磷酸铁离子筛卤水提锂热力学分析

Thermodynamics analysis of Li-extraction from brine using FePO_4 ion-sieve

针对高Mg和Li质量比盐湖卤水提锂困难的问题,提出利用LiFePO4/FePO4材料对盐湖卤水进行选择性提锂的思路。在热力学计算的基础上绘制298.15 K时Me(Li,Na,K,Mg)-Fe-P-H2O体系的-pH图,并讨论FePO4对盐湖中Na+、K+、Mg2+与Li+的选择性吸附问题。结果表明:当离子浓度为0.1 mol/L、体系氧化还原电位降到0.36 V(vs SCE)时,FePO4中+3价的铁即被还原为+2价,同时Li+嵌入FePO4晶格生成LiFePO4;而体系电位需降到0.132 V和0.073 V才分别生成KFePO4和NaFePO4,说明材料对Li的选择性优于Na和K的,而Mg0.5FePO4则在计算分析的范围内不能稳定存在,说明FePO4对Mg2+无吸附性。因而,在适当的电位范围内(本研究的计算条件下为0.173～0.33 V)即可利用磷酸铁材料实现Li与Na、K、Mg等元素的选择性提取,而吸附锂后通过调节氧化还原电位大于约0.33 V,即可实现LiFePO4材料中Li的脱出。

LiFePOa/FePO4 material was chosen as ion-sieve for selectively extracting Li from high Mg/Li mass ratio brine. Thermodynamic analysis was conducted about selectivity of adsorption of Na^6＋, K^＋, Mg^2＋ and Lit by FePO4 ion-sieve. The corresponding φ-pH diagrams of Me （Li, Na, K, Mg）-Fe-P-H2O system at 298.15K were plotted and analyzed. The results show that, under the condition of ion concentration 0.1 tool/L, the redox potential of Me （Li, Na, K, Mg）-Fe-P-H2O system is controlled not higher than 0.36V （vs SCE）, Fe3t in FePO4 is reduced to Fe2＋, and, Li＋ is inserted into FePO4 crystal lattice to form LiFePO4 synchronously. KFePO4 and NaFePO4 will not be obtained until the potential of the system is reduced to 0.132 V and 0.073 V, respectively. Lit is absorbed preferentially compared with Nat and Kt by FePO4 ion-sieve. Mg0.5FePO4 can＇t exist steadily in the studied range, which means that Mg^2＋ can＇t be absorbed by the material. Therefore, the FePO4 ion-sieve can be used to selectively extract Lit when the redox potential being set within 0.173-0.33 V. After extraction, Li can be de-intercalated from LiFePO4 by setting the potential of system above 0.33 V.