Lead sulphide, volatilization

Oxidation of the lead sulphide feed occurs with coincidental volatilization of the lead compounds PbS and PbO. Therefore, a maximum rate of PbS volatilization from the flame is noted during the intermediate step of feed oxidation. This follows from the relations of the lead sulphide volatilization rate ... 

At the same time, the observed dependence can be explained by the mechanism of PbS volatilization, taking into account the considerable amount of lead sulphide concentrate ( 25-30% of the charge) added to the oxidized feeds treated. The discussed mechanism of lead sulphide volatilization suggests that the heat supply, with no restrictions because of the use of fuel as a major contributor of the heat, causes a corrsiderable increase in the PbS volatilization rate. This explanation agrees with the data obtained in the pilot-scale unit (as appears on the same figure). In that case, the amount of sulphide concentrate in the oxidized feed varied in the range of 0-60% of the initial materials and the heat exchange conditions in the flame did not differ much. This was dictated by the necessity to use extra fuel to compensate for the heat losses in the reaction shaft despite the composition of the raw materials treated. As a result, the lower carry-over corresponded to the reduction of PbS in the feed. 

Lead The production of lead from lead sulphide minerals, principally galena, PbS, is considerably more complicated than the production of zinc because tire roasting of the sulphide to prepare the oxide for reduction produces PbO which is a relatively volatile oxide, and therefore the temperature of roasting is limited. The products of roasting also contain unoxidized galena as well as die oxide, some lead basic sulphate, and impurities such as zinc, iron, arsenic and antimony. 

Although the obvious method of preparing a binary compound would seem to be to bring the two elements together, such a procedure is not very often followed. Aluminum and sulphur can be made to combine directly, it is true, but when the finely divided substances are mixed and the mixture is heated, either the sulphur entirely distils off without any reaction taking place, or if a reaction starts it is too violent to control. We have, therefore, selected lead sulphide as a source of the sulphide radical because this substance is not volatile and cannot escape before it reacts, and the reaction is not too violent. A part of the energy furnished by the combination of aluminum and sulphur is expended in separating lead and sulphur. 

The mechanism of lead compound volatilization described above explains the relative insensitivity of the total carry-over dependence on the temperature of the lower flame (Figure 2 (b)). At the same time, based only on these laws and having no regard for the phenomena of heat and mass transfer in the gas phase of the flame, it is impossible to explain either the reduced carry-over during the industrial-scale flash smelting of sulphide feeds or the increased carry-over when smelting oxidized feeds (See Figure 2 (a) and (c)). 

Thus, a speculation on the limiting effect of heat transfer on the PbS volatilization rate allows a theoretically valid explanation of the observed low rates of volatilization under the conditions of flash smelting of lead sulphide feeds. 

Arsenic occurs primarily in sulphide minerals associated with copper ores, and to a lesser extent with zinc, lead and gold ores. Arsenic is produced as a by-product of the smelting of these metals. Primary arsenic production has now ceased in the USA and Europe, and most arsenic is now imported from China and Mexico. The volatility of arsenic represents a significant concern, and there is at present no known natural mechanism by which arsenic is immobilized in the environment. Anthropogenic activities account for an input of some 19000 tonnes into the atmosphere, compared with 12000 tonnes from natural processes, such as volcanism and forest fires (Ayres and Ayres, 1996). 

As was previously mentioned, trace elements that sublime at temperatures below those attained during coal combustion (e.g., As, Se, Hg, Zn), and are associated with thermally unstable solid phases (in particular organic matter and sulphide minerals), are subject to vaporization into furnace gases. Once these gases, and fly ash particles entrained in the gases, are vented from the combustion furnace they quickly cool, leading to the condensation of volatilized elements onto the... 

Gas chromatography has, of course, been used extensively in the analysis of many types of organic compounds with boiling points up to about 250°C, also to the analysis of organic compounds of lead, mercury, selenium, tin, manganese and silicon. Derivitisation of these compounds to produce compounds sufficiently volatile to be amenable to gas chromatography is frequently practised. Gas chromatography has also been applied to the determination of arsenic, antimony, selenium, tin, beryllium and aluminium and the common anions such as sulphate, nitrate, phosphate, sulphide, cyanide and thiocyanate. 

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