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Abstract
Horsehead Metal Products is a zinc refinery in Mooresboro, NC, treating crude Waelz oxide (WOX) to produce special high-grade zinc by a hydrometallurgical process comprising leaching, zinc solvent extraction/electrowinning, melting and casting and bleed treatment. Cadmium removal by zinc-dust cementation plays an essential role in the bleed treatment circuit to prevent cadmium buildup in the circuit. Silica, though present at a low level of approximately 0.3 percent in the WOX feed material, had a significant impact on the efficiency of cadmium cementation with zinc dust, most likely due to the passivation of zinc dust particles with silica gel. Solid/liquid separation efficiency by filter press was also affected. To identify the source of the silica issues, silica levels were mapped throughout the circuit, and adjustments, including using pregnant leach solution as the feed to cementation, and pH control in the WOX leaching and neutralization were introduced to mitigate the issues created by silica gel in the circuit.
Key words: Silica, Silicon, Silica gel, Cementation, Zinc, Polyethylene oxide, DADMAC, WOX, Waelz oxide
(ProQuest: ... denotes formulae omitted.)
Introduction
Located in Mooresboro, NC, Horsehead Metal Products Inc. is a hydrometallurgical zinc refinery processing Waelz oxide (WOX) to produce special high-grade (SHG) zinc. The flowsheet, known as the Modified Zincex process, consists of leaching, solvent extraction (SX), electrowinning (EW), melting and casting and bleed treatment (Fig. 1). Cadmium cementation, part of the bleed treatment process, provides an essential bleed of cadmium from the circuit, as metallic cement. The cadmium cementation circuit consists of agitated cementation reactors, a zinc dust addition system, filter presses, agitated purification (releach) reactors and a cadmium concentrate filter press.
At the refinery, WOX pulp is leached at a controlled pH to achieve the maximum zinc extraction, according to the main reaction:
... (1)
Other oxides present in WOX are leached through similar reactions, contributing to impurities in the pregnant leach solution (PLS). This solution then enters the SX circuit, where zinc is extracted from solution. The extraction and stripping of zinc by di2-(ethylhexyl) phosphoric acid (D2EHPA, symbolized as RH) is a reversible reaction:
... (2)
The loaded strip liquor, concentrated in zinc, is transferred to the electrowinning circuit, where the zinc cathode is produced, and the cathode is fed to the casting...