Abstract:
Methods and apparatus for the generation of energy from the oxidation of aluminum comprise means for breaking up the continuity of the passivating oxide layer on the aluminum surface by exposing said surface to a mercury-, indium- or gallium-containing substance, exposing the surface-treated aluminum to a fluid which is capable of oxidizing said aluminum, and withdrawing useful energy from the oxidation reaction.

Description:
This is a continuation-in-part of my applications Ser. No. 704,452, filed July 12, 1976, now U.S. Pat. No. 4,305,999, Ser. No. 20,967, filed Mar. 16, 1979 now U.S. Pat. No. 4,218,520 (which is a divisional application of Ser. No. 843,155, filed Oct. 17, 1977, now U.S. Pat. No. 4,150,197), and Ser. No. 080,185, filed Oct. 1, 1979 now U.S. Pat. No. 4,254,190. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to methods and apparatus for generating electrical, thermal or mechanical energy from the oxidation of aluminum or its alloys, especially from aluminum waste and other inexpensive aluminum products. 
     In my copending applications and prior patents, I have disclosed apparatus and methods for withdrawing electrical energy, heat, and an aluminum hydroxide reaction product from the electrochemical oxidation of aluminum, preferably in aqueous alkaline electrolytes. The present invention deals with apparatus and methods for generating mechanical or electrical energy from the oxidation of aluminum without necessarily resorting to aqueous alkaline systems or even to electrochemical energy conversion. 
     Electrochemical power sources usually necessitate a number of series-connected cells to provide the required voltage. Moreover, the electrochemical aluminum-consuming systems dislosed thus far usually necessitate electrolyte circulation through these cells, in which in turn introduces current leakage problems. The complexity of such series-connected systems increases their cost and may affect their reliability. 
     It is therefore an object of my invention to provide aluminum-consuming power sources which are simpler in design and operation, and cheaper to manufacture and maintain. 
     It is still another object of my invention to provide an aluminum-consuming power source having high overall energy storage and power densities. 
     In alternative embodiments of my invention, the energy conversion means is still electrochemical rather than thermal. However, it disclosed aluminum-consuming electrochemical cells in using a non-caustic or only mildly caustic electrolyte. 
     Non-alkaline primary aluminum batteries are known in the art. However, such batteries have utilized aluminum alloys of special composition. 
     It is yet another object of my invention to provide non-alkaline aluminum-consuming electrochemical cells which do not necessitate the use of special alloys. 
     It is still another object of my invention to provide aluminum-consuming power sources which can utilize disposable aluminum cans or other inexpensive forms of aluminum as their primary fuel. 
     SUMMARY OF THE INVENTION 
     Briefly, my invention consists of breaking up the continuity of the protective oxide film, which normally forms on the surface of aluminum alloys, so as to permit air-oxidation of aluminum in a thermal engine or electrochemical oxidation in a mild aqueous electrolyte. Such break-up may be effected by immersion of the aluminum in a solution containing a compound of mercury, indium or gallium, preferably the latter. These elements tend to remain at the aluminum surface, and continue to break up the continuity of the protective oxide film even after withdrawal from said solution, thereby facilitating oxidation of the aluminum. 
     Aluminum-consuming electrochemical cells can be made to yield satisfactory current densities and cell voltages when used with a non-caustic or mildly caustic electrolyte comprising a compound of mercury, indium or gallium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     My invention may be best understood with the aid of the drawings, in which: 
     FIG. 1 is a block diagram of one preferred embodiment of my invention; 
     FIG. 2a is a diagrammatic front view of one or more battery cells according to an alternative embodiment of my invention; 
     FIG. 2b is a diagrammatic top view of a single battery cell OF FIG. 2a; 
     FIGS. 2c and 2d are diagrammatic views of sections S--S and S&#39;--S&#39;, respectively, of a single cell of FIG. 2a; 
     FIG. 2e is the pattern of the aluminum anode 49 of FIGS. 2b and 2d. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiment represented by the block diagram of FIG. 1, aluminum particles are fed upon demand from a hopper 1 by means of an electrically or manually actuated control valve mechanism 2 into a slurry-mixing chamber 3. The valve mechanism 2 may be similar to one of those discussed in my copending application Ser. No. 080,185, filed Oct. 1, 1979. Chamber 3 is connected to an inlet tube 7, an outlet tube 8, and a short upper tube 11 connected to the valve mechanism 2. 
     The slurry entering chamber 3 through inlet 7 is preferably at a temperature of about 60° C., and is depleted of aluminum particles. However, after mixing with the particles added through valve 2, the slurry exiting through tube 8 is aluminum-enriched. This slurry is fed through tube 8 into an aluminum/air battery stack, which is similar in overall structure to the slurry-type zinc-air battery described in the technological literature, especially in a publication by A. J. Appleby et al. in the Proceedings of the 1975 Intersociety Energy Conversion Engineering Conference, pages 811-816, which is included herein by refernce. However, thanks to the far lower specific gravity of aluminum particles (2.7 g/cc, which is less than half of that of zinc particles) it becomes permissible to use here a less viscous, and hence more conductive, electrolyte. This, in turn, results in smaller ohmic losses, and hence in higher cell voltages at comparable current densities or in higher current densities at comparable voltages than was possible in the cited zinc/air battery. 
     the total exposed area of the aluminum particles immersed within the slurry should preferably be barely sufficient to yield the desired power output from the battery stack 20, but should not appreciably exceed this requirement so as to avoid loss of energy conversion efficiency through hydrogen generation. To minimize such losses, any hydrogen generated by reaction of water with the aluminum particles may be collected in chamber 3 in the space above the electrolyte level and fed through a tube 14 to a small auxiliary hydrogen/air fuel cell 15, comprising also an air inlet 16, positive and negative terminals 17 and 18, respectively, connected to the respective cathode and anode current-collector grids (not shown), and a warm air outlet 19. 
     the electric current from battery 20 is withdrawn via the positive and negative terminals 21 and 22 (connected to the respective anode and cathode current-collecting grids, not shown). Fresh air is supplied to the battery via inlet 23, and the partly oxygen-depleted air leaves through outlet 24. The partly aluminum-depleted slurry leaving battery 20 through outlet 25, preferably at a temperature of 70° to 90° C., is fed by pump 26 into a cyclone separator 27, wherefrom the aluminum particles are expelled into chamber 28, while the clear supernatant fluid exits through line 29 into an air-cooled heat-exchanger 30. The liquid exiting from heat exchanger 30 via outlet line 31 should be preferably at 60° C. or cooler. At this temperature, the excess aluminum ions wil precipitate out of the electrolyte when exposed to aluminum hydroxide seeds in the precipitator 32. The aluminum hydroxide slurry from precipitator 32 is forced by pump 33 through a cyclone separator 34, wherefrom the aluminum hydroxide is expelled into chamber 35, while the supernatant liquid is returned via line 36, chamber 28, and line 7 into the mixing chamber 3. When passing through chamber 28, the liquid from cyclone 34 entrains the aluminum particles and returns them to chamber 3, thereby completing one entire cycle. 
     The aluminum hydroxide expelled from cyclone 34 contains some adsorbed ions from the electrolyte. To minimize such electrolyte losses, the precipitate in chamber 35 may be rinsed with distilled water from line 37, and the rinsings may be recirculated via line 38 and pump 33 into cyclone 34. The distilled water in line 37 originates from the condenser 39, wherein humid and warm air leaving the aluminum hydroxide dryer 40 is cooled with ambient air, thereby being caused to give up its excess moisture. 
     Both the aluminum/air battery 20 and the hydrogen/air fuel cell 15 operate preferably at a temperature of 70°-90° C. The air fed to the cathodes of each of these power sources is allowed to approach a temperature of about 70° C., and is thereafter fed through lines 19, 24, and 41 into the aluminum hydroxide dryer 40, wherein it is blown through the previously rinsed aluminum hydroxide from chamber 35. Dryer 40 may comprise a plurality of porous polytetrafluoroethylene membranes (not shown) through which the air from line 41 may freely pass, but which prevent the entrainment of solid particles. the dried aluminum hydroxide may then be transferred into an aluminum hydroxide storage bag 42. 
     Of course, the aluminum hydroxide rinsing and drying operations, as well as the transfers of aluminum hydroxide from chamber 35 to dryer 40 and then to storage bag 42, may be preferably effected in discontinuous steps. 
     While the foregoing block diagram looks outwardly indentical with that of FIG. 1 of my copending application Ser. No. 080,185, filed Oct. 1, 1979, there are several major differences between the two systems. In first place, the present embodiment does not resort to an intermediary zinc/zincate couple in the battery stack 20. Secondly, the electrolyte used herein is not alkaline. A saline, mildly alkaline, or mildly acidic electrolyte may be adequate provided that it comprise a dissolved compound of mercury, indium, or, preferably, gallium in a concentration sufficient to activate the aluminum surface by breaking up the continuity of the passivating aluminum oxide film. A concentration of the order of 1 gram/liter of Ga +3  ions may usually suffice for this purpose. Thirdly, the use of a non-alkaline electrolyte permits a higher operating battery temperature without entailing excessive corrosion, which in turn improves the air cathode performance and reduces the size and weight of the required heat-exchangers. 
     For some applications it may be advantageous to add a phosphate buffer to the electrolyte. Such a buffer may improve the characteristics of the anode by tying up undesirable iron ions, and it may also improve the performance of the air cathode. The latter may also be improved by an intermediary oxidation-reduction couple such as I -  /I 2  or I -  /I 3   - . 
     The same type of electrolyte may also be used in the alternative embodiment of FIGS. 2a through 2e. FIG. 2a is a front view of one or more cells of a reserve type aluminum/air battery for marine applications. Each cell comprises an outer rectangular casing 43 made of an insulating material, preferably a molded plastic, into which are sealed two air cathodes 44, 44&#39;, each made of Teflon-bonded activated carbon particles (by methods well known to persons skilled in the art) and comprising current-collecting metal screens 45, 45&#39; connected to the positive cell terminals 46, 46&#39;. As indicated in FIGS. 2b, 2c, and 2e, the upper part of the short left side 47 of casing 43 is slotted so as to accomodate the tab 48 of an aluminum anode sheet or plate 49. The opposite short side 50 of casing 43 may comprise a groove 51 designed to accomodate the right edge 52 of anode 49. Before inserting said anode inside casing 43, the tab 48 is first covered on both sides by a gasket 53 (which may be made of two similar strips of rubber, or other suitable resilient material, bonded together along their lower edges) and is then forced into the slotted edge 47 of casing 43, with edge 52 of the anode sliding into and being held in place by groove 51 of edge 50. The electrolyte may then be introduced into chamber 54 (FIG. 2d) up to a level 55 which is sufficient to completely submerge the rectangular portion of anode 49 while remaining slightly below the lower external edges of gasket 53. 
     The main purpose of gasket 53 is to prevent access of electrolyte and air to those portions of anode 49 which are not submerged. The electrolyte breaks up the continuity of the protective oxide film on the aluminum surface thereby permitting electrochemical cell action. However, this would also permit rapid air oxidation of the tab 48 near the the air-electrolyte interface in the absence of gasket 53. The latter eliminates the triple aluminum-air-electrolyte interface, thereby preserving the integrity of the protective oxide film in that part of the anode 49 which is not submerged. 
     Alternatively, a triple aluminum-electrolyte-air interface may be prevented by covering up the non-submerged part of anode 49 by a suitable coating or sealant, such as a silicone or epoxy resin. 
     There will now be obvious to persons skilled in the art many modifications and variations of the above-disclosed embodiments which, however, will remain within the scope of my invention if defined by the following claims.