BARC Newsletter
Founder’s Day Special Issue
Autoclave Leaching of Refractory Uranium Minerals
T. Sreenivas, K. Anand Rao, M. Manmadha Rao, K.C. Rajan, Md. Serajuddin,
P. Karthikayini and N.P.H. Padmanabhan
Mineral Processing Division
This paper received the MISRA award for Best Paper in Hydro, Electro and Bio metallurgy Category
instituted by the Indian Institute of Mineral Engineers, at the International Seminar on Mineral
Processing Technology, held at Jamshedpur during Dec. 2010
Abstract
least harmful. Sustained R & D efforts in BARC have resulted in successful use of autoclave reactors, for the dissolution
of refractory uranium minerals (viz. davidite, betafite, liandratite) and ores with extremely fine dissemination of uranium
phases. This paper gives an overview of the studies.
Keywords: Uranium, Leaching, Autoclave, Pressure, Mineral processing.
Introduction
e.g. betafite & pyrochlore). In addition to the chemical
There are approximately 205 species of uranium minerals,
refractory nature of some of the uranium minerals, there
exists certain uranium ores which inspite of having simple
occurring in 14 types of uranium deposits [1,2]. Uranium
minerals can be grouped into various chemical categories
uranium oxide phases, yield low leach recovery due to
the physical nature of occurence of uranium in them. To
such as oxides (uraninite, pitchblende), hydrated oxides
(becquerelite, gummite), Nb-Ta-Ti complex oxides
this category, belong ores having uranium mineralization
in very-fine to ultra-fine sizes, either in discrete form and/
(brannerite, davidite), silicates (coffinite, uranophane,
uranothorite), phosphates (autunite, torbernite), vanadates
or as disseminations in various gangue minerals. These
ores too demand rigorous leaching conditions for
(carnotite, tyuyamunite) and hydrocarbons (thucholite,
asphaltite). Simple oxide minerals like uraninite and
quantitative dissolution, which could be elevated
temperature and pressure, finer grind, higher oxidation
pitchblende are relatively easily leachable, while the rest
of them are not so leachable. Depending upon the ease
potential, higher leachant dosage, longer contact time
etc.
of dissolution / leachability, the uranium minerals are
categorized as (a) slightly refractory (oxidation is required
Autoclave circuits have been proven as reliable unit
prior to leaching e.g. coffinite & uranothorite), (b)
moderately refractory (requires higher leach temperatures,
processes, in maximizing the leachability of valuable
minerals, which are not only chemically refractory in nature
greater free acid and oxidant concentration and longer
leach periods (e.g. brannerite, thucholite & davidite); and
but also those ores which have the required minerals, in
physically refractory form. Besides increasing the leach
(c) highly refractory (requires extreme leach conditions
efficiencies and kinetics, the autoclave reactors also give
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Autoclave circuits have been proven as reliable unit operations in hydrometallurgy, for treating refractory ores - either
physical or chemical type. They yield high leach efficiency and produce stabilized leach residues, which are environmentally
BARC Newsletter
Founder’s Day Special Issue
Table 1: Uranium plants with acid or alkaline pressure leach technology [4, 5].
Eldorado Nuclear Ltd.,Beaverlodge Mill, Uranium City,
SaskatchewanAtlas Corp., Minerals Division, Moab Utah
United Nuclear-Homestake, Partners, Grants New Mexico
Cotter Corporation
Canon City Colorado
Lodeva Mill, France
Rio Algom Corp., Lisbon Mill, Moab Utah
Key Lake Mining Corp.,Key Lake, Saskatchewan
COGEMA Resources Inc., McClean Lake, Saskatchewan
Uranium One Inc. Dominion Reefs Mill, S Africa
1953
1956
1958
1970
1980
1980
1980
1972
1983
2008
Type of
leaching
Alkaline
Alkaline
Acid
Alkaline
Acid
Alkaline
Alkaline
Acid
Acid
Acid
stabilized leach residues, which can be safely disposed
the establishment of India’s first uranium mill with alkaline
[3]. The use of autoclaves in uranium industry was in
vogue since early 1950’s [4,5]. Table 1 gives details of
pressure leach technology at Tummalapalle in Andhra
Pradesh.
some prominent uranium mills using this technology. In
the earlier days pressure leach technology was justified for
Leaching Chemistry of Uranium Minerals
only high grade uranium ores, due to its cost-intensive
nature. However, increasing demand for uranium, upward
The common oxidation states of uranium, in its minerals
trend in uranium metal price and growing environmental
awareness, triggered the use of pressure leaching
like uraninite, pitchblende, coffinite and numerous others,
are +4 and +6. Amongst the two oxidation states, U+6
technology, even for low and medium grade uranium ore
deposits. The technology is also being adopted by
is soluble in aqueous media under suitable EH – pH
conditions, while U+4 is practically insoluble. The uranium
installations processing uranium bearing mine dumps or
waste [6]. Uranium ore processing mills use either Pressure
minerals occurring in various ore deposits consist
predominantly of uranous ion (U+4), necessitating the use
OXidizing autoclaves (POX) or HIgh Pressure
Acid leach autoclave technology (HiPAL) depending
of an oxidant and other lixiviants, for quantitative
dissolution during leaching. The type of leaching - acid
upon the mineralogical characteristics of the ore, type of
leaching followed – acid or alkaline, and on the type of
or alkaline mode depends upon the host rock. Sulfuric
acid is the common leachant in acid leaching process,
oxidant as well as the level of oxidizing environment,
necessary during the leaching reaction [4,5]. Similarly
while Na2CO3 - NaHCO3, (NH4)2CO3 and NH4HCO3 are the
widely used lixiviants in alkaline leaching of uranium ores.
different types of autoclaves – vertical or cigar type/
horizontal or in some cases, pipe autoclave reactors were
The oxidant reagents could be either chemical or gaseous
in nature. A typical chemical reaction in alkaline leaching
investigated, for their suitability in uranium leaching
application [4,7]. This paper discusses the recent R & D
of UO2 with carbonate ions and oxidant (X) is given in
Equation 1, a similar equation can be written for the sulfuric
and pilot-plant scale studies, carried out in our Division,
on the utility of autoclave reactors in processing various
acid leaching process.
refractory uranium ores. An outcome of these studies is
UO2 + 3CO3-2 + X  [UO2(CO3)3 ]-4 + X-2 .
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Plant Details and Year of commencement
BARC Newsletter
Founder’s Day Special Issue
Case Studies
fraction, are accounted by discrete pitchblende, which is
Oxidative pressure leaching of uranium from a
dolomitic limestone ore
mostly associated with pyrite and collophane. Pitchblende
occurring with pyrite is present as fine orbicular cluster,
separated by thin disconnected rims of pyrite or as garlands
around pyrite.
India has a medium-tonnage, low-grade uranium ore
deposit of siliceous dolomitic phosphatic limestone type,
Atmospheric alkaline leaching studies, carried out on this
in Kadapa district of Andhra Pradesh. Detailed exploration
carried out in a stretch of about 9 km in this area,
ore sample, by varying important process parameters like
mesh-of-grind, temperature, contact time, dosages of
established a resource of 29000 t of U3O8 with a cut-off
grade of 0.025% U3O8 [8]. Mineralogical studies on an
leachants - sodium carbonate and sodium bicarbonate,
solids concentration and type of oxidant, gave a maximum
exploratory mine ore sample from this area, indicated the
occurence of uranium values predominantly as ultra-fine
U3O8 leachability of 65% (Fig. 2). Studies with other
oxidants like NaOCl, Cu-NH3, oxygen and air gave poor
dissemination, in lighter gangue minerals (specific gravity
less than 3.2). It also occurs, albeit to a minor extent, in
leachability in comparison to KMnO4, emphasizing the
need for strong oxidizing conditions during the dissolution
Siliceous minerals in the ore are quartz, feldspar and chlorite
(13%). Collophane (4%) is the only phosphate bearing
phase. Pyrite is the predominant sulphide ore mineral,
along with few grains of chalcopyrite and galena. The
iron bearing oxides are magnetite, ilmenite and goethite.
Heavy media separation of various closely-sized feed
fractions, using bromoform (BR) and methylene iodide
(MI) liquids, have indicated that about 91% of the uranium
values are present in lighter minerals (specific gravity <3.2)
(Fig. 1), as ultra-fine disseminations. The remaining 9%
of uranium values reported in methylene iodide heavy
62.8 62.3
60
Wt
U3O8 Dist.
%40
60
40
20
0
0
10
20
30
40
50
60
Contact time (hour)
Fig. 2: Atmospheric leaching of uranium from dolomitic
limestone type ore of Kadapa district (A.P.). Mesh of grind 100#, Sodium carbonate 50 kg/t, Sodium bicarbonate 50
kg/t, KMnO410 kg/t,
alternative is to carryout the leaching reaction in a pressure
reactor, using a gaseous oxidant. Since the solubility of
oxygen diminishes with increasing temperature, adoption
of higher partial pressure aids in increased dissolved oxygen
28.9
17.8
9.2 8.8
0
BRL (Sp.G 2.8)
80
process. However, as KMnO4 cannot be used as an oxidant
on commercial scale due to its expensive nature, the only
80
20
100
Merit Award
as U-Ti complex. Carbonate minerals constitute the major
gangue present in the form of dolostone (~80%).
U3O8 Leachability (%) the form of ultra-fine pitchblende in association with pyrite,
as disseminations in collophane-rich parts, coffinite and
MIL (Sp.G 3.2)
MIH (Sp.G >3.2)
Heavy Media Liquid (Specific Gravity)
Fig. 1: Distribution of mass and uranium values of a dolomitic
limestone ore, Kadapa district (A.P.) in various heavy media
fractions
concentration. Detailed analysis of the leach residue
obtained in the atmospheric leaching experiments
indicated, that uranium values associated with pyrite are
not completely leached at temperatures <1000C. Further,
some of the locked-up uranium values in various gangue
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phases, require more aggressive diffusion conditions for
aggressive conditions. About 75% of uranium values were
penetration of the leachant to the desired mineral interface.
Both these requirements can be realized only at elevated
leached at a reaction temperature of 125 - 130°C in 3 h
of contact time, using a feed ground to 65% weight finer
temperature and under sustained oxidizing conditions,
possible in an autoclave reactor. Leaching at elevated
than 200#. Increasing the fineness of grind in -200# to
85% showed, an enhancement in leachability to about
temperature and pressure was initially carried out in a
laboratory, 5 liter S.S. autoclave reactor equipped with
80%. Based on these results, large-scale leaching studies
were carried out, both on batch and continuous leach
necessary instrumentation and control to maintain preset temperature, overpressure and agitation speed of the
reactor, to generate necessary scale-up and engineering
data for industrial scale reactor, besides verifying the
impeller. All the autoclave leaching experiments were
carried out, at optimum dosage combination of sodium
reproducibility of results at higher-scale of operation. Both
the batch and cigar type continuous reactor were of 850
carbonate and sodium bicarbonate evolved in atmospheric
leaching, that is - 50 kg/ton and - 70 kg/ton respectively.
liter capacity with inconel 600 as material of construction.
The kinetic profile of the leaching reaction in pilot scale
The autoclave leaching studies mainly addressed the
dissolution of uranium associated with pyrite and the scope
batch and continuous reactor is illustrated in Fig. 5. Largescale studies confirmed the results generated in batch scale
of replacing KMnO4 with industrial oxygen. Figs. 3 and 4,
illustrate the effect of temperature and contact time on
experiments. At present, DAE is setting-up a 3000 tpd
capacity uranium mill at site, wherein two 720 m3 capacity
the leachability of uranium values, observed under
autoclave reactors with inconel 600 cladding for the wetted
parts will be used. This will be the first uranium plant,
using autoclave leaching technology in India.
80
100
60
90
P= 6Kg/cm 2
40
80
P= 8.5Kg/cm 2
P= 11Kg/c m 2
20
0
80
100
120
140
160
180
Tem perature (Centigrade)
Fig. 3: Effect of temperature on leachability of uranium from
dolomitic limestone ore in autoclave reactor. MOG 65% 200#, Solids 50%, Contact time 6 h, Na2CO3 50 kg/t, NaHCO3
70 kg/t
Cumulativ Leachability of U3O8 (%)
U3O8 leachability (%)
100
70
60
50
40
Batch Autoclave Reactor
Continuous Autoclave Reactor
30
20
10
0
0
100
2
3
4
5
6
7
8
9
Cumulative Residence Time (h)
Py rite
Leachability (%)
1
Fig. 5: Leaching kinetics of uranium from dolomitic limestone
ore in batch and continuous autoclave reactor.
80
U3O8
60
High acid pressure leaching of uraniferous alkali
syenite ore containing betafite
40
20
Uraniferrous alkali-syenite ore deposit in Vellore district
0
0
2
4
6
Con tact T ime (h ou rs )
8
Fig. 4: Effect of Contact time on the leachability of uranium
from dolomitic limestone ore in autoclave reactor. MOG
85% -200#, Solids 50%, Na2CO3 50 kg/t and NaHCO3 70 kg/
t, pO2 5.8 kg/cm2.
of Tamilnadu, analysing 0.04% U3O8 containing betafite (Ca,U)2(Ti,Nb,Ta)2O6 (OH) as the main radioactive mineral,
was studied for recovering uranium values. The other
radioactive minerals in the ore include pyrochlore,
uranothorite, thorite, cyrtolite, monazite and minor
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BARC Newsletter
Founder’s Day Special Issue
concentrations of some secondary uranium minerals.
100
Practically all the uranium-bearing phases are refractory
in nature. The main gangue minerals in the ore are feldspar
Various process options were tried to extract the uranium
values, which included physical beneficiation followed
by acid leaching of the pre-concentrate, pug-cure-leaching,
direct acid leaching of the ground ore at atmospheric
pressure and normal temperature conditions and finally
sulfuric acid leaching under aggressive temperature and
pressure. Atmospheric leaching experiments indicated the
scope of leaching to about 80% of the uranium values
60
40
20
0
0
2
4
6
8
Contact time, h
10
12
Fig. 7: Effect of contact time on autoclave leaching of betafite
bearing uranium ore.
with high sulfuric acid dosage, about 200 kg/ton, at about
90C. Though the leachability is reasonably satisfactory,
Encouraging results obtained in the autoclave reactor
generated significant interest in the exploration geologists,
the high residual SO4-2 / HSO4- in the leach liquor, limits
the efficiency of down-stream unit operations, particularly
working on this potential deposit.
ion-exchange, tailings neutralization etc. Studies carried
out at elevated temperature in an autoclave reactor, have
High acid pressure leaching of uranium from
liandratite
indeed yielded twin benefits of reducing the acid
consumption to 90 kg/ton and enhancing the leachability
High sulfuric acid pressure leaching was also found
to 88-90% at about 150 C (Fig. 6) in 8 h contact time
(Fig. 7). XRD analysis of feed and various leach residues
successful in leaching uranium values from liandratite, a
uranium bearing niobium-tantalum oxide refractory
have indicated, that the leachability of other-wise refractory
uranium mineral phases could be accomplished within
mineral, with the chemical formula [U(Nb, Ta)2 O8]. As
the chemical reactivities of uranium and Nb & Ta are
commercially acceptable operating conditions, due to the
presence of both betafite and thorite in metamic form.
different, a separate sequence of chemical conditions is
required for maximal dissolution of individual metals.
0
Leaching of niobium and tantalum oxides required, highly
acidic or highly alkaline aqueous conditions, for
100
quantitative dissolution at very high temperature. Unlike
niobium and tantalum, the acidity or alkalinity necessary
% Leachability
80
60
for uranium leaching is significantly lower. In view of
this, the scheme for extracting the metal values from a
40
liandratite bearing ore sample (Table 2), is formulated in
such a way, that in the first stage, conditions were set to
20
leach as much uranium values as possible, followed by
re-adjustment of the chemical and experimental conditions,
0
50
75
100
125
150
175
200
Temperature
Fig. 6: Effect of temperature on autoclave leaching of betafite
for dissolution of niobium and tantalum values. Optimal
process conditions gave uranium leachability of about 80%
at a reaction temperature of 1800 C, sulfuric acid dosage
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major gangue of the ore is non-acid consuming in nature,
the ore was processed by the sulfuric acid leaching route.
% Leachability
(83%), pyriboles (3.3%), iron oxides (2.9%), sulfides
(0.1%), limonite (1.6%), barite (0.9%) etc. Since the
80
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Founder’s Day Special Issue
Partial Mineralogical
Composition
Liandratite [U(Nb, Ta)2 O8]
Samarskite [(Y, Er)(Nb,Ta)2 O6]
Partial Chemical
Composition
Nb2O5 39%, Th 1.5%,
Ta2O5 5%, Fe2O3 11%,
MnO 1.0% and U3O8 21%.
of 400 kg/ton, feed ground to finer than 150 m and
contact time of 48 h. There was no necessity of adding
any additional oxidant during the leaching reaction, as
sulfuric acid itself served as a strong oxidizing agent, due
to its high concentration. The redox potential was
consistently about 450 -500 mv (vs. SCE). The solution
composition of the leach liquor indicated the presence of
References
1. Merrit, R.C., 1971, The extractive metallurgy of
uranium, Colorado School of Mines Research Institute,
Golden Colorado, United States Atomic Energy
Commission, Colorado, p 50.
2. Edwards, C.R. and Oliver, J.A., 2000, Uranium
processing: A review of currents methods and
technologies, Journal of Metals, 52 (9), p 12.
3. Tong Deng, 1995, Aqueous pressure oxidation of
minerals - a salient development in hydrometallurgy,
Mineral Processing and Extractive Metallurgy Review,
12, p 185.
4. K.S. Fraser and K.G. Thomas, 2010, Uranium extraction
rd
history using pressure leaching, 3 International
conference on uranium (URANIUM 2010), 40th
Annual Hydrometallurgy Meeting, Saskatoon.
niobium and tantalum concentration of 5 mg/l and 30
mg/l against uranium concentration of 21 g/l, indicating
that the leaching conditions are good for selective
5. BODU, R., et al., 1984, “The Lodeve plant: A complex
alkaline process for a complex uranium ore”, Mintek
dissolution of uranium values. Thus, adoption of high
acid autoclave leaching, could dissolve uranium quite
50 (Proc. Conf. Sandton, 1984), The Council for
Mineral Technology, Randburg, South Africa, p 707.
selectively from the liandratite phase.
Conclusions
6. R.J.Verster and H.J.H. Pieterse, 1985, The use of
autoclaves in uranium leach flowsheet., Technical
publications of M/s. Bateman Engineering, SA,
www.batemanengineering.co.za.
L eaching studies on different ores, having chemically
refractory uranium minerals (betafite, liandratite) and on
7. SORENSEN, E., JENSEN, E.,1985, “Uranium extraction
by continuous carbonate leaching in a pipe
an ore where the uranium phases are in very-fine to ultrafine dissemination in the host matrix, have indicated the
autoclave”, Advances in Uranium Ore Processing and
Recovery from Non-Conventional Resources (Proc.
scope of recovering uranium very efficiently, by using
autoclave reactor. The investigations provide positive
IAEA Tech. Committee Mtg Vienna, 1983), IAEA,
Vienna, p 41.
signals for exploration and exploitation even “difficult to
process” uranium ores of India. This is a welcome
8. Anjan Chaki, 2009. An Overview of Uranium
Exploration Strategy in India, International Symposium
technological advance, for augmenting the uranium
resources, in order to meet the fuel demands of our
on Uranium Raw Material for the Nuclear Fuel Cycle:
Exploration, Mining, Production, Supply and Demand,
ambitious nuclear power programme.
Economics and Environmental Issues (URAM-2009)
Vienna, Austria, p 22.
Acknowledgement
The authors are thankful to Dr. A.K. Suri, Director,
Materials Group, BARC, for his interest in the investigations
and for this encouragement.
Founder’s Day Special Issue October 2011
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Table 2: Partial mineralogical and chemical composition of
Madagascar Ore.