Abstract:
A process is provided for melting and reclaiming aluminum and lithium from aluminum-lithium scrap including heating a molten salt in a heat bay, mixing the heated salt with aluminum-lithium scrap in a charge bay to form a molten mixture, separating aluminum from impurities in the charge bay by coalescing aluminum to form a molten aluminum metal pad, chlorinating a portion of the charge mixture from the charge bay to form a chlorinated salt mixture and metal chlorides, removing metal chlorides from the salt mixture, removing lithium from the salt mixture, and feeding the salt mixture back to the heat bay. In one aspect, the chlorinating step of the present invention includes introducing carbon monoxide or, preferably, solid carbon to control oxide concentration. The process, in other aspects, further includes adding fluorides and recovering lithium and removing metal chlorides from the salt to maintain a preferred salt composition. The lithium and the metal chlorides can be removed by withdrawing volatile metal chloride gases and electrolytically reducing metal chlorides in the molten salt.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for recovering aluminum-lithium from aluminum-lithium alloy containing impurities such as from aluminum-lithium scrap or aluminum-lithium skim or dross. 
     Aluminum metal is an engineering material which can be readily recycled. Aluminum recycling involves recovery or reclamation of aluminum metal from aluminum scrap containing impurities, e.g., such as aluminum scrap or aluminum skim or dross from processes wherein molten aluminum metal comes in contact with oxygen in the air. Typically, the impurities in the aluminum exist as oxides and include aluminum oxide on the surface of the aluminum and other oxides such as surface magnesium oxide deriving from alloying elements. Aluminum skim often contains nitrides such as AlN and carbides such as AlC. Other extraneous materials are present in aluminum-lithium scrap. An example of such extraneous materials in aluminum-lithium scrap would be surface oxides containing lithium aluminate and oxides of aluminum, lithium, and magnesium. 
     Molten salt reclamation processes involve lifting non-metallic materials from aluminum or aluminum alloy during melting and preferentially wetting the non-metallic materials comprising the impurities in the incoming aluminum scrap. The molten salt preferentially wets the impurities which separate from the aluminum as the aluminum coalesces into metal droplets. The aluminum metal droplets sink to the bottom of a salt-containing process vessel and form into a continuous molten metal pad of aluminum. 
     The preferential wetting of the oxides and the resulting separation of aluminum from such contaminant oxides is a function of salt composition and the amount of solid particulate oxides and nitrides present in the salt. As scrap is fed to the salt-containing process vessel, the amount of oxides present in the salt increases with each increment of scrap added to the process. This increasing oxide content poses a serious problem in salt-based reclamation processes since the presence of oxides at levels higher than 5 to 30 wt %, as a function of particle size distribution, causes significant reductions in the recovery of aluminum from the process. In effect, the high oxide levels poison the process for reclaiming aluminum metal. This aspect of the conventional processes has been a significant drawback to the efficiency and economics of salt-based recovery systems. 
     It is an object of the present invention to provide a process for reclaiming aluminum metal from aluminum-lithium scrap containing impurities. 
     It is a further object of the present invention to provide a process for reclaiming aluminum metal from aluminum-lithium scrap containing impurities at a higher yield than in present processes. 
     It is yet another object of the present invention to provide a process for melting aluminum-lithium alloy while preventing contamination of the melt to an extent that is not now practicable. 
     These and other objects of the present invention will become apparent from the description of the invention as follows. 
     SUMMARY OF THE INVENTION 
     The process of the present invention for continuous salt-based melting and recovery of aluminum-lithium includes heating a molten salt in a heat bay, mixing the heated salt with aluminum-lithium scrap in a charge bay to form a molten charge mixture, separating aluminum metal from impurities by coalescing to form a molten metal pad of aluminum and a salt sludge containing lithium and separated impurities, chlorinating a portion of the salt sludge to form a chlorinated salt mixture containing lithium and metal chlorides, removing metal chlorides from the chlorinated salt mixture, removing lithium, and feeding the remaining chlorinated salt mixture back to the heat bay. The process chlorinating step includes using carbon or carbon monoxide to control oxide concentration. In one aspect, carbon is the preferred agent to control oxide concentration. The step for removing metal chlorides includes withdrawing metal chloride gas and further includes reducing molten metal chlorides and lithium electrolytically and removing reduced metal so produced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE is a schematic diagram depicting the overall process of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The process of the present invention represents a continuous deep salt metal reclamation technique designed and developed primarily for melting high surface area, dirty aluminum-lithium scrap. Aircraft plate and sheet are examples of such aluminum-lithium scrap, and the process of the present invention is suitably tailored to an increased volume of such aircraft plate and sheet expected to be available for recycling in the future. However, the process of the present invention is not limited to aircraft plate or sheet but can be applied to most any aluminum-lithium containing oxide impurities including aluminum-lithium skim or dross formed in melting processes wherein molten aluminum-lithium alloy comes into contact with oxygen in the air. 
     The continuous metal scrap reclamation process of the present invention involves a molten salt maintained continuously in the molten state and providing a chemical composition and oxide composition such that metal recovery is achieved at improved levels continuously. 
     The process of the present invention operates to control and maintain chemical composition concentration continuously at a level consistent with the maximum recovery of aluminum and lithium metal through reaction of the oxides with chlorine and carbon or carbon monoxide to form chlorides of two general types: volatile and non-volatile. The volatile chlorides, e.g., such as silicon tetrachloride, are removed from the continuous salt-based melting process as part of the reaction sequence in the carbon monoxide/dioxide vapor waste stream. Small amounts of all chlorides present, including aluminum chloride, are removed in the offgas stream, but most remains in the molten salt. 
     Non-volatile chlorides, e.g., such as aluminum chloride, lithium chloride, and magnesium chloride, are removed in a process step by electrolysis. The process provides a continuous melting operation in which recovery of molten aluminum and lithium is maximized by controlling the integrity of the salt-based melting medium continuously. 
     Referring now to the FIGURE, a continuous scrap remelt process involves a superheated molten salt melt medium circulated throughout the continuous process of the present invention beginning, for purposes of illustration, in heat bay 1. Salt is heated in heat bay 1 to a temperature maintained in the range of about 1400°-1450° F. The salt is pumped from heat bay 1 by pump 2 to charge bay 3 where the salt is mixed with aluminum-lithium scrap through an appropriate agitation in a swirl motion. In one aspect, scrap enters charge bay 3 continuously via a fume-controlling lock chamber (not shown). As the heated salt mixes with the aluminum-lithium scrap, a coalesced molten metal collects in a lower portion of a collection bay depicted here as collection bay 4 where an optional coalescer is located to insure substantially metal-free salt/oxide passage to chlorination bay 6. Chlorine and carbon or carbon monoxide, alternatively phosgene, is introduced to the salt stream in chlorination bay 6 to convert the required amount of oxides into chlorides to maintain a steady-state oxide concentration. In the chlorination bay 6, several gas species are provided such that carbon dioxide is the main reaction by-product as well as titanium tetrachloride and silicon tetrachloride if titanium or silicon is present as scrap contaminant which, together with excess chlorine and carbon monoxide, are withdrawn from chlorination bay 6 and passed through a scrubber (not shown). The molten salt, now enriched with aluminum chloride and magnesium chloride, passes through an optional filter (not shown) to electrolysis bay 9 in which the required amount of reduction of lithium, aluminum, and magnesium is controlled to remove the lithium and maintain a preferred steady-state salt concentration condition. Lithium and a relatively small amount of aluminum and magnesium metal formed in this step can be tapped periodically. Cleaned salt passes from electrolysis bay 9 and is fed back into heat bay 1 to be superheated again and maintained at process temperatures of about 1400°-1450° F. 
     The means for heating the salt melt medium in heat bay 1 preferably is AC resistance heating. Heating provided by AC resistance minimizes fume generation and can be carried out under an inert atmosphere. 
     The process of the present invention takes place within two or more interconnected chambers, as depicted in the FIGURE. Molten salt flows continuously through the chambers, e.g., as shown in the FIGURE in counterclockwise flow. The flow of salt in the process is induced by a molten salt pump as depicted by pump 2 in the FIGURE. Multiple electrodes can be used in the heat bay to transfer electrical energy to the salt. 
     Salt leaving heat bay 1 enters charge bay 3, the next major chamber of the process. In the charge bay, as scrap is added continuously to a swirling or agitating salt, movement of the salt aids in the separation of oxides from the aluminum during melting. Flow rates of the salt into the charge bay 3 are controlled in the process of the present invention to be directly related to the scrap charge flow rate and incoming salt temperature. In this way, the process of the present invention provides a superheated salt (above about 1350° F.) for establishing the heat necessary to melt the aluminum-lithium scrap. 
     The molten salt of the present invention comprises a salt containing about 75-98% chlorides of sodium, potassium, magnesium, aluminum, calcium, and lithium. Other chlorides such as barium chloride or others are not preferred for reasons that they are either more expensive or more hazardous to handle. For aluminum-lithium alloys, the preferred salt typically contains only chlorides of lithium and potassium. The salt further contains up to about 25%, preferably about 2-20%, and more preferably about 5-15% by weight fluorides of one or more of sodium, potassium, magnesium, calcium, aluminum, and lithium. The composition of the salt is selected to control performance of the process in the three process steps of charge mixing/coalescence, chlorination, and reduction at the lowest practical temperature. Salt melting temperature range in the process of the present invention preferably is as low as possible but must be higher than the scrap melting point. Fluoride presence in the salt is important for the reason that fluoride increases metal coalescence in the charge bay and further for the reason that kinetics of the chlorination reaction are enhanced. At high fluoride concentrations, higher temperatures would be required. However, high fluoride concentration is preferred because the chlorination reaction rate is higher, thus decreasing the size of the chlorination unit. The increased reaction rate is believed to be attributable to an increased solubility of oxide. 
     In one aspect, a salt containing 60-90% LiCl, 0-25% KCl, and 5-15% LiF is preferred. Outside this range of salt composition, coalescence and/or chlorination reaction rates decrease at the preferred operating temperature. 
     In charge bay 3, oxide films such as Al 2  O 3  and MgO from the scrap and tramp oxides present in the scrap stream such as TiO 2  and SiO 2  are wetted by the salt preferentially to form a salt sludge. The aluminum metal droplets sink to the bottom of charge bay 3 and coalesce into a continuous body or pad of molten metal, which is tapped from the chamber continuously or semi-continuously by a number of conventional tapping methods. 
     Salt flows from the charge bay into the chlorination chamber where sufficient oxides are reacted with chlorine and carbon or carbon monoxide to maintain the concentration of oxides at the desired level. The level of oxides should be maintained at less than about 10 wt %. All the oxides are chlorinated by a reaction such as: 
     
         MO.sub.x (s)+Cl.sub.2 (v)+CO(v)→MCl(l,v)+CO.sub.2 (v) 
    
     where M can be Al, Mg, Ti, Si, or other metals. 
     It has been found that solid carbon used in place of carbon monoxide gas vapor in the process of the present invention and mixed with chlorine provides an unexpectedly advantageous result. In this aspect, excess carbon is added and remains continuously with the salt. It would have been expected that carbon monoxide would react at a higher reaction rate rather than solid carbon. However, it has been found empirically that carbon in the process of the present invention provides a reaction rate two times faster than when carbon monoxide is substituted in the same process. 
     It has been found further that the chlorination step of the process of the present invention as depicted in the above equation will not go to any significant rate without the addition of carbon or carbon monoxide. 
     Metal chlorides such as silicon tetrachloride leave the chlorination chamber in an offgas stream which also contains carbon dioxide, unreacted carbon monoxide, and chlorine gas. Other metal chlorides, such as magnesium chloride, have sufficiently low vapor pressures at temperatures of about 1400° F. and do not leave with waste gas but rather build up in the salt. Aluminum chloride builds up in the salt because it readily forms low vapor pressure complexes with most chlorides, e.g., such as NaAlCl 4  or KAlCl 4 . Aluminum chloride would leave with the waste gases if the complexes were not formed since the atmospheric sublimation point of aluminum chloride is 365° F. 
     The final chamber through which the salt passes is called the reduction bay wherein the buildup of non-volatile metal chlorides such as MgCl 2 , LiCl, and AlCl 3  is controlled. In the reduction bay, a low voltage DC current passes through the salt to form electrolytic products of chlorine gas and reduced metals. The chlorine gas so formed can be recycled to the chlorination reactor of the present process. The metals are formed at the bottom of the chamber (the formed metal being the cathode) and can be removed periodically as molten lithium-aluminum-magnesium alloys. Incoming scrap containing significant lithium, e.g., about 2 wt %, reacts with the AlCl 3  and MgCl 2  in the charge bay, thereby forming aluminum, magnesium, and lithium chloride. Operative chemical reactions are: 
     
         2AlCl.sub.3 +3Mg→3MgCl.sub.2 +2Al 
    
     
         AlCl.sub.3 +3Li→3LiCl+Al 
    
     
         MgCl.sub.2 +2Li→2LiCl+Mg 
    
     In this case, the only metal recovered in the electrolytic cell to any extent is lithium, since AlCl 3  and MgCl 2  in the continuous process of the present invention will not build up and LiCl will increase in concentration in the salt at a higher rate, if electrolysis is not continuous. If the electrolysis/reduction is carried out continuously, an Al-Mg-Li alloy is formed. However, a more pure Li alloy (low Al) and Mg is formed as a by-product by allowing the AlCl 3  and MgCl 2  to react through many charge/chlorination cycles before operating a batch electrolysis reduction cell. 
     The preferred method for heating the molten salt-based melt medium of the process of the present invention is electrical resistance AC heating. However, electric radiation or other methods are suitable as substitutes. 
     The process of the present invention is controlled to minimize any contact with moist air. Otherwise, hydrolysis will occur, leading to chlorine losses. Additionally, fumes from the process would be environmentally undesirable. For these reasons, atmospheric contact is maintained at a minimum. 
     The process of the present invention provides a chlorination step and an electrolysis step operated continuously. However, such chlorination/electrolysis steps can be performed in a batch or semi-continuous reactor. 
     In one aspect, the chlorination step can be performed under a high pressure, e.g., above about 50-60 psig. Such a pressure increases reaction rates significantly. However, the process of the present invention is not limited to such higher pressures for practical reasons. 
     While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.