Acid water containing considerable amounts of zinc and sulfate ions is effluented from some process in zinc hydrometalllurgy. Such water is neutralized with slaked lime and the precipitates formed, which mainly consist of zinc hydroxide a dgypsum, are dumped into dam. In order to utilize these precipitates selective flotation of zinc hydroxide from gypsum has been investigated.
As a fundamental study, floatability of zinc hydroxide was examined using dodecylammonium acetate (DAA) or sodium dodecylsulfate (SDS) as a collector. It was found that zinc hydroxide was relatively floatable with DAA or SDS at wide pH range in alkaline medium. Then the effects of soluble starch and sodium silicate on the floatability of zinc hydroxide and gypsum were examined. It was shown that sodium silicate tended to depress zinc hydroxide effectively when SDS was used as a collector. Then, selective flotation was made for the mixture of zinc hydroxide and gypsum, and it was shown that satisfactory separation of the mixture could be achieved only when SDS was used as a collector and sodium silicate as a depressant for zinc hydroxide.
Synthetic mine water containing 1000mg/1 of zinc ions, 7328mg/1 of sulfate ions and 603mg/I of calcium ions was neutralized with slaked lime until pH 8.5. The flotation tests were conducted for the precipitates thus formed. It was touiid that zinc hydroxide was effectively separated from gypsum with SDS collector when sodium silicate was aaaea in Dotn procelss of neutralization and flotation, and that some 90% of gypsum was recovered in the froth and 87% of zinc hydroxide in te tailing solution.

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