Cyanidation kinetics of silver telluride (Ag2Te)

The extraction of precious metals from tellurides by cyanidation is more difficult than when they are in their native form, nevertheless the reason for their refractory nature has not been adequately supported. In this study, the mechanism of the cyanidation kinetics of silver telluride (Ag₂Te) was investigated. For this purpose, cyanidation experiments were carried out to: (1) study the difference between the cyanidation kinetics of elemental silver and silver telluride; (2) study the effect of temperature (i.e. 20, 25, 27, 30, 35 and 40°C) on silver telluride dissolution; and (3) elucidate the kinetic mechanism of the silver telluride cyanidation. The results obtained showed that: (1) while 83.5% of elemental silver was dissolved in 8?h, only 13.2% of silver from silver telluride was dissolved in the same time; (2) temperature has an important effect on silver extraction from silver telluride, but a minor effect on tellurium dissolution; and (3) at temperatures between 20 and 27°C, the process was controlled by the chemical reaction with an apparent activation energy of 191.9?kJ?mol^?1, whereas at temperatures between 30 and 40°C, the process was controlled by diffusion through a Ag₅Te₃ layer of products with an apparent activation energy of 25.2?kJ?mol^?1.     

Potentiometric studies on the adsorption of Au(CN)2 and Ag(CN)2 onto activated carbon

The adsorption of Au(CN) ₂ ⁻ and Ag(CN) ₂ ⁻ onto activated carbon has been correlated with potential measurements of carbon. It is proposed that Au(CN) ₂ ⁻ , Ag(CN) ₂ ⁻ , and CN⁻ are adsorbed on carbon by both an ion exchange mechanism and an oxidative reaction which leads to a decrease in carbon potential. The potential of carbon decreased according to the degree of anion adsorption in the order Au(CN) ₂ ⁻ > Ag(CN) ₂ ⁻ > CN⁻ > I⁻ > Cl⁻. Adsorption of anions is accompanied by release of OH⁻ ion which raises the solution pH and decreases the carbon potential. The potential of carbon in contact with solutions containing KAu(CN)₂, KAg(CN)₂, or KCN is much more negative than that in contact with chloride solution at the same pH. The results indicate that cyanide is oxidized on the carbon surface by oxygen or by reducible functional groups such as quinone. N. TSUCHIDA, formerly Postgraduate Student, Murdoch University, is Metallurgist with Beshi Nickel Refinery (Sumitomo Metal and Mining Co.), Niihama, Japan     

Ag(I)-binding to phytochelatins

Phytochelatins (PCs) are glutathione-derived peptides with the general structure (gamma-Glu-Cys)nGly, where n varies from 2 to 11. A variety of metal ions such as Cu(II), Cd(II), Pb(II), Zn(II), and Ag(I) induce PC synthesis in plants and some yeasts. It has generally been assumed that the inducer metals also bind PCs. However, very little information is available on the binding of metals other than Cu(I) and Cd(II) to PCs. In this paper, we describe the Ag(I)-binding characteristics of PCs with the structure (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly, and (gamma-Glu-Cys)4Gly. The Ag(I)-binding stoichiometries of these three peptides were determined by (i) UV/VIS spectrophotometry, (ii) luminescence spectroscopy at 77 K, and (iii) reverse-phase HPLC. The three techniques yielded similar results. ApoPCs exhibit featureless absorption in the 220-340 nm range. The binding of Ag(I) to PCs induced the appearance of specific absorption shoulders. The titration end point was indicated by the flattening of the characteristic absorption shoulders. Similarly, luminescence at 77 K due to Ag(I)-thiolate clusters increased with the addition of graded Ag(I) equivalents. The luminescence declined when Ag(I) equivalents in excess of the saturating amounts were added to the peptides. At neutral pH, (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly, and (gamma-Glu-Cys)4Gly bind 1.0, 1.5, and 4.0 equivalents of Ag(I), respectively. The Ag(I)-binding capacity of (gamma-Glu-Cys)2Gly and (gamma-Glu-Cys)3Gly was increased at pH 5.0 and below so that Ag(I)/-SH ratio approached 1.0. A similar pH-dependent binding of Ag(I) to glutathione was also observed. The increased Ag(I)-binding to PCs at lower pH is of physiological significance as these peptides accumulate in acidic vacuoles. We also report lifetime data on Ag(I)-PCs. The relatively long decay-times (approximately 0.1-0.3 msec) accompanied with a large Stokes shift in the emission band are indicative of spin-forbidden phosphorescence.     

机械合金化法制备Ag/SnO_2(12)材料的组织与性能研究

采用机械合金化、冷等静压成型、烧结、热挤压等粉末冶金技术集成的方法制备Ag/SnO2(12)材料,并对其组织与性能进行研究。研究结果表明:通过机械合金化的方法获得的Ag/SnO2(12)复合粉末,粉末颗粒形状不规则,为多层片状银的叠加,粒径的范围在20～50μm,SnO2颗粒细小且均匀弥散镶嵌于Ag基体中;Ag/SnO2(12)复合材料中SnO2颗粒细小,组织均匀,力学性能良好,但由于材料采用粉末冶金方法制备,不可避免的存在少量的孔隙,从而影响了材料的电学性能,电阻率偏高;其断口形貌为解理脆性断裂(宏观)和准解理断裂(微观)的综合。     

氰化金和氰化银在活性炭上的吸附行为和特征

本文重点讨论Au(CN)_2~-和Ag(CN)_2~-在活性炭上吸附的行为和特征。 当矿浆中只有Au(CN)_2~-或Ag(CN)_2~-时,它们在活性炭上的吸附过程和特征相同;当矿浆中既有Au(CN)_2~-又有Ag(CN)_2~-时,它们的吸附过程和行为又不相同。 Ag(CN)_2~-对Au(CN)_2~-在活性炭上吸附的连续性没有影响,但对吸附容量有影响。无Ag(CN)_2~-时金在活性炭上的吸附容量为q=40.71C~(0.717),有Ag(CN)_2~-时(本文叙及条件),金在活性炭上的吸附容量为q=4.4368C~(0.4701)。 q——吸附平衡时金容量,mg/g.c; C——吸附平衡时液相金品位,mg/L。 受Au(CN)_2~-的影响,Ag(CN)_2`-在活性炭上的吸附行为变化很大,可由吸附变为解吸。因此,当Au(CN)_2~-和Ag(CN)_2~-存在于同一体系时,它们在活性炭上的吸附不能同时完成,金、银必须进行分段吸附才能获得满意的吸附效果。     

Studies on role of oxygen in the adsorption of Au(CN) 2 − and Ag(CN) 2 − onto activated carbon

Comparative studies of the adsorption of Au(CN) ₂ ⁻ , Ag(CN) ₂ ⁻ , and Hg(CN) ₂ ⁻ onto activated carbon (Norit R2020) have suggested that oxygen and oxygen containing surface functional groups play a role in the adsorption process of Au(CN) ₂ ⁻ and Ag(CN) ₂ ⁻ but not in the adsorption of Hg(CN) ₂ ⁻ . Adsorption of Au(CN) ₂ ⁻ and Ag(CN) ₂ ⁻ on carbon degassed at 950 °C under 10⁻⁵ torr (1.33 × 10⁻³ P) vacuum is decreased by 50 pct compared with the adsorption on normal activated carbon. However, in the presence of oxygen in solution, the degassed carbon adsorbs Au(CCN) ₂ ⁻ to the same extent as normal carbon. The effect of organic solvents and the variation in the potential of the two types of carbon upon adsorption of Au(CN) ₂ ⁻ were also investigated. These results indicate that activated carbon behaves like an ion-exchange resin but is capable of oxidizing cyanide and cyanide complexes by chemisorbed oxygen. A dual mechanism for the adsorption of Au(CN) ₂ ⁻ and Ag(CN) ₂ ⁻ onto activated carbon is therefore proposed, in which cyanide complexes adsorb on carbon by anion exchange with OH⁻ followed by partial oxidative decomposition of Au(CN) ₂ ⁻ or Ag(CN) ₂ ⁻ to insoluble AuCN or AgCN. N. TSUCHIDA, formerly Postgraduate Student at Murdoch University,