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Article Recovery of and Cesium Resources from Brine of Desalination through t-BAMBP Extraction

Wei-Sheng Chen, Cheng-Han Lee * , Yi-Fan Chung, Ko-Wei Tien, Yen-Jung Chen and Yu-An Chen Department of Resources Engineering, National Cheng Kung University, No.1, DaxueRoad, Tainan City 70101, Taiwan; [email protected] (W.-S.C.); [email protected] (Y.-F.C.); [email protected] (K.-W.T.); [email protected] (Y.-J.C.); [email protected] (Y.-A.C.) * Correspondence: [email protected]; Tel.: +886-6-2757575 (ext. 62828)

 Received: 23 April 2020; Accepted: 6 May 2020; Published: 8 May 2020 

Abstract: 50 billion cubic meters of brine every year creates ecological hazards to the environment. In order to reuse brine efficiently, rubidium and cesium were recovered in this experiment. On the other hand, the main impurities which were needed to be eliminated in brine were , , , calcium, and magnesium. In the procedure, was distilled and evaporated first to turn into simulated brine. Perchloric acid was then added into simulated brine to precipitate potassium perchlorate which could reduce the influence of potassium in the extraction procedure. After that, t-BAMBP and ammonia were separately used as extractant and stripping agent in the extraction and stripping procedures to get rubidium solutions and cesium hydroxide solutions. Subsequently, they reacted with to get and cesium carbonate. In a nutshell, this study shows the optimal parameters of pH value to precipitate potassium perchlorate. Besides, pH value in the system, the concentration of t-BAMBP and ammonia, organic /aqueous phase ratio (O/A ratio), reaction time, and reaction temperature in solvent extraction step were investigated to get high purities of rubidium carbonate and cesium carbonate.

Keywords: solvent extraction; t-BAMBP; rubidium; cesium; brine; chemical precipitation; recovery

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 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 such as , garnet, and [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 and create the economic value of brine.

Metals 2020, 10, 607; doi:10.3390/met10050607 www.mdpi.com/journal/metals Metals 2020, 10, 607 2 of 15

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 (RbCl) and cesium nitrate (CsNO3). can be used to produce sleeping pills, sedatives, and treatment of bipolar disorder [14–17]. Cesium nitrate can be used as a 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 compositions of simulated brine are measured by inductively coupled plasma optical emission spectrometry (ICP-OES) and the concentrations are shown in Table1.

Table 1. Metal compositions of simulated brine

Element Li Na K Ca Mg Rb Cs Concentration (mg/L) 182 501,30 5914 702 139,80 7.1 43.6

In the whole process, perchloric acid (HClO4) was purchased from Sigma-Aldrich (St. Louis, MO, USA) (70%) to selectively precipitate potassium perchlorate. (NaOH) and sulfuric acid (H SO ) were separately acquired from Applichem Panreac (Barcelona, Spain) ( 98%) 2 4 ≥ and Sigma-Aldrich (St. Louis, MO, USA) ( 98%). They were used in the extraction process to ≥ adjust the pH value. 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) (HNO 65%) and diluted to 1% to be the background value and thinner for ICP analysis. 3 ≥ 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 Metals 2020, 10, 607 3 of 15 emissionMetals 2020 spectrometry, 10, x FOR PEER (ICP-OES; REVIEW Varian, Vista-MPX, PerkinElmer, Waltham, MA, USA).3 of 15 In the separated process, 445 mm 730 mm 505 mm of funnel shaker (FS-12; Shin Kwang Precision 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. 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) On the other hand, the thermostatic bath (XMtd-204; BaltaLab, Vidzemes priekšpilsēta, Rīga, Latvia) was usedwas used to maintain to maintain the the temperature temperature during during thethe extractionextraction process process and and stripping stripping process. process. In the In the procedureprocedure of producing of producing compounds, compounds, rotary rotary evaporatorsevaporators (BUCHI (BUCHI R-300; R-300; BÜCHI BÜCHI Labortechnik Labortechnik AG, AG, Flawil,Flawil, Switzerland) Switzerland) was was used used to to evaporate evaporate solutions solutions under under low low pressure pressure and and high high purity purity of rubidium of rubidium carbonatecarbonate and and cesium cesium carbonate carbonate could could be be precipitated. precipitated.

2.3. Experimental2.3. Experimental Procedures Procedures

2.3.1.2.3.1. Chemical Chemical Precipitation Precipitation ChemicalChemical precipitation precipitation procedure procedure waswas usedused to remove potassium potassium in inthis this experiment. experiment. In the In the process,process, the potassiumthe potassium which which was was in in simulated simulated brinebrine would would be be selectively selectively precipitated precipitated as potassium as potassium perchlorate by adding perchloric acid. Removal of potassium could reduce the co-extracted effect in perchlorate by adding perchloric acid. Removal of potassium could reduce the co-extracted effect in the solvent extraction process. The parameter of chemical precipitation such as effect of pH value was the solvent extraction process. The parameter of chemical precipitation such as effect of pH value investigated. Precipitation rate was calculated according to Equation (1) and the flow diagram is was investigated.shown in Figure Precipitation 1. rate was calculated according to Equation (1) and the flow diagram is shown in Figure1. – P(%)= 100 [M] [M] (1) P(%) = 0 − 100 (1) [M] × P is Precipitation rate, M is metal concentration0 of leach liquor, M is metal concentration of leach liquor after precipitation.

FigureFigure 1. 1.Flow Flow diagram diagram of chemical chemical precipitation. precipitation.

2.3.2. Solvent Extraction–Extraction Process P is Precipitation rate, [M]0 is metal concentration of leach liquor, [M] is metal concentration of leach liquorIn this after study, precipitation. 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 2.3.2.magnesium. Solvent Extraction–Extraction The extraction process Process was divided into two stages. Cesium was extracted in the first Instage, this and study, Li, Na, t-BAMBP K, Ca, Mg, was and diluted Rb were into still kerosene in the andaqueous used phase. as the Rubidium extractant was to separatethen extracted rubidium in the second stage, and other impurities were in the aqueous phase as well. and cesium from other impurities such as lithium, sodium, potassium, calcium, and magnesium. To calculate the efficiency of extraction, distribution ratio and extraction percentage were used The extractionin this process. process Distribution was divided ratio, into D, was two the stages. concentration Cesium ratio was of extracted the metal in in the the first organic stage, phase and to Li, Na, K, Ca,the Mg, metal and inRb thewere aqueous still phase in the at aqueous equilibrium. phase. The Rubidiumdistribution was ratio then can be extracted written as in Equation the second (2). stage, and other impurities were in the aqueous phase as well. [M]org Ci - Cf Vaq To calculate the efficiency of extraction,D = distribution= ratio× and extraction percentage were(2) used in [M] C V this process. Distribution ratio, D, was the concentrationaq f ratioorg of the metal in the organic phase to the metal in theCi is aqueous the initial phase concentration at equilibrium. of metal The in distribution aqueous phase, ratio C canf is the be equilibrium written as Equationconcentration (2). of metal ions in aqueous phase. Vaq and Vorg are separately the volume of aqueous phase and organic phase. [M] org Ci Cf Vaq Based on the distribution ratio,D the= extraction= percentage− can be written as Equation (3). (2) [M]aq Cf × Vorg

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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).

D Eex (%) = 100 (3) D + Vaq/Vorg · Metals 2020, 10, x FOR PEER REVIEW 4 of 15

D is the distribution ratio. Vaq and Vorg are separately the volume of aqueous phase and D organic phase. E (%) = ∙ 100 ex D + V /V (3) In the extraction process, the parameters such as theaq org pH value of aqueous phase, the concentration of t-BAMBP,D is the the O distribution/A ratio, the ratio. reaction Vaq and time, Vorg are and separately the reaction the volume temperature of aqueous were phase all and presented organic in the form ofphase. extraction percentage. In the extraction process, the parameters such as the pH value of aqueous phase, the 2.3.3. Solventconcentration Extraction–Stripping of t-BAMBP, the O/A Process ratio, the reaction time, and the reaction temperature were all presented in the form of extraction percentage. Ammonia was the stripping agent in this experiment which could strip the rubidium and cesium from the2.3.3. organic Solvent phase. Extraction–Stripping The stripping Process efficiency is written as Equation (4). Ammonia was the stripping agent in this experiment which could strip the rubidium and cesium ΣM from the organic phase. The stripping efficiency is writtenaq as Equation (4). Est (%) = 100 (4) ΣMorg × ΣMaq Est (%) = 100 (4) ΣMorg ΣMaq is the concentration of metal in aqueous phase after stripping, and ΣMorg is the concentrationΣMaq of ismetal the concentration ion in organic of phasemetal ion before in aqueous stripping. phase after stripping, and ΣMorg is the Theconcentration flow diagram of metal of whole ion in organic solvent phase extraction before processstripping. is shown in Figure2. After the first and the second stagesThe offlow extraction, diagram of the whole concentration solvent extraction of ammonia, process the is shown O/A ratio, in Figure and the2. After reaction the first temperature and the second stages of extraction, the concentration of ammonia, the O/A ratio, and the reaction were investigated in the form of stripping percentage in the stripping process. temperature were investigated in the form of stripping percentage in the stripping process.

FigureFigure 2. Flow2. Flow diagram diagram ofof wholewhole solvent solvent extraction extraction process. process.

3. Results and Discussion

3.1. Removal of Potassium

Metals 2020, 10, 607 5 of 15

3. Results and Discussion

3.1.Metals Removal 2020, 10, ofx FOR Potassium PEER REVIEW 5 of 15 Because potassium would be co-extracted with rubidium and cesium in the extraction process, Because potassium would be co-extracted with rubidium and cesium in the extraction process, the perchloric acid (HClO4) was added into simulated brine to selectively precipitate potassium the perchloric acid (HClO4) was added into simulated brine to selectively precipitate potassium perchlorate (KClO4) at 5 ◦C and 10 min. The precipitation percentages in different pH values are perchlorate (KClO4) at −5 °C and 10 min. The precipitation percentages in different pH values are shown in Figure3 and the final compositions of simulated brine are shown in Table2. In this procedure, shown in Figure 3 and the final compositions of simulated brine are shown in Table 2. In this the precipitation percentage of KClO4 was 98.5% at 5 ◦C and pH 2. procedure, the precipitation percentage of KClO4 was− 98.5% at −5 °C and pH 2.

Figure 3. Figure 3. Precipitation percentage of KClO44 in different different pH value atat 1010 min.min. Table 2. Final composition of simulated brine Table 2. Final composition of simulated brine Elements Li Na K Ca Mg Rb Cs Elements Li Na K Ca Mg Rb Cs Concentration (mg/L) 167 491,80 91 622 135,70 6.94 42.14 Concentration (mg/L) 167 49180 91 622 13570 6.94 42.14

3.2. First Stage of Solvent Extraction for Cs InIn thethe firstfirst stagestage ofof solventsolvent extraction,extraction, t-BAMBP was used to separate Cs and other metal ions. Due toto thethe property property of of t-BAMBP, t-BAMBP, only only K, Rb,K, Rb, and and Cs wereCs were able able to be to extracted be extrac effitedciently. efficiently. In this In study, this thestudy, values the values of them of in them the firstin the stage first ofstage solvent of so extractionlvent extraction were were analyzed analyzed by ICP-OES by ICP-OES and turnedand turned into theinto extraction the extraction percentage. percentage.

3.2.1. EEffectffect of pH Value ofof thethe AqueousAqueous PhasePhase Because t-BAMBP is an extractantextractant which is suitablesuitable for alkalicalkalic condition,condition, the pH valuesvalues werewere adjusted to 8–14 with 0.1 MM t-BAMBPt-BAMBP and OO/A/A ratioratio 1:11:1 atat reactionreaction timetime 1515 minmin andand 2525 ◦°CC inin thisthis experiment. Figure4 shows that the extraction percentage of K + and Rb+ were observed almost 0% to experiment. Figure 4 shows that the extraction percentage of K+ and Rb+ were observed almost 0% to 5% at any pH value and the percentage of Cs+ was observed above 99% from pH 8 to pH 12. In the 5% at any pH value and the percentage of Cs+ was observed above 99% from pH 8 to pH 12. In the conditioncondition ofof pHpH 1313 andand pH pH 14, 14, emulsification emulsification happened happened and and reduced reduced the the extraction extraction percentage. percentage. Due Due to theseto these situations, situations, the optimalthe optimal pH value pH value of the of aqueous the aqueous phase wasphase set was to pH set 8 whichto pH has8 which a high has extraction a high percentage of Cs+ and could reduce the usage of sodium hydroxide. extraction percentage of Cs+ 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

Metals 2020, 10, x FOR PEER REVIEW 5 of 15

Because potassium would be co-extracted with rubidium and cesium in the extraction process, the perchloric acid (HClO4) was added into simulated brine to selectively precipitate potassium perchlorate (KClO4) at −5 °C and 10 min. The precipitation percentages in different pH values are shown in Figure 3 and the final compositions of simulated brine are shown in Table 2. In this procedure, the precipitation percentage of KClO4 was 98.5% at −5 °C and pH 2.

Figure 3. Precipitation percentage of KClO4 in different pH value at 10 min.

Table 2. Final composition of simulated brine

Elements Li Na K Ca Mg Rb Cs Concentration (mg/L) 167 49180 91 622 13570 6.94 42.14

3.2. First Stage of Solvent Extraction for Cs In the first stage of solvent extraction, t-BAMBP was used to separate Cs and other metal ions. Due to the property of t-BAMBP, only K, Rb, and Cs were able to be extracted efficiently. In this study, the values of them in the first stage of solvent extraction were analyzed by ICP-OES and turned into the extraction percentage.

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+ and Rb+ were observed almost 0% to 5% at any pH value and the percentage of Cs+ 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 Metalsto these2020 situations,, 10, 607 the optimal pH value of the aqueous phase was set to pH 8 which has a6 high of 15 extraction percentage of Cs+ and could reduce the usage of sodium hydroxide.

Figure 4. Extraction percentage of metals at didifferentfferent pHpH valuesvalues inin thethe firstfirst stage.stage. 3.2.2. Effect of t-BAMBP Concentration Metals3.2.2. 2020Effect, 10 ,of x FOR t-BAMBP PEER REVIEW Concentration 6 of 15 The conditions of t-BAMBP concentration were set up from 0.001 M to 1 M at pH 8 and O/A The conditions of t-BAMBP concentration were set up from 0.001 M to 1 M at pH 8 and O/A ratio ratio 1:1 at reaction time 15 min and 25 ◦C in this step. Figure5 shows that the extraction percentage 1:1 at reaction time 15 min and 25 °C in this step. Figure 5 shows that the extraction percentage of Cs+ of Cs+ from 0.001 M to 0.1 M increased gradually and became stable. The reason was that a higher 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+ to be caught. However, the extraction percentage of concentration of the extractant enabled more Cs+ to be caught. However, the extraction percentage of K+ and Rb+ started to increase with a higher concentration of extractant. This was because K+ and Rb+ K+ and Rb+ started to increase with a higher concentration of extractant. This was because K+ and Rb+ were extracted by excess extractant and made an adverse effect on this system. Due to this condition, 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. the optimal parameter of t-BAMBP concentration was chosen as 0.1 M.

Figure 5. ExtractionExtraction percentage percentage of concentration of t-BAMBP in the first first stage. 3.2.3. Effect of O/A Ratio 3.2.3. Effect of O/A Ratio Figure6 shows the O /A ratios were set from 0.1 to 2 with 0.1 M t-BAMBP at pH 8 and at reaction 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 maintain above 99% time 15 min and 25 °C. The result shows that the extraction percentages of Cs+ maintain above 99% with different O/A ratios, which means that Cs+ were almost extracted. However, when the O/A with different O/A ratios, which means that Cs+ were almost extracted. However, when the O/A ratio ratio was greater than 0.5, the extraction percentage of K+ and Rb+ increased gradually above 2%. was greater than 0.5, the extraction percentage of K+ and Rb+ increased gradually above 2%. This was This was because K+ and Rb+ were extracted by excess extractant as well. Hence, in order to get a because K+ and Rb+ were extracted by excess extractant as well. Hence, in order to get a high high concentration of Cs+ and avoid the impurities, the O/A ratio of 0.1 was an optimal parameter in concentration of Cs+ and avoid the impurities, the O/A ratio of 0.1 was an optimal parameter in this this step. 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+ was very stable from 3 min to 60 min. K+ and Rb+ 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.

Metals 2020, 10, x FOR PEER REVIEW 6 of 15

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+ 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+ to be caught. However, the extraction percentage of K+ and Rb+ started to increase with a higher concentration of extractant. This was because K+ and Rb+ 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+ maintain above 99% with different O/A ratios, which means that Cs+ were almost extracted. However, when the O/A ratio was greater than 0.5, the extraction percentage of K+ and Rb+ increased gradually above 2%. This was + + Metalsbecause2020 K, 10 ,and 607 Rb were extracted by excess extractant as well. Hence, in order to get a 7high of 15 concentration of Cs+ 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 O/A ratio in the firstfirst stage. 3.2.4. Effect of Reaction Time 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 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 1:1 at 25 ◦C. In Figure7, the extraction percentage of Cs was very stable from 3 min to 60 min. K and ratio 1:1 at 25 °C. In Figure 7, the extraction percentage of Cs+ was very stable from 3 min to 60 min. Rb+ were at equilibrium and low extraction percentage as well. It shows that the reaction of t-BAMBP K+ and Rb+ were at equilibrium and low extraction percentage as well. It shows that the reaction of t- was fast and reaction time was not a significant influence in the extraction process. On account of this BAMBP was fast and reaction time was not a significant influence in the extraction process. On condition,Metals 2020, 10 3, min x FOR was PEER chosen. REVIEW 7 of 15 account of this condition, 3 min was chosen.

Figure 7.7. Extraction percentage of reaction timetime inin thethe firstfirst stage.stage. 3.2.5. Effect of Reaction Temperature 3.2.5. Effect of Reaction Temperature Figure8 shows the reaction temperature was a significant parameter in the extraction process. 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 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 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 because t-BAMBP extracted metal ions with exothermic reaction and the high temperature caused decrease of distribution ratio. Finally, the extraction percentages of Cs+,K+, and Rb+ were respectively the decrease of distribution ratio. Finally, the extraction percentages of Cs+, K+, and Rb+ were 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, respectively 99.8%, 5.2%, and 1% with the condition of pH 8 of the aqueous phase, 0.1 M t-BAMBP, 3 min reaction time, and 35 ◦C reaction temperature. Compared to other studies, this study shows O/A ratio of 0.1, 3 min reaction time, and 35 °C reaction temperature. Compared to other studies, this the high extraction percentage of Cs+ with lower pH value and O/A ratio at the low temperature and study shows the high extraction percentage of Cs+ with lower pH value and O/A ratio at the low Cs+ could be separated from Rb, K, and other impurities such as lithium, sodium, potassium, calcium, temperature and Cs+ could be separated from Rb, K, and other impurities such as lithium, sodium, and magnesium. 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+. 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

Metals 2020, 10, x FOR PEER REVIEW 7 of 15

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+ 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+ with lower pH value and O/A ratio at the low + Metalstemperature2020, 10, 607and Cs could be separated from Rb, K, and other impurities such as lithium, sodium,8 of 15 potassium, calcium, and magnesium.

Figure 8. Extraction percentage of reaction temperature in the firstfirst stage.stage. 3.2.6. Stripping of Cs from the Organic Phase through Ammonia 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 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 stripped. In this process, NH4OH was chosen as a stripping agent and the effect of NH4OH was presented in Figures9–11. Because the stripping e fficiencies of rubidium and potassium were low in 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. Figure9 shows that when the 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 concentration of NH4OH increased, the stripping efficiency increased as well and become in the situation of 1 M, 2 M, and 5 M. Therefore, the optimal concentration of NH4OH was 1 M in the equilibrium in the situation of 1 M, 2 M, and 5 M. Therefore, the optimal concentration of NH4OH procedure. Figure 10 shows that the stripping efficiency decreased drastically when O/A ratio was 4. was 1 M in the procedure. Figure 10 shows that the stripping+ efficiency decreased drastically when It means that insufficient NH4OH was unable to strip the Cs . Due to this condition, O/A ratio 2 was O/A ratio was 4. It means that insufficient NH4OH was unable to strip the Cs+. Due to this condition, the optimal condition to get a high concentration of Cs+. Figure 11 presents the stripping efficiency with O/AMetals ratio2020, 102, wasx FOR the PEER optimal REVIEW condition to get a high concentration of Cs+. Figure 11 presents8 ofthe 15 reaction temperature. Because t-BAMBP has great extracting ability at lower temperatures, it made stripping efficiency with reaction+ temperature. Because t-BAMBP has great extracting ability at lower NH4OH unable to strip Cs . However, if the temperature was above 35 ◦C, NH4OH decomposed 4 + temperatures, it made NH OH unable to+ strip Cs . However, if the temperature was above 35 °C, to NH3 first which was unable to strip Cs . Hence, the stripping temperature was chosen at a room NH4OH decomposed to NH3 first which was unable to strip Cs+. Hence, the stripping temperature temperature of 25 C in this step. Finally, the stripping efficiency of Cs+ was almost 99.9% in the first was chosen at a room◦ temperature of 25 °C in this step. Finally, the stripping efficiency of Cs+ was stripping process. almost 99.9% in the first stripping process.

Figure 9. Stripping efficiency of concentration of NH OH in the first stage. Figure 9. Stripping efficiency of concentration of NH44OH 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

Metals 2020, 10, x FOR PEER REVIEW 8 of 15 Metals 2020, 10, x FOR PEER REVIEW 8 of 15 temperatures, it made NH4OH unable to strip Cs+. However, if the temperature was above 35 °C, temperatures, it made NH4OH unable to strip Cs+. However, if the temperature was above 35 °C, NH4OH decomposed to NH3 first which was unable to strip Cs+. Hence, the stripping temperature + NHwas4 OHchosen decomposed at a room totemperature NH3 first which of 25 °Cwas in unable this step. to strip Finally, Cs .the Hence, stripping the stripping efficiency temperature of Cs+ was + wasalmost chosen 99.9% at in a theroom first temperature stripping process. of 25 °C in this step. Finally, the stripping efficiency of Cs was almost 99.9% in the first stripping process.

Metals 2020, 10, 607 Figure 9. Stripping efficiency of concentration of NH4OH in the first stage. 9 of 15 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 10. StrippingStripping efficiency efficiency of O/A O/A ratio in the first first stage.

Figure 11. StrippingStripping efficiency efficiency of reaction temperature in the firstfirst stage. Figure 11. Stripping efficiency of reaction temperature in the first stage. 3.3. Second Stage of Solvent Extraction for Rb 3.3. Second Stage of Solvent Extraction for Rb 3.3. SecondAfter theStage first of stageSolvent of Extraction solvent extraction, for Rb metals were separated into two systems. One system was After the first stage of solvent extraction, metals were separated into two systems. One system cesium, and another system was rubidium with other impurities such as lithium, sodium, potassium, was Aftercesium, the and first anotherstage of systemsolvent wasextraction, rubidium metals with were other separated impurities into suchtwo systems.as lithium, One sodium, system calcium, and magnesium. In the second stage of solvent extraction, t-BAMBP was also chosen as waspotassium, cesium, calcium, and another and magnesium. system was In rubidium the second with stage other of solventimpurities extraction, such as t-BAMBP lithium, wassodium, also potassium,extractant to calcium, separate and rubidium magnesium. with other In the impurities. second stage Among of solvent the rest extraction, of the impurities, t-BAMBP only was K could also 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+ and K+ 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 and K 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. Metals 2020, 10, x FOR PEER REVIEW 9 of 15 Metals 2020, 10, x FOR PEER REVIEW 9 of 15 chosen as extractant to separate rubidium with other impurities. Among the rest of the impurities, 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 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. and Rb were presented to analyze the optimal condition. 3.3.1. Effect of pH Value of the Aqueous Phase 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+ and K+ was The effect of pH value of the aqueous phase in the extraction and separation of Rb+ and K+ 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 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+ and K+ increased when the pH value reaction time 15 min and 25 °C. The extraction percentage of Rb+ and K+ 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+, raisedMetals 2020 up, 10and, 607 declined at pH 13 and pH 14 due to the emulsification. In order to extract more10 Rb of 15+, pH 12 value was chosen in this procedure. pH 12 value was chosen in this procedure.

Figure 12. Extraction percentage of metals at didifferentfferent pHpH valuevalue inin thethe secondsecond stage.stage. Figure 12. Extraction percentage of metals at different pH value in the second stage. 3.3.2.3.3.2. EEffectffect of t-BAMBP ConcentrationConcentration 3.3.2. Effect of t-BAMBP Concentration TheThe conditionsconditions of of t-BAMBP t-BAMBP concentration concentration from from 0.01 0. M01 to M 1 Mto at1 pHM at 12 pH and 12 O /Aand ratio O/A 1:1 ratio at reaction 1:1 at The conditions of t-BAMBP concentration from 0.01 M to 1 M at pH 12 and O/A ratio 1:1 at+ timereaction 15 min time and 15 min 25 ◦ Cand were 25 °C set were in this set step. in this Figure step. 13 Figure shows 13 thatshows the that extraction the extraction percentages percentages of Rb reactionfrom 0.01 time M to 15 1 min M were and about25 °C 50%were and set in the this percentages step. Figure of K13+ showswere about that the 40%. extraction In order percentages to get more of Rb+ from 0.01 M to 1 M were about 50% and the percentages of K+ were about 40%. In order to get of Rb+ from 0.01 M to 1 M were about 50% and the percentages of K+ were about 40%. In order to get rubidium,more rubidium, 0.5 M 0.5 t-BAMBP M t-BAMBP was chosen was chosen in this in procedure. this procedure. more rubidium, 0.5 M t-BAMBP was chosen in this procedure.

Figure 13. Extraction percentage of concentration of t-BAMBP inin thethe secondsecond stage.stage. Figure 13. Extraction percentage of concentration of t-BAMBP in the second stage. 3.3.3. Effect of O/A Ratio 3.3.3. Effect of O/A Ratio 3.3.3.Figure Effect 14of O/Ashows Ratio the O/A ratio was set from 0.1 to 2 with 0.5 M t-BAMBP at pH 12 and at reaction 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 timeFigure 15 min 14 and shows 25 ◦C. the The O/A result ratio shows was set that from the 0.1 extraction to 2 with percentages0.5 M t-BAMBP of Rb at pHmaintain 12 and aboveat reaction 50% time 15 min and 25 °C. The result shows that the extraction percentages of Rb+ maintain above 50% withtime di15ff minerent and O/A 25 ratios. °C. The However, result shows when thethat O the/A ratioextraction was greater percentages than 0.1, of theRb+ extractionmaintain above percentage 50% with different O/A ratios. However, when the O/A ratio was greater than 0.1, the extraction withof K+ differentincreased O/A gradually. ratios. Hence, Howeve in orderr, when to get the a highO/A concentration ratio was greater of Rb+ thanand avoid0.1, the the impurities,extraction percentage of K+ increased gradually. Hence, in order to get a high concentration of Rb+ and avoid percentagetheMetals O/A 2020 ratio, of10, 0.1 xK FOR+ wasincreased PEER an REVIEW optimal gradually. parameter Hence, in in this order step. to get a high concentration of Rb+ and10 of avoid 15 the impurities, the O/A ratio 0.1 was an optimal parameter in this step. the impurities, the O/A ratio 0.1 was an optimal parameter in this step.

FigureFigure 14. 14. ExtractionExtraction percentage of of O/A O/A ratio ratio in in the the second second stage. 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+ increased gradually from 3 min to 15 min and became stable. On the other hand, the extraction percentage of K+ 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+ 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+ 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+ and K+ 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+ with lower pH value and O/A ratio at low temperature.

Metals 2020, 10, x FOR PEER REVIEW 10 of 15

Metals 2020, 10, 607 11 of 15 Figure 14. Extraction percentage of O/A ratio in the second stage. 3.3.4. Effect of Reaction Time 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 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+ increased gradually from 3 min to ratio 0.1:1 at 25 °C.◦ In Figure 15, the extraction percentage of Rb+ increased gradually from 3 min to 15 min and became stable. On the other hand, the extraction percentage of K+ was almost 30% from 15 min and became stable. On the other hand, the extraction percentage of K+ was almost 30% from 3 min to 60 min. Hence, 15 min of reaction time was chosen in this process to extract rubidium. 3 min to 60 min. Hence, 15 min of reaction time was chosen in this process to extract rubidium.

FigureFigure 15. 15. ExtractionExtraction percentage percentage of of reaction reaction time time in in the the second second stage. stage. 3.3.5. Effect of Reaction Temperature 3.3.5. Effect of Reaction Temperature In Figure 16, it shows that Rb+ was influenced by the reaction temperature. The effect of reaction In Figure 16, it shows that Rb+ 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 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 percentage of Rb+ was about 98% at 5 C and decreased drastically after 5 C. The percentage of 35 C + ◦ ◦ ◦ extraction percentage of Rb was about 98% at 5 °C and decreased+ drastically after 5 °C. The was 56.8% and only about 10% at 45 ◦C. In order to extract more Rb , 5 ◦C was chosen as the optimal percentage of 35 °C was 56.8% and only about 10% at 45 °C. In order to extract more Rb+, 5 °C was parameter. Finally, the extraction percentage of Rb+ and K+ and were respectively 98.3% and 41.3% chosen as the optimal parameter. Finally, the extraction percentage of Rb+ and K+ and were with the condition of pH 12 of the aqueous phase, 0.5 M t-BAMBP, O/A ratio of 0.1, 15 min reaction respectively 98.3% and 41.3% with the condition of pH 12 of the aqueous phase, 0.5 M t-BAMBP, O/A time, and 5 C reaction temperature. Compared to other studies, this study shows the almost same ratio of 0.1, ◦15 min reaction time, and 5 °C reaction temperature. Compared to other studies, this Metalsextraction 2020, 10 percentage, x FOR PEER of REVIEW Rb+ with lower pH value and O/A ratio at low temperature. 11 of 15 study shows the almost same extraction percentage of Rb+ with lower pH value and O/A ratio at low temperature.

FigureFigure 16. 16. ExtractionExtraction percentage percentage of of reaction reaction temperature temperature in in the the second second stage. stage. 3.3.6. Stripping of Rb from the Organic Phase through Ammonia 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 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 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 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 potassium was low in this procedure, only stripping efficiency of rubidium was investigated. Figure shows that when the concentration of NH4OH increased from 0.1 M to 0.5 M, the stripping efficiency 17 shows that when the concentration of NH4OH increased from 0.1 M to 0.5 M, the stripping of Rb+ increased as well. However, efficiency became equilibrium in the situation of 0.5 M, 1 M, efficiency of Rb+ increased as well. However, efficiency became equilibrium in the situation of 0.5 M, 2 M, and 5 M. Therefore, the optimal concentration of NH4OH was 0.5 M in this step. Figure 18 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+ 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+ 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+ was almost 95% in the second stripping process. After the extraction and stripping of Cs+ 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.

Metals 2020, 10, x FOR PEER REVIEW 11 of 15

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 Metals17 shows2020, 10that, 607 when the concentration of NH4OH increased from 0.1 M to 0.5 M, the stripping12 of 15 efficiency of Rb+ 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 eefficiencyfficiency of RbRb+ waswas above above 90% 90% and decreased at O/A O/A ratio 4. Due to this condition, O/A O/A ratio 2 waswas thethe optimaloptimal condition.condition. Fi Figuregure 19 presents the strippingstripping eefficifficiencyency with + + reaction temperature. Like the situation of CsCs+,, the the stripping stripping efficiency efficiency of Rb+ waswas lower lower at at the low temperature and gradually rose with increase in temperature.temperature. Hence, the stripping temperature was + chosen as as 35 35 °C◦C in in this this procedure. procedure. Finally, Finally, the thestripping stripping efficiency efficiency of Rb of+ was Rb almostwas almost 95% in 95%the second in the + + secondstripping stripping process. process. After the After extraction the extraction and stripping and stripping of Cs+ and of Cs Rb+and, the Rboptimal, the optimalparameters parameters of solvent of solventextraction extraction are shown are in shown Tables in 3 Tables and 4,3 andand 4the, and comp theonents components of stripping of stripping solutions solutions are shown are shown in Tables in Tables5 and 6.5 and6.

Metals 2020, 10, x FOR PEER REVIEW 12 of 15 Metals 2020, 10, x FOR PEER REVIEW 12 of 15 Figure 17. Stripping efficiency of concentration of NH OH in the second stage. 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 18. Stripping efficiency of O/A ratio in the second stage. Figure 18. Stripping efficiency of O/A ratio in the second stage.

FigureFigure 19. 19. StrippingStripping efficiency efficiency of of reaction reaction temperature temperature in 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 3. Optimal parameters of solvent extraction of cesium Extractant Extractant pH value of Reaction and pH value of Concentration Reaction Reaction and aqueous Concentration O/A ratio Reaction temperature Stripping aqueous (M) O/A ratio time (min) temperature Stripping phase (M) time (min) (°C) agent phase (°C) t-BAMBPagent 8.0 0.1 0.1 3 35 Ammoniat-BAMBP 8.0- 0.1 1 0.1 2 3- 2535 Ammonia - 1 2 - 25 Table 4. Optimal parameters of solvent extraction of rubidium Table 4. Optimal parameters of solvent extraction of rubidium Extractant Extractant pH value of Reaction and pH value of Concentration Reaction Reaction and aqueous Concentration O/A ratio Reaction temperature Stripping aqueous (M) O/A ratio time (min) temperature Stripping phase (M) time (min) (°C) agent phase (°C) t-BAMBPagent 12.0 0.5 0.1 15 5 Ammoniat-BAMBP 12.0- 0.5 0.1 2 15 - 355 Ammonia - 0.5 2 - 35 Table 5. Metal compositions of the first stage of stripping (cesium hydroxide solutions) Table 5. Metal compositions of the first stage of stripping (cesium hydroxide solutions) Element Li Na K Ca Mg Rb Cs ConcentrationElement (mg/L) N.D.Li 8.13Na 10.33K N.D.Ca N.D.Mg 1.39Rb 1635Cs Concentration (mg/L) N.D. 8.13 10.33 N.D. N.D. 1.39 1635

Metals 2020, 10, 607 13 of 15

Table 3. Optimal parameters of solvent extraction of cesium

Extractant and pH Value of Concentration Reaction Time Reaction O/A Ratio Stripping Agent Aqueous Phase (M) (min) Temperature (◦C) t-BAMBP 8.0 0.1 0.1 3 35 Ammonia - 1 2 - 25

Table 4. Optimal parameters of solvent extraction of rubidium

Extractant and pH Value of Concentration Reaction Time Reaction O/A Ratio Stripping Agent Aqueous Phase (M) (min) Temperature (◦C) t-BAMBP 12.0 0.5 0.1 15 5 Ammonia - 0.5 2 - 35

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

Element Li Na K Ca Mg Rb Cs Concentration (mg/L) N.D. 8.13 10.33 N.D. N.D. 1.39 1635

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

Element Li Na K Ca Mg Rb Cs Concentration (mg/L) N.D. 0.02 6.02 N.D. N.D. 293.68 N.D.

3.4. Production of Rubidium Carbonate and Cesium Carbonate In order to get stable compounds of rubidium and cesium, ammonium carbonate was added into rubidium hydroxide solutions and cesium hydroxide solutions to produce rubidium carbonate solutions and cesium carbonate solutions. The reaction situations were shown in Equations (5) and (6).

(NH4) CO + RbOH Rb CO + NH + H O (5) 2 3(s) (aq)→ 2 3(aq) 3(aq) 2 (l) (NH4) CO +CsOH Cs CO +NH +H O (6) 2 3(s) (aq)→ 2 3(aq) 3(aq) 2 (l) The rubidium carbonate solutions and cesium carbonate solutions were then put into rotary evaporators and evaporated under low pressure and temperature and precipitate the purity of 98.0% of rubidium carbonate and 98.9% of cesium carbonate. After chemical precipitation and solvent extraction, the ICP-OES analyses are shown in Table7.

Table 7. ICP-OES analyses of rubidium carbonate and cesium carbonate

Compounds Li Na K Ca Mg Rb Cs

Rb2CO3 N.D. 0.1% 1.9% N.D. N.D. 98.0% N.D. Cs2CO3 N.D. 0.4% 0.5% N.D. N.D. 0.2% 98.9% N.D.: Not-detected.

4. Conclusions Hydrometallurgy method was used to separate rubidium and cesium effectively from the desalination brine in this study. The recommended recovery process is shown in Figure 20. The brine was treated by chemical precipitation and solvent extraction to recover rubidium and cesium. Perchloric acid was added into brine to pH 2 at 5 C to precipitate potassium perchlorate which could reduce the − ◦ influence of potassium in the extraction procedure. After that, t-BAMBP and ammonia were used as extractant and a stripping agents. The results show that 0.1 M t-BAMBP was used as extractant to separate cesium and impurities under the optimal condition of pH 8, O/A ratio 0.1, 3 min reaction time, Metals 2020, 10, 607 14 of 15

and 35 ◦C reaction temperature. Then, cesium was stripped by using 1 M ammonia under O/A ratio 2 and 25 ◦C reaction temperature. On the other hand, the results show that 0.5 M t-BAMBP was used as extractant to separate rubidium and potassium under the optimal condition of pH 12, O/A ratio 0.1, 15 min reaction time, and 5 ◦C reaction temperature. In the stripping process, 0.5 M ammonia under O/A ratio 2 and 35 ◦C reaction temperature are the optimal parameters. Finally, the rubidium carbonate and cesium carbonate were produced by adding ammonium carbonate. By these processes, the purityMetals 2020 of, rubidium10, x FOR PEER carbonate REVIEW and cesium carbonate were 98.0% and 98.9%. 14 of 15

FigureFigure 20. 20.Recommended Recommended recoveryrecovery process of of this this experiment. experiment.

Author Contributions: Conceptualization, W.-S.C. and C.-H.L.; methodology, W.-S.C. and C.-H.L.; validation, Author Contributions: Conceptualization, W.-S.C. and C.-H.L.; methodology, W.-S.C. and C.-H.L.; validation, W.-S.C.W.-S.C and. and C.-H.L.; C.-H.L.; formal formal analysis, analysis, C.-H.L. C.-H.L. andand Y.-F.C.; investigation, C.-H.L., C.-H.L., Y.-F.C. Y.-F.C. and and K.-W.T.; K.-W.T.; resources, resources, C.-H.L.,C.-H.L., Y.-F.C. Y.-F.C. and Y.-J.C.;and Y.-J.C.; data curation, data curation, C.-H.L., C.-H.L., K.-W.T., K.-W.T., Y.-J.C. andY.-J.C. Y.-A.C.; and writing—originalY.-A.C.; writing—original draft preparation, draft C.-H.L.;preparation, writing—review C.-H.L.; writing—review and editing, C.-H.L., and editing, K.-W.T., C.-H.L., Y.-J.C. K.-W.T., and Y.-A.C.;Y.-J.C. and visualization, Y.-A.C.; visualization, C.-H.L.; supervision,C.-H.L.; W.-S.C.;supervision, project administration, W.-S.C.; projectW.-S.C. administration, All authors W.-S.C. have All read authors and agreed have read to the and published agreed to versionthe published of the manuscript.version Funding:of the Thismanuscript. research received no external funding. ConflictsFunding: of Interest: This researchThe authorsreceived declareno external no conflictfunding. of interest. Conflicts of Interest: The authors declare no conflict of interest.

References

1. The United Nations World Water Development Report 2012. UNESCO: Paris, France, 12 March 2012.

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