The present invention relates to a process of removing sulfate, i.e., sulfate anions, from aqueous alkali metal chlorate solutions containing chromate and sulfate. More particularly, this invention relates to the removal of sulfate from aqueous chromate-containing sodium chlorate solutions without simultaneous removal of significant quantities of chromate.
Aqueous alkali metal chlorate solutions, e.g., sodim chlorate solutions, are conventionally produced by electrolysis of alkali metal chloride brines in electrolytic cells, usually bipolar electrolytic cells. A cell liquor containing both the alkali metal chlorate and alkali metal chloride is recovered from the cells. It is conventional to add chromate, e.g., sodium dichromate, to the solution in the cell, because chromate improves the current efficiency of the cell during conversion of sodium chloride to sodium chlorate. By the term "chromate" is meant a chromium-containing species such as a chromate ion (CrO.sub.4.sup..dbd.), a monobasic chromate ion (HCrO.sub.4.sup.-) or a dichromate ion (Cr.sub.2 O.sub.7.sup..dbd.), the particular species being determined primarily by the pH in the solution.
The alkali metal chloride brine used as feed for these cells can be obtained by dissolving solid salt, for example, by solution mining of rock salt. Brine formed from dissolving solid salt can contain various impurities including sulfate salts. While complete removal of sulfate from the brine is unnecessary, electrolysis of brine having sulfate levels above 25 grams per liter (g/l) can damage the anode coatings. In a typical process for producing the alkali metal chlorate, the cell liquor is recovered from the cell, a portion of the alkali metal chlorate product is crystallized and separated from the liquor, the liquor is resaturated with alkali metal chloride and the liquor is then recycled to the electrolytic cell. Without removal of some sulfate, sulfate levels would rise above acceptable levels. Sulfate levels may be reduced to acceptable levels by adding calcium chloride to precipitate calcium sulfate. Such sulfate removal can be achieved by treatment of a sidestream of the aqueous chlorate liquor. It has been found that chromate ions within the chlorate liquor tend to precipitate with the calcium sulfate. Such precipitation can result in calcium sulfate precipitates having chromium levels in the range of several thousand parts per million (ppm). It is desirable to minimize the chromium level in the sulfate-containing precipitate thereby reducing toxic waste problems with the precipitate.