The circular economy demands waste utilization for the production of high-value products, and this requires the development of novel processing routes. In this study, rare earth (La and Ce) oxides were completely (>99%) recovered from polishing waste by a combined novel reductive acid leaching and alkali treatment process. About 70% of rare earths were dissolved during the first leaching step. The undissolved rare earth compounds are converted to oxides/hydroxides by alkali treatment and dissolved in the acid solution – the 2nd leaching step – for the complete recovery of rare earths. The recovered rare earth oxides were used for producing in-situ high-value Al-La-Ce alloys with fused salt electrolysis. Mechanical properties of our Al-La-Ce alloys are similar to the known high temperature Al-Ce alloys. This development of new alloys by our novel process helps in utilization of both overproduced primary La and Ce oxides as well as La and Ce recovered from polishing waste.

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The management of bauxite residue (BR) is a major issue for the aluminum industry because of its high alkalinity and the large volumes generated. Therefore, the recovery of rare earth elements (REEs) with or without other metals from BR and utilization of the generated residue can contribute to a solution on the management problem of BR and it can be one of the options to meet the demand of REEs. In view of the above, the selective recovery of REEs over major elements such as iron by direct acid leaching was studied initially. From the leaching results, either the recovery of REEs was low or the dissolution of iron was high. To address that, iron was removed from BR by smelting. The slag generated after smelting was leached with mineral acids. The selectivity of REEs over iron was greatly improved. However, the high level of alumina presence in BR required a large amount of fluxes thereby increasing the energy consumption in smelting. Hence, the removal (and recovery) of alumina from BR by sodium carbonate roasting was carried out. The sample, after alumina removal, was smelted and the REEs were successfully recovered from slag by leaching with mineral acids. An alternative process, called sulfation–roasting–leaching, was also developed by which the REEs can be selectively leached. The scandium recovery, however, was low. Preliminary energy and economic analysis showed that alkali roasting–smelting–leaching and sulfation–roasting–leaching were the most promising processes for the treatment of BR.@en

One of the main applications of ceria (CeO ₂ ) is its use in glass polishing. About 16,000 tonnes of rare earth oxides, which is about 10% of total rare earth production, are used for polishing applications. The waste generated in glass polishing contains rare earths, along with other impurities. In this study, two different glass polishing waste samples were characterized and two different processes were proposed for the complete recovery of rare earths from polishing waste, i.e., an acid-based process and an alkali-based process. The polishing waste samples were characterized with inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA) and particle size analysis. Chemical analysis showed that sample A (CeO ₂ -rich waste from plate glass polishing) contained a high amount of impurities compared to sample B (CeO ₂ -rich waste from mirror polishing). XRD analysis showed that sample B contained CeO ₂ , LaO _0.65 F _1.7 and LaPO ₄ compounds, whereas sample A contained CaCO ₃ in addition to rare earth compounds. SEM-EDX analysis showed the presence of alumino-silicates in sample A. Leaching experiments were carried out at 75 ^◦ C at different acid concentrations for the recovery of rare earths from polishing waste samples. The leaching results showed that it is difficult to dissolve rare earths completely in acid solutions due to the presence of fluorides and phosphates. Hence, undissolved rare earths in the leach residue were further recovered by an alkali treatment with NaOH. In another approach, polishing waste samples were directly treated with NaOH at 500 ^◦ C. After alkali treatment followed by water leaching, rare earths can be completely dissolved during acid leaching. Rare earths from polishing waste can be recovered completely by both the acid-based process and the alkali-based process.

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A study has been carried out to understand the modification of alumina inclusions in Al-killed high sulphur steel with calcium treatment. For calcium treatment to be effective, a general practice is to desulphurise the steel to prevent the formation of solid CaS inclusions that are harmful to steel quality and final properties. To avoid this additional desulphurising step, the authors developed a new approach of calcium treatment of steel at an industrial scale. This approach involves treating the liquid steel with calcium treatment at low aluminium levels which enables formation of liquid calcium aluminate inclusions (C12A7) in the melt and then adding remaining amount of required aluminium. Based on this principle, Al-S diagram has been developed and calcium treatment has been modified accordingly. The inclusion transformation and morphology were studied using scanning electron microscope /energy dispersive spectroscopy analysis and. activity of CaS was calculated.

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Ceria is the main component in glass polishing powders due to its special physico-chemical properties. Glass polishing powder loses its polishing ability gradually during usage due to the accumulation of other compounds on the polishing powder or due to changes in the particle size distribution. The recovery of cerium from the glass polishing waste results in the efficient utilization of natural resources. This paper reviews processes for the recovery of rare earths from polishing waste. Glass polishing powder waste can be reused via physical, physico-chemical or chemical processes by removing silica and/or alumina. The removal of silica and/or alumina only improves the life span up to some extent. Therefore, removal of other elements by chemical processes is required to recover a cerium or cerium-rich product. However, cerium leaching from the polishing waste is challenging due to the difficulties associated with the dissolution of ceria. Therefore, high acid concentrations, high temperatures or costly reducing agents are required for cerium dissolution. After leaching, cerium can be extracted from the leach solution by solvent extraction or selective precipitation. The product can be used either in glass polishing again or other high value added applications.

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The present work describes the characterisation of the two different polishing waste samples. The samples were analysed with laser particle size analyser, XRD, XRF, TGA, and SEM. In the sample A, a large amount of calcite (CaCO3) together with silica and aluminosilicates were observed with SEM. In the minerology, only CaCO3, CeO2 and LnO0.65F1.7 were found. In the sample B, very small amounts of impurities (<2%) were found. The particle size of sample B was decreased compared to it’s original polishing powder. CeO2 and LnO0.65F1.7 compounds were found in the XRD analysis.@en

Cerium is the most abundant element out of the 17 rare-earth elements (REEs). The current production of CeO2 is about 54,400 t (32% of RE oxides). About 40,000 t of RE oxides are consumed by glass industry, out of which about 16,000 tons are being used for polishing applications1. Cerium dioxide is the main component (40 – 99%) in glass polishing powders2–4. It is also used for polishing silicon wafers and ceramics. The CeO2 particles in the slurry polish the glass with chemical-mechanical action. The waste slurry generated after glass polishing contains cerium together with other REEs (mainly lanthanum, neodymium and praseodymium), silicon, iron, aluminium, zinc, sodium etc. The concentration of the elements other than REEs in glass polishing waste depends on the chemical composition of the glass, additives and flocculants. The abrasive properties of polishing powder diminish due to the enrichment of the slurry with other elements with time and affect the quality of product adversely. Currently, the waste generated after polishing (CeO2: 8-80%)5,6 ends up in landfills3. Recycling of this waste not only contributes to the proper utilisation of natural resources but also makes recycling of valuable REEs possible, and it also saves the disposal cost. In addition, the recovered cerium can be converted to cerium metal and used in high value aluminium and magnesium alloys and steels. This generates highly added value from waste and closes metal loops. Therefore, this paper reviews the literature on recovery of cerium from glass polishing waste.@en