Mg(BO2)2在MgSO4和MgCl2-MgSO4水溶液中分解的相平衡与物种化学平衡

硫酸镁亚型盐湖老卤是MgCl₂、MgSO₄的饱和溶液体系,富集了Mg、B、Li等元素。本文针对含硼水盐体系热力学难以表达的问题,开展了硼在水盐体系存在形态及其平衡关系的研究。利用易于水解的Mg(BO₂)₂作为硼源,借助拉曼光谱和X射线衍射(XRD)为检测手段,定量研究Mg(BO₂)₂在MgSO₄-H₂O、MgSO₄-MgCl₂-H₂O体系中水解硼物种转化规律。结果表明:(1)Mg(BO₂)₂在MgSO₄溶液中的水解固相为MgB₂O(OH)6、MgB₄O₇·9H₂O和Mg(OH)₂；在MgSO₄-MgCl₂-H₂O溶液中水解固相为Mg₂B₆O₁₁·15H₂O、MgB₄O₇·9H₂O、Mg₂Cl(OH)₃·4H₂O。(2)Mg(BO₂)₂在MgSO₄溶液中水解,液相硼物种主要有B₃O₃(OH)-₄、B₃O₃(OH)₅2-、B(OH)-₄、B₄O₅(OH)₄2-、H₃BO₃,其分布受MgSO₄浓度影响很大,MgSO₄浓度从0增加至饱和,B₃O₃(OH)-₄始终占总硼量的50.07%以上,B₃O₃(OH)₅2-占液相总硼从4.77%上升至37.16%为第2化学物种。(3)在MgSO₄-MgCl₂-H₂O溶液中,硼物种的主要形态有B₃O₃(OH)-₄、B₃O₃(OH)₅2-、B(OH)-₄,其分布 随Cl₂2-]/(Cl₂2-]+SO₄2-])变化很大,在MgCl₂和MgSO₄的共饱溶液中,分别占液相总硼量的58.91%,14.62%和12.81%。(4)液相硼物种之间平衡关系的活度商lnQ,不仅与溶液pH、水活度有关,还与MgSO₄、MgCl₂摩尔浓度呈二元线性关系,这样就可将硼在H₃BO₃-NaOH体系物种分布关系的lnQ扩展到 (MgSO₄,MgCl₂,MgSO₄-MgCl₂)水溶液的多元体系。由此获得硼物种的量化关系,可为推算硼在水盐体系的物种分布,进而计算含硼电解质溶液热力学性质提供依据。

Phase Equilibria and Species Chemical Equilibrium of Mg(BO2)2 Decomposed in MgSO4 and MgCl2-MgSO4 Aqueous Solutions

Bittern of magnesium sulfate subtype salt lake is saturated with MgCl2 and MgSO4, and enriches elements of Mg, B and Li. In view of the difficulty in expressing the thermodynamics of water salt system containing boron, the existing species and chemical equilibrium of boron in water salt system were studied. In this paper, the decomposition and species equilibrium behaviors of Mg(BO2)2 in MgSO4 and MgSO4-MgCl2 aqueous solution were quantitatively studied by means of Raman spectroscopy and X-ray diffraction (XRD). The results showed: (1) the solid phases of Mg(BO2)2 decomposed in MgSO4 solution are MgB2O(OH)6,MgB4O7.9H2O and Mg(OH)2. And the decomposed solid species in MgSO4-MgCl2 aqueous solution are Mg2B6O11.15H2O, MgB4O7.9H2O, Mg2Cl(OH)3.4H2O, respectively. (2) Liquid species of Mg(BO2)2 decomposed in MgSO4 aqueous solution were determined to be B3O3(OH)4^-, B3O3(OH)5^2-, B(OH)4^-, B4O5(OH)4^2- and H3BO3, and with the increasing of MgSO4 concentration from 0 to saturation, B3O3(OH)4^- account for more than 50.07 % of the total boron quantity, and B3O3(OH)5^2- increase from 4.77 % to 37.16 %; (3) In MgSO4-MgCl2 aqueous solution, B3O3(OH)4^-, B3O3(OH)5^2- and B(OH)4^- are the main species, and accounting for 58.91 %, 14.62 % and 12.81 %, respectively in MgCl2 and MgSO4 co-saturated solution. (4) The activity quotient lnQ between boron species is mainly related to liquid pH, and also limited to water activity, meanwhile, it takes the linear relationship with molar concentrations of MgSO4 and MgCl2. So, lnQ in H3BO3 - NaOH system can be extended to the multicomponent system of MgSO4, MgCl2 and MgSO4- MgCl2 aqueous solution. Therefore, the quantitative relationship of boron species can be obtained, which can provide a basis for calculating the species distribution of boron in salt water system and the thermodynamic properties of boron containing electrolyte solution.