Silver removal from lead

Silver is often removed from lead by the Parkes process, described in Chapter 27. Some pure lead is made by electrolytic refining. 

Silver is almost invariably removed from lead by the addition of zinc, which forms an intermetallic Ag-Zn compound. After removal of this there remains about 0.54-0.60% Zn in the lead, depending upon the details of the desilverising process. The lead-zinc solution is at a temperature just above the melting point, and to remove the zine by distillation, the lead must be heated considerably. Distillation at atmospheric pressure would require such an excessive temperature as to be uneconomic, and therefore the process is eonducted under vacuum. Mild steel vessels can be operated to a temperature of about 600 C, and with suitable design can be made robust and vacuiun-tight. 

Crude lead contains traces of a number of metals. The desilvering of lead is considered later under silver (Chapter 14). Other metallic impurities are removed by remelting under controlled conditions when arsenic and antimony form a scum of lead(II) arsenate and antimonate on the surface while copper forms an infusible alloy which also takes up any sulphur, and also appears on the surface. The removal of bismuth, a valuable by-product, from lead is accomplished by making the crude lead the anode in an electrolytic bath consisting of a solution of lead in fluorosilicic acid. Gelatin is added so that a smooth coherent deposit of lead is obtained on the pure lead cathode when the current is passed. The impurities here (i.e. all other metals) form a sludge in the electrolytic bath and are not deposited on the cathode. 

Blocks of rich cmst are added periodically and allowed to melt. As melting takes place, the lead-rich phase sinks to the bottom and is withdrawn from the kettie by a syphon. The lighter silver—2inc phase rises and floats on the surface of the lead. After sufficient silver—2inc alloy has accumulated, it is tapped from the top section of the kettie. In this manner it is possible to achieve a 120 1 concentration of the silver in the cmst which is passed on for retorting. The lead removed from the bottom of the kettie typically contains 0.5% silver and 2% 2inc. 

Betts Electrolytic Process. The Betts process starts with lead bullion, which may carry tin, silver, gold, bismuth, copper, antimony, arsenic, selenium, teUurium, and other impurities, but should contain at least 90% lead (6,7). If more than 0.01% tin is present, it is usually removed from the bullion first by means of a tin-drossing operation (see Tin AND TIN ALLOYS, detinning). The lead bullion is cast as plates or anodes, and numerous anodes are set in parallel in each electrolytic ceU. Between the anodes, thin sheets of pure lead are hung from conductor bars to form the cathodes. Several ceUs are connected in series. 

Refining. The alloy of bismuth and lead from the separation procedures is treated with molten caustic soda to remove traces of such acidic elements as arsenic and teUutium (4). It is then subjected to the Parkes desilverization process to remove the silver and gold present. This process is also used to remove these elements from lead. 

The removal of silver from lead is accomplished by die addition of zinc to the molten lead, and slowly cooling to a temperature just above the melting point of lead (600 K). A crust of zinc containing the silver can be separated from the liquid, and the zinc can be removed from tlris product by distillation. The residual zinc in the lead can be removed eitlrer by distillation of the zinc, or by pumping chlorine tluough the metal to form a zinc-lead chloride slag. 

The solubility of the precipitates encountered in quantitative analysis increases with rise of temperature. With some substances the influence of temperature is small, but with others it is quite appreciable. Thus the solubility of silver chloride at 10 and 100 °C is 1.72 and 21.1mgL 1 respectively, whilst that of barium sulphate at these two temperatures is 2.2 and 3.9 mg L 1 respectively. In many instances, the common ion effect reduces the solubility to so.small a value that the temperature effect, which is otherwise appreciable, becomes very small. Wherever possible it is advantageous to filter while the solution is hot the rate of filtration is increased, as is also the solubility of foreign substances, thus rendering their removal from the precipitate more complete. The double phosphates of ammonium with magnesium, manganese or zinc, as well as lead sulphate and silver chloride, are usually filtered at the laboratory temperature to avoid solubility losses. 

After each series of experiments with beams of various intensity the section plate would be removed from the cell and disassembled, with radioactive silver washed out by nitric acid. Radioactivity of the solutions obtained was measured by a multichannel spectrometric scintillation y-counter with sensitivity of up to 10 G, i. e. around 10 of atoms which, according to calculations, is 10 times lower than sensitivity of ZnO sensor 10 G or 10 of Ag atoms respectively [28]. This difference in sensitivity lead to great inconveniences when exposing of targets was used in above methods. Only a few seconds were sufficient to expose the sensor compared to several hours of exposure of the scintillation counter in order to let it accumulate the overall radioactivity. It is quite evident that due to insufficient stability during a long period of exposure time an error piled up. 

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. 

Brittania A process for removing silver from lead, operated by Brittania Refined Metals in England, using ore from the Mount Isa mine in Australia. After initial concentration by the Parkes process, and removal of the zinc by vacuum distillation, the mixture, which contains silver (70 percent), lead, and some copper is treated in a bottom blown oxygen cupel in which lead and copper are removed by the injection of oxygen through a shielded lance. 

Davey A modification of the Parkes process for removing silver from lead. A water-cooled tray is floated on the molten lead. Invented by T. R. A. Davies in 1970 and operated by Penarroya in Brazil, France, Greece, and Spain. 

Parkes A process for removing silver from lead, based on the use of zinc, which forms in-termetallic compounds of lower melting point. Developed by A. Parkes in Birmingham, England, in the 1850s. Parkes also invented the first plastic (Parkesine), used for making billiard balls. 

Rozan A variation of the Pattinson process for extracting silver from lead, in which steam is blown through the molten metal. This oxidizes the zinc and antimony, which come to the surface and are removed. 

Some differences in arsenate and chromate adsorption on ODA-clinoptilolite and Pb-(Ag-linoptilolites) as well were recorded (Figs. 5 and 6). ODA-clinoptilolite exhibited more efficient arsenate and chromate removal from aqueous solutions than the inorganically exchanged modifications. However, silver exchanged clinoptilolite revealed higher capacity values for both oxyanions uptake than lead exchanged clinoptilolite did. This phenomenon supports preferred silver treated clinoptilolite utilization for specific water purification process even on the base of environmental acceptability. 

If the ratio be unity, the concentrations of the solute in each solvent will be the same if the ratio be far removed from unity, a correspondingly large proportion of the solute will be found in the one solvent which can be utilized to extract the Soln. from the other solvent. E.g. ether will remove ferric chloride from its aq. soln., and since many other chlorides are almost insoluble in ether, the process is utilized in analysis for the separation of iron from the other elements the solubility of cobalt thiocyanate in ether is utilized for the separation of cobalt perchromic acid is similarly separated from its aq. soln. by ether molten zinc extracts silver and gold from molten lead the extraction of organic compounds from aq. soln. by shaking out with ether or other solvent is much used in organic laboratories. 

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