Calcine leaching

The W z oxide is delivered as a washed raw material that still contains excess chloride and fluoride it is processed in the Walz oxide treatment plant. The WSlz oxides are washed in an alkaline environment, and subsequently leached with spent electrolyte. After leaching, the filtrate is added to the calcine leaching stage. 

The spent electrolyte volumes used for calcine leaching and Walz oxide leaching are applied as derived in section 3.2 and 3.3. 

The furnace and boiler calcines are combined. Then the mixture is conveyed to a ball mill for grinding to a particle size of 50 % below 40 pm. The ground calcine, cyclone and electrostatic precipitator flue dust are combined and fed to calcine leaching. A typical calcine contains 0.2-0.3 percent sulfide and 1.5 percent sulfate, see Figure 2. 

The traditional oxide leach plant consisted of an acid leaching step, several neutral leaching steps and, until recently, a zinc dust purification step to remove cadmium, as shown in Figure 3. In the old scheme, most of the fume and various recycle streams were leached in spent acid in the acid leach step. The resulting acid slurry was thickened, and the lead-rich solid residues were filtered, repulped in water, combined with the residue slurry from the calcine leaching plant and pumped to the smelter. 

The final overflow solution was then advanced to a small zinc dust purification circuit to cement cadmium, and the purified electrolyte was then forwarded to the calcine leaching plant. 

The zinc dust piuification circuit in the oxide leaching plant was originally conunissioned as a purification system for an old oxide cellhouse, decommissioned about 20 years ago when the oxide and calcine leaching plant electrolytes were combined. However, the oxide purification circuit continued to operate with about one gram per liter addition of dry zinc dust, principally to cement cadmium in the oxide electrolyte prior to entering the calcine leaching plant. A portion of the purification residue was then forwarded directly to the cadmium plant. Some arsenic was also removed fix>m solution. 

In mid-1999, the oxide electrolyte purification circuit shown in Figure 3 was shut down on an interim basis. The arsine outbreaks were essentially eliminated and the calcine leaching plant performance was not detrimentally affected. Therefore, the oxide purification process has been permanently decommissioned. 

Cominco s calcine leaching circuit (also called sulphide leaching) combines the products from zinc pressure leaching and oxide leaching with e mainstream calcine leaching circuit. Process details have been well described in other papers (1-3). 

Several attempts were made from late 1998 to early 1999 to add this material to the roasters. In all trials, the settling rates in the calcine leaching plant neutral thickeners increased... 

Cominco uses a water atomization process to produce the zinc dust needed for electrolyte purification in the calcine leaching plant. The zinc dust is delivered as a slurry in water from the melting plant to the leaching plant. There the zinc dust is decanted, repulped in purified electrolyte and distributed to the points of addition in the purification circuit. The... 

D. W. Ashman, O. J. Delong and W. A. Jankola, "Silica Control During Zinc Calcine Leaching at Cominco s Trail Operations", World Zinc 93. I. G. Matthew, Ed., The Australasian Institute of Mining and Metallurgy, Victoria, Australia, 1993,217-226. 

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