PROCESS FOR RECOVERING MERCURY FROM A MERCURY-CONTAINING SLUDGE

1,154,420. Electrolytically producing caustic alkali and chlorine. TOYO SODA MFG.CO. Ltd. and JAPAN ORGANO CO. Ltd. Oct. 18, 1967 [Oct. 20, 1966], No. 47430/67. Heading C7B. In the production of caustic alkali and chlorine by electrolysis of alkali metal chloride e. g. LiCl, NaCl or KCl solution in a mercury cathode cell 1, depleted brine 2 is fed via dechlorinating apparatus 3 and salt dissolving tank 4 into a brine purification tank 6, where sodium carbonate 7 and caustic soda 8 are added to remove impurities such as Ca++ and Mg++ ions together with mercury by precipitation, to form a saturated and purified brine 24. Mercury is recovered from the sludge 9 produced in the tank 6 by dissolving the sludge (or a cake 11 filtered therefrom by a filter 10) with an acid 14 e. g. HC1 or H 2 SO 4 in a tank 13 to form a solution 15 which is contacted with an anion exchanger 18 which absorbs mercury, eluting the mercury from the anion exchanger with an acid 20 e. g. HC1 or H 2 SO 4 to form an eluate 21 which is used to adjust the pH of the purified brine 24, e.g. neutralise it from 10-11 to 7-8 in a tank 23, being fed into the cell 1, where the mercury is recovered by electrolytic reduction. The solution 15 may be neutralised with alkali 17 before contacting the anion exchanger 18, which may be solid, e.g. a resin column, or liquid, e.g. contained in Kerosene solution.