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Carbonation of lime (CaO) using ethanol is studied for direct atmospheric CO2 capture. The parameters that control the carbonation of lime using air and ethanol are determined, and operational conditions that enhance the CO2 uptake are investigated. A mixture of lime and ethanol was exposed to air within a fume hood, allowing ethanol evaporation for a defined period. Results reveal that the carbonation occurs after ethanol has evaporated, when the remaining powder is exposed to the atmosphere. After contact with ethanol, the CO2 uptake of lime is enhanced because the specific surface area of the powder increases. Increasing the time of exposure to the atmosphere results in an increase in the carbonation conversion (from 0.03 gCO2/gCaO for 1 day of exposure, up to 0.44 gCO2/gCaO for 60 days of exposure). Milling the solvent-lime slurry, before exposing it to the atmosphere, also enhances the carbonation yield through the effect of size reduction and mechanical activation (from 0.30 gCO2/gCaO with 5 min of pre-milling, to 0.60 gCO2/gCaO with 10 min of pre-milling). Another way to increase the carbonation yield is through the use of alkaline wastewater. Combining the wastewater with ethanol, or milling a slurry of CaO/wastewater before exposure to the atmosphere, increases the carbonation yield further to 90.2% (0.70 gCO2/gCaO) and 96.2% (0.75 gCO2/gCaO), respectively. This study reveals that ethanol/alkaline wastewater treatments, along with their different enhancement options, represent an effective viable route for sequestering CO2 from the atmosphere, which could be applied to pure lime or potentially to lime-containing wastes.

© 2019 Elsevier B.V. During acid leaching of bauxite residue (red mud), the increase in dissolution of rare-earth elements (REEs) is associated with a substantial co-dissolution of iron; this poses problems in the downstream processing (i.e. solvent extraction or ion exchange). Six different slags generated by reductive smelting of the same bauxite residue sample were treated by high-pressure acid leaching (HPAL) with HCl and H 2 SO 4 to selectively extract REEs. Thus, up to 90 wt% of scandium was extracted from the slags using H 2 SO 4 at 150 °C, while with HCl the extraction of scandium reached up to 80 wt% at 120 °C. The extraction of yttrium, lanthanum and neodymium was above 95 wt% when HCl was used as a reagent, but it was much lower (90 wt%, 18 g L −1 ), while the concentration of the remaining iron (>60 wt%) was of 3 g L −1 in the leachate. The co-dissolution of silicon and titanium was lower than 5 wt%. ispartof: HYDROMETALLURGY vol:184 pages:162-174 status: published

In this work, the extraction of selected rare earth elements from bauxite residue by dry digestion method followed by water leaching was investigated. Kinetic studies performed with HCl and H2SO4 demonstrated that, at ambient temperatures, silica dissolution increases with increasing acid concentration, which leads to the formation of silica gel. Dissolution of silica is limited to less than 5 wt% by applying a two-step process: dry digestion of bauxite residue with HCl or H2SO4, followed by water leaching. The extraction of aluminium was low because of the low solubility of aluminosilicate compounds. The extraction of iron and titanium increased with increasing acid concentrations. High extraction of the rare-earth elements (REEs) were achieved with the HCl-based dry digestion method, but the concentration in the leachate was limited to approximately 6–8 mg L−1. About 40 wt% of scandium was recovered with a high co-dissolution of iron, due to the occurrence of scandium(III) ions in the lattice matrix of iron(III) oxide. Dry digestion method with multi-stage circulation of the acid leaching solution significantly increased the REEs concentration up to 20 mg L−1, while achieving an acid consumption of 788 g of HCl per kilogram of bauxite residue, and a significant reduction of water consumption (60%) relative to the single-stage acidic leaching method. The low water consumption allows to increase the filtration efficiency of the leach liquor due to the avoidance of silica gel formation.

© 2017 The present work considers the neutralisation of bauxite residue (red mud) with CO2 as a potential technology for reducing the acid consumption in the acidic leaching step for metal recovery. The pH of bauxite residue was reduced during neutralisation by the transformation of hydroxide ions to (hydrogen) carbonate ions. Neutralisation at high CO2 partial pressures and high temperatures reduces the alkalinity of the bauxite residue, but it leads to the stabilisation of silicate compounds such as cancrinite and grossular. After acidic leaching of the neutralised product with sulfuric acid, a decrease by 20% in the dissolution yield of Al, Fe, and Ti was observed, due to an insufficient amount of acid devoted to leaching as the transformation of calcite into bassanite and the high concentration of silicate compounds consumed part of the acid. Sc recovery by leaching of the highly neutralised bauxite residue was about 35 wt%, which depends on Fe and Ti recovery. A positive correlation between La/Nd with Sc indicates that the recovery of La/Nd indirectly depends on the extraction of Fe/Ti, as Sc is chemically associated to these major metals. ispartof: Minerals Engineering vol:112 pages:92-102 status: published

International audience; Considering the wide use and production of nano-objects since last two decades, these trendy nanomaterials (NMs) are expected to end up in thermal disposal and incineration waste plants. It seems relevant to assess the risks related to the thermal disposal and incineration of waste containing NMs (WCNMs). The objective of this work is to develop a methodology for risk management in order (1) to support decision-makers and incineration plant managers and (2) to give insights on nanosafety of exposed operators and on potential environmental risks related to the incineration and thermal disposal of WCNMs. Therefore, the main challenge is to find (a) key parameter(s) which would govern the decision related to risk management of NMs thermal disposal. On the one hand, we focused on the relevant literature studies about the incineration of NMs. On the other hand, we conducted a panel discussion. The review of this literature highlights that the nanostructure destruction of the nano-object appears as a relevant indicator of the risks related to the NMs incineration. As a consequence, we defined a “temperature of nanostructure destruction” (TND) which would be the temperature from which the nanostructure will be destroyed. For the later, this parameter has been assumed to be a consistent indicator to develop a preliminary methodology. If the combustion chamber temperature is higher than the TND of the NM or if they are close to each other, then the nanostructure will be destroyed and no risks related to NMs remain. If the TND of the NMs is higher than the combustion chamber temperature, then the nanostructure will not be destroyed and risks related to NMs have to be considered. As a result, five groups of nanomaterials have been defined. WCNMs including Carbon NMs appear to be in good position to be destroyed correctly. On the other hand, there would not be operational thermal disposal available to manage WCNMs including CeO2 and ZrO2. Finally, a decision tree has been designed. TND is used as criteria to assess if a waste can be managed safely or not by a specific thermal disposal and which safety measures have to be taken.

The potential environmental risk arising from the incineration of waste containing nanomaterials is a new field which deserves further attention. Some recent studies have begun to focus on this topic (e.g. Walser 2012, Le Bihan 2014, Ounoughene 2015) but the data are incomplete. In addition, there is a need to consider real life waste. The present study gives some insight into the fate and behavior of a commercial coating containing a commercial additive (7%) based on nano-CeO2 (aggregates of 10 to 40 nm, with elemental particles of 2-3 nm). The tests have been conducted with a system developed in the frame of the NanoFlueGas project (Le Bihan 2014). It involves the combustion parameters of incineration (temperature around 850°C, highly ventilated combustion, at least 2 s residence time for the combustion gas in a postcombustion chamber at 850°C, and high oxygen/fuel contact). Time tracking by electric low pressure impaction (ELPI) shows that the incineration produces aerosol with number concentration dominated by sub-100 nm particles. Cerium is observed by TEM and EDX analysis but as a minority in the frame of heterogeneous particles (Figure 1). ICP-MS analysis indicates that the residual material consists mainly of CeO2 (60% of the mass). Observation by TEM establishes that is in the form of 40-200 nm aggregates and suggests that sintering occurred during incineration (Figure 2). As a conclusion, incineration led mainly to the release of nano-CeO2 in the residual material, as the major component. This is consistent with a scale 1 experiment conducted by spraying nano-CeO2 onto waste in the furnace entrance (Walser 2012). Aggregates are larger compared to the initial waste, and sintering is observed. So, the incineration did not destroy the nano-structure of the waste but transformed it (and probably decreased the specific surface).

International audience; Considering the wide use and production of NMs since last two decades, these trendy nanomaterials (NMs) are expected to end up in thermal disposal and waste incineration plants (WIP). It seems relevant to assess the risks related to the thermal disposal and incineration of waste containing NMs (WCNMs). The objective of this work is to present a first approach to develop a preliminary methodology for risk management in order (1) to give insights on nanosafety of exposed operators and on potential environmental risks related to the incineration and thermal disposal of WCNMs, and (2) to eventually support decision-makers and incineration plant managers. Therefore, the main challenge is to find (a) key parameter(s) which would govern the decision related to risk management of NMs thermal disposal. On the one hand, we focused on the relevant literature studies about experimental works on incineration of NMs. On the other hand, we conducted an introductory discussion with a group of experts. The review of this literature highlights that the nano-object's nanostructure destruction appears as a relevant indicator of the risks related to the NMs incineration. As a consequence, we defined a "temperature of nanostructure destruction" (TND) which would be the temperature from which the nanostructure will be destroyed. This parameter has been assumed to be a consistent indicator to develop a preliminary methodology. If the combustion chamber temperature is higher than the TND of the NM (or if they are close to each other), then the nanostructure will be destroyed and no risks related to NMs remain. If the TND of the NMs is higher than the combustion chamber temperature, then the nanostructure will not be destroyed and risks related to NMs have to be considered. As a result, five groups of NMs have been identified. WCNMs including carbonic NMs appear to be in good position to be destroyed safely in WIP. On the other hand, based on this criterion, there would be no available thermal disposal plants to safely manage WCNMs including CeO2 and ZrO2. Finally, a decision tree has been designed. TND is used as criteria to assess if a waste can be managed safely or not by a specific thermal disposal and which safety measures have to be taken.

The potential environmental risk arising from the incineration of waste containing nanomaterials is a new field which deserves further attention. Some recent studies have begun to focus on this topic but the data are incomplete. In addition, there is a need to consider real life waste. The present study gives some insight into the fate and behavior of a commercial coating containing a commercial additive (7% w/w) based on nano-CeO2 (aggregates of 10 to 40 nm, with elemental particles of 2-3 nm). The tests have been conducted with a system developed in the frame of the NanoFlueGas project. The test protocol was designed to respect the regulatory criteria of a good combustion in incineration plants (temperature around 850°C, highly ventilated combustion, at least 2 s residence time for the combustion gas in a post-combustion chamber at 850°C, and high oxygen/fuel contact). Time tracking by electric low pressure impaction (ELPI) shows that the incineration produces aerosol with number concentration dominated by sub-100 nm particles. Cerium is observed by TEM and EDS analysis but as a minor compound of a sub-group of particles. No nanoCeO2 particles have been observed in the aerosol. ICP-MS analysis indicates that the residual material consists mainly of CeO2 (60% of the mass). Observation by TEM establishes that this material is in the form of aggregates with individual particle of 40-200 nm and suggests that sintering occurred during incineration. As a conclusion, the lab scale incineration study led mainly to the release of nano-CeO2 in the residual material, as the major component. Its size distribution is different than the one of the nano-CeO2 observed in the initial sample before incineration. Additional research is needed to improve the understanding of nanoCeO2 behavior, and to integrate experiments at lab and real scale.

© 2019, The Minerals, Metals & Materials Society. This study investigates the chemical associations of the selected rare-earth elements (Sc, Y, Ce, La, and Nd) with major element phases in the postprocessed bauxite residues and compares them with the untreated bauxite residue. The treatment of bauxite residue considers our previous published process which involved the neutralization with CO 2 , followed by leaching with H 2 SO 4 . Neutralized bauxite residue resulted with larger aggregates than the untreated bauxite residue after making contact with CO 2 as the consequence of additional CaCO 3 formation. Neutralization with CO 2 , however, has a negligible effect on the distribution of the rare-earth elements (REEs) with respect to the untreated bauxite residue, but a large amount of rare earths remained unreacted after acid leaching. Electron probe microanalysis (EPMA) confirmed the chemical associations of Sc(III) and Ce(IV) with Fe(III)- and Al(III)-containing minerals in the postprocessed bauxite residues, i.e., bauxite residues subjected to CO 2 -neutralization and neutralization–acid leaching processes. The occurrence of Nd(III) is positively correlated to that of La(III) in the untreated bauxite residue, but both of them may be associated with the same mineralogical phase as Ce(IV) after processing. Y(III) may remain associated with the Al/Si-minerals, cancrinite and chamosite. Ergo, the extractability of Sc, Y, Ce, La, and Nd from neutralized bauxite residue is more difficult in H 2 SO 4 media due to the presence of coarser particles compared to those of the untreated bauxite residue, but also due to the formation of a solid product layer (i.e., CaSO 4 ) that is presumably adsorbed on the surface of Fe(III)-rich phases (hematite and goethite) and Al(III)-containing minerals (diaspore, gibbsite, boehmite, and chamosite). ispartof: JOURNAL OF SUSTAINABLE METALLURGY vol:5 issue:1 pages:57-68 status: published