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The common industrial practice for primary aluminum production consists of the Bayer process for the production of alumina followed by the Hall-Heroult process for the production of aluminum. Both processes were developed at the end of the 19th century and despite continuous optimization, their basic thermodynamic inefficiencies and environmental issues remain till today unchanged. As a result, primary aluminum production industry is the world's larger industrial consumer of energy, is ranked among the most CO2 intensive industries, and is associated with the generation of enormous quantities of solid wastes. In this paper a detail energy and exergy analysis of the primary production of aluminum is presented and alternative sustainable processes are reviewed. [ABSTRACT FROM AUTHOR]

The aim of this article is to predict, through reactive transport modeling, the progress of the reaction front in lignite-based fly ash permeable reactive barriers by taking into account media compositional changes and degradation rates as well as the drop in reactivity due to precipitation and clogging, metal ion concentration, and removal mechanisms. In order to simulate field conditions, synthetic solutions with the quality of leachates generated at waste disposal sites (in terms of pH and toxic load) were pumped upflow in two 40-cm-long laboratory columns placed in series. In addition, this article aims to introduce engineering and science tools in understanding the mechanisms and the geochemical processes that determine the fate of contaminants during migration of acidic plumes in environmental media. The following methodological approach may be helpful in forensic investigations aiming to establish the contamination history at waste disposal sites. [ABSTRACT FROM AUTHOR]

This article discusses the fate assessment of several inorganic contaminants in zero-valent iron treatment walls used for the cleanup of acidic plumes and the prevention of groundwater contamination in active or abandoned mixed sulphide and coal mining sites. The fate assessment of contaminants provides useful information for potential forensics investigations carried out in affected mining and waste disposal sites. Laboratory studies using sodium chloride as conventional tracer were carried out to identify transport-related issues and assess the performance and long-term reactivity of iron walls. Transport parameters, such as residence time, dispersion, and heterogeneity, were determined by fitting chloride breakthrough curves (BTCs) to the convection-dispersion equation using time moment analysis and the CXTFIT 2.1 curve-fitting software. First moment analysis of breakthrough curves revealed modest increase in pore-water velocity over time, indicative of decreasing porosity within the reactive mass, mainly due to the formation of mineral precipitates. Geochemical modeling of the process, including interactions between iron filings, heavy metal ions, and sulphates; interpretation of the ionic profiles; and calculation of porosity loss was also carried out by using the speciation/mass transfer computer code PHREEQC-2 and the WATEQ4F database. [ABSTRACT FROM AUTHOR]

Modeling the chemical equilibrium in ionic solutions encountered in industrial applications, especially in the field of hydrometallurgy, still remains an unresolved issue. The complicated speciations, as well as the high ionic strengths encountered in these solutions, render the theories of analytical chemistry practically useless. The Debye–Hückel theory is examined in depth, so as to reveal the reasons of its failure. To remedy this problem, over the years, a large number of semi-empirical models have been proposed that are here reviewed, in order to help the reader find a model best suited for a system of interest. The models are classified into three main categories, based on their fundamental logic: ion interaction models describe the system through the physical interactions of the ions; ion association models describe the system through chemical equilibriums; and finally hybrid models use concepts from both previous categories. Focus is given in presenting the idea upon which each model is based rather than simply presenting the equations required for its implementation. [ABSTRACT FROM AUTHOR]

This article aims to investigate the stabilization of Pb, Zn, and Cd contained in contaminated soil from a former mining site in Montevecchio, Sardinia, using clinoptilolite-rich tuff from Pentalofos, Evros. The study included (1) batch experiments and their environmental characterization and (2) column experiments. The first tests involved 1-month pot experiments with varying soil-zeolite mixtures, and their evaluation was carried out by standard USEPA leaching tests (TCLP, EPT test, SPLP). Moderate solubility reductions were recorded according to the TCLP (Pb: 38%, Zn: 33%, and Cd: 32%) due to the introduction of competing Na ions in the solution, while the EPT test showed more significant variations (Pb: 55%, Zn: 74%, and Cd: 46%). A major decrease is achieved in both cases by 10% w/w zeolite addition. The study was complemented by column experiments involving soil-zeolite mixtures eluted by CH 3COOH solutions (0.003N and 0.05N). The solubility of Pb was reduced by 50 to 60% compared with the control column, thus indicating the considerable selectivity of clinoptilolite for Pb. In addition, Na and Ca measurements in the leachates confirmed that the immobilization of Pb was mainly attributed to ion exchange reactions. [ABSTRACT FROM AUTHOR]

The use of organic solvents in an electrolytic system for neodymium electrorecovery by electrolysis at low temperatures is studied in the current work. More specifically, an alternative route, that of the system of DMSO (Dimethyl sulfoxide) with dissolved NdCl3 has been researched and has given promising results. The study of this electrolytic system has been divided into two stages. Firstly, the characteristics of the electrolyte, the dissolution of NdCl3 in DMSO, the conductivity and the viscosity of NdCl3 solutions in DMSO at various temperatures, and the Nd complexation in the solution were studied and secondly, the electrolysis parameters and their impact on the Nd electrodeposition process were evaluated. Finally, the deposits were submitted to SEM-EDS (Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy) analysis and metallic Nd was confirmed to be electrodeposited by X-ray Photoelectron Spectroscopy (XPS) spectroscopy. [ABSTRACT FROM AUTHOR]

The effectiveness of an in situ heavy metals fixation technique aimed at converting contaminants to low solubility and low bioavailability forms, eliminating the risk posed by oxidic tailings and contaminated soils, was investigated. Calcium oxyphosphate salt (Ca(H2PO4)2 H20) was used as a stabilizing agent for oxidic tailings and contaminated soils originating from Montevecchlo, Sardinia, Italy. Stabilization was effected by mixing the contaminated soil or oxidic tailing sample with calcium oxyphosphate salt at various doses. The effectiveness of stabilization was evaluated by USEPA TCLP standard toxicity testing. Complementary EDTA extraction tests and biological tests using beans Phaseolous vulgaris asplant indicator were carried out. The toxicity of Pb and Cd was reduced below TCLP regulatory limits at calcium oxyphosphate doses higherthan 0.7 and 0.2% w/w for soils and tailings, respectively. Lead solubility according to the EDTA test decreased with phosphate dose for both materials tested. Lead uptake by plant leaves and roots from thesoil sample decreased with the phosphate addition, while Cd uptake remained almost constant. An adverse effect on plant growth and Zn uptake was observed for calcium oxyphosphate dose up to 1.1 % w/w. Basedon the results, a remediation scheme for oxidic tailings and contaminated soils is proposed. [ABSTRACT FROM AUTHOR]

Abstract: The use of microwave radiation as an energy source in mineral processing and extractive metallurgy has demonstrated both the instantaneous generation of heat by microwaves in a number of compounds and minerals, and the achievement of high temperatures for an efficient time period enable the heating and reduction of metallic oxides and ores. In the present study, the carbothermic reduction of copper oxide (CuO) and one malachite [Cu2CO3(OH)2] concentrate were investigated. To explore feasibility, the dielectric constants [real (ε′) and imaginary (ε″) permittivities] of both materials were measured at the frequencies of 2.45GHz and 912MHz, in the temperature range from 25 to 800°C using the cavity perturbation method. The high ε″ values (between 1.9 and 36.3) observed in the case of CuO suggest strong microwave absorption, while the malachite concentrate values (between 0.1 and 0.4) indicate limited microwave absorption. Experiments showed the microwave heating rate of CuO was considerably higher than that of the malachite concentrate. The carbothermic reduction of CuO oxide was examined, and the effect of power supply, carbon source, carbon content and granularity of the reducing agent on the reduction rate was studied in detail. Using an 800W power supply, and with addition of lignite as a reducing agent (with carbon content two times stoichiometric), almost complete reduction of 10g of CuO was achieved in 4min. The carbothermic reduction of one malachite concentrate was attempted with the same experimental procedure, but the reaction was not possible using only lignite as the reducing agent, since the poor microwave absorption of malachite concentrate–lignite mixture produced a maximum temperature of 200°C. This difficulty was overcome by the addition of 5% by weight of graphite powder to the mixture. The rapid heating of the malachite concentrate–lignite–graphite mixture (800°C after 2min) resulted in sequential malachite calcination and CuO reduction reactions. After 8min at a power supply of 800W, the reduction degree of CuO produced by the calcination of malachite was about 90%. [Copyright &y& Elsevier]

Abstract: Current study investigates the possibility of obtaining the removal of iron from bauxite ores, by exploiting the metabolism of iron reducing microorganisms. The experiments were carried out using the microorganism Desulfuromonas palmitatis. The effectiveness of the treatment was evaluated on six different bauxite samples, containing the alumina oxide in the form of diaspore or boehmite. The Fe2O3 content of the samples varied between 16.3% and 22.3% and occurred mainly in the form of goethite or hematite, with one sample containing also chamosite. For comparison reasons the same treatment was also applied on three samples of synthetic Fe(III)-oxides, namely ferrihydrite, goethite and hematite. The removal of iron from the six bauxite samples was found to vary between 7% and 29% and the highest extraction corresponded to the sample containing the Fe(II) rich compound, chamosite. The bioavailability of Fe in the three synthetic oxides was highly dependant on their crystallinity. The amorphous ferrihydrite was almost completely dissolved, up to 95%, whereas dissolution of crystalline goethite and hematite did not exceed 9% and 1.2% respectively. [Copyright &y& Elsevier]

Abstract: A combined chemical and biological treatment scheme was evaluated in this study aiming at obtaining the simultaneous removal of metalloid arsenic and cationic heavy metals from contaminated soils. The treatment involved the use of the iron reducing microorganism Desulfuromonas palmitatis, whose activity was combined with the chelating strength of EDTA. Taking into consideration that soil iron oxides are the main scavengers of As, treatment with iron reducing microorganisms aimed at inducing the reductive dissolution of soil oxides and thus obtaining the release of the retained As. The main objective of using EDTA was the removal of metal contaminants, such as Pb and Zn, through the formation of soluble metal chelates. Experimental results however indicated that EDTA was also indispensable for the biological reduction of Fe(III) oxides. The bacterial activity was found to have a pronounced positive effect on the removal of arsenic, which increased from the value of 35% obtained during the pure chemical treatment up to 90% in the presence of D. palmitatis. In the case of Pb, the major part, i.e. approximately 85%, was removed from soil with purely chemical mechanisms, whereas the biological activity slightly improved the extraction, increasing the final removal up to 90%. Co-treatment had negative effect only for Zn, whose removal was reduced from 80% under abiotic condition to approximately 50% in the presence of bacteria. [Copyright &y& Elsevier]

Abstract: In the present study, flow-through column experiments using a series of continuous-flow columns were conducted to investigate the performance of Fe0 permeable reactive barriers (PRB) for the treatment of acidic leachates generated in active or abandoned mining and waste disposal sites. Simulated AMD solutions of low and high metal ion concentration were pumped at different flow rates into the laboratory system. Concentration profiles under steady-state transport conditions were developed by measuring metal ion concentration at various sampling ports. Results show that metal ion removal is mainly accomplished via redox reactions that initiate precipitation of mineral phases. Sulfate green rust was identified from the analysis of precipitates formed on the iron surface; sulfate and heavy metals uptake by green rust is a secondary clean up mechanism. Experimental results as well as geochemical modeling by PHREEQC-2 indicate that cadmium and copper are removed by redox reactions while aluminum, manganese, nickel, cobalt and zinc are mainly removed as metal hydroxides. It is therefore seen than zero-valent iron barriers can be used as an environmentally and economically viable remediation technology for the clean up of acidic leachates loaded with several inorganic contaminants and sulfates and the subsequent prevention of groundwater contamination. [Copyright &y& Elsevier]

In the present paper, the potential use of lignite fly ash in the control of acid generation from sulphidic tailings disposed of at Lavrion, Greece was studied. Long-term laboratory column kinetic tests were performed on tailings containing 27% S, which were homogeneously mixed with various amounts of fly ash, ranging from 10 to 63% w/w. The drainage quality of the columns was monitored over a test period of 600 days. Chemical and mineralogical characterisation of column solid residues was performed after a 270-day test period. The hydraulic conductivity of the mixtures was also measured to evaluate the potential of fly ash to form a low permeability layer. Based on the results, the addition of fly ash to sulphidic tailings, even at the lower amount, increased the pH of the drainage at values of 8.6–10.0 and decreased the dissolved concentrations of contaminants, mainly Zn and Mn, to values that meet the European regulatory limits for potable water. Higher fly ash addition to tailings, at amounts of 31 and 63% w/w also reduced the water permeability of material from 1.2×10−5 cm/sec to 3×10−7 and 2.5×10−8 m/s, respectively. [Copyright &y& Elsevier]

In this study, the ionic liquid betainium bis(trifluoromethylsulfonyl)imide (HBetTf2N), has been applied directly as a leaching agent on a low-grade rare earth element (REE) ore called Rödberg, from the Fen carbonatite complex ore deposit located in Norway. REE in Rödberg are mainly present in the form of fluoro-carbonates of bastnäsite group minerals. Factors studied during leaching were water ratio into ionic liquid, leaching temperature, pulp density (%w/v) and retention time. Leaching of Rödberg ore with HbetTf2N resulted to 65–100% recovery of light REE and 40–60% recovery of heavy REE. Additionally > 80–90% of Ca and Mg content dissolves, whereas the HbetTf2N is selective against iron and silicon resulting in a Fe/Si rich residue. The metals dissolved into HbetTf2N after leaching are stripped with an aqueous acidic solution, regenerating the ionic liquid for reuse. Through acid stripping, REE concentration is doubled in the aqueous acidic solution, for further purification. The carboxylic acid functional group attached to the cation side of HbetTf2N is capable of dissolving bastnäsite thus enabling the development of new alternative methods for treating low-grade REE primary resources. [ABSTRACT FROM AUTHOR]

In this study the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide (HbetTf2N) has been applied directly on leaching bauxite residue, for achieving selective dissolution of rare earth elements (REE) against iron. Several factors have been examined such as leaching temperature, retention time, pulp density and water addition into the ionic liquid. It was found that moderate water addition, elevated temperatures and increased retention time provide up to 70–85% REE extraction yields with respective Fe extraction yield < 3%. Sc recovery showed a different leaching behavior from the rest of the REE with its extraction yield not exceeding 45%. Furthermore, no dissolution occurs for Ti and Si whereas 30% maximum extraction of Al was obtained, followed by almost total dissolution of Ca and Na content. This work shows that most REE and a fraction of Sc are associated with Calcium- and Aluminium-bearing phases in the bauxite residue, while the majority of Sc is associated probably with the remaining Fe and Ti-bearing phases. The metals dissolved into hydrophobic ionic liquid can be extracted with an acidic medium while the IL leaching agent is regenerated and after separation can be reused to a new portion of bauxite residue. Enabling consecutive scrubbing-stripping on the ionic liquid pregnant solution an enhanced acidic Sc solution with reduced level of impurities is obtained. [ABSTRACT FROM AUTHOR]