2.3.2. Solvent Extraction–Extraction Process

In this study, t-BAMBP was diluted into kerosene and used as the extractant to separate rubidium and cesium from other impurities such as lithium, sodium, potassium, calcium, and magnesium. The extraction process was divided into two stages. Cesium was extracted in the first stage, and Li, Na, K, Ca, Mg, and Rb were still in the aqueous phase. Rubidium was then extracted in the second stage, and other impurities were in the aqueous phase as well.

To calculate the efficiency of extraction, distribution ratio and extraction percentage were used in this process. Distribution ratio, D, was the concentration ratio of the metal in the organic phase to the metal in the aqueous phase at equilibrium. The distribution ratio can be written as Equation (2).

$$\mathbf{D} = \frac{[\mathbf{M}]\_{\mathrm{org}}}{[\mathbf{M}]\_{\mathrm{aq}}} = \frac{\mathbf{C}\_{\mathrm{i}} - \mathbf{C}\_{\mathrm{f}}}{\mathbf{C}\_{\mathrm{f}}} \times \frac{\mathbf{V}\_{\mathrm{aq}}}{\mathbf{V}\_{\mathrm{org}}} \tag{2}$$

Ci is the initial concentration of metal ions in aqueous phase, Cf is the equilibrium concentration of metal ions in aqueous phase. Vaq and Vorg are separately the volume of aqueous phase and organic phase.

Based on the distribution ratio, the extraction percentage can be written as Equation (3).

$$\mathrm{E\_{ex} \left( \% \right)} = \frac{\mathrm{D}}{\mathrm{D} + \mathrm{V\_{aq} / V\_{org}}} \cdot 100 \tag{3}$$

D is the distribution ratio. Vaq and Vorg are separately the volume of aqueous phase and organic phase.

In the extraction process, the parameters such as the pH value of aqueous phase, the concentration of t-BAMBP, the O/A ratio, the reaction time, and the reaction temperature were all presented in the form of extraction percentage.

#### 2.3.3. Solvent Extraction–Stripping Process

Ammonia was the stripping agent in this experiment which could strip the rubidium and cesium from the organic phase. The stripping efficiency is written as Equation (4).

$$\mathrm{E\_{st} \left(\%\right)} = \frac{\Sigma \mathrm{M\_{aq}}}{\Sigma \mathrm{M\_{org}}} \times 100\tag{4}$$

ΣMaq is the concentration of metal ion in aqueous phase after stripping, and ΣMorg is the concentration of metal ion in organic phase before stripping.

The flow diagram of whole solvent extraction process is shown in Figure 2. After the first and the second stages of extraction, the concentration of ammonia, the O/A ratio, and the reaction temperature were investigated in the form of stripping percentage in the stripping process.

**Figure 2.** Flow diagram of whole solvent extraction process.
