Metallurgy of Key Minerals - Thanatia: The Destiny of the Earth's Mineral Resources - page 226

chlorine gas produced in the anode is recycled for the preparation of the magnesium
chloride feed whilst the metal is removed from the cells and protected with sulphur
hexafluoride so to prevent oxidation during the casting process (IPPC, 2009)
Significant emissions involved in magnesium production are dust from dolomite
and the calcination of magnesium oxide, chlorine and HCl which form part of the
cell gases and the cell room ventilation. SF 6 used in the casting operation to prevent
re-oxidation of the molten magnesium is also a problem 23 . Likewise, dioxines and
furans are generated in the chlorination operation whilst carbon dioxide is produced
both in the chlorinator and during electricity production. Due to the high toxicity
of the off gases expelled by the chlorinator furnaces, they must all be treated in a
series of wet scrubbers and wet electrostatic precipitators before incineration. The
contaminated water also requires attention in a water treatment plant (IPPC, 2009).
Hancock (1984) and Yoshiki-Gravelsins et al. (1993) compiled data on the total
energy requirement to produce magnesium from seawater. The former reports a
figure of 467.74 GJ/t of magnesium whilst the latter states a slightly reduced one
at 406.8 GJ/t. The current power consumption ranges from 10 to 20 kWh/kg Mg,
whereas the theoretical minimum is 6.8 kWh/kg Mg 24 . The main ine ciencies
of magnesium production stem from the heat losses associated with the need to
maintain the bath temperature and the recombination of magnesium and chlorine
in the electrolyte. The IPPC (2009) shortens this range to 13-14 kWh/kg Mg. In
contrast, the production of calcium metal by electrolysis, requires about 33 - 55
kWh/kg Ca. The IPPC figure does not however account for the energy needed for
the preparation of the necessary raw materials.
8.13 Rare Earth Metals
Extractive metallurgy of rare earths elements (REE) is perhaps the most complex
of any metal group. They include the seventeen metals scandium, yttrium and
the lanthanides: lanthanum, cerium, praseodymium, neodymium, promethium, sa-
marium, europium, gadolinium, terbium, dysprosium, holmium, erbium , thulium,
ytterbium and lutetium.
Since rare earths applications and subsequently their production have recently
boomed, their metallurgy is far from optimised, neither in terms of energy con-
sumption nor environmental impact. The reactivity of rare earths is high and as
they naturally occur simultaneously, costly separation processes must be employed
for isolating each and every one of them as a commodity. Therefore, in the authors'
opinion, ground-breaking optimisations cannot be expected in the near future. Per-
haps the best source for understanding the extractive and metallurgical intricacies
of REE is that of Gupta and Krishnamurthy (2005).
23 The European Council and Parliament prohibits the use of SF 6 in magnesium die-casting as of
1st January 2008, except where the quantity involved is below 850 kg per year.
24 See http : ==www:magnesium:com=. Accessed Nov. 2011.