Coal bottom ash processing for capitalization according to circular economy concept

Highlights

• Coal bottom ash processing for capitalization according to circular economy concept.

• Metals recovery by sequential precipitation at different pH.

• Metals recovery by successive microemulsion assisted extractions.

• Metals recycle as their oxides synthesized into microemulsion templates.

• Bottom ash residue recycle as raw materials for different industrial applications.

Abstract

This work reports the semiquantitative laboratory chemical procedures developed for some Rare Earth Elements (REE) extraction and concentration from coal bottom ash (BA), including recycling the recovered valuable components. This preliminary assessment applied in case of 25 coal BA samples of Ceplea Valley landfill of Romania describes the chemical methods by using digestion with nitric acid coupled with microemulsion assisted extraction procedure. The enrichment of REE by concentrated nitric acid followed by water washing proved to be an adequate approach since water recovers 29.42% of metals comparing to that extracted by the total phase (ac + aq). The REE concentration process achieved by precipitation with NaOH, at different pH values, shows to be almost completely in the fraction at pH ≈ 0 and 3. The statistical outlook coefficient C_outl shows that evaluation of REE concentration in acid/aqueous solution falls into the “promising” field for their recovery by chemical procedures. Optimal microextraction was obtained by coupling microemulsion Winsor II ternary system consisting in Brij 30/AcOEt:AcOBu/metal ions solution, with citric acid as complexing agent. Results confirm that recovery of lanthanides as nanoparticles from microemulsion system yields over 85%. Optical characterization of oxide nanoparticles suggested applications such as for energy efficient buildings, photo-catalysts for water remediation or thin films for photonic optical devices. The ash residue can be also recycled as an alternative raw material for building materials production.

Introduction

The REEs are relatively abundant in the earth’s crust, but minable concentrations are less common than for many other ores. Globally, REE (lanthanides along with the chemically similar elements scandium and yttrium) are steadily of growing interest due to their irreplaceable role and strategic importance in many cutting-edge technologies such as: hybrid automobiles (La (Bauerlein et al., 2008), Y), fuel cells (Tb), power source for satellites (Pm (Grukh et al., 2004)); optical devices (LED and CFL), Ce (Stwertka, 2002); lasers (Eu (Becker et al., 1999), Yb (Lide, 2005), Tu); luminescent solar concentrators (Tu); optical communications (Er (Sims, 2016)); high stability atomic clocks (Yb); high-temperature superconductors (Y); permanent magnets used in drive motors for electric vehicles and generators for wind turbines (Nd, Dy, Pr, Ho and Sm); luminous paint and materials Pm (Caro, 1998); metallurgical additives and alloys (Yb, Eu (Trovarelli, 2002), Y, Tb), including for aerospace industry minor components (Sc); medical applications i.e. health care devices, dermatology, dentistry (Er, Tu, Y); meteorology (Tu); catalysts (in petroleum cracking applications (Lu), synthetic garnets (Y) etc. For this reason, nowadays there is a vital need of substantial exploration and evaluation of REE deposits around the world. Moreover, REEs are considered critical in Europe (Jones and Haygood, 2011).

Coal and coal combustion ash represent alternative unconventional resources for critical materials (Huang et al., 2018) since many coals already contain a significant amount of concentrated valuable metals hosted in coal partings and coal minerals and as fine-grained mineral relics, neo-formed minerals, and aluminosilicate glasses in coal combustion ash (Kolker et al., 2017; Valentim et al., 2018a, 2019). Such a concentration of critical metals in coal and coal ashes is known for decades (Goldschmidt and Peters, 1993, Seredin and Dai, 2012, Mayfield and Lewis, 2013), and extended research of characteristics of BA landfilled at Ceplea Valley deposit (Turceni power plant in Oltenia lignite basin, Romania) has been previously published by the authors (Valentim et al., 2018, Valentim et al., 2019). The study shows that bulk REY + Sc (rare earth plus yttrium and scandium) concentration is consistent throughout the landfill, the chemical composition and phase mineralogy of the samples studied being homogeneous both horizontally and along deposit depth. Therefore, the BA landfilled at Ceplea Valley is a potentially “promising” source of REY + Sc as shown by Valentim et al. (2019).

The REE concentrations in combustion coal products (CCP) are available in high amounts due to their original concentration in coal forming basins and to the large volumes of coal burnt to satisfy the electricity demands, generating huge amounts of organic matter free mineral phases that host REE, among other elements (Blissett et al., 2014, Dai et al., 2016, Wagner and Matiane, 2018).

Coal bottom ash processing for capitalization according to the concept of circular economy involves two approaches: one, REE recovery and, the other, waste recycling.

To recover REE a few key aspects must be considered: leaching from BA residues, extraction, and concentration from leachate. There is only little information on the environmental performance of different REEs separation procedures (Wall et al., 2017), and the concentration of REE in coal and coal combustion ash are several orders of magnitude lower than those of REE ores, which makes the extraction of REE from these materials highly challenging. REE extraction from BA is not currently of interest due to the decreasing prices during the last six years (USGS Mineral Commodity Summaries, 2020) and therefore not to be, from this perspective, profitable at present. Moreover, recycling large volumes of coal ash generated every year cannot be ignored as one of the most promising and potential non-energy secondary unconventional resource of REE in Europe (Blissett et al., 2014, Taggart et al., 2016, Yang et al., 2016, Lin et al., 2017, Phuoc and Wang, 2017). However, technologies for the extraction and recovery of REE from coal ash are not well developed, and this knowledge gap creates the need to understand the feasibility of this alternative source of REE, which is considered in this research.

Among the variety of REE leaching available procedures, four types are commonly cited in the literature (Izquierdo and Querol, 2012): batch leaching tests using deionized water; column/flow-through tests; Toxicity Characteristic Leaching Procedure (TCLP); acid extractions performed under aggressive conditions (pH < 2). Sequential batch leaching consisting of four cycles (of seven days duration each), with leaching solutions’ pH starting from strongly acidic to strongly basic were performed. Dutta et al. showed much higher mobility of elements at low pH and revealed the phenomena governing the mobilization patterns: dissolution, precipitation/re-precipitation, or co-precipitation, depending on medium pH *Dutta et al., 2009).

The most common methods to extract and concentrate REE include: collection, physical separation (by particle size, magnetic and density separation) (Lin et al., 2017) and concentration by solvent extraction (Wall et al., 2017), ion exchange, membrane technology supercritical fluid treatment, precipitation and crystallization; metal extraction and precipitation by electrowinning and/or electro refining, cementation, gas reduction (Tanaka et al., 2008). The “US Department of Energy group” developed an environmentally friendly method based on physical-chemical extraction for concentrating the REE oxides (Jones and Haygood, 2011, Peiravi et al., 2017). Results of the RAREASH project (2nd ERA-MIN Joint Call 2014) about methods developed for REE recovery and concentration of some coal fly-ashes by multi-stage leaching with sulphuric (VI) acid, precipitation (Blissett et al., 2014;. Całus Moszko et al., 2016, Kalupa et al., 2017, Pan et al., 2018, Pan et al., 2019, Świnder et al., 2017, Šwinder et al., 2017, Tang et al., 2019, Želazny et al., 2017a, Želazny et al., 2017b) and flotation (Wierzchowski et al., 2017) have been already published.

Microemulsion assisted extraction procedure (MAEP) applied to recover metals from solid and liquid wastes represent an important issue, as it is based on the one-step extraction-concentration-recovery. MAEP offers some benefits among other methods such as enhanced selectivity, no need for processing at high temperature or pressure, cost-effective and less time consuming (Hirai and Komasawa, 1991, Matsuyama et al., 1996a, Matsuyama et al., 1996b, Farha et al., 2010, Zhu et al., 2016). In case of REE extraction by MAEP a few advantages were identified such as high extraction yields of metals, sensitivity to detection being thus improved, the selectivity related to different metals (Cadar et al., 2018a). The risks of environmental contamination and the pollution can be decreased by applying MAEP, and secondary raw materials can be provided for the metal’s shortage used in the production of different electronic devices (Castro Dantas et al., 2001a, Castro Dantas et al., 2001b, Castro Dantas et al., 2002, Castro Dantas et al., 2003, Castro Dantas et al., 2009, Cadar et al., 2018b, Kumari et al., 2018, Xu et al., 2018).

Most of the authors agreed that after the recovery of metals the residue could be used for new or traditional composites suitable for building industry or road constructions (Jones and Haygood, 2011). The BA characteristics of the Ceplea Valley landfill deposit were widely investigated to find their potential use for making building materials by research of Abagiu et al. (2013). Studies have been carried out on the morphology (Predeanu et al., 2013a, Predeanu et al., 2013b, Predeanu et al., 2016), environmental impact (Popescu et al., 2013), and successful utilization of BA to produce ceramic composites, in mixtures with other industrial wastes (Anghelescu et al., 2017a, Anghelescu et al., 2017b). To use BA in making bricks, thermo-isolation concretes and compacting masses, the ash must satisfy specifications for size, density, absorption, porosity, and compressive strength as shown by Popescu et al., 2014a, Popescu et al., 2014b.

The aim of the present research which is a follow-up of those carried out by Valentim et al., 2018, Valentim et al., 2018b, Onose et al., 2017 is to develop a preliminary qualitative laboratory method to process coal bottom ash for capitalization according to the concept of circular economy. Two different chemical methods to extract fifteen REE from Romanian BA samples of Ceplea Valley are proposed. In this respect, metals - containing leaching was obtained using nitric acid, at room temperature, in dynamic conditions. Complementary solutions to recycle BA residue, others than in the construction materials industry, were also evaluated.