The process involves a two-stage precipitation, with the second stage under pH control using sodium hydroxide.
Zinc precipitates onto zinc hydroxide crystals.
The sludge is settled, converted to zinc sulfate using sulfuric acid, and is recycled. This paper discusses a process for precipitating a dense sludge of high zinc assay resulting from rayon yarn manufacturing. Zinc is used as a regeneration retardant in the acid spinning bath of the process. Since it is not consumed in any of the viscose reactions, the zinc is lost through dragout by the filaments to the subsequent wash streams, filter backwashing splashes, leaks and equipment washing. The process used at the American Enka facilities is a two-stage precipitation with the second stage taking place under careful pH control using sodium hydroxide.