The Rare Earth Elements (Lanthanum,
Cerium, Prasedymium, Neodymium, Samarium, Europium, Gadolinium, Terbium, Dysprosium,
Holmium, Erbium, Thulium, Ytterbium, Lutetium and Yttrium) are of vital
importance in many modern high-tec industries. Unfortunately, as their name
suggests they are extremely rare; present at extremely low levels in many soils
and rock formations around the world, but seldom at concentrations high enough
to make extraction viable. At the moment around 85% of the world’s Rare Earth
Elements are produced from two areas in southern China. Several other recently
deposits have been found to also contain high levels of uranium and thorium,
which creates severe environmental problems to be overcome and some theorists
have suggested that Rare Earth Elements could be extracted from the sea floor,
where Rare Earth Elements eroded from deposits on land should in theory become
concentrated over time, but as yet the technology to extract them from such an
environment does not exist, and sites looked at have not yielded particularly
high levels anyway.
In a paper published in the
journal Gondwana Research on 5 November 2014, Poul Emsbo of the United States Geological Survey, Patrick McLaughlin of the Wisconsin Geological and Natural History Survey at the University of Wisconsin-Extension and George Breit, Edward du Bray
and Alan Koenig, also of the United States Geological Survey, examine the
presence of Rare Earth Elements in phosphorites (phosphate deposits) at a
number of sites around the United States.
Phosphorites are formed in
shallow marine settings by the concentration of phosphates from land washed out
to sea over time, exactly the same scenario that has been suggested for the concentration
of Rare Earth Elements. Thus if it is in theory possible to extract Rare Earth
Elements from the sea floor, then it also ought to be possible to extract them
from these ancient marine sediments, without the bother of finding ways to mine
on the sea floor.
Emsbo et al. looked at 23 phosphorite deposits from locations across the
US, ranging from Ordovician to Pliocene in age. All of the deposits are
laterally extensive in range (i.e. found over a wide area) and already being
commercially targeted for their phosphates.
Localities and stratigraphic age of studied phosphate deposits. Ages
and colour coding in the stratigraphic chart are adapted from the International Commission on Stratigraphy. (a) Temporal distribution of major (i.e.,mined) and
minor phosphorites in the United States. (b) Spatial distribution of phosphorites
sampled by this study. (c) Distribution of major phosphorite occurrences across
the United States. Emsbo et al. (2015).
The levels of Rare Earth
Elements, and the proportion of each element within the sample, proved to be
roughly similar in phosphorites of similar ages from different areas,
suggesting that the level of Rare Earth Elements incorporated in sediments
depended on the water chemistry at the time the sediment was laid down (the
oceans are interconnected and their chemistry remains essentially the same
across the globe, but is known to have varied over geological time). Pliocene
and other more recent deposits were found to have similar levels of Rare Earth
Elements to modern marine deposits, but the levels rose sharply in Miocene
deposits, reaching levels similar to those found in commercially worked
deposits in China, and remained high in Mesozoic and later Palaeozoic deposits.
Deposits from the Late Ordovician and Mississippian were further highly
enriched in Heavy Rare Earth Elements (Europium, Gadolinium, Terbium, Dysprosium,
Holmium, Erbium, Thulium, Ytterbium, Lutetium and Yttrium), which are
particularly sought after in industry.
The discovery of high levels of
Rare Earth Elements in American phosphorite deposits has important economic
implications, particularly in the Miocene deposits, which are already heavily
worked (around 80% of all phosphorus extracted in the US is recovered from
Miocene-Pliocene sediments). Rare Earth Elements are generally recovered from
sediments by acid leaching (i.e. soaking the ore in acid for long periods to
dissolve the desired elements, then recovering these from the resultant
sludge). This is an environmentally challenging procedure, but importantly, is
also the method used to recover phosphorus.
This means that many plants in
the US are currently already extracting Rare Earth Elements from phosphorites,
but not then recovering it from the sludge (the easier and less destructive
part of the process). Emsbo et al.
show that for many Miocene deposits the value of Rare Earth Elements extracted
could be similar to that of the phosphorous, doubling the value of the strata.
The raised levels of Heavy Rare
Earth elements seen in some deposits are also interesting, not just because of
the value of these elements, but because very high levels coincide with
extinction events in the biological record. Emsbo et al. suggest that these are not so much evidence of extra
elements entering the oceans at the time (as with the iridium anomaly at the
end of the Cretaceous, which has been used as evidence of an impact by an
iridium-rich meteorite), but rather of prevailing anoxic conditions in the oceans
at these times, leading in turn to a drop in oyhyhydroxides in the ocean –
chemicals which form bonds with Rare Earth Elements and keep them in the water
column, leading to large amounts of Rare Earth Elements settling out of the
oceans (with heavier elements settling faster, so that more were deposited
before the oceans recovered).
See also…
The mining company Bosveld Phosphates is facing criminal charges in South Africa after highly acidic...
The Sea Snake Palaeophis maghrebianus was first described by Camille Arambourg from the early Eocene phosphate beds of Morocco in 1952, though like many fossil Snakes it has been known only from...
On Saturday 26 November 2011, at about 3.30 in the afternoon, a major landslide occurred at a rare earth element mine in Cangwu County in the Guangxi...
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