US 5,567,293 A · Electromembrane processes for the treatment of kraft mill electrostatic precipitator catch

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.

Claims30

Dependent claims

A single-stage electrodialysis process comprising:

(a) providing a cell comprising an anode, a cathode and at least two compartments therebetween separated by a monovalent anion-selective membrane,
(b) feeding an aqueous solution of a mixture of alkali metal salts of monovalent chloride anions and polyvalent anions into a first of said compartments to contact a first side of said monovalent anion-selective membrane, said solution being substantially free of polyvalent metal ions,
(c) feeding a liquid comprising water into a second of said compartments to contact a second side of said monovalent anion-selective membrane,
(d) passing a direct current through said cell between the anode and cathode to effect migration of monovalent chloride anions from said first compartment through said monovalent anion-selective membrane into said second compartment, and
(e) recovering a solution of alkali metal salts of the polyvalent anions depleted in salts of the monovalent chloride anions from said first compartment.

2 more elements

A single stage electrodialysis process comprising:

(a) providing a cell comprising an anode, a cathode and at least two compartments there-between defined at least in part by a monovalent anion-selective membrane and a pair of bipolar membranes, said bipolar membranes having a cation-selective side facing said cathode and an anion-selective side facing said anode,
(b) feeding an aqueous solution of a mixture of alkali metal salts MA and M.sub.x B of monovalent and polyvalent anions into a first of said compartments to contact a first side of said anion-selective membrane, and the anion-selective side of a first of said bipolar membranes, wherein M is an alkali metal cation, A is a monovalent chloride anion and B is a multivalent anion having a valency x, said solution being substantially free of polyvalent metal ions,
(c) feeding liquid comprising water into a second of said compartments to contact an anion-selective side of a first of said bipolar membranes,
(d) passing a direct current through said cell between the anode and cathode to effect:
(i) migration of said monovalent chloride anions A from said first compartment through said monovalent anion-selective membrane into said second compartment, and
(ii) splitting of water by said bipolar membrane of said second compartment with accumulation of hydrogen ions along with the monovalent anions A, in said second compartment and accumulation of hydroxide ions in said first compartment,
(e) removing accumulated HA from said second compartment, and
(f) removing accumulated MOH from said first compartment.

5 more elements

Dependencies are inferred from the claim text.

Electromembrane processes for the treatment of kraft mill electrostatic precipitator catch

Life of this patent

Electromembrane processes for the treatment of kraft mill electrostatic precipitator catch

Life of this patent

Abstract

Claims30