Peter Tom Jones

Perspectives for the recovery of rare earths from end-of-life fluorescent lamps

Abstract This vision paper discusses the advantages and disadvantages of the three main options for the recycling of rare-earth elements from end-of-life fluorescent lamps: (1) direct re-use of the lamp phosphor mixture; (2) separation of the lamp phosphor mixture into the different phosphor components; (3) recovery of the rare-earth content. An overview is given of commercial activities in Europe in the domain of recycling of materials from end-of-life fluorescent lamps and the recovery of rare earths from these lamps. The collection of end-of-life fluorescent lamps is currently driven by a legal framework that prohibited the release of mercury to the environment. The contaminations of the lamp phosphor powders by mercury and by small glass particles of crushed fluorescent lamps are limiting factors in the recycling process. Research should be directed to an advanced clean-up of the reclaimed lamp phosphor fraction, and in particular to the removal of mercury and glass fragments. The recovery of rare earths from the lamp phosphors could be facilitated by taking advantage of the differences in resistance of the different lamp phosphors by chemical attack by inorganic acids and bases.

Rare Earths and the Balance Problem

Maintaining the balance between the demand by the economic markets and the natural abundance of the rare-earth elements (REEs) in ores constitutes a major challenge for manufacturers of these elements. This is the so-called balance problem (or balancing problem). The ideal situation is a perfect match between the demand and (production) supply of REEs, so that there are no surpluses of any of the REEs. The balance problem implicates that the rare-earth industry has to either find new applications for REEs that are available in excess, or needs to search for substitutions for REEs that have limited availability and that are high in demand. Different solutions are proposed to solve the balance problem: diversification of REE resources, recycling and urban/landfill mining, substitution, reduced use and new high-volume applications. No single solution can solve the balance problem, but a combination of different strategies can. It is illustrated that the issue of thorium in REE ores is also directly related to the balance problem: presently, thorium is considered as radioactive waste, but this waste could be turned into a valuable resource by using thorium in a thorium-based nuclear fuel cycle.

Degradation mechanisms of magnesia-chromite refractories in vacumm-oxygen decarburisation ladles during production of stainless steel

Abstract Magnesia–chromite bricks are used as refractories for the refining of stainless steel in vacuum–oxygen decarburisation (VOD) ladles. Refractory wear is not uniform. In the present work, worn bricks from different zones in the ladle have been analysed, and a set of interdependent degradation mechanisms is proposed. Refractory wear as a function of position in the ladle is discussed. Slag infiltration and MgO dissolution from the refractory were observed in all samples, whereas FeOx decomposition was seen at two levels in the high wear samples. First, partial decomposition of primary chromite crystals ( (Mg) [Fe3+, Cr, Al]2 O4 ) occurred at the hot face of the brick. Three layers were distinguished in the reacted chromite crystals and a reaction mechanism is presented. Second, a decrease in the FeOx content of the magnesia phase occurred at the hot face of the brick. The negative effect of the presence of FeOx in magnesia–chromite refractories is discussed and the influence of the ferrostatic pressure is demonstrated. Finally, the consequences of the following phenomena are discussed: increase in brick porosity, slag infiltration, corrosion, erosion of the partially liquid bonded refractory system, and spalling and cracking.

Behaviour of magnesia-carbon refractories in vacuum-oxygen decarburisation ladle linings

Abstract Magnesia–chromite bricks are commonly used in the working lining in vacuum–oxygen decarburisation ladles. However, they are expensive and potentially environmentally-unfriendly. The possibilities of employing magnesia–carbon refractories were investigated through industrial testing combined with post-mortem assessments, thermodynamic calculations and mechanical considerations. Pitch bonded magnesia materials cannot be used in the slagline owing to the MgO–C reaction and extrinsic oxidation of carbon by reducible slag components (mainly CrOx), resulting in carbon removal, slag infiltration and direct MgO corrosion. Combined with turbulent slag motion this leads to hot erosion. Direct carbon burnout and the MgO reaction are the main causes for their failure in the freeboard. However, pitch bonded magnesia refractories are able to compete with magnesia–chromite bricks in the ladle bottom and in the lower metal bath region, where levels of turbulence and slag infiltration are limited. Simultaneously, both the MgO–C reaction and the extrinsic oxidation of carbon are restrained by, respectively, the inhibiting effect of a mechanical barrier and the non-wetting behaviour of steel towards the magnesia refractory component.

A study of slag-infiltrated magnesia-chromite refractories using hybrid microwave heating

Abstract Rebonded magnesia-chromite refractories are used in vacuum oxygen decarburisation ladles for the secondary refining of stainless steel. They suffer from acute wear due to the stringent chemical, thermal and mechanical conditions imposed. Although post-mortem investigation of worn refractories is indispensable to study the degradation mechanisms, its application is often hampered by evaluation difficulties due to crystallisation phenomena occurring during in situ cooling. To study the actual microstructures at elevated temperatures, industrially worn and virgin refractory samples were reheated and quenched in a cylindrical single-mode microwave furnace. Usage of a tubular susceptor allowed reproducible hybrid heating of the samples up to 1800°C. The quenched samples were analysed with SEM and EPMA-EDS. Concurrently, elemental line scans, X-ray mappings and quantitative image analyses were performed. This allowed the description of the ‘high-temperature inactivation’ mechanisms of the secondary chromite bonding phase: dissolution (1) in the periclase phase and (2) in the liquid slag, with (2) being predominant. Conclusions are drawn with respect to the refractoriness of the contributors to the spinel bonding phase: MgO.Cr2O3 (magnesiochromite)>MgO.Al2O3 (spinel)≫MgO.Fe2O3 (magnesioferrite). Hybrid microwave heating is shown to be an interesting alternative for conventional furnace experiments on refractory samples.

Effect of High Cooling Rates on the Mineralogy and Hydraulic Properties of Stainless Steel Slags

This article investigates the effect of chemical composition and cooling rate during solidification on the mineralogy and hydraulic properties of synthetic stainless steel slags. Three synthetic slags, covering the range of typical chemical composition in industrial practice, were subjected to high cooling rates, by melt spinning granulation or quenching in water, and to low cooling rates, by cooling inside the furnace. Both methods of rapid cooling led to volumetrically stable slags unlike the slow cooling which resulted in a powder-like material. Stabilized slags consisted predominantly of lamellar β-dicalcium silicate (β-C2S) and Mg, Ca-silicates (merwinite and bredigite); the latter form the matrix at low basicity and are segregated along the C2S grain boundaries at high basicities. Slowly cooled slags consist of the γ-C2S polymorph instead of the β-C2S and of less Mg, Ca-silicates. Isothermal conduction calorimetry and thermogravimetric analysis indicate the occurrence of hydration reactions in the stabilized slags after mixing with water, while calcium silicate hydrates (C-S-H) of typical acicular morphology are identified by SEM. The present results demonstrate that the application of high cooling rates can result in a stable, environmental-friendly, hydraulic binder from stainless steel slags, rich in β-C2S, without the necessity of introducing any additions to arrest the β polymorph.

Rare Earths and the Balance Problem: How to Deal with Changing Markets?

The balance between the market demand and the natural abundance of the rare-earth elements (REEs) in ores, often referred to as the Balance Problem (or the Balancing Problem), is a major issue for REE suppliers. The ideal situation is a perfect match between the market demand for and the production of REEs, so that there are no surpluses of any of the REEs. This means that the rare-earth industry must find new uses for REEs that are available in excess and search for substitutes for REEs that have either limited availability or are high in demand. We present an overview of the trends in the applications for the different REEs and show that the demand for REEs for use in magnets, catalysts, and alloys is still increasing, while the application of REEs in polishing agents, glass, and ceramics are stable. On the other hand, the use of REEs in nickel–metal-hydride (NiMH) batteries and lamp phosphors is decreasing. These changes in the REE market have an influence on the Balance Problem, because the REEs that can be recycled from fluorescent lamps, cathode-ray tubes (CRTs), and NiMH batteries have to be at least partly reused in other applications. Magnesium and aluminum alloys offer an opportunity to mitigate the Balance Problem caused by these changes in the REE market. This is illustrated for REEs that can be recycled from fluorescent-lamp phosphor waste, CRT phosphors, and NiMH batteries. At present, five REEs (Nd, Eu, Tb, Dy, and Y) are being considered as very critical by Europe, the United States, and Japan, but we forecast that in the medium term, only neodymium will remain a critical REE. This paper discusses the relationship between criticality and the Balance Problem and shows how this relationship influences the market for specific REEs.

Antimony Recovery from End-of-Life Products and Industrial Process Residues: A Critical Review

Antimony has become an increasingly critical element in recent years, due to a surge in industrial demand and the Chinese domination of primary production. Antimony is produced from stibnite ore (Sb2O3) which is processed into antimony metal and antimony oxide (Sb2O3). The industrial importance of antimony is mainly derived from its use as flame retardant in plastics, coatings, and electronics, but also as decolourizing agent in glass, alloys in lead-acid batteries, and catalysts for the production of PET polymers. In 2014, the European Commission highlighted antimony in its critical raw materials report, as the element with the largest expected supply–demand gap over the period 2015–2020. This has sparked efforts to find secondary sources of antimony either through the recycling of end-of-life products or by recovering antimony from industrial process residues. Valuable residues are obtained by processing of gold, copper, and lead ores with high contents of antimony. Most of these residues are currently discarded or stockpiled, causing environmental concerns. There is a clear need to move to a more circular economy, where waste is considered as a resource and zero-waste valorization schemes become the norm, especially for rare elements such as antimony. This paper gives a critical overview of the existing attempts to recover antimony from secondary sources. The paper also discusses the possibility of waste valorization schemes to guarantee a more sustainable life cycle for antimony.

Strengthening bottom-up and top-down climate governance

Although the UN and EU focus their climate policies on the prevention of a 2 °C global mean temperature rise, it has been estimated that a rise of at least 4 °C is more likely. Given the political climate of inaction, there is a need to instigate a bottom-up approach so as to build domestic support for future climate treaties, empower citizens, and motivate leaders to take action. A review is provided of the predominant top-down cap-and-trade policies in place – the Kyoto Protocol and EU Emissions Trading Scheme (EU ETS) – with a focus on the grandfathering of emissions entitlements and the possibility of offsetting emissions. These policies are evaluated according to two criteria of justice and it is concluded that they fail to satisfy them. Some suggestions as to how the EU ETS can be improved so as to enable robust climate action are also offered. Policy relevance The current supranational climate policy has not been successful and global leaders have postponed the adoption of a meaningful successor to the Kyoto Protocol. In view of this inaction, bottom-up approaches with regard to climate policy should be further developed. It is argued that two of the main top-down policies, grandfathering and offsetting, impede the avowed goals of EU climate policy and pose significant ethical dilemmas with regard to participatory and intergenerational justice. In order to provide a more robust EU climate policy, the EU should inter alia provide a long-term perspective for investors, reduce the volatility of the carbon price, and prepare for the possibility of carbon leakage.

The influence of curing conditions on the mechanical properties and leaching of inorganic polymers made of fayalitic slag

This study reports on the impact of the curing conditions on the mechanical properties and leaching of inorganic polymer (IP) mortars made from a water quenched fayalitic slag. Three similar IP mortars were produced by mixing together slag, aggregate and activating solution, and cured in three different environments for 28 d: a) at 20 °C and relative humidity (RH) ~ 50% (T20RH50), b) at 20 °C and RH≥90% (T20RH90) and c) at 60 °C and RH ~ 20% (T60RH20). Compressive strength (EN 196-1) varied between 19 MPa (T20RH50) and 31 MPa (T20RH90). This was found to be attributed to the cracks formed upon curing. Geochemical modelling and two leaching tests were performed, the EA NEN 7375 tank test, and the BS EN 12457-1 single batch test. Results show that Cu, Ni, Pb, Zn and As leaching occurred even at high pH, which varied between 10 and 11 in the tank test’s leachates and between 12 and 12.5 in the single batch’s leachates. Leaching values obtained were below the requirements for non-shaped materials of Flemish legislation for As, Cu and Ni in the single batch test.