RU2138582C1 - Method for production of aluminium - Google Patents

Abstract

FIELD: production of aluminium by electrolytic separation of it from aqueous solutions simultaneously with hydrogen. SUBSTANCE: use is made in the method of a pool cathode, for example gallic one. The content of aluminium in metal is increased up to 6 mass percent, the alloy is withdrawn from electrolyzer, cooled within 98 to 26 C, aluminium is separated by crystallization, and primary saturated solid solution with a content of aluminium of about 80 mass percent composition is returned to electrolysis as cathode metal, and primary solid solution is melted and subjected to recrystallization at a temperature of below 660 C, separating in succession the secondary, tertiary, etc, solid solutions from liquid until aluminium of commercial purity is obtained from them. The residual quantities of gallium are removed by isothermal holding of metal in a furnace at a temperature of 660 C, draining off the surface aluminium layer, and separately the bottom layer, enriched by gallium. EFFECT: prevented processes of passivation of the cathode surface, electrolysis is accomplished easily with guaranteed separation of aluminium to cathode alloy. 6 cl, 2 dwg, 4 tbl

Description

 The invention relates to electrometallurgy of aluminum. A well-known and used around the world method for producing aluminum by electrolysis of fluoride-oxide melts was proposed more than 120 years ago and does not meet modern requirements in terms of process efficiency or its environmental characteristics.

 Alternative methods for the production of aluminum — the carbothermic process [1], the Todt process [2], the Kuwahara process [3] electrolysis of chlorides [4], and the reduction of aluminum with sodium [5] - did not reveal any advantages over the Herou-Hall method.

 The prototype of the present invention is our previous proposal of the same name, under N 2.032.772 [6,7] The production of aluminum from aqueous solutions simultaneously with hydrogen, which is the essence of this invention, is extremely tempting, but it cannot be implemented due to the passivation processes of solid aluminum cathode oxide-hydroxide films of variable composition. Our attempts to implement the process in alkali-aluminate, sulfate, hydrochloric acid and nitric acid solutions were equally unsuccessful. In this regard, we propose to obtain aluminum and hydrogen on a flowing liquid metal cathode, for example, on gallium or consisting of an alloy of gallium with aluminum. Other fusible alloys can be used. On the flowing liquid metal cathode with a constantly refreshed metal surface, there are no passivation processes characteristic of the surface of a solid aluminum cathode. As a result, electrolysis is carried out easily and, to a first approximation, simply with guaranteed release of aluminum into the cathode alloy.

For example, electrolysis was carried out on a saturated (for 20 ^o C) aqueous solution of aluminum sulphate qualification of analytical grade at pH 3 using a drop electrode with a capillary diameter of 0.4 to 1.0 mm and a length of 10 mm (Fig. 1). The cell consisted of a glass cup 1 with a medical syringe 2 filled with gallium 3 placed in it. Drops of a gallium alloy with aluminum 4 after separation from the capillary accumulated in the form of a puddle 5 at the bottom of the glass. The anode was made of platinum wire 6 with a diameter of 0.3 mm Its working part was a spiral 7 with a diameter of 5 mm and a length of 25 mm. The current lead to the cathode metal was made of copper wire 8 with a diameter of 1.5 mm, which only contacted the gallium with the end part.

 In the case of a free flow of gallium through small diameter capillaries, on the order of 0.3-0.4 mm, the capillary sometimes clogged with particles of the solid phase and the metal flow stopped. To exclude this, the pressure flow of gallium in the capillary was used with mechanical feeding of the piston and rod 9 along the axis of the syringe. For temperature control of the electrolyte 10, the cell was placed in a glass 11 with water from the thermostat circulating through it.

 The installation scheme included a power source, adjustable external resistance, an ammeter and a copper coulometer. During electrolysis, aluminum and hydrogen were released on the surface of the gallium droplet, and oxygen was released on the platinum spiral.

The experiments were carried out at a temperature of 50 to 95 ^o C. A gallium sample was placed in the syringe, which in various experiments was 25 to 80 g, which ensured the mass flow of gallium through the capillary was of the order of (4-10) • 10 ^-5 kg • s ^-1 . The time of a single experiment was usually about 15 minutes with an amount of electricity of about 0.45 A / h.

 The alloy collected at the bottom of the cell was re-placed in the syringe, gradually increasing the concentration of aluminum in it. The full cycle of accumulation of aluminum in gallium ranged from 3 to 32 operations of a single electrolysis.

 The amount of aluminum obtained was determined by increasing the mass of the cathode metal and according to atomic absorption analysis. The results of the main experiments are given in table. 1.

 According to the same results of atomic absorption analysis, starting gallium contained only 0.008 wt.% Aluminum, i.e. Aluminum on gallium is steadily released and its concentration can be raised to several percent. At the same time, the total increase in the mass of the cathode metal is greater than the amount of aluminum obtained, which is easily explained by the contamination of the obtained alloy with dispersed inclusions of sulfate, oxides, hydroxides and hydrides.

 The aluminum current yield under the experimental conditions is from 27 to 46%, and the hydrogen current output (up to 100% difference) from 54 to 73% (Table 2), which at the present stage of development can be considered a quite satisfactory result.

The problem is different - how to further simply and efficiently separate the alloy components?
Consider the state diagram of aluminum - gallium (Fig. 2) [8,9,10]. The system has a eutectic with a gallium content of 93.5% at. (97 wt.%) And aluminum 6.5% at. (3.5 wt.%). The temperature of the eutectic horizontal is 26.4 ^o C (!). In the crystal lattice of aluminum, gallium forms solid solutions at a saturation concentration of 9.5% at. Ga (21 wt.%) For the same temperature of the eutectic horizontal at 26.4 ^o .

If gallium is considered as a solvent metal, then gallium primarily crystallizes in the pre-eutectic region and then at 26.4 ^° C it is a eutectic mixture of gallium crystals and a saturated solid solution of gallium in aluminum. The temperature range between the liquidus and solidus lines here is very small and is not more than 3.3 ^o . The task of separating pure aluminum and gallium for pre-eutectic alloys within the framework of economically viable costs seems unsolvable.

In the hypereutectic region, aluminum crystallizes primarily in which gallium dissolves to form a solid solution. The limiting concentration of aluminum in such a solid solution is 90.5% at. or 79 wt.% (!). And such an aluminum-based alloy can be easily isolated at room temperature. The primary crystallization region of the aluminum-based solid solution is huge and occupies a large part of the diagram area, and the difference in the temperatures of the beginning and end of crystallization reaches 634 ^o (!).

However, the temperature of the electrolysis process in aqueous solutions cannot be higher than 100 ^o C. An alloy containing 14% at. aluminum and 86% at. Gaul. Therefore, the process of electrolytic enrichment of the cathode alloy with aluminum can be continued until its concentration of 14 at.% Or 5.9 wt.% Is reached.

Further, such an alloy can be removed from the electrolyzer, cooled from the electrolysis temperature to ~ 26 ^° C and crystallized solid solution with aluminum content up to 79 wt.% Can be isolated by crystallization. The concentration of aluminum in the mother liquor - alloy is reduced from ~ 5.9 wt.% To a eutectic, equal to 3.5 wt.%.

 The task of further reducing the concentration of aluminum and regenerating pure gallium is unnecessary. The mother liquor, an alloy of the eutectic composition, is returned to the electrolyzer as the initial cathode metal.

Testing the possibilities of separating aluminum in the form of a primary saturated solid solution with gallium with an aluminum content of up to 79 wt.% Was performed as follows. The gallium-aluminum alloy obtained by electrolysis and containing 4.7 - 5.5% aluminum was cooled in various modes from 98 to 26 ^o C and a saturated solid solution of aluminum and gallium was isolated on a metal stirrer. The mixer was made of copper or aluminum wire with a diameter of 3 mm, bent L-shaped. This stirrer was placed in the alloy and slowly rotated at a speed of 8-10 rpm. In the horizontal section of the wire, solid solution crystallized. The wire stirrer was weighed before and after crystallization, and the crystalline precipitate was then dissolved by boiling in a 3 N HCl solution. Hydrochloric acid solutions were then sent for atomic absorption analysis. The results of experiments on the allocation of the primary saturated solid solution of Ga in Al are given in table. 3 (where * in addition to aluminum and gallium, the crystalline sediment contained up to 2% copper, as well as impurities of sulfates, oxides, and hydrides).

 As can be seen, at room temperature it is possible to obtain an alloy corresponding in composition to a saturated solid solution containing about 80 wt.% Aluminum. The experimental results indicate that with fast crystallization from an overcooled metastable state, alloys even richer in aluminum can be obtained. (Experience 3, table. 3).

Next, the primary solid solution is melted and subjected to recrystallization at temperatures of 500 - 600 ^o C. For example, the primary solid solution containing 75% Al and 25 wt.% Ga with a crystallization temperature of 600 ^o C, is melted and during cooling from 600 to 520 ^o C secrete by crystallization a secondary solid solution, the aluminum content of which is from 97.8% to 95% with an average value of 96.4%.

 The secondary solid solution is mechanically separated from the mother liquor, richer in gallium. Then it is melted, cooled at temperatures from 650 to 600 and a tertiary solid solution is isolated with an average aluminum content of 98.7% with a possible content of up to 99.6% (!).

Further purification of the obtained aluminum from gallium impurities is carried out by isothermal exposure of the metal in a furnace at a temperature of about 660 ^o C. Since the density of aluminum is 2.3 g / cm ³ and gallium about 6.0 g / cm ³ , the latter accumulates in the absence of thermal convection in the lower layers of the furnace bath and periodically or continuously merges from there. Aluminum is drained from the upper, surface layers of the liquid metal. If necessary, a deeper purification of aluminum from gallium, metal centrifugation is used. All products containing gallium are returned to the previous stages of the process.

 The proposed method in comparison with the existing one allows to abandon the production of alumina, carbon materials, fluoride salts. It has a high level of environmental friendliness and potentially low energy consumption. In contrast to the prototype proposal, the feasibility of the method is proved by the results of numerous experiments described in this text.

Sources of information
1. Peacy JG, WG Devenport J. Metals, 26 (1974), 7, p. 25-28.

 2. Grjotheim K., Welch B., J. Metals, 33 (1981), 9, p. 26-32.

 3. Kuwahara K, Franc. Pat. N 8105183, 1981.

 4. Grjotheim K., Qiu Z., Molten Salt Technology, V. II, Shenyang, p. 435.

 5. Begunov A.I., Sheveleva N.N., Yakovleva A.A. Application for the invention N 5.034.052 from 03.25.92, with pos. VNIIGPE decision.

 6. Runners A. And,. Kulkov V.N. A method of producing aluminum. RF patent N 2.032.772 dated 04/13/92.

 7. Begunov A. I, Grinberg I. S. Metallurg. Processes for the Early Twenty-First Centure. V. I. Basic Principles, pp. 477-480. San Diego, CA, USA. (1994).

 8. R.P. Elliot. Structures of double alloys. "Metallurgy", M, 1970, 455 p. (Constitution of Binary Alloys, First Supplement, R.P. Elliot, McGraw-Hill Book Company).

 9. Zoller H. Metall, 1957, Bd 11, s 378-381.

 10. Clare J.W.H. J. Inst. Metals, 1957-58, v. 86, p. 431-432.

Claims (6)

 1. The method of producing aluminum by electrolytic separation of it from aqueous solutions simultaneously with hydrogen, characterized in that they use a liquid metal cathode, for example gallium or from an alloy of gallium with aluminum.  2. The method according to claim 1, characterized in that the aluminum content in the cathode metal is increased to hypereutectic values not exceeding 6 wt.% (Not less than 3.5 and not more than 6 wt.%). 3. The method according to any one of claims 1 and 2, characterized in that the resulting alloy is removed from the electrolysis process and cooled in the range of 98 - 26 ^o C during crystallization of a saturated solid solution based on aluminum with a content of the latter of about 80 wt.%.  4. The method according to any one of claims 1 to 3, characterized in that the mother liquor - an alloy of the eutectic composition after crystallization from it of a saturated solid solution is returned to electrolysis as a cathode metal. 5. The method according to any one of claims 1 to 4, characterized in that the primary saturated solid solution is melted and subjected to recrystallization at temperatures below 660 ^o C, separating sequentially secondary, tertiary, etc. solid solutions from liquid to aluminum of technical purity. 6. The method according to any one of claims 1 to 5, characterized in that the residual amount of gallium is removed by isothermal exposure of the metal in a furnace at a temperature of 660 ^o C, merging the surface aluminum layer and separately the bottom layer enriched in gallium.

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