Although the barrens at four hours were affected by all the variables, the digestion pH and sulfate concentration were of the greatest statistical significance. This means that there is a high degree of correlation between the barrens and the sulfate and pH values. The effect of sulfate and digestion pH on the barrens is pictured in Figure 3. Several significant observations can be made. First, the digestion pH has an optimum range that does not shift as the sulfate increases (Figure 3A). The minimum value of the barrens obtained in this range is, however, dependent on the sulfate concentration. It is not surprising that the barrens increase as the pH is lowered since uranyl peroxide dissolves in acid. It is surprising, however, that as the pH increases above the optimum range, the barrens begin to increase dramatically. Apparently, uranyl peroxide will dissolve in base or large excesses of hydrogen peroxide plus base:
A second observation is that the barrens increase with increasing sulfate level though the increase in barrens is more dramatic at high sulfate levels. As the uranium concentration is increased, the effect of a given level of sulfate on the barrens is decreased (Figure 33). These effects are consistent with the formation of a di-sulfate-uranyl complex.
There is a much greater interaction between the chloride concentration and the digestion pH than was seen with the sulfate effect. Figure 4 shows that at a given pH, the barrens increase significantly with increasing chloride (Fig. 4A). For example, at pH 4.75, the barrens increase from .001g/L U3O8 to 1.44g/L U3O8 when the chloride increases from 0 to 100g/L. The corresponding change for increasing sulfate was only .09g/L to 1.44g/L U3O8. A second observation on the effect of chloride is that the pH corresponding to lowest barrens decreases with increasing chloride. Thus, at 100g/L chloride, the digestion pH should be lower than at 25g/L chloride.
Finally, the effect of a given level of chloride is lessened by increasing the uranium concentration (Fig. 4B). The similarity of the chloride effect to the sulfate effect at high digestion pH and the strong interaction between digestion pH and chloride level indicates that the simple mono-chloro-uranyl complex does not account for the total chloride effect. Apparently, higher chloro complexes are involved.
The interaction between the chloride and sulfate concentrations is pictured in Fig. 5. Several observations can be made. First, the increase in barrens is approximately equal along either axis which means that the effect of chloride or sulfate,independent of the other ion, is similar. Second, the levels of chloride and sulfate are not additive, that is, the barrens at 100g/L chloride alone are much greater than the barrens calculated for 50g/L chloride and 50g/L sulfate. And third, the pattern that is observed by increasing either ion concentration is independent of the concentration of the other ion; the barrens increase slowly from. 0-50g/L of chloride or sulfate, but increase dramatically from 50-100g/L. The magnitude of the increase is affected by the total ion concentration. These observations lead to an important principle. If the chloride strength is high and sulfate low, it is much better to add sulfate than it is to add chloride. This has a bearing on pH control, i.e., in a chloride eluant, sulfuric acid should be used for pH control, not hydrochloric.
Figure 6 shows how the uranium concentration and initial pH interact with the digestion pH. As can be expected from the reaction equation for hydrogen peroxide and uranium, increasing the uranium concentration promotes the formation of uranyl peroxide (Fig. 6A), the barrens being lowest at high uranium levels. There is a slight decrease in the optimum digestion pH as the uranium level increases, supporting the conclusion of an increase in reaction rate at higher uranium values as high pH is not needed to attain low barrens. Outside the optimum digestion pH range, the uranium concentration has little effect on the barrens. The effect of the initial pH is slight. Generally, the higher the initial pH, the lower the barrens. This effect is overshadowed by the dependence of the barrens on the digestion pH.
When the precipitation reaction is allowed to continue for a full 24 hours, the barrens at any set of conditions are improved over the four hour measurements. In Fig. 7, the barrens at 24 hours are portrayed as a function of sulfate, chloride and digestion pH. The barrens follow the same behavior pattern as was observed at four hours (compare Figs. 3, 4). The digestion pH has the largest effect and has an optimum range for low barrens. The barrens increase with increasing sulfate and chloride. However, the effect of high sulfate and chloride is more pronounced at the 24 hour reading. The barrens show a greater rise with increasing sulfate and chloride than was true at the short digestion time indicating that at high concentrations of chloride and/or sulfate, the uranyl complexes that are formed are quite stable. Consequently, it is impossible to precipitate uranyl peroxide at concentrations of chloride and/or sulfate in excess of 100g/L; a longer digestion time does lower the barrens but not sufficiently to make the process usable.
It has been demonstrated that the solution composition and the precipitation conditions have a profound impact on the precipitation of uranyl peroxide. The reaction occurs easily in solutions having less than 50 g/L of sulfate or chloride but is inhibited in higher concentrations. Above 100g/L sulfate or chloride, it is extremely difficult to reach low barrens. The initial pH should be high to help promote the formation of uranyl peroxide. The digestion pH should be kept in fairly strict limits to prevent the redissolution of uranyl peroxide. The reaction works best with high uranium concentrations. By looking at the interplay of these factors, it has been possible to gain a greater understanding of the process chemistry involved. This understanding should make, it easier to effectively apply uranyl peroxide precipitation to specific plant needs. As long as low barrens can be obtained, the uranyl peroxide itself is a sufficiently pure, easily handled product to be considered as a routine processing step.
It should be emphasized that greater purity is an inherent advantage of uranyl peroxide precipitation. The reaction of hydrogen peroxide with uranium is a specific chemical reaction under acidic conditions producing an insoluble, well characterized and crystalline product. ADU, on the other hand, is an amorphous, un-characterized uranyl oxide/hydroxide produced by raising the pH of the solution. Under these conditions it is easy to precipitate other basic metal oxides/hydroxides and thus impurities are incorporated directly into the ADU. With peroxide precipitation, most of the impurities are simply absorbed on the surface of the precipitate and can be removed with washing.