(MERAF) for the Base Metals Smelting Sector - CCME

and fume removal, and cooling, then passed through wet electrostatic
precipitators for mist removal. Mercury Removal is the final cleaning step before
the gas is processed for the recovery of sulphur dioxide.
Approximately 95 percent of the sulphur dioxide is recovered from the cleaned
gas in the Acid Plant which has three parallel lines. Some of the marketable
sulphuric acid product is used to make ammonium sulphate fertilizer.
Residual sulphur dioxide and acid mist remaining in the Acid Plant tailgas is
removed by ammonia scrubbing followed by a mist eliminator. The ammonia
scrubbing system produces an ammonium bisulphite product that is acidified with
sulphuric acid to produce an ammonium sulphate solution and a 100% SO 2 gas
stream. Both are subsequently converted to marketable products - crystalline
ammonium sulphate and liquid SO 2 .
The calcine from Roasting is cooled and then comminuted in the Calcine
Grinding step. The ground calcine is leached in sulphuric acid solutions to
produce a zinc sulphate solution and a residue. The residue is separated from
the solution in the Acid Thickening and Filtration areas, and then recycled to the
Smelting and Reduction area in the Lead Plant.
Zinc oxide fume, produced from the smelter Slag Fuming process, is also
leached in sulphuric acid to produce a zinc sulphate solution and a residue. The
residue is filtered, washed and recycled back to the Kivcet furnace. A solvent
extraction process integrated in the zinc oxide leaching circuit is used for the
production of indium and germanium products. (These operations are not shown
on the flowsheet.)
Zinc sulphate solutions produced from Oxygen Pressure Leaching and oxide
fume leaching are added to Acid Leaching, which is the first step in leaching the
roaster calcine. Acid Leaching is followed by Neutral Leaching where fresh
calcine is added to the acidic zinc sulphate solution to utilize some of the
solution’s acid. Neutral Thickening separates the solids into a slurry which is
recycled, while the overflow is pumped from the leaching area to Purification.
In Purification, sulphuric acid and zinc dust are used in various stages to
selectively remove impurities and by-products. Cadmium bars, extrusions and
balls are made for sale, thallium is produced for sale, and gypsum and
copper/cobalt/nickel cake are reverted to the Lead Plant.
The Electrowinning process utilize electrolysis to remove zinc from the purified
solution and deposit it on aluminum cathodes. The pure zinc is subsequently
stripped from the aluminum starter sheet, melted, alloyed and cast into various
shapes for market.
The Weak Acid Effluent from the scrubbing of the zinc roaster gas, other Zinc
Plant aqueous process bleeds, Lead Plant effluents and site run-off water are
treated in the Effluent Treatment Plant. Quicklime is added to precipitate a heavy
metals and gypsum sludge. The sludge is dewatered and the resulting residue is
reverted to the Kivcet furnace in the Lead Plant. Treated effluent is discharged to
the Columbia River as are other effluents via two other outfalls.
50