Study on the Removal of Inorganic and Organic Mercury in Waste Water by the Flotation Method

Abstract

The removal of inorganic and organic mercury in waste water by the flotation method was investigated as a part of the intensive studies of the water pollution control. The removal of inorganic mercury was examined by the following two methods : 1) Fe(OH)₃ co-precipitation-flotation method, 2) Fe(OH)₃ co-precipitation-Na₂S precipitation-flotation method. From the results obtained, it was recognized that inorganic mercury in waste water could be efficiently removed by the Fe(OH)₃ co-precipitation-flotation method with 80 mg/l cumulative addition of ferric ions in the pH about 9 after the third stage flotation using sodium oleate as a collector. By the first stage flotation, however, the removal of inorganic mercury in the waste water was inefficient. So, sodium sulphide was added to the waste water in order to precipitate completely the mercury. Thus, the precipitates of inorganic mercury produced with 40 mg/l ferric ions and 1 equivalent addition of sodium sulphide to the total amounts of mercury were completely removed at the pH 6.5―9.5 using sodium oleate in only the first stage flotation. The removal of organic mercury in waste water was performed by the following methods. A substance which contains organic mercury was decomposed into inorganic mercury with gaseous chlorine, followed by the Fe(OH)₃ co-precipitation-flotation method and the Fe(OH)₃ co-precipitation-Na₂S precipitation-flotation method. The decomposition of organic mercury into inorganic mercury was achieved readily by the oxidation reaction using Cl₂ gas. The optimum conditions of this reaction were found at pH below 1. The removal of excess chlorine in the flotation pulp is important for a successful flotation of the precipitates. The excess chlorine was eliminated by 8 g/l sodium thiosulphate or the aeration over 40 min. It was found that the mercury decomposed by Cl₂ gas was completely removed by the flotation method with an addition of 50―100 mg/l ferric ions and 1 equivalent of sodium sulphide to the total amounts of mercury at the pH 5.0―9.5. This new method being applied, the removal of inorganic and organic mercury in the waste water was successful in a short time by only one stage flotation using sodium oleate, and mercury in the tailing solution was not detected.