Patent Application
20130319876
(1) Field of the Invention
The invention relates to methods and apparatus for the production of alkali metals and caustic solutions and, more particularly, to a mercury-free Castner-Kellner process and apparatus.
(2) Description of the Prior Art
Electrolysis of salt solutions is widely used in the chemical industry to produce alkali metals and caustic solutions. The Castner-Kellner process has been used for over a century to produce high-purity caustic soda by utilizing a mercury electrode to separate the brine feedstock from the product. However, the use of mercury is undesirable today due to the metal's toxicity. Typically, mercury ions can be found in the caustic soda solution and spent brine, which are the products of the Castner-Kellner process. Additional filtration is necessary to remove mercury from the final product and unreacted brine. The cost of monitoring and reducing mercury levels in electrolyte solutions to safe levels poses an economical constraint on the use of mercury cathodes in industry.
Electrochemical reduction of cations at a liquid metal cathode surface can result in the diffusion of the reduced metal into the electrode, thereby changing the alloy composition. For mercury-based cathodes, the process is known as amalgamation. The alloy can be less reactive than the reduced metal in water, thereby preventing back and side reactions from taking place. Mercury cathodes were widely used in the chlor-alkali industry to produce sodium hydroxide (caustic soda) via the Castner-Kellner process, as taught in U.S. Pat. No. 452,030 to Castner. The use of liquid metal cathodes can be advantageous compared with solid cathodes when ease of surface alloying and transport of dissolved metallic element are important to the electrochemical process.
The chlor-alkali industry has largely abandoned the use of mercury cathodes and the Castner-Kellner process in favor of asbestos diaphragm and selective ion-exchange membrane cells. Both rely on the selective diffusion of sodium ions through the separatory barrier. Diaphragm cells are capable of producing 11% sodium hydroxide (NaOH), with up to 15% salt. Membrane cells produce 35% sodium hydroxide. In comparison, mercury cells can produce from 50% to 70% sodium hydroxide with very little salt present in the product. The reduction of sodium to its metallic state and amalgamation with mercury effectively blocks anion transport during the separation process, resulting in a high purity product with low salt content.
Galinstan, a eutectic alloy containing gallium (Ga), indium (In), and tin (Sn), has been tested as a replacement for mercury (Hg) in electrochemical analysis. The eutectic alloy melts at −19° C. and is liquid at room temperature. A paper by P. Surmann and H. Zeyat, “Voltammetric analysis using a self-renewable non-mercury electrode”, Anal. Bioanal. Chem. (2005) 383 (6): 1009-1013 reports that liquid galinstan electrodes are similar to mercury electrodes in terms of hydrogen overpotential, simplicity of surface renewal, and good electrochemical behavior without the danger posed by mercury toxicity.
The use of liquid gallium and gallium alloys as a mercury substitute was considered for a number of different applications, ranging from electrical and mechanical sensors to electrical contact lubrication, as taught in U.S. Pat. Nos. 5,478,978 and 5,792,236 to Taylor et al. Mercury-free thermometers using galinstan have recently been brought to market as indicated in U.S. Pat. No. 6,019,509 to Speckbrock et al.
Alloys containing bismuth, indium, and tin has been proposed for use as a cadmium-free heat transfer medium and sealing material, as taught by U.S. Pat. No. 2,649,366 to Smith et al. and U.S. Pat. No. 4,214,903 to Murabayashi et al. U.S. Pat. Nos. 4,738,858 and 5,168,020 to Jow describe the use of a rechargeable sodium alloy anode, the constituents of which may contain lead, antimony, selenium, mercury, and cadmium in addition to bismuth and tin. The presence of these toxic metals in the electrode alloy may contaminate the electrolyte solution via leaching during cell operation.
There is consequently a need for an apparatus and method for producing alkali metals and caustic solutions that does not require the use of mercury or other toxic heavy metals.
According to one aspect of various exemplary embodiments, an electrolysis cell comprises a first cell type adapted to contain a first electrolyte solution; a second cell type, separated from the first cell, the second cell type adapted to contain a second electrolyte solution; an alloy electrode disposed at a bottom of and electrically interconnecting the first cell, and the second cell, the alloy electrode being mercury-free; walls separating the first cell from the second cell, the wall intersecting the alloy electrode but not bisecting the alloy electrode; a first cell type anode disposed in the first cell; and a second cell type cathode disposed in the second cell.
According to another aspect of various exemplary embodiments, a method for the production of caustic soda comprises applying electrical current to an anode disposed in a first electrolysis cell containing an alkali metal halide solution; applying electrical current to a cathode disposed in a second electrolysis cell containing water; and interconnecting the alkali metal halide solution of the first electrolysis cell with the water contained in the second electrolysis cell with a non-toxic mercury-free alloy electrode.
According to a further aspect of various exemplary embodiments, an electrolysis system comprises a first cell adapted to contain a first electrolysis solution of an alkali metal halide; a first cell anode adapted to deliver electrical current to the first electrolysis solution; a second cell adapted to contain a second electrolysis solution; a second cell cathode adapted to deliver electrical current to the second electrolysis solution; and an alloy electrode adapted to electrically interconnect the first electrolysis solution and the second electrolysis solution, the alloy electrode formed from a bismuth-indium-tin ternary alloy.
The above and other features of the invention's exemplary embodiments, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular assembly embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
Reference is made to the accompanying drawings in which is shown an illustrative embodiment of the invention, from which its novel features and advantages will be apparent, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein:
The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention: the scope of the invention is best defined by the appended claims.
Broadly, the current invention provides apparatus and methods for the production of alkali metals and caustic solutions without the use of toxic liquid mercury-sodium amalgam electrodes. The current invention may utilize a bismuth-indium-tin eutectic alloy as a replacement for the mercury electrode used in a conventional Castner-Kellner apparatus.
Bismuth, tin and indium forms an eutectic alloy, with a composition (by mass) of 32.5% bismuth (Bi), 51.0% indium (In), and 16.5% tin (Sn), which melts at 60.5° C. The constituent elements are nontoxic and are safe to handle without protective safety equipment. The alloy may be prepared by melting commercially pure bismuth, indium and tin in an oven or furnace at 300° C. for one hour. Heating under vacuum or an inert gas atmosphere will reduce the quantity of dross formed at the surface of the crucible, but any dross present may be skimmed off during pouring and casting.
The melting point and resistivity of the ternary bismuth-indium-tin alloy may be adjusted by changing the quantity of bismuth, indium and tin used during initial fabrication. For example, an alloy may be formed from about 10-60% bismuth, about 20-80% indium, and about 5-50% tin (by mass). Other compositions may be contemplated within the scope of the current invention as defined by the appended claims.
Referring now to
The second cell type 16 may use an electrolyte solution 22 of a caustic soda, typically sodium hydroxide (NaOH), or may use water (H2O). A metallic cathode 24, typically an iron (Fe) cathode, may be disposed in the electrolyte solution 22. A bismuth-indium-tin alloy, typically the eutectic alloy described above, may be used along the bottom of the electrolysis cell 10 to form an alloy electrode 26.
The alloy electrode 26 may be disposed along a shared bottom of both cells of the cell types 14, 16. This may be achieved by having the walls 12 separating the cells intersect or dip below the level of the electrolytes, but still allow the alloy electrode 26 to flow beneath them by not bisecting the alloy electrode 26. An electric current may be applied to the anode 20 and the cathode 24 to begin the electrolysis process.
A rocking mechanism may be provided to agitate the electrolysis cell 10. The rocking mechanism may include a fulcrum 28 and a rotating eccentric 30 under the electrolysis cell 10, as is known in the art.
Upon initial operation, alkali metal ions from the electrolyte solution 18 are reduced electrochemically and alloys with the bismuth-indium-tin electrode 26. Continued operation cause the alkali metal concentration to reach an equilibrium value. The alkali-metal enriched alloy functions to transport alkali metal ions from the salt solution to the caustic solution through paired oxidation reduction reactions at the electrolyte solution 18, 22-alloy electrode 26 interfaces.
The electrolyte solution 18 of the first cell type 14 may carry out the following reactions. The reaction at the electrolyte solution 18—anode 20 interface, with sodium chloride as the alkali metal halide, is:
2Cl−→Cl2+2e−. (i)
The chlorine gas that results from (i) may be vented at the top of the first cell type 14 where in may be collected as a by-product. The reaction at the electrolyte solution 18—alloy electrode 26 interface is:
2Na++2e−→2Na. (ii)
In the second cell type 16, the reaction at the electrolyte solution 22—alloy electrode 26 interface is:
2Na (alloy)→2Na++2e−. (iii)
The reaction at the electrolyte solution 22—anode 24 interface is:
2H2O+2e−→2OH−+H2. (iv)
The net effect is that the concentration of alkali metal halide (sodium chloride, for example) in the right cell (first cell type 14) decreases, and the concentration of the caustic solution (sodium hydroxide) in the left cell (second cell type 16) increases. As the process continues, some sodium hydroxide solution may be withdrawn from the second cell type 16 as output product and replaced with water. Sodium chloride may be added to the first cell type 14 to replace sodium chloride that has been electrolyzed.
For sodium and the production of caustic soda (e.g., NaOH), the equilibrium concentration is about 1% by weight (about 6% Molar concentration). It is possible to prepare the alloy by direct incorporation of sodium metal in its initial manufacture to avoid the need for charging during initial operation.
The apparatus and methods provided in various exemplary embodiments may be used not only for the production of caustic soda and alkali metals, but also as a means of electrochemical desalination of sea water (or brackish water) using a non-toxic metal alloy as the ion transport medium. The apparatus and methods described by these embodiments may also be applied to the commercial production of hydrogen and chlorine gases.
In some embodiments of the current invention, the electrolysis cell may be used, in “reverse”, as a fuel cell. An open-cell potential of 2.1 V was observed in the circuit after external power was disconnected. Other low melting-point metals, such as gallium (Ga), and alloys (containing bismuth, indium, tin or gallium) may also be used to form amalgams for alkali metal transport in the methods and apparatus of the current invention.
The following Table shows the change in alloy composition by the use of various exemplary embodiments in an electrolytic cell with the nontoxic alloy electrode. Aqueous alkali metal solutions were electrolyzed by applying potential of 10V across the working and counter electrodes for 90 minutes. The difference between the initial and final composition of the electrode alloy demonstrates the formation of alkali metals and diffusion into the body of the electrode. These metals include aluminum (Al), sodium (Na), potassium (K), bismuth (Bi), indium (In), and tin (Sn).
While the above description focuses on the alloy electrode and use in water-based or aqueous electrolytic cells, the current invention may be applied to other electrolyte systems. For example, various embodiments may be applied to molten-salt and organic electrolytic cells.
The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is neither intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.
The invention described was made in the performance of official duties by one or more employees of the Department of the Navy, and thus, the invention herein may be manufactured, used or licensed by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
