Page 174 - Materials Chemistry, Second Edition

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3.1. Mining and Processing of 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, FeCl 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).

ð5Þ ZnO (also CuO, NiO) þ H 2 SO 4 ! ZnSO 4 þ H 2 O
autoclave (>0.6 MPa)
ZnS þ O 2 þ 2H 2 SO 4 ! 2 ZnSO 4
ð6Þ þ 2H 2 O þ 2S
þ

ð7Þ Au (in ore) þ 2O 3 þ 6H þ 4C‘ ! AuC‘ 4 þ 3=2O 2 þ 3H 2 O
D

Al 2 O 3 H 2 O ðor 2 AlOðOHÞÞ þ 2OH ! 2 ½A1O 2 Š
D
ð8Þ þ 2H 2 O ! 2 ½AlðOHÞ Š
4
D; CO 2

2½AlðOHÞ Š ! 2AlðOHÞ #ðor Al 2 O 3 3H 2 OÞ
4 ðaqÞ 3ðsÞ
2
ð9Þ þH 2 O þ CO 3
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.