Lead-sulfide deposits

Detector elements are prepared either by sublimation in the presence of a small partial pressure of O2 or by chemical deposition from alkaline solution containing a lead salt and thiourea or selenourea (63). Lead sulfide and lead selenide deposit from solutions as mirror-like coatings made up of cubic crystallites 0.2—1 p.m on a side. The reaction may nominally be represented by the following ... 

Deposits. Selenium forms natural compounds with 16 other elements. It is a main constituent of 39 mineral species and a minor component of 37 others, chiefly sulfides. The minerals are finely disseminated and do not form a selenium ore. Because there are no deposits that can be worked for selenium recovery alone, there are no mine reserves. Nevertheless, the 1995 world reserves, chiefly in nonferrous metals sulfide deposits, are ca 70,000 metric tons and total resources are ca 130,000 t (24). The principal resources of the world are in the base metal sulfide deposits that are mined primarily for copper, zinc, nickel, and silver, and to a lesser extent, lead and mercury, where selenium recovery is secondary. 

Like selenium, tellurium minerals, although widely disseminated, do not form ore bodies. Hence, there are no deposits that can be mined for tellurium alone, and there are no formally stated reserves. Large resources however, are present in the base-metal sulfide deposits mined for copper, nickel, gold, silver, and lead, where the recovery of tellurium, like that of selenium, is incidental. 

Lead sulfide films have been prepared by various deposition processes like vacuum evaporation and chemical bath deposition. Electrochemical preparation techniques have been used in a few instances. Pourbaix diagrams for all three aqueous lead-chalcogen Pb-S, Pb-Se, and Pb-Te systems, along with experimental results and cited discussion on the chemical etching and electrolytic polishing of lead chalcogenide crystals and films, have been presented by Robozerov et al. [201]. 

Cathodic deposition of lead sulfide from acidic aqueous solutions of Pb(II) ions (nitrate salts mainly) and Na2S203 on various metallic substrates at room temperature has been reported. Stoichiometric PbS films composed of small crystallites (estimated XRD diameter 13 nm) of RS structure were obtained at constant potential on Ti [204]. Also, single-phase, polycrystalline thin films of RS PbS were electrode-posited potentiostatically on Ti, Al, and stainless steel (SS) [205]. It was found that the Al and Ti substrates promoted growth of PbS with prominent (200) and (111)... 

The above mechanism (boiling, loss of CO2 and increase in pH) could also lead to the deposition of other sulfides. The reactions causing sulfide depositions by this mechanism are written as. 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Kanniainen, T. Lindroos, S. Leskela, M. 1995. Deposition of lead sulfide thin films by successive ionic layer and reaction-technique. Adv. Sci. Technol. 5 291-298. 

Puiso, J. 2004. Growth kinetics and properties of lead sulfide thin films deposited on crystalline silicon using successive ionic layer adsorption and reaction method. Ph.D. thesis. Kaunas University of Technology, Kaunas, Lithuania. 

Anderson, G. M. and G. Garven, 1987, Sulfate-sulfide-carbonate associations in Mississippi Valley-type lead-zinc deposits. Economic Geology 82,482 488. 

Cadmium occurs naturally as sulfide co-deposited with zinc, copper, and lead sulfides. It is produced as a by-product in above-mentioned metal processing. Similar to lead and mercury, this heavy metal has no known biological functions in living organisms, and accordingly its accumulation in food and water leads to undesirable consequences to biota. Cadmium toxicology is related to dangerous influence to CNS and excretion systems, firstly, on kidney. 

It is important to distinguish clearly in this scenario between the general solidification of the Earth s crust, which had the effect of freezing in variations in the U/Pb and Th/Pb ratios, and the specific mineralization event which created the galena (lead sulfide, PbS) deposits, which removed the lead from the uranium and thorium, and effectively therefore froze the isotopic composition of the lead in the galena at the values representative of the time of mineralization. 

Lead is a bluish-white, heavy metallic element with properties that are more metal-like than the properties of metalloids or nonmetals. Lead can be found in its native state, meaning that elemental metallic lead can be found in deposits in the Earth s crust. However, most lead is first mined as galena ore (lead sulfide, PbS). The galena is mixed with lead sulfate, lead sulfide,... 

Chowdhury, S. 2002. The Sargipali sulfide deposit of Orissa, India its atypical lead-high character and genesis. Journal Nepal Geological Cong. Society, 27 (Special Issue), 11-24. 

In general, substituted 3,5-diamino-l,2,4-thiadiazoles resist the action of alkaline sodium plumbite,50,1U 125 except the parent compound110 which deposits lead sulfide on brief warming. 

Carbon and metal sulfide deposits are the two main causes of deactivation of residue hydrodemetallization (HDM) catalysts. During a catalytic test, the metals contained in the feed (Ni, V) are slowly deposited on the catalyst surface leading to the build up of large particles of metal sulfides which ultimately plug the catalyst pores. Carbon, on the other hand, is known to accumulate quickly on the catalyst surface within the first days of a run until a steady state is reached (1-20). At the beginning of a run, a strong deactivation of the residue HDM catalyst rapidly occurs to which both types of deposits may contribute. However at the present time it is not clear whether this initial deactivation is mainly due to coke or metal sulfide deposits. 

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