Hand-held XRF sorting of spent refractory bricks to aid recycling Background MgO-C vs. Al2O3-MgO-C refractory bricks MC bricks are unfired refractories, generally constituted from high-purity MgO aggregate (sintered or fused), graphite, a carbon binder, and powdered antioxidants such as aluminium (Al), silicon (Si), silicon carbide (SiC), boron carbide (B4C), and Al-Si and AlMg alloys, which are added to protect the carbon against oxidation. Carbon is added to these bricks as a bonding phase and as graphite flakes to improve thermal shock resistance and to prevent wetting of the brick by slag, thus improving corrosion resistance. AMC refractories are constituted from the same raw materials as the MC bricks, but also contain alumina aggregate (bauxite and corundum). They also have a continuous matrix which contains the resin binder, graphite flakes, and antioxidant additives (Munoz and Tomba Martinez, 2012). Wear is reduced due to controlled residual expansion by in-situ spinel (MgAl2O4) formation, which improves resistance to thermal shock and spalling (Munoz, Pena, and Tomba Martinez, 2014). Chemical properties The chemical composition ranges of typical synthetic resin-bonded MC and AMC bricks, without antioxidant additions, are shown in Tables I and II. The magnesia grains are of high purity (fused and/ or sintered) with small amounts of impurity oxides such as alumina (Al2O3), haematite (Fe2O3), lime (CaO), and silica (SiO2). The Table I Chemical composition ranges of synthetic resin-bonded MC bricks without antioxidants, (mass%) (Routschka and Wuthnow, 2012) MgO Al2O3 C SiO2 Fe2O3 CaO 92-99 0.1-0.8 7-25 0.2-1.5 0.2-1.0 0.6-3 Table II Chemical composition ranges of AMC bricks without antioxidants, (mass%) (Routschka and Wuthnow, 2012) Al2O3 MgO C SiO2 Fe2O3 TiO2 carbon is usually added as graphite, with carbon contents ranging from 7 to 25%. Steelmaking ladles can contain up to 12% carbon (Kujur et al., 2018). AMC bricks with varying Al2O3: MgO ratios are produced. The impurity oxides present are governed by the amounts and types of alumina and magnesia raw materials used. Natural graphite also contributes oxide-based impurities to these bricks. Effect of antioxidants on microstructure Antioxidants protect carbon against oxidation by reacting with the carbon or the gaseous atmosphere to form carbides, oxides, and nitrides. These compounds crystallize as plates or whiskers and fill the pores or react with Al2O3 or MgO to form new solid phases such as spinel (MgAl2O4) or forsterite (Mg2SiO4), depending on the antioxidant(s) added (Munoz, Pena, and Tomba Martinez, 2014). Al and Si powders are mostly used due to their low cost and effective protection (Bag, 2011). Zhang, Marriott, and Lee (2001) examined reactions at 1200°C for 3 hours in an MC brick in which aluminium was used as an antioxidant (Figure 1). Aluminium remained as angular, unreacted metallic particles, while it also reacted with carbon to form spheroidal Al4C3 and in certain areas it oxidized to form spheroidal Al2O3 grains. Cubic MA spinel subsequently formed through the reaction of the oxidized Al antioxidant (Al2O3) and MgO at approximately 1200°C. AlN formed as 10 µm diameter whiskers between graphite flakes and on the Al4C3 grain surfaces (Figure 1). When Si was added as an antioxidant it also formed an oxide, a carbide, and a nitride during reaction, similar to the Al antioxidant. Forsterite was observed (Figure 2a), a SiO2 shell surrounding the unreacted Si particle (Figure 2b), spheroidal SiC (Figure 2a-c); as well as Si3N4 as 10 µm black whiskers within the SiC grains (Figure 2c) (Zhang, Marriott, and Lee, 2001). Munoz, Pena, and Tomba Martinez (2014) investigated the physical, chemical, and thermal characterization of different AMC refractories using several techniques such as XRF, XRD, reflected light optical microscopy, SEM, and gravimetry. The microstructures of the used AMC brick obtained by optical microscopy contained a large amount of alumina, with smaller proportions of antioxidant particles, and impurities such as mullite (3Al2O3.2SiO2), tielite (Al2O3.TiO2), and rutile (TiO2), (Figure 3). Market analysis MgO <15 mass% Corundum >70 <15 >2.0 – 15 <1.5 <0.3 <0.3 Bauxite >65 <15 >2.0 – 15 <3.5 <1.0 <3.0 Corundum >55 ≤30 >2.0 – 15 <1.0 <0.3 <0.3 Bauxite >50 ≤30 >2.0 – 15 <3.0 <1.0 <2.0 MgO 15 – 30 mass% One of the most important refractory raw materials is magnesia, which accounts for 25–30% of the total refractory mineral demand. China is one of the largest suppliers of dead-burnt and fused magnesia and controls 71.3% of of the global supply of natural graphite, 65% of the synthetic graphite, and 100% of the spherical graphite (IMFORMED, 2021). Over 82.4 Mt of bauxite was exported by Guinea in 2020, making it one of the world’s single largest exporters (Artacho, 2021). Figure 1—Microstructures of an MC brick with Al addition after 3 hours at 1200°C, (a) BEI at 300 µm; (b) and (c) secondary electron image (SEI) at 10 µm (Zhang, Marriott, and Lee, 2001) (G Graphite, FM fused magnesia, SM sintered magnesia, MA MgAl2O4) 10 JANUARY 2023 VOLUME 123 The Journal of the Southern African Institute of Mining and Metallurgy