Purification of Concentrated Aqueous Hydroxide Solutions for Electrolysis

A process for purifying an aqueous solution of 25 wt. % to 35 wt. % potassium hydroxide comprising at least one target impurity selected from the group consisting of dissolved iron (II), dissolved iron (III), sulfate anions and organic compounds, said process comprising:

• adding to the potassium hydroxide solution, at least one precipitation additive selected from the group consisting of water-soluble salts of magnesium (II) cations and calcium (II) cations in an amount up to a 30-fold excess, based on the total stoichiometric amount of the at least one target impurity;
• contacting the solution with the precipitation additive(s) for a period of time in a range from about 0.2 h to about 6 h to enable formation of at least one insoluble compound comprising the at least one target impurity or a derivative thereof as a precipitate;
• and
• filtering the precipitate from the solution.

A process for purifying an aqueous solution of 25 wt. % to 35 wt. % potassium hydroxide comprising carbonate anions as a target impurity, said process comprising:

• adding to the potassium hydroxide solution, barium (II) hydroxide as a precipitation additive in an amount up to a 3-fold excess, based on the total stoichiometric amount of the target impurity;
• contacting the solution with the precipitation additive for a period of time in a range from about 0.2 h to about 6 h to enable formation of at least one insoluble compound comprising the target impurity or a derivative thereof as a precipitate;
• and
• filtering the precipitate from the solution.