Scielo RSS <![CDATA[Journal of the Southern African Institute of Mining and Metallurgy]]> vol. 114 num. 2 lang. es <![CDATA[SciELO Logo]]> <![CDATA[<b>Precious Metals 2013</b>]]> <![CDATA[<b>President's Corner</b>]]> <![CDATA[<b>Aspects of coloured precious metal intermetallic compounds </b>]]> This paper provides a review on coloured gold, platinum, and palladium intermetallic compounds, and discusses the models that were developed to obtain these materials. These compounds have a crystal structure of high symmetry, such as the CaF2 or CsCl structures, ensuring distinct electron band structures. Various examples of coloured gold, platinum, and palladium intermetallic compounds are provided. More in-depth discussion is provided on the purple gold (AuAl2) and yellow platinum-aluminium (PtAl2) compounds with CaF2 structure, as well as the purplish-pink palladium-indium (PdIn) compound with CsCl structure. Precious metal intermetallic compounds are used in jewellery and provide a new dimension to design. Some of these compounds have also found use as barrier coatings on turbine blades for jet engines, and more recently, research has been conducted into their potential use as catalysts, electro-catalysts, sensors, capacitors, and for decorative coatings. <![CDATA[<b>Structural and electrical characteristics of printed silver and palladium nanoparticle networks</b>]]> In this study, silver and palladium nanoparticulate inks with different metal-binder ratios were screen-printed onto a paper substrate. The printed layers comprised the metal nanoparticles and an ethyl cellulose binder. The layers are not typical metal composites due to the low concentration of binder and high porosity. The electrical sheet resistivity of each particle concentration was measured, and the data fitted using the scaling law of percolation theory and the general effective media (GEM) equation. The percolation parameters, i.e. the percolation threshold and the critical exponent, for each system were determined from the fits and compared. The percolation threshold for the palladium layer (39.8 wt%) was lower than that found for the silver layer (44.9 wt%). The critical exponents determined for the palladium and silver layers were both lower than the universal value for three-dimensional percolation systems. Microscopy observations indicate that the difference in the percolation threshold for the two layers can be attributed to variations in their microstructure. <![CDATA[<b>Investigating the chromium-platinum coated system</b>]]> In this study, the effects of heat treatment on phase formation and surface morphology of the Cr-Pt coated system are investigated using X-ray diffraction and electron microscopy techniques. The CrPt and Cr4Pt (Cr3Pt - A15 type) phases were formed after heat treatment of the coated samples. Heat treating at 1000°C renders a distinct island coating morphology that is non-continuous in nature. A diffusional model is proposed for this coated system with regards to the surface morphology formation after heat treatment. <![CDATA[<b>Technical and economic aspects of promotion of cobalt-based Fischer-Tropsch catalysts by noble metals - a review</b>]]> The conversion of carbon-containing feedstock into liquid fuels can proceed via the Fischer-Tropsch synthesis, which is catalysed using supported cobalt catalysts. These catalysts contain noble metals to facilitate the reduction of oxidic cobalt species in the catalyst precursor to metallic cobalt, which is the catalytically active species for the Fischer-Tropsch synthesis. This is thought to occur via hydrogen spillover, i.e. the diffusion of atomic hydrogen over the support from the noble metal to the oxidic cobalt moieties. Noble metals may also affect the dispersion of cobalt in the active Fischer-Tropsch catalyst. The economic aspect of the addition of noble metals to the Fischer-Tropsch catalyst is discussed. <![CDATA[<b>Formation of amorphous Ti-50 at.% Pt by solid-state reactions during mechanical alloying</b>]]> Mechanical alloying of an equiatomic mixture of crystalline Ti and Pt elemental powders in a high-energy ball mill formed an amorphous alloy by solid-state reactions. Mechanical alloying was carried out in an argon atmosphere at a rotation speed of 1200 r/min and a 20:1 ball-to-powder weight ratio, for time intervals of 4 to 40 hours. At an intermediate stage of mechanical alloying, scanning electron microscopy showed the formation of characteristic layered structures of inhomogeneous composition within the powder particles. X-ray diffraction analysis showed the gradual disappearance of crystalline Bragg peaks and the emergence of broad amorphous maxima as milling progressed. The amorphization process was completed after 8-12 hours. The amorphous state of the product indicated that temperatures during processing did not exceed the crystallization temperature of the alloy. <![CDATA[<b>Ion exchange technology for the efficient recovery of precious metals from waste and low-grade streams</b>]]> Efficient recovery of precious metals from process solutions is essential for improving process economics. Traditionally, precious metals are relatively effectively recovered from waste streams via precipitation or cementation. However, these approaches have a number of drawbacks, including poor water balance, creation of environmentally unfriendly waste streams, and losses of precious metals. Ion exchange technology is an alternative for the recovery of precious metals from waste or low-grade streams. This technology allows the recovery of the precious metals to extremely low levels (micrograms per litre) with relatively high upgrade ratios from the solution onto the resin without major water balance concerns, while the impact on the environment could be minimized or avoided. Research was conducted on the recovery of platinum group metals and gold from different low-grade and waste streams from one of the precious metals refineries in South Africa by means of ion exchange. Various functionalities and matrices (granular and fibrous) of ion exchange materials were evaluated. The results from these studies indicated that in some cases ion exchange could be very effective for the recovery of precious metals, and that the PGM concentration could be reduced to < 1 mg/L. The upgrading ratios of the various PGMs onto the specific fibres were relatively high for the specific streams evaluated, which might in some cases justify incineration of the loaded material instead of stripping and recycling the adsorbent. The cost of direct incineration for one of the waste streams tested would be less than 1% of the value of the PGMs recovered. However, the adsorbent has to be carefully selected and the process design optimized for each specific stream. <![CDATA[<b>Synthesis and crystal structure of tetrakis(1,3-diphenyl-1,3-propanedionato) zirconium(IV)</b>]]> The coordination compound teirakis(1,3-diphenyl-1,3-propanediona to)zirconium(IV), [Zr(DBM)4], (DBM = dibenzoyl methane/1,3-diphenyl-1,3-propanedionol) was synthesized and characterized by single-crystal X-ray diffraction. [Zr(DBM)4] crystallizes in the monoclinic space group P21/c (a = 24.769(4) Å, b = 10.216(5) Å, c = 19.314(4) Å, and β= 101.541(5)°). The coordination geometry around the zirconium atom, formed by the eight O-donating atoms of the four β-diketonates, is found to be a near-perfect square antiprism. Crystal packing is stabilized by C-H...π interactions throughout the crystal lattice. This structure is a noteworthy example of the application of ambient condition synthesis of zirconium compounds in N,N'-dimethylformamide (DMF) as reaction and crystallization solvent, highlighting the possibilities of aerobic condition metal purification through chelation.