Electrolytic manufacture of dithionites

Claim 3

A method according to claim 1 wherein the material of the cation-active permselective membranes is selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a flurosulfonated perfluovinyl ether, and a sulfostyrenateD perflourinated ethylene propylene polymer, and the cells employed are three- and two-compartment cells.

Claim 4

A method according to claim 2 wherein the permselective membrane is of a hydrolyzed copolymer of tetrafluoroethylene and FSO2CF2CF2OCF(CF3)CF2OCF''''CF2, which copolymer has an equivalent weight of about 900 to 1,600.

Claim 5

A method according to claim 3 wherein the voltage drop across the three-compartment cell is about 3 to 6 volts, that across the two-compartment cell is about 3 to 5 volts, the current density for the three-compartment cell is about 1 to 3 amperes/sq. in., that for the two-compartment cell is about 0.1 to 2 amperes/sq. in. and the operating temperature of the three-compartment cell is about 50* to 100*C. and that of the two-compartment cell is about 3* to 40*C.

Claim 6

A method according to claim 4 wherein the membrane walls are from about 0.02 to about 0.5 mm. thick, the membranes are mounted on a network screen or cloth of filaments of a material selected from the group consisting of polytetrafluroethylene, perfluorinated ethylene propylene polymer, polyproplyene, titanium, tantalum, niobium and noble metals, which has an area percentage of openings therein from about 8 to about 80% with the filaments having a thickness of about 0.01 to about 0.5 mm.

Claim 7

A method according to claim 5 wherein the voltage drop across the three compartment cell is from 4 to 5 volts, that across the two-compartment cell is from 3.5 to 4.5 volts, the current density in the three-compartment cell is from 3.5 to 4.5 volts, the current density in the three-compartment cell is from about 1.5 to 2.5 amperes/sq. in., the current density in the two-compartments cell is 0.2 to 1 amperes/sq. in., the current density in the two-compartment cell is 0.2 to 1 ampere/sq. in., the operating temperature of the three-compartment cell if 80* to 100*C., the operating temperature of the two-compartment cell is 3* to 25*C., the feed to the anode compartment of the three compartment cell is a chloride solution containing 20 to 25% of chloride, the hydroxide removed from the cathode compartment of that cell is an aqueous solution at a concentration of 20 to 30% hydroxide, the sulfite withdrawn from the buffer compartment of the same cell is an aqueous solution at a concentration of 1 to 15% sulfite and accompanying it is sodium hydroxide, at a concentration of 15 to 1% hydroxide, the chloride feed to the anolyte compartment of the two-compartment cell is essentially the same as that of the feed of such compartment of the three-compartment cell, and the dithionite and sulfite removed from the catholyte compartment of the two-compartment cell are in aqueous solution at a concentration of 10 to 70 g./l. of the dithionite and 100 to 250 g./l. of the sulfite.

Claim 8

A method according to claim 6 wherein the anodes are dimensionally stable anodes of material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures thereof, on a valve metal, and the cathode is stainless steel.

Claim 9

A method according to claim 7 wherein the chloride is sodium chloride, the hydroxide is sodium hydroxide, the sulfite is sodium sulfite and the dithionite produced is sodium dithionite, the anolytes are recirculated and the depleted anolytes are increased in concentration to about 25% NaCl, at which concentration they are fed to the anode compartments, by dissolving solid sodium chloride therein.

Claim 10

A method according to claim 8 wherein the membrane copolymer equivalent weight is from 1,100 to 1,400, the membrane wall thickness is 0.1 to 0.3 mm., the anode is ruthenium oxide on titanium, the pH''s of the anolytes are about 2 to 4 and the dithionite withdrawn is in an aqueous solution with sodium sulfite, wherein the dithionite concentration is from 30 to 50 g./l.

Claim 13

A method according to claim 10 wherein the cation-active permselective membranes are selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluoroyinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, the wall thickness of the membranes is from about 0.02 to 0.05 mm. the hydroxide produced in the cathode compartment of the three-compartment cell is of a high concentration and chloride-free, the sulfite is of a concentration of 1 to 15%, the voltage drop across the three-compartment cell is about 3 to 6 volts, that across the two-compartment cell is about 3 to 5 volts, the current density for the three-compartment cell is about 1 to 3 amperes/sq. in., that for the two-compartment cell is about 0.1 to 2 amperes/sq. in. and the operating temperature of the three-compartment cell is about 50* to 100*C. and that of the two compartment cell is about 3* to 40*C.

Claim 14

A method according to claim 11 wherein the permselective membrane is of a hydrolyzed copolymer of tetrafluoroethylene and FSO2CF2CF2OCF(CF3)CF2OCF CF2, which copolymer has an equivalent weight of of about 1,100 to 1,400, the membrane thickness is from 0.1 to 0.3 mm., the anodes are dimensionally stable anodes of material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures thereof, on a valve metal, the cathodes are stainless steel, the chloride, hydroxide, sulfite and dithionite are sodium salts, the sodium chloride is charged to the anode compartments of the cells in an aqueous solution at a concentration of about 20 to 25% NaCl and the dithionite produced is in aqueous solution at a concentration of 5 to 50 g./l.