Coal fly ash as a resource for rare earth elements

Despite growing importance of renewable energy sources (i.e., geothermal, wind and solar) (Bertani 2010; Lund et al. 2011; Tomaszewska and Pająk 2012; Tomaszewska and Szczepański 2014; Dinçer 2011), coal’s share of the global energy mix continues to rise and by 2017 will come close to surpassing oil as the world’s biggest energy source (IEA 2012). With the global energy demand growing, coal is seen as a reliable and secure energy source and many countries are increasing its consumption (IEA 2014). Consequently, the global coal fly ash production increases and there is a pressing need of its recycling and utilisation (Yao et al. 2015; Mattigod 2003). Although, there is a number of possible fly ash applications, on average, only 25 % is utilised, the rest is considered a pollutant and disposed of as waste (Blissett and Rowson 2012; Sommerville et al. 2013). It is thought that coal combustion represent a large-scale mobilisation of all naturally occurring elements in a magnitude that is comparable to the rates of mobilisation and transport that occur as part of natural geochemical cycling of elements (Bertine and Goldberg 1971; Hendrick 1995). Hence, the recent research onto coal fly ash (CFA) has been focused on its novel applications (Blissett et al. 2014). Although there are many publications in regard to fly ash utilisation, there are only very few looking into its potential as a resource of REE, and even fewer investigate coal-biomass fly ashes for this purpose (Heebnik 2011; Blissett et al. 2014). In addition, most of investigated fly ashes come from exploratory coal samples that have been laboratory ashed for analytical investigations, not from coal fly ash stockpiles (Blissett et al. 2014). Because the use of biomass as a co-firing fuel in power plant is increasing, there is an increase in a production of ‘new kind’ of fly ashes, which have different properties from coal fly ash and consequently often are not suitable for many common applications (e.g., in cement and concrete). Hence, it is important to investigate alternative application of both coal and coal–biomass fly ashes. This work aims to study both types of fly ash as a potential source of REE, by investigating samples from ten coal-fired power plants in Poland. Rare earth elements are a group of 17 elements (15 lanthanides plus yttrium and scandium), which are particularly used in many new technologies such as fuel cells, green energy devices, hi-capacity batteries, magnets for wind power generation, to name but a few (Chen 2011; Całus-Moszko and Bialecka 2013; Hoenderdal et al. 2013).

In natural environment, REE are a group of elements with a high potential as process indicators in the system rock or soil as well as water (Grawunder et al. 2014). They can be classified (geochemically) into the following: light earth elements (LREE — La, Ce, Pr, Nd and Sm), medium (MREE — Eu, Gd, Tb, Dy and Y) and heavy (HREE — Ho, Er, Tm, Yb and Lu) (Blissett et al. 2014). However, Seredin and Dai (2012) devised a new REE classification which takes into account current market trends and is related to the likely supply and demand forecast over next few years. This classification divides REE into three groups: critical (Nd, Eu, Tb, Dy, Y and Er), uncritical (La, Pr, Sm and Gd) and excessive (Ce, Ho, Tm, Yb and Lu). Naturally, this classification makes an evaluation of the REE distribution more relevant to the industry (Blissett et al. 2014). It needs to be noted that the commercial importance of REE is not reflected in the amount in which they are used, but in the fact that they are crucial in a wide range of applications including key technologies for the clean and sustainable energy (Alonso et al. 2012; Du and Greadel 2013; Seredin et al. 2013). REE resources are not that rare (deposits have been identified in 34 countries—according to Chen (2011)), but because their mining and production are not cheap and can be associated with significant environmental impact (radioactivity of elements associated with them—uranium and thorium as reported by BGS-NERC 2010), they have low substitution and low recycling rates, most of world’s economies/countries including the European Union (EU) depend highly on the import (Massari and Rubeti 2013). In recent years, majority of countries which produce and export REE, have reduced their export to the EU in order to protect their national downstream industries. For example China, which is world’s main producer and exporter of REE (c.a. 97–98 %) has progressively increased export taxes on them and restricted their export rates (a decrease of about 40 % between 2009 and 2010) (Alonso et al. 2012; Massari and Ruberti 2013). As there is a gap between REE’s growing demand and decreasing supply, it is extremely important to look for the alternative resources.

Kertis and Yudowich (2009) estimated that CFAs contain 445 ppm of rare earth elements (REE) on an average global basis. and as Seredin and Dai (2012) noted, ‘if the average REE content in coal ashes, based on analyses of numerous samples, is comparable to some conventional ores, the wastes of coal combustion could and should be considered as possible source of these materials.’

Among all REE, lanthanum, europium, erbium and neodymium are considered the most important for the new technologies (Całus-Moszko and Białecka 2013). For example, lanthanum, which is the second most common element among all REE and in the natural environment is present in greater amount than silver and lead, is essential component in hybrid cars, while europium has been used in ultraviolet LED devices, erbium in lasers used for medical operations, neodymium—in magnets. Additionally, REE have been used in fertilisers (Pang et al. 2002).

As most of investigations into coal fly ash were concentrating on their use in the concrete/cement industry or for the production of synthetic zeolites (Querol et al. 2002; Franus et al. 2014; Wdowin et al. 2014), the investigation into CFA as a potential source of REE is considered a new research area. However, it seems to attract more and more attention and investment. REE extraction process from fly ash is considered to be far less intensive than that of mining a raw product. In addition, CFAs contain a full range of REE, whereas most mines have only a few varieties of these elements. Due to these reasons, many companies are exploring methods to extract REE from coal fly ash and investigating whether coal fly ash processing can become an economical and environmentally friendly alternative to traditional mining (Mayfield and Lewis 2013). For example, Neumann Systems reported that their process has successfully extracted more than 60 % of the available metals (including 14 rare earth and strategic metals) from fly ash samples supplied by a Colorado Springs Utilities power plant. The company estimates that it can extract about $600 worth of rare elements from every ton of coal ash and that the waste from Martin Drake plant alone can produce c.a. $49 million worth of minerals a year. Moreover, to dissolve REE out of fly ash, the extraction process uses by-products of scrubbing emissions (sulphuric and nitric acids) which is considered an additional benefit (Currie 2012).

For the complete evaluation of the coal fly ash as a raw source of REE, a number of parameters should be taken into account, including the possibility of beneficiation and simplicity and cost of hydrometallurgical recovery and various environmental issues. However, for the initial evaluation, data on REE individual contents within the investigated coal fly ash may be adequate (Seredin and Dai 2012).

With the growing energy demand and consequent increase in the use of coal (IEA 2014), the problem of coal fly ash disposal is expected to worsen (Izquierdo and Querol 2012). On the other hand, prices of strategic minerals and rare earth elements, which are essential to many advanced technologies (e.g., wind turbines, electric vehicles, permanent magnets and mobile phones), have been rising significantly due to the increasing global demands and supply shortages (Mayfield and Lewis 2013). Therefore, in addition to new mining ventures and electronic recycling schemes (Binnemans et al. 2013), many companies search for new mineral sources, among which CFA is considered a promising one (Mayfield and Lewis 2013). As extraction of REE from coal fly ash is still developing and have yet to be commercialised, there is a limited knowledge of their contents in the existing coal fly ash deposits. Hence, identification and full characterisation of current coal fly ash deposits, which are economically viable for REE (and other strategic metals) recovery is necessary.

While this initial investigation onto coal and coal-biomass fly ashes showed the variation in the total REE content between different samples, all tested materials had over 30 % of the critical REE content and an outlook coefficient >0.7, which suggests that at least the three samples (6, 7 and 11) with a total REE content close to the average global basis (445 ppm) should be considered for the further examination. Future work will include investigation of the CFA stockpiles homogeneity in terms of REE contents and an extensive chemical characterisation to determine the resources with minimal concentration of hazardous substances that require treatment and disposal. Additionally, possible extraction processes and their economic feasibility will be studied.