Method for altering the concentration of a chemical element in liquid metal via an electrochemical way

Abstract

 Method for altering the concentration of at least one chemical element of the group Ia of the periodic table of the elements in a liquid metal of the group composed of aluminium, copper, zinc, tin and lead in an electrochemical way, according to which an electrochemical cell is formed with a solid electrolyte (3), having at one side the melted metal (1) and at the other side an electricity-conducting material in which the chemical element can be absorbed, hereafter called the conductor, and whereby a DC voltage is produced between said conductor (4) and the liquid metal (1), characterized in that a material is used as a solid electrolyte (3) which conducts several ions at once.

Description

[0001] The present invention concerns a method for altering the concentration of at least one chemical element of the group Ia of the periodic table of the elements in a liquid metal of the group composed of aluminium, copper, zinc, tin and lead in an electrochemical way, according to which an electrochemical cell is formed with a solid electrolyte, having at one side the melted metal and at the other side an electricity-conducting material in which the chemical element can be absorbed, hereafter called the conductor, and whereby a DC voltage is produced between said conductor and the liquid metal.

[0002] A frequent impurity in primary aluminium is sodium, which is found in the electrolytic bath (cryolite) during the production of aluminium (Hall-Héroult cycle). Lithium is also added to the bath during this process, such that this is also present as an impurity. Lithium is also a major impurity in secondary (recycled) aluminium, since for the making of aluminium use is often made of scrap metal from the airplane industry. In the future, said scrap metal will contain more and more lithium-aluminium alloys.

[0003] Until now, for various practical reasons, scrap metal from the airplane industry must be separated from other scrap metal as long as there is no adequate method to purify the aluminium from lithium.

[0004] Alkali metals such as lithium and sodium which are present in the liquid metal encourage crack formation during the rolling of the ingots.

[0005] The purification of the liquid metal can take place at a separate purification stage with chemical additives. It is also known, however, that said purification can be done in an electrochemical way. Such a purification method is described in patent US-A-4.849.072. According to this method lithium is removed from a melted lithium-aluminium alloy by means of an electrochemical cell having a melted, lithium-containing salt as an electrolyte, with the melted lithium-aluminium alloy at one side and melted lithium at the other side.

[0006] The altering of the lithium concentration in a liquid alloy of lithium and another metal, such as tin or aluminium, by means of an electrochemical cell with a solid electrolyte is described by H. Tuller and P. Moon in "Materials Science and Engineering", (Elsevier Sequoia), volume B1, (1988), page 189. According to this method, lithium can be extracted from an aluminium alloy so as to purify it. However, no other elements but lithium can be extracted from it such as for example sodium and potassium, since the solid electrolyte used only conducts lithium ions. For a lithium borate such as LiBO₂ is used as an electrolyte.

[0007] The present invention aims to correct these disadvantages and to provide a method for altering the concentration of a chemical element via an electrochemical way according to which an electrochemical cell with a solid electrolyte is formed, but whereby not only lithium but also other chemical elements of the group Ia of the periodic table of the elements can be extracted all at once from a metal, in particular aluminium, or can be put in such a bath.

[0008] To this aim a material conducting several ions at once is used as a solid electrolyte.

[0009] Preferably, use is made of a solid electrolyte from the beta-alumina family or the SOSICON family.

[0010] The use of a beta-alumina as a solid electrolyte is also mentioned in the above-mentioned publication, but only for the electrolysis of melted sodium salts. The use of said electrolyte is obvious here, since it is known to be a good sodium ion conductor.

[0011] The applicant uses a sodium ion conductor having a mixed ion conductivity (e.g. sodium-beta-alumina). Although this material is a sodium conductor, the mobile sodium ions can nevertheless be replaced through interchange by other ions such as H₃O⁺, K⁺, Sr⁺, Li⁺, such that it can also be used to pump through other than Na⁺ ions. In particular, use is made of beta-alumina as a solid electrolyte, which is a mixed oxide composed of Al₂O₃ and Na₂O.

[0012] The working method described above is particularly interesting for altering the amount of lithium in liquid aluminium and in particular for purifying aluminium which has been polluted by lithium.

[0013] In the latter case, not only lithium is extracted from the aluminium-lithium alloy, but also other alkali metals or impurities.

[0014] It is advantageous to use bismuth or a bismuth alloy as a conductor.
Other particularities and advantages of the invention will become clear from the following description of a method for altering the concentration of a chemical element in a liquid metal via an electrochemical way according to the invention. This description is given as an example only and is not limitative in any way. The figures refer to the accompanying drawing which shows a cross section of a device for the application of the method according to the invention.

[0015] To purify melted aluminium which has been polluted by lithium, alkali metals or any other metallurgic impurities, an electrochemical cell is formed with said melted aluminium, a conductor and a solid electrolyte between the two of them, and a DC voltage is produced between the liquid metal and said conductor.

[0016] Use is made of a beta-alumina or beta''-alumina as a solid electrolyte, a mixed oxide composed of Al₂O₃ and Na₂O. Use can also be made of a "sodium super ionic conductor" (SOSICON), which is a material based on sodium, zirconium and phosphate. Bismuth can be used as a conductor.
In order to bring lithium ions from the liquid aluminium bath in the bismuth, the liquid aluminium must be connected to the positive pole of a DC source and the bismuth with the negative pole of this source.

[0017] By sending a constant current through the solid electrolyte, the concentration of the positively charged ions will drop in the aluminium and increase in the bismuth.

[0018] The amount of material that is pumped through the solid electrolyte is determined by Faraday's law. For 26.8 Ah, one mole of monovalent, equivalent, ionized metal is pumped through the solid electrolyte. How the cell can be constructed in practice becomes clear from the accompanying drawing in which three cells are set up in parallel.

[0019] The liquid aluminium 1 is situated in the gutter 2. The aluminium 1 is present in all three cells at once. Each cell contains a tube 3 of sodium-beta-alumina closed off at the bottom which is filled with liquid bismuth 4.

[0020] The bismuth 4 in the three tubes is connected to the negative pole of a common DC-source 6 via conductors 5. The three amounts of bismuth are connected in parallel whereby the current is measured in the part of the conductors 5 which is connected to the bismuth in one tube 3, for example by measuring the tension by means of a voltmeter 7 over a rheostat 8 of one Ohm, which is connected in series to the cell.

[0021] The positive pole of the common power source 6 makes an electric contact with the aluminium 1 via the conductor 9 which is immersed in said aluminium. The following reaction takes place in the aluminium 1: M -> M⁺ + e⁻, whereas in the bismuth 4 the following reaction takes place: M⁺ + e⁻ -> M, whereby M can be hydrogen, sodium, lithium, etc.

[0022] Thus at the transition aluminium/beta-alumina, an electrochemical oxidation of lithium, sodium, hydrogen, etc. takes place. The thus formed ions migrate through the solid electrolyte, i.e. the tube 3 of sodium-beta-alumina, and are reduced at the transition beta-alumina/bismuth. The more cells are formed with the liquid aluminium, the greater the total current and thus the greater the purification capacity. In practice use will always be made of a battery of cells.

[0023] With a device as shown in the accompanying drawing and with pure bismuth in the sodium-beta-aluminium tubes 3, it was possible to remove a total mass of lithium, sodium and such like to the amount of 0.0384 g from the liquid aluminium with a total conducted charge of 0.1612 Ah.

[0024] The present invention is in no way limited to the embodiments described above; on the contrary, many variations are possible while still remaining within the scope of the application and the embodiments described.

Claims

1. Method for altering the concentration of at least one chemical element of the group Ia of the periodic table of the elements in a liquid metal of the group composed of aluminium, copper, zinc, tin and lead in an electrochemical way, according to which an electrochemical cell is formed with a solid electrolyte (3), having at one side the melted metal (1) and at the other side an electricity-conducting material in which the chemical element can be absorbed, hereafter called the conductor, and whereby a DC voltage is produced between said conductor (4) and the liquid metal (1), characterized in that a material is used as a solid electrolyte (3) which conducts several ions at once.

2. Method according to the above claim, characterized in that a solid electrolyte from the beta-alumina family is used.

3. Method according to any of the above claims, characterized in that a solid electrolyte from the SOSICON family is used.

4. Method according to any of the above claims, characterized in that aluminium containing lithium is used as a liquid metal.

5. Method according to any of the above claims, characterized in that aluminium containing sodium is used as a liquid metal.

6. Method according to any of the above claims, characterized in that bismuth or a bismuth alloy is used as a conductor.

Drawing