U.S. patent application number 14/158824 was filed with the patent office on 2014-11-20 for extraction of metals from metallic compounds.
This patent application is currently assigned to Rare Element Resources Ltd.. The applicant listed for this patent is Rare Element Resources Ltd.. Invention is credited to Henry Kasaini.
Application Number | 20140341790 14/158824 |
Document ID | / |
Family ID | 51210194 |
Filed Date | 2014-11-20 |
United States Patent
Application |
20140341790 |
Kind Code |
A1 |
Kasaini; Henry |
November 20, 2014 |
EXTRACTION OF METALS FROM METALLIC COMPOUNDS
Abstract
Methods for the extraction of metals such as rare earth metals
and thorium from metal compounds and solutions. The methods may
include the selective precipitation of rare earth elements from
pregnant liquor solutions as rare earth oxalates. The rare earth
oxalates are converted to rare earth carbonates in a metathesis
reaction before being digested in an acid and treated for the
extraction of thorium. A two-step extraction method may be applied
to precipitate thorium as thorium hydroxide under controlled pH
conditions such that pure thorium precipitate is recovered from a
first step and a thorium-free rare earth solution is recovered at
the subsequent step. The resulting rare earth solutions are of
extremely high purity and may be processed directly in a solvent
extraction circuit for the separation of rare earth elements, or
may be processed for the direct production of a 99.9% bulk rare
earth hydroxide/oxide concentrate.
Inventors: |
Kasaini; Henry; (Littleton,
CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rare Element Resources Ltd. |
Lakewood |
CO |
US |
|
|
Assignee: |
Rare Element Resources Ltd.
Lakewood
CO
|
Family ID: |
51210194 |
Appl. No.: |
14/158824 |
Filed: |
January 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61754420 |
Jan 18, 2013 |
|
|
|
61902579 |
Nov 11, 2013 |
|
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Current U.S.
Class: |
423/18 ;
423/11 |
Current CPC
Class: |
Y02P 10/20 20151101;
C22B 3/46 20130101; C22B 59/00 20130101; Y02P 20/582 20151101; C22B
3/44 20130101; C22B 3/10 20130101; C22B 60/02 20130101; C22B
60/0291 20130101; C07C 51/418 20130101 |
Class at
Publication: |
423/18 ;
423/11 |
International
Class: |
C22B 60/02 20060101
C22B060/02 |
Claims
1. (canceled)
2. A method for the extraction of thorium from an acidic solution
containing solubilized thorium, the method comprising the steps of:
first contacting the acidic solution with a hydroxide precipitant
at a first pH of at least about pH 2.8 and not greater than about
pH 3.9 to precipitate thorium as thorium hydroxide and form an
intermediate thorium depleted solution and a first thorium
hydroxide product; separating at least a portion of the first
thorium hydroxide product from the intermediate thorium depleted
solution; second contacting the intermediate thorium depleted
solution with the hydroxide precipitant at a second pH of at least
about pH 3 and not greater than about pH 4, wherein the second pH
is greater than the first pH, to precipitate additional solubilized
thorium as thorium hydroxide and form a thorium depleted solution
and a second thorium hydroxide product; and separating at least a
portion of the second thorium hydroxide product from the thorium
depleted solution.
3. (canceled)
4. The method recited in claim 2, wherein at least one of the first
and second contacting steps comprises contacting the acidic
solution or the intermediate thorium depleted solution with the
hydroxide precipitant at a pH of at least about pH 3.2.
5. The method recited in claim 2, wherein at least one of the first
and second contacting steps comprises contacting the acidic
solution or the intermediate thorium deleted solution with the
hydroxide precipitant at a pH of at least about pH 3.3.
6. The method recited in claim 2, wherein at least one of the first
and second contacting steps comprises contacting the acidic
solution or the intermediate thorium deleted solution with the
hydroxide precipitant at a pH of at least about pH 3.4.
7. The method recited in claim 2, wherein at least one of the first
and second contacting steps comprises contacting the acidic
solution or the intermediate thorium deleted solution with the
hydroxide precipitant at a pH of at least about pH 3.5.
8. The method recited in claim 2, wherein at least one of the first
and second contacting steps comprises contacting the acidic
solution or the intermediate thorium deleted solution with the
hydroxide precipitant at a pH of at least about pH 3.7.
9. The method recited in claim 2, wherein at least one of the first
and second contacting steps comprises contacting the acidic
solution or the intermediate thorium deleted solution with the
hydroxide precipitant at a pH of not greater than pH 3.9.
10. The method recited in claim 2, wherein the hydroxide
precipitant is selected from the group consisting of sodium
hydroxide and ammonium hydroxide.
11. The method recited in claim 2, wherein the hydroxide
precipitant comprises ammonium hydroxide.
12. The method recited in claim 2, wherein the concentration of
solubilized thorium in the acidic solution is at least about 50
mg/l and is not greater than about 2000 mg/l.
13. The method recited in claim 2, wherein the concentration of
solubilized thorium in the acidic solution is at least about 100
mg/l and is not greater than about 1600 mg/l.
14. The method recited in claim 2, wherein the acidic solution
comprises solubilized rare earth elements.
15. The method recited in claim 2, wherein the acidic solution is a
nitric acid solution.
16. The method recited in claim 15, further comprising the step of:
leaching, before the contacting step, a rare earth containing
product to form the acidic solution, the leaching comprising the
step of contacting the rare earth containing product with nitric
acid to solubilize rare earth elements as rare earth nitrates and
solubilize thorium as thorium nitrate.
17. The method recited in claim 16, wherein the rare earth
containing product comprises at least one rare earth compound
selected from the group consisting of rare earth oxides, rare earth
hydroxides, rare earth carbonates and mixtures thereof.
18. The method recited in claim 16, wherein the rare earth
containing product comprises rare earth carbonates.
19. The method recited in claim 2, wherein the acidic solution
comprises at least two different rare earth elements.
20. The method recited in claim 2, wherein the acidic solution
comprises at least three different rare earth elements.
22. The method recited in claim 2, wherein the rare earth elements
comprise at least three different rare earth elements selected from
the group consisting of cerium, dysprosium, europium, terbium,
lanthanum, neodymium, praseodymium and yttrium.
23. The method recited in claim 19, wherein the rare earth elements
comprise at least one rare earth element selected from the group
consisting of neodymium, europium, praseodymium and terbium.
24. The method recited in claim 2, wherein at least about 95% of
the thorium in the acidic solution is precipitated as thorium
hydroxide in the thorium hydroxide product.
25. The method recited in claim 2, wherein at least about 98% of
the thorium in the acidic solution is precipitated as thorium
hydroxide in the thorium hydroxide product.
26. The method recited in claim 2, wherein the thorium depleted
solution comprises not greater than about 0.01 at. % thorium.
27. The method recited in claim 2, wherein the thorium depleted
solution comprises not greater than about 0.005 at. % thorium.
28. The method recited in claim 2, wherein metals solubilized in
the thorium depleted solution comprise at least about 99%
solubilized rare earth metals.
29. The method recited in claim 2, wherein at least about 99% of
rare earth metals solubilized in the acidic solution are
solubilized in the thorium depleted solution.
30-47. (canceled)
48. The method recited in claim 2, wherein the first pH is from
about pH 3.0 to about pH 3.3.
49. The method recited in claim 2, wherein the second pH is from
about pH 3.5 to about pH 4.0.
50. The method recited in claim 2, wherein the first pH is from
about pH 3.0 to about pH 3.3 and the second pH is from about pH 3.5
to about pH 4.0.
51. The method recited in claim 2, further comprising the step of
recycling at least a portion of the second thorium hydroxide
product to the first contacting step.
52. The method recited in claim 51, wherein the second thorium
hydroxide product comprises at least about 20 at. % rare earth
elements.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority benefit under 35 USC
.sctn.119 of U.S. Provisional Application No. 61/754,420 filed on
Jan. 18, 2013, and claims the priority benefit under 35 USC
.sctn.119 of U.S. Provisional Application No. 61/902,579 filed Nov.
11, 2013. The disclosure of each of these applications is
incorporated herein by reference in its entirety.
FIELD
[0002] This disclosure relates to the field of extractive
metallurgy, such as for the extraction of rare earth metals and/or
thorium from feedstocks containing these elements.
BACKGROUND
[0003] Rare earth elements (REEs) comprise seventeen elements in
the periodic table, specifically the 15 lanthanide elements plus
scandium and yttrium. REEs are a group of metallic elements with
unique chemical, catalytic, magnetic, metallurgical and
phosphorescent properties, and as such find use in a wide variety
of modern devices including high-strength magnets, batteries,
displays, lighting, and high performance metal alloys.
[0004] REEs are relatively plentiful in the earth's crust. However,
REEs are typically highly dispersed and are not often found as
concentrated rare earth minerals in economically exploitable ore
deposits. The extraction of REEs from mineral deposits is also
challenging because mineral deposits containing REEs typically also
contain appreciable levels of radioactive elements such as thorium
(Th) and uranium (U) that must be safely separated from the REEs
during processing of the ore.
[0005] Other ore deposits, such as those containing tantalum (Ta)
and/or niobium (Nb), may also contain appreciable amounts of
thorium that must be safely removed from the metals during
processing of the ore.
SUMMARY
[0006] It is one objective to provide a method for the selective
extraction of rare earth elements from base metals by precipitation
of pregnant liquor solutions to form rare earth oxalates. The rare
earth oxalates may be converted to rare earth carbonates in a
metathesis reaction before being digested in an acid and treated
for the extraction of thorium.
[0007] It is also an objective to provide a method for the
extraction of thorium by precipitating the thorium as thorium
hydroxide under controlled pH conditions so that the thorium
precipitates without precipitating substantial amounts of rare
earth metals. The resulting rare earth solutions are of extremely
high purity and may be processed in a solvent extraction circuit
for the recovery of high purity rare earth metals, or may be
treated to convert the solutions to rare earth oxides.
DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic flowsheet illustrating a method for
the selective precipitation of thorium as thorium hydroxide from an
acidic solution.
[0009] FIG. 2 is a schematic flowsheet illustrating a method for
the selective precipitation of thorium as thorium hydroxide from an
acidic solution using multiple hydroxylation steps.
[0010] FIG. 3 is a schematic flowsheet illustrating a method for
the precipitation of thorium as thorium hydroxide from an acidic
solution including the recycle of acid from a solvent extraction
circuit.
[0011] FIG. 4 is a schematic flowsheet illustrating a method for
the precipitation of thorium as thorium hydroxide from an acidic
solution including the precipitation of rare earth element
hydroxides from a nitric acid solution.
[0012] FIG. 5 is a schematic flowsheet illustrating a method for
the conversion of a rare earth oxalate product to a rare earth
carbonate product by metathesis.
[0013] FIG. 6 is a schematic flowsheet illustrating a method for
the conversion of a rare earth oxalate product to a rare earth
carbonate product by metathesis including the recycle of acid from
a solvent extraction circuit.
[0014] FIG. 7 is a schematic flowsheet illustrating a method for
the precipitation of rare earth elements as rare earth oxalates
from a pregnant liquor solution.
[0015] FIG. 8 is a schematic flowsheet illustrating a method for
leaching a rare earth ore concentrate to form a pregnant liquor
solution.
[0016] FIGS. 9A and 9B are flowsheets illustrating comprehensive
methods for the extraction of rare earth elements from an ore
incorporating various embodiments of the present disclosure.
[0017] FIGS. 10A and 10B illustrate the effect of pH on selective
thorium precipitation when using a hydroxide precipitant to
precipitate thorium as thorium hydroxide.
DESCRIPTION OF THE EMBODIMENTS
[0018] In some embodiments, the present disclosure relates to
methods for the selective precipitation of thorium (Th) from acidic
solutions of metals, such as acidic solutions containing rare earth
elements ("REEs"), such as an acidic solution that is derived from
a pregnant liquor solution ("PLS") formed by acid leaching of an
ore (e.g., a mineral ore concentrate) containing the REEs. In some
embodiments, the present disclosure relates to methods for
preparing the acidic solutions, such as from rare earth oxalates
(REE-oxalates) or other rare earth compounds, which may be derived
from a mineral ore. In some embodiments, the present disclosure
relates to methods for the precipitation of REEs from a solution
(e.g., a PLS) in the form of REE-oxalates. In other embodiments,
the present disclosure relates to unique products that may be
formed by the disclosed methods when applied alone or in
combination.
[0019] REEs comprise 17 elements in the periodic table, namely the
15 lanthanide elements plus scandium and yttrium. Specifically, the
REEs include scandium (Sc), yttrium (Y), lanthanum (La), cerium
(Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium
(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium
(Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and
lutetium (Lu). All of these REEs except Pm are found in nature,
e.g., in mineral deposits. Many REEs find use in modern devices and
have a very high value do to their relative scarcity. Of
particularly high value are the REEs yttrium, praseodymium,
neodymium, europium, terbium and dysprosium.
[0020] Many mineral deposits containing REEs also contain
radioactive elements, such as thorium and uranium. Radioactive
elements may also be found with other non-REE mineral deposits,
such as uranium deposits, tantalum deposits and niobium deposits.
It is highly desirable to separate these radioactive elements from
the non-radioactive metals before final processing to extract the
non-radioactive metals, e.g., from a solution of the
non-radioactive metals.
[0021] In a first embodiment, a method for the selective
precipitation of thorium from an acidic solution containing
solubilized thorium is provided. The method may be applicable to
solutions that contain other solubilized metals in addition to the
thorium, such as solubilized REEs, uranium, tantalum or niobium. In
one example, the acidic solution includes significant amounts of
solubilized REEs (i.e., an acidic REE solution), such as an acidic
solution that is derived from a rare earth ore concentrate. In one
particular example, the acidic solution includes one or more of
yttrium, praseodymium, neodymium, europium, terbium and dysprosium.
Although the following description primarily describes the
extraction of thorium from such acidic REE solutions, the thorium
precipitation method of this embodiment may be applicable to other
acidic solutions containing solubilized thorium, such as acidic
solutions containing Group 5 metals such as tantalum and/or
niobium.
[0022] The method of this embodiment includes the precipitation of
thorium in the form of thorium hydroxide (e.g., Th(OH).sub.3 or
ThO(OH).sub.3) from an acidic solution. For example, the method may
include precipitating thorium as thorium hydroxide by contacting
the acidic solution with a hydroxide precipitant, e.g., by
contacting the acidic solution with a compound that includes a
hydroxide group, such as sodium hydroxide (NaOH) and/or ammonium
hydroxide (NH.sub.4OH). Thorium hydroxide may be precipitated from
the acidic solution while maintaining a substantial portion of
other valuable metals (e.g., REEs) in solution for subsequent
recovery of the other metals, such as in a solvent extraction
circuit. In one example, the acidic solution may have a relatively
low free acid content, such as about 5 g/l (grams per liter) of
acid.
[0023] FIG. 1 illustrates a schematic flowsheet of a method for the
precipitation of thorium from an acidic solution according to this
embodiment. As illustrated in FIG. 1, an acidic solution 102
containing at least solubilized thorium is contacted with a
hydroxide precipitant 104 in a hydroxylation step 110, e.g., by
contacting the acidic solution 102 and the hydroxide precipitant
104 in a reactor 112 to cause thorium in the acidic solution 102 to
precipitate as thorium hydroxide. After at least a portion of
thorium in the acidic solution 102 has precipitated from the acidic
solution 102 as thorium hydroxide, a thorium depleted solution 106
may be separated from a thorium hydroxide product 108 in a
separating step 114, e.g., using a filter 116.
[0024] The acidic solution 102 contains at least solubilized
thorium. As is discussed in more detail below, the acidic solution
102 may be derived from the leaching of a mineral ore (e.g., an ore
concentrate) containing REEs or other high-value metals. Thorium is
among the elements that are commonly found in mineral deposits
containing REEs and the resulting acidic leach solutions typically
contain undesirable concentrations of thorium. In one example of
this embodiment, the concentration of solubilized thorium in the
acidic solution 102 is at least about 50 mg/l (milligrams per
liter), such as at least about 100 mg/l of solubilized thorium in
the acidic solution 102, or even at least about 200 mg/l of
solubilized thorium.
[0025] The acidic solution 102 may also include one or more REEs,
i.e., REEs that are also solubilized in the acidic solution 102.
For example, the acidic solution 102 may include REEs in a
concentration of at least about 10 grams per liter (g/l). In
certain characterizations, the acidic solution 102 includes a
relatively high concentration of REEs, such as at least about 15
g/l REEs, at least about 20 g/l REEs, at least about 30 g/l or even
at least about 50 g/l REEs, where the REEs are solubilized (e.g.,
dissolved) into the acidic solution 102. Typically, the acidic
solution 102 will include not greater than about 100 g/l REEs. In
one particular characterization of this example, the acidic
solution 102 includes at least one or more REEs of particularly
high value, such as one or more of praseodymium, neodymium,
europium, terbium and dysprosium.
[0026] The solution 102 is acidic and may have a pH of not greater
than about pH 3.8, such as not greater than about pH 4.2, prior to
being contacted with the hydroxide precipitant 104. In one example,
the acidic solution includes nitric acid (HNO.sub.3), although
other acids such as sulfuric acid (H.sub.2SO.sub.4) may also be
useful in the embodiments disclosed herein. For example, the acidic
solution 102 may comprise nitric acid (HNO.sub.3) and may be
obtained from the acid digestion of rare earth compounds, e.g., the
acid digestion of earth oxide (RE-oxides), rare earth hydroxides
(RE-hydroxides), rare earth oxalates (RE-oxalates) and/or rare
earth carbonates (RE-carbonates) with nitric acid to form
solubilized nitrate compounds of REEs. Nitric acid is particularly
useful, as the thorium hydroxide precipitated during hydroxylation
110 will becomes stable and thus will not dissolve, even at a
relatively low pH.
[0027] The acidic solution 102 comprises nitric acid, and in one
particular example, the acidic solution 102 has a free acid
concentration in the range of from about 0.5 g/l to about 55 g/l
HNO.sub.3. When the acidic solution 102 comprises nitric acid, the
solubilized elements (e.g., thorium and REEs) may be in the form of
solubilized nitrate salts. It is an advantage of this embodiment
that the acidic solution 102 may have a relatively low free acid
concentration, and therefore may require relatively small
quantities of the hydroxide precipitant 104 to precipitate thorium
hydroxide and to avoid diluting metal species in solution, which
favors crystallization to precipitate thorium.
[0028] The acidic solution 102 may also include traces of non-REE
elements that are solubilized in the acidic solution 102. For
example, the non-REE elements may include metallic elements, such
as: alkali metals such as sodium (Na) and potassium (K); alkaline
earth metals such as magnesium (Mg), calcium (Ca), strontium (Sr)
and barium (Ba); transition metals such as nickel (Ni), copper
(Cu), zirconium (Zr), iron (Fe), manganese (Mn) and titanium (Ti);
post-transition metals such as lead (Pb) and aluminum (Al);
metalloids such as silicon (Si); and radioactive metals (e.g.,
actinides) such as thorium (Th) and uranium (U). The non-REE
elements may also include non-metallic elements such as sulfur (S)
and phosphorous (P).
[0029] In one example, however, the acidic solution 102 includes
primarily REEs and thorium, with little or no other non-REE
elements (e.g., base metals) that are solubilized in the acidic
solution 102. For example, the acidic solution 102 may comprise not
greater than about 5 wt. % non-REE elements, such as not greater
than about 3 wt. % non-REE elements. Method for the formation of
such acidic solutions are described in more detail below.
[0030] The hydroxylation step 110 includes contacting the acidic
solution 102 with a hydroxide precipitant 104, such as sodium
hydroxide or ammonium hydroxide, to precipitate a thorium hydroxide
product 108 (e.g., predominately containing particulate thorium
hydroxide). For example, the reactants may be contacted in a
reactor 112 under conditions such that at least a portion of the
thorium solubilized in the acidic solution 102 precipitates as a
thorium hydroxide product 108.
[0031] It is an advantage of the method of this embodiment that the
thorium may be precipitated from the acidic solution 102, while a
substantial majority of the REEs contained in the acidic solution
102 remain solubilized in a thorium depleted solution 106 that is
separated from the thorium hydroxide product 108. To ensure that
sufficient quantities of thorium precipitate from the acidic
solution 102 and that a substantial majority of REEs in the acidic
solution 102 remains solubilized, it has been found that the pH
during the hydroxylation step 110 should be maintained at a pH that
enables high selectively for thorium, i.e., to preferentially
precipitate thorium from the acidic solution 102. In one
characterization, the pH during the hydroxylation step 110 is
within the range of at least about pH 3 and not greater than about
pH 4. It has been found that increasing the pH within this range
may increase the amount of thorium precipitated from the acidic
solution 102 as a thorium hydroxide product 108. In one
characterization, the pH during the hydroxylation step 110 is
maintained at a pH of at least about pH 3.0, such as at least about
pH 3.1, at least about pH 3.2, at least about pH 3.3, at least
about pH 3.4 or even at least about pH 3.5, such a at least about
pH 3.6. However, as the pH approaches about pH 4, an increasing
quantity of REEs may also precipitate from the acidic solution 102
(e.g., as particulate REE-hydroxides). In the embodiment
illustrated in FIG. 1, to avoid the precipitation of undesirable
quantities of REEs from the solution, the pH should be maintained
at less than ph 4, such as not greater than pH 3.9 or not greater
than pH 3.8. In one example, the pH during the hydroxylation step
110 may be maintained at the desired pH level by controlling the
quantity of hydroxide precipitant 104 that is added to the reactor
112 during the hydroxylation step 110, e.g., during the
precipitation of thorium from the acidic solution 102.
[0032] It has also been found that the desirable range of pH values
for the selective precipitation of thorium is dependent upon the
concentration of solubilized thorium in the acidic solution 102. In
particular, it has been found that increased pH values within the
range of about pH 3.5 to pH 4 may be utilized to selectively
precipitate thorium as a thorium hydroxide product 108 without
precipitating significant amounts of REEs when the concentration of
thorium in the acidic solution 102 is relatively low. That is, as
the concentration of the thorium in the acidic solution 102
decreases, the pH during the hydroxylation step 110 may be
increased to remove additional thorium without removing undesirable
quantities of REEs. In one example, the acidic solution 102 can be
diluted (e.g., with water) to reduce the thorium concentration, and
the hydroxylation step 110 may carried out at a higher pH (e.g., pH
3.5 to pH 3.9) without precipitating undesirable quantities of
REEs. In one characterization, the acidic solution 102 comprises
not greater than about 800 mg/l of thorium, such as not greater
than 500 mg/l, or even not greater than about 200 mg/l thorium, and
the contacting step is carried out at a pH of at least about pH
3.5, such as at least about pH 3.6, at least about pH 3.7, and even
at least about pH 3.8, but not greater than pH 4, such a not
greater than pH 3.9. However, it is believed that at least about 50
mg/l of thorium is required in the solution for precipitation of
thorium to occur.
[0033] The acidic solution 102 and the hydroxide precipitant 104
should remain in contact (e.g., in reactor 112) for a period of
time sufficient to precipitate a majority (e.g., at least about
50%) of the thorium from the acidic solution 102 and form a thorium
depleted solution 106 and a thorium hydroxide product 108. In one
characterization, the time of contact (e.g., the average residence
time in the reactor) during the hydroxylation step 110 may be at
least about 30 minutes and may be not greater than about 90
minutes. It is an advantage of this embodiment that the
hydroxylation step 110 may be carried out at ambient temperatures,
e.g., the step does not typically require the reactor 112 to be
heated or cooled. Further, the hydroxylation step 110 may be
carried out at ambient pressures, e.g., the step does not require a
sealed or otherwise pressure-controlled reactor 112.
[0034] After the contacting step 110, the thorium hydroxide product
108 may be separated from the thorium depleted solution 106 in a
separating step 114. For example, a filter 116 may be used to
filter the output stream 107 containing thorium hydroxide and the
thorium depleted solution 106 from the reactor 112 and retain the
thorium hydroxide product 108 on the filter 116. The thorium
depleted solution 106 (i.e., the filtrate), containing high levels
of REEs and very low levels of thorium, may be further treated as
is discussed below. The thorium hydroxide product 108 may
advantageously be of high purity, i.e., the product may comprise at
least about 99 wt. % thorium hydroxide, such as at least about 99.9
wt. % thorium hydroxide. The thorium hydroxide product 108 may be
disposed of, or may be a salable commodity particularly in view the
high purity of the thorium hydroxide product 108.
[0035] As is noted above, thorium precipitation from the acidic
solution may be enhanced with increased pH (e.g., up to about pH 4)
and with a decreased concentration of thorium in the acidic
solution and with low free acid content. In one example of this
embodiment, this finding may be applied in a multi-step (e.g.,
two-step) process. Specifically, the thorium extraction method of
this embodiment may include a first hydroxylation step that
includes contacting an acidic solution with a hydroxide precipitant
at a first pH, e.g., of at least about pH 3 and not greater than
about pH 4, to precipitate a thorium hydroxide product containing
very low amounts of REEs and form an intermediate thorium depleted
solution, i.e., having a lower concentration of thorium than the
acidic solution. The intermediate thorium depleted solution may
then be subjected to a second hydroxylation step where the
intermediate thorium depleted solution is contacted with a
hydroxide precipitant at a second pH of at least about pH 3.1 and
not greater than about pH 4.2, where the second pH is greater than
the first pH to remove additional thorium. In one particular
characterization of this method, the pH during the first
hydroxylation step is from about pH 3.0 to about pH 3.3, and the pH
during a second hydroxylation step is from about pH 3.5 to about pH
4. In this regard, the pH in the second hydroxylation step may be
carried out at such higher pH to aggressively remove thorium, even
in the event some REEs may precipitate with the thorium hydroxide
product, as is discussed below.
[0036] In this embodiment, some additional solubilized thorium is
precipitated as thorium hydroxide in the second hydroxylation step
to form a thorium depleted solution, i.e., having a lower
concentration of thorium than the intermediate thorium depleted
solution, and that also has a relatively high concentration of REEs
in solution. As compared to the embodiment described with respect
to FIG. 1, the thorium depleted solution from the second
hydroxylation step may be recycled to the first hydroxylation step
so that only small concentrations of REEs report with the thorium
hydroxide product.
[0037] Referring now to FIG. 2, this exemplary method may include a
first hydroxylation step 110a where the acidic solution 102 is
contacted with a first hydroxide precipitant 104a, such as in a
first reactor 112a, under conditions such that at least a portion
of the thorium in the acidic solution 102 precipitates as a first
thorium hydroxide product 108a and a substantial majority of the
REEs (e.g., at least about 99 at. % of the REEs) remain solubilized
in an intermediate thorium depleted acidic solution 106a. For
example, at least about 50 at. % of the thorium in the acidic
solution 102 may be precipitated in reactor 112a and removed in a
first separation step 114a, e.g., using a filter 116a. In one
particular characterization, at least about 60 at. % and not
greater than about 90 at. % of the thorium in the acidic solution
102 is separated from the intermediate thorium depleted solution
106a in the separation step 114a as a thorium hydroxide product
108a. As a result, the intermediate thorium depleted solution 106a
recovered from the separation step 114a has a lower concentration
of thorium than the acidic solution 102.
[0038] As is discussed above, the lower concentration of thorium in
the intermediate thorium depleted solution 106a advantageously
enables a higher pH to be utilized in a second hydroxylation step
110b (i.e., as compared to the first hydroxylation step 110a),
e.g., in a second reactor 112b. Thus, in a second separation step
114b, a second thorium hydroxide product 108b is separated from the
thorium depleted solution 106. The thorium depleted solution 106
from the separation step 114b may advantageously include not
greater than about 5% of thorium contained in the acidic solution
102, such as not greater than about 2% of the thorium contained in
the acidic solution 102. Further, due to the high selectivity of
the process, at least about 95%, such as at least about 98%, of
REEs in the acidic solution 102 may remain solubilized in the
thorium depleted solution 106. Although illustrated as a two-step
process in FIG. 2 (e.g., including two hydroxylation steps), the
method may include additional incremental steps if desired for
enhanced thorium precipitation and/or enhanced REE recovery.
[0039] Further, the amount of thorium hydroxide product 108b that
is separated from the thorium depleted solution 106 may be
relatively small, as compared to the amount of thorium hydroxide
product 108a that is separated from the intermediate thorium
depleted solution 106a. Further, the thorium hydroxide product 108b
may include some REEs (due to the higher pH used in hydroxylation
step 110b). In one characterization, the thorium hydroxide product
108b may include up to about 20 at. % REEs on a metals basis.
Therefore, in one example, the thorium hydroxide product 108b may
be recycled back to the first hydroxylation step 110a, so that the
recovery of REEs in the thorium depleted solution 106 is increased.
That is, any increase in the amount of REEs precipitated as
REE-hydroxides in hydroxylation step 110b may be mitigated by
recycling the thorium hydroxide product 108b to hydroxylation step
110a, keeping the REEs in the circuit. Thus, in this example, all
of the thorium hydroxide may be extracted from the circuit with the
thorium hydroxide product 108a.
[0040] In one example of the foregoing embodiments, ammonium
hydroxide is utilized as a hydroxide precipitant 104/104a/104b to
precipitate thorium as thorium hydroxide. For example, ammonium
hydroxide may be added as an aqueous solution having a
concentration of from about 10% to about 20% ammonium hydroxide,
e.g., about 15%. As a result, the thorium depleted solution 106
recovered from the separation step(s) 114 will contain substantial
amounts of ammonium nitrate (NH.sub.4NO.sub.3), dissolved in the
thorium depleted solution 106. As is discussed in more detail
below, it may be desirable to continuously or intermittently
extract the ammonium nitrate, which is a valuable and salable
by-product.
[0041] In some embodiments of the present disclosure, methods for
the formation of the acidic solution are provided. Further, methods
for the extraction of REEs from the thorium depleted solution are
provided. In some examples, it may be advantageous to integrate the
method(s) described above for the precipitation of thorium from an
acidic solution with a solvent extraction circuit for extracting
REEs from the thorium depleted solution. It may also be
advantageous to integrate a method for the formation of the acidic
solution, before hydroxylation, by acid digestion of rare earth
compounds, particularly acid digestion of REE-carbonates. In one
particular embodiment, reagent consumption may be reduced and
overall operating expenses of the process reduced by recycling
nitric acid from a solvent extraction circuit to an acid digestion
step to form the above-described acidic solution. In one
characterization, nitric acid consumption may be reduced to almost
zero, with only make-up nitric acid being added to the process to
compensate for normal evaporation and leakage losses.
[0042] In one example, the acidic solution is formed by the acid
digestion of an REE-carbonate product, such as one that has a high
purity with respect to REEs. As illustrated in FIG. 3, an
REE-carbonate product 174 may be contacted with an acid 120 (e.g.,
fresh nitric acid or sulfuric acid) in an acid digestion step 122,
such as in a reactor 124. The resulting acidic solution 102 may be
an acidic solution substantially as described above with respect to
FIGS. 1 and 2. The acidic solution 102 may be contacted in a first
hydroxylation step 110a with a hydroxide precipitant 104a to
precipitate a thorium hydroxide product 108a from the acidic
solution 102. The thorium hydroxide product 108a may be separated
from the thorium depleted solution 106b in a separation step 114a.
Thereafter, as illustrated with respect to FIG. 2, the intermediate
thorium depleted solution 106b may be contacted in a second
hydroxylation step 110b with a second hydroxide precipitant 104b to
form the thorium depleted solution 106. The thorium depleted
solution 106 may then separated from the second thorium hydroxide
product 108b in a separation step 114b.
[0043] As is noted above, the amount of thorium hydroxide product
108b may be relatively small and there may be appreciable
quantities of REEs in the thorium hydroxide product 108b. To reduce
losses of REEs, the thorium hydroxide product 108b may be recycled
back to the process, and as illustrated in FIG. 3, the second
thorium hydroxide product 108b is recycled back to the acid
digestion step 122 where the thorium is re-digested with the
REE-carbonate product 174. In this manner, all of the thorium
hydroxide is removed from the acidic solution 102 with the first
thorium hydroxide product 108a. When the thorium hydroxide product
108b is separated in separating step 114b, the resulting thorium
depleted solution 106 is a relatively high purity RE-nitrate
solution.
[0044] The high purity RE-nitrate solution 106 may then be
subjected to a solvent extraction circuit 126 to extract REEs from
the thorium depleted solution 106. It is an advantage of this
embodiment that having the REEs solubilized in nitrate media may
reduce the expenses associated with a solvent extraction circuit.
The solvent extraction circuit 126 may include the steps of solvent
extraction 128 and solvent stripping 130 with a stripping solvent
132. Solvent extraction circuits for the recovery of REEs are known
in the art and will not be described here in additional detail.
However, because the thorium depleted solution 106 described herein
is of extremely high purity, the solvent extraction circuit 126 may
advantageously be operated at a reduced capital expense and reduced
operating expense. The resulting products are very high purity and
high value REEs 134.
[0045] As is noted above, the thorium depleted solution 106 may
include substantial quantities of highly salable ammonium nitrate.
Thus, an ammonium nitrate removal step 136 may be utilized to
continuously or intermittently remove ammonium nitrate 138 from the
solution 106. As illustrated in FIG. 3, the ammonium nitrate is
removed after the solvent extraction circuit 126, as the presence
of ammonium nitrate in the thorium depleted solution 106 is not
believed to impair the efficacy of the solvent extraction circuit
126. However, it will be appreciated that the ammonium nitrate
separation step may also occur before the solvent extraction
circuit 126 if desired.
[0046] The ammonium nitrate separation step 136 may include cooling
the thorium depleted solution to a reduced temperature (e.g., below
about 10.degree. C.) to crystallize ammonium nitrate 138. Because
ammonium nitrate 138 is highly soluble in acid, it may only be
necessary to intermittently operate the separation step 136 to
remove ammonium nitrate 138. Ammonium nitrate is valuable and
salable by-product that is widely used in the fertilizer industry
and may represent a significant source of revenue from the
process.
[0047] As illustrated in FIG. 3, after separation of the ammonium
nitrate 138 (intermittently or continuously), the nitric acid 140
(e.g., recycled nitric acid) may be recycled back to the process,
e.g., back to the acid digestion step 122. Thus, the acid (e.g.,
input at 120) may be contained in an essentially "closed loop"
within the process. Additional nitric acid may be generated during
the solvent extraction circuit due to cationic ion exchange
releasing protons into solution. In this regard, a substantial
quantity of the nitric acid required for the acid digestion step
may be provided by the recycled nitric acid 140, and only a small
amount of fresh nitric acid 120 may be required for the process
once steady state and continuous operations are achieved and
maintained.
[0048] FIG. 3 illustrates the integration of a solvent extraction
circuit for the extraction of high purity REEs as metals from the
nitrate solution containing the REEs. In other embodiments, it may
be advantageous to integrate the method(s) described herein for the
precipitation of thorium from an acidic solution with a circuit for
precipitating the REEs, e.g., as REE-oxides and/or
REE-hydroxides.
[0049] In this regard, FIG. 4 illustrates an example of an
integrated process similar to the process illustrated in FIG. 3,
but where an REE precipitation circuit replaces the solvent
extraction circuit of FIG. 3. Thus, the thorium depleted and
REE-nitrate rich solution can be treated to precipitate high purity
REE-compounds such as REE-oxides and/or REE-hydroxides which, for
example, may be shipped to a separate facility for extraction of
the REEs as metals.
[0050] Referring to FIG. 4, the thorium depleted solution 106 from
the separation step 114b will typically have a pH in the range of
about pH 3.6 to about pH 4 (e.g., about pH 3.8) and will be rich in
REE-nitrates and may contain no, or extremely low levels of,
thorium and/or uranium. For example, the solution 106 may include
not greater than about 1 ppm thorium and/or uranium. As illustrated
in FIG. 4, this solution 106 is conveyed to an REE precipitation
step 142, where the solution 106 is contacted with an REE
precipitation agent 144. In one characterization, ammonium
hydroxide is used for precipitation in both the precipitation step
144 to precipitate REEs and in the hydroxylation step(s) 110a/110b
to precipitate thorium. The REE precipitation agent 144 may be
added to the solution 106 in sufficient quantities to increase the
pH of the solution, such a by increasing the pH to at least about
pH 4.5, such at least about ph 4.9. In one characterization, the pH
during the precipitation step 144 is not greater than about pH 6
and may be about pH 5.5. At these pH levels, the REEs will
precipitate from the solution 106 as REE-hydroxides 146, which may
be separated from an REE-depleted nitrate solution 148 in a
separation step 150.
[0051] The REE-hydroxides 146 may then be converted from the
REE-hydroxides to REE-oxides. As illustrated in FIG. 4, the
REE-hydroxides 146 are conveyed to a drying step 152 where the
REE-hydroxides are heated to a drying temperature that is
sufficient to convert a substantial majority of the REE-hydroxides
146 to REE-oxides 154. For example, the drying step 152 may include
heating the REE-hydroxides 146 to a temperature of at least about
100.degree. C., such as at least about 120.degree. C., and not
greater than about 160.degree. C., such as not greater than about
150.degree. C. In one example, the REE-hydroxides 146 are conveyed
to a screw feed dryer for the substantially continuous production
of the REE-oxides 154. In another example, the REE-hydroxides 146
may be stockpiled as necessary and dried batchwise.
[0052] It is an advantage of this embodiment that the resulting
REE-oxide product 154 will have a very high purity, particularly
with respect to base metals and radioactive metals such as uranium
and thorium. In one example, the REE-oxide product 154 has a purity
of at least about 99.8%, i.e., the REE-oxide product 154 comprises
at least about 99.8% REE-oxides, such as a purity of at least about
99.9%. For example, the REE-oxide product 154 may comprise not
greater than about 1 ppm thorium. The uranium content may be not
greater than 0.1 ppm, for example, such a not greater than about
0.01 ppm.
[0053] An REE-depleted nitrate solution 148 may also recovered from
the separation step 158, and may have a high content of ammonium
nitrate, such as from about 30 g/l to about 50 g/l ammonium
nitrate. The solution 148 may be recycled to preserve nitrates and
in particular to preserve ammonium in the process. As illustrated
in FIG. 4, the REE-depleted nitrate solution 148 may be conveyed to
a vessel 156 where ammonium hydroxide is stored for use in the
process, i.e., where the recycled nitrate solution 148 is added to
fresh ammonium hydroxide 158. An ammonium hydroxide product 160
such as an ammonium hydroxide solution may then be conveyed as
needed to the process, e.g., to hydroxylation steps 110a/110b
and/or to REE precipitation step 142. Because the recycled
REE-depleted nitrate solution will contain ammonium nitrates, it
may be desirable to remove the ammonium nitrates from the ammonium
hydroxide vessel 156 on a continuous or intermittent basis. In this
regard, a portion 162 of the solution contained within vessel 156
may be periodically bled off from the vessel 156 and subjected to
an ammonium nitrate precipitation step 164 to crystallize an
ammonium nitrate by-product 166 and recycle an ammonium nitrate
depleted solution 168 back to the vessel 156. The ammonium nitrate
by-product 166 will be of high purity and a valuable by-product of
the process.
[0054] While one example for the precipitation of REE compounds
from the thorium depleted solution have been described in detail,
it will appreciated that other methods may be applied. For example,
in some examples, it may be desirable to directly precipitate the
REEs as REE-nitrates from the thorium depleted solution.
[0055] As is noted above, the acidic solution 102 may contain REEs
in addition to thorium, and may be formed by the dissolution of a
variety of compounds in an acid (e.g., dissolution by acid
digestion). In some of the embodiments disclosed herein, it is
desirable that the REEs are in the form of REE-oxalates, e.g.,
RE.sub.2(C.sub.2O.sub.4).sub.3 or RE.sub.3(C.sub.2O.sub.4).sub.3,
where RE is a rare earth element. However, the solubility of
REE-oxalates in acid is very low. Thus, in one example, the acidic
solution 102 is formed by the dissolution of carbonate compounds,
such as RE.sub.2(CO).sub.3.xH.sub.2O where RE is a rare earth
element, and Th(CO.sub.3).sub.2.xH.sub.2O, as illustrated above in
FIG. 3. The REE-carbonates may be formed by a variety of methods,
and in one example the REE-carbonates are formed from REE-oxalates
by a metathesis reaction to render the REEs soluble in an acid such
as nitric acid.
[0056] In the embodiment illustrated in FIG. 5, an REE-oxalate
product 170 is converted in a metathesis step 172 to a
REE-carbonate product 174 for subsequent dissolution of the
REE-carbonate product 174 in an acid, e.g., to solubilize the REEs
and thorium in an acidic solution 102 (FIG. 3). In this embodiment,
an REE-oxalate product 170 is contacted with a carbonate compound
176 such as sodium carbonate (Na.sub.2CO.sub.3) in the metathesis
step 172, along with a solvent 178 such as water, which may be
introduced with the other reactants or introduced separately. For
example, the metathesis step 172 may include contacting the
reactants in a reactor 180 for a period of time sufficient to
convert at least about 98%, such as at least about 98.5% of the
REEs in the REE-oxalate product 170 to REE-carbonates in the
REE-carbonate product 174. Similarly, the metathesis step 172 may
be carried out for a period of time sufficient to convert at least
about 98%, such as at least about 98.5% of thorium in the
REE-oxalate product 170 from thorium oxalate to thorium carbonate
in the REE-carbonate product 174. The only by-product of the
metathesis step 172 is a high-purity carbon dioxide stream 182
which may be captured as a by-product.
[0057] In a separation step 184, the REE-carbonate product 174
(e.g., REE-carbonate particulates) may be separated from an oxalate
solution 186 such as by using a filter 188. The oxalate solution
186 will include substantial amounts of dissolved oxalates (e.g.,
Na.sub.2C.sub.2O.sub.4.yH.sub.2O when the carbonate compound 176 is
sodium carbonate) and in some examples discussed below, the oxalate
solution 186 may advantageously be recycled to a step where REEs
are precipitated as the REE-oxalate product 170.
[0058] Another embodiment of the present disclosure is directed to
the integration of several of the above-described methods in a
process for extracting REEs from an REE-oxalate product by applying
a metathesis reaction to convert the REE-oxalates to
REE-carbonates, digesting the REE-carbonates in an acid to form an
REE-rich solution, and selectively precipitating thorium from the
REE-rich solution. The resulting high purity REE-nitrate solution
may then be treated in a solvent extraction circuit to extract the
REEs, or may be processed to recover a dry powder of REE-nitrates,
REE-oxides or REE-hydroxides.
[0059] As illustrated in FIG. 6, a metathesis step 172 may be
carried out by contacting an REE-oxalate product 170 and a
carbonate compound 176 such as sodium carbonate in a reactor to
form an REE-carbonate product 174 and an oxalate solution 186,
e.g., an oxalate solution containing dissolved sodium carbonate.
The REE-carbonate product 174 is then subjected to an acid
digestion step 122 where the REE-carbonate product 174 may be
contacted with nitric acid, e.g., fresh nitric acid 120 and/or
recycled nitric acid 140 such as from a subsequent solvent
extraction circuit 126. Recycled thorium hydroxide product 108b
from a downstream hydroxylation step 110b may also be added to the
acid digestion step 122.
[0060] The resulting acidic solution 102 containing dissolved
carbonates may then be subjected to hydroxylation in steps 110a and
110b to form a thorium hydroxide product 108a which is a high
purity thorium hydroxide product containing very small
concentrations of REEs. The thorium depleted solution 106 my then
be separated from the thorium hydroxide product 108b and subjected
to a solvent extraction circuit 126 to extract REEs therefrom, as
is described with respect to FIG. 4. The REE-depleted nitrate
solution 148 may be recycled, e.g., also as described with respect
to FIG. 4.
[0061] In another embodiment of this disclosure, a method for the
formation of REE-oxalates from a solution, such as a pregnant
liquor solution ("PLS") is provided. The method may include the
extraction of the REEs from a PLS in the form of a precipitation
product that includes REE-oxalate particulates. In accordance with
this embodiment, the precipitation of oxalate compounds (e.g.,
REE-oxalates) from a PLS containing REEs and other elements (e.g.,
base metals, uranium and other metals) advantageously may result in
a very high purity REE-oxalate product having a very low
concentration of non-REE elements. Further, certain embodiments
provide for the recycling of oxalic acid and/or oxalate compounds
to reduce the overall consumption of oxalic acid by the
process.
[0062] FIG. 7 schematically illustrates one such method for the
formation of an REE-oxalate product having low concentrations of
non-REE elements. As illustrated in FIG. 7, a PLS 190 is contacted
with oxalic acid (H.sub.2C.sub.2O.sub.4) 192 in an oxalate
formation step 194. The PLS 190 may include one or more REEs, i.e.,
REEs that have been dissolved (e.g., solubilized) in the PLS 190.
For example, the PLS 190 may be an acidic solution (e.g., from a
chloride leach) and the REEs may be present as dissolved salts,
such as dissolved chloride salts. In one example, the PLS 190
includes a concentration of REEs of at least about 20 g/l. For
example, the PLS 190 may include at least about 25 g/l REEs, such
as at least about 30 g/l REEs, at least about 35 g/l REEs, or even
at least about 40 g/l REEs.
[0063] The PLS 190 may also include non-REE elements that are
solubilized in the PLS 190. The non-REE elements may include
metallic elements, particularly: alkali metals such as sodium (Na)
and potassium (K); alkaline earth metals such as magnesium (Mg),
calcium (Ca), strontium (Sr) and barium (Ba); transition metals
such as nickel (Ni), copper (Cu), zirconium (Zr), iron (Fe),
manganese (Mn) and titanium (Ti); post-transition metals such as
lead (Pb) and aluminum (Al); metalloids such as silicon (Si); and
radioactive metals (e.g., actinides) such as thorium (Th) and
uranium (U). The non-REE elements may also include non-metallic
elements such as sulfur (S) and phosphorous (P). Among the
foregoing, and in certain characterizations, the PLS 190 may
particularly include Mn in concentrations of at least about 10 g/l
and/or may include Fe in concentrations of at least about 20
g/l.
[0064] It is a particular advantage of the oxalate formation step
194 of this embodiment that a substantial majority of the non-REE
elements do not report with the REE-oxalate product 170, i.e., they
remain solubilized in an REE-depleted solution 198.
[0065] Exemplary compositions of pregnant liquor solutions are
illustrated in Table I.
TABLE-US-00001 TABLE I PLS Example 1 PLS Example 2 PLS Exemplary
Element (mg/l) (mg/l) Range (mg/l) F 3610 3380 3000-4000 Al 4555
4593 4000-5000 Ba 2126 2147 2000-2500 Ca 3045 3151 3000-3200 Fe
22838 22733 22000-30000 K 1638 1653 1000-2000 Mg 1892 1995
1000-2000 Mn 13349 13514 10000-14000 Na 17733 -- 10000-180000 P 70
71 60-80 Pb 1011 1058 1000-1200 S <100 <100 10-80 Si 47 49
40-50 Th 40 70 40-70 Ti 296 309 250-350 U 40 34 30-60 Zn 1210 1297
1000-3000 REEs 35737 36369 >35000
[0066] As can be seen from Table I, pregnant liquor solutions,
e.g., from the leaching of a rare earth ore concentrate with HCl,
may also contain appreciable amounts of non-REE elements, including
base metals and other undesirable metals such as uranium and
thorium. It is a significant advantage of this embodiment that
REE-oxalates may be precipitated from the PLS, while a substantial
majority of the non-REE elements remain in solution, i.e., do not
form oxalate compounds during the oxalate formation step.
Particularly, very low concentrations of elements such as Al, Fe,
Ca, Mg, n. P, Pb, S, Ti, U and/or Zn will precipitate with the
REE-oxalates. As a result, the REE-oxalate product is of very high
purity and a substantial proportion of the base metals and other
metals such as uranium can be removed prior to extraction of the
REEs, e.g., in a solvent extraction process.
[0067] The oxalate formation method of this embodiment to extract
REEs includes contacting the PLS 190 with oxalic acid 192 in an
oxalate formation step 194, such as by contacting the reactants in
a reactor 194 (e.g., a sealed reactor) under conditions such that
REE-oxalate compounds (e.g., RE.sub.2(C.sub.2O.sub.4).sub.3,
RE.sub.3(C.sub.2O.sub.4).sub.3, where RE=rare earth) precipitate
from the PLS 190. It will be appreciated that the REE-oxalate
compounds may also be hydrated, e.g.,
RE.sub.2(C.sub.2O.sub.4).sub.3.xH.sub.2O. The PLS 190 is an acidic
solution and may, for example, include free chloride ions. For
example, the PLS 190 may comprise hydrochloric acid (HCl) and may
be obtained from the leaching of rare earth minerals (e.g., a rare
earth ore concentrate) with HCl. In one example, the PLS 190 has a
free acid concentration (HCl) in the range of from about 0.5M to
about 1M (e.g., about 18.2 g/l to about 35.5 g/l HCl).
[0068] A sufficient amount of oxalic acid 192 is contacted with the
PLS 190 in the reactor 202 to precipitate a majority of the REEs as
REE-oxalates in an REE-oxalate product 170. For example, the oxalic
acid 192 (e.g., fresh oxalic acid) input to reactor 202 may be an
aqueous solution having a concentration of fresh oxalic acid in the
range of at least about 38.4 g/l to about 52.5 g/l. Excess oxalic
acid may be required and may be obtained by recycling various
product streams in the process as is described herein.
[0069] Sodium oxalate (Na.sub.2C.sub.2O.sub.4) 210 may also be
contacted with the PLS 190 in the oxalate formation step 194, such
as by adding the sodium oxalate 210 to the oxalic acid 192, or by
adding the sodium oxalate 210 directly to the PLS 190 in reactor
194. As is illustrated in FIG. 7, the sodium oxalate 210 may
advantageously be recycled from a subsequent process step, such as
from crystallization step 206, describe below. Alternatively, or in
addition to, fresh sodium oxalate may be added to the oxalate
formation step 194. In one example, the ratio of (fresh) oxalic
acid 192 to sodium oxalate 210 may be greater than 1, and in one
particular characterization, the ratio of oxalic acid 192 to sodium
oxalate 210 may be at least about 3:1 and not greater than about
4:1, such about 3.5:1. The addition of recycled sodium oxalate 210
to the oxalate formation step 194 may advantageously reduce the
total consumption of oxalic acid by the process. This reduction in
oxalic acid consumption may represent a significant cost savings
for the process.
[0070] The oxalate formation step 194 may be carried out under
reaction conditions such that the formation of REE-oxalates is
favored over the formation of most non-REE oxalates from the PLS
190. In one characterization, the oxalate formation step 194 is
carried out by maintaining the reactants (e.g., PLS 190, oxalic
acid 192 and optionally sodium oxalate 210) at an elevated
precipitation temperature (e.g., a controlled precipitation
temperature above ambient) of at least about 50.degree. C. and not
greater than about 90.degree. C., such as at least about 60.degree.
C. or at least about 70.degree. C., and not greater than about
85.degree. C. It has been found that a very high proportion of the
REEs dissolved in the PLS 190 will precipitate as REE-oxalate
particulates at such precipitation temperatures, while a
comparatively low quantity of most non-REE elements (e.g., with the
exception of thorium) will precipitate from the PLS 190.
Precipitation temperatures at the higher end of this range (e.g.,
from about 75.degree. C. to about 85.degree. C.) may result in
higher purity metal oxalates, i.e., a high content of REE-oxalates
in the REE-oxalate product 170 and a relatively low content of
non-REE oxalates in the REE-oxalate product 170. In one
characterization, the oxalate formation step 194 may be carried out
by maintaining a precipitation temperature (e.g., in reactor 202)
for a sufficient amount of time to precipitate at least about 75
at. % of the REEs in the PLS 190 as particulate REE-oxalates such
as, at least about 85 at. % of the REEs., at least about 90 at. %
of the REEs, at least about 95 at. % of the REEs, or even at least
about 98 at. %, at least about 99 at. % or 99.5 at. % of the REEs.
In one example, the oxalate formation step 194 may be carried out
for at least about 30 minutes and not greater than about 120
minutes, such as for about 60 minutes. The reactants may also be
agitated (e.g., mixed) in the reactor 202 during the oxalate
formation step 194.
[0071] After formation of oxalate precipitates in the reactor 194,
the metal oxalate precipitates may be allowed to crystallize (e.g.,
to grow) over a period of time and the REE-oxalate product 170 may
then be separated from an REE-depleted solution 212 in a separation
step 208. For example, the mixture 214 may be allowed to cool over
a period of time to allow crystallization of the metal oxalates to
form the REE-oxalate product 170.
[0072] If the mixture 214 from the oxalate formation step 194 is
allowed to cool, it may take a long period of time (e.g., several
days) for the metal oxalate precipitates to completely crystallize
so that the REE-oxalate product 170 may be readily separated from
the REE-depleted solution 212. Alternatively, the temperature of
the reactants may be increased to a second crystallization
temperature (e.g., greater than the first precipitation
temperature) to enhance (e.g., to accelerate) crystallization and
growth of the metal oxalate precipitates, particularly of the
REE-oxalates. It has been found that because a majority of the
initially available oxalate ion (C.sub.2O.sub.4.sup.2-) is consumed
in the formation step 194 at the precipitation temperature, the
increase in temperature in the crystallization step will not cause
a substantial amount of non-REE elements to precipitate from the
PLS 190. The crystallization temperature is greater than the
precipitation temperature, and in one characterization, the
crystallization temperature is at least about 5.degree. C. greater
than the precipitation temperature, such as at least about
7.degree. C. greater than the precipitation temperature. In another
characterization, the crystallization temperature is at least about
90.degree. C., such as at least about 92.degree. C. and is not
greater than about 100.degree. C., such as not greater than about
98.degree. C. The crystallization of the oxalates may be carried
out in the same reactor as the precipitation of the oxalates (e.g.,
in reactor 194), or may be carried out in a separate reactor (not
illustrated).
[0073] The crystallization temperature may be maintained for a time
sufficient to grow the REE-oxalate particulates to a size that is
suitable for subsequent separation 208 from the remaining
REE-depleted solution 212. In one characterization, the
crystallization temperature is maintained for a period of time
sufficient to grow the REE-oxalate precipitates to an average size
(e.g., diameter) of at least about 50 nm, such as at least about 65
nm. In one particular characterization, the REE-oxalate
precipitates are crystallized to an average size of from about 50
nm to about 85 nm. For example, the crystallization temperature may
be maintained for at least about 4 hours and not greater than about
8 hours, such as for about 6 hours.
[0074] After crystallization, REE-oxalate product 170 may be
separated from the REE-depleted solution 212 in a separation step
208. For example, the separation step 208 may include the use of a
micro-filter 216 to separate the REE-oxalate product 170 from the
REE-depleted solution 212.
[0075] The REE-oxalate product 170 comprises predominately
REE-metal oxalates. It is an advantage of the oxalate formation
step 194 that the REE-oxalate product 170 may be of high purity.
For example, the total non-REE metals (e.g., Ba, Na, K, Si, Sr
and/or Th) may constitute not greater than about 5 wt. % of the REE
oxalate product 170, such as not greater than about 3 wt. % or even
not greater than 1 wt. %. Table II illustrates the elemental metal
concentrations of exemplary REE-oxalate products, i.e., expressed
as percentages of the total metal content, as determined by
inductively coupled plasma (ICP) analysis.
TABLE-US-00002 TABLE II Ex. 1 Concentration Ex. 2 Concentration
Element (at. % of total metals) (at. % of total metals) REEs ~98.2
~92.5 F 0.00 0.00 Al <0.01 <0.01 Ba 0.44 1.00 Ca 0.16
<0.10 Fe 0.16 0.58 K 0.14 <1.00 Mg <0.01 <0.01 Mn
<0.1 <0.1 Na 0.08 <0.1 P 0.04 0.24 Pb 0.04 <0.10 S 0.04
0.04 Si 0.02 <0.50 Th 0.52 0.58 Ti 0.02 <0.10 U 0.00 0.00 Zn
<0.01 <0.10 Total Non- ~1.74 ~7.04 REEs Th + U ~0.52
~0.58
[0076] As is illustrated by Table II, the oxalate formation step
194 may advantageously selectively precipitate REE-oxalates from
the PLS 190, e.g., to the exclusion of non-REE elements such as
base metals.
[0077] Thus, the REE-depleted solution 212 may be an acidic
solution that includes solubilized metals that did not precipitate,
e.g., to form a metal oxalate, during the oxalate formation step
194. For example, the REE-depleted solution 212 may include
solubilized elements as listed in Table II such as Fe, Mn, Th, U,
F, Al, Ca, K, Mg, Na, Sr, Zn, P, S, Pb and Ti. In one
characterization, the REE-depleted solution 212 includes not
greater than about 0.5 g/l REEs, such as no greater than about 0.25
g/l REEs. In another characterization, the REE-depleted solution
212 contains no greater than about 10 ppm thorium, such as from
about 1 ppm to 10 ppm thorium. The REE-depleted solution 212 may
have high free acid content, and the free acid content may be
higher than the free acid content of the PLS 190. For example, the
free acid content (HCl) of the REE-depleted solution 212 may be
greater than about 100 g/l, such as greater than about 110 g/l. In
another characterization, the free acid content of the REE-depleted
solution 212 may be at least about 1.5 times greater than the free
acid content of the PLS 190, such as at least about 2 times
greater.
[0078] As is describe above, the REE-oxalate product 170 may
include a high concentration of the REEs in the form of
REE-oxalates. In one characterization, at least about 95% of the
total metallic elements in the REE-oxalate product 170 are REEs,
such as at least about 97% and even at least about 99% of the total
metallic elements. Any remaining non-REE metal oxalates (i.e.,
impurities) may comprise, for example, oxalates of Ba, Na, K, Si
and Th. Stated another way, based on the total metals content of
the REE-oxalate product 170, the product 170 may include no greater
than about 5 at. % non-REE metals, such as no greater than about 3
at. % non-REE metals and even no greater than about 1 at % non-REE
metals.
[0079] The REE-depleted solution 212 may be recycled to conserve
acid (e.g., HCl acid), which may be particularly advantageous due
to the relatively high free acid content of the REE-depleted
solution 212. For example, as illustrated in FIG. 7, the
REE-depleted solution 212 may be transferred to a thickening step
218 such as in a thickener 220. After thickening 218, a separation
step 222 may be carried out to separate oxalates 224, which then
may be recycled to the oxalate formation step 194, from an acidic
solution 226, e.g., an acidic solution that includes a high
concentration of chloride ions. The solution 226 may be subjected
to a distillation step 228 using a distiller 230 to recover water
232, which may be used as process water in other process steps, and
acid 238, which may also be recycled to other process steps such as
a leaching step described below. Residue 234 may be further treated
in a crystallization step 236 to recover and recycle oxalic acid
crystals 242 and a by-product 244 that contains metals. The
by-product 244 may be processed to recover further metals of value,
such as gold, uranium, aluminum, manganese, iron, magnesium,
strontium and zinc. The residue may be transferred to a
crystallization step 236, where oxalic acid crystals 176 may be
recovered and recycled.
[0080] As is noted above, the PLS 190 may be derived from the
leaching of rare earth mineral ore, such as an ore concentrate.
FIG. 8 illustrates a schematic flowsheet for one such leaching
process. It will be appreciated that the leaching process
illustrated in FIG. 8 is only exemplary, and that other leaching
processes for forming a pregnant liquor solution may also be
employed in accordance with this disclosure.
[0081] As illustrated in FIG. 8, the leaching process may include
leaching a rare-earth ore concentrate 246 with an acid 248 such as
HCl acid in a counter-current flow to enhance leaching efficiency
and reduce acid consumption. As is known to those skilled in the
art, the ore concentrate 246 may be derived from rare earth
containing minerals such as bastnaesite, monazite, carbonatite,
loparite, or similar rare earth containing minerals. After
separation from waste rock and other debris, the rare earth
minerals may be beneficiated (e.g., milled) to reduce particle size
and increase surface area of the minerals, and subjected to further
separation such as by flotation and/or magnetic separation. A
typical rare earth ore concentrate will include about 30% to about
70% rare earth oxides.
[0082] To extract metal values from the rare earth ore concentrate
246, the concentrate 246 may be first contacted with recycled PLS
250 (e.g., containing HCl) in a pre-leaching step 252. In addition
to the ore concentrate 246 and the recycled PLS 250, the
pre-leaching step 252 may optionally include the addition of a
reducing compound 254, for example a sulfur-containing compound
such as sodium sulfite (Na.sub.2SO.sub.3). The addition of a
sulfur-containing compound such as sodium sulfite may
advantageously precipitate barium (Ba) and radium (Ra) as their
sulfates from the PLS 246. Further, the sulfur may reduce iron (Fe)
in the PLS 246. Specifically, the compound 254 may be selected to
reduce at least a portion of the Fe in the PLS 246 from a +3
oxidation state to a +2 oxidation state. As is described herein,
the leaching process may be integrated with a step that includes
precipitating REE-oxalates from the PLS 246 by the addition of
oxalic acid. However, Fe.sup.3+ may disadvantageously consume
oxalic acid, and may increase the reagent costs for the overall
process. By reducing the Fe.sup.3+ concentration in the PLS 246 and
maintaining a substantial majority of the iron in the Fe.sup.2+
state, downstream consumption of oxalic acid may be reduced.
[0083] The pre-leaching step 252 is advantageously integrated
(e.g., in counter-current flow) with a primary leaching step 256.
In the primary leaching step 256, pre-leached ore concentrate 246a
is contacted with additional acid 248 (e.g., HCl) to leach metals
from the pre-leached ore concentrate 246a. For example, the primary
leaching step 256 may include contacting the pre-leached ore
concentrate 246a with fresh HCl 248 and/or recycled HCl 238, e.g.,
recycled PLS from distillation 228 (FIG. 7). A reducing compound
142 may also be used in the primary leaching step 256, as is
described above with respect to pre-leaching step 252. The primary
leaching step 256 may be carried out at an elevated temperature,
such as at least about 40.degree. C. to not greater than about
95.degree. C., for a period of time and under conditions (e.g.,
agitation) sufficient to solubilize substantially all of the REEs
(e.g., at least about 95 wt. % of the REEs) in the pre-leached ore
concentrate 246a. In one particular example, the primary leaching
step is carried out of a temperature of from about 50.degree. C. to
about 70.degree. C. to reduce dissolution of barium. The use of
lower leaching temperatures (e.g., 50.degree. C.) may also reduce
the capital expense of the reactor by enabling fiberglass reactors
to be utilized. In one characterization, the primary leaching step
256 is carried out for about 6 hours (e.g., for an average
residence time of about 6 hours).
[0084] After primary leaching 256, a solid/liquid separation step
258 may be carried out to separate an acidic solution 250
comprising REEs from leach solids 260, which then may be treated
(e.g., with hydroxides or carbonates) to neutralize the leach
solids before disposal as tailings. The acidic solution 250 may be
conveyed to the pre-leach step 252, after which the final PLS 190
is separated from the pre-leached ore concentrate 246a in a
separation step 262, e.g., in a substantially continuous
process.
[0085] The RE ore concentrate 246 will typically include other
elements in addition to the REEs, including metallic elements and
non-metallic elements. Table I above illustrates the predominant
elements that may be found in an exemplary pregnant liquor solution
(PLS) extracted from the acidic leaching of an RE ore
concentrate.
[0086] As is discussed above, it is an advantage of the methods
disclosed herein that the final REE product is of very high purity,
and includes low concentrations of non-REE elements such as base
metals, uranium and thorium. Such a high purity REE product may be
produced by combining the oxalate formation step described above
with a metathesis step to convert the REE-oxalates to
REE-carbonates, digesting the REE-carbonates in an acid and
selectively precipitating thorium as thorium hydroxide from the
solubilized REEs
[0087] Comprehensive flowsheets incorporating various embodiments
of the foregoing methods are illustrated in FIGS. 9A and 9B. In
accordance with these flowsheets, a rare earth ore concentrate is
leached in hydrochloric acid to form a pregnant liquor solution.
Rare earth metals in the form of REE-oxalates are then precipitated
from the pregnant liquor solution, and the REE-oxalates are then
converted to REE-carbonates in a metathesis reaction. The
REE-carbonate product, which also includes thorium carbonate, is
then digested in nitric acid and the thorium is precipitated as
thorium hydroxide by the addition of a hydroxide precipitant,
leaving a nitrate solution that is rich in REEs and contains a very
low concentration of other metals, including thorium, uranium and
base metals. This nitrate solution can then be treated in a solvent
extraction circuit to extract high purity rare earth metals (FIG.
9A), or can be treated to form REE-oxides (FIG. 9B).
[0088] Referring to both FIG. 9A and FIG. 9B, a rare earth ore
concentrate 246 is subjected to a leaching circuit to form a PLS
190 substantially as described with respect to FIG. 8. The leaching
process may include leaching a rare-earth ore concentrate 246 with
an acid 248 such as HCl acid in a counter-current flow to enhance
leaching efficiency and reduce acid consumption.
[0089] The concentrate 246 may be first contacted with recycled PLS
250 (e.g., containing HCl) in a pre-leaching step 252. In addition
to the ore concentrate 246 and the recycled PLS 250, the
pre-leaching step 252 may optionally include the addition of a
reducing compound 254, for example a sulfur-containing compound
such as sodium sulfite (Na.sub.2SO.sub.3) to precipitate barium
(Ba) and/or radium (Ra), and/or to reduce Fe.sup.3+ to Fe.sup.2+ in
the PLS 246. The pre-leaching step 252 is advantageously integrated
(e.g., in counter-current flow) with a primary leaching step 256.
As illustrated in FIGS. 9A and 9B, recycled HCl 238, e.g., recycled
PLS from a subsequent distillation step 228 may be contacted with
the incoming ore concentrate 246. In the primary leaching step 256,
pre-leached ore concentrate 246a is contacted with additional acid
248 (e.g., HCl) to leach metals from the pre-leached ore
concentrate 246a. A reducing compound 142 may also be used in the
primary leaching step 256, as is described above with respect to
pre-leaching step 252. The primary leaching step 256 may be carried
out at an elevated temperature, such as at least about 40.degree.
C. to not greater than about 95.degree. C., for a period of time
and under conditions (e.g., agitation) sufficient to solubilize
substantially all of the REEs (e.g., at least about 95 wt. % of the
REEs) in the pre-leached ore concentrate 246a. In one particular
example, the primary leaching step is carried out of a temperature
of from about 50.degree. C. to about 70.degree. C. to reduce
dissolution of barium. The use of lower leaching temperatures
(e.g., about 50.degree. C.) may also reduce the capital expense of
the reactor by enabling fiberglass reactors to be utilized. In one
characterization, the primary leaching step 256 is carried out for
about 6 hours (e.g., for an average residence time of about 6
hours).
[0090] After primary leaching 256, a solid/liquid separation step
258 may be carried out to separate an acidic solution 250
comprising REEs from leach solids 260. The leach solids may be
treated (e.g., with hydroxides or carbonates) to neutralize the
leach solids 260 before disposal as tailings. The acidic solution
250 may be conveyed to the pre-leach step 252, after which the
final PLS 190 is separated from the pre-leached ore concentrate
246a in a separation step 262, e.g., in a substantially continuous
leaching circuit.
[0091] In addition to REEs, the PLS 190 may also include non-REE
elements that are solubilized in the PLS 190, as is discussed
above. It is a significant advantage of this embodiment that
REE-oxalates may be precipitated from the PLS 190, while a
substantial majority of the non-REE elements remain in solution,
i.e., do not form oxalate compounds during the oxalate formation
step. Particularly, very low concentrations of elements such as Al,
Fe, Ca, Mg, n. P, Pb, S, Ti, U and/or Zn will precipitate with the
REE-oxalates. As a result, the REE-oxalate product is of very high
purity and a substantial proportion of the base metals and other
metals such as uranium can be removed prior to extraction of the
REEs, e.g., in a solvent extraction process.
[0092] After formation of the PLS 190, the PLS 190 is subjected to
an oxalate formation step 194 where REEs and thorium are
precipitated from the PLS 190, while a large proportion of other
metals (e.g., base metals and uranium) advantageously remain in
solution. The oxalate formation step 190 includes contacting the
PLS 190 with oxalic acid 192 under conditions such that REE-oxalate
compounds precipitate from the PLS 190. A sufficient amount of
oxalic acid 192 is contacted with the PLS 190 to precipitate a
majority of the REEs as REE-oxalates in an REE-oxalate product 170.
Sodium oxalate (Na.sub.2C.sub.2O.sub.4) 210 may also be contacted
with the PLS 190 in the oxalate formation step 194, such as by
adding the sodium oxalate 210 to the oxalic acid 192, or by adding
the sodium oxalate 210 directly to the PLS 190. As is illustrated
in FIGS. 9A and 9B, recycled sodium oxalate 224 may advantageously
be input to the oxalate formation step 194 from a subsequent
process step, such as from a thickening step 218 and separation
step 222. Alternatively, or in addition to, fresh sodium oxalate
210 may be added to the oxalate formation step 194. In one example,
the ratio of (fresh) oxalic acid 192 to sodium oxalate 210 may be
greater than 1, and in one particular characterization, the ratio
of oxalic acid 192 to sodium oxalate 210 may be at least about 3:1
and not greater than about 4:1, such about 3.5:1. As is noted
above, the addition of recycled sodium oxalate 224 to the oxalate
formation step 194 may advantageously reduce the total consumption
of oxalic acid by the process, i.e., by the oxalate formation step
194. This reduction in oxalic acid consumption may represent a
significant cost savings for the process.
[0093] As is described above with respect to FIG. 7, the oxalate
formation step 194 may be carried out under conditions such that
the formation of REE-oxalates is favored over the formation of most
non-REE oxalates from the PLS 190. In one characterization, the
oxalate formation step 194 may be carried out by maintaining a
precipitation temperature for a sufficient amount of time to
precipitate at least about 75 at. % of the REEs in the PLS 190 as
particulate REE-oxalates such as, at least about 85 at. % of the
REEs., at least about 90 at. % of the REEs, at least about 95 at. %
of the REEs, or even at least about 98 at. %, at least about 99 at.
% or 99.5 at. % of the REEs. In one example, the oxalate formation
step 194 may be carried out for at least about 30 minutes and not
greater than about 120 minutes, such as for about 60 minutes.
[0094] After formation of oxalate precipitates, the metal oxalate
precipitates may be allowed to crystallize (e.g., to grow) over a
period of time and the REE-oxalate product 170 may then be
separated from an REE-depleted solution 212 in a separation step
208. For example, the mixture 214 may be allowed to cool over a
period of time to allow crystallization of the metal oxalates to
form the REE-oxalate product 170. Alternatively, the temperature of
the reactants may be increased to a second crystallization
temperature (e.g., greater than the first precipitation
temperature) to enhance (e.g., to accelerate) crystallization and
growth of the metal oxalate precipitates, particularly of the
REE-oxalates. The crystallization of the oxalates may be carried
out in the same reactor as the precipitation of the oxalates, or
may be carried out in a separate reactor.
[0095] After crystallization, REE-oxalate product 170 may be
separated from the REE-depleted solution 212 in a separation step
208. The REE-oxalate product 170 comprises predominately REE-metal
oxalates and the REE-oxalate product 170 may be of very high
purity. For example, the total non-REE metals (e.g., Ba, Na, K, Si,
Sr and/or Th) may constitute not greater than about 5 wt. % of the
REE oxalate product 170, such as not greater than about 3 wt. % or
even not greater than 1 wt. %.
[0096] The REE-depleted solution 212 may be an acidic solution that
includes solubilized metals that did not precipitate, e.g., to form
a metal oxalate, during the oxalate formation step 194. For
example, the REE-depleted solution 212 may include solubilized
elements as listed in Table II above such as Fe, Mn, Th, U, F, Al,
Ca, K, Mg, Na, Sr, Zn, P, S, Pb and Ti. The REE-depleted solution
212 may have high free acid content, and the free acid content may
be higher than the free acid content of the PLS 190. As a result,
the REE-depleted solution 212 may be recycled to conserve acid
(e.g., HCl acid). As illustrated in FIGS. 9A and 9B, the
REE-depleted solution 212 may be transferred to a thickening step
218. After thickening 218, a separation step 222 may be carried out
to separate oxalates 224 (e.g., sodium oxalates), which then may be
recycled to the oxalate formation step 194. The acidic solution 226
will contain a high concentration of chloride ions, and the
solution 226 may be subjected to a distillation step 228 to recover
water 232 and an acid 238. The water may be used as process water
in other process steps, such as in a subsequent metathesis step
172, for filter washing etc. The 238 may also be recycled to other
process steps such as a leaching step 252 and/or 256. As
illustrated in FIGS. 9A and 9B, the acid is recycled to the
pre-leaching step 252 in a closed loop. Residue 234 from the
distillation may be further treated in a crystallization step 236
to recover and recycle additional oxalic acid crystals 242 to the
oxalate formation step 194, further reducing the consumption of
oxalic acid by the process. The by-product 244 from the
crystallization step 236 contains metals, and may be processed to
recover further metals of value, such as gold, uranium, aluminum,
manganese, iron, magnesium, strontium and zinc.
[0097] The foregoing process steps described with respect to FIGS.
9A and 9B illustrate various ways that reagents (e.g., hydrochloric
acid and oxalic acid) may be recycled within the process to
significantly reduce reagent consumption and reduce operating
expenses associated with the process.
[0098] As is described above with respect to FIGS. 5 and 6, the
high purity REE-oxalate product 170 is converted in a metathesis
step 172 to a REE-carbonate product 174 for subsequent dissolution
of the REE-carbonate product 174 in an acid, e.g., to solubilize
the REEs and thorium in an acid digestion step 122. In this
embodiment, an REE-oxalate product 170 is contacted with a
carbonate compound 176 such as sodium carbonate (Na.sub.2CO.sub.3)
in the metathesis step 172, along with a solvent such as water
(e.g., recycled water 232), which may be introduced with the other
reactants or may be introduced separately. The only by-product of
the metathesis step 172 is a high-purity carbon dioxide stream 182
which may be captured as a by-product.
[0099] In a separation step 184, the REE-carbonate product 174
(e.g., REE-carbonate particulates) may be separated from an oxalate
solution 186 such as by using a filter 188. The oxalate solution
186 will include substantial amounts of dissolved oxalates (e.g.,
Na.sub.2C.sub.2O.sub.4.yH.sub.2O when the carbonate compound 176 is
sodium carbonate) and the oxalate solution 186 may advantageously
be recycled back to the oxalate formation step 194 to further
reduce the consumption of fresh reagents.
[0100] After formation of the high purity REE-carbonate product
174, the product 174 may be contacted with nitric acid 120 (e.g.,
fresh nitric acid or sulfuric acid) in an acid digestion step 122.
The resulting acidic solution 102 may be an acidic solution
substantially as described above with respect to FIGS. 1 and 2
above. The acidic solution 102 may then be contacted in a first
hydroxylation step 110a with a hydroxide precipitant 104a to
precipitate a thorium hydroxide product 108a from the acidic
solution 102. The thorium hydroxide product 108a may be separated
from the thorium depleted solution 106b in a separation step 114a.
Thereafter, the intermediate thorium depleted solution 106b may be
contacted in a second hydroxylation step 110b with a second
hydroxide precipitant 104b to form the thorium depleted solution
106. The thorium depleted solution 106 may then separated from the
second thorium hydroxide product 108b in a separation step
114b.
[0101] The amount of thorium hydroxide product 108b may be
relatively small and there may be appreciable quantities of REEs in
the thorium hydroxide product 108b. To reduce losses of REEs, the
thorium hydroxide product 108b may advantageously be recycled back
to the acid digestion step 122 where the thorium is re-digested
with the REE-carbonate product 174. In this manner, all of the
thorium hydroxide is removed from the acidic solution 102 with the
first thorium hydroxide product 108a. When the thorium hydroxide
product 108b is separated in separating step 114b, the resulting
thorium depleted solution 106 is a relatively high purity
RE-nitrate solution (or RE-sulfate solution in the event sulfuric
acid is utilized in the digestion step 122).
[0102] Referring now to FIG. 9A, the high purity RE-nitrate
solution 106 may then be subjected to a solvent extraction circuit
126 to extract REEs as metals from the thorium depleted solution
106. Having the REEs solubilized in nitrate media (nitric acid) may
reduce the expenses associated with a solvent extraction circuit
126. The solvent extraction circuit 126 may include the steps of
solvent extraction 128 and solvent stripping 130 with a stripping
solvent 132. Because the thorium depleted solution 106 described
herein is of extremely high purity, the solvent extraction circuit
126 may advantageously be operated at a reduced capital expense and
reduced operating expense. The resulting products are very high
purity and high value REE metals 134.
[0103] The thorium depleted solution 106 may also include
substantial quantities of highly salable ammonium nitrate. Thus, an
ammonium nitrate removal step 136 may be utilized to continuously
or intermittently remove ammonium nitrate 138 from the solution
106. As illustrated in FIG. 9A, the ammonium nitrate is removed
after the solvent extraction circuit 126, as the presence of
ammonium nitrate in the thorium depleted solution 106 is not
believed to impair the efficacy of the solvent extraction circuit
126. However, it will be appreciated that the ammonium nitrate
separation step may also occur before the solvent extraction
circuit 126 if desired.
[0104] The ammonium nitrate separation step 136 may include cooling
the REE-depleted acidic solution 148 to a reduced temperature
(e.g., below about 10.degree. C.) to crystallize ammonium nitrate
138. Because ammonium nitrate 138 is highly soluble in acid, it may
only be necessary to intermittently operate the separation step 136
to remove ammonium nitrate 138. Ammonium nitrate is valuable and
salable by-product that is widely used in the fertilizer industry
and may represent a significant source of revenue from the process.
After separation of the ammonium nitrate 138 (intermittently or
continuously), the resulting acid 140 (e.g., recycled nitric acid)
may be recycled back to the process, e.g., back to the acid
digestion step 122. Thus, the acid (e.g., input at 120) may be
contained in an essentially "closed loop" within the process.
Additional acid may be generated during the solvent extraction
circuit due to cationic ion exchange releasing protons into
solution. In this regard, a substantial quantity of the acid
required for the acid digestion step may be provided by the
recycled acid 140, and only a small amount of fresh acid 120 may be
required for the process once steady state and continuous
operations are achieved and maintained.
[0105] Referring now to FIG. 9B, an REE precipitation circuit
replaces the solvent extraction circuit of FIG. 9A. Thus, the
thorium depleted and REE-nitrate rich solution can be treated to
precipitate high purity REE-compounds such as REE-oxides and/or
REE-hydroxides which, for example, may be shipped to a separate
facility for extraction of the REEs as metals. See FIG. 4 described
above.
[0106] The thorium depleted solution 106 from the separation step
114b will typically have a pH in the range of about pH 3.6 to about
pH 4 (e.g., about pH 3.8) and will be rich in REE-nitrates and may
contain no, or extremely low levels of, thorium and/or uranium.
This solution 106 is conveyed to an REE precipitation step 142,
where the solution 106 is contacted with an REE precipitation agent
144, such as ammonium hydroxide, in sufficient quantities to
increase the pH of the solution, such a by increasing the pH to at
least about pH 4.5, such at least about ph 4.9. In one
characterization, the pH during the precipitation step 144 is not
greater than about pH 6 and may be about pH 5.5. At these pH
levels, the REEs will precipitate from the solution 106 as
REE-hydroxides 146, which may be separated from an REE-depleted
nitrate solution 148 in a separation step 150.
[0107] The REE-hydroxides 146 may then be converted from the
REE-hydroxides to REE-oxides. The REE-hydroxides 146 are conveyed
to a drying step 152 where the REE-hydroxides are heated to a
drying temperature that is sufficient to convert a substantial
majority of the REE-hydroxides 146 to REE-oxides 154. In one
example, the REE-hydroxides 146 are conveyed to a screw feed dryer
for the substantially continuous production of the REE-oxides 154.
In another example, the REE-hydroxides 146 may be stockpiled as
necessary and dried batchwise.
[0108] It is an advantage of this embodiment that the resulting
REE-oxide product 154 will have a very high purity, particularly
with respect to base metals and radioactive metals such as uranium
and thorium. In one example, the REE-oxide product 154 has a purity
of at least about 98%, i.e., the REE-oxide product 154 comprises at
least about 98% REE-oxides. Further, the REE-oxide product 154 may
have a purity of at least about 99%, such as at least about
99.5%.
[0109] An REE-depleted nitrate solution 148 may also recovered from
the separation step 158, and may have a high content of ammonium
nitrate, such as from about 30 g/l to about 50 g/l ammonium
nitrate. The solution 148 may be conveyed to a vessel 156 where
ammonium hydroxide is stored for use in the process, i.e., where
the recycled nitrate solution 148 is added to fresh ammonium
hydroxide 158. An ammonium hydroxide product 160 such as an
ammonium hydroxide solution may then be conveyed as needed to the
process, e.g., to hydroxylation steps 110a/110b and/or to REE
precipitation step 142. Because the recycled REE-depleted nitrate
solution will contain ammonium nitrates, it may be desirable to
remove the ammonium nitrates from the ammonium hydroxide vessel 156
on a continuous or intermittent basis. In this regard, a portion
162 of the solution contained within vessel 156 may be periodically
bled off from the vessel 156 and subjected to an ammonium nitrate
precipitation step 164 to crystallize an ammonium nitrate
by-product 166 and recycle an ammonium nitrate depleted solution
168 back to the vessel 156. The ammonium nitrate by-product 166
will be of high purity and a valuable by-product of the
process.
[0110] The flowsheets illustrated in FIGS. 9A and 9B may provide at
least one or more of the following advantages.
[0111] The metathesis step produces a REE-carbonate product that is
soluble in relatively dilute concentrations of acid, e.g., 6 wt. %
or lower, as compared to other REE compounds such as REE-oxalates
and REE-oxides. This results in a lower overall acid consumption
and therefore reduced operating expense.
[0112] The metathesis advantageously removes uranium from the
product, as uranium carbonate does not form during metathesis. In
one example, at least about 95%, such as at least about 97% of the
uranium contained in the pregnant liquor solution will be rejected
in the during the metathesis step, leaving less than 5%, such as
less than 3% of the initial uranium in the REE-carbonate
product.
[0113] The metathesis step advantageously enables the oxalate
reagents to be recycled back to the oxalate formation step, thereby
reducing the consumption of fresh oxalic acid.
[0114] The resulting REE-nitrate solution is of extremely high
purity, and contains extremely low quantities of radioactive
elements such as radium, thorium and/or uranium. Essentially all
radium may be removed with sulfites during leaching of the ore
concentrate and subsequent oxalate formation steps. The bulk of the
uranium is rejected at the oxalate formation step, and most
remaining uranium is rejected during the metathesis step. Thorium
is removed as thorium hydroxide when precipitated with a hydroxide
precipitant. The REE-nitrate solution also is substantially free of
suspended solids, such as silicate particulates, thereby
substantially reducing crud or mud formation in the solvent
extraction.
[0115] The REE-nitrate solutions may also reduce the capital
expenses associated with the solvent extraction circuit as compared
to other solutions such as REE-chloride solutions. For example,
chloride solutions typically require titanium coated vessels to
carry out the extraction. The use of a nitrate solution may
eliminate this requirement.
[0116] The process may utilize several recycle streams and
therefore is cost effective with respect to the reagents.
EXAMPLES
[0117] A pregnant liquor solution containing REEs is contacted with
oxalic acid to precipitate metal oxalates. The precipitation
temperature is about 70.degree. C. No sulfite was added to the PLS,
and therefore Fe.sup.3+ was present. No recycle was performed. The
concentration of oxalic acid is varied from 90 g/l to 115 g/l to
140 g/l to assess the effect of oxalic acid concentration on the
purity of the precipitate product (i.e., the metal oxalates).
Results are shown in Table III.
TABLE-US-00003 TABLE III Oxalate @ Oxalate @ Oxalate @ 90 g/l 115
g/l 140 g/l H.sub.2C.sub.2O.sub.4 H.sub.2C.sub.2O.sub.4
H.sub.2C.sub.2O.sub.4 Element (wt. %) (wt. %) (wt. %) REEs.sup.1 Ce
16.511467 16.61625 16.36584 La 8.549287 8.323717 7.840823 Nd
5.490483 5.67714 5.908091 Pr 1.594311 1.635.82 1.66676 Y 0.075021
0.075069 0.7525 TOTAL REEs 32.220569 30.6922 32.534 Impurity
Elements Th 0.193692 0.218965 U 0.001687 0.001574 0.001865 Si
0.491763 0.46509 0.470356 Au <LOD <LOD 0.002628 As 0.002351
0.0011.58 0.002418 Se <LOD <LOD <LOD Pb <LOD <LOD
<LOD Zn <LOD <LOD <LOD Cu 0.012393 0.012785 0.013937 Ni
0.032589 0.034371 0.03619 Co 0.071752 0.070868 0.070785 Fe 0.513656
0.530995 0.564361 Mn <LOD <LOD <LOD Cr <LOD <LOD
<LOD V <LOD <LOD <LOD Ti <LOD <LOD <LOD Ca
<LOD <LOD <LOD K <LOD <LOD <LOD Zr <LOD
<LOD <LOD Mo 0.000372 0.000516 0.000781 Nb 0.000466 0.000678
<LOD Sr 0.00376 <LOD <LOD Mn <LOD <LOD <LOD Cr
<LOD <LOD <LOD V <LOD <LOD <LOD Ti <LOD
<LOD <LOD Ca <LOD <LOD <LOD K <LOD <LOD
<LOD Al <LOD <LOD <LOD Mg <LOD <LOD <LOD Zr
<LOD <LOD <LOD .sup.1other REEs not analyzed <LOD =
below the limits of detection
[0118] As demonstrated by Table IV, REE-oxalates with a high
proportion of REEs and a relatively low proportion of non-REEs can
be obtained by oxalate precipitation over a range of oxalic acid
concentrations, even at a precipitation temperature of about
70.degree. C. In particular, it is noteworthy that many prior
processes for separation of REEs from a pregnant liquor solution
also precipitate many non-REE elements with the REEs, for example
U, Si, As, Pb, Zn, Fe, Mn, Mo, Nb, Cr, Ti, Ca, K, Al and Zr.
[0119] In the following Example, thorium is precipitated from an
acidic solution using a hydroxide precipitant at various pH levels
to observe the effect of pH on the precipitation of thorium and of
REES.
[0120] For these tests, 400 grams (326 ml) of a nitric acid
solution having a free acid content of about 5 g/l and a specific
gravity of 1.227 is added to a one liter vessel having a mixer. A
1M solution of ammonium hydroxide (NH.sub.4OH) is added dropwise to
the vessel until the target pH level is reached, and the target pH
is maintained for one hour. A temperature of about 25.degree. C. is
maintained during the precipitation step. After 60 minutes, the
vessel contents are filtered and the weight, specific gravity and
free acid content of the filtrate are measured. The retentate is
washed with deionized water and dried.
TABLE-US-00004 TABLE IV Acidic Solution Assay % % % % % (mg/l or
Precipitated Precipitated Precipitated Precipitated Precipitated
Element g/tonne) @ pH 1.0 @ pH 2.0 @ pH 2.5 @ pH 3.0 @ pH 3.5 La
20000 7 5 4 0 2 Ce 13500 7 5 3 0 2 Pr 3150 3 0 7 0 19 Nd 11000 4 1
3 0 3 Sm 1580 4 0 8 0 19 Eu 352 3 0 7 0 19 Gd 814 2 0 5 0 15 Tb 62
3 0 8 4 22 Dy 204 2 0 5 0 17 Ho 24 2 0 4 1 19 Y 494 5 0 7 0 18 Er
40 3 0 7 0 15 Tm 4 4 4 3 0 18 Yb 19 7 6 5 2 21 Lu 3 5 5 5 2 21 Sc
<5 0 0 3 22 70 Th 734 4 1 8 62 95 U 1 0 0 0 0 24
[0121] The foregoing data is graphically illustrated in FIG. 10A.
This data demonstrates that at pH 3.0, 62% of the thorium in the
acidic solution may be precipitated as thorium hydroxide. When the
pH is increased to pH 3.5, 95% of thorium is precipitated, however
increasing amounts of REEs also begin to precipitate from the
solution.
[0122] However, if thorium concentration in the solution is
decreased, it is found that the pH can be increased without
precipitating significant quantities of REEs from the solution.
FIG. 10B illustrates the results of increasing the pH of a solution
over a range from pH 3.0 to pH 3.8, where the initial thorium
concentration is decreased to 117 mg/l. As is illustrated in FIG.
10B, pH levels at least as high as pH 3.8 can be utilized to
extract a high percentage of the thorium without precipitating
significant amounts of the REEs. The results for the tests at pH
3.5, pH 3.6 and pH 3.8 for a solution containing 117 mg/l thorium
are given in Table V.
TABLE-US-00005 TABLE V Feed Final Solution Percent Final Solution
Percent Final Solution Percent Assay Assay @ pH 3.5 Removed Assay @
pH 3.6 Removed Assay @ pH 3.8 Removed Element (mg/l or g/tonne)
(mg/l or g/tonne) @ pH 3.5 (mg/l or g/tonne) @ pH 3.6 (mg/l or
g/tonne) @ pH 3.8 La 2710 2200 1 2220 0 2160 2 Ce 1870 1520 1 1540
0 1500 2 Pr 473 386 0 390 0 380 2 Nd 1630 1344 0 1364 0 1332 0 Sm
232 189 0 191 0 187 1 Eu 51.0 42 0 42 0 41 1 Gd 122 99 0 102 0 100
0 Tb 9.8 8 0 8 0 8 0 Dy 30.8 23 1 25 0 25 0 Ho 3.52 5 1 3 0 3 0 Y
74.4 60 1 61 0 59 3 Er 6.15 5.06 0 4.84 4 4.96 1 Tm 0.58 0.46 3 0.5
0 0.48 0 Yb 2.83 2.32 0 2.30 1 2.22 4 Lu 0.35 0.30 0 0.28 2 0.28 2
Sc 0.71 0.62 0 0.40 31 0.48 17 Th 117 69 28 65 32 51 46 U 0.22 0.18
0 0.18 0 0.18 0
[0123] As is illustrated in this Example, high levels of thorium
can be extracted from a relatively dilute solution at increased pH
levels, without extracting high levels of REEs from the
solution.
[0124] While various embodiments have been described in detail, it
is apparent that modifications and adaptations of those embodiments
will occur to those skilled in the art. However, is to be expressly
understood that such modifications and adaptations are within the
spirit and scope of the present disclosure.
* * * * *