Elemental Sustainability and the Importance of Scarce Element Recovery

ANDREW J. HUNT, THOMAS J. FARMER AND JAMES H. CLARK

Green Chemistry Centre of Excellence, Department of Chemistry, The University of York, Heslington, York, YO10 5DD, UK

1.1 The Issue of Elemental Sustainability

Important topics including climate change and peak oil have been making headlines with increasing intensity over the past decade. The subject of green, clean sustainable energy, fuels and chemicals is an important topic of focus for the scientific community and is a fundamental component of the long term wellbeing of planet Earth. The necessity to be carbon neutral is well known and as a consequence solutions are being sought to lessen our dependence on fossil resources. New legislation and a growing movement towards the development of "low carbon technologies" are driving this technological change towards a sustainable carbon future. Unfortunately there is a serious problem, as many "low carbon technologies" including wind turbines, electric cars, energy saving light bulbs, fuel cells and catalytic converters, require rare and precious metals for their production and use. Traditional supplies of such elements are "running out", thus creating other challenges in the form of a resource deficit. In fact, such elements are not running out or being destroyed but are being quickly dispersed throughout our human environment or what has been referred to as the technosphere. This makes recapture of these unique elements both highly problematic and costly. Such challenges must be tackled through the development of multidisciplinary partnerships and a sustainable holistic approach to the extraction, processing, use and recovery should be adopted for all elements within the periodic table. The only exception to this would be radioactive materials which cannot be recovered in the initial state once decay has occurred. As such, it is essential to develop new sustainable routes and strategies for the recovery and reuse of these elements.

Elemental sustainability is a concept whereby the sustainability of each element in the periodic table is guaranteed. For an element to be sustainable, its use by this current generation should not impair or restrict future generations from also utilising that same element. Within these constraints, it is also important to consider the triple bottom line of sustainability, that is, the environmental, societal and economic effects of these elements and their use. All elements within the Earth's crust are available in finite amounts, although some, like aluminium, iron and silicon, are available in many orders of magnitude higher abundances than others, like platinum, silver and selenium. Each element in the periodic table also has varying levels of demand. This demand varies as new technological advances come on-stream and others become obsolete. Rising demand for some elements is caused by both developed and developing nations which require advanced materials for consumer goods products (e.g. mobile phones and flat screen televisions) and the level of demand for each element often varies from nation to nation and region to region. As the world's population continues to rise, the growing middle classes will continue to demand a higher standard of living, fuelling a need for consumer goods and cleaner energy. This combination of known availability of certain elements and their current level of demand has caused some to have been flagged up with concern. Although we should endeavour to use all elements in the periodic table sustainably, those whose current rates of use risk depleting known reserves in the near future should be of greatest focus in the short to mid-term. Reserves are known tonnages of metals that can be economically and legally extracted using existing technologies. These reserves represent only a small proportion of the element compared to the significant abundance in the Earth's crust, while the resources of elements are represented in the locations or concentrations of that element or ore that have reasonable prospects of being recovered in the future.

As shown in Figure 1.1 numerous elements fall into the range where current known reserves will be consumed in less than 50 years if current rates of extraction are retained. Some of these are at high risk as a result of exceptionally low crustal abundances and these include the precious metals where the annual production of the majority is below 200 tonnes. It is not only elements with low crustal abundances and small annual productions that are of concern as known reserves of both strontium (Sr) and manganese (Mn)would be consumed in less than 50 years at current annual production levels of 380 000 and 16 000 tonnes, respectively. However, both the consumption and reserves of these finite elements are continually changing in response to movements in markets, discovery of new mineral deposits, development of new applications, advances in extraction technologies and improvements in the efficiency of use, recovery and recycling. As such, care must be taken when using the rate of consumption versus known reserves as a metric for the criticality of elements (Figure 1.1). Current known reserves of indium, an element which is vital for the production of display devices, solar cells and semiconductors, may run out in as little as 13 years at the current rate of consumption, thus fuelling concerns over the security of supply. If investment is made into developing technologies for recovery at end-of-life, in addition to using the remaining reserves more efficiently, it is hoped that supplies of this element will not be depleted and thus by utilising them in a sustainable manner reserves will be left for future generations.

1.2 What are Critical Elements?

An element that is classed as critical can be defined in various ways depending on the purpose of the assessment (e.g. for a specific application such as mobile phones) and the different needs of the individual country or territory. Many assessments of critical elements have been made, all of which apply different criteria and as such generate a diverse list of critical elements (Table 1.1), although in all instances there is an appreciation that current and projected demands for that element will result in rapid depletion of known reserves. Often elements that are considered to be critical in one territory, nation or company may be omitted from the...