Mineral Resource estimation of rare earth elements
Dean O’Keefe FAusIMM, Principal Advisor, MEC Mining
Close up of europium.
JORC Code compliant Mineral Resource estimation (MRE) of rare earth elements (REEs) follows similar criteria as estimation for other commodities like precious metals. This article outlines key factors and considerations when conducting an MRE for REEs.
There are 15 REEs in the lanthanide series; additionally, yttrium and scandium are often grouped with the lanthanide REEs. Scandium and yttrium are included because they share Group IIIA in the Periodic Table with lanthanum (atomic number 57) and have comparable properties. Among the 15 lanthanoids, promethium (Pm) does not occur in nature. Therefore, the REE series contains 16 elements. These are scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) (McNulty, Hazen, and Park, 2022).
There are more than 200 minerals that contain REEs (e.g. monazite, xenotime, bastnäsite, apatite, zircon, etc). REE-rich mineral species are widely distributed across all geological contexts. The main types of REE deposits are REEs associated with carbonatites and alkaline igneous rocks, heavy mineral sands (e.g. monazite-xenotime), and supergene ion-adsorption clay deposits. Some of the most economic REE-rich minerals are monazite and bastnäsite. Monazite is a REE phosphate whereas Bastnäsite is a fluorocarbonate (McNulty, Hazen, and Park, 2022).
According to Dr Ignacio Gonzalez-Alvarez, Principal Geochemist at CSIRO: “In this new context of REE exploration, REE ion adsorption clay deposits (REE IADs) have emerged as potential economic low-cost and low-environmental impact sources of HREEs. The REE IADs are believed to originate from intense lateritic weathering. During this process, chemical reactions release and mobilise REE into surface water. In specific geological contexts, these mobilised REEs can selectively be adsorbed onto clay minerals, forming REE IADs. Many potential REE IADs have been reported in South Australia and Western Australia in the last two years.”
China had been producing most of the REE minerals in the world. However, geopolitical divisions and a supply rift, where in 2010 China cut its production quota for REE concentrates by 25 per cent and its export quotas for various REE products by 37 per cent, led to a significant rise in REE prices and created a new high-risk environment for the REE supply chain. For example, the Chinese-export price of neodymium oxide increased from approximately $25/kg in early 2010 to a peak value of $340/kg in July 2011.
Further geopolitical divisions since the 2010 Chinese quota cuts have resulted in a geopolitically polarised world. In 2022, Australia joined the Minerals Security Partnership along with the United States, Canada, Finland, France, Germany, Japan, the Republic of Korea, Sweden, the United Kingdom, and the European Commission. According to the Hon Madeleine King, MP, Minister for Resources and Minister for Northern Australia, “The partnership seeks to catalyse public and private investment for mining, processing and recycling projects that adhere to the highest environmental, social and governance (ESG) standards”.
Out of the 16 REEs, neodymium and samarium can be used to create resilient magnets that withstand high temperatures, making them perfect for mission-critical electronic and defence applications. Neodymium and samarium are impervious to extreme temperatures and are used in fighter jet fin actuators, missile guidance, control systems, aircraft and tank motors, satellite communications and radar and sonar systems (Neumann, 2022). The greatest demand for REE is for both military applications and clean energy applications.
REE have been designated Critical & Strategic metals. Critical minerals are defined as being fundamental to high-technology and green-technology with no viable substitute, and that face potential for disruption to supply. REE are Critical Minerals listed on the registers of USA, Australia, the EU, Japan, Canada, and China; only antimony, chromium, cobalt, lithium, and tungsten are common to the critical minerals register of all of these countries in addition to the REE. Given the government support and the instruction to regulators to fast-track development of projects in certain jurisdictions, such as security pact countries, then provided that the REE project satisfies the RPEEE conditions, it may be argued that the downstream processing options or supply of concentrate for processing, are now far more likely to be satisfied eventually.
The typical MRE criteria to consider are outlined in Table 1 of the JORC Code (2012), which sets out the minimum standards for public reporting. Since there are 16 REEs, there are variations to the approach required to meet these standards for reporting the MRE.
When performing QA/QC, this will include the usual consideration of blanks, duplicates, assay precision, assay bias, certified reference materials (CRM) results, etc. The CRM or standards are inserted to check that the analysis is reproducing known values. A number of different CRM for REEs are available and should be selected based on the dominant REE credits. The full range of REEs will be contained within the CRM, and additional elements such as zirconium, niobium, and tantalum. Results may be checked on a batch basis to determine if a batch may require reanalysis should there be significant departure from the known result, and also checked to calculate the total percentage average departure from expected value.
For sample preparation, sample fragments are reduced using a jaw crusher or a combination of jaw crusher and cone-crusher. A subsample of the crushed material is usually obtained by riffle splitting. The subsample is ground to an agreed specification such as 90 per cent passing 150 mesh (100 micron) by means of a pulveriser. An example of analysis is the ALS methodology of Li-fusion, where the samples are fused by sodium peroxide in a muffle furnace, leaching the melt with hydrochloric acid, transferring the solution to a volumetric flask, diluting to the mark, shaking well, and then reading on inductively coupled plasma mass spectrometry equipment. This type of analysis produces an REE metal value only.
Both the assay results and the geology should be considered to define the domains to be modelled and then used for the MRE. The assay results are values that may be presented as elemental values or oxides.
The metal values are used for the variography studies and the subsequent interpolation. For variography, it is often unrealistic to model experimental variograms for multiple elements, particularly where some elements may have low grade but contribute nonetheless to the Total REO% (Total REE content presented as the sum of all REEs in oxide form wt%). An approach to avoid spending excessive time and effort on the variogram modelling would be to select the most dominant credit elements, for example dysprosium, and to then model the experimental variogram for dysprosium and then to default this variogram to other elements, such as other heavy REEs. One or more elements may be used for the grouping, the dominant light REE may be grouped with other light REEs. Should an interpolation method such as Ordinary Block Kriging be used, then top or balancing cuts should be applied to mitigate the influence of extreme values (should they be present).
One of the more important aspects of a JORC Code compliant MRE is consideration of the Reasonable Prospects of Eventual Economic Extraction (RPEEE) hurdle. For REE this is a complex issue; historically there have been few processing hubs and there have been major environmental issues with processing. REE deposits contain thorium and uranium, with lesser concentrations in ionic clays; these radioactive elements may pose a health hazard and disposal problems.
At the conclusion of the interpolation into the Ore Block Model (OBM), the REE grades require conversion from the elemental to the oxide value. For example the element Cerium (Ce) would require an adjustment factor of 1.23 to convert to CeO2. All elements will require conversion to oxide values.
Typically, the MRE will report on Total Rare Earth Oxides (TREO%), Critical Rare Earth Oxides (CRE0%), and the dominant individual credit REO. The MRE may also state the Light REE and the Heavy REE. An economic cut-off grade may be calculated, or a peer project cut-off grade may be used, but this cut-off will need to be carefully selected and justified, to satisfy the RPEEE.
A critical factor for a rare earth project is the understanding of the REE-rich mineralogy. REE-mineralogy dictates the economic viability of the project. Mineralogical categorisation and metallurgical test work for rare earth samples is fundamental to evaluate the REE recovery during mineral processing to produce an upgraded mineral concentrate for chemical treatment. It is important that off the shelf techniques exist for extraction of the REE to again satisfy the RPEEE criteria. As just one example, off-the-shelf processing technology exists for Bastnäsite.
Here is an example of a ‘mine to magnet’ process for an REE: (1) mining of ore; (2) processing to obtain a concentrate; (3) cracking for the intermediate product; (4) separating for the oxide; (5) refining to obtain the metal; and then finally (6) alloying.
The extraction of REE usually involves the dissolution of the ore using acidic or alkaline solutions depending on the mineralogy of the REE-containing phases and reactivity of gangue phases. Typically, the use of acidic solutions is more common.
Depending on mineralogy, the extraction step often involves roasting of the REE ore at 400–500 °C in concentrated sulphuric acid to remove fluoride and CO2, and to change the mineral phase to make it more water-soluble. Generally, separation techniques such as solvent extraction, ion exchange, and precipitation are often used for the recovery of REE from pregnant leach solutions (PLS) obtained from acid or alkali leaching. Solvent extraction is generally accepted as the most appropriate commercial technology for separating REE due to the need to be able to handle larger volumes (Balaram, 2019).
Factors to consider for MRE classification are the underlying QA/QC, the drillhole spacing, and variance. Rather than linking the categories directly to distance, the variogram model of grade variability can be used to help define the Mineral Resource classification. Values on the sills of the directional variograms may be used to determine ranges to separate various Mineral Resource categories. The variogram ranges may then be used to select distances for the search ellipses for categorisation.
Consideration of the modifying factors, including ESG, processing, etc, that allow the conversion of Measured Mineral Resources and Indicated Mineral Resources, to Probable Reserves and Proved Reserves following conclusion of a Prefeasibity or Feasibility Study, is the ultimate application of the MRE.
In summary, the MRE for REE may require conversion of element values to oxide values; domaining based on grade and geology; grouping of elements for variography based on dominant credit elements; classification of Mineral Resources based on QA/QC and/or underlying data, and variance; statement of REE as TREO%, CREO%, HREO%, LREO%, and the dominant credit elements such as dysprosium, Dy%. All JORC Code compliant MRE state that the hurdle of RPEEE can be resolved. The RPEEE criteria should be applied to all Mineral Resources, regardless of Mineral Resource category.
MEC Mining greatly appreciates the review insights provided by Dr Ignacio Gonzalez-Alvarez, Principal Geochemist, CSIRO (https://people.csiro.au/g/i/ignacio-gonzalez-alvarez).
About the author
Dean is a geologist and geostatistician and is Principal Advisor at MEC Mining in Western Australia. Dean has both managed and completed JORC and NI43-101 compliant MRE studies over a period of 34 years for submission to the Australian, Canadian, Hong Kong, and London markets. Dean is a Fellow of the AusIMM and a qualified Quarry Manager with ten years opencut experience and more than ten years management of a Global Consulting group based in Beijing, P.R China. Dean has signed off as CP and QP for various mineralisation styles, for commodities including precious metals, PGE, base metals, ferrous metals, and REE.
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The Hon Madeleine King, MP, Minister for Resources and Minister for Northern Australia, 2022 [Online]. Available from: https://www.minister.industry.gov.au/ministers/king/media-releases/australia-joins-global-minerals-security-partnership
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Neumann N, 2022. ‘Securing the rare earth supply chain is crucial for defence’, Army Technology [Online]. Available from: https://www.army-technology.com/features/securing-the-rare-earth-supply-chain-is-crucial-for-defence/