Silver sulfide impurities

Precipitation can also occur upon chemical reaction between the impurity and a precipitating agent to form a compound insoluble in the molten metal. The refining of cmde lead is an example of this process. Most copper is removed as a copper dross upon cooling of the molten metal, but the removal of the residual copper is achieved by adding sulfur to precipitate copper sulfide. The precious metals are separated by adding zinc to Hquid lead to form soHd intermetaHic compounds of zinc with gold and silver (Parkes process). The precious metals can then be recovered by further treatment (see Lead). 

Metals less noble than copper, such as iron, nickel, and lead, dissolve from the anode. The lead precipitates as lead sulfate in the slimes. Other impurities such as arsenic, antimony, and bismuth remain partiy as insoluble compounds in the slimes and partiy as soluble complexes in the electrolyte. Precious metals, such as gold and silver, remain as metals in the anode slimes. The bulk of the slimes consist of particles of copper falling from the anode, and insoluble sulfides, selenides, or teUurides. These slimes are processed further for the recovery of the various constituents. Metals less noble than copper do not deposit but accumulate in solution. This requires periodic purification of the electrolyte to remove nickel sulfate, arsenic, and other impurities. 

Sulfides Metal ion(s) + sulfur ion Galena (composed of lead sulfide), a metal ore, from which lead and also silver (which occurs in galena as an impurity, are extracted Pyrite (composed of iron sulfide), an iron ore... 

Donors and acceptors exist in silver halides both as intrinsic and extrinsic centers. Ionized donors have been identified as interstitial silver ions and substitutional uncompensated divalent cation impurities from their IR spectra [77-79]. Ionized acceptors are probably halide ions surrounding silver ion vacancies [74,80,81] and possibly some incompletely compensated substitutional divalent anion impurities such as sulfide or selenide. Carriers trapped at donors and acceptors can undergo radiative recombination by tunneling if the spatial separation of the donor-acceptor pair is not too large [6,82], The emission energy of a donor-acceptor (D-A) pair separated by a distance r in an isotropic medium is given by... 

Electrolysis is another important purification technique. The copper metal obtained by roasting copper sulfide usually contains impurities such as zinc, iron, silver, and gold. The more electropositive metals are removed by an electrolysis process in which the impure copper acts as the anode and pure copper acts as the cathode in a sulfuric acid solution containing Cu ions (Figure 20.6). The reactions are... 

Impurities sensitize some azides. For example, Gray and Waddington [18] introduced thallous sulfide into thallous azide and found that the initiation temperature was lowered. For pure lead azide the initiation temperature was 763°K whereas for the impure lead azide it was 693°K. Similarly, lead azide crystals containing 1% of carbon particles of size less than 1 /im initiate at temperatures below that of the pure lead azide [19]. An interesting method for introducing small quantities of impurities into silver azide was used by Deb et al. [20]. They prepared silver azide by using radioactive Ag, which decays by j3-emission into Cd. This method allowed 10" % of impurity to be introduced. It is important that even such small quantities affected not only the sensitivity but also the... 

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