Chromate removal by precipitation

Claim 1

A method of removing chromate from aqueous alkali metal chlorate solutions which comprises: adding to a chromate-containing aqueous solution of an alkali metal chlorate, at a temperature of about 40° to about 60° C, a water-soluble sulfide in an amount ranging from about 70 to about 200% of the stoichiometric value required to react with said chromate, said aqueous solution having an initial pH in the range of 7-8; then adding to said solution a water-soluble ferrous salt in an amount ranging from about 30 to about 100% of the stoichiometric value required to react with said chromate, and adjusting the pH of the solution such that the final pH is in the range of about 1.8 to about 3.0, whereby said chromate is converted to an insoluble trivalent chromium-containing material and the sulfide is completely converted to elemental sulfur; and separating said insoluble trivalent chromium-containing material from said solution.

Claim 2

A method of removing chromate from aqueous alkali metal chlorate solutions which comprises: adding to a chromate-containing aqueous solution of an alkali metal chlorate, at a temperature of about 40° to about 60° C, a water-soluble sulfide in an amount ranging from about 70 to about 200% of the stoichiometric value required to react with said chromate, said aqueous solution having an initial pH in the range of 7-8; then adding to said solution a water-soluble ferrous salt in an amount ranging from about 30 to about 100% of the stoichiometric value required to react with said chromate, and adjusting the pH of the solution such that the final pH is in the range of about 1.8 to about 3.0, whereby said chromate is converted to an insoluble trivalent chromium-containing material and the sulfide is completely converted to elemental sulfur; and separating said insoluble trivalent chromium-containing material from said solution.

Claim 11

A method of removing chromate from aqueous alkali metal chlorate solutions which comprises: adjusting the pH of a chromate-containing aqueous solution of an alkali metal chlorate to a value within the range of from about 7 to about 11, said solution having a temperature of about 40° to about 60° C; adding to said solution the product obtained by reacting a ferrous salt with an alkali metal hydroxide in relative proportions of 1 part ferrous salt to 1.5-1.8 parts of said alkali metal hydroxide, the amount of said ferrous salt employed being from about 3 to about 5 times the stoichiometric amount required to reduce the chromate present in the solution, said addition being effected slowly, and simultaneously with strong agitation, and said pH being controlled to a value within the range of 10-12, whereby said chromate is converted to an insoluble trivalent chromium-containing material; and separating said insoluble trivalent chromium-containing material from said solution.

Claim 12

A method of removing chromate from aqueous alkali metal chlorate solutions which comprises: adjusting the pH of a chromate-containing aqueous solution of an alkali metal chlorate to a value within the range of from about 7 to about 11, said solution having a temperature of about 40° to about 60° C; adding to said solution the product obtained by reacting a ferrous salt with an alkali metal hydroxide in relative proportions of 1 part ferrous salt to 1.5-1.8 parts of said alkali metal hydroxide, the amount of said ferrous salt employed being from about 3 to about 5 times the stoichiometric amount required to reduce the chromate present in the solution, said addition being effected slowly, and simultaneously with strong agitation, and said pH being controlled to a value within the range of 10-12, whereby said chromate is converted to an insoluble trivalent chromium-containing material; and separating said insoluble trivalent chromium-containing material from said solution.