**1. Introduction**

According to the investigation of the United Nations, more than 1 billion people in the world currently live in the areas with scarce water resources. Moreover, this number will reach 1.8 billion by 2025 [1]. In response to the shortage of freshwater resources, seawater desalination technology has developed rapidly since the beginning of the 20th century [2]. However, as seawater desalination is common now, waste brine also causes considerable harm to the environment. For example, waste brine will change the composition of seawater and affect the ecosystem. In order to reuse waste brine, some elements such as lithium, magnesium, calcium, and chlorine are recycled from brine recently [3]. In this study, rubidium and cesium were extracted from brine through the hydrometallurgy method. It is expected to achieve the goal of recovering valuable metals, reducing waste, and protecting the environment.

Based on the report of the U.S. Geological Survey (USGS), the rubidium and cesium resources are mainly from primary minerals such as carnallite, garnet, and lepidolite [4–8]. The main reservoirs are Namibia, Zimbabwe, Afghanistan, and some other countries. Rubidium and cesium respectively have only 90,000 tons of reserve, and the difficulty of mining makes them rarer. Although rubidium and cesium are not common, they are valuable and useful in many areas. Therefore, it is important to recover rubidium and cesium or its compounds from waste brine which can reduce the reliance of primary mineral and create the economic value of brine.

Rubidium and cesium have a wide range of using [9–12]. For industry activities, rubidium and cesium metals are the material of television, radar, infrared filter, radiant energy receiver, and reconnaissance telescope [13]. On the other hand, rubidium carbonate (Rb2CO3) and cesium carbonate (Cs2CO3) can be raw materials of glasses and increase stability and durability. Additionally, rubidium carbonate and cesium carbonate are easy to turn into other compounds such as rubidium chloride (RbCl) and cesium nitrate (CsNO3). Rubidium chloride can be used to produce sleeping pills, sedatives, and treatment of bipolar disorder [14–17]. Cesium nitrate can be used as a light refraction regulator in the optical fiber industry and glass industry [18]. Due to the unlimited development of rubidium and cesium compounds, many countries try various methods to get rubidium and cesium and apply them in industries.

Nowadays, rubidium and cesium are mainly recovered from saline lake and ores through a solvent extraction method with t-BAMBP extractant [19–22]. On the other hand, because impurities could be removed efficiently through chemical precipitation and make higher purification of compounds, it was chosen to be the procedure before solvent extraction in this experiment. During the chemical precipitation process, perchloric acid (HClO4) was added into the brine to selectively precipitate potassium perchlorate (KClO4). Due to the reduction of potassium, it could avoid the adverse effects which potassium create in the follow-up processes. Moreover, t-BAMBP and ammonia were used in the solvent extraction procedure to separate rubidium, cesium, and other impurities such as lithium, sodium, potassium, calcium, and magnesium efficiently. To sum up, the chemical precipitation method and solvent extraction method were used in this experiment to get high purities of rubidium and cesium resources and made them be able to reuse in the industries.

#### **2. Material and Methods**

#### *2.1. Materials*

In this experiment, the simulated brine was got from seawater through distillation and evaporation. The metal compositions of simulated brine are measured by inductively coupled plasma optical emission spectrometry (ICP-OES) and the concentrations are shown in Table 1.

**Table 1.** Metal compositions of simulated brine


In the whole process, perchloric acid (HClO4) was purchased from Sigma-Aldrich (St. Louis, MO, USA) (70%) to selectively precipitate potassium perchlorate. Sodium hydroxide (NaOH) and sulfuric acid (H2SO4) were separately acquired from Applichem Panreac (Barcelona, Spain) (≥98%) and Sigma-Aldrich (St. Louis, MO, USA) (≥98%). They were used in the extraction process to adjust the pH value. Kerosene was purchased from CPC Corporation (Kaohsiung, Taiwan) to dilute the extractant. t-BAMBP was purchased from Realkan Corporation (Beijing, China) (≥90%) for the extraction process, and ammonia (NH4OH) was purchased from Sigma-Aldrich (St. Louis, MO, USA) (30–33%) for the stripping process. In the final process, ammonium carbonate ((NH4)2CO3) was acquired from Sigma-Aldrich (St. Louis, MO, USA) (≥90%) to produce rubidium carbonate and cesium carbonate. During the analysis procedure, ICP rubidium standard solution, ICP cesium standard solution, and ICP multi-element standard solution were acquired from High-Purity Standards, Inc. (North Charleston, SC, USA). The nitric acid (HNO3) was purchased from Sigma-Aldrich (St. Louis, MO, USA) (HNO3 ≥ 65%) and diluted to 1% to be the background value and thinner for ICP analysis.

## *2.2. Equipment*

The materials and products were analyzed by X-ray fluorescence spectrometer (XRF; ZSX100s, SPECTRO Analytical Instruments Inc., Kleve, Germany) and inductively coupled plasma optical emission spectrometry (ICP-OES; Varian, Vista-MPX, PerkinElmer, Waltham, MA, USA). In the separated process, 445 mm × 730 mm × 505 mm of funnel shaker (FS-12; Shin Kwang Precision

Industry Ltd., New Taipei City, Taiwan) was used to shake 500 mL of separating funnels at 3000 rpm. On the other hand, the thermostatic bath (XMtd-204; BaltaLab, Vidzemes priekšpilseta, R ¯ ¯ıga, Latvia) was used to maintain the temperature during the extraction process and stripping process. In the procedure of producing compounds, rotary evaporators (BUCHI R-300; BÜCHI Labortechnik AG, Flawil, Switzerland) was used to evaporate solutions under low pressure and high purity of rubidium carbonate and cesium carbonate could be precipitated.
