采用稀释成盐法从富硼浓缩盐卤中合成的镁硼酸盐化合物及其结构与性质

彭姣玉　林锋　杨波　王立平　Dinnebier E.Robertc　董亚萍＊,,2　李武

(1中国科学院青海盐湖研究所，西宁810008)（2中国科学院大学，北京100049）(3中国科学院盐湖资源综合高效利用重点实验室，西宁810008) (4德国马克斯-普朗克固体研究所，斯图加特70569)

采用稀释成盐法从富硼浓缩盐卤中合成的镁硼酸盐化合物及其结构与性质

彭姣玉1,2,3林锋1,2,3杨波1王立平1,2,3Dinnebier E.Robertc4董亚萍＊,1,2李武1

(1中国科学院青海盐湖研究所，西宁810008)（2中国科学院大学，北京100049）(3中国科学院盐湖资源综合高效利用重点实验室，西宁810008) (4德国马克斯-普朗克固体研究所，斯图加特70569)

通过稀释成盐法在富硼浓缩盐卤体系Na-K-Mg-Cl-SO4中合成了一种新的六硼酸镁Mg[B6O7(OH)6]·5H2O化合物。根据X射线粉末衍射数据对其晶体结构进行了精修，并采用红外及拉曼光谱法对其结构进行了表征，分析了其光谱及结构特征。结果表明，该化合物由1个Mg原子、1个B6O7(OH)6基团和5个H2O分子构成，Mg原子以六配位形式与氧结合形成畸变MgO6八面体构型；热重分析表明，高温分解过程该化合物脱水转化为四硼酸镁MgB4O7；通过紫外可见漫反射法求得其禁带宽度为4.44 eV。

镁硼酸盐；含硼盐卤；稀释成盐法；结构；热稳定性

0Introduction

Boron and its compounds are important inorganic salt products.Especially materials enriched in boron-10arewidelyusedinnuclearpower,military equipment,and pharmaceuticals,etc,based on the property that boron-10 can absorb thermal neutrons. For example,the boric acid enriched in boron-10 used as neutron absorber in nuclear power can effectively ensure the nuclear reactor′s safe operation[1]. Furthermore,boron minerals,owing to their high heat resistance,light weight,fire retardency,nonlinear optics and anti-wear properties,can be widely applied inceramics,metallurgy,buildingmaterialsand electronic areas[2-5].In recent years,with the increasing demandofboronproductsandthedifficulty exploitationoflowgradeboronores,boron exploitation from salt lake brine has become an important means to produce boron products[6].

Lake Da Qaidam is one of the magnesium sulfate subtype salt lakes found on Qinghai Tibet Plateau in western China.The brine of which is abundant in sodium,potassium,magnesium,lithium and boron mineral resources.The main composition of the brine is Na-K-Mg-Cl-SO4.And the crystallization path about summer brine during evaporation was[7]:

halite→halite+epsomite→halite+epsomite+ hexahydrite+sylvite→halite+hexahydrite+sylvite+ carnallite→halite+hexahydrite+carnallite+bischofite.

When the brine evaporation path went into bischofite crystallization,the brine is rich in boron and lithium resources and can be used as raw material for the production of boron and lithium products.Gao and Li have investigated the chemical behavior of the borate during brine evaporation[8].They found that the borate in this brine,in general,does not crystallize out but accumulates in the highly concentrated brine,in the form of polyborate anions of“tetraborate”bystatistics.Thesupersolubilityof boron enriched in the brine can reach up to 5.82%in B2O3[9].However,the fact will be exactly the opposite when diluting the boron-bearing highly concentrated brine with some water.Kinds of Mg-borate salts would participate after diluting the brine for a period of time and the deposit salts changed with the dilution ratio of brine[10].This phenomenon is quite different to crystallization by evaporation.Gao called it as“crystallization by dilution”.The salts precipitated from diluted brine contain magnesium borate hydrate(MgO(B2O3)3·6H2O),macallisterite(Mg2[B6O7(OH)6]2·9H2O),admontite(MgO(B2O3)3·7H2O),hungchsaoite(Mg(H2O)5B4O5(OH)4·2H2O),kurnakovite(MgB3O3(OH)5(H2O)4·H2O) andinderite(MgB3O3(OH)5(H2O)4·H2O).The above results of“crystallization by dilution”indicate a possible economic extraction of boron resources from brine. Thus,we just call it as dilution method compared with traditional methods of acid precipitation and solvent extraction.During the dilution crystallization process, we found a new Mg-borate compound of Mg[B6O7(OH)6] ·5H2Ofromthehighlyconcentratedbrine.The present study aims to report the synthesis,structural characterization,optical and thermal properties of this new compound.

1　Experimental

1.1Materials and measurements

Mirabilite ores(Na2SO4·10H2O)were used to remove parts of Mg2+ion from brine for further evaporation purpose.The Raman spectra were recorded using a Raman spectrometer(ALMEGA-TM,Therm Nicolet,American)with the linearly polarized 532.0 nm line of a diode laser.The spectra were taken from 300 to 1 500 cm-1since the characteristic peaks of borate compounds were given in the range of 1 500～500 cm-1[11].The FTIR spectrum was obtained on a Fourier transform IR spectrometer(NEXUS,Therm Nicolet,American)with KBr pellet in the range of 400～4 000 cm-1at room temperature.The DSC and TGmeasurementswereperformedsimultaneously using a scanning calorimeter(NETZSCH STA,449F3) heating from 30 to 1 200℃in nitrogen atmosphere with a constant heating rate of 5℃·min-1.The Mg and B elements of the product was investigated by the inductivecouplingplasma(ICP)spectrometer (Thermo,6500)and the O element was measured by an Oxford series X-ray energy dispersion spectrometer(EDS).The UV/Vis diffuse spectrum for Mg[B6O7(OH)6] ·5H2O was recorded with a Agilent Cary5000 spectrophotometer in the range from 200 to 800 nm at room temperature.

1.2Preparation of the concentrated boron-bearing brine

When the brine evaporation path went into bischofite crystallization,the boron content in brine is about 1.3%in B2O3which is too low to extract boron from brine by dilution method.Further evaporation is needed to concentrate the boron element.However, since the brine with saturated bischofite is extremely high-saline,viscous and hygroscopic and tends to absorb humidity from the air rather than drying.This brine can hardly be further concentrated by natural evaporation.In this study,according to the following reaction: some mirabilite ores were added into the brine at 25℃to react with the magnesium chloride generating magnesium sulfate and sodium chloride.Then the brine saturated with epsomite and halite can be easily further evaporated at room temperature.Therefore,the concentrated boron-bearing brine can be obtained by evaporation.

1.3Synthesis of the magnesium borate of Mg[B6O7(OH)6]·5H2O

The concentrated boron-bearing brine with boron content above 4%B2O3was used as materials to synthesize the new compound of Mg[B6O7(OH)6]·5H2O. First,an amount of the concentrated brine was diluted with some water,the mass ratio of brine and water was 1∶0.25.The solution was stirred for about 20 min each day at room temperature until the participation of solid phase,then aged and filtrated after a week. The sediment was washed by water and absolute alcohol,respectively,and dried in a vacuum drying oven for 24 h.The chemical composition of the obtained product was analyzed by titration.

1.4X-ray powder diffraction crystallography

A laboratory powder diffractometer(X′Pert PRO, 2006 PANalytical,Cu Kα1),with a tube voltage and current of 40 kV and 30 mA,was used to confirm the Mg[B6O7(OH)6]·5H2O phase purity.The powder pattern was measured in the scanning range from 3.502 0°to 109.998 0°,2θ with a scan step of 0.002°and at a fixed counting time of 16 s per step.

Owing to the difficulty to grow crystals suitable for single-crystal structure determination,the crystal structure was solved and refined using conventional X-ray powder diffraction techniques.The TOPAS 4.2 program suite[12]was used for indexing,obtaining structure solution,and refinement of the crystal structure model.The iterative least squares algorithm(LSI)[13]was employed to index the pattern,and resulted in a primitive monoclinic lattice.The unit cell and profile parameters were refined by a Pawley fit[14]using the fundamental parameters approach[15].The background was modelled by a Chebychev polynamial of 10th order.

2Results and discussion

Fig.1Crystallization path of brine DL1at metastable diagram of Na+,K+,Mg2+/Cl,SO42-//H2O at 25℃

2.1Evaporation crystallization path

Thebrineevolutionduringevaporationwas representedgraphicallyonthemetastablephase diagram of Na+,K+,Mg2+/Cl,SO/H2O at 25℃[16](Fig.1).Its composition was listed in Table 1.In Fig. 1,DL0was the residue bittern of Da Qaidam salt lake during evaporation and its system was located in the bischofite phase.After removing parts of Mg2+ion by adding some mirabilite ores,the brine system“DL1”gone into the epsomite region and is easy to concentrate by evaporation.When the crystallization pathevolved from epsomite(DL1)to bischofite(DL3),the boron content in brine was concentrated to about 3% in B2O3(Table 1).This brine can be used to extract boron by dilution method.But,for the synthesis of the new compound,it will be further evaporated until the boron content is above 4%B2O3.Therefore,the concentrated boron-bearing brine DL5or DL6obtained byevaporationwereusedasmaterialsforthe synthesis of the new compound.

2.2XRD pattern and crystal structure characterization

Fig.2 shows the X-ray diffraction pattern of Mg[B6O7(OH)6]·5H2O and the results of the Rietveld refinement[17].The refined structural parameters are presented in Table 2.The obtained product was phase pure according to the X-ray powder diffraction.The chemical elements analyzed by titration and the crystal water measured by TG are shown in Table 3. The measured composition values fit well with the given formula.

Table 1　Evolution of major ion concentrations during evaporation at ambient temperature

Fig.2　X-ray diffraction pattern of Mg[B6O7(OH)6]·5H2O and results of the rietveld refinement

In Fig.2,the X-ray diffraction pattern shows remarkable similarity to the pattern of Ni[B6O7(OH)6]· 5H2O[18].Forcomparability reasons,the standard crystallographic space group setting P21/c of Ni[B6O7(OH)6]·5H2O was keptforMg[B6O7(OH)6]·5H2O (Element analysis:Mg 6.38%,B 15.99%,O 73.20%) leading to lattice parameters of a=0.898 87(3)nm,b= 2.179 35(7)nm,c=0.720 79(2)nm,γ=99.8759(6)°, and V=1.391 07(8)nm3.

In the crystal structure of Mg[B6O7(OH)6]·5H2O (Fig.3),the asymmetric unit consists of one Mg atom, one B6O7(OH)6cluster and five H2O molecules.The B atoms are in both 3-and 4-coordinated environments forming BO3triangles and BO4tetrahedra.Three BO3and three BO4units are connected by sharing a common O atom to form B6O7(OH)6.The Mg atom is 6-coordinatedwithsixOatomstoformaMgO6octahedron.In each units MgO6octahedron,the Mg atom shares two common O atoms with one B6O7(OH)6cluster and four common O atoms with four H2Oforming a 3D Mg(H2O)4B6O7(OH)6framework.Furthermore,the last one H2O connects with the Mg(H2O)4B6O7(OH)6framework to form the whole structure of Mg[B6O7(OH)6]·5H2O.

In the B6O7(OH)6cluster,the B-O distances of BO3triangles are in the range of 0.135 7～0.138 5 nm (average 0.136 8 nm),and the B-O distances of BO4tetrahedra are in the range of 0.143 8～0.152 4 nm (average 0.147 7 nm).The O-B-O angles of the BO3triangles and the BO4tetrahedra are in the range of 115.018°～122.712°and 107.226°～111.398°,respectively.The Mg-O distances are in the range of 0.203 5～0.213 7 nm.The bond distances and angles of the compoundareinagreementwithotherborate compounds reported previously[19-21].

Table 2Crystallographic and rietveld renement data for Mg[B6O7(OH)6]·5H2O

Table 3Titration analysis of the Mg[B6O7(OH)6]·5H2O phase

Fig.3Crystal structure of the Mg[B6O7(OH)6]·5H2O compound

2.3FTIR and raman spectroscopy

Fig.4and Fig.5 show the FT-IR and Raman spectra of Mg[B6O7(OH)6]·5H2O,respectively.In the FT-IR spectrum,the bands at high wavenumbers of 3 200～3 600 cm-1belonged to stretching of hydroxyl(ν(O-H));and the peak at 1 663 cm-1corresponded to bending of H-O-H.According to literatures[11,22],The bands at 1 420,1 350 cm-1were assigned to asymmetric stretching of the three-coordinate boron(νas(B(3)-O)). The bending of B-O-H was observed at band of 1 253 cm-1.The peaks between 1 174 and 1 053 cm-1belonged to asymmetric stretching of four-coordinated boron (νas(B(4)-O)).The symmetric stretching of B(3)-O and B(4)-O was observed in the range of 807～944 cm-1.And the band around 682 was out-of-plane bending of B(3)-O.

Fig.4FT-IR spectrum of the Mg[B6O7(OH)6]·5H2O compound

Fig.5Raman spectrum of the Mg[B6O7(OH)6]·5H2O compound

In the Raman spectrum,based on the assignment of borates[22-24],weak bands in the region of 1 200～1 400 cm-1and 1 110～1 000 cm-1were asymmetric stretching of the three-coordinate boron(νas(B(3)-O))and four-coordinate boron(νas(B(4)-O)),respectively.The band at 937 cm-1was assigned to symmetric stretching ofthethree-coordinateboron(νs(B(3)-O));and the bands around 899,808 cm-1were νs(B(4)-O).Generally, the bands in the range of 610～650 cm-1were associated with the symmetric pulse vibration of triborate anion or hexaborate anion[22].In this study,the strong band at 612 cm-1belonged to the characteristic peaks of νp(B6O7(OH)62-).Besides,the peaks at and below 477 cm-1were assigned to bending of four-coordinate boron(δ(B(4)-O)).

3.4Thermal analysis

TG and DSC analyses of the Mg[B6O7(OH)6]· 5H2O phase are shown in Fig.6.Three endothermic peaks(151,211 and 994℃)and one exothermic peak (694℃)occurred in the DSC curve.In the first step, the weight loss is about 20%which was similar to the theoretical values of five water molecules weight of 22.90%,indicating there were five water molecules in the compound.In the second step,the weight loss was about 17%,which can be regarded as the weight of six hydroxyls(theoretical value of 13.75%)and the remaining water.The above total weight loss is about 37%correspondingtotheweightoffivewater molecules and six hydroxyl(theoretical values of 36.65%)in the compound.In the third step,the structure of the compound was changed after removing five water molecules and six hydroxyl.The fourth step indicates the melting point of the calcined crystals.

Fig.6TG and DSC analyses of the Mg[B6O7(OH]6·5H2O compound

To study the thermal decomposition behavior,the dehydrated products calcined at 400,800 and 1 000℃were confirmed by XRD analysis(Fig.7).Before XRD analysis,the dehydrated product calcined at 800℃was treated with methanol solution by esterification reaction to remove B2O3which occurred together with the dehydrated product during calcination process.In Fig.7,the exothermic peak around 694℃marked the transition of the dehydrated amorphous product toMgB4O7(PDF card:00-01-0927).While the calcined temperature went up to about 1 000℃,the MgB4O7crystal began to melt and stuck in the alumina crucible during cooling process.Therefore,a whole decomposition process is presented by the following chemical reactions:

3.4Results of optical measurements

Fig.8 shows the absorption spectrum of Mg[B6O7(OH)6]·5H2O compound.The absorption data were calculated from the following Kubelka-Munk function: F(R)=(1-R)2/(2R),where R is the reflectance.The energy gap Egwas calculated by the function:Eg=1 240/ λ,where λ is the wavelength.In Fig.8,the energy gap Egof Mg[B6O7(OH)6]·5H2Ocompounddetermined from extrapolation of high energy part of absorption spectra is about 4.44 eV.

Fig.7XRD patterns of dehydrated products

Fig.8Optical absorption spectra of Mg[B6O7(OH)6]·5H2O compound

3Conclusions

A new magnesium borate mineral,Mg[B6O7(OH)6] ·5H2O has been synthesized by dilution method from boron-bearing salt lakes for the first time at room temperature.The low reaction temperature used in this study provides a green chemistry approach for the synthesis of magnesium borate.The dilution crystallization method also provides a possible economic extraction of boron resources from salt lake brine.The crystal structure of this new compound was solved by laboratoryX-raypowderdiffractiondata.The compound crystallizes in the monoclinic space group P21/c.Its crystal structure consists of infinite chains of B6O7(OH)6clusters,intercalated by MgO6and H2O molecules,forming a 3D framework.The vibrational spectroscopy of FTIR and Raman reveals the presence of BO3triangles,BO4tetrahedra,water H2O and the characteristic B6O7(OH)62-anion in the compound, which further verifies the structural characterization by X-ray powder diffraction.The thermal analysis(TG and DSC)showed that there were at least four phases occurred during decomposi-tion process.The thermal behaviorgoesthroughthetransformationfrom amorphous to crystal phase of MgB4O7.The optical measurement found that the energy gap of the new magnesium borate is about 4.44 eV.

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Synthesis,Structure and Properties of a Magnesium Borate in Concentrated Boron-Bearing Salt Lake Brine by Dilution Method

PENG Jiao-Yu1,2,3LIN Feng1,2,3YANG Bo1WANG Li-Ping1,2,3Dinnebier E.Robertc4DONG Ya-Ping＊,1,2LI Wu1
(1Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining 810008,China) (2University of Chinese Academy of Sciences,Beijing 100049,China) (3Key Lab of Comprehensive and Highly Efficient Utilization of Salt Lake Resource, Chinese Academy of Science,Xining 810008,China)
(4Max-Planck Institute for Solid State Research,Stuttgart 70569,Germany)

A new magnesium borate mineral,Mg[B6O7(OH)6]·5H2O was synthesized via dilution method from boron-bearing Na-K-Mg-Cl-SO4salt lakes.The crystal structure of the new phase was solved and refined using X-ray powder diffraction data and characterized with FTIR and Raman.Its asymmetric unit consists of one Mg atom,one B6O7(OH)6cluster and five H2O molecules.The Mg atom is 6-coordinated with six O atoms to form a MgO6octahedron.Thermal gravimetry(TG/DSC)was used to investigate the thermal behavior of the new compound.During the high temperature decomposition process the dehydrated product MgB4O7formed.The optical absorption characteristic of this new mineral was investigated by UV-Vis spectrometer and its energy gap Egis about 4.44 eV.

magnesium borate;boron-bearing salt lake;dilution method;structure;thermal behavior

O611.4

A

1001-4861(2015)02-0305-08

10.11862/CJIC.2015.042

2015-07-13。收修改稿日期：2015-10-21。

国家青年科学基金(No.21501187)，青海省科技支撑项目(No.2013-G-138A)，青海省(应用)基础研究计划项目(NO.2013-Z-705)，中国科学院仪器设备功能开发技术创新项目(No.Y410031012)资助。

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