1. ORE PREPARATION and LEACHING the First Step in This
1. ORE PREPARATION and LEACHING
The first step in this process is physical preparation of uranium ore and acid leaching of the uranium values. Ore chosen for this process is soft, non-specimen-quality, calciferous / dolomitic Chinle sandstone from the Moab, Utah area, collected in person. A sledge hammer is used to convert about 5 lb. of “large rocks into small rocks,” ideally pea-sized or less, in a utility bucket. This particular ore does not need to be milled prior to leaching because of the high carbonate concentration, which leads to considerable agitation and generation of fine sediment as the carbonates partially neutralize the leach acid. Muriatic acid from Lowe's, HCl(aq) at 32% w/w concentration sold by the gallon, is used as the lixiviant. For this batch approximately 1/3 gallon was applied, but that is likely to be greatly in excess of necessity. Add enough water to cover the crushed ore and be careful to contain the aggressive foaming caused by CO2 evolution. Primary uranium ores such as those from SE Utah typically contain both the uranium(IV) and uranium(VI) oxidation states, of which only the latter is readily soluble in HCl(aq). To help oxidize uranium to the soluble hexavalent state, about 500 mL of 3% hydrogen peroxide is added. Laundry bleach can also be added (this must be done outdoors, obviously!).
Figure 1. Batch of ore in a pail, awaiting technological enhancement with the steel instrumentation at right.
Figure 2. Uranium ore leaching in a utility pail, in a strong solution of hydrochloric acid, hydrogen peroxide, and hypochlorite bleach (i.e., free chlorine in this acid environment).
Ore is leached for several days. During this time the leach liquor develops a deep green ++ coloration and contains uranium as the green uranyl cation, [UO2] . Care should be taken that acid is always in great excess (pH < 1) as the carbonates in the ore consume it. H2O2 should not be added in quantities that give rise to precipitates. 2. ALKALINE PRECIPITATION
Uranium in the leachate may be separated from alkaline-soluble components by precipitation with ammonia. The precipitate also contains most other transition metals, so it is not selective for uranium in particular. However, ammonia is preferable to sodium hydroxide because it forms soluble complexes with several major transition metals (e.g. Zn) at high concentration. We begin by decanting the raw acid leachate (Fig. 3) with a siphon, and filtering with coffee filters to remove fine sediment that causes turbidity. This task can be time-consuming since coffee filters are neither large nor particularly permeable to flow. Clarified leachate is shown in Fig. 4. The ammonia of choice for chemical work is sold specifically as “janitorial strength” ammonia, 10% w/w, in gallon jugs at Ace Hardware stores nationwide. This is one of the few reliable sources of technical-grade NH3(aq) free of surfactants, dyes, perfumes, and other adulterants. Ammonia is stirred into the leachate while precipitates form. Continue to add ammonia in excess of equivalence. The solution should cause a red color with phenol red indicator. The brownish precipitate containing uranium as ammonium diuranate, (NH4)2U2O7, will settle out and the supernatant solution will lose its green coloration and become water-white. To avoid pickup of CO2 from the air, this and all subsequent alkaline solutions should be covered while not in use. Step 2 is completed by decanting the supernatant solution into a waste bucket; washing the voluminous, muddy precipitate several times with dilute ammonia water; and finally, drying the precipitate in an oven. The resulting brownish-yellow cake can be sieved to a fine powder (Fig. 5).
Figure 3. Raw pregnant Figure 4. Filtered leachate is the color Figure 5. Washed, dried, and leachate carrying uranium and of an apple-flavored “Jolly Rancher” sieved precipitate with ammonia, many other cations including and exhibits ultraviolet fluorescence. containing insoluble ammonium those of iron, copper, vanadium, diuranate along with alkali- radium and molybdenum in insoluble hydroxides of many solution, and various debris and contaminants. This is the silicates as suspended solids equivalent of very-finely-milled causing extreme turbidity. uranium ore; the acid leach and first alkaline precipitation are principally conceived as a non- mechanical milling method rather than a separation of uranium. 3. CARBONATE-PEROXIDE PURIFICATION
The pairing of carbonate and peroxide uranium chemistry was chosen to isolate uranium from most of its contaminants. Uranium is one of few metals that form soluble carbonate complexes (Eq. 1), and also one of few metals to form very stable, acid-insoluble peroxides (Eq. 2). Higher purity may theoretically be attained by repeating a cycle of carbonate dissolution and peroxide precipitation. Purified solid ammonium diuranate (“yellowcake”) is an intermediate in my process, and while it is a convenient means of storing solid material for continuation of the process later, it is not clear that separating it effects any theoretical improvement in the final purity; as stated before, alkaline precipitation of uranium is not very selective.
2− 4− − NH 4 2 U 2O 7 s 3H2 Ol 6CO3 aq 2UO2 CO3 3 aq 2NH4 aq 6OH aq [1]
2 . UO2 aq H 2 O2 aq 2H 2 Ol UO4 2H2 O s 2H aq [2]
My carbonate lixiviant was 1M Na2CO3 and 0.33M NaHCO3 in boiling distilled water. The solids from Step 2 remained in contact with this solution in an airtight container for several days, with occasional microwave heating and stirring, to form a lime-green solution (Fig. 6). The solution was decanted and filtered to separate it from the waste solids. The latter were transferred to the waste bucket mentioned above. Na2CO3 can be obtained in grocery stores as Arm And Hammer Washing Soda in the laundry detergents section, or as Du Pont “pH Up” in pool supply areas of Home Depot or Lowe's. Carbonate and bicarbonate form a buffered solution that resists the rise in pH that would otherwise accompany the dissolution of uranium by carbonate (see Eq. 1). Values of pH > 10 are to be avoided as they favor precipitation of uranium as diuranates. In addition to uranium, the carbonate leaching presumably dissolves molybdenum (as the molybdate anion), vanadium (as the vanadyl carbonate anion), and the actinide decay chain products such as thorium and protactinium as carbonate complexes to some degree. To form intermediate yellowcake from ammonia in high yield, the carbonate leachate must first be neutralized (until effervescence ceases) with muriatic acid. With carbonate interference out of the picture, 10% NH3(aq) was added until further additions ceased forming precipitate. The lemon-yellow ADU yellowcake has an unavoidable muddy consistency, but settles faster if the ammonia is added slowly and with minimal stirring. The cake was washed several times in boiled distilled water, oven- dried, and seived (Fig. 7).
Figure 6. Uranyl tri- Figure 7. Intermediate- carbonate complex in purity ammonium di- solution. uranate (ADU) yellowcake. To form uranyl peroxide and isolate uranium from vanadium and molybdenum, I dissolved the ADU yellowcake in the minimum necessary quantity of hot muriatic acid and then diluted the solution with distilled water to ~3 l / cup of yellowcake (Fig. 8). Three equivalents of 27% hydrogen peroxide (“Baquacil,” a pool oxidizer sold by the gallon in pool stores) were added to the solution. A dramatic color change accompanies this action—a reddish-brown, blood-hued solution results, I believe due to a vanadium peroxo complex (Fig. 9), and a fine pale lemon-yellow precipitate forms. 10% ammonia was added slowly while stirring. More precipitate was formed and the reddish-brown color dissipated. The role of ammonia is to raise the pH and counteract the buildup of acid liberated per Eq. 2. Peroxide precipitation of uranium is known to occur with the best purity and yield at a pH of 3-4, so too much ammonia should be avoided. The hydrous UO4 precipitate was settled, washed several times with dilute H2O2 in distilled water, and dried in a ~80 deg. C oven. The solid peroxide (Fig. 10) is chalky and easy to handle. I attempted to process it further to form Fiestaware-orange UO3 by heating for a prolonged period at 200 deg. C. I wanted a compound with better-known stoichiometry than the UO4 -water adduct, and also wanted to eliminate the strong oxidizing reactivity of the peroxide in preparation for possible ion exchange or solvent extraction later on. The resulting oxide is shown in Fig. 11. Yield for the batch was 69 g. Some small but potentially dangerous peroxide remains, as evidenced by the generation of oxygen gas when a sample is strongly acidified with cold sulfuric acid in the presence of manganese dioxide. In the future I will probably not spend the effort to produce UO3, but will probably redissolve the peroxide into a carbonate solution for ion exchange on a strong-base anionic resin.
Figure 8. Uranyl solution for Figure 9. Red-brown soluble complex forms in solution while peroxide precipitation. adding hydrogen peroxide.
Figure 10. Uranyl peroxide. Figure 11. Anhydrous uranyl oxide.