A review on the recycling processes of spent auto-catalysts: Towards the development of sustainable metallurgy.

Spent auto-catalysts are considered as promising platinum group metals (PGMs) resources based on their rapidly increasing demand along with the underlying uncertainty of the sustainability and long-term availability of PGMs. Recycling spent auto-catalysts presents attractive advantages, particularly for the conservation of primary resources reserves, and for the reduction of negative environmental impact due to exploitation. PGM reclamation is the major aim of recycling operations despite their minor concentration in spent auto-catalysts, which implies that the remaining materials are disposed of as unwanted solid waste after the extraction process. This poses a genuine challenge, as well as a motivation to develop recycling processes for spent auto-catalysts capable of recovering all components/valuable metals, while moderating environmental pollution and global warming. The focus herein involves the description of the available technologies, including pyro- and hydro-metallurgical processes, to recover PGMs from spent auto-catalysts, and specifically an analysis of the developmental trends in recycling methods to ensure "sustainable metallurgy".

Total recycling of all the components from spent auto-catalyst by NaOH roasting-assisted hydrometallurgical route.

Total recycling of all valuable metals such as PGMs, Ce, Al and Mg from the spent automobile catalysts has been explored. The alkali roasting was performed under NaOH, 0.5-3.0 g; temperature, 300-800 °C; and time, 10-60 min. The phase transformation from cordierite to the soluble products (NaAlO2, Na2MgSiO4) was influenced by the temperature, following the diffusion-controlled model (Ea(roasting), 6.4 kJ/mol). XRD analysis of the roasted mass revealed that the refractory phases of cordierite and γ-alumina could be eliminated at ≥600 °C at sample-to-NaOH mass ratio, 1:1. The leaching of the roasted mass followed an intermediate-controlled mechanism for aluminum leaching with (Ea(Al-leaching) value of 20.3 kJ/mol), while it was diffusion-controlled for magnesium leaching (Ea(Mg-leaching), 8.9 kJ/mol). At the optimum leaching condition (1.0 M H2SO4, 90 °C, 60 min, yielding >95% aluminum and magnesium), a significant amount of PGMs was also leached. Thereafter, the cementation process was investigated with Al0-powder that could precipitate >99% PGMs within 15 min at 90 °C. The yielded concentrate of PGMs and CeO2 was subsequently leached in 6.0 M HCl with 2.0 M NaClO3 that dissolved >97% PGMs, leaving the residue as the CeO2 concentrate. Individual metals can be recovered by following established separation and purification techniques.

Effects of Heat-Treatment on Photoluminescence Spectra and Photocatalytic Properties of Solution-Combusted ZnO Nanopowders.

ZnO nanopowders were prepared by a solution combustion method (SCM). The SCM ZnO nanopowders were heat-treated at 200, 400, 500, or 700 °C for 30 min in air and the photoluminescence (PL) of the nanopowders was evaluated. Two strong PL emission peaks are generally recognized as the unique PL signature of ZnO, one is from the band-edge emission and the other corresponds to green emission. The green emission is derived from crystalline defects, and is a critical obstacle for the electrooptical applications of ZnO. Surprisingly, the PL spectra of the SCM ZnO powders showed a single sharp peak near 390 nm. Furthermore, the intensity of this blue emission doubled when the synthesized ZnO powder was heat-treated at 400 °C. The green emission appeared for the sample treated at 500 °C, and was the highest for that treated at 700 °C. To comfirm the photocatalytic activity of the ZnO powder heat-treated at 400 °C, the removal of Ag ions from a used photofilm developer was evaluated, with complete removal within 10 min. The removal of the Ag ions by the ZnO powder heat-treated at 400 °C was more than two orders of magnitude faster than that achieved with the SCM ZnO powder. The relation between PL and photocatalytic activity was explained in terms of recombination of the photogenerated electrons. These results might be very useful for highly efficient photocatalyst applications.

Photoluminescence and Electrical Properties of ZnO Nanopowders Prepared by a Solution Combustion Method for Optoelectronic Device Application.

Nano-sized powders of ZnO phosphor were prepared by a solution combustion method (SCM). The ZnO powder prepared using Zn(OH)2 and glycine as the oxidant and fuel, respectively, (fuel/oxidant = 0.8), show good powder characteristics such as an average grain size of 30 nm and specific surface area of 120 m²/g. Single-phase ZnO powders were obtained regardless of the fuel/ oxidant ratio. The photoluminescence spectra of the obtained ZnO powders show a single sharp peak near 390 nm corresponding to a deep blue color. This was confirmed by cathodoluminescence measurement and CIE color coordinate values. The PL spectra, powder characteristics and electrical properties show very good consistency. Furthermore, the electron carrier concentration of the ZnO nanopowders prepared by SCM is more than 3 orders of magnitude higher than that of the purest ZnO single crystal, which is commercially available. These powders could be potentially applied as blue phosphors of displays such as A.C. powder electroluminescent devices and plasma display panels.

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.

High Temperature Corrosion of TiAlCrSiN Films in N2/0.1%H2S Gas.

Nanomultilayered TiAlCrSiN film was corroded in N2/0.1%H2S-mixed gas at 900 °C for 5-300 h. It corroded to TiO2, α-Al2O3, and Cr2O3. From the early corrosion stage, not only the outward diffusion of Al, Ti, Cr, and Si but also the inward migration of oxygen occurred. As the corrosion proceeded, the outermost TiO2 layer, outer (Ti-depleted, (Al,Cr)-rich)-oxide layer, inner (Ti-rich, (Al,Cr)-depleted)-oxide layer, and partially oxidizing innermost film formed on the oxygenaffected film.

High-Temperature Corrosion of Nano-Multilayered TiAISiN Thin Films in Ar-0.2%SO2 Gases.

Nano-multilayered TiAlSiN films with a composition of 26Ti-16.3Al-1.2Si-56.5N (at.%) were deposited onto steel via arc ion plating, and corroded at 800-900 °C for 30 h in Ar-0.2%SO2 gases. The films were corrosion resistant, because the oxidation process dominated sulfidation. The scales consisted primarily of Al2O3 and TiO2, where a small amount of Si dissolved.

Effect of Al2O3 Nano-Filler on Properties of Alkali/Alkaline-Earth Borosilicate Glass Composite Sealants.

The effect of adding Al2O3 nano-filler (5 and 10 vol%) to two different alkali/alkaline-earth borosilicate glass sealants, particularly on the viscosity and electrical characteristics of the glass composite sealants, was investigated to improve the cyclic sealing performance. The effects of the filler and base glass composition on the viscosities, electrical conductivities, and phase transformations of the sealants were investigated. The glass viscosity was decreased by replacing 20 mol% SrO with alkali and zirconium oxide in a base alkaline-earth glass. Alumina filler increased the high-temperature electrical conductivities, as well as the viscosities, of the heat-treated glass composite sealants. The replacement of 20 mol% SrO with alkali and zirconium oxide in the base alkaline-earth glass decreased the electrical conductivity of the heat-treated glass containing Al2O3 nano-filler.

Electrical and Photocatalytic Property Change of Solution-Combusted ZnO Nanopowder by Heat-Treatment.

ZnO nanopowder was synthesized by a solution combustion method. This nanopowder was char- acterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), four point probe, Hall measurement and photocatalytic reaction. The nanopowder was also investigated after heat-treating at 400 °C and 700 °C. The carrier concentrations were 8 x 10(20) cm(-3), 6 x 10(21) cm(-3) and 1.5 x 10(18) cm(-3) for the non heat-treated, the 400 °C heat-treated and the 700 °C heat-treated ZnO nanopowders respectively. Electrical resistivities of 1 Ω cm, 0.6 Ω cm and 2.6 Ω cm were obtained for the three kinds of heat-treated ZnO nanopowders respectively. These three kinds of nanopowders were then employed as photocatalysts to recover silver ions from wastewater. At 5 min of photocatalytic reaction time, the reaction removed 66.7%, 100% and 10.8% of the silver ions. The carrier concentration and photocatalytic efficiency of the 400 °C heat-treated one were 7.5 fold and 1.5 fold higher than those of the non heat-treated one respectively. However, the 700 °C heat-treated one showed far worse values than the non heat-treated one. This phenomenon was explained by carrier trap centers.

Effect of Heat Treatment on Properties of Glass Nanocomposite Sealants.

The objective of this study was to investigate the effect of heat treatments on the viscosities and electrical conductivities of glass sealants to be used in solid oxide fuel cells. Glass-based sealants, both with and without an alumina nanopowder added as a nanofiller, were heat treated at temperatures ranging from 750 degrees C to 770 degrees C for periods of up to 240 h. The effects of heat treatments on the viscosities, electrical conductivities and phase transformations of the sealants were investigated. The results showed that alumina nanopowder added to the glass increased both high-temperature electrical conductivities and the viscosities of the sintered glass nanocomposite sealants. However, lengthy heat treatments decreased the electrical conductivities of the glass nanocomposite sealants. This decrease in the conductivities of the heat-treated glass nanocomposites was attributed to the crystallization of glass phase, owing to the dissolution of the alumina nanofiller in the sealing glass.

Oxidation of Nano-Multilayered CrAlCuN Thin Films Between 800 and 1000 degrees C in Air.

Thin CrAlCuN films with a composition of 21.5Cr-18.6Al-5.6Cu-54.3N (at.%) were deposited on steel substrates by the cathodic arc plasma deposition, and oxidized for up to 50 h in air. They consisted of alternating crystalline CrN/AlN nano-multilayers where Cu was incorporated. At 800 degrees C, a thin Cr2O3 layer formed. At 900 and 1000 degrees C, an outer Cr2O3 layer and an inner (α-Al2O3, Cr2O3)-mixed layer formed. Copper diffused outward to a small extent during oxidation. The film had good oxidation resistance, owing to the formation of the protective Cr2O3 and α-Al2O3.

Liquid-liquid extraction of Cd(II) from pure and Ni/Cd acidic chloride media using Cyanex 921: a selective treatment of hazardous leachate of spent Ni-Cd batteries.

The present paper is focused on solvent extraction of hazardous Cd(II) from acidic chloride media by Cyanex 921, a new extractant mixed with 10% (v/v) TBP in xylene. The optimum conditions for extraction and stripping of Cd(II) were investigated with an aqueous feed of 0.1 mol/L Cd(II) in 2.0 mol/L HCl. McCabe-Thiele diagram was in good agreement with the simulation studies, showing the quantitative extraction (99.9%) of Cd(II) within two counter-current stages utilizing 0.30 mol/L Cyanex 921 at O/A ratio of 3/2 in 10 min. Stoichiometry of the complexes extracted was determined and confirmed by numerical treatment and graphical method, revealing the formation of HCdCl3 · 2L and HCdCl3 · 4L for Cyanex 921(L) concentration in the range 0.03-0.1 mol/L and 0.1-1.0 mol/L, respectively. The thermodynamic parameters for the extraction of cadmium were also determined. The stripping efficiency of cadmium from the loaded organic with 0.10 mol/L HCl was 99.6% in a three-stage counter-current process at an O/A ratio of 2/3. Cyanex 921 was successfully applied for the separation of Cd(II) from Ni(II) in the simulated leach liquor of spent Ni-Cd batteries. The study demonstrates the applicability of the present hydrometallurgical approach for the treatment of hazardous waste, the spent Ni-Cd batteries.

Effect of alumina nanofiller on properties of heat-treated glass composite sealants.

Alkali/alkaline-earth borosilicate glass-alumina composites containing 10 vol% Al2O3 were prepared for use as solid oxide fuel cell sealants. The effect of heat treatment and Al2O3, addition on the viscosities and electrical conductivities was investigated to improve cyclic sealing performance. Upon a 48-h heat treatment, the viscosity of the glass-alumina composites at 750 degrees C was approximately four orders of magnitude higher than that of the base glass owing to the crystallization of the glass in the presence of Al2O3. Heat treatment increased the electrical conductivities of both the base glass and the glass-alumina composites. The electrical conductivities of glass-alumina composites in the range from 400 degrees C to 550 degrees C were three times higher than those of the base glass regardless of heat treatment. This increase in the conductivities and viscosities by heat treatment was attributed to the devitrification and structural densification of the sealing glass and the partial dissolution of the Al2O3 filler in alkali/alkaline-earth borosilicate sealing glass.

Effect of solution combusted TiO2 nanoparticles added to the dielectric layers for low power consumption AC powder electroluminescence devices.

A unique synthesis method was developed, which is called solution combustion method (SCM). TiO2 nanoparticles were synthesized by this method. These SCM TiO2 nanoparticles (~35 nm) were added to the dielectric layers of AC powder electroluminescence (EL) devices. The dielectric layers were made of commercial BaTiO3 particles (~1.2 µm) and binding polymer. 0, 5, 10 and 15 wt% of SCM TiO2 nanoparticles were added to the dielectric layers during fabrication of AC powder EL devices respectively. Dielectric constants of these four kinds of dielectric layers were measured. The brightnesses and current densities of AC powder EL devices were also measured. When 10 wt% of SCM TiO2 nanoparticles was added, dielectric constant and brightness were increased by 30% and 60% respectively. Furthermore, the current density was decreased by 60%. This means that the addition of SCM TiO2 nanoparticles resulted in four fold higher luminous efficiency.

High temperature oxidation of ZrO2/Al2O3 thin films deposited on steel.

Thin ZrO2/Al2O3 films that consisted of alternating monoclinic ZrO2 nanolayers and amorphous Al2O3 nanolayers were deposited on a tool steel substrate using Zr and Al cathodes in a cathodic arc plasma deposition system, and then oxidized at 600-900 degrees C in air for up to 50 h. The ZrO2/Al2O3 films effectively suppressed the oxidation of the substrate up to 800 degrees C by acting as a barrier layer against the outward diffusion of the substrate elements and inward diffusion of oxygen. However, rapid oxidation occurred at 900 degrees C due mainly to the increased diffusion and subsequent oxidation of steel as well as the crystallization of amorphous Al2O3.

Effect of solution combusted TiO2 nanopowder within commercial BaTiO3 dielectric layer on the photoelectric properties for AC powder electroluminescence devices.

A unique synthesis method was developed, which is called solution combustion method (SCM). TiO2 nanopowder was synthesized by this method. This SCM TiO2 nanopowder (-35 nm) was added to the dielectric layer of AC powder electroluminescence (EL) device. The dielectric layer was made of commercial BaTiO3 powder (-1.2 microm) and binding polymer. 0, 5, 10 and 15 wt% of SCM TiO2 nanopowder was added to the dielectric layer during fabrication of AC powder EL device respectively. Dielectric constant of these four kinds of dielectric layers was measured. The brightness and current density of AC powder EL device were also measured. When 10 wt% of SCM TiO2 nanopowder was added, dielectric constant and brightness were increased by 30% and 101% respectively. Furthermore, the current density was decreased by 71%. This means that the brightness was double and the power consumption was one third.

Effect of Al2O3 nano-filler on properties of glass-based seals for solid oxide fuel cells.

This study compares the viscosity and strength of three glass-based seals prepared with or without nano or micron-sized alumina powder used as filler material. Measurements of the viscosity and bending strength of the glass-based seals showed that addition of the nano-sized alumina powder to the glass increased both the high-temperature viscosity and the strength of the sintered glass matrix. Strength tests and observations of the microstructure of the fracture surface of the seal samples confirmed the strengthening of the glass network structure. Conversion of non-bridging oxygen to bridging oxygen is presumed to occur upon the addition of alumina to the glass sample. The strengthening of the alumina-glass composite seal was attributed to the alumina nano-filler and prolonged heat treatment at elevated temperatures.

Disassembly and physical separation of electric/electronic components layered in printed circuit boards (PCB).

Although printed circuit boards (PCBs) contain various elements, only the major elements (i.e., those with content levels in wt% or over grade) of and precious metals (e.g., Ag, Au, and platinum groups) contained within PCBs can be recycled. To recover other elements from PCBs, the PCBs should be properly disassembled as the first step of the recycling process. The recovery of these other elements would be beneficial for efforts to conserve scarce resources, reuse electric/electronic components (EECs), and eliminate environmental problems. This paper examines the disassembly of EECs from wasted PCBs (WPCBs) and the physical separation of these EECs using a self-designed disassembling apparatus and a 3-step separation process of sieving, magnetic separation, and dense medium separation. The disassembling efficiencies were evaluated by using the ratio of grinding area (E(area)) and the weight ratio of the detached EECs (E(weight)). In the disassembly treatment, these efficiencies were improved with an increase of grinder speed and grinder height. 97.7% (E(area)) and 98% (E(weight)) could be accomplished ultimately by 3 repetitive treatments at a grinder speed of 5500 rpm and a grinder height of 1.5mm. Through a series of physical separations, most groups of the EECs (except for the diode, transistor, and IC chip groups) could be sorted at a relatively high separation efficiency of about 75% or more. To evaluate the separation efficiency with regard to the elemental composition, the distribution ratio (R(dis)) and the concentration ratio (R(conc)) were used. 15 elements could be separated with the highest R(dis) and R(conc) in the same separated division. This result implies that the recyclability of the elements is highly feasible, even though the initial content in EECs is lower than several tens of mg/kg.

Leaching studies for tin recovery from waste e-scrap.

Printed circuit boards (PCBs) are the most essential components of all electrical and electronic equipments, which contain noteworthy quantity of metals, some of which are toxic to life and all of which are valuable resources. Therefore, recycling of PCBs is necessary for the safe disposal/utilization of these metals. Present paper is a part of developing Indo-Korean recycling technique consists of organic swelling pre-treatment technique for the liberation of thin layer of metallic sheet and the treatment of epoxy resin to remove/recover toxic soldering material. To optimize the parameters required for recovery of tin from waste PCBs, initially the bench scale studies were carried out using fresh solder (containing 52.6% Sn and 47.3% Pb) varying the acid concentration, temperature, mixing time and pulp density. The experimental data indicate that 95.79% of tin was leached out from solder material using 5.5M HCl at fixed pulp density 50 g/L and temperature 90°C in mixing time 165 min. Kinetic studies followed the chemical reaction controlled dense constant size cylindrical particles with activation energy of 117.68 kJ/mol. However, 97.79% of tin was found to be leached out from solder materials of liberated swelled epoxy resin using 4.5M HCl at 90°C, mixing time 60 min and pulp density 50 g/L. From the leach liquor of solder materials of epoxy resin, the precipitate of sodium stannate as value added product was obtained at pH 1.9. The Pb from the leach residue was removed by using 0.1M nitric acid at 90°C in mixing time 45 min and pulp density 10g/L. The metal free epoxy resin could be disposed-of safely/used as filling material without affecting the environment.

Gas nitriding and subsequent oxidation of Ti-6Al-4V alloys.

Ti-6Al-4V alloys consisting of α-Ti grains and intergranular ß-Ti islands were nitrided at 850°C for 1 to 12 h under a nitrogen pressure of 1 Pa. With increasing nitriding time, the Ti-N compound layer became thicker, and the α-Ti diffusion zone containing dissolved nitrogen became wider. In the Ti-N compound layer, the initially formed Ti2N became TiN as the nitriding progressed. The nitride layers were oxidized to rutile-TiO2 after oxidation at 700°C for 10 h in air.