Rare earth elements: deposits, uncertainties and wasted opportunities
Rare earths are more abundant than their name suggests. The real issues that need addressing, say researchers from Monash University in Melbourne, Australia, are extraction, processing and the subsequent environmental impacts.
From palm-sized smartphones to gargantuan wind turbines, recent advances in the technological uses of rare earth elements (REEs) has led to a significant increase in demand for these metals. Denoting the lanthanide element series plus scandium (Sc) and yttrium (Y), REEs are generally split into the light and the heavy rare earth elements, or LREEs and HREEs (see table below).
However, the word rare in REE is something of a misnomer. Several are not rare and are in fact, on average, present in Earth’s crust at higher concentrations than other metals not generally considered rare. A good example is cerium (Ce), which has an average continental crustal abundance of 63 parts per million (ppm) – higher than that of other economically important metals, such as copper (47ppm), lead (17ppm) and nickel (28ppm).
REEs are enriched within a wide variety of different mineral deposits, although only some deposit types are currently exploited for these elements, either as a target commodity or as a by-product of other metal production. These include:
- carbonatite (for example, Bayan Obo in China and Mount Weld in Australia)
- alkaline magmatic intrusion (for example, Kvanefjeld in Greenland)
- REE-bearing clays (for example, Long Nan and Yian Xi, both in China)
- pegmatites (for example, Strange Lake in Canada)
- iron oxide-copper gold (IOCG – for example, Olympic Dam in Australia)
- granite-related (for example, Nolan’s Bore in Australia)
- placer and heavy mineral (for example, WIM Group in Victoria, Australia)
- shale (for example, Talvivaara in Finland and Buckton in Canada)
- quartz-pebble conglomerate uranium deposits (for example, Eco-Ridge in Canada)
Each deposit type contains varying abundances of REEs, and may have resources that are dominated by the LREE, HREE or a combination of both (although the majority of known deposits contain significantly larger amounts of LREEs than HREEs). The various REE deposits vary widely in terms of size, ore grade and LREE/ HREE ratio. Ore grades can differ as much as 1–3 orders of magnitude. At the bottom end of the scale is the low-grade Round Top project in the USA, where REE concentrations are often only marginally higher than average crustal abundances.
However, considering the recent increased interest in REEs, there are still a number of significant uncertainties surrounding their resources and production. The Bayan Obo deposit in China (currently the world’s most important source of REEs) is a good example of this – although some statistics on the resources and production are available, they vary significantly. For instance, while one source reports that the Bayan Obo deposit has reserves of 600Mt ore at 5% rare earth oxides (REO), another reports resources of 1,460Mt ore at 3.9% REO. Such wide variation in data is just one of the major uncertainties that arises when attempting to assess available global REE resources.
One of the main issues with REE supply is processing. High-tech end uses mean that the majority of demand is for high-purity single REEs, hence processing of ores does not simply mean concentrating ore minerals (as is the case for base metals such as copper, nickel, lead and zinc), but instead selectively removing these elements from their host minerals and concentrating each one individually. In addition, REEs are hosted by a variety of different minerals that are difficult to process, in sharp contrast with base and precious metal resources, which are generally hosted by one or two easily processed minerals in a given deposit. The sheer variety of REE minerals (allanite, apatite, bastnäsite, brannerite, eudialyte, monazite, gadolinite and xenotime, to name just a few) means that REE processing and extraction is both difficult and time-consuming. Furthermore, the REEs’ similar chemical behaviour makes isolating individual elements problematic, resulting in mineral processing techniques that are both energy and chemical intensive.
Such difficult and expensive extraction and processing has led to significant wastage of potential resources in today’s minerals industry. This is exemplified by the world-class Olympic Dam Cu-Au-U IOCG deposit in South Australia. Geoscience Australia estimates that this deposit contains around 53Mt of REO, none of which are currently extracted but are worth more than one trillion Australian dollars. It is not yet clear whether it will be possible to extract the huge REE resource at Olympic Dam via extraction from tailings and by-products of mineral processing, or if these resources will be permanently rendered inaccessible and unprocessable.
The importance of other elements in making a REE resource profitable is highlighted by the Bayan Obo deposit. Very few people realise that it is actually an iron mine, with REEs merely produced as a by-product – albeit a very important one. Another tailings site is Australia’s old Mary Kathleen uranium mine. Here, some 0.2Mt REO remains in the approximate 9Mt of tailings, while only 9kt triuranium octoxide (U3O8) was extracted during mining.
The other significant uncertainty on global REE resources is the environmental impact of extraction and processing. The relatively small and somewhat poorly documented nature of global REE production means that little research has focused on their cradle-to-grave environmental impacts – in terms of mine site, processing, production, manufacturing and recycling (or lack thereof).
In addition, REE mineralisation is often associated with enrichments in the radioactive elements uranium and thorium, as well as a wide variety of other harmful elements, which can potentially cause significant environmental and public health problems during processing and waste disposal.
The inherent difficulties in processing REE ores are also illustrated by a 2009 report in the China Daily, which indicated that production of a single tonne of refined REE oxide from Bayan Obo yielded 63,000m3 of harmful sulphur and fluorine-bearing gases, 200m3 of acidic water and 1.4t of radioactive waste (especially thorium-related wastes). It also reported numerous occupational hazards – such as pneumoconiosis (a type of lung disease) and occupational poisoning from lead, mercury, benzene and phosphorous. This is in addition to the pollution resulting from the energy used for extraction and processing. In China, the energy is primarily provided by coal power stations that produce significant amounts of CO2, sulphur dioxide (SO2) and other fine particulates. It is therefore ironic that REEs are used in technologies that reduce and remediate environmental impacts – for example, replacing harmful elements such as cadmium and lead, or increasing battery rechargeability – as well as having more of a direct impact on emissions, for example when used to increase lightbulb efficiency or in renewable energy generation, such as wind turbines.
Although there appears to be abundant potential sources of REEs that can underpin the future of a many environmental and high-tech applications, the real constraints for the industry – constraints that already exist and will only become tighter as the industry expands – are centred around managing its environmental footprint and addressing social concerns regarding these impacts.
This article was co-authored by Simon Jowitt, Zhehan Weng and Gavin Mudd from Monash University in Melbourne, Australia, as part of a PhD project between Monash and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) on the sustainability of the REEs. For more information, contact Simon Jowitt, firstname.lastname@example.org