Process for removing residual hydrazine from caustic solutions
AssigneeVulcan Materials Company
InventorGyaneshwari P. Khare
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Abstract
In a process for manufacturing food-grade caustic such as sodium hydroxide wherein hydrazine or a derivative or salt thereof is used as a corrosion inhibitor, residual hydrazine is removed from the concentrated caustic product solution by oxidation. The oxidation of residual hydrazine is accomplished by adding to the hydrazine-containing concentrated caustic solution, which is characterized by a pH of 14 or higher, a 5- to 10-fold stoichiometric excess of hydrogen peroxide or an equivalent peroxy compound or hypochlorite.
Claims7
In a process for making a concentrated caustic solution containing between about 40% and about 80% sodium or potassium hydroxide by dehydration of a dilute aqueous solution containing between about 10% and about 35% of said hydroxide and chlorate as an impurity, wherein said dehydration is conducted in a plurality of consecutive evaporation stages at consecutively higher temperatures such that at least one of said stages is at a temperature in the range between about 130.degree. and about 175.degree. C. while in contact with a metal surface composed of nickel as a major component and wherein a corrosion-inhibiting amount of hydrazine or an inorganic or organic derivative of hydrazine is added to said caustic solution that is to be evaporated, the improvement which comprises adding to said concentrated caustic solution at least one oxidizing agent selected from the group consisting of hydrogen peroxide, alkali metal peroxides, peroxides or calcium, magnesium and zinc, peroxy sulfuric acid, peroxy acetic acid, peroxy phosphoric acid, perboric acid, percarbonic acid and nontoxic metal salts of any of said acids, and hypochlorites of nontoxic metals, in an amount sufficient to oxidize residual hydrazine remaining in said caustic solution, and mixing the resulting mixture at a pH of at least 14 and at a temperature between about 20.degree. and 75.degree. C. until said residual hydrazine is substantially destroyed.
In a process for making concentrated caustic soda wherein a relatively dilute aqueous solution comprising sodium hydroxide, sodium chlorate as an impurity and hydrazine or an equivalent amount of an inorganic or organic derivative of hydrazine as a corrosion inhibitor is concentrated by dehydration at a temperature between about 100.degree. and 175.degree. C. in contact with a metal surface composed in major part of nickel, the improvement which comprises adding to said dehydrated solution an oxidizing compound selected from the group consisting of peroxygen compounds and hypochlorite salts of an alkali or alkaline earth metal in an amount sufficient to react with residual hydrazine remaining in said dehydrated solution, and agitating the resulting mixture at a pH of at least 14 and between about 20.degree. and at a temperature 75.degree. C. until the residual hydrazine is substantially destroyed.
A process for removing hydrazine from an aqueous caustic solution characterized by a pH of at least 14 and containing about 40 to 80% sodium hydroxide and from 5 parts per billion to about 5 parts per million hydrazine or an inorganic or organic derivative of hydrazine, which process comprises adding to said solution a five- to ten-fold stoichiometric excess of an oxidant relative to the amount of hydrazine present in the solution, said oxidant being selected from the group consisting of hydrogen peroxide, peroxides of non-toxic mono- or divalent metals, peroxy acids and non-toxic metal salts thereof, and hypochlorites of non-toxic metals and agitating the resulting mixture at a pH of at least 14 and at a temperature between about 20.degree. and 75.degree. C. until the residual hydrazine is destroyed.
Dependencies are inferred from the claim text.