The Foundations of Chemistry

METALLURGIES OF SPECIFIC METALS

The metallurgies of Mg, Al, Fe, Cu, and Au will be discussed as specific examples. The

order of increasing standard reduction potentials of these metals indicates the order of

increasing ease of reduction to the free metals.

Standard Reduction

Reduction Half-Reaction Potential E^0 , Volts

Mg^2  2 e88nMg 2.37

Al^3   3 e88nAl 1.66

Fe^2   2 e88nFe 0.44

Cu^2  2 e88nCu 0.337

Au^3  3 e88nAu 1.50

MAGNESIUM

Magnesium occurs widely in carbonate ores, but most Mg comes from salt brines and

from the sea (Figure 22-5). Sea water is 0.13% Mg by mass. Because of its low density

(1.74 g/cm^3 ), Mg is used in lightweight structural alloys for such items as automobile and

aircraft parts.

Magnesium ions are precipitated as Mg(OH) 2 by addition of Ca(OH) 2 (slaked lime) to

sea water. The slaked lime is obtained by crushing oyster shells (CaCO 3 ), heating them

to produce lime (CaO), and then adding a limited amount of water (slaking).

22-5

908 CHAPTER 22: Metals I: Metallurgy

Mg and Al are active metals, Fe and
Cu are moderately active, and Au is
relatively inactive.

CaCO 3 →CaO + CO 2 CaO + H 2 O→Ca(OH) 2

Lime kilns Slaker

MgCl 2 + Ca(OH) 2 →

Mg(OH) 2 + CaCl 2

Settling

tank

Mg(OH) 2 + 2HCl→

MgCl 2 + 2H 2 O

Filter

Mg(OH) 2

HCl

Chlorine

gas

Dilute

MgCl 2

soln.

Evaporators

MgCl 2 soln.

Electrolytic cells

MgCl 2 →Mg + Cl 2

Mg

Dryers Conc.

Hydrochloric

acid plant

Oyster

shells

CaCO 3

Ocean water

INTAKE FLUME

Strainers

Figure 22-5 Schematic diagram of an industrial plant for the production of magnesium

from the Mg^2 ions in sea water.