Process and apparatus for controlling impurities and pollution from membrane chlor-alkali cells
AssigneeIonics, Incorporated
InventorWayne A. McRae
Life of this patent
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Abstract
An improved integrated and cyclic process and apparatus are disclosed wherein portions of various process and waste fluids are recycled to control impurities in the influents to membrane chlor-alkali cells and simultaneously to minimize liquid pollutants from the process. The improved process and apparatus reduces energy costs and wastes while simultaneously controlling impurities important to operation of membrane chlor-alkali electrolytic cells.
Claims10
A process wherein a substantially saturated aqueous alkali metal chloride brine, containing minor amounts of non-monovalent metallic cations, is electrolyzed in a multiplicity of membrane chlor-alkali cells having anode compartments containing anodes, said anodes capable of generating chlorine containing minor amounts of oxygen from aqueous chloride solution and a pH in the range of from about 0.5 to about 5, cathode compartments containing cathodes and substantially fluid impermeable, cation permselective perfluorocarbon membranes separating said anode compartments from said cathode compartments and wherein at least part of the gaseous chlorine effluent from said anode compartments is burned with a substantially chemically equivalent amount of the gaseous hydrogen effluent from said cathode compartments and thereafter absorbed in water to produce hydrochloric acid, which process comprises:
- (a) as a first brine purification step, mixing said brine with at least one compound selected from the group consisting of phosphoric acid, alkali carbonates, alkali bicarbonates, alkali phosphates, alkali acid phosphates and mixtures thereof, adding a first portion of the alkali metal hydroxide liquid effluent from said cathode compartments to produce a pH of at least about 10 thereby precipitating a substantial portion of said non-monovalent cations, removing said precipitate and recovering the thus pretreated brine;
- (b) as a subsequent brine purification step, contacting substantially all of the recovered brine of step (a) having a pH of at least about 9 with a chelating cation exchanger in the monovalent cation form thereby maintaining the concentration of any non-monovalent metallic cation in the brine effluent at a concentration of not more than about 2 parts per million;
- (c) thereafter flowing said purified brine into said anode compartments and adding thereto a first portion of said hydrochloric acid so controlled as to maintain the pH of the liquid immediately effluent from said anode compartments in the range of from about 2 to about 4;
- (d) maintaining the liquid effluent from said cathode compartments at a concentration of at least about 8 percent by weight of alkali metal hydroxide and at a temperature of at least about 70.degree. C.;
- (e) substantially regenerating said chelating exchanger with a second portion of said hydrochloric acid solution and thereafter contacting said exchanger with a second portion of the alkali metal hydroxide liquid effluent from said cathode compartment to substantially convert said exchanger to the alkali metal form;
- (f) adjusting the pH of substantially all the liquid effluent from said anode compartments to a value less than or equal to about 2 by adding a third portion of such hydrochloric acid solution thereby substantially converting any hypochlorite therein to chlorine, thereafter removing and recovering dissolved chlorine in said liquid effluent to produce a low chlorine liquid effluent, and;
- (g) substantially resaturating at least part of the resulting low chlorine liquid effluent of step (f) with alkali metal chloride and returning it to said first brine purification step.
Apparatus for producing chlorine comprising a multiplicity of cells having anode compartments containing anodes capable of generating chlorine containing minor amounts of oxygen from aqueous chloride solutions having at least one gram equivalent of free chloride per liter of solution and a pH in the range of about 0.5 to 5, cathode compartments containing cathodes and substantially fluid impermeable, cation permselective perfluorocarbon membranes separating said anode compartments from said cathode compartments which comprises:
- (a) first pretreatment means for mixing a substantially saturated aqueous chloride solution containing minor amounts of non-monovalent metallic cations with at least one compound capable of forming an insoluble calcium compound, means for measuring the pH of the mixture and pH responsive means for controlling the addition of a first portion of the liquid effluent from said cathode compartments to said mixture;
- (b) second pretreatment means for contacting said recovered solution with a chelating cation exchanger;
- (c) means for maintaining the pH of the liquid, immediately effluent from said anode compartments in the range of from about 2 to about 4;
- (d) means for maintaining the liquid effluent from said cathode compartments at a concentration of at least about 8 percent by weight of alkali metal hydroxide;
- (e) means for maintaining the liquid, immediately effluent from said cathode compartments, at a temperature of at least about 70.degree. C.;
- (f) means for combining at least part of the hydrogen effluent from said cathode compartments with a substantially equivalent amount of chlorine effluent from said anode compartments and with water;
- to produce aqueous hydrochloric acid;
- (g) means for substantially regenerating said chelating cation exchanger with a first part of said aqueous hydrochloric acid solution;
- (h) means for subsequently converting said exchanger substantially to the alkali metal form by contacting with a second portion of the liquid effluent from said cathode compartments;
- (i) means for measuring the pH of the liquid effluent from said anode compartments, means for adding a second part of said hydrochloric acid solution to said effluent, said means being responsive to said pH measuring means, means for removing the dissolved chlorine from said liquid effluent, means for recovering said chlorine and said liquid effluent and;
- (j) means for substantially resaturating at least part of said recovered anode effluent with chloride and returning it to said pretreatment means.
Dependencies are inferred from the claim text.