METHOD OF REMOVING LITHIUM FROM ALUMINUM-LITHIUM ALLOYS

Abstract: The invention is a method of removing lithium from aluminum-lithium alloys. Aluminum-lithium alloys (64) and a metal halide flux (68) are fed into a rotary furnace (10) at a raised and cool end thereof. The temperature at the cool end is above that of the melting point of the flux (68) and below that of the melting point of the alloy (64). The flux is thus melted as it enters the furnace. It coats the solid alloy and the furnace refractory lining (46) to prevent contact between the refractories and the molten metal until the lithium content of the aluminum has been reduced to an acceptable level. A rotation of the furnace moves the alloy (64) and molten flux (68) toward a lower and hotter end at which the temperature is such to melt the aluminum. At this time the lithium is re­ moved from the alloy (64) and is absorbed within the flux (68). The aluminum (70) is then drained from the furnace at its lower end.

Description: This invention relates to reclaiming aluminum-lithium scrap for use in secondary casting alloys by the removal of lithium from the original aluminum lithium alloy. Aluminum alloys containing lithium are significantly less dense than conventional aluminum alloys and thus offer weight savings of around 10% for airplane structures in alloys containing between 2% and 2 1/2% of lithium. At present it is considered that aluminum alloys containing more than about 2 1/2% of lithium tends to weaken the alloy so that it would not be acceptable for use in commercial aircraft. Aluminum lithium alloys are now becoming available on a commercial basis and are being considered for use on current and future airplanes. Aluminum ores are widely available, but the extraction process is energy-intensive and the cost of aluminum metal is largely dependent upon the expenses incurred during extraction. For this reason scrap aluminum alloys are of substantial economic importance, particularly in view of the fact that a large airplane manufacturer would have something of the order of 30 million pounds (13 600 tons) of aluminum scrap available for sale each year. Normally, commercial aluminum scrap is recycled and converted back to usable form at a fraction of the cost of producing virgin metal. Primary producers generally reprocess their own scrap, but scrap produced by their customers is normally sold to secondary producers who process it into a variety of alloys, which are mainly used for casting purposes. Aluminum-lithium alloys, however, present a special problem in that they are very reactive when molten, oxidize readily in air, and attack the refractories in furnace linings. It has, therefore, been postulated by the major primary producers that considerable difficulty could be encountered by secondary producers in attempting to reclaim aluminum lithium alloys by normal methods. They have also claimed that contamination of other alloys by small amounts of lithium could seriously affect their mechanical properties and render them unsuitable for die-casting processes. The currently recommended procedure of one primary producer is to strictly segregate all aluminum-lithium scrap from other aluminum scrap in order to prevent cross-contamination. Furthermore, it is recommended that all aluminum-lithium scrap be recycled only by primary producers who need to be assured that it is not contaminated, and that if more than one aluminum-lithium alloy composition is used, they must be separated from each other. Such requirements are extremely costly in terms of man power, facilities, and transportation and could eliminate much scrap from the secondary market when aluminum-lithium becomes widely used. If the advice of the primary producer or producers must be followed, the only alternative to recycling through the primary producers is to eliminate all aluminum-lithium material from the scrap loop by methods such as burial. This would still require segregation from other alloys and there would still be a risk of accidental contamination of the more conventional alloys by aluminum lithium alloys. As indicated above, recycling of aerospace alloys into the secondary aluminum market is a multimillion dollar industry and elimination of such recycling would have a significant economic impact on aerospace production costs. In addition to the foregoing, two basic methods are known in the aluminum industry for the removal of magnesium from molten aluminum alloys. The generally accepted method is to bubble chlorine gas through the molten alloy until sufficient magnesium has been removed as magnesium chloride. A less commonly used method is to cover a molten bath with a flux of metal fluoride. This method is less noxious and poses fewer safety problems, but is much slower than the chlorine process. Although the second method utilizes nonnoxious chemicals, resultant particulate emissions are considered hazardous waste. Aluminum smelting facilities use standard "baghouses" that readily control these emissions. It is known that lithium behaves similarly to magnesium and would react with chlorine or fluorine. Both of the above methods could be used for lithium extraction but because aluminum-lithium alloys oxidize readily in air and attack the refractory furnace linings, these methods are not practical for normal extraction operations, because they require mixing the flux into a molten metal bath or bubbling reactants through the bath. The methods are also time consuming. In addition, the large primary producers of aluminum-lithium alloys have had to use special refractories to keep reactions to a minimum. DE-C-816,018 discloses a process for extracting magnesium from aluminum scrap. The aluminum scrap is added to a furnace in which the flux is pre-melted. The flux consists of on the one hand fluoride compounds and on the other chloride compounds. The flux mixture comprises 93% by weight chloride compounds and 7% by weight fluoride compounds. The temperature in the furnace is 750-850°C, although heating is interrupted during the addition of the fluoride compound and of the aluminum scrap. The invention relates to a method of removing lithium from aluminum-lithium alloys, comprising the steps of: i) feeding the aluminum-lithium alloy and a metal halide flux into a relatively cool end of a rotary furnace provided with a refractory lining, the maximum temperature of the cool end being about 704°C (1300°F) and being maintained above that of the melting point of the flux and approximately at or below the melting point of the alloy, so that the flux is melted, whereby the alloy is coated with the molten flux preventing the alloy from oxidizing, and whereby the furnace refractory lining is coated with the molten flux preventing contact between the refractory lining and molten alloy until the lithium content of the aluminum alloy has been reduced to an acceptable level; ii) rotating the furnace to move the mixture of the alloy and the molten flux toward a hotter end of the rotary furnace, the temperature of the hotter end being in the range of 730 to 816°C (1350 to 1500°F) sufficient to melt the alloy which is adjacent the hotter end, so that lithium is removed from the alloy and is chemically absorbed by the flux; and iii) separating the molten, lithium comprising flux from the molten, lithium leaner alloy. The temperature of the cool end is maintained above that of the melting point of the flux and approximately at or below the melting point of the alloy. The flux is thus melted and the molten flux coats the solid alloy to prevent the alloy and the lithium from oxidizing. The refractory lining of the furnace is also coated with the flux to prevent contact between the refractories and the molten metal until the lithium content of the aluminum has been reduced to a low level. Refractory life is therefore not affected by the presence of the lithium. The furnace is rotated to transfer the alloy and the molten flux toward a lower and hotter end. The furnace is heated sufficiently to melt the alloy adjacent the hotter end. Thus, as the alloy is melted, the lithium is removed therefrom and is chemically absorbed into the flux. The molten aluminum is then drained from the lower end of the furnace, or held there to be alloyed with other metals. By this method the lithium in an alloy is reduced to less than thirty parts per million. Magnesium in the alloy may, if required, also be be removed along with the lithium. The surrounding of the alloy by the flux prevents the aluminum and lithium from oxidizing as they are melted. This is necessary because the aluminum-lithium alloy is very reactive when molten and will oxidize very rapidly, compared to nonlithium containing alloys. By preventing oxidation, the economics of the process is enhanced in that a higher percentage of available aluminum is recovered. The invention also provides a rapid method of extraction of lithium. The lithium is removed as the alloy is melted, but a large bath of metal is not formed until the lithium is extracted and absorbed by the flux. A primary purpose of the invention is to remove lithium from aluminum-lithium scrap for converting it into alloys suitable for die casting or sand casting. The method also is suitable for reclaiming aluminum from mixtures of aluminum-lithium and other aluminum alloy scrap. This method also has the advantage of reprocessing aluminum-lithium scrap without using noxious chemicals and enables a high percentage of aluminum to be recovered. Further advantages of the invention will be brought out in the following part of the specification wherein small details have been described for the competence of the disclosure, without intending to limit the scope of the invention which is set forth in the appended claims. Referring to the accompanying drawings which are for illustrative purposes; Fig. 1 is a side elevational view of a rotary barrel smelting furnace; Fig. 2 is a cross-sectional side view of the furnace shown in Fig. 1; Fig. 3 is a cross-sectional transverse view of the furnace, taken along the line 3--3 in Fig. 1; and Fig. 4 is a cross-sectional transverse view of the furnace, taken along the line 4--4 in Fig. 1. Referring again to the drawings, therein is shown a rotary barrel furnace, generally designated as 10, having a generally cylindrical body 12 mounted on supports 14 and 16. The furnace is shown to be sloping at an angle of scrap about 5° but may be positioned to slope in the range of approximately 0 to 10°, depending on the type of scrap being processed. In some situations a slope is not required. On the top of the supports are rollers 18 engaged in supporting and rotational contact with tracks 20 and 22, surrounding and fixed to the cylindrical body 12. A motor 26 is fixed on a support 28 and has a spur gear 32 on its drive shaft in driving engagement with an external ring gear 34 fixed on the external surface of the body 12. The motor may rotate the furnace in the range of 1 to 10 RPM but typically is adapted to rotate at the rate of 2 RPM. The furnace is gas or oil heated and the gas/oil is supplied through a tube 38 extending through a fixed nonrotatable end plate 40, supported by means not shown. A flame 36 is adjusted to heat the upper end of the furnace at a relatively low temperature to melt the flux and is adapted to heat the area of the furnace adjacent the lower end to a higher temperature to melt the aluminum. An inlet hopper 42 extends through the plate 40 and it is open to the interior of the furnace. The furnace is lined with refractories 46 inside of the cylindrical body except for an opening 50 in the plate 40 and an area of the discharge opening 52 closed by a door 54 during operation. Positioned at the lower and hotter end of the furnace is a drain spout 60 for discharging molten material into a basin 62. In operation, the furnace 10 is used in a batch process to remove lithium from aluminum-lithium alloys. Magnesium can also be removed and in the same process further alloying can be accomplished. Typically, the alloys 64 are in the form of scrap and the scrap and a flux 68 is fed into the cooler and higher end of the furnace through the hopper 42. The flux 68 is comprised of a mixture of aluminum fluoride and potassium fluoride, known as KALF, and a mixture of potassium chloride and sodium chloride. The fluorides are in the range of 35 to 75 weight percent of the flux and the chlorides are in the range of 25 and 65 weight percent of the flux. The fluoride mixture is formed of 50% of aluminum fluoride and 50% of potassium fluoride and in the chloride mixture 40% is potassium chloride and 60% is sodium chloride. A common flux is formed of 65% of the fluorides and 35% of the chlorides by weight. For every 45,4kg (100 lbs.) of the alloy between 7.3 kg and 10 gkg 16 lbs. and 24 lbs.) of the flux is added, the ratio being typically 5 to 1, metal to flux. The flame 36 from the burning fuel extends about 2/3 of the way into the furnace. The maximum temperature for the cool end is about 704°C (1300° F). and the typical operating temperature is about 620°C (1150° F). The temperature adjacent the lower and hotter end of the furnace is in the range of 730 to 816°C (1350° to 1500° F). which is sufficient to melt the aluminum in the alloy. Thus, the flux melts as soon as it enters the furnace so as to surround the heavier alloy and to coat the entire refractory surface of the furnace as the furnace rotates. Some of the molten flux drops down from the upper part of the furnace but the continuous rotation adds molten flux to the coating. As the alloy and molten flux are mixed together, they are slowly fed along the furnace during the rotation and down the slope toward the hot end. As the alloy pieces melt, they give up their lithium to the flux; that is, the lithium is chemically absorbed by the flux. Thus, the furnace refractories never come into contact with molten aluminum-lithium alloy and only molten aluminum 70, from which the lithium has been extracted, comes into contact with the refractories at the lower end of the furnace as it is melted and is in position for discharge through the openings 52 into the trough 60. Thus, in the operation the aluminum-lithium alloy and flux are so mixed so as the flux melts it forms a coating on the solid alloy before it can oxidize. By preventing oxidation of the aluminum, as well as the lithium, a high aluminum recovery rate is assured and the lithium content of the molten aluminum is reduced to less than thirty parts per million. The aluminum 70 discharged into the basin 62 is ready for converting it into alloys suitable for die casting or sand casting. The method also has the advantage of reprocessing aluminum-lithium scrap utilizing nonnoxious chemicals. By keeping the alloy scrap coated and the lithium absorbed in the flux after it is freed from the alloy, the refractories are protected so as to prevent their deterioration. The molten flux is discharged from the furnace after all of the molten aluminum has been discharged.

Claims: A method of removing lithium from aluminum-lithium alloys, comprising the steps of: i) feeding the aluminum-lithium alloy and a metal halide flux into a relatively cool end of a rotary furnace provided with a refractory lining, the maximum temperature of the cool end being about 704°C (1300°F) and being maintained above that of the melting point of the flux and approximately at or below the melting point of the alloy, so that the flux is melted, whereby the alloy is coated with the molten flux preventing the alloy from oxidizing, and whereby the furnace refractory lining is coated with the molten flux preventing contact between the refractory lining and molten alloy until the lithium content of the aluminum alloy has been reduced to an acceptable level; ii) rotating the furnace to move the mixture of the alloy and the molten flux toward a hotter end of the rotary furnace, the temperature of the hotter end being in the range of 730 to 816°C (1350 to 1500°F) sufficient to melt the alloy which is adjacent the hotter end, so that lithium is removed from the alloy and is chemically absorbed by the flux; and iii) separating the molten, lithium comprising flux from the molten, lithium leaner alloy. A method according to claim 1, including: sloping the rotary furnace so that the cool end is higher than the hotter end. A method according to claim 1 or 2, including: alloying the molten aluminum in the furnace with other metals. A method according to claim 1-3, including: reducing the lithium in the alloy to less than 30 parts per million. A method according to claim 1-5, in which the flux is comprised of: a mixture of aluminum fluoride and potassium fluoride, the fluoride mixture forming between 35% and 75% by weight of the flux; and a mixture of potassium chloride and sodium chloride, the chloride mixture forming between 25% and 65% by weight of the flux. A method according to claim 2-5, in which: the fluoride mixture is formed of 50% of aluminum fluoride and 50% of potassium fluoride by weight: and the chloride mixture is formed of 40% potassium chloride and 60% sodium chloride by weight. A method according to claim 1-5, in which the flux is comprised of: a mixture of aluminum fluoride and potassium fluoride, the fluoride mixture forming 65% of the flux by weight; and a mixture of potassium chloride and sodium chloride, the chloride mixture forming 35% of the flux by weight. A method according to claim 1-7, including: feeding between 7.3 kg (16 lbs.) and 10.9 kg (24 lbs.) of flux into the furnace for 45,4 kg (100 lbs.) of alloy. A method according to claim 2-8, including: sloping the furnace at angles in the range of 0° to 10°. A method according to claim 1-9, including: heating the cool end of the furnace to about 620°C (1150°F); and heating the hotter end of the furnace to the range of 730°C to 816°C (1350°F to 1500°F). A method according to claim 1-10, including: rotating the furnace at the rate of between 1 RPM and 10 RPM. A method according to claim 1-12, in which: the alloy contains lithium in an amount up to about 2 ½ %. A method according to at least claim 2, including: sloping the furnace at an angle of about 5°. A method according to claim 1-13, including: feeding approximately 0.45 kg (1 lb.) of flux into the furnace for 2.3 kg (5 lbs.) of alloy.

Inventors: BADGER, DOUGLAS VICTOR, SMALL, BILLY LEE, STANSFELD, PHILIP KARL
Applicants: THE BOEING COMPANY
Application year: 1987
Main classification: C22B21
IPC codes: C22B26, C22B21, C22B7