Abstract: The invention relates to a method for removing calcium ions and silicon compounds from various liquors produced or obtained in an alkali metal chlorate process, thereby reducing the operating cost and improving the possibility to close the process further. The calcium ions and silicon compounds are removed by addition of carbonate ions for precipitating calcium carbonate and by addition of an iron-containing compound for forming and precipitating a complex of iron ions and a silicon compound, whereupon the precipitates are co-separated from the liquor. The removal of calcium and silicate ions is substantially facilitated by the simultaneous separation of the precipitates. Subsequently, the excess of carbonate ions can be reduced by acidifying the electrolyte for releasing carbon dioxide.

Abstract: The invention relates to a method for separating sulphate from an aqueous electrolyte in an alkali metal chlorate process, thereby reducing the operating cost and increasing the possibility of further closing the electrolytical production of alkali metal chlorate. Calcium ions are added to a portion of the electrolyte, for precipitating calcium sulphate. By adjusting the pH within the range of from 4.6 up to about 7, and combining the temperature, content of ions and the molar ratio between sulphate and calcium ions added in a suitable manner, the calcium sulphate will precipitate predominantly as gypsum and essentially without inclusions and/or co-precipitation of chromium-containing compounds or chlorate. The precipitate is subsequently removed from the portion of electrolyte, whereafter the thus purified portion of electrolyte is recycled to the chlorate process.