Min-seuk Kim

Selective recovery of gold from waste mobile phone PCBs by hydrometallurgical process

The leaching of gold from the scrap mobile phone PCBs by electro-generated chlorine as an oxidant and its recovery by ion exchange process was investigated. The leaching experiments were carried out by employing separate leaching reactor connected with the anode compartment of a Cl(2) gas generator. The leaching of gold increased with increase in temperature and initial concentration of chlorine, and was favorable even at low concentration of acid, whereas copper leaching increased with increase in concentration of acid and decrease in temperature. In a two-stage leaching process, copper was mostly dissolved (97%) in 165 min at 25°C during the 1st stage leaching in 2.0 mol/L HCl by electro-generated chlorine at a current density of 714A/m(2) along with a minor recovery of gold (5%). In the 2nd stage gold was mostly leached out (93% recovery, ∼67 mg/L) from the residue of the 1st stage by the electro-generated chlorine in 0.1 mol/L HCl. Gold recovery from the leach liquor by ion exchange using Amberlite XAD-7HP resin was found to be 95% with the maximum amount of gold adsorbed as 46.03 mg/g resin. A concentrated gold solution, 6034 mg/L with 99.9% purity was obtained in the ion exchange process.

Resource recycling of superalloys and hydrometallurgical challenges

Superalloys are high melting temperature, excellent creep resistance, anti-corrosive, and oxidation resistant alloys; they are predominantly used in gas turbines of aircraft engines and power plants. Today, a series of superalloys is available according to their composition and applications. The tough nature of superalloys makes it difficult to recycle them after they are formed; this increases the demand and shortage of the energy-critical elements that are their raw materials and makes their processing one of the most demanding form of recycling today. A few processes are available, and some of these unique recycling systems are in use; however, a lack of data and/or information exists for two reasons: (i) scrap recycling is most frequently performed by superalloy manufacturers themselves (not always by the same unit) by recharging them into molten charges, and (ii) the recycler does not want to produce business for their rivals. This article presents an overview of the processes investigated to recycle superalloys, including the advantages and disadvantages of each. The available processes are not very technologically or economically feasible; therefore, further efforts are necessary to explore these processes and to make a value-added product by recycling superalloys rather than just re-melting them. Considering the continuously increasing number of alloyed elements in advanced superalloys, this paper also notes to the challenges associated with hydrometallurgical recycling of superalloys and the need for future studies on this topic.Graphical Abstract

Highly selective separation of individual platinum group metals (Pd, Pt, Rh) from acidic chloride media using phosphonium-based ionic liquid in aromatic diluent

An effective solvent extraction-based method has been developed to recover individual platinum group metals (PGMs) (i.e. Pd, Pt and Rh) with high purity from acidic chloride media using phosphonium-based ionic liquid Cyphos IL 101 ([P66614]+Cl−) diluted in xylene used as a model of industrial diluents such as SOLVESSO 150. The system showed the selective co-extraction of Pd(II) and Pt(IV) from a feed solution containing 100 mg L−1 Pt(IV), 55 mg L−1 Pd(II), and 25 mg L−1 Rh(III) in HCl via an anion exchange mechanism, leaving Rh(III) in the raffinate. A McCabe–Thiele diagram demonstrated the quantitative extraction of Pd(II) and Pt(IV) with 0.6 g L−1 [P66614]+Cl− with two counter-current stages at an organic/aqueous (O/A) phase volume ratio of 3/2 at 298 K. The stoichiometry of complexes formed in the organic phases was determined using Jobs method and further characterized by 1H, 13C, 31P NMR and UV-vis spectroscopy. More importantly, a two-step stripping strategy was successfully adapted to treat the loaded organic phases with low aggressive media NaSCN and thiourea/HCl for the selective recovery of Pt(IV) and Pd(II), respectively, with high purity >99.9% (w). Three counter-current stages are required for the quantitative stripping of Pt(IV) with 0.1 mol L−1 NaSCN at an O/A ratio of 3, whereas the total stripping of Pd(II) with 0.01 mol L−1 acidic thiourea requires only one stage at an O/A ratio of 5. A series of extraction-stripping processes of up to 5 cycles indicated a possible recirculation of regenerated solvent without loss of performance. Simulated experiments in continuous modes suggest that phosphonium-based ionic liquid such as Cyphos IL 101 can be advantageously used as extractant to recover individually Pd, Pt and Rh from acidic chloride feed solutions as those encountered in the PGMs recycling industry.

Simple recycling of copper by the synergistic exploitation of industrial wastes: a step towards sustainability

A “Green Recycling” process for copper recovery by the synergistic treatment of two different industrial wastes is presented. Firstly, the solid waste generated during the processing of epoxy modeling compounds was treated with the waste nitric acid solution produced in the sputter cleaning. A faster leaching rate of copper without any effect of temperature was noticed, which can be attributed to the intermediate-controlled kinetics along with the autocatalytic behaviour of Cu2+ present in the waste solution. An increase in pulp density above 25% adversely affected the solubilisation of copper because of the decreased lixiviant acidity corresponding to the metals being leached. Subsequently, the key experimental parameters for liquid–liquid separation of copper using LIX 84-I after Fe-removal with D2EHPA, viz. equilibrium pH, extractant concentration, temperature, and phase ratio (O/A), were optimized and correlated with complexation chemistry. The extraction equilibrium constant (log Kex = 1.67) under the studied conditions of nitrate media was determined by the log–log plot of D*[H+]2aqvs. ([HR]2)org Thermodynamic properties indicated a strong affinity of LIX 84-I towards Cu2+ due to the emergence of spontaneous inner-sphere coordination with disrupted hydration. A counter-current study for the separation and recovery of copper was performed under the optimized conditions, which has successfully validated the results of extraction and stripping isotherms exhibited by the McCabe-Thiele plots. The unfolded hydrometallurgical process described in this study has a potential for sustainable utilization of wastes to recover the metal of interest at a 46% lower energy cost than the processing of primary ores.

Efficient recycling of WC-Co hardmetal sludge by oxidation followed by alkali and sulfuric acid treatments

We present a process to recycle strategic metals, viz. tungsten and cobalt, from a WC-Co hardmetal sludge (WCHS) via oxidation followed by a two-step hydrometallurgical treatment with alkali and acid solutions. The oxidation of WCHS was investigated in the temperature range of 500 to 1000 °C and optimized at 600 °C to transform the maximum WC into an alkali-soluble WO3. The conditions for the selective dissolution of WO3 in stage-I were optimized as follows: 4.0 M NaOH, pulp density of 175 g/L, and temperature of 100 °C for 1 h, yielding maximum efficacy. Subsequently, in the second step, the optimal conditions for cobalt leaching from the alkali-treated residue were established as follows: 2.0 M H2SO4, 25 g/L pulp density, and 75 °C temperature for 30 min. Downstream processing of the obtained metal ions in solutions was also easier, as the only impurity of dicobaltite ions with the Na2WO4 solution was precipitated as Co(OH)3 under atmospheric O2; meanwhile, the CoSO4 solution obtained through the second step of processing can be treated via electrolysis to recover the metallic cobalt. The present process is simpler in operation, and the efficient use of eco-friendly lixiviants eliminates the previously reported disadvantage.

Hydrometallurgical recycling of surface-coated metals from automobile-discarded ABS plastic waste

The ammoniacal leaching of surface-coated metals from automobile-discarded ABS plastics followed by their recovery through solvent extraction has been investigated. The leaching of ABS (typically containing 4.1% Cu, 1.3% Ni, and 0.03% Cr) could efficiently dissolve the ammine complexes of Cu and Ni, leaving Cr unleached as fine particles. The optimization studies for achieving the maximum efficiency revealed that the leaching of metal ions in different ammoniacal solutions follows the order CO32- > Cl- > SO42-. The leaching carried out in a carbonate medium by maintaining the total NH3 concentration 5.0 M at a NH4OH/(NH4)2CO3 ratio of 4:1, pulp density of 200 g/L, agitation speed of 400 rpm, temperature of 20 °C, and time of 120 min yielded the optimum efficiency of >99% Cu and Ni (i.e., 8.14 g/L and 2.57 g/L, respectively, in the leach liquor). Subsequently, the solvent extraction of metals from ammoniacal leach liquor as a function of extractant (LIX 84-I) concentration and organic-to-aqueous (O:A) phase ratio was examined. Based on the extraction data, a three-stage counter-current extraction at O:A = 1:1 was validated using 0.8 M LIX 84-I, yielding the quantitative extraction of both metals into the organic phase. Thereafter, the stripping of metals in acid solutions indicated that 0.5 M H2SO4 could quantitatively strip Ni from the loaded organic phase; however, ∼27% Cu was also co-stripped. The rest of Cu from the Ni-depleted organic phase was separately stripped with 1.0 M H2SO4 that can be directly sent to the electrowinning process. On the other hand, the co-stripped metals from the acidic solution can be easily separated, again using LIX 84-I as the extractant, by adopting the pH-swing method. Finally, a process has been proposed for the hydrometallurgical recovery of surface-coated metals from waste ABS plastics; that does not affect the physicochemical characteristics of the polymer substances for their reuse.

Extraction of the Surface-Coated Metals from Waste Acrylonitrile Butadiene Styrene Plastics in an Ammoniacal Solution

Leaching of the surface-coated metals from waste acrylonitrile butadiene styrene plastics (WABSP) has been investigated in the ammoniacal solution. The ammoniacal leaching of WABSP (typically containing 4.1 wt% Cu, 1.3 wt% Ni, 0.03 wt% Cr) efficiently dissolved copper and nickel, leaving unleached chromium as fine particles. The results indicate that the anions of different buffer media for metals leaching follow the order: CO32− > Cl− > SO42−. A declined leaching with high NH4OH–(NH4)2CO3 ratio (8:1) and a steep rise in extraction with high pulp in solution revealed the influence of anion (CO32−) and cation (Cu2+) present in the system. The leaching performed in carbonate solution by maintaining 5.0 M total NH3 concentration as NH4OH–(NH4)2CO3 ratio, 4:1; pulp density, 200 g/L; agitation speed, 400 rpm; temperature, 20 °C and time, 120 min yielded the maximum extraction (>99%) of copper and nickel. The study reveals a plausible recovery of the surface-coated metals from the WABSP without altering the properties of polymer material that can be recycled separately.

Hydrometallurgical Extraction of Lead in Brine Solution from a TSL Processed Zinc Plant Residue

Brine leaching of a TSL processed zinc-plant residue (ZPR), containing 83.3% anglesite, has been investigated for lead recovery . A prior treatment of ZPR in acid solution did not show any significant effect of acid concentration in zinc dissolution. The study reveals that the solubility limits of PbCl2 is as an important factor with respect to the brine concentration and pulp density in lixiviant. The dissociated sulfate ions hinder the formation of PbCl2 on prolong leaching (above 60 min), while exhibiting the reverse solubility of lead as PbSO4. A 3-step leaching process could yield >92% leaching efficiency when the parameters were maintained as: 10% pulp density in a 250 g/L NaCl solution, 80 °C temperature, and 60 min time. This leaves the insoluble ferrite, sulfides and oxy-sulfates of zinc and lead in the final residue as revealed by the XRD characterization.