Editors’ Corner

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The theme of the Spring 2015 American Chemical Society National Meeting was “Chemistry of Natural Resources.” In accordance with this theme, it might be interesting to look at a particular type of natural resource, metallic ores, and at the chemical processes used to separate metals not only from nonmetallic elements such as oxygen and sulfur, but also from other metals. Since this is a bulletin on chemical information, the sources used to find this information would be at least as much of interest as the information itself. Here I describe the literature sources used to learn about the recovery of lead from its principal ore, galena, with emphasis on processes to separate silver from lead.

ImageGalena is primarily lead monosulfide (PbS). Notably, the CAS Registry Number (RN) of galena (12179-39-4) is different from the CAS RN of lead monosulfide (1314-87-0). In addition to lead, galena often contains commercially valuable amounts of silver and sometimes gold. Copper, tin, arsenic, antimony, and bismuth are also frequently found in galena. (Image source: http://en.wikipedia.org/wiki/Galena)

The first chemical step in galena processing is roasting, which converts the sulfides to oxides. The equation for this is nominally:

PbS + 1.5 O2 → PbO + SO2.

However, the PbO combines with other oxides, and most of the lead ends up as a silicate, Pb3Ca2Si3O11. The silicate is either processed in a lead blast furnace, or in an Imperial smelting furnace. In the lead blast furnace, carbon monoxide is used to reduce the lead-containing silicate as follows:

Pb3Ca2Si3O11 + 3 CO → 3 Pb + CaSiO3 + SiO2 + 3 CO2.

The oxides separate as a slag and one obtains lead bullion. In the Imperial smelting furnace, lead and zinc are processed together. Once again carbon monoxide is used to reduce the target metal oxides: liquid lead settles to the bottom and is withdrawn to form lead bullion, while zinc vapor rises to the top, where it is condensed.

Metals and metalloids which are not reduced by the carbon dioxide form a slag. Whichever furnace is used to produce the lead bullion, a large portion of the copper separates on cooling as "dross;" the dross is a mixture of copper, lead, and other impurities, and is processed separately. Meanwhile, additional copper is removed from the bullion by reaction with copper monosulfide:

Cu(Pb) + CuS → Cu2S.

The bullion is then “softened,” either by blowing air through the molten lead bullion (the classical process) or by treatment of the lead with molten sodium hydroxide containing sodium nitrate (the Harris process). Either of these softening processes removes arsenic, antimony, and tin from the lead.  Bismuth, zinc, and silver remain with the lead. It is at this point that the silver is separated from the lead.

Four methods exist for separating silver from lead; the term desilverizing may be used to denote any of them. Cupellation is a process in which lead bullion is oxidized by blowing air through the melt; lead oxide separates out, and silver and other noble metals remain in the metallic state.

The Pattinson process separates lead from silver by melting the mixture and letting it cool until crystals form. The crystals are enriched in lead, and the melt is enriched in silver. The process is repeated several times. In the Parkes process, molten zinc is added. It does not mix with the molten lead, and silver is extracted into the zinc layer. The zinc is then distilled off. Finally, silver may be separated from zinc electrochemically, with the silver being recovered from the anode slime. 

The information in the preceding paragraphs was obtained from one classic reference and two more up-to-date references. The latest reference is Alain Vignes, Extractive Metallurgy, Wiley-ISTE, 2011 (ISBN 1848212925). This is a three-volume set, and is rich in figures and equations. Among the desilverizing processes, only the Parkes process is mentioned in the section on lead refining.  Somewhat older, but more comprehensive, is Fathi Habashi, Handbook of Extractive Metallurgy, Wiley-VCH, 1998 (ISBN 3527287922). This is a four-volume set and the volumes are much larger than those of the Vignes reference. All four of the desilverizing methods are described in the chapter on silver; the chapter on lead describes only the Parkes process. The classic reference, J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans, Green & Co., 1923, also describes all four desilverizing methods. The desilverization of lead has its own section, in the chapter on silver in volume 3. One should also note that the Kirk-Othmer Encyclopedia of Chemical Technology (5th edition, 2007, ISBN 0471484967) has a chapter on lead and lead alloys, and that Ullmann’s Encyclopedia of Industrial Chemistry (ISBN 0471484967) has a section on the refining of lead bullion in its chapter on lead. Kirk-Othmer mentions cupellation, the Parkes process, and electrochemical refinement, whereas Ullmann’s mentions only the Parkes process.

Of course, for the latest developments in desilverization technology, one must go to the recent literature. In addition to Chemical Abstracts and the usual patent databases, one might mention the database METADEX in this regard. METADEX is the electronic version of the print publications Metals Abstracts, Metals Abstracts Index, and Alloys Index, and is produced by ProQuest LLC. It provides information on the properties, fabrication, applications, and development of metals and alloys, including the processes for extraction of metals from their ores. METADEX is available via STN (but not SciFinder) as well as via ProQuest. 

Several reference resources have been described which discuss the removal of silver from lead.  (This is not intended to be a comprehensive list of such resources). Four general methods of removing silver from lead were described in these resources. All references described the Parkes process (extraction using zinc), which appears to be the most commonly used process today. Three other methods: cupellation, electrochemical separation, and the Pattinson process, are described in some of the references. Although the search here was focused on the separation of one pair of metals, these same references should be useful when investigating other metallurgical separation processes.

David Shobe, Assistant Editor, Chemical Information Bulletin