Patent Application
20060042413
The present invention relates to a method of producing low carbon aluminum in a single reactor compartment carbothermic furnace with control to lower or raise the temperature of reactants within the interior of the reactor compartment.
The direct carbothermic reduction of alumina has been described in U.S. Pat. No. 2,974,032 (Grunert et al.) and U.S. Pat. No. 6,440,193 B 1 (Johansen et al.) as well as in Proceedings 6th Conference on Molten Slags Fluxes and Salts, Edited by S. Seetharaman and D. Sichen “Carbothermic Aluminum”, K. Johansen, J. Aune, M. Bruno and A. Schei, Stockholm, Sweden-Helsinki Finland, Jun. 12-17, 2002. It has long been recognized that the overall reaction:
Al2O3+3C=2Al+3CO (1)
takes place, or can be made to take place, generally in steps such as:
2Al2O3+9C=Al4C3+6CO (vapor) (2)
Al4C3+Al2O3=6Al+3CO (vapor) (3)
Al2O3+2C=Al2O (vapor)+2CO (vapor) (4)
Al2O3+4Al=3Al2O (vapor) (5), and
Al=Al (vapor) (6).
Reaction (2) takes place at temperatures below 2000° C. and generally between 1900° C. and 2000° C. Importantly, reaction (3), which is the aluminum producing reaction, takes place at higher temperatures of about 2050° C., and requires substantial heat input. Very importantly, in addition to the species stated in reactions (2) and (3), volatile species including gaseous Al, reaction (6), and gaseous aluminum suboxide that is Al2O, are formed in reaction (4) or (5). In the overall carbothermic reduction process, the Al2O and Al gases are recovered by reacting them with carbon in a separate reactor usually called the vapor recovery unit or vapor recovery reactor.
Other patents relating to carbothermic reduction to produce aluminum include U.S. Pat. No. 4,099,959 (Dewing et al.), where dual reaction zones are described and where off gases are passed through granular carbon material and countercurrent to fresh coal or “green” coke in a gas scrubber. U.S. Pat. Nos. 4,033,757 and 4,388,107 (both Kibby) teach reduction of carbon content by heating the surface of the melt to about 2100° C. while maintaining a lower temperature of about 1850° C. in the slag thereby lowering C (carbon) in the metal. This however would seem to be difficult in operation and would appear to cause excessive vaporization. The former Kibby '757 patent uses arc heating and a plasma jet in a process that starts at 1850° C.-1950° C., then arc heats to 2100° C., producing Al with <10 wt. % C. The latter Kibby '107 utilizes a secondary furnace or separate decarbonization zone requiring transfer of very hot metal and slag to and from the furnace.
Other art in this area, includes, for example, U.S. Pat. Nos. 4,334,917 and 4,533,386 (both Kibby) which appear to teach either multiple reactors or additional decarbonization zones. U.S. Ser. No. 10/646,507, filed Aug. 23, 2003 (J. Aune et al.-Docket 03-0668) teaches an electrode arrangement for a single reactor compartment carbothermic furnace, where side wall electrodes, each connected to the other, substitute for a bottom lining as an electrical contact and vertical electrodes are submerged in the liquid slag both.
In the carbothermic process, the use of dual reaction zones or a plurality of furnaces, adds expense to the process, and unnecessary complication. What is needed is an efficient and simple method for recovering lower carbon containing aluminum. Therefore, it is one of the main objects of this invention to provide a more cost and energy effective, improved aluminum production process, by use of a single reactor compartment, carbothermic furnace with temperature control of the reactor compartment.
The above needs are met and the above problems solved by providing a method of using a single carbothermic reactor to produce aluminum with low carbon content, comprising: (a) providing a single furnace having a single hollow, interior reactor compartment with a plurality of bottom resistance heating electrodes and one or more optional vertical top electrodes; and then; (b) adding Al2O3 and C for start-up of the process to the inside of the furnace and melting their mixture, to provide a (Al2O3—Al4C3) slag and excess Al4C3 having a temperature between about 1875° C. and 2000° C.; and then (c) adding Al4C3 to the slag, and raising the temperature of the furnace to form a top Al phase with about 6 wt % to 7 wt % C and a bottom slag phase having a temperature between about 2050° C. and 2100° C.; and then (d) adding Al2O3 to the Al—C/slag, which Al2O3 addition results in producing Al2O3 rich slag and in lowering the temperature to between about 1800° C. and 1900° C., to produce a decarbonization reaction within the single reactor compartment, providing a top Al phase with less than (<) 5 wt % C and a bottom (Al2O3 rich-Al4C3) slag having a temperature between about 1800° C. and 1900° C.; and then (e) tapping the top Al<5 wt % C phase; and (f) repeating steps (b) to (e). This slag is then used to begin the next cycle. The next cycle is begun by adding some C and Al2O3 to the bottom slag and repeating steps (c) to (e). Preferably the tapped aluminum phase is Al<3 wt % C and the Al4C3 added in step (c) is from a vapor recovery unit associated with the reactor.
In step (b), arc heating using retractable, at least one vertical top electrodes are preferably used to provide slag. In step (d), addition of Al2O3 at this stage, very importantly, lowers the temperature within the furnace and changes the slag composition transferring a substantial amount of C from aluminum to the slag. This provides a very simple method to produce lower carbon containing aluminum, where only one furnace or reactor is used in the process.
The invention is further described with reference to the accompanying non-limiting drawings in which:
In
The gas from reactor 5 contains primarily CO, and possibly some H2 from the volatile part of the charcoal reactor charge and little or no Al or Al2O. The off gas from reactor 5 has a high energy value as hot CO and could be used to produce electrical energy in a gas turbine or conventional boiler. The aluminum vapor species will have reacted to carbide, condensed to Al2O3 and C or formed an Al2O3—Al4C3 slag. The Al4C3—Al2O3 slag and unreacted carbon is fed into the second stage of the carbothermic smelter via conduit 6. An Al—C liquid alloy exits smelter stage 2 as shown in
In the first step 10 of
In summary, in the process, we have:
Slag Making: To start up the process, Al2O3 and carbon are added to make a liquid slag, 77% Al2O3-23% Al4C3 (mole percent) at about 1900° C.-2000° C. and some excess Al4C3. Some Al2O and Al vapors are formed and go to the vapor recovery reactor 5. Once the process is at a steady state, the starting point for slag making is the slag remaining after decarburization in the previous cycle.
Metal Making: Metal is produced by the following reaction at about 2050° C.-2100° C.:
Al2O3+Al4C3=6Al+3CO
Aluminum carbide is added from the vapor recovery reactor 5. About 17% of the Al will vaporize as Al2O and Al. It is not possible to react all of the slag since the energy is supplied by slag resistance heating through the slag and some slag must remain in the furnace. About 20% of the slag does not react and remains for resistance heating. Some decarburization can occur by raising the temperature after all the carbide is added and reducing the carbide content of the slag and carbon in the metal but this will result in large amounts of Al2O and Al vaporization.
Decarburization: Al2O3 is added to the furnace to remove carbon from the metal. Some electric power is necessary to heat and melt the Al2O3 while some of the energy comes from the sensible heat of the slag since its temperature is higher than required for decarburization The slag-metal system is allowed to cool to about 1850° C. The slag becomes rich in Al2O3 and carbon is transferred from the metal to the slag (Al2C3). The metal is tapped and the resulting Al2O3 rich liquid slag is the starting point for return to slag making.
After the metal is tapped the temperature is increased to about 1900° C.-2000° C. and Al2O3 and carbon are added once more, to produce the desired liquid slag compositions and excess Al4C3 for metal making. In the process, substantial amounts of CO are produced which carry Al as Al and Al2O gaseous species. These are converted to Al4C3 in the vapor recovery reactor 5 and returned to the furnace during metal making, all as shown in
Generally, in step 10, a single furnace 11, having side walls and a bottom, and a single, hollow reactor compartment 13, as shown in
The electrodes 12 and 16 can be made from carbon, graphite, or non-consumable inert ceramic materials, where each is individually supplied with electricity by electric current means 19. The bottom resistance heating electrodes are preferably horizontal and used in metal making to reduce super heating the metal and causing excessive vaporization. The bottom electrodes 16 are also preferably disposed at/adjacent to the bottom phase molten slag phase/level 22, as shown in steps 20, 30 and 40. In step 10 and 20, Al2O, vapor, CO and Al exit as streams 3 and 3′. The Al2O3, C, Al4C3 supply means in steps 10 to 30 are preferably gas tight. The purified aluminum stream 26 may then be passed to any number of apparatus, for example, degassing apparatus to remove, for example, H2 fluxing apparatus to scavage oxides from the melt and eventually to casting apparatus to provide unalloyed primary shapes such as ingots or the like of about 50 lb. (22.7 Kg) to 750 lb. (341 Kg). These ingots may then be remelted for final alloying in a holding or blending furnace or the melt from fluxing apparatus may be directly passed to a furnace for final alloying and casting as alloyed aluminum shapes.
Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.
An example of how this process would work beginning with the metal making stage, 100 moles of a 77% Al2O3-23% Al4C3 slag. As the temperature increases, reaction (3) occurs. Al4C3 would be added to the slag to maintain the slag composition. The reaction proceeds until there are 15 moles of Al2O3 and 5 moles of carbide remaining in the furnace. The process will produce 372 moles of Al but 62 moles will vaporize leaving 310 moles of liquid Al containing about 7.5 wt % C.
The Al vaporized will produce about 15 moles of carbide. During slag making enough Al is vaporized to produce 10 moles of carbide. A total of 62 moles of carbide are required in the metal making step. With 28 moles of carbide reacting from the slag and about 25 moles from the vapor recovery reactor (“VRR”) there is a deficit of about 9 moles of Al4C3. This additional carbide can be produced in slag making so the actual starting point is:
For metal making, the slag +Al4C3 is heated to a higher temperature (2050° C.-2100° C.) producing 310 k moles aluminum metal containing about 7.5 wt. % C. About 20 k moles of slag remain for resistance heating.
For decarburization 75 k moles of Al2O3 is added making the resulting slag 90 k moles Al2O3-and 12 moles Al4C3. The temperature is decreased to about 1850° C. At the lower temperature the carbon distribution ratio between slag and metal increases. The carbon content of the metal is reduced from about 6.0% to 2.5%. This is based on a carbon distribution ratio between slag and metal of 2 and 8904 kg of metal and 9900 kg of slag. The metal is tapped and the remaining slag, 90 k moles Al2O3-12 k moles Al4C3, is the starting point for slag making.
After the metal is tapped the temperature is increased to about 2000° C. and Al2O3 and carbon are added to produce the desired liquid slag composition and excess Al4C3 for metal making. This will require about 225 k moles of C and 37 k moles of Al2O3. After the slag is made the metal making step is repeated.
Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.
