Table of Contents
Despite severe slippage in the growth rate of nuclear power, current estimates still show that the U.S. uranium demand will exceed the production capacity from known resources by about 1981. Projected demands also show that a fourfold to sixfold increase in uranium production will be necessary to meet the expected 1990 requirements. Surging demand and rapidly rising uranium prices mean that ores much lower in grade than the current average mill feed grade of about 0.16 percent U3O8 will have to be processed in the United States.
Current Bureau of Mines leaching studies are concentrating on systems for recovering uranium from ores containing 0.01 to 0.1 percent U3O8. Studies are in progress on a number of these materials. Information is being developed on both the processing characteristics of individual materials and the variability between different materials. The objective of the program is to develop a fund of information on the processing characteristics of lower grade uranium ores.
This report presents the results of characterization studies conducted on a granitic uranium-bearing material from Wyoming. The available samples of this material contained up to 1,200 parts per million (0.12 percent) U3O8. The average uranium content of the earth’s crust is 2 to 4 parts per million U3O8 , while some large bodies of granite such as the Conway granite in New Hampshire contain 15 to 20 parts per million. The enrichment in the Wyoming granite makes this material a considerably more attractive uranium resource than normal granites. Although the size of the deposit is not known, it does appear likely that significant amounts of uranium are present in the area.
The Bureau of Mines characterization studies investigated the effect of variables such as time, temperature, and reagent quantities in both acidic and alkaline leaching. Percolation leaching was examined to test the feasibility of heap leaching. The chemical response during in situ leaching was simulated by leaching with dilute carbonate solutions.
Description of the Samples
Two composited drill core samples of approximately 20 pounds each were used in the present studies. The chemical analyses are listed in table 1. This brecciated material is associated with a fault zone. A microscopic examination showed that the material is a partially altered granite. The most abundant minerals include quartz, microcline, plagioclase, and chlorite. The chlorite is apparently an alteration of the biotite present in normal granite. Pyrite and limonite are visible with a hand lens. A heavy mineral fraction with specific gravity greater than 2.94 containing pyrite, magnetite, epidote, apatite, and zircon was obtained by a heavy medium separation. Small amounts of chalcopyrite , arsenopyrite , and hematite are also present. Individual grains which were hand-picked from a magnetic fraction were coated with a reddish earthy material that appears to be gummite. No uranium minerals were identified. Although this material contains significantly more uranium than a normal granite, the source of the enrichment is not known.
Wet screen analyses were made on the samples after crushing, with the results shown in table 2. In both cases the uranium content increases as the particle size decreases. Since the samples were crushed from fragments much larger in size, this indicates that the uranium may be concentrated somewhat along natural fracture surfaces in the mineral. An alternative explanation of the uranium concentration in the fines is that uranium minerals tend to be softer than the matrix minerals in granite.
In leaching uranium-bearing materials, the most commonly used lixiviants are sulfuric acid and carbonate-bicarbonate solution. Sulfate and carbonate ions can complex the uranyl ion, UO2²+, to form anions, for example, UO2(SO4 )3 4- and UO2(CO3)3 4-, which can subsequently be removed from solution by anion exchange methods.
Several types of tests were made on the two samples. Agitation leaching with both types of solution was done to determine the effects of various reagents, temperature, and time on the uranium extraction. Percolation leaching tests were made with sulfuric acid solution to determine reagent consumption, the time needed for the extraction, and the solution flow rate through a packed bed.
Leaching With Sulfuric Acid
Dilute sulfuric acid effectively dissolves uranium in its oxidized form, UO3. The reduced form, UO2, can usually be converted to UO3 by the addition of an oxidizing reagent. Very often, however, acid-consuming components such as CaO and CaCO3 will be present; this increases the amount of reagent required and can make acid leaching impractical.
Several series of batch agitation leaching tests were done on the samples to determine the acid and oxidant requirements and the most effective ways of using these reagents. In these tests, 150 to 200 grams of crushed material were leached in a 1-liter beaker. The percent uranium extraction was calculated on the basis of the U3O8 content of the products.
To determine the acid and oxidant requirements in batch leaching for sample 1, tests were made with H2SO4 additions of 50 and 100 pounds per ton and NaClO3 additions up to 5 pounds per ton. As illustrated by the data in table 3, this material was effectively leached by 50 pounds per ton H2SO4 and 3 pounds per ton NaClO3. Larger amounts gave very little increase in uranium extraction.
The data also illustrate unexpected particle size effects. The terminal pH values for the minus 10-mesh grind were lower than those with the minus 35-mesh grind. Higher uranium extractions were also obtained with the minus 10-mesh grind. The results indicate that 18 hours is ample time for leaching minus 10-mesh material and also that the more finely ground material consumes more acid during this time.
In these tests, the oxidant was not added at the start of the leach. The initial reaction of the acid and gangue materials may produce reducing compounds which would consume the oxidant added at that time. These compounds can be oxidized by the atmosphere during the initial stage of the leach.
Effect of Leaching Time
Several tests with durations of 4 to 18 hours were made on minus 10-mesh and minus 35-mesh material to determine the effect of time on uranium extraction. The results are given in table 4. The best result was obtained when minus 10-mesh material was leached for 18 hours. Also, the solution pH values indicate that acid continued to be consumed throughout the duration of the test.
Effect of Oxidant Addition
Different oxidizing reagents are not equally effective on all ores. Comparisons were made using NaClO3 and MnO2. Sodium chlorate is soluble, but to dissolve MnO2 the manganese must first be reduced to Mn²+ by reaction with Fe²+ in solution (6, p. 64). It seems likely that MnO2 takes longer to react, and for this reason the MnO2 was added at the beginning of the leach in these experiments. This practice is followed in some uranium mills.
A series of tests was made to determine the effect of both NaClO3 and MnO2 additions. The relative efficiencies of these oxidants are shown in table 5.
The data do not indicate any significant differences. Assuming that in their respective reactions MnO2 is converted to Mn²+ and ClO3- to Cl-, 2.45 pounds of MnO2 is needed to oxidize the same amount of material as 1 pound of NaClO3. Hence, 3 pounds of NaClO3 and 7.5 pounds of MnO2 are nearly equivalent.
To determine if the time of addition of NaClO3 is important, tests were done in which the oxidant was added from 0 to 2 hours after the start of the leach. The results are shown in table 6. The terminal pH values were lower for the delayed addition; this indicates that the earlier the NaClO3 is added, the more acid is consumed.
Addition of a Surfactant
The addition of a surface active agent (surfactant) could possibly accelerate the removal of UO2²+ from the surface of a mineral by lowering the surface energy between the solid and liquid phases. Some previous experimental results indicate that the use of such agents reduces the consumption of other reagents. To investigate this effect, leaching tests were made with the addition of Aerosol OS, manufactured by American Cyanamid Co. The results in table 7 show that no beneficial effects were observed.
Comparison of the Two Samples
Reagent requirements may vary considerably for materials from different regions of the same deposit. Comparisons were made on the two available samples. Although they are of similar appearance and composition (see table 1), their leaching characteristics are somewhat different, as shown in table 8. Sample 1 was effectively leached with 50 pounds per ton H2SO4, whereas, sample 2 required more acid. The percolation tests, discussed in a later section, gave similar results.
Leaching With Carbonate-Bicarbonate Solution
If a uranium ore contains large amounts of acid consuming components, the carbonate-bicarbonate leaching system may prove to be more economical than acid leaching. Another advantage of carbonate leaching is that fewer undesirable elements such as iron are dissolved. Leaching with carbonate solutions, however, does have disadvantages. The uranium dissolution normally takes longer and the reagents must be recovered and recycled. The response of each new ore to carbonate leaching should be evaluated so that the specific relationship between favorable and unfavorable effects can be determined.
Effect of Oxidant Addition, Temperature, and Time
Leaching tests were made to explore the effects of time and temperature variations and oxidant addition. Test conditions varied from 12 hours at 85° C to 12 days at ambient temperature of 22° C; the leach solution contained 40 grams per liter Na2CO3 and 20 grams per liter NaHCO3. The results are given in table 9.
These tests indicated that both oxidant additions and higher temperatures decreased the retention times required for extractions greater than 90 percent. In the 72-hour test using an 8-pound-per-ton oxidant addition at 22° C, 96 percent of the uranium was extracted. This result strongly suggests that retention times less than 72 hours may be feasible at the 22° C leaching temperature. At a leaching temperature of 85° C, only 12 hours was required to extract 90 percent of the uranium.
Leaching With Dilute Solution
An ore body can be considered for in situ leaching if it is below the water table and at least partially enclosed by impermeable strata to prevent the loss of solution. The ore itself must be both permeable to the solution and amenable to leaching. In situ leaching normally uses dilute solutions, for example, 1 gram per liter NaHCO3 or NH4HCO3. More concentrated solutions may lower the permeability of the ore body. Several tests were done at ambient temperature for durations up to 21 days to determine if the uranium could be dissolved effectively. The results are given in figure 1.
Although after 21 days the maximum uranium recovery was only 38 percent, the amount of U3O8 dissolved was still increasing at the rate of 1 percent per day for the Na2CO3 and Na2CO3-NaHCO3 solutions and at 0.2 percent per day for the NH4HCO3 solution. The final extraction figures could be much higher over longer periods of time.
The lower efficiency of the ammonium system was probably due to the loss of NH3 during the leaching. This possibility should be investigated if in situ leaching is to be considered; ammonia loss underground would not be as great as in open leaching.
Acid and carbonate solutions can be used not only in agitation leaching but also in percolation leaching. In the latter method, the ore is in open
heaps or vats, and leach solution, which is distributed over the top of the ore, percolates through the bed and is collected at the bottom for uranium recovery and recirculation. This method requires that the particle size of the ore be large enough to permit reasonable percolation rates but small enough so that the uranium minerals are accessible to the solution during the leaching process.
Several small-scale percolation tests were made on both samples to explore the potential for heap leaching. Five hundred grams of material were crushed to either minus 3 mesh or minus ½ inch and placed in a 1½-inch diameter column, forming a bed approximately 12 inches deep. Due to the limited amount of material available it was necessary to prepare the minus 3-mesh material from sample 1 and the minus ½-inch material from sample 2. The solution was added at a rate which maintained a ¼-inch layer of liquid on top of the bed, that is, a “flooded” column operation. Effluent was collected and recycled as needed. Figure 2 shows the laboratory column arrangement.
Uranium Extraction and Solution Flow Rate
During the percolation tests, measurements were made to determine the flow rates, effluent pH, and uranium extraction. Both the final results and data at intermediate times during the tests are shown in tables 10-11.
The 50 pounds per ton addition of H2SO4 was very-effective for sample 1, the maximum extraction of 95 percent being reached in 3 days. The pH values show that acid consumption continued for both samples for more than 10 days and that sample 2 consumed more acid than sample 1 even at the ½-inch particle size. The extraction from sample 2 was appreciably less than that obtained from sample 1. This decrease is probably due to a combination of the larger particle size and the higher acid requirement of sample 2. This additional acid requirement was also observed during the test results presented previously in table 8. It is quite possible that uranium was beginning to reprecipitate when the pH reached the 3.1 level, and it is probable that the extraction from sample 2 would have been higher if more acid had been used.
The flow rate data suggest that adequate percolation flow rates should be possible with these materials, since successful heap leaches have been conducted with percolation rates as low as 2 gallons per square foot per day.
Oxidation by Exposure to Air
Some ores can be partially oxidized by exposure to the atmosphere, which could occur during stockpiling, for example. Several percolation leaching tests were done on a portion of sample 2 which had been air-oxidized by placing 15 pounds of minus ½-inch material in a column and forcing humidified air up through the bed at the rate of 100 milliliters per minute for 105 days. After 30 days, 100 milliliters of water was dripped on top of the bed, wetting the top 4 inches. After 60 days, an additional 1,000 milliliters was added; the entire bed was wetted, and 360 milliliters of water was collected at the bottom of the column.
Tables 12-13 give a comparison between the leaching characteristics of oxidized and unoxidized material. Maximum extraction was reached in 1 day for the oxidized material, but 4 days was needed for the unoxidized material.
Note.—See text for procedure for air oxidation of material.
The results of a more extensive series of tests on the oxidized material are given in tables 14-15. When 100 pounds per ton H2SO4 was used, the addition of an oxidant increased the extraction only slightly. Acid additions greater than 100 pounds per ton also gave little improvement. The pH values show that acid was consumed throughout the duration of the test. The solution flow rates illustrate the wide range of flows occurring in a series of supposedly identical small-scale tests. The data show that the rate may also increase or decrease significantly during the course of a particular test. Larger scale tests would be required to predict actual flow rates.
Conclusions and Recommendations
Although limited amounts of material were available and conditions could not be optimized for all types of tests, the exploratory studies resulted in the following conclusions:
- The reagent requirements varied considerably between similar samples. Fifty pounds per ton H2SO4 was sufficient for one of the samples, but 50 to 100 pounds per ton was needed for the other.
- The granitic material was effectively leached by H2SO4 in 18 hours, with 93 percent U3O8 extraction being achieved. Acid consumption continued after maximum uranium dissolution had been reached.
- An oxidizing agent was necessary in acid leaching. The addition of a surfactant gave no beneficial result. When the oxidant was NaClO3, the terminal pH was higher when the NaClO3 was added at the start than if added later. This result suggests that delayed oxidant addition reduces acid consumption.
- In leaching with sodium carbonate-bicarbonate solution, up to 94 percent U3O8 extraction was attained. The leaching rate was increased greatly by elevated temperatures and the addition of an oxidant.
- From the limited test work done, it appears that leaching with dilute (for example, 1 gram per liter) carbonate-bicarbonate solution is feasible. This would make in situ leaching an attractive alternative if the uranium is concentrated along natural fracture surfaces in the ore body, and if the structural geology and water table relationship are suitable.
- In acid percolation leaching, the extraction was as high as that in agitation leaching and required no more reagent for optimum extraction. The solution flow rates were high enough to suggest that this method is practical. Since acid consumption continued after maximum extraction had been reached, the timing of heap or in situ leaching would have to be carefully controlled to avoid unnecessary acid addition.
- The characteristics of this material can be changed by exposure to the atmosphere. In percolation leaching, a sample exposed to airflow for 105 days was leached in 1 day, compared with 4 days for the unoxidized material.
The following recommendations are made for treating this type of materials
- Reagent requirement tests should be conducted on both fresh and aged samples, since these requirements may be considerably different.
- The time needed for maximum uranium extraction in acid leaching should be examined carefully, since reagent consumption continues after the extraction is completed.
- The amenability to leaching with dilute carbonate solution should be examined if heap or in situ leaching is considered. The ore body can be considered for in situ leaching if the chemical characteristics are suitable, the formation is permeable to the solution, and the solution can be contained within the ore body. The possibility of heap leaching should be examined by larger scale percolation tests.
- Acid solution may not be suitable for in situ leaching, due to the continual consumption of acid. If the pH becomes higher than 2, uranium precipitation may occur; this precipitation has been observed in some of the percolation tests.