Fahlman B.D. Materials Chemistry
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sulfur-based sedimentary deposits. Metamorphic rocks are those formations that
have changed composition and morphology over time through the influence of
temperature and pressure. Examples of these rocks include marble, slate, gneiss,
and yield similar metals as igneous formations.
Most metals are obtained through physi cal and chem ical modification of ores that
contain a sufficiently high conce ntration of the desired metal. Table 3.1 lists some
metals, regions of their primary occurrence, and some applications for the metallic
materials. Quite often, the processing steps used to isolate and purify metals from
Table 3.1. Natural Occurrence of Selected Metals and Materials-Based Applications
Metal Selected applications
a
Primary locations
b
Li Batteries, glasses, aerospace components US, Chile, China, Argentina
Na Dyes, pigments, lamps, photoelectric cells Chile, Peru, US
K Nuclear reactors, respiratory equipment Ger., Can., France, Spain
Be X-ray tubes, ship/aircraft navig. systems US, Africa, Brazil, Rs.
Mg Auto parts, coatings, photoengraving US, Can., S. Amer., Asia
Ca Lighter flints, lights, plastics stabilizer US, Can., S. Amer., Asia
Sr Glasses, ceramics, paints, TV tubes Mexico
Sc High-intensity lamps, nuclear reactors China, US, Aus., Ind.
Y Coatings, capacitors, superconductors, lasers China, US, Aus., Ind.
Ti Golf clubs, surgical implants, jet engines US, Aus., S.A., Norway
Zr Prosthetic devices, nuclear reactors Ind., US, Aus., S.A.
V Tools, ceramics, batteries, magnets US, Mex., Peru, Rs., S.A.
Nb Heat shields, electromagnets, spaceships Japan, Rs., Can., Brazil
Ta Aircraft turbines, rocket nozzles, HT vessels Aus., Brazil, Thailand, Can.
Cr Corrosion-resistant coatings, tools S.A., Turk., Rs., Cuba, Phil.
Mo Lubricants, lamp filaments, integ. circuits US, Aus., Sweden
W Filaments, coatings, missiles, tools, paints US, Asia, S. Amer., Rs.
Mn Steels, paints, batteries US, Can., France, Ger., Ind.
Fe Autos, tools, structural materials US, Can., China, Brazil, Rs.
Ru Coatings, superconductors, electrodes S.A., Rs., US
Os Fountain pen tips, pacemakers, forensics S.A., Rs., US
Co Magnets, batteries, recording media Can., N. Africa, Swe.
Rh Catalytic converters, coatings, crucibles S.A., Rs., US
Ir HT crucibles, spark plugs S.A., Rs., US
Ni Batteries, coatings, currency Can., Rs., Aus.
Pd Jewelry, dental crowns, surgical instruments S.A., Rs., US
Pt Jewelry, coatings, surgical implants, lighters S.A., Rs., US
Cu Conductive wire, pipes, currency Chile, US, Can.
Ag Jewelry, dental fillings, photography Mex., Peru, US, Can.
Au Jewelry, integ. circuits, heat shields, coatings S.A., US, Aus., Can.
Zn Coatings, pipes, highway guard rails US, Can., Aus., China
Cd Alloys, batteries, nuclear reactors Canada, Aus., Mexico
Al Auto parts, ceramics, coatings, paints Brazil, Jamaica, Asia
Sn Alloys, plating, flat glass Bolivia, Brazil, Malaysia
Pb HT vessels, pipes, sound abs. materials Aus., US, China, Peru
Ce Lighter flints, ceramic capacitors, magnets China, US, Aus., Ind.
Eu Television screens, nuclear reactors China, US, Aus., Ind.
Er Lasers, catalysts, phosphors China, US, Aus., Ind.
a
These selected applications may be for the pure metal or other compounds/alloys.
b
Can. ¼ Canada, Rs. ¼ Russia, Aus. ¼ Australia, Ind. ¼ India, S.A. ¼ South Africa.
158 3 Metals
ores have a significant impact on the overall microstructure, which will affect the
physical properties of the materials fabricated from them.
To obtain the ores in sufficient quantities needed to recover a desirable amount of
metal, either open pit or shaft mining is used depending on how accessible the ore is
located. We are all familiar with the picture of treacherous mine shafts that have
even been featured in movies such as Coal Miner’s Daughter . However, such “brute
force” is sometimes not required. For instance, to recover coal that is used in power
plants to generate electricity, there are often sufficient resources near the surface that
may be obtained using large cranes. Pit mining for metals such as copper, iron, and
precious metals is commonly used throughout the US and Canada.
Of course, once the ore is obtained from its deposit, the actual work of extracting
the desired metal has yet to be accomplished. In addition to metals, a variety of other
substances comprise natural minerals. Since aluminum and silicon are the most
prevalent elements in the Earth’s crust, most of the metals exist naturally as alumi-
nates, silicates,
[1]
or aluminosilicates. The most common minerals are feldspars
(comprising 60% of the Earth’s crust) and clays. These materials have been used
since ancient times for the production of materials such as pottery, brick, and china.
An example of a feldspar is K
2
Al
2
Si
6
O
16
, which corresponds to a mixture of
potassium superoxide, alumina, and silica (K
2
O·Al
2
O
3
·6SiO
2
). Upon contact with
water and carbon dioxide, a weathering reaction results in kaolinite, an aluminosi li-
cate clay (Eq. 1). However, in addition to these oxidized sources of metals, there are
substances such as alkaline carbonates, sulfates, phosphates, as well as organic matter
that need to be removed to yield the desired metal. As you would expec t, the yield for
this process is quite low; ores typically possess less than 1% of the desired metal!
K
2
Al
2
Si
6
O
16
þH
2
O þ2CO
2
!Al
2
Si
2
O
7
þ2 KHCO
3
þ4 SiO
2
ð1Þ
The most common mineral classes from which metals are extracted include:
i. Silicates – e.g., feldspars, quartz, olivines (Mg/Fe), pyroxenes (XY(Si, Al)
2
O
6
),
garnets (X
3
Y
2
(SiO
4
)
3
), and micas (X
2
Y
46
Z
8
O
20
(OH, F)
4
)
ii. Carbonates – e.g., calcite and aragonite (both CaSO
4
), dolomite (Mg/Ca),
siderite (Fe)
iii. Sulfates – e.g., anhydrite (CaSO
4
), celestine (SrSO
4
), barite, gypsum (hydrated
CaSO
4
), chromate, molybdate, selenate, sulfite, tellurite, tungstate
iv. Halides – e.g., fluorite (CaF
2
), halite (NaCl), sylvite (KCl), sal ammoniac
(NH
4
Cl)
v. Oxides – e.g., hematite (Fe
2
O
3
), magnetite (Fe
3
O
4
), chromite (Fe/Cr), spinel
(Mg/Al), ilmenite (Fe/Ti), rutile (TiO
2
)
vi. Sulfides – e.g., pyrite (FeS), chalcopyrite (Cu/Fe), pentlandite (Ni/Fe), galena
(PbS), sulfosalt (S/As), selenide/telluride/arsenide/antimonide minerals
vii. Phosphates – e.g.,AO
4
(A ¼ P, As, Sb, V), apat ite (Ca
5
(PO
4
)
3
(OH, F, Cl))
The variety of procedures that are used to obtain metals from their ores is known as
extractive metallurgy. Following mining of the ore from the deposit, a flotation process
is first used to separate the metal from the gangue. This consists of grinding the ore
into a powder and mixing it with water, oil, and surfactants (i.e., collectors) to yield
3.1. Mining and Processing of Metals 159
hydrophobic surface charges on the particulates. The most common collector molecules
are alkali salts of the O-alkyl dithiocarbonates (I) commonly known as xanthates, which
are widely used for the concentration and separation of metals from sulfide-based ores.
The mixture is vigorously stirred into a frothy mixture; the coated grains attach
themselves to air bubbles that float to the solution surface, where they are removed
through a skimming/filtration process. This technique is very useful in the separation of
lead and zinc components, often found together in natural sulfide deposits.
Once the bulk ore is concentrated from its many impurities, the metal is reduced
from its natural oxides. The most common method that is used is referred to as
smelting, where the ore is heated in the presence of a reducing agent such as a
carbonaceous material, or carbon monoxide (Eqs. 2 and 3, respectively). Although
carbon-based materials were used exclusively by early civilizations, modern extrac-
tive methods commonly employ other reducing agents such as Na, Mg, Ca, Al, or
alloys such as Zn/Mg. However, with the advent of biomass as an alternative fuel,
organic-based waste materials are now being used as an eff ective reducing agent.
Often, the ore is not pure following flotation; therefore, a flux such as limestone or
dolomite is added to remove the gangue as slag – a mixture of metal oxides (e.g.,
SiO
2
,Al
2
O
3
, CaO, MgO, etc.), sulfides and impurity metals.
M
x
O
y
þ yC
!
12001500
C
xMþ yCOð2Þ
M
x
O
y
þ yCO
!
15002000
C
xMþ yCO
2
ð3Þ
If the ore consists of metal sulfides, it must first be converted to the oxide through
reaction with oxygen at high temperature (Eq. 4):
2MSþ 3O
2
!
15002000
C
2MO þ 2SO
2
ð4Þ
The temperature required for tin and lead smelting is only on the order of 300–
400
C; hence, it is no surprise that early civilizations discovered these metals from
heating local miner als over campfires, which yields a temperature of ca. 900
C.
However, this temperature regime is not sufficient for copper smelting, which
requires temperatures in excess of 1,100
C. Consequently, copper metal was most
likely discovered serendipitously by using higher temperature coal-fired pottery
kilns that contained the copper-rich mineral malachite.
Let’s consider some tangible examples of metal extraction/refining, starting with
vanadium metal. There are approximately 50 types of minerals that will yield a
viable amount of V metal, such as vanadinite, patronite, bauxite, and carbon deposits
such as crude oil, coal and butim inous matter. The oxide obtained from these
deposits via flotation is first mixed with NaCl or Na
2
CO
3
at 900
C to yield sodium
orthovanadate, NaVO
3
. This salt is dissolved in water and acidified with sulfuric
acid to a pH between 2 and 3 to yield a red solid, which is hydrated V
2
O
5
.
[2]
160 3 Metals
This solid is then melted at 690
C in the presence of a reducing agent (either C or Ca
under an inert atmosphere) to yield pure vanadium metal.
Titanium metal is refined using the Kroll process, which is effective in separating
Ti from Zr, which is almost always present in significant concentrations in the
particular ore. A mineral such as rutile is mixed with coke and subjected to
1,000
C in a fluidized bed reactor to yield an impure form of titanium metal. This
is then reacted with chlorine gas at elevated temperatures to yield TiCl
4
, which is
separated from other volatile chlorides by fractional distillation. Magnesium metal
is then added to TiCl
4
at 800–850
C under argon gas to yield Ti metal, as a porous
metallic sponge. Aqueous HCl or liquid sodium leaching is then used to drive off the
MgCl
2
byproduct. To refine Zr metal rather than Ti, baddeleyite ore (ZrO
2
) is used,
which contains significant concentrations of Fe. In this process, FeC l
3
must be
separated from ZrCl
4
by fractional distillation, and 1,100
C is used instead of ca.
800
C for the magnesium reduction reaction above.
The above techniques that involve high-temperature processes are known
as pyrometallurgy. Another common technique involves the electrolytic reduction
of metal compounds, often referred to as hydrometallurgy or electrorefining, depend-
ing on whether the procedure is carried out before or after the metal has already been
separated from its ore, respectively. Hydrometallurgy consists of three steps:
(i) Leaching – a caustic or acidic solution is added to the ore to yield a concen-
trated solution containing the metal of interest. This may be done in situ (e.g.,
solution added directly to uranium deposits), or in heaps or vats of the ore. For
these latter methods, the crushed ore is placed over a storage pond and irrigated
with the leaching solution, known as a lixiviant. The solution collected follow-
ing slow gravity-controlled leaching is referred to as the pregnant solution.
Equations 5-9 illustrate some example lixiviants that are used to leach a variety
of ores. It should be noted that refractory ores such as ZnS require high
pressures to achieve effective leaching (Eq. 7).
ZnO (also CuO, NiO) þ H
2
SO
4
! ZnSO
4
þ H
2
Oð5Þ
ZnS þO
2
þ2H
2
SO
4
!
autoclave (>0.6 MPa)
2 ZnSO
4
þ 2H
2
O þ 2Sð6 Þ
Au (in ore) þ2O
3
þ 6H
þ
þ 4C‘
! AuC‘
4
þ 3=2O
2
þ 3H
2
Oð7Þ
Al
2
O
3
H
2
O ðor 2 AlOðOHÞÞ þ 2OH
!
D
2 ½A1O
2
þ 2H
2
O !
D
2 ½AlðOHÞ
4
ð8Þ
2½AlðOHÞ
4
ðaqÞ
!
D; CO
2
2AlðOHÞ
3ðsÞ
#ðor Al
2
O
3
3H
2
OÞ
þH
2
O þ CO
3
2
ð9Þ
Approximately 85% of the resulting hydrated alumina is converted into
aluminum metal, with the remaining converted to various forms of alumina
(Figure 3.1), for applications ranging from flame-retardants to polishing media
and sapphire production.
3.1. Mining and Processing of Metals 161
(ii) Solution Concentration – two methods are used to separate the metallic com-
pound from the many impurities generated during leaching: solvent extraction
and chemical precipitation (e.g., Eq. 10). Whereas the former is used to
concentration metals from dilute low-value leaching solutions, the latter is
used for more concentrated solutions.
NiSO
4
þ H
2
S
ðgÞ
! NiS
ðsÞ
#ð10Þ
Figure 3.1. Processing routes for the conversion of bauxite into alumina and aluminum metal.
Reproduced with permission from Cardarelli, F. Materials Handbook, Springer: New York, 2008.
Copyright 2008 Springer Science and Business Media.
162 3 Metals
(iii) Metal Recovery – the metal is recovered using electrolytic reduction of the
concentrated solution, which is known as electrowinning. This is suitable for
most metals, and is most commonly used for the alkali metals, rare earths, Pb,
Cu, Zn, Ni, Co, Au, Ag, Al, Cr, and Mn.
For the purification of sodium, a fused salt is used at high temperatures (Figure 3.2a).
As is often necessary for ionic salts, a solid solution is necessary to reduce the melting
point of the salt. The addition of calcium chloride effectively reduces the melting point
of the solution from 800
C (for NaCl alone) to 600
C. To circumvent the high melting
point for aluminum (2,030
C), anhydrous aluminum oxide (i.e.,alumina, m.p.
2,050
C) is dissolved in molten cryolite (Na
3
AlF
6
,m.p.1,009
C) and excess AlF
3
,
resulting in a more reasonable operating temperature (920–980
C) for electrolysis,
known as the Heroult-Hall process (Figure 3.2b).
It is estimated that iron constitutes 90% of all applications that involve metals.
Hence, it is not surprising that the purification and post-processing for iron is the
most widely practiced. The most primitive method that was used in the nineteenth
century to purify iron from its ore is called bloomery. This method used pure carbon
in the form of charcoal to reduce the metal (Eq. 2). In this process, the temperature is
Figure 3.2. Schematic of electrometallurgical processes used to purify metals. Shown is the (a) Downs cell
used to purify sodium metal and (b) Hall cell used to purify aluminum metal. Reproduced with permission
from Chemistry: The Central Science, 8th ed., Brown, LeMay, Bursten. Copyright 2002 Prentice-Hall.
3.1. Mining and Processing of Metals 163
not sufficient to completely melt the iron, so a spongy mass consisting of iron and
silicates are formed. Through heat/hammering treatments, the silicates mix into the
iron lattice, creating wrought iron. This form of iron was used exclusively by early
blacksmiths, since the heating of wrought iron yields a malleable, bendab le, and
extremely easy compound to work with.
Most modern applications for metallic iron are steel related, exploiting its high
hardness, ductility and tensile strength. Figure 3.3 shows a flowchart for the various
procedures that are used for modern ste elmaking. The first step uses a blast fur-
nace
[3]
that is comprised of a massive, refractory-lined steel column wherein
pelletized iron ore, charcoal, and calcium sources (from limestone and dolomite)
are poured into the top, and a large jet of pre-heated (ca. 1,050
C) air is blown in
from the bottom. As mixing of the components occurs at various temperature
regimes, the various oxides present in the ore are reduced to metallic iron. From
the coolest – hottest portions of the blast oven, corresponding to the highest – lowest
regions, respectively, the following oxides are reduced (Figure 3.4):
1. 500–600
C: Hematite (Fe
2
O
3
)
2. 600–900
C: Magnetite (Fe
3
O
4
)
3. 900–1,100
C: Wustite (FeO)
4. >1,100
C : FeO
0.5
Since iron ore is largely comprised of aluminosilicate minerals, a byproduct is also
formed within the blast furnace, known as slag (ca. 30–40 wt.% SiO
2
, 5–10 wt.%
Al
2
O
3
, 35–45 wt.% CaO, 5–15 wt.% MgO, and 5–10 wt.% CaS).
It should be noted that it takes 6–8 h for the native iron ore to descend toward the
bottom of the blast furnace, but only ca. 8 s for the pre-heated air to reach the top of
the furnace. Oftentimes, a fused solid known as sinter is also added to the blast
furnace, which is comprised of fine particulates of iron ore, coke, li mestone and
other steel plant waste materials that contain iron. The reducing agent within the
blast furnace (coke) is comprised of 90–93% carbon, and is formed by heating coal
to remove the volatile components such as oil and tar. The coke is ignited at the
bottom of the blast furnace immediately upon contact with the air blast; since there is
excess carbon in the furnace, the active reducing species is CO rather than CO
2
.
The molten iron collects at the bottom of the blast furnace and is cooled into a
form known as pig iron. This form of iron is comprised of ca. 93.5–95% Fe, 0.3–
0.9 T Si, 0.025–0.05% S, 0.55–0.75% Mn, 0.03–0.09% P, 0.0 2–0.06% Ti, and 4.1–
5% C. Due to the relatively high carbon concentration, this form of iron is too brittle
and hard for any structural applications. It should be noted that re-heating pig iron
with slag and hammering to rem ove most of the carbon will yield wrought iron.
However, it is often most useful to convert this form into high-strength steel.
While impurities such as silicon, calcium, aluminum, and magnesium were largely
removed in the slag, phosphorus and sulfur impurities remain behind, and are
dissolved into the molten iron.
The next step involved in steelmaking is reducing the carbon concentration of the
pig iron. The molten iron formed in the blast furnace is transported to the basic
oxygen furnace (BOF) via transfer ladles (Figure 3.5a). Lime (CaO) is added
164 3 Metals
Figure 3.3. Flowchart illustrating the various steps involved in steelmaking and processing. Reproduced
with permission from the American Iron and Steel Institute. May be found online at http://steel.org/
AM/Template.cfm?Section ¼ How_Steel_is_Made&CONTENTID¼4800&TEMPLATE¼/CM/Content
Display.cfm.
3.1. Mining and Processing of Metals 165
directly to the transfer ladle at 1,350–1,400
C (Figure 3.5b) in order to facilitate
removal of sulfur (Eq. 11):
CaO þ FeS ! CaS þ FeOð11Þ
The resultant slag is raked from the ladle and the remaining molten iron is charged
into the BOF along with steel scraps (Figure 3.4c ). A supersonic jet of oxygen is
used within the BOF to yield a temperature of 1,620–1,720
C. Under these extreme
oxidizing conditions, most of the remaining C, P, Si and Mn impurities are depleted
from the hot metal, resulting in a steel with a concentration of 0.03–0.07 wt.%
carbon. The entire oxidation process within the BOF lasts only 20–25 min
(Figure 3.6).
The next process of steelmaking consists of fine-tuning the composition of the
steel generated from the BOF. If low-carbon steel is desired, the molten steel is
degassed to yield a product concentration of <0.005 wt.% C (Figure 3.7). Alterna-
tively, desired dopants (to alter overall properties – see section 3.2) may be added
directly to the molten iron within the alloying ladle as a metal wire or powder.
Alloying is afforded by lowering arc furnace electrodes into the ladle; argon gas is
injected at the bottom of the ladle to prevent oxidation and induce stirring that
is required for homogenizat ion of the steel.
Once the desired composition of the steel has been achieved, the processes of bulk
forming may begin. Long blocks of steel (ca. 30
0
) are formed by slab casting,
whereby the molten steel is poured into a water-cooled copper mold via a tundish.
As shown in Figur e 3.8, a series of rollers slowly draw the solidified steel downward
Figure 3.4. Schematic of a blast iron. Shown are: 1 – incoming pre-heated air from the Cowper stove;
2 – melting zone; 3 – reduction zone of FeO; 4 – reduction zone of Fe
2
O
3
; 5 – pre-heating zone; 6 – feed
of iron ore, limestone, and coke; 7 – exhaust gases; 8 – column of ore, coke, and limestone; 9 – removal
of slag; 10 – tapping of molten pig iron; 11 – collection of waste gases. Image courtesy of Robert Blazek.
166 3 Metals
Figure 3.5. (a) Transfer of pig iron from the blast furnace to basic oxygen furnace (BOF) in transfer
ladles, (b) sulfur removal via CaO addition, and (c) charging the BOF with pig iron and steel scraps.
Images courtesy of Severstal Steel, Dearborn, MI.
3.1. Mining and Processing of Metals 167