China
has the largest resource of rare earth elements(REE) in the world,
the reserve of REE in China takes up over 40% of the world reserve.
The resources of rare earth elements in China are mainly divided
into four types of deposits which consist of 95% total REE resources
of China. They are the Bayan
Obo REE−Fe−Nb Ore Deposit
in Inner Mongolia, the Mianning
REE Ore Deposit
in Sichuan Province, the Weishan
REE Ore Deposit in
Shandong Province and the Weathered
Crust Elution−Deposited
Rare Earth Ore (The Ion Adsorption
Type Rare Earth Ore) mainly
in Jianxi Province, but also distributed in Guandong, Fujian and
Guanxi Provinces.
The
Bayan Obo REE−Fe−Nb Ore Deposit
Mineralogy
The
Bayan Obo REE‐Nb‐Fe deposit is situated in Inner Mongolia on the
northern edge of the North China Kraton, 135 km northwest of Baotou
(110°E, 41°47′N). It is a giant polymetallic rare earth element
(REE)‐Fe‐Nb ore deposit of hydrothermal origin. It was first
discovered as a Fe deposit by Prof. Ding Daoheng in 1927. The
estimated total reserves of Fe (average grade Fe 35 wt%), Nb (average
grade of 0.13 wt%) and REE (average grade REO 6 wt%) are 1.5 Bt, 27
Mt and 48 Mt respectvely.
It is the world’s largest known REE ore deposit. A high
percentages of the
three
main light rare earth elements La, Ce and Nd are found in Bayan Obo
REE−Fe−Nb Ore Deposit, Sichuan Mianning REE Ore Deposit and
Shandong Weishan REE Ore Deposits.
The ore composition in the Bayan Obo is very complex, 71 elements and
170 minerals are found, one element could exist in several or more
than ten different minerals, mineral symbiosis relationship is close
and complicated, and dissemination size is fine. More than 90% rare
earth elements in ore exist in independent minerals, and about 4% ~
7% of REE are
dispersed
in iron minerals and fluorite. A total of 15 kinds of rare earth
minerals are found, but the principal ones are bastnaesite
[(Ce,La,Nd)(CO3)F]
and monazite [(Ce,La,Nd)PO4]
with the ratio of 7:3 or 6:4, whereas magnetite and hematite are the
dominant Fe‐ore minerals.
Several
kinds of niobium minerals are contained in the ore such as columbite
(FeNb2O),
aeschynite ((Y,Ca,Fe)(Ti,Nb)2(O,OH)6) and fersmite
((Ca,Ce,Na)(Nb,Ta,Ti)2(O,OH,F)6).
The main gangue minerals are quartz, apatite, dolomite and fluorite .
These minerals consist of both primary and secondary metamorphic
minerals of the host rocks and the epigenetic ore and gangue minerals
introduced by hydrothermal solutions.
Most
of the Bayan Obo minerals, particularly in the banded ore, are very
fine to extremely fine-grained. Most Fe−REE−Nb minerals are
closely associated with fine grain sizes. The grain sizes of REE
minerals are in the range of 0.01 – 0.074 mm and the amount of less
than 0.04 mm is 70% ~ 80%.
Beneficiation
techniques
Compared
with domestic and abroad single bastnaesite ores, the Bayan Obo ores
are much more difficult to upgrade due to its similar physical and
chemical properties with iron minerals and gangue minerals. A
lot of experimental studies by many research institutes in China on
the rational exploitation of the Bayan Obo REE deposits had been
conducted since the late 1950s and more than 20 beneficiation
techniques have been reported.
A
significant progress on REE minerals processing had been made in the
early 1990s and the mixed and separate REE concentrates with
high‐grade and high recovery were obtained from the iron ores. The
flowsheet of low-intensity magnetic separation (LIMS) – high
intensity magnetic separation (HIMS) – flotation was considered the
most successful one to be used in industrial plants which were
developed by Changsha Metallurgical Research Institute in 1990.
After
crushing the ore is ground to 90 – 95% passing 0.074 mm. By LIMS
(rougher and cleaner) magnetite is recovered in the LIMS concentrate.
The tailings of LIMS is processed through HIMS rougher at the
magnetic field strength of 1.4T to recover another Fe mineral,
hematite, and most REE minerals. Hematite
and REE minerals contained in the HIMS rougher concentrate are
separated by HIMS cleaner at the magnetic field strength of 0.6T.
Combined LIMS and HIMS concentrate reported to the Fe reverse
flotation to get the final Fe concentrate and the HIMS cleaner
tailings containing most REE minerals is processed by flotation to
get REE concentrates. At the REE flotation circuit, the feed grade of
REO is 9.78−12% (HIMS cleaner tailings). The flotation is performed
at low alkaline condition (pH 9) and the flotation reagents contained
naphthyl hydroxamic acid as the collector of REE minerals, sodium
silicate as the depressant of silicates and J10 asthe frother.
The feed solids are 35‐45% wt. One stage of rougher flotation, plus
one scavenger flotation and two cleaner flotation stages produced an
REO concentrate at a grade of 55% and a secondary REO concentrate at
a grade of 34% with the combined recovery of 72−75%. However, these
concentrates are a mixture of REE minerals mainly bastnaesite and
monazite. The separation of bastnaesite and monazite has been reached
by further flotation using phtalic acid or benzoic
acid as the collector of bastnaesite and alum as the depressant of
monazite. Different flotation reagents had been tested. A good
result
of up to 60% REO flotation concentrate was obtained by processing the previous concentrate of REO 25% ~ 30% from the
gravity separation concentrator when the major adjustments of the
compatible depressants were made. In 1976, the plant test was
successful,
and commercial production began in 1978. By application of the
reagents system, the rare earth concentrate of REO> 60%, flotation
recoveries of 60% to 65% were obtained. Since then, high‐ grade
commercial rare earth concentrates were produced.
Because
the selectivity of cyclic alkyl hydroxamic acid and alkyl hydroxamic
acids was found poor the collector of H205,
as the representative of aromatic hydroxamic acid, was successfully
developed in 1986 by using naphthalene as raw material through
sulfonated ‐ hydrolysis‐alkali fusion hydroxylation for
preparation of intermediates naphthol, and then acylation reaction
and condensation system with hydroxylamine (‐NH‐OH), to make
naphthyl isobutyl group hydroxamic acid. The rare earth grade and
recovery could significantly be improved by using the new generation of
rare earth mineral collector H205
with only water glass and the activator sodium fluoride in the the flotation process was not needed. After the 1990s, a hydroxamic acid
with dual activating group improved from H205,
has been used in the Bayan Obo rare earth plant as a collector.
By
the end of 2012, the Bayan Obo rare earth flotation concentrator had
been developed into an annual output of 250,000 t of REE concentrates
(contained 50% REO).
The
Sichuan Mianning REE Deposit
Mineralogy
The
Sichuan Mianning REE Ore Deposit was found between 1985–1986. It is
an alkaline pegmatite carbonate type rare earth deposit. The
average grade is
3.7% REO.
It is rich in light REE and heavy REE. Bastnaesite is the main rare
earth mineral and chevkinite and parisite are found in the ore. Other
associated minerals are barite (BaSO4),
fluorite (CaF2),
iron and manganese minerals and a small amount of galena. The ore is
divided into granule and powder types. The granule ore has coarse
grain size usually >1 mm. The grain size of bastnaesite is between
1 to 5 mm. The powder ore is the weathered product of the
original
ore and takes up about 20% of total ore weight. The grade is about
REO 3−7% and the grain size is 80% passing 325 mesh.
Beneficiation
techniques
Three
types of flowsheets are industrially used: the gravity concentration
flowsheet, the magnetic – gravity concentration flowsheet and the
gravity – flotation flowsheet. The coexistence of massive and
powdered black sludge, Fe‐Mn weathered amorphous aggregates slime,
greatly influences the floatability of the
REE
minerals.
The
ore is
ground to 62% passing 200 mesh and hydro classified into the four
size fractions. The shaking tables are
used to process the fractions separately. Three different grade
bastnaesite concentrates are
obtained with the grades of 30%, 50% and 60%. The overall recovery
iss
75%.
After
grinding the ore is
concentrated by LIMS and HIMS separation and a
magnetic concentrate with the grade of 5.64% is
obtained. The recovery of magnetic circuit is
74.2% and yield 42%.
Then
the magnetic concentrate is
classified into four size fractions and processed separately by
shaking tables. The final concentrate with the grade of REO 52.3% is
obtained. The overall recovery of RE is
around 55%.
The
ore is
primarily ground to 50% passing 200 mesh and hydro classified into
four size fractions. The classified fractions are
concentrated by the shaking tablesseparately. The grade of overall
gravity concentrate is
REO 30% with the RE recovery of 74.5%. The gravity concentrate is
reground to 70% passing 200 mesh for flotation. C5−9 hydroximic
acid (H205)
and phthalate in the ratio of 1:1, sodium carbonate, sodium silicate
are
used as the flotation reagents at pH 8‐9. By one rougher, one
cleaner and one scavenger flotation the concentrate with the grade of
REO 50−60% is
acquired at a
RE
recovery of 50−60%.
The
Shandong Weishan REE Deposit
Mineralogy
The
ore deposit was first found in 1958 and the exploration was completed
in 1975. The
average grade is
3.13%. It is a quartz−barite−carbonate type RE ore deposit.
The main RE minerals are bastnaesite and parisite and the main
associated minerals are barite, calcite, quartz and fluorite etc. The
grain sizes of RE minerals are coarse in the range of 0.04−0.5 mm.
Beneficiation
techniques
The
Weishan REE flotation plant was built in 1982. The ore was ground to
65−75% passing 200 mesh and REE minerals were floated with once
rougher, three scavengers and three cleaners. In 1980s oleic acid and
kerosene were used as the collectors of RE minerals at acidic
condition of pH5 using sulfuric acid. After 1991 when
the ore grade dropped to 3‐4%, a specific collector with formula
C6H4OHCONHOH
was used with the addition of sodium silicate and frother L101. The
flotation was run at the weak alkaline condition of pH 8−8.5. The
RE concentrate with a
grade of REO >60% at the recovery of 60−70% was obtained. A
secondary RE concentrate with the grade of REO 32% at a
recovery of 10−15% was acquired. According to market demand, REE
concentrate with the grade of 45~50% REO at the recovery of 80~85%
has been produced and barite has also been recovered by flotation
from REE flotation tailings.
The
Weathered Crust Elution‐deposited REE Ore (The Ion Adsorption REE
Deposit)
Mineralogy
China’s
weathered crust elution−deposit rare earth ore or ion adsorption
rare earth ore is unique in the world. It was found in 1969 in Jianxi
Province containing two types, a
light REE type and a
heavy REE type, and was also found in other provinces Fujian, Hunan,
Guandong and Guanxi. The deposit is
considered to form by many years weathering of granite and extrusive
rocks. It has the characteristics of shallow ore body coverage, soft
ore and very fine grain size. The 80%−90% REE in the state of
positive hydrated ions are adsorbed on the surface of clay minerals
such as kaolinite, halloysite and illite. The ore is relatively
low‐grade, generally only 0.05% to 0.5% REO, with high heavy REE.
Extraction
Techniques
Because
the REE in the ore mainly occur in the state of hydrated ions
adsorbing on the surface of clay minerals they can’t be effectively
concentrated by conventionally physical methods but are
extracted
by the ion−exchange method. The adsorbed REE ions are not dissolved
in water or ethyl alcohol but they are dissolved in the electrolyte
solutions of NaCl, (NH4)2SO4
and NH4Cl
by ion–exchange. Over
80% of REE are ion exchangeable. In the
1970s as a major extraction method NaCl was used as the leaching
solvent to dissolve REE by ion exchange. But high solvent
concentration and long leaching time were required. And the solvent
consumption and processing cost were high. Meanwhile, the grade of RE
concentrate can only reach 70% REO. Since 1980 (NH4)2SO4
has been used to replace NaCl as the solvent. A high grade (REO 92%)
of RE concentrate has been achieved. The process is usually run
in a cement bath with a volume of about 10−20 m3. When the bath
is filled with the ore to the height of about 1.5 m the solvent of
(NH4)2SO4
solution with the concentration of 1−4% is poured through the ore
for leaching. The pregnant solvent is obtained at the bottom of the
bath. Oxalic acid is used as the precipitant to get the mid product of
oxalic acid−rare earths and further processed by burning to get
final rare earths product (>REO 92%). Ammonium bicarbonate also
has been used as a competitive precipitant. The in‐situ leaching
process also called the solution mining was studied from the 1980s to
resolve the ecological environmental problems in the exploitation by
the bath leaching technology. The wells are drilled in the natural
orebody and the leaching reagent
is injected. REE ions were selectively leached. Collected leachate is
precipitated by the solution of oxalic acid or ammonium bicarbonate
to obtain rare earth oxide products. The water is recycled. It was
reported that two key technical problems must be solved during the
in‐situ leaching process. One is how to avoid the leaching reagent
solution spreading around the wells to cause environmental pollution
and the other is how to recover the leachate so as optimize the
leaching rate. The technology has been utilized in Wenfeng, Jiangxi
province. More than 200 t of REE are extracted annually at the REE
recovery of 70%.
Baotou
Iron and Steel and Rare Earth Company
is
the owner and operator of
Bayan Obo..
HEFA
Rare Earth Canada Co. Ltd.
sells
rare earth products directly from Its parent company, Baotou
HEFA Rare Earth in
China.
