Rare Earth Minerals and Metals Processing R&D
Each rare earth deposit is unique and consists of different ore bodies containing several rare earth elements in varying proportions. Consequently, processes uniquely suitable for separating the minerals from each ore deposit have to be developed. Sometimes there are other minerals found in the ore that carry other metals of economic values. These minerals also need to be separated. Extensive analytical and mineralogical characterization of ores is required to design environmentally sound physical and chemical separation or enrichment techniques. These enrichment techniques will upgrade the low concentration of the metals (e.g. 2% in the ore) to a higher concentration (e.g. 40% in the concentrate). The initial step is to crush and grind the ore to a very fine particle size to disengage the minerals containing rare earth elements from other minerals and the waste (host) rocks. The next step is to separate those minerals that contain rare earth metals selectively from the others as efficiently as possible. This initial separation technique is based on the differences in physical, chemical and mineralogical characteristics of each mineral and host rock from which the minerals have to be separated. If there is no sufficient difference in magnetic or density or mineral surface chemistry between the minerals and the waste rocks, then the separation process can be very inefficient. The concentrates so produced are further subjected to chemical dissolution. It is from this solution that mixed rare earth compounds are extracted and subsequently refined to produce high purity metals. A comprehensive research is required to enhance and exploit the physical and chemical differences that exist between minerals and the differences that exist between metals to engineer efficient production processes. In recognition of that, a multidisciplinary research on characterization, physical separation, hydrometallurgy and environmental aspects of processing various rare earth ores has been initiated at NRCAN (CanmetMINING).
1. Mineralogical Characterization
Carbonatite-hosted REE ores such as the Bayan Obo and Mountain Pass (USA) are highly enriched in light REE (LREE) and there is some consensus that the present reserves of specific REE, in particular the heavy (HREE), will become insufficient for the increased demand in the future. At the moment, the HREE world supply is highly dependent on the “ion-adsorption clay” deposits of Southern China. There are presently significant exploration activities in Canada for REE deposits, with significant emphasis on the HREE. To ensure sufficient supply of these critical metals within Canada, the support of research activities focusing on REE processing to ensure the creation of an economically-viable and environmentally-responsible REE mining industry is a priority of Natural Resources Canada. Considering the structural, morphological and compositional diversity of REE minerals, and the complexity of natural (e.g. weathering) and engineered (e.g. physical concentration, leaching) REE fractionation and enrichment mechanisms, mineralogical characterization is a key component at all steps ranging from exploration to production. Detailed characterization of the chemical, structural, textural, morphological, and surface properties of REE minerals within a deposit and a thorough understanding of the evolution of these properties are the basis for the identification of efficient concentration and extraction processes. The objective of the characterization project is to provide such critical information for important REE deposit types. It is achieved through the development of robust mineralogical characterization approaches that will take advantage of the respective strengths of advanced micro analytical techniques.
2. Physical Separation
There are over 170 minerals known to contain rare earth metals (REM). However, there are only about 60 of them containing these elements in significant amounts. To date, there are only three of these minerals, namely bastnaesite (Ce,La,Y)CO3F, monazite (Ce,La,Nd,Th)PO4 and xenotime (Y(PO4)), which have been the real sources for the current supplies of REM besides the clay-adsorbed types. The minerals containing REMs are extensively dispersed and intergrown with several oxide, carbonate and phosphate minerals. Consequently they are very difficult to liberate at coarse grinding and are thus difficult to separate without special methods and reagents to obtain high grade rare earth mineral concentrates. Although the REMs in the same mineralogical class have similar behaviors, the minerals in which they occur vary in their chemical and mineralogical nature. Therefore, unlike base metal sulphide minerals, there are no reagents that may be applied across the board for efficient flotation. Due to the relatively low volume of annual production, the chemical industry didn’t invest in the development of specific collectors (chemicals) for flotation of individual RE minerals. The relatively low metal recovery and concentrate grade during processing may be attributed to the lack of selectivity of the reagents (chemicals) used in flotation, which are not synthesized particularly for RE minerals. The overall processing challenges and issues to be resolved through systematic research and innovations may be summarized as follows:
- lack of mineral specific collectors and or depressants to selectively separate minerals from ores into individual concentrates and tails
- lack of sufficient understanding of individual RE mineral physical and chemical behaviours (responses) to develop efficient processing flowsheets
- achieving high separation efficiencies with low reagent, water and energy consumptions
- Producing high grade concentrates to reduce high consumption of acids and hydroxides required to dissolve all RE elements before extraction.
3. Hydrometallurgy
Bastnasite, monazite, and xenotime are the three minerals known to contain REE in sufficient quantity. Most concentrates produced by the physical separation processes such as flotation, magnetic separation, gravity or their combinations contain these minerals and other minor minerals also containing REE. These primary REE minerals are the major REE resources in North America. Secondary REE resources have also been discovered in Canada and the elements in these deposits occur in the form of absorbed metal ions on clays or shales. The hydrometallurgical extraction technologies applied to the mineral concentrates are different from those applied to the adsorbed ion types. Metals separation from the concentrates involves an initial process of chemically dissolving the minerals. Cracking is a complete chemical dissolution of the REE minerals. Cracking may require roasting for effective chemical attack. Inadvertently, other rock minerals that were not successfully separated at the physical concentration step will also dissolve contributing to increased chemical consumptions and complication of the metal extraction processes. The amount of chemicals required depends on the purity of the concentrate. Because each concentrate produced from each ore deposit is unique the chemical separation process and its complexity is also unique. The solution that contains the dissolved metals is further manipulated to yield mixed but simpler rare earth compounds. The key processing techniques are precipitation, solvent extraction and ion exchange. The mixed rare earth compounds are further refined to produce separate metals with more than 99.9% purity depending on applications. It is for this reason that a wealth of knowledge has to be generated through extensive research to produce these uniquely strategic metals. The project will provide technical and scientific support to Canadian companies in the business of developing deposits for the production of these strategic metals.
4. Environment
There are two aspects of environmental issues related to REM productions. Many primary REM deposits contain significant amount of radioactive metals, mainly thorium and uranium in lesser extent. The second aspect is the toxicity of the individual rare earth elements. This project deals with the latter issue.
In aquatic toxicity testing of metals, the behaviour of the free divalent metal cation - the species of metal largely considered to be the most toxic - is well understood, particularly with respect to binding with natural organic compounds. These natural organic compounds are very important in determining toxicity to aquatic life because they bind to metal cations and render them unavailable, i.e. non-toxic. However, the same cannot be said for rare earth elements. The toxicity and behaviour of rare earth elements (REE) in natural environments has not been studied to any great extent, and given the trivalent charge of many REE, it is largely unknown how toxic they are to aquatic species, and how they interact with natural organic compounds.
Other data gaps relevant to this work pertain to the impact of REE mining in the North. Of the available information in the literature, none examines the effect of REE on Northern species. A recent literature review of the aquatic toxicity of REE by a research group at Wilfrid Laurier University identified some other critical data gaps, including the effect of manipulating water chemistry in REE exposures. Our goal is to address some of these data gaps discussed above to further understand the behaviour of REE in the environment. Our goal is to determine the effects of rare earth elements to Northern aquatic species. The overall objective of this project is to provide an ‘end of pipe’ view of REE mining by investigating potential toxicity of ‘effluent’ produced by leaching REE ores to aquatic species. The impact of altering water chemistry on REE toxicity to determine effect of water pH, hardness and/or concentration of dissolved organic matter will be investigated. These metals are considered to be ‘data-poor’ and any knowledge gained from this work on their behaviour in the environment will help fill critical data gaps. With interest in REE mining being developed in Canada, these data will help identify potential risks and environmental issues.