Page 172 - Materials Chemistry, Second Edition

159
3.1. Mining and Processing of 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 aluminosili-
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 expect, the yield for
this process is quite low; ores typically possess less than 1% of the desired metal!
ð1Þ K 2 Al 2 Si 6 O 16 þH 2 Oþ2CO 2 ! Al 2 Si 2 O 7 þ2KHCO 3 þ4SiO 2
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 4 6 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), apatite (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