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An in-situ electro-generated chlorine leaching with a closed recycling unit of unconsumed gas was developed for precious metal dissolution and applied to the leaching of platinum from a spent automotive catalyst containing 0.13 wt% Pt. Cathodic current in a cathodic compartment of the chlorine generation cell reduces the unconsumed chlorine gas into chloride ions for further chlorine generation. The electro-reduction efficiency was 99.4% in alkaline 2 M NaCl solution, whereas 80.8% in 35% HCl solution at 0.62 mmol min−1 supply rate of unconsumed chlorine gas. The 98.3% platinum from the spent automotive catalyst leached without discharging the unconsumed chlorine gas into the air at 100 g L−1 solid/liquid ratio and 80°C in a 7 M HCl solution. Increasing the solid/liquid ratio to 400 g L−1 drastically decreased the leaching percentage to 16.5% in 90 minutes by possible adsorption of leached Pt to the porous surface of the automotive catalyst carrier. [ABSTRACT FROM AUTHOR]

In this study, deep eutectic solvents (DESs) were employed as environmentally friendly alternatives of inorganic acid to selectively leach REEs from waste NdFeB magnets. Three DESs were tested, effective for light REE leaching, and preliminary experiments were conducted to study the selective leaching of REEs, using synthetic materials, Nd 2 O 3 , Fe, Fe 3 O 4 , and Fe 2 O 3. The findings suggest that neodymium and iron should exist as Nd 2 O 3 and Fe 2 O 3 , respectively, for selective leaching. To enhance the selective leaching of the NdFeB magnet, three pretreatment methods were applied: oxidative roasting, NaOH digestion, and NaOH digestion-oxidative roasting. During the oxidative roasting, NdFeO 3 was formed, and it hindered REE leaching, achieving the highest REE leaching efficiency of 22.5% in the guanidine hydrochloride (GUC)-lactic acid (LA) DES. To counteract NdFeO 3 formation, the NaOH digestion was introduced, yielding Nd(OH) 3 and Fe 3 O 4. These were then converted to Nd 2 O 3 and Fe 2 O 3 through oxidative roasting. With the NaOH-digested product, selective REE leaching was achieved solely with the ethylene glycol (EG)-maleic acid (MA) DES, while it was feasible in all three DESs with the NaOH digestion-oxidative roasting. The EG-MA DES displayed the highest selectivity, with a leaching efficiency of 97.3% Nd and 0.8% Fe. Additionally, the solvent could be reused at least twice, and the leaching efficiencies of 97% for Nd and 0.7% for Fe were maintained. This selective leaching technique benefits from using environmentally friendly solvents compared to the traditional inorganic acid leaching and demonstrates high REE selectivity and solvent reusability, suggesting a novel method for REE recovery via DESs. • Deep eutectic solvents (DESs) can selectively leach REEs from NdFeB magnets. • For selective leaching, NdFeB should be decomposed by a proper pretreatment. • NaOH digestion-oxidative roasting was the most effective, which yielded Nd 2 O 3 and Fe 2 O 3. • The EG-MA DES displayed the highest selectivity, with 97% Nd and 0.8% Fe leached. • The used DES can be recycled in the process at least two cycles. [ABSTRACT FROM AUTHOR]

Abstract The effect of heavy metals-containing gold ore mineralogy on gold oxidation kinetics in solutions with various free cyanide (CN) and dissolved oxygen (DO) concentrations was studied by electrochemical tests. To effectively simulate the cyanide gold leaching conditions, the electrochemical experiments were carried out in the slurries of the ores. The results of the linear sweep voltammetry tests showed three significant gold oxidation peaks. The peak potentials and the corresponding current densities were dependent on the composition of the ores in the test slurries, and the CN and DO concentrations of the test solutions. The heavy metal concentration of the ores also effectively changed the electrochemical response of the tests. The highest gold oxidation kinetics were observed in a pyritic sample containing 1800 ppm Pb, 640 ppm Ag and 195 ppm Hg (sample 1). The fastest gold oxidation kinetics were registered at −0.2 V vs. Ag/AgCl when DO was 8.3 mg/L, and the corresponding Au oxidation peak current densities were 4.75 and 7.07 mA/cm2 at 600 and 900 mg/L free CN, respectively. The Au oxidation was assisted by cementation reaction between the aqueous Hg and/or Ag species, and metallic gold. The weak lead deposition was also observed. However, the positive effect of Hg on the gold oxidation kinetics was depressed at higher DO (37.0 mg/L) concentrations. Unlike Hg, the effect of Ag on increasing the gold oxidation kinetics was shown to be independent of the solution DO. The second sample of the study, sample 2, was a sulfidic gold ore with 0.25% galena (PbS). The results showed that the presence of galena in an electrochemical gold oxidation test could increase the gold oxidation kinetics. In tests with 8.3 mg/L DO, the current densities of 2.80 and 5.23 mA/cm2 at 600 and 900 mg/L CN solutions, respectively, were obtained at −0.4 V vs. Ag/AgCl. Increasing DO to 37.0 mg/L did not improve the gold oxidation kinetics. Based on the results of this study, the presence of the specified heavy metals is most effective in gold oxidation kinetics if the free CN concentration is high and the DO concentration is low. Besides, the electrochemical methodology introduced in this study could offer a solution for determining the leaching parameters. Highlights • Hg, Ag and Pb improve gold oxidation kinetics at low overpotential range at high CN and low DO concentrations. • At high DO condition, Hg and Pb are not very effective for gold oxidation. • Silver can enhance gold oxidation kinetics regardless of DO concentration. [ABSTRACT FROM AUTHOR]

The selective precipitation of rare earth elements (REEs) in acidic media often plays a key role in the effective extraction of these elements from various sources such as ores and recycling streams. In this study, the precipitation characteristics of REEs with sulfate, a frequently used precipitant, were carefully examined, and the results were compared with those of other precipitants, such as phosphate, oxalate, and fluoride/carbonate systems. Emphasis is given on various forms of precipitates, such as anhydrous sulfate, octa-hydrated sulfate, and sodium double salt, in which the sodium double salt was compared with the anionic double salt precipitation of the fluoride-carbonate system. It was found that anions such as Cl−, NO3−, and SO42− play an important role in the precipitation behavior, particularly through complexation with the dissolved REEs. In general, the effectiveness of precipitation follows the order of sodium double salt, a hydrated form of sulfate, and anhydrous sulfate. In this study, it was observed that the synergistic role of a double salt precipitation, either cationic or anionic, is frequently as effective as that of oxalate and phosphate, even in a low pH range. [ABSTRACT FROM AUTHOR]

Selective leaching behavior of the spent NdFeB magnets was investigated to find out how controlled phase system through the roasting conditions gave an impact on the performance of the selective leaching. The pre-formed Nd 2 O 3 by the selective oxidation made it possible to prevent the formation of NdFeO 3 at high roasting temperature. This resulted in the ideal Nd 2 O 3 and Fe 2 O 3 phase system, which was obtained by roasting at 660 °C, suitable for the selective leaching of Nd. While the roasting at 600 °C was still not enough to get full oxidation, the roasting at 720 °C made the NdFeO 3 formed. These characteristics with the roasting conditions affected leaching behavior. The NdFeB powder sample of 32 μm particle diameter, roasted at 600 °C showed the highest Nd leaching efficiency as well as the highest Fe leaching efficiency, which seemed to be ascribed to the existence of the metallic Fe. The poor leaching efficiency for the large powder size after roasting was ascribed to the poor diffusion characteristic of the H 2 SO 4 solution. With the controlled phase system, the 100% leaching efficiency of Nd could be obtained by reducing the particle size to 16 μm. The formation of NdFeO 3 as a co-product, which had resistance to the reaction with the leaching agent, could prevent the dissolution of Nd 2 O 3 around the NdFeO 3 layer. Thus, the 720 °C roasting condition showed the lowest leaching efficiency. The co-product Fe 2 O 3 also showed good resistance to leaching so its leaching efficiency was just 5–6% after the leaching time of 3–4 h. • Roasting over 500 °C could be applicable in manipulating phases for selective leaching in case Nd was oxidized in advance through selective oxidation. • Good controlled phase system enabled to achieve the improved leaching efficiency as well as selectivity. • The powder size played an important role in determining the rate constant and finally, leaded to the improved leaching efficiency. [ABSTRACT FROM AUTHOR]