Process for the removal of inorganic salts from a water stream

Claim 1

A process for the removal of inorganic salts from an unpotable feed water stream containing said salts, said process comprising the steps of: (a) passing said feed water stream sequentially through at least two ion exchange beds, one of which is in the anion form and the other of which is in the cation form, to remove a significant portion of the anions and cations in the stream, and to form a filtrate stream, (b) regenerating at least a portion of the anion exchange bed by passing an aqueous sodium ion-containing regenerant stream through the anion exchange bed portion while step (a) is discontinued with respect to it, to form a spent anion exchange regenerant stream, including ions from said unpotable water previously retained in said anion exchange bed, (c) regenerating at least a portion of the cation exchange bed by passing an aqueous chloride ion-containing regenerant stream through the cation exchange bed portion while step (a) is discontinued with respect to it to form a spent cation exchange regenerant stream including ions from said unpotable water previously retained in said cation exchange bed. (d) mixing said spent anion and cation exchange regenerant streams together with an acidifying agent in the presence of soluble calcium ion to precipitate a calcium salt of at least one anion selected from the group consisting of sulfate, phosphate, silicate and fluoride, in the spent anion exchange regenerant stream, the amount of said soluble calcium being maintained in excess of equivalency to the sum of the sulphate, phosphate, silicate and fluoride remaining in said spent regenerant stream, (e) separating the product stream of step (d) into a precipitate portion and a soluble regenerant stream portion, (f) mixing said soluble regenerant stream portion from step (e) with a causticizing agent in the presence of carbonate ion to precipitate at least one cation selected from the group consisting of calcium, magnesium, ferrous or manganous, in the insoluble hydroxide or carbonate form, the amount of the said carbonate being maintained in excess of equivalency to the sum of the said calcium, magnesium, ferrous and manganous cations, (g) separating the product stream of step (f) into a soluble sodium chloride solution stream and a precipitate portion, (h) separating the soluble sodium chloride stream into a more concentrated sodium chloride stream and a less concentrated sodium chloride stream, (i) electrolyzing said more concentrated sodium chloride stream to form an aqueous sodium hydroxide stream, and an aqueous hydrochloric acid stream, and (j) recovering said less concentrated sodium chloride stream as a purified product water stream.