Geology Reference
In-Depth Information
consumption of the global process of 1,582 GJ/t
12
. Consequently, these numbers
show that PGMs production is more energy and material intensive than either gold
or silver. If the world demand for PGMs is constantly increasing, sooner or later
they will need to be beneficiated from laterites (Proenza et al., 2010), leading to
a sharp increase in the figures provided by Mudd (2007b) and Glaister and Mudd
(2010). A fact which stresses the need to decrease consumer demand and increase
secondary production. The latter emits 30 to 180 less SO
2
per kg of PGM produced.
Increasing recycling rates for autocatalysts and electronic products, for example,
could also lead to environmental benefits with 30% less SO
2
emitted (Saurat, 2006).
8.10 Mercury
The main mercury sulphide ore is cinnabar. It has a volcanic origin and is deposited
from hot alkaline waters. Almaden mine in Spain has been the most important
mercury supplier in the world since ancient times. However, as it is now known
that mercury is highly toxic its market and production have declined. Currently,
mercury is primarily obtained as a byproduct from copper, lead and zinc production
and hence it is recovered in the sulphuric acid scrubbers where the gases from the
copper, lead and zinc ores are roasted. The level of recovery ranges from 0.02 to
0.8 kg/t of metal Cu, Zn or Pb (IPPC, 2009).
Mercury production consists of roasting crushed cinnabar at 600-700
o
C. This
oxidises the sulphides of base metals and separates evaporated mercury which is
afterwards collected in a condensing column in the form of liquid metal. Secondary
mercury can be also obtained from dental amalgams, silver oxide batteries, lamps
or from precious metal amalgams. In a retort raised at 700
o
C and under vacuum
pressure conditions, mercury evaporates after a typical residence time of 18 hours.
Mercury gases are later incinerated to remove organic impurities, condensed and
collected as liquid metal at room temperature.
The main environmental concern with mercury is the vacuum mercury itself.
However, it presents a lesser degree of harm than, for instance, the lead that was
freely dispersed via gasoline combustion in vehicles for decades. That said, the effect
of mercury poisoning was recorded by the Ancient Romans. The real danger stems
from organic compounds of mercury such as methyl mercury which is neurotoxic
and notoriously di
cult to eliminate. It is produced from the mercury catalysts
used in plastic manufacturing whereby it is discharged to wastewater. The best
way to reduce this issue is prevention - that is to say the avoidance of mercury in
chemical compounds such as paints or pesticides with dissipative uses.
12
According to the LCA on silver production obtained as byproduct of copper carried out by
Kennecott Utah Copper (2006c) 12% of the energy is used for smelting and 8% for refining.
The remaining energy is used for mining, 27%, concentration, 54% and 1% for tailings (see also
Fthenakis et al. (2007)).
Search WWH ::
Custom Search