Magnesium bicarbonate as an in situ uranium lixiviant

In the subsurface solution mining of mineral values, especially uranium, in situ, magnesium bicarbonate leaching solution is used instead of sodium, potassium and ammonium carbonate and bicarbonates. The magnesium bicarbonate solution is formed by combining carbon dioxide with magnesium oxide and water. The magnesium bicarbonate lixivant has four major advantages over prior art sodium, potassium and ammonium bicarbonates.

BACKGROUND OF THE INVENTION 
This invention concerns the solution mining os uranium with novel 
lixiviant. The novel lixiviant is magnesium bicarbonate formed by 
combining carbon dioxide with magnesium oxide and water. 
In known processes for leaching uranium values from underground formations 
in situ, an oxygenated aqueous solution of an alkaline or acid leaching 
agent is delivered to the uranium-bearing formation through one or more 
injection wells. Conventional alkaline leaching agents are sodium, 
potassium and ammonium carbonates and bicarbonates. The acid or alkaline 
leaching solution utilized in conjunction with the oxidant transforms the 
uranium mineral deposit into a soluble salt. The uranium mineral is 
leached from the formation, dissolved in the leaching solution and 
subsequently produced from an offsetting production well. The production 
fluid is then processed for the extraction of the uranium therefrom, with 
the spent leaching solution and oxidant being either reconstituted for 
reinjection into the formation or discarded. 
Acid and alkaline leaching solutions, and sodium, potassium and ammonium 
carbonates or bicarbonates present distinct problems. Sodium and potassium 
cause clay swelling thereby affecting formation permeability and solution 
sweep efficiencies. Similar undesirable effects arise when sodium and 
potassium carbonate or bicarbonate cause calcite and gypsum precipitation. 
Acid leach solutions cause gypsum formation. Acid solutions react with 
certain formation minerals. Ammonium ions can cause adverse environmental 
effects which render it necessary to remove the ammonium ions after 
leaching. Alkaline solutions precipitate alkaline metal from the leach 
solution causing a decrease in injectivity, permeability and sweep 
efficiency. Continued injection of sodium, potassium and ammonium 
carbonates ot their respective bicarbonates in conjunction with oxidants 
results in a build up of these alkaline metal ions which aggrevates the 
forming of undesirable precipitates. 
In the improved uranium leaching process of this invention, most of the 
problems associated with sodium, potassium and ammonium carbonates and 
bicarbonates are overcome or substantially reduced. 
SUMMARY OF THE INVENTION 
The objects of this invention are accomplished using magnesium bicarbonate 
solution to replace sodium, potassium and ammonium carbonates and 
bicarbonates. The magnesium bicarbonate solution is formed by combining 
carbon dioxide with magnesium oxide and water. The magnesium bicarbonate 
lixiviant composition has at least four significant advantages. The pH of 
the solution may be maintained at approximately 7 thus minimizing calcite 
formation and the other adverse effects of acid or alkaline solutions. In 
addition, magnesium ions tend to shrink clays thereby enhancing 
permeability rather than reducing it. Magnesium ions eliminate the 
environmental necessity of removing ammonium ions after leaching. Divalent 
magnesium ions form an uncharged complex with sulfate thereby reducing 
gysum precipitation. Magnesium bicarbonate does not exist except in 
solution; therefore, the solution of magnesium bicarbonate will usually be 
formed on site. The magnesium bicarbonate solution is used in the same 
fashion as sodium, potassium and ammonium carbonate and bicarbonate 
solutions are used in known uranium in situ leaching processes.

DETAILED DESCRIPTION OF THE INVENTION 
Uranium values or minerals and other oxidizeable, leachable substances like 
thorium, vanadium, copper, nickel, molybdenum, rhenium and selenium 
frequently occur in underground or subterranean siliceous rocks and 
sedimentary deposits or formations. Uranium generally occurs as a mixture 
of the insoluble tetravalent form and the soluble hexavalent form. In the 
basic solution mining process of this invention, an oxidant or oxidizing 
agent in injected or introduced into a subterranean deposit to contact the 
mineral substance and to oxidize the mineral in place to a soluble form. 
Air is usually used as the oxidizing agent, but oxygen and hydrogen 
peroxide are also suitable oxidizing agents. Other chemical oxidants like 
permanganates may be used, but the cost of such chemicals and the 
difficulty or removing them from some formations render such chemicals 
economically unattractive. The preferred concentration of oxidizing agent 
on a free oxygen basis is between 25 and 250 parts per million. 
The oxidized mineral substance, e.g., hexavalent uranium, is contacted in 
situ by injecting magnesium bicarbonate leaching solution into the 
formation to solubilize the hexavalent uranium and form a pregnant liquor 
of the mineral. This pregnant liquor is recovered or extracted from the 
mineral deposit. The oxidation of the mineral can be carried out as a 
separate step or simultaneously with the magnesium bicarbonate leaching 
step. Preferably, however, the process is operated continuously and the 
oxidizing agent and leaching solution are injected simultaneously. 
Since magnesium bicarbonate does not exist except in water in the presence 
of some free carbon dioxide, the magnesium bicarbonate solution injected 
into the formation formed by combining carbon dioxide and water with 
magnesium oxide or magnesium carbonate, for example, the carbon dioxide 
may be bubbled through a water-magnesium oxide mixture under pressure. 
Preferably, the magnesium bicarbonate leach solution is formed at the 
injection site just prior to the injection. Preferably, the pH of the 
leach solution is maintained between 6 and 8 and is kept as close to 7 as 
is feasible. The maximum concentration of magnesium bicarbonate depends on 
the type of water used to form the leach solution, the ratio of the volume 
of the solution injected to the volume of the liquid produced, and other 
factors well understood by those skilled in the art. Preferably, the 
bicarbonate ion concentration will be between 250 and 1500 parts per 
million. 
In a conventional fashion, the magnesium bicarbonate leaching solution is 
brought into contact with the subterranean deposit through one or more 
injection wells which penetrate the subterranean deposit. The leaching 
solution is introduced into an injection well under sufficient pressure to 
force it out into the adjacent deposit. Continued injection of the 
magnesium bicarbonate leaching solution drives pregnant solution through 
the deposit to one or more spaced-apart production wells where the 
solution is recovered for subsequent extraction of the mineral values. The 
leaching solution may also be driven by a follow-up drive fluid. The drive 
fluid may be air, water, flue gas, brine or any other suitable fluid for 
displacing the leaching solution. 
The number of injection and production wells and the spacing therebetween 
can be varied to best suit the nature of the formation. It is preferred 
that the injection and production wells either be drilled in concentric 
patterns about each other with a single production well contained within 
the center of the pattern, for example a typical five-spot pattern, or 
that the injection and production wells be drilled in offsetting line 
patterns so as to create a line drive sweep mechanism within the uranium 
formation. Generally, the distance between the injection and production 
wells will be from 20 to 500 feet. Particular engineering conditions of 
the formation such as depth, thickness, permeability, porosity, water 
saturation, and economic and recoverable value of the uranium mineral in 
the formation control the design of the well pattern for a specific 
formation. 
Alternatively, a given volume of leaching solution can be injected into a 
well to percolate into the surrounding formation. Following this injection 
phase, the injected leaching solution may be recovered from the same well 
into which it had been injected. If desired, one or more of the production 
wells may be turned into an injection well. Also each stage or variation 
of the process may be followed or preceded by one or more periods of 
noninjection with or without continued production. Also, each stage or 
variation of the process may be followed or preceded by one or more 
periods of nonproduction with or without continued injection. Therefore, 
through patterned well completion and other variations of the type 
mentioned, the process may be used sequentially across the deposit so that 
the entire deposit is treated. 
The process of this disclosure may be preceded by one or more buffer zones 
to improve or control sweep patterns or to remove deleterious substances. 
Moreover, surface active agents, clay swelling inhibitors, solubility 
improvers, and other additives used in subsurface formations for improved 
results may be used. 
The pregnant mineral enriched solution that enters a production well is 
recovered by conveyance to the surface. At the surface, the recovered 
pregnant solution is processed in any desired way to recover the mineral 
value. For example, the pregnant solution may be filtered and passed 
through an ion exchange resin. The resin is then treated with sodium 
chloride solution with or without added carbon dioxide or the like. The 
recovered mineral may then be further prepared for commercial use if 
desired. 
It is possible in the above described manner to lixivate uranium from any 
strata containing extractible values with the magnesium bicarbonate 
solution, including granites and granitic deposits, pegmatites and 
pegmatitic dikes and other formations and sedimentary deposits including 
sandstones, oil sands, etc., and uranium deposits of secondary character 
where for example the mineral values leached from say, pegmatitic sources 
have been naturally redeposited in some conveniently located porous 
sedimentary stratum. 
The above indicated solution mining processes for recovering mineral 
values, especially uranium, from a subsurface formation with magnesium 
bicarbonate leach solution an illustration of the wide variety of 
available procedures for in situ solution mining of recoverable minerals 
like uranium. This invention is not concerned especially with the 
provision of any particular method for mining the mineral from a 
subsurface formation. Any convenient or desirable method may be employed 
for this purpose so long as it includes the basic steps of injecting an 
oxidant, injecting a magnesium bicarbonate leach solution, recovering a 
mineral pregnant liquor, and recovering the mineral from the pregnant 
liquor. It is the magnesium bicarbonate that provides the aforementioned 
advantages of this process over prior processes of this nature.