The Exergy Replacement Costs of Mineral Wealth - Thanatia: The Destiny of the Earth's Mineral Resources - page 353

mining technique (either underground or open-pit), has a particular effect on the
energy consumption due to a variety of different factors such as ore grade, grind
size, nature, depth and processing route.
Such factors have been analysed for different commodities including copper,
nickel, aluminium and iron through the life cycle assessment (Norgate and Jahan-
shahi, 2010; Norgate and Haque, 2010; Norgate and Jahanshahi, 2011).
Bearing in mind the above limitations and the energy data available for mining
processes (which is usually very scarce) the authors assume that the same technology
is applied for the entire range of concentration between the ore grade x m in the mine
and the pre-smelting grade, x r , than between the dispersed state found in Thanatia,
x c and x m . For this reason, the average energy vs. ore grade trends for different
minerals is analysed and the corresponding unit exergy cost values are calculated
and extrapolated to ore grades corresponding to those present in Thanatia.
Summarising, the first step in obtaining the unit exergy cost requires knowledge
of their real energy consumptions in mining and concentrating processes (going
from x m to x r ) as a function of the ore grade (x m ). Such information can be
obtained from data published in the literature. At the same time, the theoretical
exergy of the same process is calculated as the difference in concentration exergy
(Eq. (9.30)) when x = x m and x = x r . Finally, the unit exergy costs are calculated
with Eq. (12.4) in function of the ore grade. The latter can be extrapolated to
obtain the unit costs at the crepuscular grade x c , which will eventually serve for
the calculation of exergy replacement costs of the mineral wealth on Earth with
Eq. (12.3). The values used for the crepuscular grade are x c obtained in Chap. 10.
Average values for x m have been taken from Cox and Singer (1992). The procedure
is depicted in Fig. 12.1.
Energy consumption values as a function of the ore grade are di cult to find.
Indeed very little studies compile such tendencies. Relevant material relating to this
issue are those of Mudd (2007b, 2010a, 2008) or Norgate and Jahanshahi (2010).
Chapman and Roberts (1983) proposed a general theoretical formula to describe
the tendencies of energy consumptions for metals mining through Eq. (12.6). These
authors estimate the energy consumption as a function of two components: 1) the
energy used in mining and concentrating the ores, which is inversely proportional
to the ore grade and 2) the energy used in smelting and refining.
F = F o =x m + F s (12.6)
Where F o is the energy used per tonne of ore, x m the ore grade, and F s represents
the energy used in the smelting and refining stage. In this topic the authors only
focus on the first term of the equation, given that only the mining and concentration
energies for exergy replacement cost calculations are of interest. Chapman and
Roberts (1983) differentiated between the energy used in mining (F m ) from that
used in concentrating (F c ) in F o . For this reason, it is important to include the
quantity of material handled by each operation through the recovery e ciency of a
metallurgical process, which is defined as the ratio of metal contained in the selected