Production of ultra pure salt
This invention relates to methods and installations for producing ultra pure sodium chloride salt crystals primarily for use in saturating depleted brine resulting from the electrolytic decomposition of saturated brine in chlor alkali membrane cells for the production of chlorine, caustic soda and hydrogen. More particularly, this invention relates to the production of ultra pure sodium chloride salt crystals by processing primary treated brine by first acidifying the primary treated brine, then stripping the carbonic acid produced by acidification as carbon dioxide, and then returning the brine to a pH of about 6 or higher which is sufficient for processing it in evaporation equipment where the ultra pure salt crystals are produced.
1. The method for producing ultra pure salt, containing below 500 parts per billion (ppb) of calcium and other multivalent metal ions, which comprises:
acidifying primary treated brine from pH 9.5 to 12 to pH 2 to 4 by adding hydrochloric acid, thereby converting the calcium, barium, strontium, and other reactive multivalent metal compounds to soluble chlorides, such acidification also converting excess sodium carbonate and hydroxides to salt, water, and dissolved carbon dioxide,
removing the carbon dioxide by stripping,
neutralizing the stripped material to a pH of at least 6 to prevent metals corrosion but not exceeding 8 to prevent carbon dioxide uptake from the surroundings,
concentrating the neutralized stripped material and washing the crystals so produced, to produce ultra pure salt crystals.
2. The method of claim 1 wherein the ultra pure salt produced contains 5 to 200 ppb calcium and other multivalent metal ions.
3. The method of claim 1 wherein the concentrating is by evaporation.
4. The method of claim 1 wherein the stripping is by an inert gas.
5. The method of claim 1 wherein the stripping is by air stripping.
6. The method of claim 1 wherein the stripping is by vacuum stripping with nitrogen.
7. The method of claim 1 wherein the stripping is by steam stripping.
8. The method of claim 1 wherein aqueous concentrated hydrochloric acid is employed in acidifying the primary treated brine.
9. The method of claim 8 wherein the hydrochloric acid is introduced into the primary treated brine through a pipeline reactor.
10. The method of claim 1 wherein liquid concentrated sodium hydroxide is employed to neutralize the stripped material.
11. The method of claim 1 wherein the stripped material is introduced into a water scrubber to reduce emissions of brine droplets.
12. The method of claim 1 wherein condensate obtained in the concentrating step is employed in washing the crystals.
13. The method of claim 1 wherein the primary treated brine has a pH of about 10 and is acidified to a pH of about 3.