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metallic lead, copper, antimony, arsenic, and noble metals. In some blast

fitmaces

the zinc

contained in the sinter remains in the slag. In others, zinc off-gas is condensed to liquid zinc metal. Noble metals are corrosion-resistant, metallic elements (e.g., the platinum group, gold, silver, and rhenium) that exchange electrons according to their position in the electrochemical series of metallic elements. Because of this preferential transfer of electrons, noble metals are largely dissolved in the lead product (bullion), with small amounts distributed to sulfide matte and slag. The copper contained in the sinter is captured in the bullion in the form of sulfide or metallic copper depending on the amount of sulfur present. Copper can report to the slag along with the zinc. Antimony and arsenic compounds are reduced to their respective metals,

which dissolve in the lead bullion. The products of the lead blast furnace are

Lead bullion, which must be further refined.

  • Slag that may contain large quantities of zinc that may be further processed.

o

Flue and

baghouse

dusts that may contain cadmium. Leaching may be used to separate

cadmium before dust is recycled to sintering. Off-gas, which may contain about 4% carbon monoxide. Matte, a sulfidic product of blast furnace smelting, forms only if the concentration of

sulfur is high.

Normally, subsequent reverberatory melting of lead bullion dross yields copper matte, lead metal, and slag. Speiss is formed when the concentration of arsenic, nickel, cobalt, and antimony is so high that these metals can no longer dissolve in the matte. Speiss is undesirable, because it dissolves large quantities of noble metals and is also expensive to refine.

Lead blast-furnace slag often contains sufficient zinc to make recovery profitable using fuming techniques. Zinc oxide in the slag is reduced to metallic zinc (vapor) by carbon, oxidized by secondary air, and exits in the furnace gases for recovery of zinc oxide dust.

Zinc process

ORNL engineers evaluated zinc-mining operations, as described below, compiled flow diagrams of unit operations [ and identified process residues with by-product recovery potential (Figure 3). The U.S consumes 18% of the total world’s zinc production while it

121

produces 11% of the world’s zinc output [

131.

The U.S. zinc commodity sector generates

metric tons per year of process residues. The content of zinc in the raw ore is used to determine how the ore will be processed. Five industrial ‘processes produce zinc-one

9,800,OOO

electrolytic and four pyrometallurgical. If zinc content in the ore is greater than zinc is produced using pyrometallurgy. If the zinc content of the ore is below

40%, 40%,

then then

electrolytic production of zinc may be more economical. Zinc produced electrolytically does not need to be refined. However, crude zinc produced by the pyrometallurgy contains Pb, up to 0.2% Fe, up to 0.3% Cd, and sometimes small amounts of As, depending upon the production method and the ore composition. Residues and slag from pyrometallurgical processes may contain 8-15% zinc, representing an opportunity for further recovery.

O-7-3%

Zinc ore

Zinc is a chalcophilic element, like copper and lead, occurring as the sulfide. The most important zinc mineral today is zinc blende, containing 67.09% Zn and 32.91% S on a

theoretical basis. The most important impurity in

ZnS

is

FeS,

and concentrations can vary

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