Process for the purification of electrolysis brine

A process for purifying electrolysis brine comprising dividing the electrolysis brine into first and second streams, adding calcium chloride to the second stream thereby substantially quantitatively to obtain a first precipitate comprising calcium sulfate in the form of CaSO.sub.4 . 2 H.sub.2 O with some magnesium compound admixed therewith, separating off the first precipitate, combining the second stream freed from the first precipitate with the first stream, adding sodium carbonate and alkali hydroxide to the re-combined streams thereby substantially quantitatively to obtain a second precipitate comprising calcium and any magnesium present therein, the calcium being present as CaCO.sub.3 in said second precipitate and any magnesium being present as Mg(OH).sub.2, separating off the second precipitate to leave a purified brine, introducing the purified brine into the electrolysis cell, reacting the CaSO.sub.4 . 2 H.sub.2 O first precipitate under pressure with sodium hydroxide and CO.sub.2 to form CaCO.sub.3 and Na.sub.2 SO.sub.4 . 10 H.sub.2 O, removing the Na.sub.2 SO.sub.4 . 10 H.sub.2 O, reacting the CaCO.sub.3 with hydrochloric acid to form CO.sub. 2 and a solution of CaCl.sub.2 having some magnesium compound dissolved therein, and recycling the CaCl.sub.2 to react with a further quantity of said second stream.