Not costing the Earth - unconsidered waste materials
Dr Alex Finlay from the Geochemical Reclamation of Industrial Minerals and Elements (GRIME) research group at Durham University, UK, outlines plans for the investigation and development of unconsidered waste materials as a green rare earth element resource.
World demand for rare earth elements (REEs) has been growing, leading to their rating as the fifth most at-risk group by in the British Geological Survey 2011. (Click here to see figure 1, British Geological Survey relative supply risk index, in popup window.) New technologies, including green renewable energy, have created this surge in demand because they rely heavily on the properties of elements such as neodymium and cerium. At the same time China has emerged as the dominant supplier of REEs, producing around 97% of global supply. However in 2010 China’s rare earth export quota was easily outstripped by world demand, meaning that economically developed countries need to find new, accessible sources of these rare earth and strategic elements.
The team at Durham University believes it is on the verge of a paradigm change in delivering REE resources that could help meet rising demand. Importantly, destruction of the environment in the quest for these materials is not being advocated, but rather suggesting a more careful look at REE recycling, a more careful look at REE recycling through reclamation from previously unconsidered waste products is suggested.
Many emerging technologies, particularly those described as ‘green’, use rare earth elements. For example, neodymium and several other REEs make up around 30% of the magnets used in large wind turbines – weighing up to two tonnes. Hybrid and electric vehicles are similarly dependent on REEs with a combination of batteries, braking systems, motors and glass containing approximately 20kg of REEs per car (depending on model and make). Plasma screens use europium and terbium whose spectral properties respectively lead to red and green luminescence. Cerium is used in self-clean ovens and promotes healing of burn victims and even the alloys used in high temperature environments such as jet engines are made more stable with use of rare earth elements.
Given this utility, production of REEs rose by more than 50% between 2000 and 2009. Moreover demand will continue to rise as low-carbon technologies increase in importance. More than 200 REE-bearing minerals have been described. Unfortunately, in most cases the concentrations of these minerals in the host rock are so low that their use is uneconomical, and for others extraction of the rare earth elements is not possible. The main source for rare earth elements is carbonatites – unusual igneous rocks that are more than 50% carbonate minerals (mainly calcite and more rarely dolomite, magnesite and siderite). Secondary sources for rare earth elements include placer deposits where REE minerals are concentrated by the denudation of the primary ores. Only a few of the rare earth element deposits are of sufficient size and concentration to merit economic exploitation.
Bastnäsite and monazite are the most important REE–bearing minerals (containing about 70% rare earth oxides by weight). A few carbonates minerals, for example, parasite and synchysite, and the phosphate xenotime are also important. A number of minerals with much lower quantities of rare earth elements are also economical to mine. These include the oxide loparite that is mined for niobium despite containing only one per cent by weight cerium oxide.
Going to waste
Primary industrial processes generate significant volumes of waste materials throughout the industrial process. The purpose of processing is to concentrate required components from the raw materials. For example retorting – the process of artificially separating oil from oil shale generates hydrocarbons while the residual shale is discarded. Similarly, metal refining concentrates the metal, such as iron, from the ore residual in the form of slag. Clearly, by fractionating one suit of elements into the desired product, another set of elements will be concentrated in the waste stream.
The Durham team’s approach was to examine large-scale industrial waste deposits to identify whether potential valuable materials were being overlooked. The fact that investigations have been concentrated into large-scale waste deposits means that even if grades are low, they may be economical due to their scale. To date the waste materials from several industrial processes have been looked at, such as shale-oil extraction, coal mining and steel production.
Pilot analysis has been undertaken on shale and waste from the retorting of the Scottish Lothian oil shale and associated spoil heaps (Bings; Figure 3, top) as well as a separate world class oil shale, waste rock from coal mining and waste slag from iron (blast furnace slag) and steel manufacture (basic oxygen steel slag).
One surprising feature common to all the results is that they contain similar proportions of light and heavy REEs. This is different from that seen in nature. For example, leachates separated from shales associated with the Durham coalfields contain concentrations of the light REE such as cerium and lanthanum, which are below typical bastnäsite ore. However, the rarer, more valuable, heavy REEs including europium, gadolinium and terbium are found in concentrations similar to and greater than traditional ore. This is also seen in blast furnace slag that contains concentrations of ytterbium and lutetium similar to bastnäsite ore. Therefore, the value of the waste materials has potentially increased.
It is no surprise that the investigated spent oilshale samples vary in their REE levels. Some are disappointing, containing REE abundances lower than bastnäsite ore, however the retorting process does enrich the concentration of REEs in the waste material relative to the unprocessed shale. Despite the modest concentrations in some samples, the availability of high quantities of waste means that the total resource for the site can be high. For example, the Broxburn Bing, near Edinburgh (Figure 3, top) contains 0.03Mt of REE resources. When this is calculated for all the West Lothian Bings (based on 1963 volume) a REE reserve of ~0.6Mt is produced, greater than many recognised global deposits (Figure 2).
In a separate example, an oil shale being exploited in Asia has a relatively low REE content, but the large quantity of the deposit means the calculated REE resource is ~0.3Mt. As previously mentioned, these materials all contain similar amounts of the more valuable heavy REE, as light REE making them potentially more valuable than traditional ores. Indeed, a rough calculation shows the Broxburn Bing alone is worth ~US$2bln in terms of its REE content (at 2010 wholesale prices).
The large REE resources will be of little interest to industry if the elements cannot be economically extracted from the waste material using environmentally benign techniques. To do this, methodologies have been borrowed from environmental geochemistry and examined leachates that might occur naturally from the weathering of spoil tips. The first such test was conducted on leachates associated with shales from the Durham coal mining industry and returned a surprising result. The REE concentrations in this material are the highest the team has recorded (~2.7‰ total REE). This suggests some of these waste materials may yield their REE resource with comparative ease.
Other analysed samples are blast furnace and basic oxygen slags. The blast furnace slag contains higher REE abundances than the basic oxygen steel slag, suggesting the majority of REEs are removed from the iron during the fusion of limestone with the ash from coke that makes the slag within the blast furnace. Interestingly, despite containing low concentrations of REEs, the basic oxygen steel slag contains extremely elevated levels of titanium, vanadium, chromium and manganese. Research is ongoing to investigate how these elements as well as the REEs may be feasibly separated from slag.
Full economic evaluation of these resources has yet to be undertaken, but the fact that these materials and minerals are close to market and are otherwise considered waste gives us hope that extracting them may prove financially attractive. The work of the team demonstrates that industrial waste materials such as slags and oil-shale residues can contain elevated quantities of REEs. The partitioning of the REEs into soluble phases during natural processes also means that reclaiming the REEs from waste may not be technically onerous or environmentally problematic. Therefore, with better use of industrial waste streams the global demand for REEs may be satisfied at a reasonable price.
Dr. Alex Finlay, Centre for Research into Earth Energy Systems, Durham University. Tel: +44 (0) 191 33 42357. Email: email@example.com Thanks to GRIME members Professor Jon Gluyas, Dr Fred Worrall, Dr Chris Greenwell and Miss Helen Foster. GRIME forms part of the Centre for Research into Earth Energy Systems, Department of Earth Sciences, Durham University. Website: http://www.dur.ac.uk/cerees