Research ArticleRecovery of Boron from Underground Brine by ContinuousCentrifugal Extraction with 2-Ethyl-1,3-hexanediol (EHD) andIts Mechanism

Xuebing Fan, Xiaoping Yu , Yafei Guo , and Tianlong Deng

Tianjin Key Laboratory of Marine Resources and Chemistry, College of Chemical Engineering and Materials Science,Tianjin University of Science and Technology, Tianjin 300457, China

Correspondence should be addressed to Xiaoping Yu; yuxiaoping@tust.edu.cn and Tianlong Deng; tldeng@tust.edu.cn

Received 1 May 2018; Accepted 29 August 2018; Published 9 October 2018

Academic Editor: Carlos A. Mart́ınez-Huitle

Copyright © 2018 Xuebing Fan et al. .is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In order to economical and environmental-friendly recycle of boron from underground brine, the extraction of boron was carriedout in this work by the continuous centrifugal technique using 2-ethyl-1,3-hexanediol (EHD) + sulfonated kerosene (SK) ex-traction system, and the extraction mechanism was also investigated by the combination of FT-IR with slope method. .e resultsshowed that boron can be effectively extracted from underground water with the concentration of boron 5.43 g·L−1 by five-stagecentrifugal extraction using 30% EHD+70% SK at pH� 2.0–3.0, R(O/A)� 1 : 2, and the extraction rate reached 98.46%. Boron inthe organic phase can be well five-stage back-extracted by 1.0mol·L−1·NaOH at R(O/A)� 1 :1 with a back-extraction rate of97.00%. About 88.32% boron in the aqueous phase obtained by back-extraction can be recycled in H3BO3 form by evaporationcrystallization after acidified to pH< 2.5..e extractionmechanism indicated that the extraction is completedmainly based on theesterification reaction between alcoholic hydroxyl in EHD and -OH in B(OH)3 at the stoichiometric ratio 1 :1 to generate a stablesix-membered ring structure of boric acid ester.

1. Introduction

Boron and its compounds are widely applied in modernindustry due to their excellent characteristics such as heatand wear resistance, high strength, and rigidity [1]. Boronmainly exists in solid boron ores and liquid ores in the formof boric or borate salts [2], but after several decades ofexploitation, the solid boron resources have become in-creasingly exhausted. It is well known that there are plentifulof brine resources containing abundant boron all over theworld, including salt lake brine, underground brine, oil-fieldbrine, and so on. However, resulting from the interference ofhigh concentration of coexisting ions such as Na+, K+, Mg2+,and Cl− in brine, the industrial production of boron frombrine has not yet been well carried out [3]. .erefore, thedevelopment of a swift and low-cost method for the sepa-ration and extraction of boron from brine will be of greatsignificance and will bring about considerable economicbenefits [4].

At present, many separation techniques have alreadybeen reported to separate boron from brine, such as ad-sorption [5], chemical precipitation [6], reverse osmosis[7], solvent extraction [8, 9], and so on. Wherein, solventextraction is one of the nonenergy intensive mass transferprocesses and is widely used to separate and recyclevaluable elements from complex matrix for its high se-lectivity and recovery. Various extractants have beendeveloped to extract boron from brine, especially based onthe extraction system containing alcohol, such as mono-hydric alcohols, dibasic alcohols, and mixed alcohols. Forexample, n-hexanol, n-octanol, isooctyl alcohol, n-dec-anol, and other fatty alcohols were used by Hejda andJedináková [10, 11] to extract boron from radioactive wastewater, and the results indicated that isooctanol had the besteffect. 2,2,5-Trimethyl-1,3-hexanediol was used by Kar-akaplan et al. [12] to extract boron from aqueous solution,and the extraction rate reached to 96.8% at the optimizingconditions. Kwon et al. [13] studied the extraction

HindawiJournal of ChemistryVolume 2018, Article ID 7530837, 8 pageshttps://doi.org/10.1155/2018/7530837

mailto:yuxiaoping@tust.edu.cnmailto:tldeng@tust.edu.cnhttp://orcid.org/0000-0002-5897-492Xhttp://orcid.org/0000-0003-0698-3565http://orcid.org/0000-0002-1728-2943https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2018/7530837

equilibrium during boron extraction and stripping with2-butyl-2-ethyl-1,3-propanediol, and the result showsthat boron extraction was intensive in acidic solution,while the stripping took place dominantly in the alkalinesolution. Usually, dibasic alcohols have higher extractionability when compared with monohydric alcohols due tothe generation of more stable extracted complexes.

.e conventional solvent extraction equipment used inindustrial production is the mixer-settler, by which theextraction and separation are achieved mainly based on thedensity difference of two immiscible phases [14, 15]. It isnoted that if the aqueous phase and organic phase have littledensity differences, the gravity-based settling will be im-practicability. However, this problem can be effectivelysolved by the introduction of the centrifugal extractiontechnique, by which it can significantly enhance the ex-traction efficiency and velocity. For example, both Gao et al.[16] and Guo [17] extracted boron from brine by the cen-trifugal extractor, and the results revealed the feasibility ofboron extraction from brine by the centrifugal extractiontechnique.

Although dibasic alcohols present excellent propertiesfor boron extraction from brine, few studies were carriedout based on the centrifugal extraction technique. Asthe retention time of aqueous phase and organic phase (orthe contact time between aqueous phase and organicphase) in the extractor is shorter when compared with themixer-settler, many factors need to be investigated in-cluding extraction stage, extraction and back-extractioneffects, and so on. In this paper, 2-ethyl-1,3-hexanediolwas chosen as the extractant to extract boron from un-derground brine. .e extraction and back-extraction werecarried out on a continuous centrifugal extractor, andthe extraction conditions and mechanism were in-vestigated systematically based on relevant chemical andinstrumental analysis.

2. Experimental

2.1. Apparatus. .e extraction operation was carried outusing a five-stage continuous centrifugal extractor (50model, Tianyi extraction, China). .e concentrations ofmetal ions were determined by an inductively coupledplasma optical emission spectrometer (ICP-OES, ProdigyXP ICP, Leman Corporation, USA). .e acidity of brine wasdetermined by an acidometer (PHSJ-5, Leici, China). .edensity of brine was measured by a densimeter (DMA4500M, Anton Paar, Austria). .e viscosities of organicphase and aqueous phase were determined by an Ubbelohdeviscometer.

2.2. Materials and Reagents. .e underground brine col-lected from Dazhou in China was concentrated to increasethe concentration of boron prior to the extraction. .ecompositions of brine after sevenfold concentration areshown in Table 1, and its density and viscosity at roomtemperature (298.15K) were determined to be 1.2967 g·mL−1and 1.86·mPa·s, respectively.

.e extractant 2-ethyl-1,3-hexanediol (EHD;ρ� 0.9405 g·mL−1 and µ� 323.06mPa·s) was supplied by theTianjin Heowns Bio. Technology Co. Ltd. .e diluentsulfonated kerosene (SK; ρ� 0.8205 g·mL−1 andµ� 2.25mPa·s) was purchased from the Tianjin JinweierChemical Co. Ltd. .e other reagents used during the ex-periment were supplied by the SinopharmChemical ReagentCo. Ltd. Doubly deionized water (DDW) with a conductivityless than 1× 10−4 S·m−1 and pH� 6.60 at room temperaturewas used throughout the experiment.

2.3. ExperimentalMethods. .e organic phase was preparedusing EHD and SK at certain proportion previous to theextraction operation. .e brine and organic phase wereintroduced to the centrifugal extractor from different inletsby two peristaltic pumps, as shown in Figure 1.

When the extraction was equilibrated (about 20min),5 mL of raffinate and initial brine was withdrawn to an-alyze the content of boron by the alkaline titrimetricmethod with an uncertainty of ≤±0.3% in mass [18].Boron content in the organic phase was calculated bysubtraction based on the mass conservation. .e othermetal ions in the aqueous phase were detected by ICP-OES with an uncertainty ≤± 0.5% in mass [19]. .e or-ganic phase was analyzed by Fourier-transform infraredspectrometry (FT-IR, TENSOR-27, Swit Bruker Corpo-ration, Germany) before and after the extraction. .estripping solution was acidized and concentrated tocrystallize, and the solid phase was identified by an X-raydiffractometer (X’pert PRO, Spectris. Pte. Ltd., Nether-lands). .e experimental results were evaluated by ex-traction rate (E), distribution ratio (D), and back-extraction rate (BE). .ey were defined as

E(%) �mO

mA + mO× 100%,

D �CO

CA,

BE(%) �mA

mO× 100%,

(1)

where mA and mO are the masses of boron in aqueous phaseand organic phase, respectively; CA and CO are the molarconcentrations of boron in aqueous phase and organicphase, respectively.

3. Results and Discussion

3.1. Extraction of Boron

3.1.1. Effect of pH and EHD on the Extraction of Boron..e analogous esterification reaction between alcoholichydroxyl group and boric acid results in the extraction ofboron by the organic phase. .erefore, the influences of pHand EHD on the extraction rate and distribution ratio ofboron were investigated by using a single centrifugal ex-tractor at phase ratio R(O/A)� 1 : 2, and the results areshown in Figure 2.

2 Journal of Chemistry

Di�erent dissolved boron forms in brine will betransformed into boric acid when brine is acidized topH< 2.5, by which it contributes to the extraction of boron.�e extraction rate and distribution ratio of boron decreasewith the increase of pH (Figure 2(a)) as well as the decreaseof EHD fraction in the organic phase (Figure 2(b)). Withthe increase of pH, the amount of boron in brine in theform of associating anions is increased while the amount ofboric acid is decreased, resulting in the decrease of e�ectivemolecules for esterication so that the extraction rate isdecreased.�e lower the acidity is, the higher the extractionrate becomes. But in the actual production process, thelower acidity will corrode the extraction equipment andincrease the acid consumption. �erefore, pH � 2 was se-lected in this work. When the content of EHD was 30%, the

single centrifugal extraction rate reached to 70%, sug-gesting that both the reaction rate between EHD and boronas well as the mass transfer process between aqueous phaseand organic phase were fast. However, when the volumefraction of EHD is larger than 30%, this variation is notobvious. It is speculated that this result mainly results fromthe high viscosity of the organic phase (Table 2), by whichthe mass transfer process between aqueous phase andorganic phase is suppressed.

3.1.2. E�ect of Phase Ratio R(O/A) and Extraction Stage onthe Extraction of Boron. Phase ratio R(O/A) and extractionstage are the main parameters to inuence the extraction ofboron by continuous centrifugal extraction; thus, they were

Raffinate

Organic phase Underground brineBoron loaded

Organic phase

Figure 1: Schematic of experimental setup.

0 1 2 330

40

50

60

70

80

pH

E (%

)

2.0

2.5

3.0

3.5

4.0

4.5

D

ED

(a)

0.1 0.2 0.3 0.4 0.5

40

60

80

ED

Volume fraction of EHD

E (%

)

1

2

3

4

5

6

7

D

(b)

Figure 2: E�ects of brine pH (a) and EHD (b) on the extraction of boron by single-stage centrifugal extraction at R(O/A)� 1 : 2.

Table 1: Composition of underground brine.

Component K+ Na+ Ca2+ Mg2+ Cl- SO42- B2O3 Br- Li+

Concentration, g·L−1 55.66 63.91 3.58 22.44 225.76 2.53 17.48 3.2 0.679

Journal of Chemistry 3

investigated in detail at the conditions of extraction by 30%EHD+70% SK at pH� 2.0. .e results are shown inFigure 3.

Based on Nernst’s distribution law, the higher the R(O/A)the higher the extraction rate. As it is shown in Figure 3(a)that both extraction rate and distribution ratio of boronincrease with the increase of R(O/A). However, it is noted thatthe concentration of boron in the organic phase as well as inthe aqueous phase after back-extraction will decrease with theincrease of R(O/A). Accordingly, it will sharply increase theenergy consumption during the evaporation and concen-tration of the aqueous phase. .erefore, it is not necessary toblindly pursue a high single extraction rate by increasing theextraction phase ratio. .is contradiction between extractionrate and phase ratio can be effectively solved by multistagecentrifugal extraction. Based on the results in Figure 3(b), wefound that although the single extraction rate is low, it willobviously increase with the extraction stage. After five-stagecontinuous extraction, a recovery rate of 98.46% was finallyobtained, and the concentration of boron in the organic phasecan be concentrated up to 10.79 g·L−1.

3.2. Back-Extraction of Boron. At present, extensive reportsrelevant to boron extraction mainly focus on the extraction,and only few involve the back-extraction. Boron isextracted based on the reaction between boric acid andglycol, while the boric acid ester is hydrolyzed during theback-extraction at the action of alkali or acid. After single-stage back-extraction, respectively, by 0.5mol·L−1·HCl and

0.5mol·L−1 NaOH, we found that the back-extraction rateof NaOH (58.3%) was much higher than that of HCl(11.6%). .erefore, both the concentration of NaOH andphase ratio were further investigated by five-stage cen-trifugal extraction. .e results are shown in Figure 4.

.e contact time between aqueous phase and organicphase during the centrifugal back-extraction is quite short;thus, the back-extraction rate of boron is usually low only bysingle-stage back-extraction. In order to obtain a high re-covery of boron, it is necessary to increase the concentrationof NaOH or decrease the phase ratio. Figure 4 shows that thestripping rate increased with the increase of NaOH con-centration as well as the decrease of the phase ratio. Just as itwas described above that a low phase ratio will lead to largeevaporation quantity of water, it is best to enhance therecovery rate by increasing the concentration of NaOH orthe stage of back-extraction. We found in Figure 4(a) that97.00% boron in the organic phase was stripped by five-stageback-extraction using 1.0mol·L−1 NaOH at R(O/A)� 1 :1,while it was 88.69% in Figure 4(b) using 1.5mol·L−1 NaOHat R(O/A)� 2 :1. It is noted that high concentration ofNaOH will probably lead to seriously emulsification of theorganic phase; thus, we should decrease the concentration ofNaOH as far as possible on condition that the recovery issatisfactory.

3.3. Recovery of Boron after Back-Extraction. When theaqueous phase obtained during back-extraction was di-rectly evaporated, we found that only a small quantity of

1:5 1:4 1:3 1:2 1:140

50

60

70

80

90

100

Phase ratio R (O/A)

E (%

)

4

5

6

7

8

ED

D

(a)

1 2 3 4 5

30

40

50

60

70

80

90

100

E (%

)Stage

(b)

Figure 3: Effects of phase ratio R(O/A) (a) and extraction stage (b) on the extraction of boron by 30% EHD+70% SK at pH� 2.0.

Table 2: Viscosity of organic phase at different EHD fractions (298.15K).

EHD (%) 10 20 30 40 50Viscosity (mPa·s) 28.06 38.85 70.79 89.03 154.32

4 Journal of Chemistry

the solid phase was obtained, and the recovery of boron wasno more than 35%. It is noted that NaOH will react withH3BO3 in solution to generate low solubility of sodiumtetraborate (Na2B4O7) (2), while it will produce high solubilityof sodiummetaborate (NaBO2; Table 3) when the molar ratiobetween NaOH and H3BO3 is larger than 1 : 2 (3). Due to themolar ratio between Na and B is far higher than 1 : 2 duringthe back-extraction, it is certain that there was a great deal ofNaBO2 existed in the aqueous phase after back-extraction:

4H3BO3 + 2NaOH � Na2B4O7 + 7H2O. (2)

H3BO3 + NaOH � NaBO2 + 2H2O. (3)

For the reasons mentioned above, the low recovery rateof boron by evaporating directly is reasonable. We foundthat the solid phase was identified as tetraborate pentahy-drate (Na2B4O7·5H2O; Figure 5(a)) rather than the usuallyexpected borax (Na2B4O7·10H2O). .is situation is specu-lated to be the effects of NaBO2 and the excessive NaOH inthe aqueous phase.

In order to obtain a high recovery of boron, based onthe solubility data of different boron compounds in Table 3,the aqueous phase can be acidized to pH < 2.5 and thenevaporated to obtain the H3BO3 product. .e resultsshowed that pure H3BO3 was acquired (Figure 5(b)), andthe recovery rate of boron was no less than 88.32%.

3.4. Extraction Mechanism of Boron by EHD. .e organicphase containing 30% EHD and 70% SK was identified byFourier-transform infrared spectroscopy (FT-IR) before andafter the extraction of boron at pH� 2.0 and R(O/A)� 1 : 2,and the results are shown in Figure 6.

Based on the comparison of FT-IR spectrograms inFigure 6, we found that there appears new absorption peaksat 663 cm−1, 1271 cm−1, 1336 cm−1, and 1419 cm−1 afterboron extraction. Wherein, the 663 cm−1 is the deformedvibration peak of BO3 and the 1271 cm−1 is the de-formation vibration peak of B–O–H, while the 1336 cm−1

and 1419 cm−1 are the stretching vibration peaks of B–Obonds in borate ester. No B–O characteristic absorptionpeak of boric acid was present in Figure 6(b), indicatingboron in the organic phase does not exist in the boric acidform. .erefore, it is certain that the extraction of boronby EHD mainly results from the esterification reactionbetween EHD and H3BO3 rather than the physicalextraction.

In order to further reveal the extraction process andmechanism, it was investigated by the slope method. SK onlyacts as a diluent during the extraction of boron; thus, thereaction between H3BO3 and EHD is expressed in thefollowing:

Table 3: Solubility of different boron compounds [20].

Temperature (°C)Solubility (g·100 g water)

H3BO3 Na2B4O7·10H2O Na2B4O7 NaBO210 3.65 1.6 —a 20.820 4.87 2.5 — 25.425 5.74 3.2 — 28.230 6.77 3.9 — 31.440 8.90 6.4 8.8 40.450 11.39 10.5 — —60 14.89 17.4 — 63.9aNo data.

1 2 3 4 50

20

40

60

80

100BE

(%)

Stage

0.5 mol/L1.0 mol/L1.5 mol/L

(a)

0.5 mol/L1.0 mol/L1.5 mol/L

1 2 3 4 50

10

20

30

40

50

60

70

80

90

BE (%

)

Stage

(b)

Figure 4: Back-extraction of boron by five-stage centrifugal extraction with NaOH solution at R(O/A)� 1 :1 (a) and 2 :1 (b)

Journal of Chemistry 5

H3BO3(aqueous phase) + mR(OH)2(organic phase)⟶

H3BO3 · mR(OH)2(organic phase),(4)

where R(OH)2 denotes the EHD and m is the stoichiometricnumber. .e apparent extraction equilibrium constant K in(4) can be assumed as

K �H3BO3 · mR(OH)2 O

H3BO3 A · R(OH)2 Om, (5)

where H3BO3 is a weak acid and it will hydrolyze in wateraccording to the following hydrolysis reaction [4]:

H3BO3 + H2O↔reversible

B(OH)−4 + H+ pKa � 9.23. (6)

.erefore, the extraction equilibrium distribution co-efficient D can be expressed as

D �H3BO3 · mR(OH)2 O

H3BO3 A + B(OH)4−

A�

H3BO3 · mR(OH)2 OH3BO3 A × 1 + 10pH−pKa( )

.

(7)

Based on Equations (5) and (7), the distribution co-efficient D can be reformulated into Equation (8) or furthersimplified into Equation (9) by considering the logarithm:

D �K × R(OH)2 O

m

1 + 10(pH−pKa)( , (8)

lgD � m × lg R(OH)2 O + constant. (9)

Equation (9) shows that there presents a linear relationbetween lgD and lg[R(OH)2]O at certain conditions, and theslope is the stoichiometric numberm in Reaction (4). Duringthe batch experiment of boron extraction, the molar con-centration of boron in brine was kept constant, while thecontent of EHD in the organic phase was gradually changed.

0 10 20 30 40 50 60 70 80 90

0

200

400

600

800

1000

1200In

tens

ity

2θ (degree)

(a)

0 10 20 30 40 50 60 70 80 90

0

5000

10000

15000

20000

25000

Inte

nsity

2θ (degree)

(b)

Figure 5: X-ray diffractograms of the solid phases generated during the evaporation of aqueous phase after back-extraction of boron. (a)Direct evaporating and (b) evaporating after acidification.

4000 3000 2000 1000 0Wavenumbers (cm–1)

Tran

smitt

ance

a

b

6633543 1271

13361419

Figure 6: FT-IR spectrograms of organic phases before (a) and after (b) the extraction of boron.

6 Journal of Chemistry

When the lgD and lg[R(OH)2]O was linear fitted, the resultsare shown in Figure 7.

Based on the result in Figure 7(a), we found that thelinear equation is expressed as y� 0.92237x+0.09896, andthe correlation coefficient (r) reaches to 0.9973, suggestinga good linear correlation between lgD and lg[R(OH)2]O. .eslope is approximated to 1.0 with a standard error of 3.4%;thus, the extraction of boron by EHD is mainly based on

the reaction between H3BO3 and EHD at stoichiometricratio 1 : 1. H3BO3 can also be expressed as B(OH)3, and theesterification reaction between alcoholic hydroxyl and -OHin B(OH)3 will generate a stable six-membered ringstructure of boric acid ester after producing two moleculesof water; thus, the accurate reaction of R3 is expressed inthe following reaction:

RR

OH

OH

OHOH

OHCH

CHHO

H2C

H2C

H2C2H2O

H2C

O

BO

++ B (10)

4. Conclusions

.e recovery of boron in underground brine was carried outin this work by using continuous centrifugal extraction with2-ethyl-1,3-hexanediol, and the extraction process andmechanismwere also investigated by the combination of FT-IR with the slope method..e results showed that boron canbe effectively extracted and separated from undergroundwater by five-stage centrifugal extraction using 30% EHD+70% SK at pH� 2.0–3.0, R(O/A)� 1 : 2. .e extraction ratefor a real brine containing 17.48 g·L−1 B2O3 was 98.46% atthe aforementioned conditions. Boron in the organic phasecan be well five-stage back-extracted by 1.0mol·L−1 NaOH atR(O/A)� 1 :1 with a back-extraction rate of 97.00%. Onlya small quantity of tetraborate pentahydrate was obtainedby the direct evaporation of the aqueous phase obtainedby back-extraction, while about 88.32% boron can be

recycled in the H3BO3 form by evaporation after acidized topH

Acknowledgments

.is study was funded by the National Natural ScienceFoundation of China (grant numbers U1607129, U1607123,and 21773170), the Foundation of Tianjin EngineeringCenter of Marine Chemical Engineering & Technology(grant number hyhg201701), and the Yangtze Scholars andInnovative Research Team of the Chinese University (grantnumber IRT_17R81).

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Journal ofNanomaterials

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