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Rare Earths and their technical challenge. – part 1

The buoyant market for rare earth is the result of a succession of decision and announcement from China that suggests a potential shortage in Japan and the West. As a reminder, during this year, China has significantly reduced its export quotas and has in this way clearly indicated its strategy to protect and promote its domestic market. Rare earths outside China have become so coveted by Western and Japanese industry. This new need for securing the supply source of raw material has had an immediate effect on the mining sector and specifically on advanced projects from Lynas, Molycorp, Alkane Resources, Arafura Resources or Great Western Minerals. 
This sudden revival of mining for rare earths, forgotten for decades, has seen the emergence of a multitude of exploration projects. Some junior companies have also understood that the marketing effect that produces the rare earths could be salvage for their financing. In this context it is important to understand the peculiarities and constraints that generate the potential deposits of rare earths.

>REE deposits are not comparable to precious or base metals deposits


The rare earth deposits are in no way comparable with the deposits of gold, base metal or platinum. Mineralogical and chemical aspects of rare earth deposits are very special and mean that research in this area are extremely specialized. The reason is that the lanthanides elements (REE’s) have similar radii and oxidation states, which allow for number of substitution into various crystal lattices causing a wide dispersion in the earth’s crust and generating multiple occurrences of REE’s.
We know that Diamonds are found in kimberlites, whereas REE’s are found in a large variety of rock forming types such as carbonitites, laterites, alkali rocks, hydrothermal complex and even in specific clays. Furthermore, each of these rocks is formed with a large diversity of mineral combinations, in which several lanthanide elements can be included without knowing which one might be extractable. 


Within the fifteen chemical elements of the lanthanides (Atomic number 57 to 71), we differentiate the light Rare earths (LREE: atomic number 57 to 62) and the heavies (HREE: atomic number 63 to 71). 
 These categories are defined based on their ionic radii and their atomic number, which is known as the 'lanthanide contraction'. This makes the chemistry of lanthanides predominantly ionic and the HREE have a higher affinity to Yttrium, being chemically associated. The rarity of HREE is not explained by their content in rocks but by the difficulty to separate them from Yttrium and to finely get the desired oxyde powder.


Today, there is not a favourite orogenic model, which provides the guarantee to get an economic REE deposit, however some preferred geological targets are worth to be further investigated. 

Iron REE deposits, with the Iron –LREE-Niobium deposit of Bayan Obo in Inner Mongolia which still dominate the worldwide production of Light Rare earths. This group might also be interesting due to its potential of re-treating certain mill tailings of magnetite and hematite deposits.

Carbonatite deposit: a very large number of REE-bearing carbonatite occurrences have been identified however only one is at the stage of production with the Montain pass deposit in California. We believe applied researches in this sector might bring some interesting perspectives due to the amount of academic work made. 

Lateritic deposits formed over carbonatites and alkaline igneous rocks. The weathering process of carbonatite laterites represents an interesting exploration target. Mount Weld deposit in Australia is a supergene REE occurrence. Some others exist around the world, but there is no yet evidence that REE can be extracted. The mineral concentration is one of the key factors to get a REE deposit. Among different types of deposits, the supergene mineralisation (a result of near surface chemical process of enrichment) of REE’s represents a high grade potential of REO. The tropical regions will surely be re-evaluated in that perspective.

The peralkaline igneous Rocks (alkaline rocks with alumina deficiency vs alkali ) are generally low grade but enriched in Yttrium and zirconium. However today the massif of Lovozero in the Kola Peninsula (Russia), with its REE-bearing loparite (Na-REE-Ti-Nb oxide), might be of a revived interest for HREE. In this group of deposit we find new potential REE-bearing minerals such as eudialyte, gadolinite, britholite.

Concentration type deposits as the placer deposit (heavy sands) or paleo-placers consist mainly of titanium mineral placer with by-product such as zircon and monazite. However the radioactive content from Thorium can negatively impact the recovery of REE’s.

Other deposits which involved REE substitution, REE as by-products or REE in phosphorites represent also potential economic targets. The allanite and apatite type deposit appear as economically challenging due to the refractory1 nature of minerals and their low quantities of Y-REE.


1 refractory minerals are those which require specific physical-chemical treatments to crack them. Refractory nature of minerals comes from their crystallisation during particular Pressure-Temperature conditions.


The discovery of an economic deposit of HREE (Heavy Rare Earth Elements) deposit outside of China represents today the main challenge for exploration companies and for metallurgists. In addition the viability of a deposit requires:


1. favourable mineralogy (lanthanide distribution and liberation classes)
2. realistic grade and tonnage according to an extractable REE basket
3. cost effective mining and processing methods
4. values of thorium, uranium and other impurities staying below the pollution threshold or managed to minimize the environment impact.


>Grade of a REE rock bearing has to be taken with caution, as they can be locked in minerals

 


With diamonds, the size frequency distributions (SFDs) tells you about continuity across diamond sizes, with gold mineralisation, the grade coupled with conventional mineralogical analysis and with the know-how of standardized processing methods provide good projections about the economic potential. But with REE’s the commonly used TREO2(Total rare Earths oxydes) value relates to a basket of elements and it will never tell you about the economic potential of a deposit. Many rock forming minerals and accessory minerals can participate to the TREO value and thus the REE’s can be widely disperced through a variety of minerals rather than being concentrated in an ore mineral. The complexity with REE stands in the need to fully understand the microscale before to get the macroscale figures such as the project economics.

To illustrate the issue with TREO, Dr Anton Chakhmouradian, an expert of alkaline and carbonititic systems at the University of Manitoba, explained in an interview made by Clint Cox,  that 50% of carbonites contain over 0.3% of  REE, however the issue is to find a single REE mineral, which could include REE at an economic viable level. He also added that over 60% of of all proven REE reserves in Russia are ‘’locked’’ in apatite in the huge alkaline intrusions at Kola pinninsula, which contains up to 8% TREO. But none of those REE’s are extracted, even from the Khibiny apatite, mined for phosphate.

2 Total Rare Earths oxides: it includes all rare earths oxide content identified in a rock sample.


>The extraction from an economic REE resource is highly dependent on its REE mineralogy.



Even though the TREO content is attractive, it is required to engage a detailed mineralogical study to evaluate the basket of lanthanide elements and to locate them across the assemblage of mineral phases. Such preliminary analysis is a key point in the project assessment and for the amenability of a basket of elements to a mining and mineral processing.A limited number of height productive and potentially productive mines for REE are active worldwide. Among about ten commercially potential REE-minerals, today, the major rare earth sources are represented by the following minerals:


 Bastnäsite, a fluorocarbonate found in carbonatites,
 Monazite, a REE-phosphate, found in hydrothermal complex or in beach sands, 
 Xénotime, an Yttrium phosphate, also found in hydrothermal complex or in beach sands, 
 Loparite, a titanate of the perovskite group, which is the principal source of rare metals for the Russian industry and is found in Alkaline igneous massif,
 Clays of South China, REE and Yttrium clays, which are formed by ion adsorption. It provides the bulk of HREE to the market place.
 and Uraninite from which REE and Yttrium are recovered as by-product during the refining of uranitite. 


Because of the complexity of the mineralogical setting of REE’s, any new exploration projects worldwide tend to show specific characteristics, which make them unique from each other. With the rising number of projects coming up, their comparisons might pinpoints useful similarities.
Thanks to modern techniques, such as QEMSCANTM technology (Quantitative Evaluation of Materials by Scanning Electron Microscopy) and Electron Microprobe Analysis, REE mineralisation in a representative rock sample can be investigated for the distribution of the REE, the mineral abundance by size, and also to determine the locking and association characteristics of the Rare Earth Element and thorium-bearing minerals.  

On a microscale, the potential of a REE deposit will depend on the following questions:


In which minerals are trapped the REE's?


After a modal mineralogical study using qualitative X-ray diffraction, the Electron Microprobe Analysis (EMPA) allows to determine the mineral chemistry and to better understand the distribution of Thorium and the rare earth elements. 

Example of a normalized distribution of REE's in identified mineral phases.
Image: Example of a normalized distribution of REE's in identified mineral phases.


How REE minerals are trapped?


Scanning Electron Microscope equipped with an Energy Dispersive Spectrometer (SEM-EDS) can be used to obtain textural information of certain mineral phases.
  
Example of SEM-BSE Image with Corresponding Spectra Showing Fergusonite

 Image: Example of SEM-BSE Image with Corresponding Spectra Showing Fergusonite


How much of specific REE are present in each mineral phase for different fraction size?


This specific study also named 'Elemental Deportment' help to evaluate the mass content of the most popular REE in each of the mineral phase identified and according to different fraction size. It is generally calculated using the mineral abundance from QEMCSANTM analysis and average compositions from the EMPA.

Example of Elemental Deportment for Neodymium (Mass % Nd)


Image: Example of Elemental Deportment for Neodymium (Mass % Nd)


How much of a mineral can liberated and what are their association to mineral phases?


Certainly the key question about evaluating the potential of the REE basket present in a deposit and its radioactive risk. It will also give basic and semi-quantitative appreciations for preparing a scoping study. For each of the mineral phases QUEMSCANTM analysis quantifies the normalised mass of preferred minerals according to different fraction size.

The final interpretation aims to determine the best candidate (mineral phase) which might be extractable without liberating Thorium. As you can easily suspect the grain size play a significant role and REE liberated from a coarse fraction would be preferred, as it might avoid liberating Thorium locked into finer fraction size of a mineral phase. It is also a way to evaluate the mineral association which can orientate the chemical processing.


 >Knowledge status: a new start boosted by the old mentors


When China took its leadership role in the market for rare earth oxide, the knowledge has migrated to China, leaving the specialized geologists and mineralogists to fall back on other research or teaching. Thus knowledge is somehow remained in an early state to explain all the specifics of an economic rare earth deposit. But now with the revitalization of the sector and the availability of new analytical techniques, hopes of improving understanding of deposits and targeting new economic resources are real. The research will be oriented toward understanding the chemical factors of variations in rare earth minerals in order to change the petrogenetic model. 

According to Dr Anton Chakhmouradian, an expert in REE, …’’REE’s are to us now like aluminium was to people of late 1800’s. Few people realized the real value of aluminium at that time, but can you imagine the aerospace, automobile and even the packaging industries of today without this metal?’’

Today many key issues of REE geochemistry and mineralisation remain unsolved. Few experts are sharing the knowledge and the expertise of REE geochemistry. These experts are also sharing their advisory between multiple exploration and mining companies, as many exploration geologist like to find the implementable answers to their questions.


>No unique chemical Processing standard exist to precipitate REE’s


The processing techniques are limited to the small number of producing mines. Mountain Pass, Maoniuping and Bayan Obo have established physical and chemical processing plants. 
The Mt Weld project in Australia (Lynas) is progressing with building the concentration plant on site, and with the material plant in Malaysia, where chemical processing will use a sulphuric acid digestion and water leaching followed by selective dilution and precipitation. The estimated operational expense is largely dominated by the chemical processing and the reagent and energy cost.
At the Dubbo zirconia project in Australia (Alkane Resources), a heavy rare earth circuit have been recently added to the demonstration plant, which has been operating since 2008.  A development decision is expected in late 2010.


>Conclusion


The REE mining industry implies major technical requirements link to extractability of a basket of REE and its environmental impact, but also to the involvement of a high degree of expertise to evaluate the amenability of mining and processing a REE deposit. The evaluation starts at an early stage, when studying the microscale mineral phases, which fully participate to assess the resource potential. The reviving of the sector will have very positive impact on the applied research in geochemistry, mineralogy and metallurgy.
With that in mind investors should be aware of the risk that many REE-exploration companies will never be able to produce any REE-oxides.

LK.