Rare Earth Element Mining and Processing
Part
1: Geology of REEs
Rare Earth Elements (REEs)
do not occur as native elemental metals in nature, only as part of a
host mineral. Due to this, the recovery of Rare Earth Minerals
(REMs) must be done using a complex processing method to chemically
break down the minerals containing the REEs.
While there are more than
200 known REE-bearing minerals, only three are generally considered
ores economically feasible for extraction: bastnasite, xenotime, and
monazite:
·
Bastnasite, the most abundant among the three REE mineral ores, is a
carbonate mineral found mainly enriched in Light Rare Earth Elemts
(LREEs) (e.g., cerium, lanthanum, and yttrium). Bastnasite is found
in vein deposits, contact metamorphic zones, and pegmatites. It
forms in carbonate-silicate rocks occurring with and related to
alkaline intrusions (e.g., Mountain Pass mine).
·
The two phosphate minerals, xenotime and monazite, can occur
together, but crystallize in different temperatures and pressures
from an igneous environment. While these minerals can contain any of
the REEs (i.e., Heavy Rare Earth Elements (HREEs) or LREEs),
enrichment of specific REEs is variable and a function of the
temperature and pressure in which they formed. Monazite commonly
occurs in placer deposits; xenotime can occur along with monazite,
but generally occurs as a more minor constituent of these types of
deposits. Deposits of phosphate rare earth ores provide the
opportunity to produce co-products of phosphates and REEs. Thorium
and uranium may also be found and produced as a co-product, or may
represent a significant management challenge. A further description
of these two minerals follows:
o
Monazite is generally enriched with the LREEs cerium, lanthanum, and
neodymium, but can also contain HREEs, particularly yttrium. The
predominance of LREEs is due to the lower crystallization
temperature and pressures of this mineral; however, it typically
contains more HREEs than bastnasite ore deposits. It occurs in
acidic igneous rocks (primarily pegmatites), metamorphic rocks, and
some vein deposits. Monazite is resistant to weathering and occurs
in many placer deposits as the host rocks are eroded. Thorium may
also be associated with monazite in various amounts.
o
Xenotime crystallizes under higher temperatures and pressures;
therefore, its crystalline structure more readily accommodates a
higher ratio of HREEs (terbium through lutetium, and yttrium) than
is commonly found in monazite. It is primarily a yttrium phosphate
mineral and occurs as a minor constituent of granitic and gneissic
rocks. Although not always present in significant quantities,
uranium and thorium can also occur as constituents of xenotime.
·
There
are two other important REE-containing minerals:
o
Euxenite which contains yttrium, erbium, and
cerium. It is found mostly in placer deposits in Idaho, and occurs
as a tantaloniobates (e.g., minerals where Ta and Nb form the
compound) of titanium, rare earths, thorium, and uranium.
o
Allanite is an epidote mineral and contains cerium, lanthanum, and
yttrium. It occurs in igneous, metamorphic, and hydrothermal
environments and is disseminated in pegmatite or occurs in vein
deposits.
These five minerals are considered to represent the principal
occurrences and the potentially more significant REE reserves in the
United States. However, many other minerals containing REEs do
occur, and deposits of these minerals could be found in the United
States and prove to be viable for mining. It is also not uncommon
for REEs to be produced as a coproduct or byproduct of other mineral
production.
Mineralogy
Rare-earth minerals occur
chiefly in association with highly alkaline (pH Basic) volcanic
(igneous) intrusive rocks (plutons) and in placers derived from
them. The rare-earth elements
may partially or wholly replace calcium in minerals such as fluorite
and apatite.
Monazite contains rare
earths of the cerium and lanthanum subgroups, plus as much as 30%
thorium and minor yttrium. It is a yellowish to reddish brown
monoclinic mineral having both hardness and a specific gravity of 5.
It occurs in commercial concentrations in beach and stream placers
and in lesser amounts in veins. It also occurs as accessory minerals
in igneous and metamorphic rocks.
Bastnaesite contains as
much as 75% rare-earth oxides of the cerium subgroup. It is a
light-yellow to brown hexagonal mineral having a hardness of 4.5 and
a specific gravity of 5. It occurs chiefly in carbonatite plutons
and subordinately in veins, pegmatites, and skams.
Xenotime is an
yttrium-subgroup phosphate that occurs in igneous and metamorphic
rocks, pegmatites, and placers. It is a pale yellow to
brownish-green tetragonal mineral having both a hardness and
specific gravity of 4.5. Rare-earth elements replace part of the
calcium in some apatite, particularly in some carbonatite plugs,
alkalic magnetite deposits, and marine collophane deposits.
Cerite is a brown to gray,
calcium, cerium subgroup hydroxyl silicate, having a hardness of 5.5
and a specific gravity of 4.9. It occurs in carbonatites,
pegmatites, and skarns. Gadolinite is a ferrous, beryllium,
yttrium-subgroup orthosilicate. It is a brown to black monoclinic
mineral with a hardness of 6.7 and a specific gravity of 4.4, which
occurs chiefly in pegmatites.
Multiple-oxide minerals
occur chiefly as brown to black, heavy (4 to 5.7), hard (4.5 to
6.5), radioactive, metamict minerals in pegmatites, alkalic igneous
rocks, and related veins.
and placers. The five commercial
minerals are niobate tantalate-titanates. Samarskite, however, lacks
titanium, while brannerite contains titanium only. All contain
uranium, and all but betafite contain thorium and yttrium-subgroup
rare earths. Betafite, brannerite, and euxenitc contain calcium,
while brannerite and samarskite contain iron. Euxenite and
samarskite contain cerium-subgroup rare earths; fergusonite and
samarskite contain erbium.
Classification
of Deposits
Rare-earth mineral
deposits have an stong relation to alkalic igneous rocks, especially
carbonatites. Such ores are found in association with a late
carbonatite pluton and in the veins that fill the fractures that
accompanied its emplacement. Carbonatite
stocks represent excellent exploration targets for large deposits of
rare-earth ores in economic concentrations.
Small quantities of exotic
rare-earth minerals occur in igneous pegmatites formed from residual
fluids that were derived from nearly complete crystallization of
presumably alkalic or subalkalic magmas. Such pegmatitic occurrences
are of manly academic interest, unless labor costs are low enough to
permit them to be economic.
Rare-earth elements also
occur both as discrete minerals and in apatite, in association with
high-temperature, metamorphic, nontitaniferous magnetite deposits.
Rare-earth concentrates are logical coproducts derived from the
beneficiation of iron ores of this type. Less commonly, rare-earth
minerals such as allanite can occur in economic concentrations in
skarns. Economic amounts
of monazite, bastnaesite, and xenotime have been found in veins in a
few places. .
Tertiary and Recent beach
placers in Brazil, India, Australia, and the United States are major
sources of monazite recovered as a coproduct from the mining of
magnetite, ilmenite, and rutile sands. Euxenite and brannerite have
been mined from recent alluvial placers in south-central Idaho.
Yttrium occurs in certain
marine phosphatic shales such as the Phosphoria Formation of Permian
age in Idaho. Although the apatite in these rocks contains only a
small amount (as much as 1000 ppm) of yttrium, it might be feasible
to extract an yttrium concentrate as a byproduct during the
beneficiation of the phosphate rock.
Carbonatite plutons and
ancient, as well as modern, placers should continue to be the
principal sources of rare-earth ores.
Part
2: Mining & Processing
Mining Methods
The technology for mining
monazite beach placers is similar to that employed for diamonds,
gold, or cassiterite. Offshore operations use floating dredges
having either suction or bucket elevators, with capacities up to
1200 tph of sand. Slurry
is carried hydraulically from the dredges to the beneficiation
plant.
Subaerial deposit's are
worked by draglines or power shovels. However, in places where
scattered pockets of ore must be mined selectively, bulldozers and
scrapers are used, and the ore is trucked to the concentrator.
In underdeveloped nations
where capital is scarce and labor cheap and abundant, primitive
methods dating back to Jason are used, in which hand-dug pits and
simple sluice boxes recover black-sand concentrates.
Conventional open-pit
mining methods are employed in the bastnaesite deposit at Mountain
Pass, Calif. The ore is drilled, blasted, and loaded into trucks by
power shovels, then hauled to the mill.
Milling
Techniques
Pure monazite contains
approximately 70% rare-earth oxides. Standard acceptable grades for
monazite-sand concentrates are 55. 60, and 66% rare-earth oxides.
Primary beneficiation of
beach sands is effected either on floating dredges or in land based
plants. After oversized particles have been screened, a black-sand
concentrate is recovered mechanically using jigs, sluice boxes.
shaking tables, and/or spiral concentrators. Through the use of
induced-roll electromagnetic separators and high-tension
electrostatic roll separators, a monazite concentrate, which may
represent only 1 % of the total black sands, can be recovered in the
magnetic nonconductive fraction. Flotation cells with oleic acid are
used to enrich Indian monazite sand concentrates.
Pure bastnaesite also
contains approximately 70% rare-earth oxides. The crude ore at
Mountain Pass, Calif., contains 7 to 10% rare-earth oxides.
Following primary and secondary crushing, the ore is passed through
a rod mill and a classifier. The slurry is then heated and passed
through flotation cells which depress the barite gangue and yield a
63% rare-earth concentrate. This, in turn, is then leached by
hydrochloric acid and countercurrent decantation to remove calcite,
thereby upgrading the concentrate to 72%. Finally, this concentrate
is calcined to remove carbon dioxide from the carbonates, yielding a
92% concentrate of rare earth oxides and fluorides.
Processing
Techniques
The caustic process and
the acid process are two common methods for treating monazite
concentrates. In the caustic process, monazite is digested in hot
sodium hydroxide, and filtered. The insoluble thorium and rare-earth
hydroxides are separated by treatment with weak hydrochloric acid,
which dissolves the rare-earth hydroxides and leaves solid thorium
hydride. The thorium hydride is then dissolved in nitric acid, and
thorium is recovered by solvent extraction.
In the acid process,
monazite is digested in hot sulfuric acid. Rare-earth sulfates are
dissolved and removed by filtration. If present (which is common)
thorium is then precipitated as a pyrosulfate, leaving the
rare-earth ions in solution. Next, the rare-earth elements are
precipitated as oxalates or as sodium-rare-earth sulfates. These, in
turn, can be roasted to form oxides, which are then dissolved in
nitric acid. The rare-earth elements are then separated from each
other by solvent extraction.
Four products may be
recovered from the treatment of bastnaesite concentrates: (1)
europium oxide, (2) a lanthanum-rich mixture of rare-earth metals,
(3) a heavy-subgroup mixture chiefly composed of samarium and
gadolinium, and (4) technical-grade cerium. The bastnaesite
concentrates are roasted, calcined and leached with hydrochloric
acid. Cerium oxide is filtered from the solution. The europium and
heavy rare-earth elements are separated from the solution by solvent
extraction. Then the
lanthanum-rich product is precipitated. After further solvent
extraction, europium sulfate is precipitated, leaving samarium and
gadolinium in solution.
Thorite concentrates are
digested in hot nitric acid, and filtered. The thorium is removed
from solution by solvent extraction and purified by countercurrent
solvent extraction. Such extraction yields a high-quality thorium
nitrate from which pure thorium oxide, tetrachloride, or
tetrafluoride may be produced.
Thorium metal may then be
made by metallothermic reduction or thermal decomposition of thorium
tetrachloride through reduction of thorium halide or oxide with
calcium, or by fused-salt electrolysis. Metallic yttrium is produced
by direct reduction of yttrium trifluoride.
Separation and
purification of the lanthanide elements is a major problem. Ion
exchange is an effective way to separate individual rare-earth
elements. EDTA (ethylene diamine tetraacetate) solution is the best
elutant for removing the lanthanide elements and yttrium from the
resin in that order. Solvent extraction can be used effectively to
separate rare-earth subgroups, and to isolate yttrium. Cerium and
europium can be separated from other lanthanide elements by valency
change reactions. Lanthanide sulfates can be subdivided by adding
cold aqueous solutions containing excess alkali sulfates to
alkali-rare-earth sulfates. The heavy lanthanides and yttrium remain
in solution. Fractional
recrystallization permits the selective separation of lanthanum,
praseodymium, and neodymium.
Rare-earth metals are
produced most successfully by electrolysis or by metallothermic
reduction of rare-earth halides. Mischmetal is produced by
electrolytic fusion of mixed anhydrous rare-earth chlorides. A pilot
plant for the production of mischmetal by electrolytic reduction of
sulfides and pure rare earths is being constructed.
