Tom Van Gerven

From NdFeB magnets towards the rare-earth oxides: a recycling process consuming only oxalic acid

A chemical process which consumes a minimum amount of chemicals to recover rare-earth metals from NdFeB magnets was developed. The recovery of rare-earth elements from end-of-life consumer products has gained increasing interest during the last few years. Examples of valuable rare earths are neodymium and dysprosium because they are important constituents of strong permanent magnets used in several large or growing application fields (e.g. hard disk drives, wind turbines, electric vehicles, magnetic separators, etc.). In this paper, the rare-earth elements were selectively dissolved from a crushed and roasted NdFeB magnet with a minimum amount of acid, further purified with solvent extraction and precipitated as pure oxalate salts. The whole procedure includes seven steps: (1) crushing and milling of the magnet into coarse powder, (2) roasting to transform the metals into the corresponding oxides, (3) the selective leaching of the rare-earth elements with acids (HCl, HNO3) to leave iron behind in the precipitate, (4) extracting remaining transition metals (Co, Cu, Mn) into the ionic liquid trihexyl(tetradecyl)phosphonium chloride, (5) precipitating the rare earths by the addition of oxalic acid, (6) removing the precipitate by filtration and (7) calcining the rare-earth oxalates to rare-earth oxides which can be used as part of the feedstock for the production process of new magnets. The magnet dissolution process from the oxides utilized four molar equivalents less acid to dissolve all rare earths in comparison with a dissolution process from the non-roasted magnet. Moreover, the less valuable element iron is already removed from the magnet during the dissolution process. The remaining transition metals are extracted into the ionic liquid which can be reused after a stripping process. Hydrochloric acid, the side product of the rare-earth oxalate precipitation process, can be reused in the next selective leaching process. In this way, a recycling process consuming only air, water, oxalic acid and electricity is developed to recover the rare earths from NdFeB magnets in very high purity.

Antimony leaching from MSWI bottom ash: Modelling of the effect of pH and carbonation

Development of treatment methods to reduce Sb leaching from municipal solid waste incinerator (MSWI) bottom ash, such as accelerated carbonation, is being complicated by insufficient understanding of Sb geochemistry. The leaching of antimonate (Sb(V)) and antimonite (Sb(III)) in MSWI bottom was studied as a function of pH and degree of carbonation. While total (Sb(V)+Sb(III)) leaching was lowest (1.2 mg kg(-1)) at the natural pH (i.e. 10.6) of uncarbonated bottom ash, HPLC-ICP-MS analysis showed that acidification and carbonation increased Sb(V) leaching, but decreased Sb(III) leaching, probably because Sb(III)(OH)(4)(-) became less stable. PHREEQC geochemical modelling suggested that Sb(V) concentrations approached equilibrium with the romeites, i.e. calcium antimonates, Ca(1.13)Sb(2)(OH)(0.26)·0.74H(2)O at pH=10.6 and Ca[Sb(OH)(6)](2) at pH=8. It is hypothesised that not interaction with ettringite but dissolution of romeite controls antimonate leaching in the pH range 8-11 in MSWI bottom ash, because while Ca is preferentially leached from romeite, the mineral structures containing more Ca at higher pH are less soluble. A model was proposed where acidification and carbonation both lead to lower Ca(2+) and/or hydroxyl concentration, which removes Ca(2+) and hydroxyls from the romeite structure and leads to comparably higher Sb(V) concentration in equilibrium with romeite. Sb solubility depends on pH and Ca(2+) availability in this model, which has implications for bottom ash valorisation and risk assessment.

Potential release of selected trace elements (As, Cd, Cu, Mn, Pb and Zn) from sediments in Cam River-mouth (Vietnam) under influence of pH and oxidation.

Since contaminated river-bed sediments in the Cam River-mouth (Vietnam) are regularly dredged and disposed on land, an understanding of the influence of time, pH and oxidation on the leaching behavior of heavy metals (Cd, Cu, Mn, Pb and Zn) and arsenic is necessary for the management of these dredged materials. A 96 h pH(stat)-leaching test to examine the leaching behavior of elements at pre-set pH values (2, 4, 6, 8 (natural), 9 and 11) and a BCR 3-step extraction to clarify the element fractionation, were performed on a freshly-collected wet suboxic sediment and a dry oxidized sediment. All heavy metals and arsenic display a V-shaped pH-dependent leaching pattern with important releases at pHs 2 and 11. At the investigated pH values, the release of As, Mn, Pb and Zn from the oxidized sediment is slower and lower if compared with the suboxic sediment while the opposite trend is found for Cd and Cu at pHs 2-8. The transfer from the acid-soluble (exchangeable and carbonate-bound) fraction to the reducible (Fe and Mn hydr/oxide-bound) fraction is consistent with the lower leachability of As, Mn and Zn at pHs 2-8 and Pb at pHs 4-8 after oxidation, while the transfer from the oxidizable (organic matter and sulfide-bound) fraction to the reducible fraction relates to the higher leachability of Cd and Cu at pHs 2-8. The lower leachability of all elements at alkaline pHs 9-11 is due to lower leached concentration of organic matter from the oxidized sediment. Sulfides only play a minor role in controlling the leachability of heavy metals and arsenic.

Recovery of scandium from leachates of Greek bauxite residue by adsorption on functionalized chitosan–silica hybrid materials

Bauxite residue (red mud) is a waste residue that results from the production of alumina by the Bayer process. Since it has no large-scale industrial application, it is stockpiled in large reservoirs. Nevertheless, it should be considered as a valuable secondary resource as it contains relatively large concentrations of critical metals like the rare earths, scandium being the most important one. In this work, we investigated the recovery of scandium from real leachates of Greek bauxite residue. In the separation of scandium from the other elements, the biggest challenge arose from the chemical similarities between scandium(III) and iron(III). This hampers high selectivity for scandium, especially because iron, as one of the major elements in bauxite residue, is present in much higher concentrations than scandium. In order to achieve selectivity for scandium, chitosan–silica particles were functionalized with the chelating ligands diethylenetriamine pentaacetic acid (DTPA) and ethyleneglycol tetraacetic acid (EGTA). Both organic ligands were chosen because of the high stability constants between scandium(III) and the corresponding functional groups. The adsorption kinetics and the influence of pH on hydrolysis and adsorption were investigated batchwise from single-element solutions of scandium(III) and iron(III). In binary solutions of scandium(III) and iron(III), it was observed that only EGTA-functionalized chitosan–silica appeared to be highly selective for scandium(III) over iron(III). EGTA–chitosan–silica shows a much higher selectivity over state-of-the-art adsorbents for the separation of scandium(III) from iron(III). The latter material was therefore used as a resin material for column chromatography in order to effectively separate scandium from bauxite residue. Full separation was achieved by eluting the column with HNO3 solution at pH 0.50; at this pH all other elements had already eluted.

Photochemical recycling of europium from Eu/Y mixtures in red lamp phosphor waste streams

Europium is one of the most critical rare-earth elements due to the combination of a high demand for the production of red lamp phosphors and the limited supply of this element. Hence, the recycling of europium from end-of-life lamp phosphors has gained a lot of interest. Separation of europium from rare-earth mixtures can be done very efficiently by selective reduction of Eu(III) to Eu(II) and subsequent removal of Eu(II) by EuSO4 precipitation. The present study shows that full separation of europium from non-equimolar binary europium/yttrium mixtures, which reflect the rare earth composition of red lamp phosphors, can be achieved by photochemical reduction of Eu(III). Eu/Y molar ratios up to 1/20 were tested in the presence of an isopropanol radical scavenger, a less harmful and toxic compound than the commonly used formic acid scavenger. Moreover, in contrast to using the very acidic formic acid, higher pH values could be reached with isopropanol, which is advantageous for the formation and the stability of the reduced Eu(II) species. Faster europium removal was obtained at higher pH values up to pH 4, halving the illumination time to reach 88% of europium recovery. Efficiencies of over 95% were reached, with purities of 98.5% of the EuSO4 precipitate. Europium recovery of industrial YOX powder was also achieved from a Eu/Y 1/30 ratio, with 50% yield of EuSO4 and a purity of 96%.

Recent developments and perspectives on the treatment of industrial wastes by mineral carbonation — a review

Besides producing a substantial portion of anthropogenic CO2 emissions, the industrial sector also generates significant quantities of solid residues. Mineral carbonation of alkaline wastes enables the combination of these two by-products, increasing the sustainability of industrial activities. On top of sequestering CO2 in geochemically stable form, mineral carbonation of waste materials also brings benefits such as stabilization of leaching, basicity and structural integrity, enabling further valorization of the residues, either via reduced waste treatment or landfilling costs, or via the production of marketable products. This paper reviews the current state-of-the-art of this technology and the latest developments in this field. Focus is given to the beneficial effects of mineral carbonation when applied to metallurgical slags, incineration ashes, mining tailings, asbestos containing materials, red mud, and oil shale processing residues. Efforts to intensify the carbonation reaction rate and improve the mineral conversion via process intensification routes, such as the application of ultrasound, hot-stage processing and integrated reactor technologies, are described. Valorization opportunities closest to making the transition from laboratory research to commercial reality, particularly in the form of shaped construction materials and precipitated calcium carbonate, are highlighted. Lastly, the context of mineral carbonation among the range of CCS options is discussed.

Susceptibility of mineral phases of steel slags towards carbonation: mineralogical, morphological and chemical assessment

Process limitations have thus far prevented mineral carbonation of alkaline wastes from been widely applied. These barriers are caused by inefficient processing, but also by mineralogical aspects inherent to the materials. Better understanding and predictability of the effects of mineral carbonation on alkaline materials could be obtained by studying the carbonation susceptibility of constituent minerals separately, allowing for detailed and accurate analysis of their reaction kinetics and maximal conversions and of the carbonate products formed. For this purpose, this paper presents the synthesis and carbonation of the seven most abundant alkaline minerals found in AOD, CC and BOF slags, namely: akermanite (Ca 2 MgSi 2 O 7 ), bredigite (Ca 7 Mg(SiO 4 ) 4 ), cuspidine (Ca 4 Si 2 O 7 F 2 ), β– and γ–C 2 S (Ca 2 SiO 4 ), merwinite (Ca 3 Mg(SiO 4 ) 2 ), and srebrodolskite (Ca 2 Fe 2 O 5 ). Two experimental approaches to mineral carbonation of increasing levels of process severity are utilized: (mild) incubator carbonation, and (accelerated) pressurized slurry carbonation. In addition, the slags and two free oxides (CaO and MgO) are equally carbonated and evaluated. Data regarding CO 2 uptake, mineral conversion and formed carbonate and non-carbonate products in the samples were obtained through TGA, QXRD (Rietveld refinement) and SEM techniques. Reduction in material basicity and evolution of particle morphology were also assessed. The synthesized mineral purities (>70 wt% target mineral phase) were found sufficient for more accurate assessment of carbonation behaviour of the individual minerals. Bredigite was found to be the most reactive mineral under all processing conditions; C 2 S and wollastonite were more reactive under slurry carbonation, while srebrodolskite and calcium monoferrite were found to be more reactive under moist carbonation. Merwinite and diopside had the slowest carbonation conversions. Calcite and aragonite were the dominant carbonate products formed, whereby aragonite formation was promoted in Mg-containing materials. The morphology of aragonite crystals and the packing density of its product layer were found to vary depending on the parent mineral. Characteristic slag carbonation products, not observed as extensively from synthetic mineral samples, were magnesian calcite from slurry carbonation, and monohydrocalcite and vaterite from moist carbonation. Wollastonite was the main crystalline non-carbonate product, occurring predominantly from slag carbonation, while silica-rich amorphous matter formed in all samples proportionally to CO 2 uptake. Free lime, when present, controlled material basicity above pH 12, while silicates were found to typically possess pH in the range of 11.3–11.9, and Ca-carbonates eventually controlled the pH of well carbonated samples to values under 10.

Effect of ultrasonic homogenization on the Vis/NIR bulk optical properties of milk

The size of colloidal particles in food products has a considerable impact on the products physicochemical, functional and sensory characteristics. Measurement techniques to monitor the size of suspended particles could, therefore, help to further reduce the variability in production processes and promote the development of new food products with improved properties. Visible and near-infrared (Vis/NIR) spectroscopy is already widely used to measure the composition of agricultural and food products. However, this technology can also be consulted to acquire microstructure-related scattering properties of food products. In this study, the effect of the fat globule size on the Vis/NIR bulk scattering properties of milk was investigated. Variability in fat globule size distribution was created using ultrasonic homogenization of raw milk. Reduction of the fat globule size resulted in a higher wavelength-dependency of both the Vis/NIR bulk scattering coefficient and the scattering anisotropy factor. Moreover, the anisotropy factor and the bulk scattering coefficients for wavelengths above 600 nm were reduced and were dominated by Rayleigh scattering. Additionally, the bulk scattering properties could be well (R(2) ≥ 0.990) estimated from measured particle size distributions by consulting an algorithm based on the Mie solution. Future research could aim at the inversion of this model to estimate the particle size distributions from Vis/NIR spectroscopic measurements.

Thermal Conductivity Prediction of Pure Liquids Using Multi-Layer Perceptron Neural Network

H EAT transfer of fluids is important in many industrial sectors, including energy supply, transportation, production, and electronics. Tomodel the heat transfer process, thermal conductivity data are required. Knowing thermal conductivity values of liquids is of great importance in the design of chemical engineering; however, these values are not easily calculable in an accurate manner. Through different investigations, the thermal conductivity has been calculated and estimated for many liquids, and there are two general trends for estimation of thermal conductivity: 1) using relations based on the theory of liquids thermal conductivity; and 2) using correlations relating thermal conductivity to other properties. In the second class, properties can be referred to as critical temperature, critical pressure, and molecular weight; and they are all simply measurable. Widespread application of a neural network has been obvious in many fields of chemical engineering over the last years. The thermal conductivity of liquids has been predicted by using this kind of network and compared with experimental outcomes. The artificial neural network (ANN) has shown successful performance in various fields of modeling, such as engineering systems, mathematics, medicine, economics, and meteorology [1,2]. Based on experimentally measured variables, Krzywanski and Nowak [3] developed an ANN model to predict the local overall heat transfer coefficient for membrane walls in the 260 megawatts of electrical power circulating-fluidized-bed boiler. Kurt and Kayfeci [4] proposed an ANN model to estimate the local overall heat transfer coefficient of ethylene glycol/water solutions at various temperatures and different concentrations. Boniecki et al. [5] investigated the options of applying the ANN as a predictive instrument for modeling ammonia emissions released in composting sewage sludge. The present study is intended to establish a neural network model to predict the thermal conductivities of pure liquids at atmospheric pressure over a wide range of temperatures and many types of substances. In this regard, the properties of a liquid’s temperature, critical pressure, critical temperature, boiling temperature, and molecular weight are set as network inputs.

The effects of ultrasound on micromixing.

The Villermaux-Dushman reaction is a widely used technique to study micromixing efficiencies with and without sonication. This paper shows that ultrasound can interfere with this reaction by sonolysis of potassium iodide, which is excessively available in the Villermaux-Dushman solution, into triiodide ions. Some corrective actions, to minimize this interference, are proposed. Furthermore, the effect of ultrasonic frequency, power dissipation, probe tip surface area and stirring speed on micromixing were investigated. The power and frequency seem to have a significant impact on micromixing in contrast to the stirring speed and probe tip surface area. Best micromixing was observed with a 24kHz probe and high power intensities. Experiments with different frequencies but a constant power intensity, emitter surface, stirring speed, cavitation bubble type and reactor design showed best micromixing for the highest frequency of 1135kHz. Finally, these results were used to test the power law model of Rahimi et al. This model was not able to predict micromixing accurately and the addition of the frequency, as an additional parameter, was needed to improve the simulations.