnace is rotated at a speed of 5 r.p.m or lower. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Figure illustrates a converter 1 whose center axis 2 defines an acute angle with the horizontal plane. The angle may be, 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 that extends perpendicularly to and passes through the center axis 2 of the converter 1. The upwardly directed end of the converter 1 narrows into an opening 4. A protective hood 9 is mounted on the upstream side of the opening. The hood 9 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 of the burner 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 opening 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 hydrocarbon, such as 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-containing gas used may be comprised of oxygen having a concentration of about 93% oxygen from a PSA-plant. The remainder of the gas may consist 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, that is, an amount that corresponds to from 80 to 100% of the oxygen required for complete combustion, oxidation of metal in the converter 6 is minimized. 
     When the starting material, for instance aluminum-containing 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 that encapsulate an aluminum core and have a surrounding protective aluminum-oxide shell to deform and the protective shell to disintegrate. 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 residues, 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 invention, 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 prevent ingress of atmospheric oxygen into the converter, and, therewith, oxidize any aluminum that 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 preferably argon. As illustrated in the drawing, the shielding gas is introduced through a lance that is connected to a source of shielding gas. Alternatively, the gas may be introduced 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 manufacture of a series 3000 aluminum alloy 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 converter opening, at a short distance therefrom. The burner had a power output of 1 MW and was operated with propane and pure oxygen, that is an oxygen-containing gas which comprised 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 ton of dross introduced, while the oxygen consumption corresponded to 44 normal cubic meters for each ton of dross introduced. The total process time was 32 minutes. The resultant rest product comprised of pure oxides of the aforesaid metals and aluminum. 
     Highly satisfactory results have been achieved when the starting material charged to the converter was comprised 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 that will prevent atmospheric oxygen from penetrating to the metal. As before mentioned, an overpressure is preferably maintained in the converter, by supplying a shielding gas, such as argon, to the converter.