Precipitation of the hydroxide neutralizes acid zinc solutions and reduces zinc in the same process. If only lime is added in a single step after extended settling, there is still less than 1% hydroxide in the sludge. Often such sludges must be stored indefinitely in lagoons because they dry

FIG. 8.3.6 Zinc recovery flowsheet (Reprinted from the U.S. Environmental Protection Agency (EPA). 1971. Zinc precipitation and recovery from viscose rayon wastewater. [American Enka Co. Project No. 12090 ESG. January.])

very slowly. A novel system (Figure 8.3.6) precipitates zinc by repeated adsorption on hydroxide particles (Rock 1971). These become dense spheroids concentratable to 5-10% hydroxide. Residual zinc in the effluent is less than 1 mg/l, independent of the zinc concentration in the feed (Chamberlain and Anderson 1971). The zinc is reused in a rayon plant. Other processes obtain improved precipitates of zinc carbonate (Courtaulds Ltd.) and sometimes use inert nuclei. Others reduce the zinc by adsorption and precipitation in activated sludge systems (Offhaus 1968) if the waste sludge is removed without additional digestion.

For the process in Figure 8.3.6 the operating and maintenance costs for recovery of the soluble waste zinc depend on the sulfuric acid-zinc sulfate ratio in the waste and on the amount of zinc recovered daily. When recovering 2000 lb Zn daily from a waste with a ratio of 5 to 6, the operating and maintenance costs are 12.5-14.0 cents/lb of Zn. The cost of purchased zinc oxide is 15.6 cents/lb equivalent zinc.

—E.G. Kominek, I.M. Abrams, S.E. Smith, E.C. Bingham, L.J. Bollyky, A.F. McClure, Jr., R..D. Buchanan, W.C. Gardiner, G.J. Crits, L.W. Canter, R.A. Conway, D.M. Rock

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