The key to this zinc recovery process is a two-stage precipitation,26 with the second precipitation taking place under careful pH control, using sodium hydroxide in contact with circulating slurry of zinc hydroxide crystals. All of the zinc precipitates in the second step, most of the impurities in the first.
The elements of the process are as follows. Acid and alkaline wastestreams are collected in a neutralization tank. Here sufficient lime is added to raise the pH to 6.0. At this point, no zinc hydroxide will precipitate but a portion of the iron, calcium sulfate, and other impurities will form a light precipitate. With a coagulant aid, the mixture is sent to a clarifier where a clear overflow containing the dissolved zinc is obtained.
This clear overflow is contacted in a reactor with a circulating stream of previously precipitated sludge containing zinc hydroxide. The pH is raised subsequently to 9.5 to 10.0 with sodium hydroxide. The bulk of the zinc precipitates onto the existing crystals in the circulating slurry. At steady-state conditions, the withdrawal rate of the circulating slurry stream is made equivalent to that of the zinc being added. This dense sludge is then settled. The settled sludge of 4 to 7% zinc assay is converted back to zinc sulfate with sulfuric acid and sent back to the spinning bath. If desired, the sludge can be filtered or centrifuged to 18% solids before dissolving with acid.
The zinc content of the overflow water from the densator-reactor is set by the pH-solubility relationship of zinc in water and results in a zinc content of 0.5 to 1 mg/L at pH = 10. Once the precipitated zinc is removed from the wastewater, the pH can be readjusted to a lower value.
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