Base metals impurities

Precious metals such as silver and gold, which are seldom oxidized even at high temperatures, are often refined by cupellation, a process for removing from them base metal impurities such as lead and tin, with which they are associated in many ores. Hot lead and tin are easily oxidized. In the cupellation process, a crude, impure precious metal is placed in a shallow cup or crucible made of bone ash, known as a cupel, and is then heated by a blast of hot air. At high temperatures, the base metal impurities are oxidized by oxygen in the hot air, and the oxides thus formed are absorbed by the porous bone ash. The Chaldeans are said to have been the first to have utilized (ca. 2500 b.c.e.) cupellation to remove lead and purify silver from lead-silver ores. 

Ordinary silver bullion processed in air may contain ca 200 ppm oxygen. Commercial silver bullion prepared under nitrogen may contain as Htde as 25 ppm oxygen. During casting some oxygen may be introduced to convert base metal impurities into oxides. Because these oxides do not enter into the soHd solution, they have no effect on the annealing and recrystaUization temperature of the silver critical to the silversmith. 

Eisner 1977 Wood et al. 1977). In addition to rejecting other base-metal impurities (except for Fe(III)) (Figure 5.11), the halides can be effectively rejected from the circuit by appropriate control of the SX operating conditions. The process stream is upgraded from 30 to 100 g/1 Zn, providing a solution suitable for EW. The selectivity of D2EHPA for Zn over Ca eliminates gypsum precipitation in the EW circuit, which is a major contributor to cellhouse downtime in conventional circuits. 

Treatment of impure gold is largely via the Miller process (30) in which chlorine is bubbled through the molten metal and converts the base metals to chlorides, which volatilise. Silver is converted to the chloride, which is molten and can be poured. The remaining gold is less pure (99.6%) than that produced by the WohlwiU process and may require additional treatment such as electrolysis. If platinum-group metals (qv) are present, the chlorine process is unsuitable. 

Volatilization. In this simplest separation process, the impurity or the base metal is removed as a gas. Lead containing small amounts of zinc is refined by batch vacuum distillation of the zinc. Most of the zinc produced by smelting processes contains lead and cadmium. Cmde zinc is refined by a two-step fractional distillation. In the first column, zinc and cadmium are volatilized from the lead residue, and in the second column cadmium is removed from the zinc (see Zinc and zinc alloys). 

Benzyl chloride is manufactured by the thermal or photochemical chlorination of toluene at 65—100°C (37). At lower temperatures the amount of ring-chlorinated by-products is increased. The chlorination is usually carried to no more than about 50% toluene conversion in order to minimize the amount of benzal chloride formed. Overall yield based on toluene is more than 90%. Various materials, including phosphoms pentachloride, have been reported to catalyze the side-chain chlorination. These compounds and others such as amides also reduce ring chlorination by complexing metallic impurities (38). 

Another point is related to the high acidity level of the final solution, which leads to certain limitations in the subsequent technological steps. Specifically, the high acidity of the initial solution eliminates any possibility for selective extraction, i.e. sequential separation of tantalum and then of niobium. Due to the high concentration of acids, only collective extraction (of tantalum and niobium together) can be performed, at least at the first step. In addition, extraction from a highly acidic solution might cause additional contamination of the final products with antimony and other related impurities. In order to reduce the level of contaminants in the initial solution, some special additives are applied prior to the liquid-liquid extraction. For instance, some mineral acids and base metals are added to the solution at certain temperatures to cause the precipitation of antimony [455 - 457]. 

Rh(N02)6 is of some importance in the traditional extraction of rhodium. Impure RI1CI3 is neutralized and treated with NaN02 Na3Rh(N02)6 is soluble under these conditions (though base metals precipitate), but when ammonium chloride is added, (NH4)3Rh(N02)6 precipitates. The potassium salt is similarly relatively insoluble. All these salts are believed... 

The rate of peroxide decomposition and the resultant rate of oxidation are markedly increased by the presence of ions of metals such as iron, copper, manganese, and cobalt [13]. This catalytic decomposition is based on a redox mechanism, as in Figure 15.2. Consequently, it is important to control and limit the amounts of metal impurities in raw rubber. The influence of antioxidants against these rubber poisons depends at least partially on a complex formation (chelation) of the damaging ion. In favor of this theory is the fact that simple chelating agents that have no aging-protective activity, like ethylene diamine tetracetic acid (EDTA), act as copper protectors. 

When corrosion occurs under focal-cell action, it may happen that the impurity concentration at the metal surface increases with advancing dissolution of the base metal hence, the corrosion rate will increase. 

Elemental distribution and chemical state of ppm metal impurities can be measured using synchrotron-based X-ray fluorescence ( iXRF) and X-ray absorption spectroscopy ( iXAS), both with a I -2 irn2 spatial resolution [314]. 

Silver halide fibres (AgClxBri x) have the widest spectral range in the mid-IR, well into the fingerprint range. Due to their crystalline nature, they have a superior flexibility. Problematic is their tendency to decompose upon contact with UV radiation or base metals. Also sulphides will chemically destroy the fibre material. Other points against are the high intrinsic attenuation due to absorption by impurities or scattering at inclusions or micro-crystals and the non-availability of (applicable) core-clad fibres. 

Gold has been used for many years as a minority carrier lifeline controller in Si. As such, it is introduced in a controlled manner, usually by diffusion into transistor structures to decrease the carrier lifetime in the base region in order to increase the switching speed (Ravi, 1981). Conversely, the uncontrolled presence of Au is clearly deleterious to the performance of devices, both because of the increased recombination within the structure and the increase of pipe defects, which can cause shorting of the device. These pipe defects consist of clusters of metallic impurities at dislocations bounding epitaxial stacking faults. 

According to Jabir, the fundamental qualities of metals are the Aristotelian hot, cold, dry, and moist. But the immediate qualities are two principles sulphur and mercury. All metals are deemed to be mixtures of sulphur and mercury. In base metals they are impure in silver and gold they attain a higher state of purity. The purest mixtures of this sulphur and mercury yield not gold but the Holy Grail of alchemy, the Philosopher s Stone, the smallest quantity of which can transform base metals to gold. 

---