Electromembrane processes for the treatment of kraft mill electrostatic

Abstract

A process for the recovery of alkali metal hydroxide and acid from the alkali metal salts of monovalent anions which are mixed with the alkali metal salts of multivalent anions employs a water-splitting system composed of bipolar membranes in conjunction with ion-selective membranes; a two-compartment cell employs monovalent anion-selective membranes to define salt/base and acid compartments with the bipolar membranes, and a three-compartment cell employs, monovalent anion-selective and cation-selective membranes to define with the bipolar membranes, acid, salt and base compartments; the process has particular applicability to the conversion of the sodium/potassium chloride portion of the Electrostatic Precipitator (ESP) Catch of the recovery boiler of coastal and/or closed-cycle kraft pulp mills, a mixture of mostly sodium/potassium sulphate and chloride, into sodium/potassium hydroxide and hydrochloric acid; the remaining sodium/potassium sulphate solution, depleted in chloride, can thus be used as sodium/sulphur make-up to the recovery cycle or converted to sulphuric acid and sodium/potassium hydroxide; a simpler version of the process, can be employed for the electrodialytic separation of ESP catch into its component sodium chloride and sodium sulphate; when a three-compartment water-splitting or electrodialysis system is used, potassium ions are preferentially removed from the feed compartment, over sodium, thus removing this undesirable non-process element from the alkali metal sulphate portion which is returned to the recovery cycle; there is also described approaches through which a partially or totally effluent free (TEF) kraft pulp mill can be achieved in terms of the elements sodium, sulphur and chlorine.