Topics Covered
Introduction
Metal Extraction and
Refinement
Lead Extraction from Galena
About ASM International
Introduction
Most metallic elements occur naturally as minerals or ores, which
are compounds that result from a reaction between metallic and
nonmetallic atoms. Extractive metallurgy is the science and study of
the processes used to separate and concentrate metals from their raw
material compounds.
Common native and mineral occurrences of metals are listed in Table
1. Most copper, zinc, and lead ores are sulfides, while iron and
aluminum are generally found in oxide ores.
Minerals are frequently brightly colored, iridescent, and
attractive. Because of such attractiveness, ancient man (and modern
miners as well) collected mineral samples. For example, a typical
copper-containing sulfide mineral is bornite, which contains copper
(Cu), iron (Fe), and sulfur (S) in the ratio of five copper atoms to
one iron atom to four sulfur atoms, designated as Cu5FeS4. This ore is
reddish purple when freshly broken and becomes iridescent blue or
blue-green when exposed to the atmosphere. This change in color
(tarnishing, due to atmospheric exposure) results from the chemical
reaction of the ore with oxygen in the atmosphere. Chalcopyrite is a
yellow copper sulfide ore that also contains iron, while covellite is a
deep blue copper ore that contains only copper and sulfur.
Extraction of metals from their respective ores requires work or
energy to separate the metal from the sulfur or oxygen. This energy can
be supplied in the form of heat, and the first extractions were
probably accidental: A fire got out of hand and mineral samples that
had been collected for their beauty were heated, resulting in copper
metal; a fire was built near or against a copper sulfide mineral
deposit and copper was produced; or it is even possible that during an
act of worship an ancient man intentionally cast bright stones into a
fire and subsequently found that the “gods” had turned the stones into
copper metal.
Table 1. Common Native Materials and Mineral Occurrences
Type of Ore
|
Elements and Compounds
|
Carbonates
|
CaCO3, CaCO3 ⋅ MgCO3, MgCO3,
FeCO3, PbCO3, BaCO3, SrCO3, ZnCO3, MnCO3, CuCO3 ⋅ Cu(OH)2, 2CuCO3 ⋅
Cu(OH)2, K2CO3, (BiO)2CO3 ⋅ H2O
|
Fluorides
|
CaF2
|
Halides
|
NaCl, KCl, AgCl, KCl ⋅ MgCl2 ⋅
6H2O, NaCl and MgCl2 in sea water
|
Native Metals
|
Cu, Ag, Au, As,
Sb, Bi, Pt (Os, Ir, Pd), Mn (nodules on ocean floor)
|
Oxides
|
Al2O3, Fe2O3, Fe3O4, SnO2, MnO2,
TiO2, FeO ⋅ Cr2O3, FeO ⋅ WO3, Cu2O, ZnO, ThO2, Bi2O3, (Fe, Mn) (Nb,
Ta)2O6
|
Phosphates
|
LiF ⋅ AlPO4,
Th3(PO4)4 ⋅ X (Re)(a) PO4
|
Silicates
|
Be3AlSi6O18, ZrSiO4, Sc2Si2O7,
NiSiO3 ⋅ X MgSiO3, ThSiO4, LiAlSi2O6
|
Sulfates
|
BaSO4, SrSO4,
PbSO4, CaSO4 ⋅ 2H2O, CuSO4 ⋅ 2Cu(OH)2
|
Sulfides
|
Ag2S, Cu2S, CuS, PbS, ZnS, HgS,
FeS ⋅ CuS, FeS2, Sb2S3, Bi2S3, MoS2, NiS, CdS, FeAs2 ⋅ FeS2 (Fe, Ni)9
(S, Te)8, (Tl, Pb)S
|
Miscellaneous
|
(Fe, Mn) WO4,
CaWO4, (Co, Ni) As2, (Co, Fe) As2, NiSb, PtAs2, (Cu, Tl, Ag)2Se
|
 |
Created from content provided by ASM
International in the book "Metallurgy for the Non-Metallurgist, Second
Edition
Editor(s): Arthur C. Reardon"
|
Metal Extraction and
Refinement
Pyrometallurgy is the extraction of metal from ores by chemical
reaction at high temperatures in fuel-fired furnaces. Pyrometallurgy
may involve roasting and/or smelting for the extraction or refinements
of metals. Smelting refers to melting processes that separate metals in
fused form from nonmetallic materials. Other more modern methods of
extraction and refinement include hydrometallurgy and
electrometallurgy. Hydrometallurgy is the leaching or removal of the
metal from an ore body by passing a strongly acidic or alkaline
solution over the ore. Electrometallurgy involves the extraction of
metals from their ores by the application of large amounts of
electrical energy.
Experimentation with copper smelting began in the Chalcolithic
period at some time after the melting of copper was discovered. As the
supply of native copper and readily available copper sulfide ores was
depleted, the metallurgist was forced to turn to the extraction of
metals from other ore deposits to obtain a supply of copper. The
extraction of copper from the copper sulfide ores and from other ore
deposits required that the early metallurgist reverse a reaction that
had occurred between copper and another element found in the earth’s
crust. Melting of copper also resulted in the discovery of alloying and
the Bronze Age.
Although modern extraction processes are complex, the smelting
process can be very simple. For example, early North American settlers
smelted lead sulfide (galena) to produce lead balls for bullets used by
most colonial woodsmen. Galena is the most common lead ore, and the
lead was extracted by building a fire inside a hollow tree stump and
roasting the galena ore on the fire. This process resulted in the lead
sulfide (galena) reacting with oxygen in the air to form molten lead
and sulfur dioxide.
Lead Extraction from
Galena
Although extraction of lead from galena had been widely practiced
for many years, it was not widely recognized until relatively recent
times that silver is associated with lead extraction from galena. Lead
sulfide ore generally contains appreciable quantities of silver, and
because because of the potential for silver recovery, lead smelting is
currently practiced under very controlled conditions.
A schematic of a modern lead blast furnace for smelting galena ore is
illustrated in Figure 1. Basically, such processes are the key to any
pyrometallurgical extraction process. An ore is heated in the presence
of a selected and (in most cases) controlled environment. This
environment reacts with the ore to produce both the desired metal and
the other (frequently less desirable) products of the smelting
reaction. To illustrate the undesirable nature of some smelting
reaction products, note that the hydrogen sulfide gas produced by lead
smelting is poisonous. Lead smelting was successful because at
temperatures commonly reached in a wood fire, oxygen in the air reacted
with the lead sulfide to free the lead and produce both sulfur dioxide
and hydrogen sulfide gas.
Figure 1. Lead blast furnace, open-top type. This furnace
is termed a blast furnace because of the air blast used to smelt the
ore/coke mixture. Source: United States Smelting, Mixing, and Refining
Co.
A Blast furnace is one way of extracting Lead from Ore. However, there
are many techniques varying from the very simple and cheap to the
extremely complicated and expensive (Electrolysis - For the extraction
of Aluminum)
. The Science of Extractive Metallurgy covers the
physical and chemical processes used to produce metals from their
natural compounds. However, it does not necessarily cover the finished
product. Often, another step is required and the purer, more
concentrated metals will need further processing.
About ASM International
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This Article was created from Material provided by ASM
International in the book "Metallurgy
for the Non-Metallurgist, Second Edition" Edited by A.C. Reardon.
For More information please contact ASM
International