WO1993012034A1

WO1993012034A1 - Process for producing lithium perchlorate

Info

Publication number: WO1993012034A1
Application number: PCT/US1992/010229
Authority: WO
Inventors: Ronald L. Dotson; Harry J. Loftis
Original assignee: Olin Corporation
Priority date: 1991-12-12
Filing date: 1992-11-25
Publication date: 1993-06-24
Also published as: AU3227093A

Abstract

A process for producing lithium perchlorate comprises oxidizing an aqueous solution of chloric acid, substantially free of ionic impurities, to produce an aqueous solution of perchloric acid, reacting the aqueous solution of perchloric acid with lithium hydroxide, lithium carbonate or mixtures thereof to produce a slurry of lithium perchlorate crystals, and, recovering the lithium perchlorate crystals. High purity lithium perchlorate can be produced which has substantially reduced concentrations of ionic impurities. Figure 2 depicts a graph of the cell voltage at selected current densities for the electrochemical production of perchloric acid from chloric acid.

Description

PROCESS FOR PRODUCING LITHIUM PERCHLORATE

This invention is related to the production of lithium perchlorate. More particularly, this invention is related to the production of lithium perchlorate from high purity perchloric acid. Lithium perchlorate is a solid oxidizer which is used in propellant and pyrotechnic compositions and more recently as a component in battery electrolytes. Lithium perchlorate has been prepared by the reaction of lithium hydroxide or lithium carbonate with perchloric acid. The lithium carbonate is admixed with concentrated perchloric acid and sufficient water to produce the lithium perchlorate trihydrate (LiC10₄.3H₂0) .

Lithium perchlorate trihydrate is dehydrated and the anhydrous salt recrystallized, for example, by extraction with ether and evaporation of the ether to recover the purified lithium perchlorate.

Now it has been discovered that a highly pure lithium perchlorate can be produced directly without requiring the recrystallization of the anhydrous product. High purity perchloric acid can be produced directly by oxidation of chloric acid which is substantially free of ionic impurities. The oxidation, for example, electrolytically using chloric acid as the anolyte, can be operated with high current loads at low voltages and increased current efficiences to .produce perchloric acid at reduced capital and operating costs. Thus lithium perchlorate can be produced which is of very high purity and at reduced costs.

These and other advantages are accomplished in a process for producing lithium perchlorate which comprises the steps of:

(a) oxidizing an aqueous solution of chloric acid substantially free of ionic impurities to produce an aqueous solution of perchloric acid,

(b) reacting the aqueous solution of perchloric acid with lithium hydroxide, lithium carbonate and mixtures thereof to produce a slurry of lithium perchlorate crystals, and,

(c) recovering the lithium perchlorate crystals. The process of the invention is depicted by the following FIGURES:

FIGURE 1 schematically illustrates one embodiment of the novel process of the present invention.

FIGURE 2 depicts a graph of the cell voltage at selected current densities for the electrochemical production of perchloric acid from chloric acid.

FIGURE 1 shows an electrolytic cell 4 divided into anode compartment 10 and cathode compartment 30 by cation permeable ion exchange membrane 16. Anode compartment 10 includes anode 12 which is spaced apart from cation permeable ion exchange membrane 16. Cathode compartment 30 includes cathode 32 which is placed in contact with cation permeable ion exchange membrane 16. The chloric acid solution is fed to anode compartment 10 of electrolytic cell 4. Following electrolysis, the perchloric acid solution produced is recovered.

A cell voltage curve is plotted in FIGURE 2 for a series of current densities, employed in the anodic oxidation of a 40% solution chloric acid to directly produce an aqueous solution of perchloric acid. The novel process of the present invention employs as the starting material a solution of high purity perchloric acid, HCIO.. The starting material, perchloric acid has been produced previously by the electrochemical oxidation of alkali metal chlorates in an electrochemical cell without a separator between the anode and the cathode. The chlorate solutions, having a pH in the range of 6-7, are electrolyzed at low temperatures, i.e. 30-45°C, and at current densities of

2 about 3 KA/m where the cell voltage is in the range of 6.5 to 7 volts. The process requires the addition of a salt such as an alkali metal chromate or dichromate to minimize cathodic reduction of the perchlorate ions formed. The alkali metal perchlorate solution produced is removed from the cell and fed to a tank where it is treated with an acid which converts the perchlorate to perchloric acid and forms a removable salt by-product. These prior art processes are energy inefficient and require multiple processing steps to isolate and purify the perchloric acid by removing chromates as well as cations such as chromium, sodium, potassium and ammonium, for example, by treatment with ion exchange resins.

In the process of the present invention, the production of high purity perchloric acid initially begins with solutions of high purity chloric acid, HC10- . High purity chloric acid solutions which are substantially free of ionic impurities such as metal ions and chloride ions can be produced by the oxidation of high purity hypochlorous acid solutions. One process suitable for producing the chloric acid heats the hypochlorous acid solution at a temperature in the range of from about 25 to about 120° C. and recovers a solution of chloric acid. This process is represented by the following reactions:

3HOC1 > HCIO3 + 2HC1 (1)

2HOC1 + 2HC1 > 2C1₂ + 2H₂0 (2)

5HOC1 > HCIO3 + 2C1₂ + 2H₂0 (3)

Another process for producing the high purity chloric acid utilizes anodic oxidation of hypochlorous in an electrolytic cell having an anode compartment, a cathode compartment, and an cation exchange membrane separating the anode compartment from the cathode compartment. In operation, the process includes feeding an aqueous solution of hypochlorous acid to the anode compartment, and electrolyzes the aqueous solution of hypochlorous solution at a temperature of from about 0° to about 40° C. to produce a chloric acid solution.

The process is represented by the following equation:

HOC1 + 2H₂0 > HC10₃ + 4H⁺ + 4e^~ (4)

High purity HOCl solutions to be used in the production of chloric acid are produced by a process in which gaseous mixtures, having high concentrations of hypochlorous acid vapors and chlorine monoxide (dichlorine monoxide, C1_0) gas and controlled amounts of water vapor are generated, for example, by the process described by J. P. Brennan et al in U.S. Patent No. 4,146,578, issued March 27,1979, or WO 90/05111 published May 17, 1990 by J. K. Melton, et. al.

Hypochlorous acid solutions produced by these processes contain concentrations of from about 35 to about 60, and more preferably from about 40 to about 55 percent by weight of HOC1. The hypochlorous acid solutions are substantially free of ionic impurities such as chloride ions and alkali metal ions as well as metal ions such as nickel and copper or mercury, among others.

Chloric acid solutions suitable for use in the process of the present invention, prepared by either of the processes for oxidizing hypochlorous acid described above, are substantially free of ionic impurities i.e. metal ions, including alkali metal ions, and chloride ions. Suitable chloric acid solutions include those in the range of from about 10 to about 50 percent, preferably from about 30 to about 50, and more preferably from about 30 to about 40 percent by weight of HC10„. In the novel process of the present invention the high purity chloric acid is oxidized to perchloric acid. One process for producing perchloric acid reacts the chloric acid with an oxygen-containing gas such as ozone. The reaction is believed to be represented by the following equation:

3HC10₃ + 0₃ > 3HC10. (5)

In a preferred embodiment, the chloric acid is fed as the anolyte to the anode compartment of an electrolytic cell which includes a cathode compartment, the anode compartment, and a separator such as a cation exchange membrane positioned between the anode compartment and the cathode compartment.

During cell operation the temperature of the chloric acid solution in the range of from about 40 to about 95°C, and preferably from about 45° to about 80°C. An electrolytic cell as illustrated in FIGURE 1 with membrane-cathode contact provides electrical continuity for proton transport from the anode chamber across the membrane and discharge on the back side of the open cathode surface. The cathode reaction is believed to be represented by the following equation:

2e^" + 2H⁺ > H₂ (6)

This "zero cathode gap" permits the cell to be operated at very high current densities while operating at surprisingly low voltages.

The anode is spaced apart from the membrane a sufficient distance to provide an anode-membrane gap which allows nascent oxygen generated to react with the chloric acid anolyte. The anode reactions are believed to be represented by the following equations:

H_0 > OH^" + H⁺ > O + 2H⁺ +2e^" (7) HC10₃ + 0 > HC10₄ (8)

Suitable anode membrane gaps are those up to a few centimeters, for example, gaps on the order of about 0.05 to about 4 centimeters, and preferably from about 0.1 to about 2 centimeters. Maintenance of the anode to membrane gap can be accomplished, for example, by operating the cell with a higher pressure in the anode compartment than the cathode compartment, or by placing a fine non-conductive porous spacer between the anode and the membrane. The anolyte is rich in hydrogen ions (protons) which are transported or tunnelled through the cation exchange membrane which separates the anode compartment from the cathode compartment. -

During cell operation the anode potential is maintained at a level sufficient to generate nascent oxygen. While the anode potential may vary with the concentration of perchloric acid produced, the anode potential should be 2 volts or greater, preferably in the range of from about 2.4 to about 2.9 volts. Current densities employed include those in the range of from

2 about 2 to about 12 KA/m , and preferably from about 4

₂ to about 10 KA/m . To achieve high current efficiencies at high current densities while maintaining low cell voltages, the anode structure is selected to provide low surface areas, being substantially flat and smooth. The anode surface is of a material which provides a high oxygen overvoltage, and additionally must be stable in the highly acidic and oxidative chloric acid media. Materials which can be employed in the anode structures include platinum and platinum group metals, metal substrates coated with platinum or platinum group metals, platinum group metal coated substrates, glassy carbon, fluorinated carbons, lead dioxide and metal substrates coated with lead dioxide, noble metal oxides, and metal substrates coated with noble metal oxides

Suitable metal substrates include valve metals such as titanium and niobium among others. Especially useful as an anode is a platinum coated niobium expanded metal having, for example, 100-200 mils of platinum metal bonded to the niobium substrate.

The cathode is placed in contact with the ion exchange membrane to minimize interference of hydrogen ion (proton) transfer to the back side of the cathode to produce hydrogen gas. Any suitable materials which readily evolve hydrogen gas in an acidic media may be employed in the cathode such as graphite or carbon, stainless steel, nickel alloys, platinum group metals, metals plated with platinum group metals, lead dioxide, etc. The cathode material should be insoluble in the acidic catholyte media while under current load, and preferably insoluble without cathodic protection. A preferred cathode is a perforated Hastelloy® metal plate or mesh.

As the catholyte, any suitable electrolyte may be employed such as a mineral acid i.e., sulfuric acid, phosphoric acid, or hydrochloric acid, as well as deionized water. In one embodiment, the catholyte contains particles of a solid state acid such as a perfluorosulfonic acid resin (sold commercially by E.I. DuPont de Nemours & Company, Inc., under the trademark "NAFION") to increase the conductivity of the catholyte. When using a solid state acid as the catholyte, small - amounts of hydrochloric acid are added to the cathode compartment.

Similarly, small concentrations of an electrolyte such as chloric acid may be present in the catholyte to improve the conductivity of the catholyte. The cation exchange membrane selected as a separator between the anode and cathode compartments is a chemically stable membrane which is substantially impervious to the hydrodynamic flow of the electrolytes and the passage of any gas products produced in the anode or cathode compartments.

Cation exchange membranes are well-known to contain fixed anionic groups that permit intrusion and exchange of cations, and exclude anions from an external source. Generally the resinous membrane or diaphragm has as a matrix, a cross-linked polymer, to which are attached

_2 charged groups such as —SO_ . The resins which can be used to produce the membranes include, for example, fluorocarbons, vinyl compounds, polyolefins, hydrocarbons, and copolymers thereof. Preferred are cation exchange membranes such as those comprised of fluorocarbon polymers having a plurality of pendant sulfonic acid groups and/or phosphonic acid groups. The term "sulfonic acid group" is meant to include compounds of sulfonic acid which when hydrolyzed produce sulfonic acid such as sulfonyl chloride and sulfonyl fluoride. Similarly, the term "phosphonic acid group" is meant to include compounds which when hydrolyzed produce phosphonic acid. During the process hydrogen ions are generated in the anode compartment which pass thru the cation exchange membrane into the cathode compartment. Oxygen gas which is also produced is removed from the anode compartment. Perchloric acid solutions produced by the process. of the invention have a high degree of purity and can include concentrations of up to 70 percent by weight of HCIO.. While any concentration of perchloric acid may be fed to the crystallizing zone, to minimize the energy requirements for vaporizing water present in the solution, it is preferred to employ perchloric acid solutions containing at least 30 percent, and preferably at least 50 percent by weight of HC10.. Solutions produced by the process of the invention may be concentrated prior to their reaction with the lithium base compound.

The high purity perchloric acid is reacted with a lithium base compound such as lithium hydroxide or lithium carbonate. Preferably the lithium base compound is in solid form. The reaction is carried out by admixture of the lithium base compound with the perchloric acid in a manner which maintains the reaction mixture acidic. Preferably the pH of the reaction mixture is at about 3 or less. During the reaction the temperature is maintained in the range of from about 10 to about 100°C, preferably at from about 15 to about 50°C, and more preferably at from about 20 to about 35°C. Lithium perchlorate crystals are produced in a slurry in perchloric acid. The crystals of LiC10 .3H₂0 are recovered and dried, for example, by vacuum dessication or ether extraction after dehydration to produce the anhydrous product.

To further illustrate the present invention, the following example(s) are presented without any intention of being limited thereby. All parts and percentages are by weight unless otherwise indicated.

EXAMPLE

Chloric acid (35 percent by weight of HC10-) was fed to the anode compartment of an H-type electrolytic membrane cell of the type illustrated in FIGURE 1. The cation exchange^'membrane which separated the anode compartment from the cathode compartment was Nafion® 117 (E.I. DuPont de Nemours & Company). The electrodes employed in the cell were a platinum coated niobium expanded fine mesh anode and a Hastelloy® metal cathode. The cathode was placed in direct contact with the cation exchange membrane while the anode was spaced apart from the cation exchange membrane to allow oxygen formation and release. The cell was operated at a current density of 1.995 KA/m_ and a cell temperature of 71°C for a period of about 26 hours. -The initial cell voltage was about 2 volts, with the final cell voltage being in the range of 5-6 volts. A perchloric acid solution containing 39 percent by weight of HCIO. was produced. To 194.76ml of the solution of perchloric acid was added 42 grams of lithium hydroxide monohydrate at a rate which maintained the pH of the reaction mixture at about 3. An additional 1.73 grams of lithium hydroxide monohydrate were added and the precipitate which was formed filtered to remove the solution and placed in a dessicator under full vacuum to dry. Lithium perchlorate trihydrate was recovered in an amount of 46.77 grams.

Claims

WHAT IS CLAIMED IS:

1 A process for producing lithium perchlorate characterized by the steps of:

(b) reacting the aqueous solution of perchloric acid with lithium hydroxide, lithium carbonate or mixtures thereof to produce a slurry of lithium perchlorate crystals, and,

(c) recovering the lithium perchlorate crystals.

2. The process of claim 1 characterized in that the chloric acid is anodically oxidized by electrolysis at a current density of from about 1 to about 12 KA/m².

3. The process of claim 2 characterized in that the aqueous solution of chloric acid contains from about 30 to about 45 percent by weight of HC10- .

4. The process of claim 2 characterized in that said anodic oxidation is accomplished by maintaining an anodic potential sufficient to form nascent oxygen.

5. The process of claim 1 characterized in that the aqueous solution of perchloric acid has a concentration of at least 30 percent by weight of

HCIO4 _A .

6. The process of claim 1 characterized in that the process for producing lithium perchlorate is accomplished by maintaining the reaction of the aqueous solution of perchloric acid with solid lithium hydroxide or lithium carbonate mixture at a temperature in the range of from about 10 to about 100°C.

7. The process of claim 1 characterized in that the process for producing lithium perchlorate is accomplished by maintaining the reaction mixture acidic.

8. The process of claim 1 characterized in that the pH of the reaction mixture is about 3 or less.