Process for the in-situ leaching of uranium

Process for the in-situ leaching of uranium employing an alkaline lixiviant and an alkali metal or alkaline earth metal hypochlorite as an oxidizing agent. The use of the hypochlorite oxidant results in significantly higher uranium recoveries and leaching rates than those attained by the use of conventional oxidants. The invention is particularly suitable for use in subterranean deposits in which the uranium mineral is associated with carbonaceous material which retards access to the uranium by the lixiviant.

BACKGROUND OF THE INVENTION 
The present invention relates to the recovery of uranium from subterranean 
ore deposits and more particularly to an in-situ leaching employing an 
alkaline lixiviant with a hypochlorite oxidizing agent. 
The various techniques for the production of uranium ore deposits may be 
characterized as falling within two general classes. One involves a 
surface milling operation in which uranium ore obtained by mining is 
crushed and blended and then subjected to a leaching procedure in which an 
acid or alkaline lixiviant is employed to extract uranium from the milled 
ore. The uranium is then recovered from the pregnant lixiviant by a 
suitable technique such as solvent extraction, direct precipitation, or by 
adsorption and elution employing an ion exchange resin. The other involves 
in-situ leaching in which a lixiviant is introduced into a subterranean 
ore deposit through a suitable injection system. The lixiviant may be an 
acidic or alkaline medium which solubilizes uranium values as it traverses 
the ore body. The pregnant lixiviant is then withdrawn from the ore body 
through a production system and treated to recover uranium therefrom by 
suitable techniques such as noted above. Mill leaching and in-situ 
leaching operations are similar in some respects and quite dissimilar in 
others. In both cases, the nature of the lixiviant is dictated to some 
extent by the nature of the uranium ore or the subterranean deposit. An 
acid lixiviant is used in most mill leaching operations since it is more 
effective with most ores and does not require that the ore be ground to as 
fine a state as in the case of an alkaline lixiviant. The use of acid 
lixiviant is somewhat limited in milled ores of high carbonate content 
which may lead to excessive consumption of acid. The presence of carbonate 
in subterranean rock deposits containing uranium also limits the use of 
acid lixiviants, not only with respect to acid consumption, but also due 
to the precipitation of reaction products, such as calcium sulfate which 
may result in plugging of the formation when sulfuric acid is used. Thus 
the use of an alkaline lixiviant is strongly indicated in many in-situ 
leaching operations, not only because of the carbonate content of the rock 
but also since alkaline lixiviants are more selective with respect to 
uranium dissolution than acid lixiviants. 
In both milling and in-situ leaching operations, an oxidizing agent is 
employed in conjunction with the lixiviant in order to ensure that the 
uranium is oxidized to or retained in the hexavalent state at which it is 
solubilized by the acid or alkaline leach medium. 
In milling operations employing an acid lixiviant, the oxidizing reaction 
requires the presence of iron and the principal oxidizing agents employed 
are manganese dioxide and sodium chlorate as disclosed in Merritt, Robert 
C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines, 
Research Institute, U.S.A. (1971), p. 63. In an alkaline leach system, the 
tetravalent uranium is directly oxidized to the hexavalent state. As 
disclosed by Merritt, pages 104 and 105, the most widely used oxidant in 
milling operations employing an alkaline lixiviant is potassium 
permanganate. Other oxidizing agents disclosed by Merritt include sodium 
hypochlorite, hydrogen peroxide, potassium persulfate and copper sulfate. 
In in-situ leaching operations employing an alkaline lixiviant, the most 
commonly employed oxidizing agents are hydrogen peroxide as disclosed in 
U.S. Pat. No. 2,896,930 to Menke and air as disclosed in U.S. Pat. No. 
2,954,218 to Dew et al. A significant distinction between mill leaching 
and in-situ leaching resides in the fact that the latter procedure is 
carried out in a massive subsurface formation where the uranium mineral is 
present in small concentrations without the intimate contact between the 
lixiviant and ore found in milling operations where the ore is rubblized 
before leaching. Thus a rapid intrinsic leaching rate is more important in 
in-situ leaching than in leaching carried out after a milling operation. 
Also many of the so-called refractory ores are much more difficult to 
leach in an in-situ environment than in conjunction with a milling 
operation. For example, as noted in the aforementioned patent to Dew et 
al., some uranium ore bodies contain carbonaceous material which retards 
the leaching action of the lixiviant. While such ores can be leached after 
milling, leaching under in-situ conditions is extremely slow without 
special procedures such as the in-situ combustion procedure disclosed in 
the Dew et al. patent. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a new and 
improved process for the recovery of uranium from a subterranean deposit 
by employing an alkaline lixiviant and a hypochlorite oxidant. In carrying 
out the invention, there is introduced into the uranium-containing deposit 
via a suitable injection system an aqueous alkaline lixiviant having a pH 
of at least 7.5. The lixiviant contains an alkali metal or alkaline earth 
metal hypochlorite. As the lixiviant traverses the subterranean uranium 
deposit, the hypochlorite oxidant functions to oxidize uranium from the 
tetravalent to the hexavalent state at which the uranium is solubilized in 
the lixiviant. The pregnant lixiviant containing uranium is then produced 
from the deposit by a production system and treated to recover uranium 
therefrom. 
In a further embodiment of the invention, uranium is recovered from the 
pregnant lixiviant through precipitation by reducing uranium from the 
hexavalent state to a water-insoluble tetravalent state. The pregnant 
lixiviant may be heated in order to carry out the reduction step at an 
elevated temperature. Subsequent to the precipitation of uranium, the 
heated barren lixiviant is then circulated to the injection system where 
it is employed to form fresh lixiviant which is introduced into the 
deposit in a heated state. A preferred application of the present 
invention is in refractory deposits which contain uranium associated with 
carbonaceous material.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
This invention relates to an in-situ leaching process employing a 
hypochlorite oxidizing agent under certain conditions of pH and 
concentration. The hypochlorite is in the form of an alkali metal or 
alkaline earth metal salt and thus may take the form of sodium 
hypochlorite, potassium hypochlorite, calcium hypochlorite, or magnesium 
hypochlorite. Normally it will be desirable to employ sodium hypochlorite 
for reasons of economics and ease of preparation at the leaching site. 
Sodium hypochlorite is a strong oxidizing agent and has been proposed for 
use in a chemical oxidation of uranium ores in carbonate leach milling 
operations. As disclosed in U.S. Pat. No. 3,647,261 to Stenger et al., it 
also has been proposed for use in the in-situ leaching of noble metals 
such as silver and gold which require extremely high oxidation potentials. 
However, the hypochlorites have not heretofore been used as oxidizing 
agents in in-situ leaching of uranium employing alkaline lixiviants. In 
this regard it will be noted that uranium, unlike the noble metals, is 
highly active and is readily oxidized from the tetravalent to the 
hexavalent state. Thus, while sodium hypochlorite is recognized in Merritt 
as a most economical oxidant, its use is attended with several 
disadvantages including the buildup of chlorides as byproducts of the 
leaching reaction. This high chloride content is inconsistent with the use 
of the anionic ion exchange resins as normally employed in extracting the 
uranium from the spent lixiviant. This coupled with the fact that sodium 
hypochlorite has appeared to be only moderately more effective than other 
oxidants such as hydrogen peroxide, dipotassium sulfate, and potassium 
permanganate has negated the use of hypochlorites in in-situ leaching 
operations. Stated otherwise, since uranium is highly active, the moderate 
increase in uranium recovery heretofore associated with the use of these 
extremely strong oxidants has been more than offset by the deleterious 
side effects associated with their use. 
As indicated by the experimental data presented hereafter, the present 
invention employs an alkali metal or alkaline earth metal hypochlorite as 
an oxidant in in-situ alkaline leach operations under conditions at which 
a severalfold increase in uranium recovery is attained. For example, the 
percent of uranium extracted employing sodium hypochlorite in accordance 
with the present invention is several times that obtained while employing 
hydrogen peroxide. This may be contrasted with the increase of less than 
20% in aboveground milling operations as noted, for example, in Table 5-9 
of Merritt at page 105. 
The efficacy of employing a hypochlorite oxidant in an alkaline lixiviant 
in accordance with the present invention is illustrated by comparative 
laboratory experiments employing different oxidants. Two general 
experimental procedures were followed. In one referred to herein as the 
"batch" technique, the experimental procedure involved the addition of 50 
cm.sup.3 of lixiviant to a container containing 10 grams of uranium ore. 
The container was then placed in a shaker where it was agitated at room 
temperatures. After 3 hours of agitation, the lixiviant was withdrawn and 
filtered and the filtrate then analyzed for uranium by the colorimetric 
method. For each test, this identical procedure was followed on a second 
sample of the same ore with the exception that the agitation continued for 
a period of 24 hours. The uranium leached from the ore sample at the end 
of the 3-hour and 24-hour periods was then employed to calculate a first 
order rate constant in accordance with the following equation: 
EQU K=(LnC.sub.0 -LnC.sub.1)/t (1) 
wherein 
K is the rate constant in hours.sup.-1, 
C.sub.0 is the uranium content of the ore sample after leaching for a first 
period, i.e. 3 hours, 
C.sub.1 is the uranium content of the ore sample after leaching for a 
second period, i.e. 24 hours, and 
t is the elapsed time between the two leaching periods, i.e. 21 hours. 
In the pack leaching procedure, the uranium ore was packed into a vertical 
plastic pipe having an internal diameter of 3/4 inch and a length of 12 
inches. The lixiviant was flowed downwardly through the pack and the 
pregnant lixiviant recovered from the bottom of the pack and analyzed 
daily for uranium by the colorimetric method. 
In a first suite of experiments, batch leaching tests were conducted on a 
number of ore samples of a composite ore obtained from the same core hole 
penetrating a subterranean uranium deposit. The ore contains uranium in 
the form of coffinite occurring as individual grains and aggregates of 
grains in a matrix of carbonaceous material. The matrix contains other 
minerals such as pyrite, apatite, anatase or rutile and chlorite. The 
carbonaceous material occurs in a poorly sorted sandstone consisting of 
detrital quartz, feldspar and rock fragments. Locally abundant kaolinite 
or chlorite, calcite and the carbonaceous material are the primary 
ccementing agents. 
This suite of experiments was carried out employing a carbonate lixiviant 
and sodium hypochlorite, hydrogen peroxide, sodium chlorate, or potassium 
permanganate as the oxidant. The results of the experimental work are set 
forth in Table I in which the second column indicates the lixiviant 
composition in terms of the concentration of oxidant and sodium 
bicarbonate, the third column the pH of the lixiviant, and the fourth 
column the first order rate constant, K, times 10.sup.3 as calculated in 
accordance with equation (1). The oxidant and bicarbonate concentrations 
are set forth in weight percents. In describing the present invention and 
the supporting laboratory data, percents are calculated on a weight 
(solute)/volume (solution) basis. Thus, in test 1, for example, the 5% 
sodium hypochlorite solution contained 5 grams of sodium hypochlorite per 
deciliter of solution. The lixiviants employed in the batch test were 
formed by adding the oxidizing agent and sodium bicarbonate to distilled 
water and then adding hydrochloric acid in an amount as necessary to give 
the indicated pH. In run 6, the lixiviant solvent was formulated from 
equal parts of distilled water and acetone and in run 7 from equal parts 
of distilled water and methanol. As shown in the table, sodium bicarbonate 
was employed in all of the runs with the exception of test 9 where the 
carbonate lixiviant was formed by the addition of sodium carbonate and in 
run 11 by ammonium bicarbonate. 
TABLE I 
______________________________________ 
Run 
No. Lixiviant Composition 
pH K 
______________________________________ 
1 5.0% NaOCl .21% NaHCO.sub.3 
8.3-9.6 
12-15 
2 1.0% NaOCl .21% NaHCO.sub.3 
8.3-9.3 
10 
3 0.5% NaOCl .21% NaHCO.sub.3 
8.3 2.3 
4 2.1% H.sub.2 O.sub.2 
.21% NaHCO.sub.3 
8.3 2.3 
5 0.21% H.sub.2 O.sub.2 
.21% NaHCO.sub.3 
8.0-8.3 
2.0 
6 0.21% H.sub.2 O.sub.2 
.21% NaHCO.sub.3 
8.3 1.3 
7 0.21% H.sub.2 O.sub.2 
.21% NaHCO.sub.3 
8.3 &lt;1 
8 0.2% NaClO.sub.3 
.21% NaHCO.sub.3 
8.3 &lt;1 
9 0.21% H.sub.2 O.sub.2 
.25% Na.sub.2 CO.sub.3 
11.3-12.0 
&lt;1 
10 0.5% KMnO.sub.4 
.21% NaHCO.sub.3 
8.3 &lt;1 
11 2.1% H.sub.2 O.sub.2 
Sat. NH.sub.4 HCO.sub.3 
8.3 &lt;1 
______________________________________ 
From an examination of the data set forth in Table I, it can be seen that 
sodium hypochlorite was capable of producing a much higher leaching rate 
than the hydrogen peroxide, the next most effective oxidant. The other 
oxidants tested, sodium chlorate and potassium permanganate, resulted in 
K-values of less than 1.times.10.sup.-3 hours.sup.-1. 
Turning now to the drawings, FIG. 1 is a graphical presentation of the 
leaching rates observed for runs 1 through 4 of Table I. In FIG. 1, curves 
1a, 2a, 3a, and 4a are graphs of the log of uranium concentration of the 
ore, C expressed as a percentage of the original uranium concentration in 
the ore plotted on the ordinate versus the leaching time in hours plotted 
on the abscissa for runs 1, 2, 3, and 4, respectively. Curve 5a is a 
similar plot for two batch leaching tests employing acid lixiviants. In 
one case, the lixiviant contained sufficient sulfuric acid to provide a pH 
of 2 and in the other case hydrochloric acid to provide the same pH. The 
oxidant employed in each of these cases was hydrogen peroxide in a 
concentration of 0.21 weight percent. In reviewing the data presented in 
FIG. 1 and also in Table I, it will be noted that the stoichiometric 
weight ratio of sodium hypochlorite to hydrogen peroxide is about 2.2. Or, 
stated otherwise, about 1 gram of hydrogen peroxide is stoichiometrically 
equivalent to about 2.2 grams of sodium hypochlorite in the oxidation of 
uranium from the tetravalent to the hexavalent state. Thus the amount of 
hydrogen peroxide employed in the test corresponding to curve 4a was 
significantly greater than the sodium hypochlorite employed in the test 
associated with curves 2a and 3a and only slightly less than that for the 
test associated with curve 1a. 
In further experimental work, a pack leaching test was carried out 
employing the equipment and format described previously. In this test, the 
packed column was first preflushed with 5.4 pore volume of an alkaline 
lixiviant containing 0.1% sodium bicarbonate and 0.3% sodium chloride. The 
preflush lixiviant, which did not contain an oxidizing agent, was injected 
at a rate of about 1.8 pore volumes per day. During this preflush, less 
than 1% of the uranium originally present in the sample was recovered. 
Thereafter, an alkaline lixiviant containing a hypochlorite oxidant in 
accordance with the present invention was injected into the column. This 
lixiviant contained 1.0 weight percent sodium hypochlorite, 0.4 weight 
percent sodium bicarbonate, and 0.8 weight percent sodium chloride. The 
lixiviant was introduced into the pack in a sequence which provided for 
injection for a period of 1 hour followed by a 2-hour off period to 
provide an average injection rate of 0.6 pore volume per day. 
The results of the pack run are illustrated in FIGS. 2 and 3. In FIG. 2, 
curve 7 is a graph of the log of the uranium concentration, C.sub.0, 
remaining in the ore expressed as a percent of the original uranium 
content plotted on the ordinate versus the amount of lixiviant injected in 
pore volumes on the abscissa. Curve 9 in FIG. 3 is a plot of the log of 
produced uranium concentration, C.sub.1, in parts per million of U.sub.3 
O.sub.8 in the pregnant lixiviant on the ordinate versus the amount of 
lixiviant injected in pore volumes on the abscissa. From a review of FIGS. 
2 and 3, it can be seen that nearly 80% of the uranium in the pack was 
recovered after the injection of 10 pore volumes of lixiviant. Further, it 
will be noted that good leaching action was obtained in the presence of 
the relatively high sodium chloride concentration. The results of the pack 
leaching experimental procedure are, of course, particularly significant 
since this technique more closely simulates the conditions encountered in 
in-situ leaching than does the batch leaching technique. 
Table II sets forth the results of further experimental work carried out 
with carbonate lixiviants and presents a direct comparison of the efficacy 
of hydrogen peroxide and sodium hypochlorite on a wide variety of ore 
samples. Two standard lixiviant solutions were employed. The first 
contained 0.46 weight percent hydrogen peroxide and 0.21 weight percent 
sodium bicarbonate and the second contained 1.0 weight percent sodium 
hypochlorite and 0.21 weight percent sodium bicarbonate. In each case, the 
pH of the lixiviant was 8.5. It will be noted that the amounts of oxidant 
in the two lixiviants were stoichiometrically equivalent since the 
hydrogen peroxide and the sodium hypochlorite were employed in the same 
molar concentrations. In Table II, the first column identifies the test 
number with the same numerical designation employed to identify samples 
obtained from a common core hole but at different depths. Thus, tests 12a, 
12b, and 12c, for example, were run employing ore samples obtained from 
different depths of the same well. Tests 14, 15, 16, and 17 were carried 
out on samples obtained from the same coffinite deposit as described 
previously. The other samples employed in the tests were obtained from 
other subterranean deposits within several miles of each other in which 
the uranium was in the form of cofffinite associated with carbonaceous 
material as described previously. 
Column 2, of Table II, gives the original uranium content of the samples 
expressed as weight percent of U.sub.3 O.sub.8 and the third and fourth 
columns give the rate constants calculated in accordance with equation (1) 
for the lixiviants containing hydrogen peroxide and sodium hypochlorite, 
respectively. The last column sets forth the ratio obtained by dividing 
the rate constant for the hypochlorite lixiviant by the rate constant of 
the hydrogen peroxide lixiviant. As illustrated by the data presented in 
Table II, the leaching rates for the lixiviant employing sodium 
hypochlorite were much greater than for the lixiviant employing hydrogen 
peroxide. In every case, the leaching rate for the hypochlorite lixiviant 
was greater than the leaching rate for the hydrogen peroxide lixiviant by 
a factor of 2 of more. 
TABLE II 
______________________________________ 
Ratio 
Run Rate Constants (hr.sup.-1 .times. 10.sup.3) 
K(NaOCl) 
No. % U.sub.3 O.sub.8 
K(H.sub.2 O.sub.2) 
K(NaOCl) K(H.sub.2 O.sub.2) 
______________________________________ 
12A 0.046 1.9 9.1 5 
12B 0.017 1.6 3.3 2 
12C 0.108 0.9 8.2 9 
13 0.041 1.4 5.4 4 
14 0.047 1.3 3.4 2 
15 0.029 2.9 24.7 8 
16A 0.027 2.4 6.8 3 
16B 0.061 1.8 15.1 9 
16C 0.054 1.2 10.1 9 
16D 0.143 1.3 34.8 26 
16E 0.147 2.1 32.1 15 
17A 0.071 1.1 15.0 13 
17B 0.242 1.3 24.2 18 
17C 0.030 0.9 5.8 7 
17D 0.046 1.3 3.3 3 
17E 0.026 1.2 4.8 4 
18 0.142 1.1 11.3 10 
19 0.296 2.5 15.1 6 
20A 0.087 1.6 8.0 5 
20B 0.047 1.4 4.2 3 
21 0.150 8.4 37.3 4 
22A 0.030 0.9 1.9 2 
22B 0.260 0.3 1.9 7 
23 0.205 2.7 26.2 10 
24 0.187 1.0 11.4 11 
25A 0.067 0.7 6.8 9 
25B 0.150 1.1 9.5 8 
26A 0.026 1.2 2.0 2 
26B 0.153 3.0 16.8 6 
27 0.042 2.2 12.2 6 
28 0.082 2.0 14.9 8 
______________________________________ 
The average leaching rate for all of the samples tested was eight times 
faster with the hypochlorite than with the peroxide. The greatest 
improvement was observed for ore samples having an original uranium 
content of at least 0.1 weight percent U.sub.3 O.sub.8. In this case, the 
lixiviant employing the hypochlorite averaged eleven times faster than the 
lixiviant employing the hydrogen peroxide. 
Alkaline lixiviants normally employ carbonate ions added as alkali metal 
carbonates or bicarbonates or mixtures thereof to complex the uranium in 
the form of the water-soluble uranyl tricarbonate ion and the present 
invention may be carried out employing the hypochlorite oxidant and an 
otherwise standard carbonate lixiviant. The hypochlorite oxidant may be 
employed in any suitable concentration, but usually will be present in a 
concentration of at least 0.01 weight percent. Preferably the hypochlorite 
concentration of the lixiviant is within the range of about 0.1-1.0 weight 
percent. While concentrations greater than 1.0 weight percent may be 
employed, this usually should be avoided since the attendant higher 
leaching rates are more than offset by the increased chemical costs. As 
noted previously, the pH of the lixiviant is at least 7.5 to avoid 
decomposition of the hypochlorite. Preferably, the pH of the lixiviant is 
within the range of 8.0-10.0 for the most effective solubilization of the 
tetravalent uranium. 
A preferred application of the invention is in those deposits containing 
uranium associated with carbonaceous material as described previously. The 
carbonaceous material is present in intimate contact with the uranium 
mineral and retards access to the uranium by the lixiviant. The 
hypochlorite functions to not only oxidize the uranium to the hexavalent 
state, but also to disrupt the carbonaceous material so that the uranium 
is exposed to the solubilizing action of the lixiviant. In most cases, the 
carbonaceous material is present in the uranium deposit in an amount of at 
least 0.1 weight percent expressed as total organic carbon. The 
concentration may range up to about 2% total organic carbon. 
The present invention may be carried out utilizing injection and production 
systems as defined by any suitable well arrangement. One well arrangement 
suitable for use in carrying out the invention is a five-spot pattern in 
which a central injection well is surrounded by four production wells. 
Other patterns such as seven-spot and nine-spot patterns also may be 
employed as well as the so-called "line flood" pattern in which injection 
and production wells are located in generally parallel rows. Typically the 
spacing between injection and production wells will be on the order of 50 
to 200 feet. In some instances, particularly where the subterranean 
uranium deposit is of a limited areal extent, injection and production may 
be carried out through the same well. Thus, in relatively thick uranium 
deposits, dually completed injection-production wells of the type 
disclosed, for example, in U.S. Pat. No. 2,725,106 to Spearow may be 
employed. Alternatively, injection of fresh lixiviant and withdrawal of 
pregnant lixiviant through the same well may be accomplished by a 
"huff-and-puff" procedure employing a well system such as disclosed in 
U.S. Pat. No. 3,708,206 to Hard et al. 
FIG. 4 is a schematic illustration of an in-situ leaching circuit which may 
be employed in carrying out the invention. In this circuit fresh alkaline 
lixiviant containing the appropriate amount of hypochlorite oxidant is 
introduced into the subterranean uranium body via the injection system. 
The hypochlorite preferably is incorporated into the lixiviant by adding 
stoichiometric amounts of elemental chlorine and the appropriate alkali 
metal or alkaline earth metal hydroxide which react to form the 
corresponding hypochlorite with sodium chloride as a byproduct. The 
lixiviant is displaced through a desired portion of the subterranean 
deposit to solubilize uranium values and the pregnant lixiviant is then 
withdrawn through the production system and treated at the surface to 
recover uranium therefrom. In the specific embodiment illustrated in FIG. 
4, the uranium is recovered by a direct precipitation technique which 
involves reducing the uranium to a water-insoluble tetravalent state. 
After uranium is recovered, the barren lixiviant is then recirculated to 
the injection system where it is employed in formulating fresh lixiviant. 
The fresh lixiviant is then recirculated into the subterranean ore deposit 
via the injection system. 
More particularly, and as shown in FIG. 4, there is illustrated a 
subterranean uranium deposit 10 penetrated by spaced injection and 
production wells 12 and 14, respectively. Lixiviant is transferred from a 
blending system 15 of any suitable design by means of a pump 16 to the 
injection well and then displaced into the deposit 10. In the system 
illustrated, the lixiviant comprises sodium hypochlorite as the oxidant 
and sodium carbonate and/or bicarbonate as the complexing agent. The 
lixiviant may be formulated by supplying makeup water from a suitable 
source (not shown) to the blending system via line 18. Sodium hydroxide, 
carbon dioxide, and chlorine gas are also supplied to the blending system 
via lines 20, 21, and 22, respectively. The sodium hydroxide and chlorine 
react to produce sodium hypochlorite as described previously and the 
sodium hydroxide and carbon dioxide react to produce bicarbonate or 
carbonate ion, depending upon the pH of the lixiviant. At the higher pH's 
produced by adding relatively large amounts of sodium hydroxide, the 
complexing agent will be predominantly in the form of the carbonate ion. 
When lower amounts of sodium hydroxide are employed, bicarbonate will 
predominate. 
At the production well, the pregnant lixiviant is produced to the surface 
by either downhole or uphole pumping means (not shown) and transferred to 
a heat exchanger 24 where it is heated to a temperature which normally 
will be in excess of 100.degree. C. Preferably, the pregnant lixiviant 
will be heated to a temperature within the range of 
150.degree.-250.degree. C. The heated lixiviant is transferred from the 
heat exchanger to a precipitation zone 25 where it is reduced from the 
hexavalent state to the tetravalent state. Reduction may be accomplished 
by treating the pregnant lixiviant with hydrogen under a pressure within 
the range of 10 to 50 atm in the presence of a suitable catalyst such as 
nickel, platinum, or freshly precipitated uranium dioxide. For a more 
detailed description of a suitable hydrogen reduction operation, reference 
is made to the aforementioned publication by Merritt, pages 235-237. 
Yellowcake (UO.sub.2) is recovered from the precipitation zone via 
effluent line 26 and the now barren lixiviant is recirculated to the 
blending system 15. A small fraction of the barren lixiviant may be 
discarded to waste disposal and, if desired, the barren lixiviant may be 
passed to a desalinization unit before it is circulated to the blending 
system. The heated lixiviant is recharged with carbonate and hypochlorite 
as described prebiously and then reintroduced into the production well.