Process for Recovering Uranium and Other Base Metals

One convenient method of recovering metal values from underground ore deposits incorporates the in-situ leaching of the deposit. A lixiviant or leach solution that will dissolve the metal is pumped or injected into the underground deposit. The enriched or pregnant leach solution containing the dissolved metal value is raised up above ground where the metal value is recovered. After the recovery, the now barren solution is typically reinjected into the deposit. The ability to essentially continuously recirculate or recycle the leach solution is at least eminently desirable if not necessary in such in-situ solution mining schemes. To maintain the efficacy of the spent leach solution, fortification with the solubilizing compounds will be necessary at least periodically.

Several problems attend the in-situ solution mining techniques and militate against the success of the technique. The leaching solutions often react with minerals other than the desired metal values so that the solutions become contaminated with undesired materials. The dissolved contaminants may be difficult to separate from the dissolved desirable metal values. Substantial amounts of expensive leaching compounds may be lost beyond recovery in the underground deposit. Long continuous recirculation of leach solutions may adversely affect the aquifer because ions may be captured by the mineral deposits. These ions may be retained in or near the mining zone long after the mining operation is terminated and be released to the aquifer to adversely affect water quality. Extensive restoration activities may be mandated to prohibit or minimize this adverse effect. Water available at the site is a valuable resource for purposes other than mining and should not be contaminated so as to destroy its other utilities.

Underground water saturated ore bodies containing oxide, sulfide or carbonate minerals of base metals such as uranium, copper, nickel, molybdenum, rhenium, selenium and vanadium are exemplary candidates for in situ solution mining. Uranium is an example of particularly valuable base metal. The known solution mining techniques for the recovery of uranium and the other base metals may include both acid and alkaline leach solutions. When the ore deposit contains substantial amounts of calcite, the alkaline carbonate-bicarbonate lixiviant has particular advantages. Ammonium or sodium carbonate-bicarbonate lixiviants are both known to be advantageous because they are less corrosive than acid lixiviants, are more selective in dissolving and then separating the uranium from other metals. Uranium is now believed to be present in the deposit in its reduced, e.g. insoluble tetravalent form and may have to be oxidized to its soluble hexavalent form either before or during leaching.

Deposits that are suitable for in situ solution mining typically include a permeable stratuum disposed between impermeable strata. The leaching solutions can then be restricted to the permeable base metal rich stratuum. That permeable stratuum should be afterwards restored as nearly as possible to its original state. Where the permeable stratuum is capable of yielding considerable quantities of water to wells or springs, great attention and effort are directed to disturbing the original water quality as little as possible and/or restoring it to its original quality if it is adversely affected by the injected recirculating compositions. Initial or baseline measurements on the aquifer or formation waters provide data on the original water composition, e.g. baseline TDS data (total dissolved solids). After the base metal is extracted but before the in situ mining operation is considered completed, the formation water ordinarily must be restored to some acceptable level near the baseline quality.

In the previously known processes, making the initial and/or reconstituting the recirculating lixiviant have been particularly troublesome because of their perturbative environmental effects, particularly on the formation waters. Restoration to or near the original or base line levels has, in some instances, been difficult, time consuming and, therefore, expensive.

U.S. Pat. No. 3,792,903 discloses a solution mining process in which a buffer zone is formed within the mineral deposit area with soft water; cations can be removed by ion exchange.

U.S. Pat. No. 3,853,981 is directed to recovery of coppe and zinc by a process which removes zinc contaminating ions from copper recovery steps.

U.S. Pat. No. 3,156,644 discloses a deionization process which permits both cations and anions to be removed from water.

A Rohm and Haas publication entitled "DESAL PROCESS" discloses desalination processes. (30 pp., 1971.)

U.S. Pat. No. 2,992,887 uses low pressure CO.sub.2 to maintain the integrity of Na.sub.2 CO.sub.3 in a lixiviant for uranium.

U.S. Pat. No. 3,130,960 uses a solution of CO.sub.2 to leach uranium.

U.S. Pat. No. 4,066,297 discloses a process of solution mining uranium with an alkaline leach solution and the removal of metal ions from the recirculating solution.

It is an object of this invention to provide a process for generating and maintaining or reconstituting a lixiviant or leaching solution in a manner that makes it particularly suitable for in-situ solution mining because it is free of or minimizes environmental perturbations.

The lixiviant is generated and/or reconstituted by converting potentially pertubative inactive ion species that are present in available water or spent aqueous based lixiviants into active leaching ion species. Perturbative anions such as Cl.sup.- and SO.sub.4.sup.= are converted into the active metal leaching HCO.sub.3.sup.- ions to either initially create or reconstitute spent lixiviants. Using anion exchange resins to effect the conversion to HCO.sub.3.sup.- ions avoids the introduction of perturbating cations and permits the lixiviant to be formed and the recirculating lixiviant to maintain its HCO.sub.3.sup.- concentration with no, little or minimal adverse effects on the environment.

FIG. 1 is a schematic flow diagram illustrating an example of preparing a leach solution where the base composition of the formation water has a relatively high anionic strength;

FIG. 2 is a schematic flow diagram illustrating an example of preparing a leach solution where the base composition of the formation water has a relatively low anionic strength; and

FIG. 3 is a more detailed schematic flow diagram illustrating a process of an in-situ solution mining with a continuous flow or recirculation of lixiviant wherein the bicarbonate strength of the continuously circulating lixiviant is maintained at a level to dissolve desired metal values.

One aspect of my invention begins with the conversion into a leaching solution of a saline brackish formation water which could be found, for example, in the bore holes at the site of a new solution mining well field. In accordance with sound environmental practices, one desires to use the available water in a manner that will not adversely affect the mining zone or permit relatively convenient restoration to or near its original condition. This is especially true of formation water. The dissolved solids in the formation water, which can, of course, be characterized a solution, will primarily be chloride, sulfate, carbonate and/or bicarbonate salts of sodium, calcium, magnesium and potassium. For the purposes of my invention, a suitable relatively high total dissolved solids (TDS) is a sufficient amount of dissolved solids to provide water having a cationic strength of about 1000 ppm expressed in HCO.sub.3.sup.- equivalents. Either of the following baseline compositions of formation waters available at two different Texas mining sites can be conveniently converted by my invention into bicarbonate leaching solutions that will leach soluble uranium values from underground deposits without contaminating the aquifer:

Water L had a pH of 7.5 and Water B had a pH of 7.6. It should be noted that while the actual analyses also reported the cations, it is the natural anion concentration of the water that determines the potential strength of the leach solution to be generated.

After the initial baseline analysis of the formation water, the cationic strength in HCO.sub.3.sup.- equivalents is calculated by adding together the actual concentration of the HCO.sub.3.sup.- ions and the actual concentration of the other anions expressed as HCO.sub.3.sup.- equivalents, the latter calculated by the formula: ##EQU1## It should be understood that the 580 ppm of SO.sub.4.sup.= can be converted to 720 ppm of HCO.sub.3.sup.- by exchanging the SO.sub.4.sup.= anions in the water for HCO.sub.3.sup.- ions available from the basic form or preferably the bicarbonate form of an anion exchange resin.

As will become more apparent hereinafter, one of the most significant advantages of my invention is not only the generation of the HCO.sub.3.sup.- ion from other anions but also that the generation avoids the addition of cations. One way of accomplishing the conversion involves the use of a strongly basic anion exchange resin. The exchange between the resin in bicarbonate form and the Cl.sup.- anion, may be represented or illustrated by the equation:

Regenerating the exhausted bed of the strongly basic anion exchange resin requires a considerable amount of NaHCO.sub.3. Because they are more readily regenerated, I prefer to use weakly basic anion exchange resins. The exchange for these resins, in basic form, and the Cl.sup.- anion, may be represented by the following formulae. The generation of the weakly basic anion exchange resin is illustrated by the equation:

With the resin in the bicarbonate form the general representation would be:

For reasons which will appear later in this description, I prefer to use the bicarbonate resin form and to use a resin prepared with tertiary amines. The preference for the tertiary amines is because that is the weakest basic resin form and it requires no significant excess of material in the resin regeneration step. My preference for the HCO.sub.3.sup.- form of the resin, rather than the basic form, is related to the ability to more easily avoid metal precipitation on the resin because of localized high basicity. This will be more fully described later in connection with the description of regenerating recirculating lixiviants.

The desired exchange of anions on the preferred tertiary amine weakly basic anion exchange resin may be represented by the equation: ##STR1## wherein R is aryl, aralkyl or an alkyl group in the C.sub.8 to C.sub.11 range or a mixture of alkyl groups in which the average number of carbons falls in the range of 8 to 11. Such weakly basic anion exchange resins are known in the art and are described, for example, in U.S. Pat. No. 3,156,644. They are particularly desirable in this invention because of the ease with which the bicarbonate form of the resin can be generated and regenerated. The regeneration can be accomplished with a lime slurry or any alkaline reagent, e.g. Na.sub.2 CO.sub.3, NaOH or NH.sub.3, with stoichiometric quantities or only a slight excess. The CO.sub.2 required for the reaction can be readily generated at the mining site with fuel oil fired conventonal CO.sub.2 generators or water scrubbed stack gases. The following reactions take place: ##STR2## Suitable weakly basic anion exchange resins are commercially available. Examples of suitable commercial resins are Amberlite IRA-68 and Amberlite IRA-94 (Rohm and Haas); Dowex 44 and Dowex MWA-1 (Dow Chemical Co.); Duolite A-7 (Diamond Shamrock); and Ionac A-260 (Ionac Corp.).

The bicarbonate form of anion exchange resins, especially the weakly basic form, is particularly advantageous because it is highly selective for most all common anions such as SO.sub.4.sup.=, Cl.sup.-, etc. This means that formation water ions other than bicarbonate ions will be easily and conveniently converted into bicarbonate ions in the formation water and thus generate an effective bicarbonate leach solution. Such leach solutions can be employed to leach base metals such as uranium from underground deposits without adversely affecting the mining zone. Using a resin in the bicarbonate form, it should be emphasized, permits the other anions to be converted to bicarbonate ions without the addition of other cations, e.g. Na.sup.+ or NH.sub.4.sup.+. Other methods of forming bicarbonate leaching solutions would require the addition of compounds such as ammonium carbonate, sodium carbonate, potassium carbonate and/or their respective bicarbonates. It should be apparent that while the addition of these compounds to the water would indeed increase the bicarbonate concentration and form a solution that would leach uranium, the concentration of cations such as Na.sup.+, NH.sub.4.sup.+, etc. would be increased in the aquifer when the leaching solution was recirculated through the deposit. Potential exchanges or reactions between minerals or compounds in the mining zone and these added cations would cause these cations to be retained in the aquifer and may require subsequent removal in restoration processes if the limits above baseline are exceeded. With my invention, the base line cation concentrations are not affected and the natural equilibrium is retained.

Where the water at the mining site has a low salinity or is below about 500 ppm in HCO.sub.3.sup.- equivalents, the desired HCO.sub.3.sup.- concentration cannot be attained with only the above-described conversion. In such cases, i.e. where the formation or available water has a relatively low TDS, carbon dioxide may be injected into the water while it is being circulated through the ore deposit so that the TDS and the HCO.sub.3.sup.- concentration is increased. If a satisfactory anionic strength is not reached in a reasonable time, then a minimal amount of compounds may be added to raise the anionic strength to a level where the desired concentration of HCO.sub.3.sup.- can be obtained. The measured baseline data can be helpful in deciding what compounds should be added to the water.

The following is a base line analysis of a relatively low TDS formation water having a pH of 8.7 available at a mining site:

It is, of course, apparent from the HCO.sub.3.sup.- Eq. (equivalents) of 360 ppm, that the water cannot be converted to a bicarbonate solution with the desired HCO.sub.3.sup.- concentration of at least about 500 ppm. Carbon dioxide is injected into the formation water and the water is circulated through the ore body until a suitable solution pH is reached. Bicarbonate, carbonate, chloride and/or sulfate salt of sodium are/is added until the Na.sup.+ reaches about 380 ppm, when the HCO.sub.3.sup.- equivalence will equal about 1023 ppm. Now the Cl.sup.- and SO.sub.4.sup.= ions can be converted to HCO.sub.3.sup.- ions as they were above.

It should be noted that with the above low TDS water, the choice was not to add ions which were at a low level in the baseline measurements, but rather ions which were already at a higher concentration were added. If, for example, 120 ppm of cation is to be added, it is preferable to increase the Na.sup.+ to 250 ppm than to increase either the K.sup.+ or Mg.sup.++ an equivalent amount. The reason is that it is simpler to restore from 250 ppm to 130 ppm than it would be to restore from 122 ppm to 2 ppm.

Referring now to FIG. 1, formation water is withdrawn from a well and passed into a saturator vessel where CO.sub.2 is injected into the water. The water containing dissolved CO.sub.2 is then passed to the IX column (resin exchange) containing a bed of weakly basic anion exchange resin in solid particulate form. The HCO.sub.3.sup.- concentration of the water emanating from the IX column is checked to assure the conversion of sufficient SO.sub.4.sup.= and Cl.sup.- to provide a desirable amount of HCO.sub.3.sup.-. The solution may be recirculated as indicated by the broken line in the event that one pass through the IX column is not sufficient to generate the desired HCO.sub.3.sup.- concentration.

FIG. 2 is a simple schematic diagram illustrating a process similar to that presented in FIG. 1 except that the water is circulated through the deposit and solids are added to generate the desired HCO.sub.3.sup.- concentration from a formation water having a relatively low TDS. The earlier description of Water I is a detailed example of this process.

The foregoing descriptions have been limited to the initial preparation of a leaching solution or lixiviant to focus on the mechanisms of conversion or exchange of anions such as SO.sub.4.sup.= and Cl.sup.- to or for HCO.sub.3.sup.-. It should be understood that the most significant benefits will attend the described conversion or exchange in a continuous or long running in situ uranium mining operation where a bicarbonate lixiviant is recirculated in a pattern of injection wells extending into the porous ore deposit and extracted from a pattern of productions wells extending up from the ore deposit. As the lixiviant is recirculated, primary aboveground production operations include the recovery of uranium dissolved in the pregnant lixiviant and lixiviant fortification.

The pregnant lixiviant is contacted with a strong base anion exchange resin to remove uranium which is loaded on the resin as a complex carbonate anion. Two fixed bed IX columns may be employed and they may be alternately cycled between loading and elution. Dowex 21K (Dow Chemical Co.) is a suitable commercially available resin. It is typically in Cl.sup.- form and after loading may be eluted with a chloride solution. The uranium may be recovered from the pregnant eluate in any of various known methods to provide, for example, an ammonium or sodium diuranate. It should be noted that the chloride exchange and leakage from the resin constitutes a source of chloride ion build-up in the recirculating lixiviant.

The barren lixiviant must be reconstituted or fortified before it can be reinjected into the ore deposit. While any HCO.sub.3.sup.- present is effective at least about 500 ppm would be generally used. From about 1000 to about 2000 ppm is considered the ideal bicarbonate concentration range. Some increase in uranium dissolution may be noted up to about 5000 ppm but there is a leveling off and greater concentrations are uneconomical with most underground deposits.

It should be understood that the leaching solution has been described in terms of HCO.sub.3.sup.- or bicarbonate ion concentration. This is a term of art that should be understood to actually include other ions in an equilibrium relation consistent with general carbonate-bicarbonate and dissolved carbon dioxide equilibrium aqueous solutions. The term of art is used for convenience but it will be understood to actually include all of the foregoing species, i.e. HCO.sub.3.sup.-, CO.sub.3.sup.= and dissolved CO.sub.2.

Where, for example, ammonium ions, sodium ions or other cations are added to lixiviant to restore the desired HCO.sub.3.sup.- concentration in the spent barren lixiviant, ecological perturbations which are difficult to reverse may occur. A significant degree of ion exchange is believed to occur between the lixiviant and the host minerals of the ore deposit. Cation exchange predominates although some degree of anion exchange also occurs. A significant tendency to exchange Ca.sup.++ in the host mineral for monovalent cations (such as ammonium, sodium and potassium) in the lixiviant have a significant impact on the lixiviant reconstitution requirements and restoration phases. The net loss of leach chemicals across the ore deposit (generally about 500 ppm of HCO.sub.3.sup.-) necessitates continuing reconstitution of lixiviant. A significant portion of any cations added in the continuing reconstitution are accumulated in the host minerals. There is, of course, a strong public interest that requires the existing ecological balances to be minimally disturbed. Restoration to or near to the baseline quality is desirable and, indeed, mandated by governmental agencies particularly where water or aquifer quality is a prime concern. The profound advantages of continuously reconstituting the spent lixiviant without adding cations should now be apparent. Oxidation of NH.sub.4.sup.+ ions can be a source of contaminating NO.sub.3.sup.- ions.

Before the HCO.sub.3.sup.- reconstituted lixiviant is injected back into the ore deposit, an oxidant such as hydrogen peroxide, oxygen, air, etc. is added so that uranium which is in the insoluble reduced form will be oxidized to the soluble form and dissolve in the bicarbonate solution. It should be noted that the oxidant also oxidizes portions of sulfides in the deposit and causes SO.sub.4.sup.= ions to build up in the recirculating lixiviant. Chlorides may also be built up and it should be apparent that these anions are eliminated in a useful manner in my process, i.e. they are exchanged for HCO.sub.3.sup.- ions in the reconstitution step.

The anion exchange for the reconstitution may be conducted before or after the uranium removal by ion exchange but is preferably conducted afterwards, i.e. on the barren solution. As was noted earlier hereinabove, a weakly basic anion exchange resin is preferred over strongly basic anion exchange resins. The weakly basic anion exchange resin may be in its basic form in the IX column. CO.sub.2 may be added or injected into the solution prior to or in the IX column. Anions such as SO.sub.4.sup.=, Cl.sup.- and NO.sub.3.sup.- will be converted into HCO.sub.3.sup.- before emerging from the IX column. Alternatively, the resin in the IX column can be first converted into the HCO.sub.3.sup.- form. Again, anions such as SO.sub.4.sup.=, Cl.sup.- and NO.sub.3.sup.- will be converted into HCO.sub.3.sup.- before the lixiviant emerges from the IX column. NO.sub.3.sup.- anions can be introduced in oxidant stabilizers, for example.

Initial experimental results with barren lixiviants that retain small amounts of metal cations such as uranium, vanadium, etc., indicates that the localized high pH present in the resin bed causes some metal precipitation which coats or contaminates the resin beds. This fouling is eliminated when CO.sub.2 is injected into the lixiviant prior to entry into the IX column and the resin is already in the bicarbonate form. The IX column itself must be partially pressurized or at least blanketed with CO.sub.2. A saturator vessel may be included prior to the IX column. The CO.sub.2 in this vessel is at a pressure of at least several psi. The solution will have a pH in the range of 5 to 7 as it emerges from the saturator. Two fixed bed IX columns may also be utilized here so that loading and elution can be alternated thus providing continuous flow.

The subsequent steps of calcium removal and CO.sub.2 injection may sometimes be advantageously included to provide pH control to either minimize possible calcium precipitation and consequent flow blocking or simply to make the injected lixiviant conform to the baseline pH. Ordinarily, it is desirable to maintain the pH of the injected solution in the range of about 6 to 9.

Referring now to FIG. 3, we have illustrated the steps of injecting the bicarbonate leach solution, the in situ or leaching in place of uranium at the underground deposit and recovering the pregnant leach solution. Now in above-ground operations, the uranium is recovered by loading and eluting on a strongly basic anion exchange resin. Shown as alternating cycles in two IX columns, it should be understood that other IX techniques, e.g. resin bed transfer could be employed. As described in detail heretofore, the barren solution is saturated with CO.sub.2 and then anions such as SO.sub.4.sup.=, Cl.sup.- are NO.sub.3.sup.- and exchanged for HCO.sub.3.sup.- ions in the IX columns. The columns may be alternately loaded and eluted. The reconstituted or fortified lixiviant now has an HCO.sub.3.sup.- concentration between about 1000 ppm and 2000 ppm. The optimal steps of calcium removal and CO.sub.2 injection provide a convenient means for controlling the pH of the recycled lixiviant. After the addition of the oxidant, the solution is injected back into the ore deposit. Although not illustrated, it should be understood that a sufficiently high concentration of HCO.sub.3.sup.- ions may, in some instances, be generated by passing only a portion of the barren solution from the uranium IX column to the CO.sub.2 tank and diverting the remainder to the recycle leach.

The foregoing description may be characterized, for example, as a minimum perturbation process. The principles of the invention, however, may also be used in what may be more appropriately characterized as a restorative in-situ mining process. Where, for example, an initial process has employed ammonia or compounds containing cations to continuously reconstitute the lixiviant, these cations may already be retained in the mineral deposit because of the underground exchange and retention of monovalent cations described hereinabove. The subsequent use of this process would provide non-equilibrium conditions at the ore deposit, causing those cations to revert to the circulating lixiviant as the uranium is being mined. The undesired or contaminating cations can be continuously removed from the recovered lixiviant above ground and the aquifer will be slowly restored to or nearer its baseline quality.

It should be understood that profound environmental advantages attend this invention because of the conversion of otherwise contaminating anions into useful bicarbonate ions. As the in-situ mining operation continues, there is no accumulation of the anions (and their associated cations) in the recirculating lixiviant. At mining sites having relatively high TDS (and high anionic strength) formation water, the Cl.sup.- and SO.sub.4.sup.= (and associated cations such as sodium), for example, will essentially stay near their baseline levels. Thus, restoration after mining is considerably simplified, if not completely eliminated insofar as anions such as Cl.sup.- and SO.sub.4.sup.= are concerned. With this process, the carbonate-bicarbonate levels are raised above baseline levels but not to any levels greater than processes which add bicarbonate salts of sodium or ammonium for maintaining leaching strength. Moreover, it is not difficult to restore the carbonate-bicarbonate levels back to or near baseline levels after the mining is ceased by precipitation, for example, with lime. Because TDS levels remain relatively constant and because the levels of SO.sub.4.sup.=, Cl.sup.- and other such anions are not increased, the consequences of leach solution migrations and excursions are not serious. At mining sites having relatively low TDS (and low anionic strength), only the cation levels (e.g. Na.sup.+) are increased because of solid salt additions. The Cl.sup.- and SO.sub.4.sup.= levels are not increased. Again, the carbonate-bicarbonate levels are not increased more than with other processes. The increase in sodium, moreover, is limited to that caused by the initial singular salt addition, a relatively small increase compared to continuous additions.

Base metals other than uranium can be susceptible to solution in bicarbonate lixiviants, particularly those where the mineral deposit contains the metal in oxide or sulfide form. Uranium, as well as other metals, can be in-situ solution mined with acid lixiviants, e.g. H.sub.2 SO.sub.4. By using appropriate cation exchangers above ground one may design a continuous leaching process with a minimal effect on the environment. Indeed, the use of certain ion exchangers could be advantageous ecologically in above-ground leaching processes where lixiviants can be used repeatedly or continuously so that reconstitution is possible.