Two-Stage Uranium in Situ Leaching Process

This invention relates generally to the field of in situ leaching of uranium, in particular to the leaching of uranium values from ores which are not amenable to alkaline or neutral carbonate leaching processes.

It has been found that some uranium ores are not amenable to alkaline or neutral carbonate leaching. These refractory ores are readily leached using dilute sulfuric acid (about 0.1-1% by weight) and oxygen under pressure. From the standpoint of cost-effectiveness, sulfuric acid is the most desirable leaching chemical for use in the in situ leaching of uranium ores. HCl is more costly and interferes with the subsequent ion exchange processes used for the recovery of uranium values from the leachate solution that is produced from the formation. HNO.sub.3 is very expensive, and its potential for water pollution makes it unsuitable as a leaching chemical. Other acids such as acetic acid are much more expensive than sulfuric acid and interfere with the recovery of uranium values by ion exchange.

Unfortunately, sulfuric acid has a long recognized disadvantage as a leaching chemical for ores which contain calcite (or CaCO.sub.3), namely, the potential plugging of the formation by the precipitation of CaSO.sub.4. Various methods have been devised in the past to circumvent this problem of CaSO.sub.4 plugging and the resulting loss in the formation permeability, but they have not been particularly successful. The importance of this problem is underscored by the fact that uranium-bearing ores are most often found in sandstone formations. There is always some calcium carbonate present in sandstone. For example, the CaCO.sub.3 content of uranium ores at Crownpoint, New Mexico, which are refractory to alkaline and neutral carbonate leaching, is in the range of from 0.6% to 6.20%.

The problem of calcite control in an in situ leaching operation is addressed in U.S. Pat. No. 4,103,963, which describes an in situ leaching operation which is representative of the prior art to this invention. The mechanics of the invention may generally follow the disclosure of that patent. Thus, it is believed to be readily apparent to persons skilled in this art upon reading the disclosure of U.S. Pat. No. 4,103,963 in conjunction with the disclosure herein how to practice the present invention.

In view of the large amount of uranium ore in the United States which is not amenable to in situ carbonate leaching processes, it has long been a goal of the industry to develop a process using H.sub.2 SO.sub.4 /O.sub.2 for leaching uranium out of CaCO.sub.3 containing refractory ores which is economical and which overcomes the loss of formation permeability heretofore observed. This goal is satisfied in accordance with this invention by conducting, in combination, a two-stage in situ leaching process. The first stage involves the removal of "labile" CaCO.sub.3 and the second stage involves conventional sulfuric acid leaching of the treated formation.

In the first step of the process in accordance with this invention, labile calcium carbonate is removed by pumping the necessary amount of carbonic acid (formed by the introduction of high pressure CO.sub.2 into water at the bottom of the well bore) or hydrochloric acid through the formation. By the term "labile" as used in this application, it is meant that calcite in the formation that can readily react with the dilute H.sub.2 SO.sub.4 used in the leaching step. Not all of the calcite in the formation can be attacked by the dilute sulfuric acid leaching solution due to the physical configuration and accessibility of the calcite in the formation. It is estimated that the labile CaCO.sub.3 content of the formation is about one-half that of the total CaCO.sub.3 content of a typical formation. The first step removes the labile calcite so that it does not form insoluble CaSO.sub.4 in the formation as a result of reaction with dilute H.sub.2 SO.sub.4 in the second, acid leaching step of the process.

The preferred acid for use in the first step is carbonic acid, formed, as indicated above, by the introduction of high pressure CO.sub.2 into the water that is pumped through the formation. The effluent solution from the first step is stripped of CO.sub.2 in an apparatus such as a Spiractor to recover the CaCO.sub.3 in solution as calcite. The CO.sub.2 gas evolved is recovered for recycling. If hydrochloric acid is used, the effluent is ordinarily disposed of, unless calcium chloride in solution can economically be recovered as a by-product. The necessary duration of the first stage operation in accordance with this invention is signaled by the achievement of a predetermined pH of the effluent solution, about 4 if carbonic acid is used, and about 3 if HCl is used. In addition to the removal of labile calcite, this first-stage treatment of the formation with carbonic acid or hydrochloric acid greatly improves the permeability of the formation.

Upon the reaching of the predetermined pH of the effluent water produced from the formation during the first stage of the process, a few (about 3) pore volumes of water are flushed through the formation in preparation for the leaching step. A leaching solution consisting of about 0.1-1% by weight of H.sub.2 SO.sub.4, and which is treated with gaseous O.sub.2 at the bottom hole pressure, is pumped through the formation to leach uranium mineral values therefrom. The uranium value is recovered from the leachate using conventional techniques, such as ion exchange or solvent extraction and precipitation to obtain uranium as yellow cake.

In the second, leaching stage, the sulfate ion concentration in the leached solution should be adjusted by varying the amount of H.sub.2 SO.sub.4 dissolved therein depending on the content of calcium ions so as not to exceed greatly the solubility product of CaSO.sub.4. Generally speaking, the working range of H.sub.2 SO.sub.4 concentration in the leaching solution is between 0.1 and 1% by weight, as the acid concentration is dictated by the potential loss in formation permeability due to plugging of the formation by CaSO.sub.4 by-product. It is also within the scope of this invention to use additives such as aluminum ions, or polyelectrolytes and other flocculants, in the leaching solution in order to improve formation permeability. The use of these additives may also make it possible to use leaching solutions of higher H.sub.2 SO.sub.4 concentration than 1% by weight. The use of leaching solutions of higher H.sub.2 SO.sub.4 concentration will speed up the leaching process and thus improve the cost-effectiveness of the method.

For reasons which are not entirely understood, it is necessary to use sulfuric acid as the acid leaching component in the second stage in order to produce uranium values effectively from the formation. The use of carbonic acid or a CO.sub.2 /O.sub.2 mixture as the leaching solution was found to be ineffective.

The foregoing description of this invention has been directed to particular details in accordance with the requirements of the Patent Act and for purposes of explanation and illustration. It will be apparent, however, to those skilled in this art that many modifications and changes may be made without departing from the scope and spirit of the invention. It is further apparent that persons of ordinary skill in this art will, on the basis of this disclosure, be able to practice the invention within a broad range of process conditions. It is my intention in the following claims to cover all such equivalent modifications and variations as fall within the true scope and spirit of my invention.