Note: Descriptions are shown in the official language in which they were submitted.
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SINGLE STAGE PURIFICATION FOR URANIUM REFINING
FIELD OF THE INVENTION
The present invention relates to a process for the preparation of nuclear
grade pure
uranium dioxide, natural metallic uranium and uranium hexafluoride from yellow
cake containing boron, rare earth and other metallic impurities. More
particularly, the
present invention relates to a process for the preparation of pure uranium
dioxide
that meets the nuclear grade specifications of the equivalent boron content
being
less than 4p,g/g on uranium basis as per ASTM C ¨ 753-99.
BACKGROUND AND PRIOR ART
Yellowcakes are uranium concentrates, which represent an intermediate step in
the
processing of uranium ores. Yellowcakes are usually obtained through the
milling
and chemical processing of uranium ore forming a coarse powder, which is
insoluble
in water and contains about 60,80% of uranium oxide depending on type of
Yellowcakes such as Magnesium Di-uranate, Ammonium Di-uranate or Uranium
peroxide. In the process conventionally used within the art, the ore is first-
crushed to
a fine powder by passing the starting raw uranium ore through crushers and
grinders
to produce the pulped ore. The pulped ore is thereafter processed with
concentrated =
acid or an alkaline solution to leach out the uranium and the eluate is
subjected to
precipitation of Uranium concentrates that is then filtered and dried to
produce
yellowcake. This yellowcake usually contains boron, rare earths and other
metallic
impurities.
The yellowcake thus produced is thereafter converted to nuclear grade pure
uranium
dioxide, natural metallic uranium and uranium hexafluoride using various
processes
conventionally known in the art. In one of the known processes i,e Solvent =
Extraction, the yellowcake is first dissolved in nitric acid and thereafter
feed
preparation is done to adjust the nitric acid and uranium concentration. This
feed is
thereafter passed through a multi-stage counter current slurry extractor
wherein
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uranyl nitrate is extracted using a mixture of 33% tributyl phosphate and
kerosene
leaving behind impurities in the mother liquor known as raffinate. The organic
phase
containing pure uranyl nitrate is further subjected to another separation step
using
de-mineralized water to produce pure uranyl nitrate solution. This pure urnayl
nitrate
solution is co-precipitated with ammonia to produce ammonium diuranate (ADU),
which is thereafter converted to produce nuclear grade uranium dioxide or
metallic
uranium.
_
The conventionally known solvent extraction process, which involves the use of
carcinogenic materials such as tributyl phosphate, highly inflammable kerosene
and
hazardous ammonia needs lots of monitoring and safety regulations for
industrial
, scale operations. This process also produces degraded tributyl phosphate due
to
reaction with nitric acid and radioactivity, which requires complicated
disposal
method involving further treatment and incineration facilities, generates lots
of solid
and liquid wastes containing nitrates which are difficult to dispose off. The
generated
solid waste contains a preponderance of nitrates, which therefore cannot be
recycled for the recovery of uranium without the removal of nitrates. The
removal of
nitrates from the solid wastes generated requires special treatment steps as
nitrate
contamination of the ground water may lead to methemoglobinemia and stomach
cancer. The conventionally used solvent extraction process is also
disadvantageous
in that it generates multiple streams of wastes. The liquid waste stream
typically
contains about 100 ppm of uranium along with the soluble nitrates which are
disposed off in large solar ponds. The disposal of the liquid wastes in the
large solar
ponds requires large space and a continuous monitoring of the ground water
around
the solarpond.
In another "dry refining process", the starting yellowcake is directly
palletized and
reduced with hydrogen to produce uranium dioxide at a temperature between 550
¨
650 C in a fluidized bed reactor. The uranium dioxide is thereafter converted
to
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uranium tetrafluoride and uranium hexafluoride in the fluidized bed/Flame
reactor. The
thus produced uranium hexafluorides are thereafter "refined" using a two-stage
pressure
distillation process. Further, this process of refining uranium fluorides by
pressure
distillation is a technically difficult and potentially hazardous process.
Refining is a process aimed at reducing the harmful impurities to an
acceptable level,
particularly to meet the nuclear grade specification that the equivalent boron
content
(EBC) may not exceed 4 g/g on uranium basis as per ASTM C - 753-99. The
process
according to the present invention surprisingly brings down the initial EBC of
180 g/g
in the starting yellowcake to about less than 1.0 gig of EBC in the final
product.
A further approach for the production of pure uranium grades has been to use
purer
forms of the starting yellowcakes using hydrogen peroxide for the reduction of
Mo, V,
P1 Zr, As, Ca, Mg, Na, Si and other sulfates to produce purer yellowcakes in
the form
of uranium peroxide from eluate solutions of sulfate nature compared with
Ammonium
diuranate and Magnesium diuranate are used as the starting materials. However,
it has
been found that the yellowcake produced in the form of uranium peroxide using
the
aforesaid approach also contains substantial levels of rare earth impurities
that remain
in the yellow cake thus necessitating further refining steps.
Without wishing to be bound by theory, the inventors believe that hitherto, it
has been
impossible to remove boron, cadmium, heavy metals and rare earth impurities
such as
gadolinium, europium and samarium in a single refining step using Hydrogen
peroxide
as precipitation route because the process involves stringent pH control by
the addition
of alkaline solutions, which interferes with the simultaneous removal of
boron,
cadmium, and rare earth impurities such as gadolinium, europium and samarium
in
single refining step.
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US 4 024 215 describes a process for the preparation of yellowcake, with, a
reduced
content of sodium and vanadium from the eluate. However, the disclosed is not
suitable for the removal of the rare earth impurities. Further, the yellowcake
produced is further refined using the wet solvent 'extraction process to
produce
nuclear grade uranium, which suffers from the above identified deficiencies.
US 2 770 521 teaches the treatment of sulfated uranium with a peroxide to
produce
uranium peroxide di-hydrate and separation of relatively pure uranium
peroxide.
However, the starting material for the process disclosed in this US patent is
uranium
present in the slag after the magneto-reduction of uranium tetrafluroide,
which is
already free of the rare earth ,impurities sought to be removed according to
the
present' invention. A further disadvantage of the process disclosed in this US
patent'
is that the disclosed process requires strict pH control for the complete
precipitation
of uranium peroxide, which is cumbersome.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide a single step
precipitation route for
refining yellow cake to produce nuclear grade pure uranium.
It is a further object of the invention to provide a process for refining
yellowcake that
is eco-friendly producing only one waste liquid stream that contains a ppm
level of
uranium and wherein the level of nitrate impurities is substantially less than
that
obtained by the solvent wet extraction process.
It is a further object of the present invention to provide a single step
precipitation ,
route for refining yellowcake to meet Nuclear Purity Standard as specified by
ASTM
C 753 ¨ 99 that avoids the use of hazardous and toxic chemicals.
=
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It is a further object of the present invention to provide a process for
refining
yellowcake to produce nuclear grade pure uranium that permits a simultaneous
removal
of boron, heavy metals, and rare earth metals such as gadolinium, dysprosium,
samarium, europium and the like.
5 It is a further object of the present invention to provide a process for
refining
yellowcake to produce nuclear grade pure uranium that brings down the level of
boron
and rare earth metals such as gadolinium, dysprosium, samarium, europium and
the like
to less than 0.1 ppm in a single precipitation step followed by washing.
It is a further object of the present invention to provide a process for
refining
yellowcake to produce nuclear grade pure uranium that brings down the
Equivalent
boron concentration (EBC) of the yellowcake to less than 1.0 microgram per
gram of
uranium.
It is a further object of the present invention to provide a process for
refining
yellowcake to produce nuclear grade pure uranium that does not require
stringent pH
control conditions.
SUMMARY OF THE INVENTION
A process for refining crude yellow cake containing significant levels of rare
earths like
Gadolinium, Europium, Samarium, and Dysprosium and other elements like Boron,
Cadmium and Iron to produce nuclear grade uranium, using a single step
precipitation
route for simultaneous removal of above said impurities, comprising of
dissolving the
yellow cake in nitric acid under mild agitation and adding hydrogen peroxide
at pre-
defined pH and temperature to electively precipitate uranium peroxide hydrate,
having
total equivalent boron content (EBC) less than 4.0 prg/g on U weight basis and
individual impurity elements as per ASTM C 753.
In another aspect, the invention provides a process for refining crude yellow
cake
containing impurities comprising boron, cadmium, rare earth metals comprising
Gadolinium, Europium, Samarium, and Dysprosium which are thermal neutron
poisons
and heavy metals to produce nuclear grade uranium, using a single step
precipitation
route for simultaneous removal of the impurities, comprising of dissolving the
yellow
cake in nitric acid under mild agitation and adding hydrogen peroxide at pH of
below 2
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and temperature between 15 to 30 C to selectively precipitate uranium peroxide
hydrate, followed by one or more dimineralized water washing to achieve final
uranium
peroxide hydrate, having total equivalent boron content (EBC) less than 4.0
gig on U
weight basis and individual impurity elements as per ASTM C 753.
Preferably, the pH is not controlled during precipitation by addition of any
chemical.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process for refining yellowcake to produce
nuclear
grade uranium using a single step precipitation route followed by washing for
the
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simultaneous removal of heavy metals, boron and other rare earth metals
comprising dissolving the yellowcake in nitric acid under mild agitation and
adding
hydrogen peroxide at pre-defined pH and temperature to selectively precipitate
uranium peroxide hydrate.
According to the present invention, the starting yellowcake is preferably
magnesium
diuranate (MDU), which may be prepared from the uranium ore by processes that
are conventionally known in the art. The starting material MDU typically
contains
various impurities having an equivalent boron concentration as high as 180
gig on
uranium basis.
. In the process of the present invention, the starting MDU is dissolved in
nitric acid to
produce crude uranyl nitrate containing all the impurities in dissolved form.
In a preferred embodiment, the said nitric acid , used has strength of about 2
N to
about 6 N.
In a subsequent step of the process of the present invention, uranium peroxide
hydrate is selectively precipitated out by the addition of commercial hydrogen
peroxide to "above uranyl nitrate solution at a predetermined pH. The pH of
the
solution at the time of addition of hydrogen peroxide is selected such that
all the
impurities remain in soluble form in the reaction system.
In a preferred embodiment, the commercial hydrogen peroxide is about 30 to 70%
w/w. The pH of the reaction system is preferably, less than about 2, at which
pH, it
was surprisingly found that all the impurities were present in the reaction
system in
soluble form, which enabled precipitation of surprisingly pure uranium
peroxide
hydrate.
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It was surprisingly found that the process outlined above led to the end
product i.e.
uranium peroxide hydrate that had a substantially reduced equivalent boron
concentration less than 1.0 gig of EBC, which is well below the specification
depicted
in ASTM C 753 - 99. It was also found that the process herein described led to
a
simultaneous reduction of boron, cadmium, heavy metals, rare earth metal
content such
that of gadolinium, europium, and samarium and other heavy metals such as
iron,
nickel, cobalt, calcium and magnesium in one refining step, which was hitherto
not
possible using hydrogen peroxide precipitation route.
A qualitative analysis of the refined uranium peroxide hydrate according to
the process
described herein presented the following results.
Elements Impurity in MDU Impurity in
Uranium
( g/g of U) Peroxide
hydrate
prepared according to
the invented process
(p.g/g of U)
= 0.1 (decontamination
factor= 200)
Gd 29 0.04 (decontamination
factor = 750)
Cd 3.1 <0.1
Eu 18 <0.1
Sm 2.8 <0.1
Fe 6000 22
, Mg 82200 19
Ca , 12500 = 77 =
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The present inventors have also found that completion of precipitation as well
as the
efficiency of refining depended on the stoichiometric ratios of the reactants,
system
pH and temperature. While the role of pH in the purity of the final product is
not
clear, the present inventors believe that the reaction aCcording to the
invented
process leads to an in-situ formation of nitric acid, which increases the
tendency for
the precipitated end product to get dissolved back favoring a reverse
reaction. A
careful monitoring of the pH to below 2 presumably strikes balance between the
prevention of co-precipitation of impurities along with uranium peroxide
hydrate, and
a comparatively slow reverse reaction favoring a low dissolved uranium
peroxide -
hydrate content in the filtrate. It is also believed that a , low temperature
of the
reaction system favors lower uranium content in the solution as the solubility
of
uranium peroxide hydrate in nitric acid is found to be low at room
temperature. This
ensures that a concentration of uranium content in the filtrate could be
maintained
below 300 ppm.
In a further optional embodiment, the dissolved uranium content (less than 300
ppm)
could be optionally further reduced to less than 50 ppm by further adding
hydrogen
peroxide over a sufficient time period to allow delayed precipitation of
uranium
peroxide hydrate with addition of suitable flocculants like INDFLOC or by Ion
Exchange with suitable cation or anion based resins. Thus, it was found that
the
overall uranium content loss from the system could be kept at as low as 20- 50
ppm
which is well below the overall loss of uranium content in the conventional
solvent ,
extraction process.
The suspended uranium content in the system could be possibly be also removed
by
micro filtration after heavy metal removal.
In a preferred embodiment, the amount of commercial hydrogen peroxide added
- was 1.5-6 times the stoichiometric amount of the uranium in the starting
MDU. The
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temperature at which hydrogen peroxide is added preferably varies from about
15 to
about 30 C to produce relatively pure uranium peroxide hydrate.
The reaction according to the invented process is instantaneous and is
preferably_,
carried out in a stainless steel container under controlled addition of
hydrogen
peroxide. The entire system is kept under mild turbulence by turbine type
agitator at
150 ¨ 200 ,rpm to complete the precipitation process. The yellow colored
uranium
peroxide thus produced needs a proper separation from the residual solution,
which
is carried out in a neutche filter backed up by a vacuum pump or by'
centrifuge
followed by de-mineralized water washing cycles. It was found that preferably,
two
washing cycles with de-mineralized water was sufficient to produce uranium
= -peroxide hydrate having desired purity levels..
, The washed solid is fed to a co-current spray drier to remove the
additional 'moisture
content at an inlet air temperature of 200 ¨ 300 C and an exhaust temperature
of
100 ¨ 130 C. The dried powder in the size range of 2 )¨ 10 micron was the end
product obtained by the herein described process.
=
An advantage of the process according to the present invention is that there
is a
single waste stream (liquid) that is more convenient to handle and dispose off
vis-à-
vis the solid and the liquid waste streams conventionally produced in the
state of the
art. This wastewater stream was found to carry a few ppm, of uranium, which
could
be micro filtered to discharge a very clear permeate as effluent containing
only
soluble rare earths:
Another significant advantage of the process according to the present
invention is
that the entire 'process requires only nitric acid and commercial grade
hydrogen
, peroxide as the reaction chemicals thereby eliminating the need for
tributyl
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phosphate, kerosene, sodium carbonate, ammonia and other waste treatment
chemicals, which are conventionally used in the solvent extraction process.
The present process therefore provides an efficient refining of yellowcake in
a much
5 more simple way, reducing load of waste streams, chemicals inventory and
offering
much more in-built safety features. The process is also compact, both capital
cost
and running cost saving and could be integrated to milling operation thus
eliminating
requirement of separate refining facility.
10 The invention shall now, be described with reference to the following
specific -
examples. It should be noted that the example(s) appended below illustrate
rather
than limit the invention, and that those skilled in the art will be able to
design many
alternative embodiments without departing from the scope of the appended
claims.
In the claims, the word 'comprising' does not exclude the presence of other
elements
or steps than those listed in a claim. The mere fact that certain measures are
recited
in mutually different dependent claims does not indicate that a combination of
these
measures cannot be used to advantage.
Other than in the operating examples provided hereunder, or where otherwise
indicated, all numbers expressing quantities of ingredients or reaction
conditions are '
to be understood as being modified in all instances by the term "about".
EXAMPLE 1
MDU powder was dissolved, in nitric acid (2 ¨ 6 N) at 80 - 90 C and the
uranium
content in initial solution kept at 250 ¨ 300 g/lit and free acidity up to
maximum of
3N. The solution was then diluted with DM water to keep uranium concentration
in
the range of 10 ¨ 50 Olt. Commercial hydrogen peroxide in the concentration
range
of 30 ¨ 70% (w/w) was then gradually added to the crude uranyl nitrate
solution at
temperature range of 15 ¨ 30 C. A turbine type agitator kept the whole system
in
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mild turbulence (150 ¨ 200 rpm) so as to maintain uniform reaction as well as
unhindered growth of particles. The whole reaction was carried out in SS
container.
The yellow colored uranium peroxide hydrate precipitated out instantaneously
and
was subjected to filtration. This part required efficient separation of solid
and liquid
followed by DM water washing cycles . The washed cake was collected and fed to
a
co-current spray drier (in S No. 5* & 6* below) at inlet air temperature of
200 ¨ 300 C
and exhaust air temperature of 100 - 130 C in effect to a dry granular
product. The '
filtrate contained 150 ¨ 300 ppm of dissolved uranium content, which was
further
brought down to less than 50 ppm before discharging to solar pond by proper pH
adjustment, and addition of little quantity of hydrogen peroxide and
flocculants
causing delayed precipitation.
=
Exp No. Solution U (gm) Quantity of DM wash Total EBC
Volume (ml) H202 (m1) water
(m1)
1 400 20 172 3000 <1
2 1500 75 112.5 1400 <1
3 1500 75 150 1400 <1
4 20000 1000 6000 7500 <1
5* 20000 1000 900 12000 <1
6* 90000 5000 6000 35000 <1
20
