Membrane Technology set to play a major role in the processing of Hi-Tech metals

As we approach the era of autonomous and electric vehicles, advanced robotics, and increased use of renewable energy, advanced computing and IT, Rio Tinto, through research by Boston’s Massachusetts Institute of Technology, has detailed the metals that are expected to be the most impacted by new technology. In a Rio presentation it was predicted that tin would be the metal most affected by technology, followed by lithium, cobalt, silver, nickel and gold.

This is good news for Cornwall, where development on the re-opening of the South Crofty tin mine is well underway, as well as the extraction of lithium from Cornish brines (posting of 23rd February).

There will be a massive demand for lithium, for use in Li-ion batteries, and new resources will have to be found to satisfy that demand. Extraction of lithium from brines will become increasingly important, and innovative methods developed to treat these brines. Not all brines are found in arid areas which allow traditional evaporation, and innovative technologies are being developed to treat these brines.

Membrane technology is one of these techniques, and a recent paper in Sciences Advances describes the development, by researchers from Australia's Monash University and CSIRO, and the University of Texas at Austin, of a membrane made from a metal-organic framework, or MOF. This material mimics the filtering abilities found in organic cells, which allow some atoms or molecules to pass through it, while stopping others.

Schematic illustration of ion transport through a ZIF-8/GO/AAO membrane with ~3.4 Å pore windows
for ion selectivity and ~11.6 Å pore cavities for fast ion transport. The inset indicates the crystal structure of ZIF-8

Porous membranes with ultrafast ion permeation and high ion selectivity are highly desirable for efficient mineral separation, water purification, and energy conversion, but it is still a huge challenge to efficiently separate monatomic ions of the same valence and similar sizes using synthetic membranes. The paper reports metal organic framework (MOF) membranes, including ZIF-8 and UiO-66 membranes with uniform subnanometer pores consisting of angstrom-sized windows and nanometer-sized cavities for ultrafast selective transport of alkali metal ions. The angstrom-sized windows act as ion selectivity filters for selection of alkali metal ions, whereas the nanometer-sized cavities function as ion conductive pores for ultrafast ion transport. The ZIF-8 and UiO-66 membranes showed a LiCl/RbCl selectivity of ~4.6 and ~1.8, respectively, which are much greater than the LiCl/RbCl selectivity of 0.6 to 0.8 measured in traditional porous membranes. Molecular dynamics simulations suggested that ultrafast and selective ion transport in ZIF-8 was associated with partial dehydration effects. The study reveals ultrafast and selective transport of monovalent ions in subnanometer MOF pores and opens up a new avenue to develop unique MOF platforms for efficient ion separations in the future.

A paper from China in the May issue of the journal Separation and Purification Technology reviews the use of membranes in the extraction of rare earth elements (REE) also crucial for modern applications, particularly for the production of powerful rare earth magnets.

China has major rare earth deposits, but after decades of uncontrolled exploitation, it is facing serious challenges in pollution abatement and control. Multi-stage solvent extraction, the most utilised method for separating REEs, produces large amounts of acidic and alkaline wastewaters, and now China has put constraints on the mining of REE resources to protect the environment and to encourage sustainable utilisation (see also posting of 11th February 2013). Recent advanced developments have been reported, such as novel solvent extraction techniques, solid extraction and biosorption. Work has also been focussed on efficient recycling of REEs from wastes such as permanent magnets, fluorescent powders, television tubes, flat panel displays and other end-of-life electronic products.

Of all these improved techniques, membrane separation technique stands out as a process which combines extraction and stripping simultaneously, without thermal heating operations such as distillation, drying or evaporation. Apart from energy savings, no volatile organic compounds are required, which makes it suitable as a promising green separation process.

We hope to hear more of the modern methods which are now being developed to process the "Hi-Tech" metals, at Hi-Tech Metals '18 in Cape Town in November (see also posting of 5th June 2017). There is a call for abstracts, which should be submitted by the end of May. The conference immediately follows Process Mineralogy '18, where Prof Frances Wall will present a keynote lecture on the importance of mineralogy in the processing of rare earths.

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