Johnson Matthey Technol. Rev., 2017, 61, (2), 126
为实现可持续发展而开发的解决方案通常对技术都有着复杂而严苛的要求；并且对于“高技术”原材料的依赖程度很高。其中很多材料都面临着严重的供货风险。企业、政府、国家和国际组织对于这些关键原材料的 (CRM) 的关注程度也越来越高。本文以稀土元素和磁铁为例，介绍针对管理供货风险正在发展的各种战略和创新方案。寻找 CRM 替代材料、提高使用和回收利用效率，这些持续的需求为基础性研究和商业创新提供了激动人心的机会。
Materials Challenges for a Transforming World
Developments for a sustainable future: the example of rare earths
- By Richard Miller
- Miller-Klein Associates Ltd, Saith Ffynnon, Downing Road, Whitford, Flintshire CH8 9EN, UK
- Email: firstname.lastname@example.org
The solutions being developed for a sustainable future are technologically complex and demanding; relying on ‘high-tech’ raw materials. Many of these materials face significant supply risk. Business, government, national and international organisations are increasingly focusing on these critical raw materials (CRMs). This paper describes the strategies and innovations being developed to manage supply risk using rare earth elements and magnets as examples. The ongoing need to find substitute materials and improve efficiency, recycling and recovery of CRMs provides exciting opportunities for fundamental research and commercial innovation.
The world is undergoing transformational change. John Beddington, a previous Chief Scientific Adviser to the UK Government, wrote about a ‘Perfect Storm’ of population growth, climate change and resource crunches, that would require radical changes for a sustainable future (1). These factors interact in complex ways. The population is not only growing, it is rapidly urbanising, increasing the pressure on city infrastructures; climate change will drive migration that will be a further challenge for cities. Climate change is not only a threat to food production, but directly endangers global cities through rising sea levels and temperatures. Resource challenges range from the obvious food, water and land, to subtler issues such as the energy and materials we use to build our societies.
These challenges have a direct impact on national and global economies. The gross value added (GVA) for the UK economy is about £1.7 trillion per annum. About £500 billion per annum of that will be delivered in a very different way in the future; in sectors such as energy, transport, food, construction and healthcare (2). It is not known what new products, services and business models will appear, but they will be different. Business as usual is simply not viable in order to meet the United Nations (UN) Sustainable Development Goals (3) and their local implementations. Over the coming years thousands of innovations will appear, and some of these will come to replace current solutions that will no longer be appropriate. Innovations will be forced to balance the needs of people, planet and profit.
Increasing Dependence on ‘High Tech’ Raw Materials
Many of the solutions currently being developed depend heavily on electrical, electronic, computing and communications technologies. These in turn rely on the sophisticated use of specific ‘high-tech’ raw materials, many of them only currently available in limited quantities. Finite supply leads to concerns about security of supply and price. Prices for many of these key materials have risen significantly in recent years and perhaps more importantly for business, have shown notable volatility (4, 5). Figure 1 shows how supply constraints in rare earth oxides can cause prices to suddenly spike 30–40 fold.
To give a simple example of this complexity, a modern smart phone contains at least 70 out of the 83 naturally stable elements in the periodic table (6). They use almost the complete palette. Figure 2 shows where some of the key elements are used (7).
The recognition of the importance of these CRMs has focused businesses, governments, and national and international organisations on their source and flows. Many governments have published lists of the materials they consider of most importance to their economy and where there are significant risks to supply security (8). Supply risks have been explored by the European Union (EU) (9), the USA (10) and Japan (11). These lists are predominantly populated by materials used in the electrical and electronics industry, in catalysts and alloys.
For example, the EU lists 20 CRMs. Three of the CRMs are groups of closely related materials: platinum group metals, light rare earths and heavy rare earths (9). Of the 20 CRMs identified by the EU, seven are mainly used in electrical and electronic equipment, including the groups of rare earths, and seven are used in the production of metallic alloys.
Different authorities use slightly different methods to assess the supply risk for each CRM, but common factors taken into account are:
Concentration of primary supply – how many countries are significant suppliers?
Reserve distribution – how widely distributed are proven reserves?
Political stability of suppliers – what is the risk of disruption of primary production?
Companion fraction – how much is produced as a side stream from a bulk material and therefore dependent on the economics of the primary product?
Substitutability – are alternatives available for the key applications?
Recyclability – can it be recycled at end of life and is it being recycled?
One factor that might appear important that is not usually assessed is the absolute crustal abundance of the material. This is rarely a defining feature of the availability and economics and supply security of a CRM.
The British Geological Survey, UK, publishes a regularly updated risk list of economically important elements (12). Their current list of 41 elements range in supply risk from the rare earth elements at the top to gold at the bottom, with the platinum group metals somewhere in the middle. An extract of some of the key materials and their risk scores is given in Figure 3.
The rare earth elements are at the top because China produces 96% of global supplies and controls 42% of known reserves. More than two-thirds is produced as a co-product from other economically important materials. Rare earth elements are hard to substitute and poorly recycled.
This adds up to a considerable supply risk, and given the criticality of rare earth elements to the drive for a low-carbon and efficient global economy, there is much government interest in these specific materials.
Strategies to Manage Supply Risk
Governments have responded by raising awareness of the challenge and encouraging innovation. There is a huge amount of effort in gathering and disseminating information about CRMs, bringing together industry players to collaborate, and sponsoring industry and academic research to develop new solutions. The EU, Japan and the USA have held tri-lateral workshops to promote sharing of knowledge and approaches to risk mitigation (13).
Government-industry initiatives that have been set up to address supply risk in CRMs include:
The broad strategies for addressing CRM risk are simple. The upstream option is to improve primary production by increasing the reserves that are available to exploit and the efficiency of extraction, while decreasing the costs and overall environmental impact. In the case of the rare earth elements this is hard to achieve. The concentration of production and proven reserves in China make it difficult for any other player to materially shift the balance, and the materials security risk remains high.
Moving downstream there are more opportunities to improve security and reduce costs. As with any problem material the main options are:
substitution – find an alternative material with a lower supply risk
efficiency – get more of the desired effect out of less material
reuse and recycling – having expensively and with difficulty obtained the CRM, keep it above ground and circulating productively for as long as possible.
One of the key tasks being undertaken by the academic world is mapping the stocks and flows of critical raw materials in the global economy. A leading light in this work for many years has been Graedel (18). His meticulous work in industrial ecology has unravelled much of the detail of what happens to industrially important materials through their life cycle and laid the foundation for both government policy and commercial action. Knowing what is actually happening in a complex system is a key step in deciding what action to take to shift it to a more stable position.
Innovation in Rare Earth Magnets
There is a great deal of research and innovation targeting the CRMs with the greatest materials security risk, far too much to offer a comprehensive analysis here. But it is possible to illustrate some of the recent approaches using examples from rare earth magnets. These are used for everything from wind-power generation, through electric cars to computer hard drives and smartphones. Current generation wind turbines use about 500 kg MW–1 (19), an electric car uses 2.5 kg for the electric motor and an electric bike uses 80 g (20). A smartphone (21) uses much less than a gram.
The rare earth elements used have the highest supply risk (12) and it can be predicted that current technological solutions for a sustainable economy will dramatically increase their use. The ratio in which the different rare earth elements are produced is not the same as the market demand. This leads to a supply balance problem which is proving difficult to manage (22).
In neodymium-iron-boron (NdFeB) magnets, some of the neodymium is replaced by dysprosium to enable the magnets to perform at high temperatures. Unfortunately, the low ratio of dysprosium to neodymium in primary extraction and low levels of recycling mean that there is a structural shortage of dysprosium available for these applications. The obvious solution is to find a substitute for the dysprosium and despite the general difficulty of finding alternatives for rare earth elements there has been progress.
For example, one approach has been to reduce the grain size in sintered NdFeB magnets from a typical 5–10 μm to around 80 nm. Movement of the magnetic domains is reduced by the larger number of grain boundaries increasing the thermal stability (23). This has allowed the demonstration of magnets suitable for traction motors without adding any dysprosium.
Another discovery recently reported by Oak Ridge National Laboratory, USA, was that dysprosium could be replaced with cerium co-doped with cobalt (24). Since cerium is the most common rare earth element and dysprosium one of the scarcest, there is potential to reduce the cost of high performance NdFeB magnets by 20%–40%. Cerium on its own does not work as it reduces the temperature at which the magnetism is lost, but the new alloy with cobalt maintains its performance to temperatures higher than dysprosium doped magnets.
Work is also going on to reduce the amount of rare earth elements needed to produce a magnet with the required performance. Improvements in recipes and alloys are always being sought, but recently there has also been research on using three-dimensional (3D) printing to create precise magnet forms with minimal material. A magnet can be designed by computer modelling to provide the precise field strength and distribution using as little magnetic material as possible. 3D printing can then produce these forms with negligible waste. Suess and colleagues have shown this approach for polymer bonded magnets (25), as have a team from Oak Ridge National Laboratory and the Ames Laboratory, USA (26). A collaboration between Oak Ridge National Laboratory and Arnold Magnetic Technologies, USA, demonstrated that it was possible to use laser heating to produce near net form sintered magnets (27), although with diminished performance.
Further development towards commercial production of rare earth magnets can be expected using a variety of additive layer manufacturing techniques.
At present the potential for reuse and recycling of rare earth magnets depends on the application and the size of the magnets. At one extreme the large magnets used in wind turbines (~500 kg MW–1) are in a known location and will be subjected to controlled deconstruction at their end-of-life. At the other extreme mobile phones and other small consumer electronics carry small magnets, and are distributed thinly across the surface of the globe. Wind turbine magnets are a valuable resource and are reused or recycled, but less than 1% of the rare earth elements from consumer electronics are captured and recycled (28).
There are a number of processes for recycling rare earth magnets (28), varying in cost, efficiency and environmental risk. One of the most interesting is hydrogen decrepitation, where hydrogen gas is used to break down the magnets into a fine powder that can be re-sintered into new magnets (29). The key attraction of the process is that the magnets do not have to be mechanically separated before recycling. So in applications like hard disk drives, time consuming disassembly steps can be avoided.
A different approach has been taken by Oak Ridge National Laboratory who have developed an automated disassembly method for hard disk drives (30). This uses a library of hard disk configurations and advanced robotics to correctly orient and locate the drives for robotic disassembly. It is estimated that 1000 tonnes of rare earth magnets could be recovered in the USA and reused, remanufactured or recycled. The system has now been licensed for commercial trials.
The researchers hope to extend the work to consumer electronics such as smart phones. However, the diversity of designs and the lack of any system for recovering phones at end-of-life will be a problem. If a circular economy in rare earth elements is to be made possible, products need to be designed for reuse and recycling, and business models need to allow the efficient recovery of consumer products containing CRMs. Improved methods to recover CRMs are of little use if there is no stream of consumer products to process.
The only currently workable route to a sustainable global economy that balances economic viability, environmental responsibility and social acceptability requires wide deployment of a whole range of advanced technologies. These in turn depend on the availability of CRMs that face challenges of security of supply and of price volatility.
Governments in industrialised countries will continue to encourage academia and industry to come up with ways to mitigate the CRM risks, and industry will persist in seeking solutions that can be commercialised. This interest and focus is unlikely to be substantially reduced by any temporary change in market conditions. Until the supply risks can be drastically and permanently lessened, there will always be pressures to make more effective use of the CRMs. Radical new technologies that do not use current CRMs, and that do not replace them with alternatives that are equally risky, could change the picture completely, but none are visible on the horizon and it would be many years before they could have a significant impact.
This means there will be exciting opportunities for both fundamental research and commercial innovation in substituting, using more efficiently and recycling CRMs. Opportunities in everything from fundamental chemistry and materials science to the design and production engineering of consumer products.
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Trained as a chemist Richard Miller has spent over 30 years in industrial research and development, and 23 years working on sustainability and resource efficiency. He has been a Research Director in fast moving consumer goods and chemicals companies, worked with public agencies to support industrial innovation and currently runs a consultancy.