3.2.1. Effect of pH Value of the Aqueous Phase

Because t-BAMBP is an extractant which is suitable for alkalic condition, the pH values were adjusted to 8–14 with 0.1 M t-BAMBP and O/A ratio 1:1 at reaction time 15 min and 25 ◦C in this experiment. Figure 4 shows that the extraction percentage of K<sup>+</sup> and Rb<sup>+</sup> were observed almost 0% to 5% at any pH value and the percentage of Cs<sup>+</sup> was observed above 99% from pH 8 to pH 12. In the condition of pH 13 and pH 14, emulsification happened and reduced the extraction percentage. Due to these situations, the optimal pH value of the aqueous phase was set to pH 8 which has a high extraction percentage of Cs<sup>+</sup> and could reduce the usage of sodium hydroxide.

**Figure 4.** Extraction percentage of metals at different pH values in the first stage.

#### 3.2.2. Effect of t-BAMBP Concentration

The conditions of t-BAMBP concentration were set up from 0.001 M to 1 M at pH 8 and O/A ratio 1:1 at reaction time 15 min and 25 ◦C in this step. Figure 5 shows that the extraction percentage of Cs<sup>+</sup> from 0.001 M to 0.1 M increased gradually and became stable. The reason was that a higher concentration of the extractant enabled more Cs<sup>+</sup> to be caught. However, the extraction percentage of K<sup>+</sup> and Rb<sup>+</sup> started to increase with a higher concentration of extractant. This was because K<sup>+</sup> and Rb<sup>+</sup> were extracted by excess extractant and made an adverse effect on this system. Due to this condition, the optimal parameter of t-BAMBP concentration was chosen as 0.1 M.

**Figure 5.** Extraction percentage of concentration of t-BAMBP in the first stage.

#### 3.2.3. Effect of O/A Ratio

Figure 6 shows the O/A ratios were set from 0.1 to 2 with 0.1 M t-BAMBP at pH 8 and at reaction time 15 min and 25 ◦C. The result shows that the extraction percentages of Cs<sup>+</sup> maintain above 99% with different O/A ratios, which means that Cs<sup>+</sup> were almost extracted. However, when the O/A ratio was greater than 0.5, the extraction percentage of K<sup>+</sup> and Rb<sup>+</sup> increased gradually above 2%. This was because K<sup>+</sup> and Rb<sup>+</sup> were extracted by excess extractant as well. Hence, in order to get a high concentration of Cs<sup>+</sup> and avoid the impurities, the O/A ratio of 0.1 was an optimal parameter in this step.

**Figure 6.** Extraction percentage of O/A ratio in the first stage.

#### 3.2.4. Effect of Reaction Time

The effect of reaction time was set from 3 min to 60 min with 0.1 M t-BAMBP at pH 8 and O/A ratio 1:1 at 25 ◦C. In Figure 7, the extraction percentage of Cs<sup>+</sup> was very stable from 3 min to 60 min. K<sup>+</sup> and Rb<sup>+</sup> were at equilibrium and low extraction percentage as well. It shows that the reaction of t-BAMBP was fast and reaction time was not a significant influence in the extraction process. On account of this condition, 3 min was chosen.

**Figure 7.** Extraction percentage of reaction time in the first stage.

#### 3.2.5. Effect of Reaction Temperature

Figure 8 shows the reaction temperature was a significant parameter in the extraction process. The effect of reaction temperature was set from 5 ◦C to 65 ◦C with 0.1 M t-BAMBP at pH 8 and O/A ratio 1:1 at 3 min. The percentage of extraction decreased drastically from 35 ◦C to 45 ◦C. This is because t-BAMBP extracted metal ions with exothermic reaction and the high temperature caused the decrease of distribution ratio. Finally, the extraction percentages of Cs+, K+, and Rb<sup>+</sup> were respectively 99.8%, 5.2%, and 1% with the condition of pH 8 of the aqueous phase, 0.1 M t-BAMBP, O/A ratio of 0.1, 3 min reaction time, and 35 ◦C reaction temperature. Compared to other studies, this study shows the high extraction percentage of Cs<sup>+</sup> with lower pH value and O/A ratio at the low temperature and Cs<sup>+</sup> could be separated from Rb, K, and other impurities such as lithium, sodium, potassium, calcium, and magnesium.

**Figure 8.** Extraction percentage of reaction temperature in the first stage.

3.2.6. Stripping of Cs from the Organic Phase through Ammonia

After the extraction process, 420 mg/L of cesium was in the organic phase and needed to be stripped. In this process, NH4OH was chosen as a stripping agent and the effect of NH4OH was presented in Figures 9–11. Because the stripping efficiencies of rubidium and potassium were low in this procedure, only stripping efficiency of cesium was investigated. Figure 9 shows that when the concentration of NH4OH increased, the stripping efficiency increased as well and become equilibrium in the situation of 1 M, 2 M, and 5 M. Therefore, the optimal concentration of NH4OH was 1 M in the procedure. Figure 10 shows that the stripping efficiency decreased drastically when O/A ratio was 4. It means that insufficient NH4OH was unable to strip the Cs<sup>+</sup>. Due to this condition, O/A ratio 2 was the optimal condition to get a high concentration of Cs+. Figure 11 presents the stripping efficiency with reaction temperature. Because t-BAMBP has great extracting ability at lower temperatures, it made NH4OH unable to strip Cs<sup>+</sup>. However, if the temperature was above 35 ◦C, NH4OH decomposed to NH3 first which was unable to strip Cs+. Hence, the stripping temperature was chosen at a room temperature of 25 ◦C in this step. Finally, the stripping efficiency of Cs<sup>+</sup> was almost 99.9% in the first stripping process.

**Figure 9.** Stripping efficiency of concentration of NH4OH in the first stage.

**Figure 10.** Stripping efficiency of O/A ratio in the first stage.

**Figure 11.** Stripping efficiency of reaction temperature in the first stage.

#### *3.3. Second Stage of Solvent Extraction for Rb*

After the first stage of solvent extraction, metals were separated into two systems. One system was cesium, and another system was rubidium with other impurities such as lithium, sodium, potassium, calcium, and magnesium. In the second stage of solvent extraction, t-BAMBP was also chosen as extractant to separate rubidium with other impurities. Among the rest of the impurities, only K could be extracted by t-BAMBP due to the property of extractant. In this case, the values of K and Rb were presented to analyze the optimal condition.

#### 3.3.1. Effect of pH Value of the Aqueous Phase

The effect of pH value of the aqueous phase in the extraction and separation of Rb<sup>+</sup> and K<sup>+</sup> was shown in Figure 12. The pH values were adjusted to 8 to 14 with 0.1 M t-BAMBP and O/A ratio 1:1 at reaction time 15 min and 25 ◦C. The extraction percentage of Rb<sup>+</sup> and K<sup>+</sup> increased when the pH value raised up and declined at pH 13 and pH 14 due to the emulsification. In order to extract more Rb+, pH 12 value was chosen in this procedure.

**Figure 12.** Extraction percentage of metals at different pH value in the second stage.

#### 3.3.2. Effect of t-BAMBP Concentration

The conditions of t-BAMBP concentration from 0.01 M to 1 M at pH 12 and O/A ratio 1:1 at reaction time 15 min and 25 ◦C were set in this step. Figure 13 shows that the extraction percentages of Rb<sup>+</sup> from 0.01 M to 1 M were about 50% and the percentages of K<sup>+</sup> were about 40%. In order to get more rubidium, 0.5 M t-BAMBP was chosen in this procedure.

**Figure 13.** Extraction percentage of concentration of t-BAMBP in the second stage.

#### 3.3.3. Effect of O/A Ratio

Figure 14 shows the O/A ratio was set from 0.1 to 2 with 0.5 M t-BAMBP at pH 12 and at reaction time 15 min and 25 ◦C. The result shows that the extraction percentages of Rb<sup>+</sup> maintain above 50% with different O/A ratios. However, when the O/A ratio was greater than 0.1, the extraction percentage of K<sup>+</sup> increased gradually. Hence, in order to get a high concentration of Rb<sup>+</sup> and avoid the impurities, the O/A ratio 0.1 was an optimal parameter in this step.

**Figure 14.** Extraction percentage of O/A ratio in the second stage.

#### 3.3.4. Effect of Reaction Time

The effect of reaction time was set from 3 min to 60 min with 0.5 M t-BAMBP at pH 12 and O/A ratio 0.1:1 at 25 ◦C. In Figure 15, the extraction percentage of Rb<sup>+</sup> increased gradually from 3 min to 15 min and became stable. On the other hand, the extraction percentage of K<sup>+</sup> was almost 30% from 3 min to 60 min. Hence, 15 min of reaction time was chosen in this process to extract rubidium.

**Figure 15.** Extraction percentage of reaction time in the second stage.

#### 3.3.5. Effect of Reaction Temperature

In Figure 16, it shows that Rb<sup>+</sup> was influenced by the reaction temperature. The effect of reaction time was set from 5 ◦C to 65 ◦C with 0.5 M t-BAMBP at pH 12 and O/A ratio 0.1:1 at 15 min. The extraction percentage of Rb<sup>+</sup> was about 98% at 5 ◦C and decreased drastically after 5 ◦C. The percentage of 35 ◦C was 56.8% and only about 10% at 45 ◦C. In order to extract more Rb+, 5 ◦C was chosen as the optimal parameter. Finally, the extraction percentage of Rb<sup>+</sup> and K<sup>+</sup> and were respectively 98.3% and 41.3% with the condition of pH 12 of the aqueous phase, 0.5 M t-BAMBP, O/A ratio of 0.1, 15 min reaction time, and 5 ◦C reaction temperature. Compared to other studies, this study shows the almost same extraction percentage of Rb<sup>+</sup> with lower pH value and O/A ratio at low temperature.

**Figure 16.** Extraction percentage of reaction temperature in the second stage.

3.3.6. Stripping of Rb from the Organic Phase through Ammonia

After the second stage of the extraction process, 70 mg/L of rubidium was in the organic phase and needed to be stripped. In this process, NH4OH was chosen as a stripping agent as well and the effect of NH4OH concentration was presented in Figures 17–19. Because the stripping efficiencies of potassium was low in this procedure, only stripping efficiency of rubidium was investigated. Figure 17 shows that when the concentration of NH4OH increased from 0.1 M to 0.5 M, the stripping efficiency of Rb<sup>+</sup> increased as well. However, efficiency became equilibrium in the situation of 0.5 M, 1 M, 2 M, and 5 M. Therefore, the optimal concentration of NH4OH was 0.5 M in this step. Figure 18

shows that the stripping efficiency of Rb<sup>+</sup> was above 90% and decreased at O/A ratio 4. Due to this condition, O/A ratio 2 was the optimal condition. Figure 19 presents the stripping efficiency with reaction temperature. Like the situation of Cs+, the stripping efficiency of Rb<sup>+</sup> was lower at the low temperature and gradually rose with increase in temperature. Hence, the stripping temperature was chosen as 35 ◦C in this procedure. Finally, the stripping efficiency of Rb<sup>+</sup> was almost 95% in the second stripping process. After the extraction and stripping of Cs<sup>+</sup> and Rb+, the optimal parameters of solvent extraction are shown in Tables 3 and 4, and the components of stripping solutions are shown in Tables 5 and 6.

**Figure 17.** Stripping efficiency of concentration of NH4OH in the second stage.

**Figure 18.** Stripping efficiency of O/A ratio in the second stage.

**Figure 19.** Stripping efficiency of reaction temperature in the second stage.


**Table 3.** Optimal parameters of solvent extraction of cesium

**Table 4.** Optimal parameters of solvent extraction of rubidium


**Table 5.** Metal compositions of the first stage of stripping (cesium hydroxide solutions)


**Table 6.** Metal compositions of the second stage of stripping (rubidium hydroxide solutions)

