3.0 HYDROMET PLANT PROCESS DESCRIPTION
The Hydromet Plant will process mineral pre-concentrate from the Physical Upgrade (PUG)
Plant. It is designed with a single processing circuit and will be expanded in Year 9 to
accommodate the increased mine production. The Hydromet Plant is designed to process the
pre-concentrate production from the PUG Plant. The rare earth oxide production rate will vary
based on the pre-concentrate production rate and grade. The rare earth concentrate produced
by the Hydromet Plant will have an approximate rare earth equivalent content of 97% percent
total rare earth oxide.
Attachment D provides a conceptual block process flow diagram of the Hydrometallurgical Plant
and plan view drawings of the layout of plant facilities. The Hydrometallurgical Plant consists of
two processes. The first part of the process focuses on direct acid digestion of minerals in
chloride media at moderate temperature (113 - 194
o
F). About 97-99 percent of rare earths are
digested and transferred into the pregnant leach solution (PLS). Subsequently, the rare earths
are selectively precipitated from the PLS by means of oxalate reagent. All the un-precipitated
metals, such as iron, manganese magnesium, calcium, aluminum, zinc, potassium, barium and
traces of uranium etc. remain in the barren PLS. At this point, about 95-99 percent of the rare
earths are selectively transferred to the oxalate precipitate solids together with the thorium. The
RE oxalates are subsequently recycled and stored separately. The acidic effluent solution or
barren PLS, full of base metals, is transferred to the storage tanks next to the distillation column.
Because the hydrometallurgical plant is designed to be a zero-effluent process, the barren PLS
is subjected to distillation or boiling to vaporize and capture the free acid and water leaving
behind a smaller residual liquor to be neutralized with an alkaline solution. The metals in the
residual liquor are precipitated as hydroxides, which will be co-mingled with leach residual solids
plus alkaline media to produce a paste for dry-stacking at the Tails Storage Facility (TSF). Some
of the base metals in this residual liquor may be recovered as a by-product using conventional
processing methods.
The recovered hydrochloric acid and water are reused in the hydrometallurgical process mainly
at the leach and dewatering stages.
The rare earth oxalate powder (with 97 percent REE) contains thorium. Therefore, the second
part of the process is designed specifically to remove thorium from the oxalates and discard it
via a licensed third party disposal facility. There are two main components in this part of the
process, namely; 1) selective precipitation of thorium and 2) final precipitation of all the rare
earths as hydroxides.
However, before thorium impurities are removed from the RE-oxalate solids, it is necessary to
convert these RE oxalates to the rare earth carbonate form through metathesis (proprietary).
The rare earth carbonates are dissolved in nitric acid to facilitate downstream thorium
precipitation. The thorium-free nitrate solution is transferred to another precipitation circuit
where the rare earths are finally recovered as hydroxides and then calcined (or dried) to
produce the final REO powder concentrate (>99 percent pure). The filtrate solution at this point,
contains ammonium nitrate which may be recovered as a valuable by-product and the water
fraction is recycled back to alkaline and acid preparation steps.
The radionuclides are efficiently extracted from the initial feed ore and transferred into the PLS.
Subsequently, thorium is sequestered from the PLS together with REEs and transferred to
oxalate solids in a proprietary process. The minor amount of uranium is not transferred to the
oxalates but is rejected and remains in the barren PLS to be neutralized together with all base
metal impurities.
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