Molecular Recognition Technology: Clean Chemistry Applied to 21st Century Rare Earth Separation
Beneficiation of rare earth elements (REE, singular or plural) is an increasingly important field in the West, especially given the uncertainty of supply originating from China, and the growing reliance of the world’s leading economies on ever faster, lighter and more efficient technologies. However, our dependence on REE has come at a remarkable price; one that has been shouldered primarily by China by way of an environmental deficit that cannot easily be reversed or repaid. In seeking to develop non-Chinese REE resources, the world needs a better mousetrap to capture REE; one without the toll that comes with wholesale disposal of effluents, reagents and caustic byproducts. MRT is such a technology, and the prospect of its deployment at production scale is very near.
Historical REE Separation & the Costly Effects of Legacy Technology
REE have essential, critical and unique uses in our global high-tech military, industrial, and commercial economy sectors. These include the use of REE in phosphors, high performance magnets, catalysts, and glass polishing . Each of these applications requires high purity individual REE. Graedel, et al.  have pointed out that potential substitutes for a dozen critical metals, such as Dysprosium (Dy), Europium (Eu), Yttrium (Y), Thulium (Tm), and Ytterbium (Yb) are either inadequate or do not exist. In other words, substitution of less expensive metals or other materials for these critical metals is highly unlikely either now or in the future.
Since recycling rates for REE are <1% , their continuing supply in the near term must come from mined ore. It would be expected that this supply requirement would place a premium on the processing of REE by efficient, economical, and environmentally friendly green chemistry separation methods. Sadly, this is not the case. Nearly all mining, beneficiation, separation and purification operations involving REE are carried out in China, where enormous quantities of waste are generated and legislative restrictions either do not exist or are weakly enforced. In 2014, a European Commission Memo reported that China’s share of the light REE global market was 87%, but its heavy REE global market share was 99% . A small amount of production, primarily involving light REE, is done in the U.S. by MolyCorp at Mt. Pass in California [1,5] and in Australia by Lynas .
Conventional methods used in all REE separation processes are based primarily on solvent extraction (SX) and ion exchange (IX). These methods unfortunately rank very low relative to their ability to meet stringent standards of clean chemistry, efficiency and economics. In fact, they are responsible, in large part, for severe environmental and human health problems associated with REE mining world-wide. These collateral problems are particularly evident in China, where most REE are mined and processed via low selectivity separation methods. The result is the discharge of generated waste into surrounding air, water, and land sites. Lax enforcement of existing regulatory laws have only served to exacerbate the problem [6,7].
High Selectivity & the Movement Towards Elegant REE Separation
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The first principle of a successful clean chemistry separation methodology is that it be capable of attaining high selectivity. By “selectivity”, we mean the ability to not only surgically target the desired metal(s) over other metals present in the solution at each stage of the separation process but to do so at high efficiency (i.e., >99%). High selectivity is important because it enables one to remove one metal at a time from a mixture, usually in a column mode. This separated metal is then eluted from the column in pure form. Thus, in an ideal processing scenario, the valuable metals are targeted early and impure metals are removed without delay. In this way, one is not continually faced with the need, downstream, to separate impurity metals from the one of interest, since these are not present.
In the case of the REE, high selectivity is imperative. The goal is the separation of the entire set of 16 REE from the surrounding gangue metals as quickly and efficiently as possible. In this way, the processor graduates from a mixed ore concentrate (akin to a mixed stew) to a suite of pure REE (comparable to a high quality bouillon). With the impurities removed at an early stage, the remaining concentrate is then ready for the selective capture of each individual REE.
An Historical Problem
Traditional methods of separating REE from gangue materials, such as SX and IX, have very low selectivity and are relatively inefficient in the percentage of valuable REE that they retain. In most cases, the retention factor is far less than 99%, and can be as low as 80%. The result is that appreciable quantities of these gangue metals remain in the process flow and follow the REE downstream as retained impurities. As a result, the need to capture these impurities in subsequent steps, where individual REE are being separated from each other, can have a significant impact on capital expense (Capex) and operating expense (Opex) costs, with much waste generation, requiring additional costs for treatment and disposal. Further, most rare earth ores contain thorium and uranium. If these are not separated from the REE in the initial separation stage, they are eventually discharged downstream into the commons, following separation from the target REE. The end result can be severe environmental and human health problems. This has been seen both in China, and in the United States in the 1990s and earlier, at the Mt. Pass REE operation in California .
In 2014, IBC Advanced Technologies, Inc. (IBC) undertook to address this problem. Under the direction of Ucore, Hazen Laboratories in Golden, CO provided IBC with a pregnant leach solution (PLS) prepared from ore taken from the Bokan Dotson-Ridge REE deposit in Southeast Alaska. The objective was for IBC scientific personnel to develop a flow sheet, based on their experience with Molecular Recognition Technology (MRT), for the separation, using a proprietary SuperLig® green chemistry hydrometallurgical process, of individual REE from this PLS at the >99% purity level. The resultant flow sheet was then to be tested, using IBC’s SuperLig® technology (MRT), with the PLS solution to make individual REE separations at the >99% purity level. The MRT process for selective metal separations was developed by IBC over two decades ago and has been used successfully since that time in making commercial separations of numerous metals [8-10].
A clean, initial separation of the set of 16 REE at the >99% level was made from the PLS using SuperLig® technology . Having this pure set of REE greatly simplified subsequent separation of individual REE from each other. The ability to remove impurity metals at this stage was a high priority for IBC, since conventional separation processes largely fail at this important juncture, and are a major contributor to increased Capex, Opex and waste generation. For example, the mining and processing of REE in China exhibit a high level of negative externality, with inherently inefficient and low selectivity separations technology resulting in unmanageable quantities of waste and cleanup expense. In the long term, the costs of remediation are not incorporated into the processing cost. Rather, they are borne by society at large, that has to deal with the resultant pollution and environmental degradation. This is a daunting task for which the accounting is essentially impossible [6,7].
A REE Separation Breakthrough
Through MRT processes, IBC successfully generated a high purity REE mixed concentrate, early in the process cycle and almost completely free of impurity metals. Greater than 99% of the REE were separated from the non-REE. This achievement highlighted a great advantage of MRT over traditional legacy technologies: the ability to remove very nearly all nuisance metals at the start of the process circuit, while recovering essentially all of the REE. The positive economics stemming from this accomplishment are clear, especially when compared with the relatively low recovery rates of legacy technologies, which can be as poor as 80%. With this important benchmark now achieved, IBC researchers were at liberty to focus on an objective that had long proven elusive within the metallurgy community: The separation of the individual REE from each other, without the use of potentially costly and environmentally invasive methods such as IX and SX.
As the final outcome of the Bokan ore study, IBC was able to accomplish this important objective. Our researchers elected to first separate Ce and Sc from the set of 16 REE, using customized SuperLig® procedures. The residual REE were then separated into a light plus yttrium (La, Pr, Nd + Y) group, and a heavy + samarium (Sm-Lu) group, followed by individual selective separations of the REE in each group. With the end goal thus achieved, each rare earth metal was precipitated as a high purity carbonate salt.
To verify the results, two separate analyses were made of REE purity. First, at IBC, solutions containing the separated, individual REE were analyzed using an inductively coupled plasma (ICP) mass spectrometry procedure. All purity levels were >99%. Second, Ucore contracted with an independent analytical laboratory to determine purity levels of the individual REE carbonate salts. All purity levels were >99%. These results confirmed the separation of the individual REE from each other at the target >99% level.
The REE were eluted from the SuperLig® columns with small quantities of acid, producing concentrated solutions of the pure metal for easy and economical salt production that is compatible with the Ucore flow sheet. Precipitation of REE carbonate salts requires minimal reagents and no heating. If desired, these salts can easily be converted to rare earth oxides by heating.
Successful completion of the project was announced in a press release on March 2, 2015 . The announcement represented the separation of each individual REE at >99% purity (with the exception of Sm and Gd, which had been separated as a pair). A second press release dated April 28, 2015  announced the successful separation of Sm and Gd from the combined pair at the >99% level. Photographs of the individual carbonate salts were published on March 2 by way of the Ucore web site .
MRT & the Future of REE Processing
With the increasing profile of REE in the media and investment communities in recent years, numerous methods have been proposed for the separation of the individual lanthanides. Remarkably, outside of the separations recently achieved by IBC using MRT, to our knowledge, there has been no evidence presented or reported that any of these methods has been successful in obtaining the complete recovery of the REE and the subsequent separation of individual REE, derived from PLS, at high purity. We note that non-MRT technologies generally rely upon separation processes of low selectivity. These processes, based on SX, IX, electrophoresis, and chromatography, are inherently problematic, due to their low selectivity and their requirement of multiple separation stages to produce highly purified metals.
Beyond the obvious benefits of selectivity, MRT is noteworthy for the simplicity of its process architecture and its ease of automation. The result is a remarkable decrease in Capex, Opex, processing time, and environmental footprint associated with REE refining operations. This augmented simplicity follows from the high metal selectivity designed into the MRT system. Labor, space, and material costs are lowered relative to conventional systems, since waste generation is minimized and many fewer separation stages are required. No flammable solvents are utilized. Thus, there is no need for their accommodation and eventual disposal as hazardous materials. Further, the ability of the MRT process to separate and recover metals at the mg/L and lower concentration levels significantly reduces waste and permitting issues.
For the interested reader, SuperLig® technology (MRT) has been described in recent articles authored by me and my associates [7,8]. We welcome feedback and insight from you, as subscribers to InvestorIntel. For now, our near term mission is a clear one: the design and construction of a pilot plant to further demonstrate separations of the individual REE at process scale.
- Van Gosen, B.S., Verplanck, P.L., Long, K.R., Gambogi, Joseph, and Seal, R.R., II, 2014, The rare-earth elements—Vital to modern technologies and lifestyles: U.S. Geological Survey Fact Sheet 2014–3078, 4 p., click here
- Graedel, T. E.; Harper, E. M.; Nassar, N. T.; Reck, B. K. (2015). On the materials basis of modern society, Natl. Acad. Sci. USA 112, 4257-4262.
- Reck, B. K.; Graedel, T. E. (2012). Challenges in metal recycling, Science, 337, 690-695.
- European Critical Raw Materials Review, European Commission, Memo, Press Release Database Brussels, Belgium, 26 May 2014
- Pradip; Fuerstenau, D.W. (2013). Design and development of novel flotation reagents for the beneficiation of Mountain Pass rare-earth ore, Minerals & Metallurgical Processing, 30, 1-9.
- Yang, X. J.; Lin, A.; Li, X.-L.; Wu, Y.; Zhou, W.; Chen, C. Q. (2013). China’s ion-adsorption rare earth resources, mining consequences and preservation, Environmental Development, 8, 131-136.
- Izatt, R. M., Izatt, S. R., Bruening, R. L., Izatt, N. E., Moyer, B. A. (2014). Challenges to achievement of metal sustainability in our high-tech society. Chemical Society Reviews, 43, 2451-2475.
- Izatt, R. M., Izatt, S. R., Izatt, N. E., Krakowiak, K. E., Bruening, R. L., Navarro, L; (2015). Industrial applications of Molecular Recognition Technology to green chemistry separations of platinum group metals and selective removal of metal impurities from process streams. Green Chemistry, 17, 2236-2245.
- Izatt, S. R., Bruening, R. L., Izatt, N. E. (2012). Status of metal separation and recovery in the mining industry. Journal of Metals, 64, 1279-1284.
- Izatt, N. E., Bruening, R. L., Krakowiak, K. E., Izatt, S. R. (2000). Contributions of Professor Reed M. Izatt to Molecular Recognition Technology: From laboratory to commercial application. Industrial & Engineering Chemistry Research, 39, 3405-3411.
- Press release, Mar 2, 2015. Molecular Recognition Technology: Environmentally Friendly, Cost Efficient Separation of Each Individual Rare Earth Element
- Press release, April 28, 2015. Ucore Updates on the Separation of Individual Rare Earth Elements
- Molecular Recognition Technology: Environmentally Friendly, Cost Efficient Separation of Each Individual Rare Earth Element
Reed M. Izatt received a BS degree in Chemistry from Utah State University (1951) and a PhD degree in Chemistry with an Earth Sciences minor ... <Read more about Reed M. Izatt, PhD>