用于盐湖卤水镁锂分离的纳滤技术研究进展

Nanofiltration Technology Used for Separation of Magnesium and Lithium from Salt Lake Brine: a Survey

作为"推动世界前进的重要元素"的能源金属,锂对国民经济的发展具有重要的战略意义。目前各行业如玻璃、电器和制药等对锂资源的需求量都大幅提高,因此对锂资源的提取和储备至关重要。相比于传统矿石提锂的方法,从盐湖卤水中提锂不需要大量的药剂投入,生产成本大幅降低,因此从盐湖卤水中提取锂资源成为获取锂资源的重要途径。盐湖中镁锂共生存在,镁和锂化学性质相似且它们的离子水合半径差异较小,要实现盐湖高效提锂首先需要解决镁锂分离困难的问题。目前,用于盐湖镁锂分离的方法有很多,但这些工艺存在一些缺陷,如能源消耗大,经济效益低、对环境不友好或不适用于高镁锂比盐湖等。纳滤是一种介于超滤与反渗透之间的压力驱动膜分离技术,纳滤膜因特殊的孔径范围和荷电性质,能有效分离单价、二价及多价离子,在盐湖卤水的镁锂分离领域表现出显著优势。因此,纳滤分离技术成为一种新兴的镁锂分离手段,被国内外研究者视为今后镁锂分离研究的一个重要方向。研究表明,纳滤膜的分离机理主要包括静电排斥和空间位阻效应,溶质在分离过程中除了受到溶质尺寸与孔径的影响,膜表面的电荷性质对溶质的分离起到了极大的作用。目前,用于镁锂分离的多为商业化纳滤膜,一般带有负电。由于镁锂的离子水合半径非常接近,因此在膜分离过程中孔径筛分作用影响较小,膜分离镁锂主要通过静电排斥来实现。近年来的研究结果表明,荷正电纳滤膜在镁锂分离过程中表现出优异的镁锂选择分离性能。由静电排斥效应可知,荷正电纳滤膜在分离镁和锂的过程中,膜对二价阳离子Mg2+的排斥作用明显高于一价阳离子Li+,因此可使一价阳离子Li+更容易透过膜进入渗透液,而二价阳离子Mg2+则被截留。本文从盐湖锂资源的分布和提取的角度出发,对盐湖卤水镁锂分离的多种方法进行了全面的归纳。以纳滤膜的结构特点和作用机理为切入点对纳滤膜的相关特性进行了详细的分析概述,重点评述了纳滤法分离镁锂技术的应用现状及其存在的问题。同时针对我国盐湖卤水中镁和锂的特殊结构和分布状态,展望了应用于盐湖镁锂分离的纳滤技术的发展方向。

As an energy metal that“pushes the world forward”,lithium plays a significant role in the development of the national economy.Currently,there has been consistent growth in demand for lithium resources of various industries like glass,electrical appliances and pharmaceuticals,etc.Hence,it is of great importance in extraction and storage of lithium resources.Compared with the traditional method of extracting lithium from ores,extracting lithium from salt lake brine show notable superiority,because it is free for large amount of chemical input,and economical with substantially reduced cost,which makes it become a primary way for achieving lithium resources.Unfortunately,lithium and magnesium exist as a symbiosis in the salt lake brine,and the chemical properties of magnesium and lithium are similar with small difference in their ionic hydration radius.As a result,the problem of separating magnesium and lithium must be figure out first,in order to achieve high-efficiency lithium extraction from salt lakes.Presently,there are many ways for separating magnesium and lithium from salt lake.Nevertheless,these methods still suffer from large energy consumption,low economic benefit,environmental unfriendliness,and unfriendly unavailability for high magnesium-lithium ratio salt lake.Nanofiltration(NF)is a pressure-driven membrane separation technique between ultrafiltration and reverse osmosis.Thanks to the special pore size range and charge properties of nanofiltration membranes,monovalent,divalent and multivalent ions can be separate effectively.Consequently,nanofiltration technique has become an emerging method for magnesium and lithium separation,and shows remarkable advantage in the field of separation of magnesium and lithium in brine of salt lake,which is regarded as an important research direction by researchers at home and abroad.Research results show that the separation mechanism of NF membrane mainly includes electrostatic repulsion and steric resistance.In addition to the impact of solute size and pore size,the charge properties of the membrane surface exert a great effect on the separation of solute.Currently,the commercial NF membranes that have been mostly employed for magnesium and lithium separation are negative charged membranes.As the difference in ionic hydration radius of magnesium and lithium is not obvious,the steric hindrance effect plays a minor role in the separation process,and the separation of magnesium-lithium by membrane is mainly realized by electrostatic exclusion.In recent years,researchers have verified that positive charged NF membrane presents excellent performance in selective separation of Mg^2+and Li+.According to the electrostatic repulsive effect,high valence cations(Mg^2+)own a higher positive charge than low valence(Li+),leading to a stronger exclusion between high valence cations and positive charged NF membrane and resulting in a stronger rejection to Mg^2+than Li+.From the point of view of the distribution and extraction of lithium resources in salt lake,this article summarizes various methods for magnesium and lithium separation from brine.Based on the structure and mechanism of nanofiltration membrane,the relevant characteristics of nanofiltration membrane are analyzed and summarized comprehensively,and the application status and existing problems of magnesium and lithium separation by nanofiltration are reviewed.In view of the special structure and distribution of magnesium and lithium in salt lake brine in China,the prospect of nanofiltration technology applied to the separation of magnesia and lithium in salt lake is proposed.