Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2001-01-16
2002-04-23
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S554000, C423S551000
Reexamination Certificate
active
06375824
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to manufacturing of sodium hydroxide. More particularly, the invention relates to manufacturing of potassium hydroxide and potassium sulfate by electrolyzing sodium sulfate.
BACKGROUND OF THE INVENTION
Demonstrated worldwide demand for some sodium-based chemicals, particularly for sodium hydroxide (caustic soda), has been on the rise in recent years. This strong demand, which is forecast to continue, keeps this chemical in tight supply position, thereby holding the price at a high level. This trend is not the same with respect to all sodium-based chemicals. In particular, the demand for sodium sulfate and, as a consequence, the price of this chemical is declining at the same time as the demand for caustic soda is rising.
This declining trend in the demand for and prices of sodium sulfate combined with the strong demand for and relatively high prices of other sodium-based chemicals, in particular of caustic soda, created a need for a simple and economical process for producing sodium hydroxide from sodium sulfate as feedstock. This need is even more strongly perceived in countries endowed with vast natural resources of sodium sulfate. This is, for example, the case in Canada, which has large deposits of natural sodium sulfate located in Southern Saskatchewan.
The most direct process for producing sodium hydroxide from sodium sulfate is the electrolytic conversion of an aqueous solution of sodium sulfate into aqueous solutions of sulfuric acid and caustic soda. Numerous implementations of this process are known in the prior art. Most of them make use of electrolytic cells employing diaphragms or ion permeable membranes to separate the product solutions from the feed solution, thus avoiding contamination of the products by the feedstock material.
U.S. Pat. No. 2,829,095, issued Apr. 1, 1958, to Oda et al., discloses a process for the production of acidic and alkaline solutions by electrolysis of a salt solution in a multi-compartment electrolytic cell partitioned by a plurality of anion and cation exchange membranes. The patent also discloses the use of the process for direct production of sodium hydroxide and sulfuric acid from Glauber's salt (sodium sulfate decahydrate).
U.S. Pat. Nos. 3,135,673, issued Jun. 2, 1964, to Tirrell et al., and 3,222,267, issued Dec. 7, 1965, to Tirrell et al. claim a method and apparatus for converting aqueous electrolytic salt solutions to their corresponding acid and base solutions. A three or four compartment electrolytic cell separated by a cation exchange membrane and one or two porous, non-selective diaphragms is used for this purpose. When a solution of sodium sulfate is used as the salt solution, solutions of sodium hydroxide and sulfuric acid or sodium bisulfate are produced.
U.S. Pat. No. 3,398,069, issued Aug. 20, 1968, to Juda, claims a process for the electrolysis of an aqueous saline electrolyte in a multicellular device having cells separated by gas permeable electrodes and further partitioned by microporous fluid-permeable diaphragms or ion-permselective membranes. When applied to a solution of sodium sulfate, the process produces solutions of sodium hydroxide and sulfuric acid.
U.S. Pat. No. 3,907,654, issued Sep. 23, 1975, to Radd et al., discloses an electrolytic cell particularly useful in electrolysis of sodium sulfate to form sulfuric acid and sodium hydroxide. The cell, which does not employ any ion permeable membranes, comprises a housing having a parent solution chamber and two electrode compartments located on the lower side of the housing and separated from each other but in communication with the parent solution chamber and positioned vertically beneath or above. Mounted within the electrode compartments are an anode and a cathode, each of which is porous to permit passage of a product solution therethrough. The product solutions of sodium hydroxide and sulfuric acid separated by gravity forces are withdrawn through the porous electrodes.
U.S. Pat. No. 4,561,945, issued Dec. 31, 1985, to Coker et al., claims a process for producing sulfuric acid and caustic soda by electrolysis of an alkali metal sulfate in a three-compartment membrane cell having a hydrogen depolarized anode. Hydrogen gas in the anode chamber is oxidized to produce hydrogen cations which migrate to the central (buffer) chamber through a membrane and combine with the sulfate anions from the alkali metal sulfate solution to produce sulfuric acid. Alkali metal ions are transported across another membrane to the cathode chamber to produce caustic and gaseous hydrogen. Both membranes used in the cell are cation selective membranes.
A similar process for increasing concentration of sulfuric acid in solutions containing an alkali metal sulfate, sulfuric acid and alkaline earth metal ions is disclosed in U.S. Pat. No. 4,613,416, issued Sep. 23, 1986, to Kau et al. Also in this case the anode compartment and the cathode compartment of a three-compartment cell are each bounded by cation exchange membranes.
In U.S. Pat. No. 5,445,717 issued to Karki et al., issued Aug. 29, 1995, there is disclosed a method for the simultaneous production of alkali metal or ammonium peroxodisulphate salts and alkali metal hydroxide. In the reference, the electrolytic phase of the method is performed in a three-compartment electrolytic cell with the middle space conducting alkali metal sulfate, the anode space ammonium or alkali metal sulfate or a mixture thereof and into the cathode space water diluted alkali metal hydroxide.
This patent proceeds according to a different process to that described herein and does not provide for the preparation of potassium sulfate from a sodium sulfate starting material.
A further variation on electrosynthesis is demonstrated in Toomey, U.S. Pat. No. 5,290,404, issued Mar. 1, 1994. In this reference, an electrochemical cell is employed for producing an alcohol or carboxylic acid from a corresponding metal salt. Metal cations and residues are also recovered during the process. This process does not employ a three-compartment desalination cell, but rather employs a standard two compartment cell divided by cation permeable membrane.
In U.S. Pat. No. 5,246,551, issued Sep. 21, 1993, to Pletcher et al., an electrochemical method for the production of alkali metal hydroxides without co-production of chlorine is disclosed. In this reference, the use of a specific group of salts such as alkali metal carbonates, alkali metal bicarbonates and the like are electrolyzed in a single membrane-two solution cell with hydrogen consuming anodes. One of the advantages of this reference is the lack of the co-production of chlorine, however, the use of these specific salts is not necessary and has been overcome by the instant application.
Martin, in U.S. Pat. No. 5.230,779, issued Jul. 27, 1993, provides an electrochemical process for the production of sodium hydroxide and sulfuric acid from acidified sodium sulfate solutions. The process particularly takes place in a two compartment electrolytic cell and does not provide any teachings with respect to potassium sulfate generation, potassium chloride or ammonium sulfate generation.
Other references generally related to electrolysis and electrosynthesis include U.S. Pat. Nos. 4,033,842 and 5,286,354.
It would be desirable if there were a process where electrosynthesis or other electrochemical methods could be employed to produce useful potassium compounds such as potassium sulfate, potassium chloride as well as fertilizer compositions, namely ammonium sulfate. The present invention employs additional unit operations onto existing processes to result in the preparation of these desirable compounds.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an improved electrochemical process for preparation of potassium sulfate and potassium hydroxide.
A further object of one embodiment of the present invention is to provide a process for producing potassium sulfate in an electrolytic cell, comprising the steps of:
passing a solution of sodium sulfate thr
Airborne Industrial Minerals Inc.
Phasge Arun S,.
Sharpe Paul S.
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