In-situ uranium leaching process

The present invention provides an improved process for the in-situ recovery of mineral values, particularly uranium, from subterranean deposits that exhibit heterogeneous permeabilities in the formation zones. Aqueous solutions of thickening agents or viscosity building agents are utilized to control the mobility of the leaching solutions as they traverse the subterranean formation to solubilize the mineral values therein.

FIELD OF THE INVENTION 
The present invention relates to a method for improving the recovery of 
mineral values such as uranium from subterranean ore bodies subjected to 
in-situ leaching by controlling the flow behavior of the leaching 
solution. More specifically, the present invention relates to an improved 
process for recovering mineral values such as uranium from a subterranean 
formation wherein improved sweep efficiency is provided through the use of 
mobility control agents. 
BACKGROUND OF THE INVENTION 
Conventionally, in in-situ solution-mining processes, the leaching solution 
is brought into contact with the subterranean deposit through a suitable 
injection system. The leaching solution or lixiviant may be an alkaline or 
acidic medium which solubilizes the mineral values as it traverses the ore 
body. Conventionally, the mineral values in an ore body are subjected to 
an oxidation step in order to convert the mineral values to a soluble 
form. For example, the tetravalent uranium must be oxidized to its soluble 
hexavalent form for leaching. The pregnant lixiviant is then withdrawn 
from the ore body through a suitable production system and treated to 
recover mineral values therefrom by suitable techniques such as solvent 
extraction, direct precipitation or by absorption and elution employing an 
ion exchange resin. The above method and modifications thereof work most 
efficiently when a fairly uniform formation is the subject of the leaching 
process. All too often, however, and in fact in the majority of cases, the 
formations are not uniform as to both porosity and permeability. In some 
zones, the strata are sufficiently heterogeneous as to severely alter flow 
patterns of the leaching fluids. Leaching fluids follow the higher 
permeability streaks thus by-passing portions of the ore body which 
results in loss of recoverable mineral values due to the lack of contact 
by leaching fluids. For example, in many uranium reservoirs, 30 to 50 wt.% 
or more of uranium values may not be recoverable via in-situ leaching 
because of channelling of leachate through the high permeability zones. 
In secondary and tertiary oil recovery processes, the problem of 
channelling and fingering has also been recognized. Various methods 
utilizing polymeric material as viscosity builders or solution thickneners 
have been used as indicated by U.S. Pat. No. 3,434,542 to Dotson et al, 
U.S. Pat. No. 3,888,308 to Gale et al, U.S. Pat. No. 4,129,182 to Dabbous, 
U.S. Pat. No. 3,530,938 to Cooper, U.S. Pat. No. 4,066,126 To Waite el al, 
U.S. Pat. Nos. 3,292,698 and 4,042,030 to Savins et al, and U.S. Pat. No. 
4,018,281 to Chang. The polymeric thickener or viscosity builder is 
normally utilized in either a water bank or a surfactant bank injected 
into the formation to drive the oil to a production system. 
However, such polymeric material as utilized in secondary and tertiary oil 
recovery degrades and loses it's viscosity building effects when subjected 
to oxidants. As stated above, oxidants are essential to the in-situ 
recovery of mineral values such as uranium. 
Accordingly, the present invention provides a new method wherein mobility 
control methods, utilizing polymeric material as thickeners or viscosity 
builders, are applied to the in-situ recovery of mineral values even when 
an oxidation step is necessary. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides an improved process for the 
in-situ recovery of mineral values, particularly uranium, from 
subterranean deposits that exhibit heterogeneous permeabilities in the 
formation zones. The formation is penetrated by suitable injection and 
production systems. An oxidant is injected into the formation to oxidize 
the mineral values therein to their soluble forms. After the desired 
degree of oxidation is achieved, an aqueous leaching solution which 
contains a leaching agent and is substantially free of oxidant is injected 
into the formation to solubilize the mineral values therein. The leaching 
solution is displaced through the subterranean formation by means of a 
mobility control aqueous solution which contains a sufficient amount of 
thickening agent to give it a greater viscosity than the leaching 
solution. In another aspect of the invention, an aqueous solution 
containing a thickening agent is injected into the formation, after 
oxidation but prior to the injection of the leaching solution, in order to 
plug the higher permeability zones thus preventing the channelling of the 
leaching fluids. This alternate process could be preceeded by a 
conventional leaching process to recover the mineral values from the 
higher permeability zones. Furthermore, a thickening agent, that exhibits 
an increase in viscosity with increasing shear rate, may be added to the 
leaching solution to give it better sweep efficiency. The above processes 
substantially reduce the fingering and channeling of the leaching solution 
thus increasing the mineral recovery not by leaching action but through 
the provision of a more favorable mobility and sweep of the formation. The 
pregnant lixiviant containing mineral values is produced via the 
production system and is subsequently subjected to processes for the 
recovery of the mineral values.

Further advantages of the process of the present invention will be apparent 
from the following more detailed description thereof. 
DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
While the present invention is hereinafter described in relation to the 
in-situ recovery of uranium, it should be understood that the invention is 
also applicable to the in-situ recovery of inorganic substances capable of 
reacting with aqueous solubilizers to form solutions miscible with water. 
These inorganic substances especially include phosphates, iron, aluminum, 
titanium, copper, nickel, silver, gold, lead, zinc, manganese, cobalt, 
chromium, and molybdenum. Other substances soluble in aqueous solubilizers 
will be apparent to those skilled in the art. 
The present invention may be carried out utilizing injection and production 
systems as defined by any suitable arrangement of wells. The injection and 
production wells can be arranged in any convenient pattern designed to 
achieve maximum contact of the uranium-containing zones by the leaching 
fluids, such as the conventional "five spot" pattern wherein a central 
well is surrounded by four somewhat symmetrically located injection wells. 
Another of the conventional flooding patterns that can be employed in the 
practice of this invention is the "line drive" pattern in which the 
injection wells are arranged in a line so that the injected fluids advance 
through the formation toward one or more spaced production wells that can 
also be arranged in a line substantially parallel to the line of injection 
wells. Other suitable patterns include staggered line drive, four spot, 
seven spot, circular flood patterns and others. 
Uranium minerals frequently occur as a mixture of the insoluble tetravalent 
form and the soluble hexavalent form. The tetravalent form must be 
oxidized to its soluble hexavalent form for leaching. Conventionally, the 
oxidizing agent and the leaching solution are injected simultaneously with 
the preferred practice being to solubilize the oxidizing agent in the 
leaching solution. Because of the adverse effects that oxidants have on 
polymeric thickening agents, it is essential in accordance with the 
present invention to subject the formation to a pre-oxidation step prior 
to the injection of the leaching solution, thus minimizing the contact 
between the oxidants and thickening agents. Although it is preferred that 
the leaching solution be substantially free of oxidant, this does not 
preclude the presence of mild oxidants, such as oxygen or air, in minor 
amounts in the leaching solution. As is known in the art, these oxidants 
exhibit low solubility in aqueous solutions. 
Any of the conventionally used oxidizing agents can be employed as the 
oxidants in the present invention with preference given to the milder 
oxidants such as oxygen, air, and oxygen-containing gases. For example, 
oxygen, air, oxygen-containing gases, or mixtures thereof may be injected 
into the formation until break-through at the production wells, 
subsequently, the production wells are shut-in to allow oxidation of the 
formation. This process may be repeated until the desired degree of 
oxidation has taken place. Alternatively, the above preferred oxidizing 
gases may be injected into the formation in an aqueous medium. In 
addition, potassium permanganate, potassium ferricyanide, sodium 
hypochlorite, potassium peroxydisulfate, and hydrogen peroxide can be 
employed as oxidants. Oxygen and oxygen-containing gases are the preferred 
oxidants. These include air, CO.sub.2 /O.sub.2, and oxygen/steam systems. 
After the oxidation of the formation is completed, an aqueous leaching 
solution or lixiviant is injected into the formation to solubilize the 
uranium therein. As is well known in the art, the lixiviant may be an 
acidic or alkaline medium which solubilizes uranium values as it traverses 
the ore body. For example, carbonate leaching systems employing alkali 
metal carbonates and/or bicarbonates are suitable leaching solutions for 
application in the present invention. Additionally, systems utilizing 
carbon dioxide as the leaching agent may be applied in accordance with the 
present invention. The above represent examples of leaching solutions and 
are not intended to be limiting. Other leaching solutions may be utilized 
depending on the thickening agent used. 
In many ore deposits the strata are sufficiently heterogeneous as to 
severely alter flow patterns of the leaching solution. Leaching fluids 
follow the higher permeability streaks thus by-passing portions of the ore 
body. Tests show that in many reservoirs 30 to 50% or more of uranium ore 
values may not be recoverable via in-situ leaching because of channeling 
of leachate through the high permeability zones. This is especially true 
in a formation having a low permeability matrix which has been extensively 
fractured or which has high permeability streaks running through the basic 
formation matrix. In such a situation, the fractures or streaks have a 
permeability which is quite high and is drastically different from the 
unfractured or base matrix. 
While the uranium flooding process of this invention is particularly 
adopted for the improving the recovery of uranium from heterogeneous 
formations, as a practical matter, most uranium formations exhibit some 
heterogeneity, and thus recoveries are improved in most naturally-occuring 
uranium formations by treatment with the processes of this invention. By 
heterogeneity, it is meant that the formation is comprised of stratified 
layers of varying permeability, or that it contains fractures, cracks, 
fissures, streaks, vuggs, or zones of varying permeability that cause 
injected fluids to advance through the formation nonuniformly. Thus, the 
formations that are particularly amenable to treatment by the process of 
this invention are those formations that have strata or zones of different 
permeabilities, or which otherwise are structurally faulted so that the 
injected leaching fluid does not advance through the formation at a 
substantially uniform rate. 
Several techniques, involving the use of thickening agents, are proposed in 
order to improve the sweep efficiency of the injected uranium leaching 
medium and thus avoid premature breakthrough at one more of the wells 
comprising the production system. While various thickening agents can be 
used, the discussion below will refer to polymers since polymers are the 
most commonly used thickening agents. The polymer solution used for 
mobility control is a water solution of a water-soluble polymer especially 
selected for its ability to reduce fluid mobility in the more permeable 
zones without causing substantial complete plugging or stoppage of flow 
within these zones. Hence the polymer must not exhibit any substantial 
chemical reaction with the formation rock, the connate formation fluids, 
or the leaching solution, that would cause cross-linking or precipitation 
of the polymer, or that would result in any substantial amount of 
absorption of the polymer by the reservoir rock causing complete plugging 
of any particular strata or zone. The type of polymer employed for 
mobility adjustment, its concentration in the aqueous polymer solution, 
and the amount of polymer solution injected into the reservoir are 
selected upon consideration of the permeability of the formation to the 
injected fluids, the differences in permeability between the various 
zones, and the reservoir volume to be treated. The reservoir structure can 
be predicted from core analysis, well logs, and the history of previous 
fluid injection programs where applicable. The optimum mobility control 
can be verified by conventional laboratory core tests. Typically, mobility 
control can be achieved in most reservoirs by the injection of between 
about 0.005 and 0.15 pore volume of an aqueous polymer solution containing 
between about 0.01 and 0.20 weight percent polymer. 
Various thickening agents which may be employed in carrying out the present 
invention include such natural materials as guar gum or karaya gum or such 
synthetic products as the ionic polysaccharide B-1459 produced by 
fermentation of glucose with the bacterium xanthomonas campestris NRRL 
B-1459, USDA, and available from the Kelco Chemical Company under the 
trade name "Kelzan"; and poly(glucosylglucan)s such as disclosed in U.S. 
Pat. No. 3,372,749 to Williams and available from the Pillsbury Company 
under the trade name "Polytran." Other thickening agents which may be 
employed include carboxymethyl cellulose, polyethyleneoxide, hydroxyethyl 
cellulose, and the partially hydrolized polyacrylamides available from the 
Dow Chemical Company under the trade name of "Pusher Chemicals." While the 
above are specific examples, it is understood that any thickening agent 
compatable with the formation involved may be employed in the invention. 
There are several means in which sweep efficiency of a leaching solution 
can be improved through the utilization of a mobility control aqueous 
solution that contains a thickening agent. In one aspect of the invention, 
the formation is subjected to a preoxidation step as described above. 
Subsequently, an aqueous solution containing a leaching agent is 
introduced into the subterranean uranium-containing formation through a 
suitable injection system. Since economics severely limit the total 
quantity of leaching solution that can be injected, it is beneficial to 
displace the leaching solution with a much less expensive fluid. The 
viscosity of the fluid utilized to drive the leaching solution through the 
formation should be greater than the viscosity of the leaching solution in 
order to eliminate unwarranted fingering effects. This is achieved by 
displacing the leaching fluid with water containing a thickening agent, 
thus providing the necessary mobility reduction through increase in 
viscosity. The leaching solution may be an acidic or alkaline solution 
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 solvent extraction, direct precipitation or by 
absorption and elution employing an ion exchange resin. The above process 
may be repeated if necessary. 
Since the majority of formations do not have a substantially uniform 
matrix, channeling of the lixiviant occurs, thus by-passing regions of the 
ore. The foregoing disadvantage can be eliminated by using a thickened 
aqueous solution of a water soluble thickening agent. Prior to the 
injection of the leaching solution or after the first slug of leaching 
solution, a thickened aqueous solution is introduced into the formation. 
The thickened aqueous solution will traverse the formation by flowing 
through the higher permeability zones. When a sufficient amount of the 
thickened solution is introduced to occupy the higher permeability zones, 
additional leaching solution is introduced which will traverse the 
previously unswept or the lower permeability zones thus solubilizing more 
uranium. This result is accomplished because of the substantial reduction 
in mobility in the higher permeability zones due to the presence of more 
viscous fluids. After the production cycle, additional cycles or slugs of 
thickened solution and leaching solution can be utilized until such 
operations become uneconomical. 
An alternate form of the invention is to inject with the leaching solution 
a thickener of the type that exhibits an increase in viscosity with 
increasing shear rate. The advantage gained here is that the viscosity 
increases in the naturally occuring paths of higher flow because of the 
higher shear rate. The increased viscosity thus creates a higher pressure 
drop resulting in less flow in this region. The leaching solution is then 
diverted to the regions of lower permeability heretofore by-passed, thus, 
resulting in the solubilization of more uranium. Some examples of 
materials which exhibit an increase in viscosity with increasing shear 
rate include various starch suspensions, heavy metal phosphates, sodium 
borate, and polyvinyl alcohols. 
The viscosity of the aqueous mobility control medium should normally be the 
viscosity of the leaching solution and the reservoir fluids. The viscosity 
of the mobility control medium may be within the range of 1 to 4 times 
that of the leaching solution and the reservoir rock with the upper limit 
being due to economic constraints. The desired viscosities are usually 
achieved by using up to 3 weight percent of a thickener. As will be 
understood by those skilled in the art, the viscosity of the thickener 
solution as referred to herein is the viscosity at shear rates and at 
temperature conditions prevailing throughout most of the formation volume 
traversed by the mobility control medium as it travels between the 
injection and production systems. 
Although the present invention has been described with preferred 
embodiments, it is to be understood that modifications and variations may 
be resorted to, without departing from the spirit and scope of this 
invention, as those skilled in the art will readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the appended claims.