Abstract:
The invention provides efficient and effective processes for recovering metals such as aluminum, magnesium and lithium from mixed waste sources such as auto shredder residue, aluminum cans, waste particles of aluminum alloy and municipal waste. The metal-waste source is dissolved in a more noble metal solvent at a temperature at which contaminants do not dissolve. The purified metal is then recovered from the solvent, preferably by electrorefining.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to methods for recovering aluminum from sources of mixed metals and metals mixed with organics. 
       BACKGROUND OF THE INVENTION 
       [0002]    Processes for effectively converting organics to carbon and steam have recently been disclosed. For example, co-pending PCT publication WO 2004/096456 A2 and U.S. patent publication 2004/0253166 A1 disclose means of delaquering, decoating, and recovering aluminum that is mixed with organics. Secondary aluminum occurs in these mixtures from many diverse sources, such as auto shredder residue, aluminum cans, waste particles of aluminum alloy and municipal waste. These resources represent very large potential sources of aluminum that are now only being exploited in very limited amounts. 
         [0003]    There are two problems with these sources of aluminum metal that prevent the full recovery and use of these resources. First, the aluminum in foil and used beverage containers is very thin. When melted into a liquid aluminum at 700° C. using the conventional techniques, it is difficult to prevent substantial losses due to burning. Salt coverings are used to reduce these losses, but these coverings also become contaminated and therefore are another source of aluminum loses. Second, the material in auto shredder residue and municipal waste is a mixture of many different kinds of aluminum alloys and other metals such as copper and zinc. Many of these aluminum alloys are not compatible with each other and are harmed by the presence of other metals. A number of complex sorting and separation schemes are known, but they are expensive, inconclusive, and generally ineffective given the small particle size associated with metal recovery from these sources. Because of these problems, very large tonnages of secondary aluminum are currently exported to countries with low labor costs for stripping and sorting these diverse aluminum sources prior to purification. This practice of exporting these sources of aluminum results in a need to import very large amounts of purified aluminum. 
         [0004]    Thus, to overcome these costly practices of shipping, sorting and importing secondary metals, there is a need for a process capable of efficiently recovering high yields of pure aluminum from low-grade, secondary sources that may be contaminated with other metals and/or organic materials. It would also be desirable to jointly recover valuable aluminum alloying materials such as magnesium and lithium as master alloys for recycle in these processes. 
       SUMMARY OF THE INVENTION 
       [0005]    The processes of the present invention meet these goals by providing a means to recover and recycle aluminum from a large number of sources. The processes have the very desirable effect of converting low grade, secondary aluminum into commercially pure aluminum ready for use. The process has two interdependent steps including the dissolution of the solid aluminum into a carrier metal, such as molten zinc or tin, followed by recovery of the purified aluminum from the more noble metal solvent. 
         [0006]    The aluminum is recovered in high yield because the dissolution occurs at temperatures below that at which severe oxidation occurs. Yield is not hurt by the thinness or small particle size of the aluminum. Indeed, these factors enhance the rapid dissolution of the aluminum metal. 
         [0007]    In the processes of the invention, secondary aluminum is not melted at the conventional temperatures of 700° C. to 750° C., but rather at temperatures which are 200° C. to 250° C. lower. A melting temperature between about 500° C. and about 600° C. is used, while a temperature between about 530° C. and about 560° C. is preferred. In this temperature range, it is possible to vary the aluminum content of the alloy while reducing the amount of impurities carried with dissolved aluminum. While it is desirable to carry as much aluminum as practical by using higher temperatures, the use of a lower temperature will eliminate more of the impurities as solids. For example, at about 500° C., an alloy of 73 wt % zinc and 27 wt % aluminum is formed. At a temperature of about 550° C., the alloy formed is 58 wt % zinc and 42 wt % aluminum. At a temperature of about 600° C., an alloy of 33 wt % zinc and 67 wt % aluminum is formed. At about 500° C., the solubility of copper in zinc is 5 wt %, whereat at about 600° C. it is 10%. The solubility of manganese in zinc at about 500° C. is 5 wt % and at about 600° C. is 12 wt %. The solubility, and therefore the separation of impurities, improves at lower temperatures. At about 550° C. the zinc solvent is an efficient carrier with a high removal of impurities. 
         [0008]    Many impurities present in the sources of secondary aluminum, including silicon and chromium, have a very limited solubility in zinc at 550° C. and are therefore readily removed by drossing. At about 550° C., manganese and very active metals, such as magnesium, can be furtler depleted by the addition of zinc chloride to the melt producing the corresponding metal chlorides for elimination and additional zinc metal. 
         [0009]    The second step in the processes of the present invention is the recovery of the melted aluminum, preferably by electrorefining of the aluminum from the more noble zinc solvent. Methods of such recovery by electrorefining are generally known in the art. For example, the principles of such electrorefining are described in European Patent No. 272 803 and U.S. Pat. No. 4,118,292, both of which are incorporated herein by reference. The teachings of these patents to Fray are well suited to provide the low voltage refining from the zinc aluminum alloy. In a preferred embodiment of the present invention, the Fray packed bed electrorefining and electrolysis cell described in U.S. Pat. No. 4,118,292 is modified from its vertical position to a horizontal position and the packed bed is replaced by weirs, over which the alloy flows to create turbulence so that the aluminum depleting from the surface is replaced. This cell differs from the conventional fused salt, tliree layer cell, in that the thick salt layer is made thin by imposing an inert woven ceramic pervious to the salt(s) but not the molten metal, which contains the salt and provides a thin, low resistance barrier between the anodic heavy aluminum and the cathodic pure aluminum. 
         [0010]    While zinc is the preferred solvent in the methods of the present invention, tin may be substituted as the solvent when a very high purity aluminum is desired. While tin has a lower carrying ability, it has the advantage of an extremely low vapor pressure, which results in a very low transfer of solvent between the alloy and the pure aluminum. 
         [0011]    At operating temperatures, zinc has a substantial vapor pressure that will approach 680° C. to 700° C. at the salt interface between the liquid aluminum metal and the lower temperature alloy. In the electrorefining cells of the present invention, the temperature of the incoming alloy is controlled to maintain a temperature that is lower than the molten aluminum above the incoming alloy. 
         [0012]    A current density used is between about 500 amperes per square foot and about 2000 amperes per square foot, with a current density of about 1000 amperes per square foot being preferred. The resistance of the thin salt in the bed is still sufficient to create a high heat flux at these referred high current densities. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The present invention is drawn to a process that effectively and efficiently reclaims secondary aluminum from mixed waste sources. In this process, the aluminum is dissolved in a solvent and thereby separated from undesirable wastes present. The aluminum is then recovered from the solvent. In instances in which magnesium or lithium are present, these metals may also be recovered by the methods of the present invention. In one embodiment of the invention, the waste containing the secondary aluminum is dissolved in a zinc solvent at a temperature between about 500° C. and about 600° C. and the aluminum is recovered from the solvent by electrorefining. 
         [0014]    The secondary aluminum may be dissolved into the zinc solvent in any convenient dissolver apparatus within the temperature range. In a preferred embodiment, the dissolution of the aluminum, and magnesium or lithium where present, is conducted in a rotary furnace, indirectly heated on its shell. Internal to the furnace is a spiral which acts in the same way as a cement truck in that, as the shell rotates in one direction, solids are moved forward and exit the front, while the liquid alloy passes through holes in the spiral. Zinc solvent is fed to the inside of the shell. Depleted aluminum alloy returned from an electrorefining cell may also be fed to the inside of the shell simultaneously with or in a similar manner to the zinc solvent. A feeder, such as a screw feeder, feeds the secondary metal/waste to the inside of the furnace. Certain insoluble materials, such as silicon, will tend to float on the aluminum-zinc alloy. At the rear of the furnace there is a dross ladle which passes through the surface of the alloy and removes floating solids which are discharged to a conveyor on the rear center line. The pregnant alloy overflows a weir to a launder, which carries it to a temperature controlled holding vessel for transfer to the refining cells. The furnace, spiral, and drossing fixtures are all protected by suitable refractory coatings. 
         [0015]    The retention time for the aluminum-containing feed is a function of its thickness or massiveness. Generally, one-half hour to about one hour will be a sufficient retention time to effectively dissolve the secondary aluminum present in the source materials. 
         [0016]    The heat needed for the endothermic dissolution of aluminum is preferably provided by the recycling alloy and by heat on the external surface of the shell. This heat may be provided by gas combustion, but the gas stream from the cooling of the refining cells may also be used as a heat source for the dissolution process. 
         [0017]    There are many suitable electrorefining apparatuses known in art that are suitable for recovering the aluminum dissolved in the solvent. In one embodiment of the invention, the refining cell in is a vertical configuration as illustrated in U.S. Pat. No. 4,118,292, which is incorporated herein by reference. This configuration is particularly desirable in instances when magnesium or lithium are present and must be removed from the zinc-aluminum alloy prior to the transfer of the bulk of the aluminum. 
         [0018]    More preferably, an electrorefining cell having a horizontal configuration is employed. In one embodiment, a graphite block is machined to provide channels in which the aluminum-zinc alloy (typically having 40 wt % aluminum) is flowed. This is connected to the cell anode. Multiple weir are configured on the bottom of the channels causing the flow to be turbulent and bring fresh alloy to the depleting surface. 
         [0019]    Ceramic woven cloths that function as a diaphragm are positioned in the channels. Suitable woven ceramics for use in these cells include aluminum silicates, high alumina ceramic, zirconia and other ceramics. These ceramic cloths are fed molten salt from a down corner to provide a layer of salt in the cloth and below it. This provides a barrier to the liquid anodic alloy and the cathodic aluminum or aluminum-magnesium alloy. The salt is chosen to be lighter than the alloy but heavier than the pure aluminum. Suitable salts include equimolar sodium chloride-aluminum chloride with magnesium chloride and sufficient barium chloride for the needed density. The salt is provided from a down spout to seep along the cloth, overflow a weir and be recycled or purified as needed. 
         [0020]    The top ridges of the channels are covered with a ceramic to provide support to the ceramic cloth diaphragm and to insulate the anode from the pure aluminum cathode. The depleted zinc solvent passes over a weir and out of the cell to be returned to the dissolver apparatus, or to further purification processing as required. 
         [0021]    The electrorefining cell may be divided into two zones. A first zone may refine out magnesium as an aluminum-magnesium master alloy. A 40 wt % aluminum-magnesium alloy is a very useful and valuable product and the refining may be ceased with the production of this alloy, if desired. 
         [0022]    The cathode is preferably a graphite block machined to provide cathodic leads into the aluminum cathode. As aluminum accumulates in this cathodic section, it overflows a weir and exits the cell. An argon blanket may be used to protect the top surface of the aluminum. 
         [0023]    By machining the top surface of the cathode into the anode configuration, a bipolar cell may be constructed and a stack of such bipolar units may be prepared for commercial operations. 
         [0024]    A high current density is preferably used for the electrorefining process. The current density is preferably in the range of about 500 amps per square foot to about 1500 amps per square foot at 0.5 to about 1.5 volts depending on the salt layer thickness prepared. 
         [0025]    At these high current densities, a great deal of heat is generated. In one embodiment, this heat is removed by holes drilled in the anodic section of the electrorefining cell below the level of the zinc and fitted with ceramic pipes. These pipes, in combination with air cooling can control the cell temperature. Preferably, the pure aluminum is maintained at about 700° C., but the zinc alloy is kept below about 600° C. This heat may be usefully transferred to and used in the dissolving stage. 
         [0026]    The cell is provided with a surrounding ceramic that is at salt level and extends up to the level of the cathodic aluminum and down to the zinc alloy level. This provides containment with shifting levels of the components. 
         [0027]    Where very high purity aluminum is desired, a tin solvent may be substituted for the zinc. Tin has a very low vapor pressure at aluminum melting temperatures. It may then be desirable to refine the aluminum a second time by taling aluminum refined from a zinc solvent and re-processing that aluminum using the processes of the present invention with a tin solvent. This second dissolution and refining from tin results in a much higher purity aluminum, but, because tin is more expensive and less efficient than zinc, this re-processing should be reserved for applications in which a very high purity aluminum is required. 
         [0028]    Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination of the following examples thereof, which are not intended to be limiting. 
       EXAMPLES 
     Example 1 
       [0029]    To ascertain the effectiveness of dissolving secondary aluminum in zinc on a commercial scale (as opposed to melting aluminum), the yield of a large amount of dissolved secondary aluminum was measured. Three tons of molten zinc solvent where placed in a die casting kettle and held at 525° C. Three additions of shredded, delaquered used beverage cans totaling 1984 pounds were added to the zinc solvent. The average recoveries from these aluminum additions were aluminum 88% and magnesium 98%. Drossiig lowered the manganese content nearly 50% to about 0.8%. This illustrates the high yields obtainable by dissolving secondary aluminum as opposed to melting, even with a very thin source of secondary aluminum. 
       Example 2 
       [0030]    Similar to Example 1 above, the yield obtained by dissolving scrap aluminum alloys in zinc was tested by dissolving 1992 pounds of alloy 380 borings and shavings a three tons of molten zinc in three batches. The 380 alloy is nominally 92% aluminum, 8.5% silicon and 0.5% manganese. Despite the fineness of the feed materials, the recovery of aluminum in the three additions averaged about 92%. Drossing lowered the level of manganese combined with silicon to about 0.01% and the silicon to about 1.35%. This illustrates the high yields and purifications obtainable by the zinc dissolution system. 
         [0031]    The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiment described hereinabove is further intended to explain the best mode known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.