Concentrates, lead

Of key importance is the lead to sulfur ratio of the concentrate, since low levels (corresponding with low lead grades) will markedly restrict the lead throughput of the sulfur elimination stage of the smelting process (see Chapter 4, Table 4.2). 

Also of major importance is the content of piecions metals, particularly silver, which contributes significant value, and the presence of the critical impurity elements arsenic, antimony and bismuth, which attract cost penalties. Smelters often have a limited capacity to remove penalty elements within the lead bullion refining operation, and hence need to balance the intake of impurities so as to remain within various capacity limitations. This can often require careful blending of a range of feed concentrates to obtain the optimum feed mix. 

The average composition range for traded lead concentrates is given in Table 3.3, which also shows the maximum limits commonly preferred for critical impurity elements. 

Element Normal range Preferred maximum limits 

A range of impurity elements need to be separated in the lead refining process. If the concentrate is relatively clean with few impurities, it may be possible to produce a primary bullion with minimal impurity separation procedures to produce an acceptable refined lead. If impurities are in excess of the minimal levels, then cost penalties can be applied to cover the additional operations and associated costs of removing those impurities. In particular, this applies to arsenic, antimony and bismuth. Alternatively, the material may be unacceptable or will only be accepted in limited amounts as part of a feed blend, depending on the capabilities of the particular smelter and associated refinery. 

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The lead concentrate from rougher flotation ceUs is upgraded by additional flotation steps. The final concentrate is dewatered by settling in thickeners to a moisture content of 50%. Vacuum filtering further decreases the moisture level to 15%. 

The analysis of partially dried lead concentrate, ready to be treated in a series of processes to produce a commercial grade of lead, is presented in Table 2. 

Dried lead concentrate, flux, and return dust are added to the converter through a lance during the smelting cycle. Oxygen enriched air is injected at the same time to carry out the smelting reactions and maintain the temperature. Once smelting is completed, the air is shut off and the reduction carried... 

The lead-bearing components ate released from the case and other nordead-containing parts, followed by the smelting of the battery plates, and refinement to pure lead or specification alloys. The trend toward battery grid alloys having Httle or no antimony, increases the abiHty of a recovery process to produce soft lead (refined). As requited in the production of primary lead, each step in the secondary operations must meet the environmental standards for lead concentration in ait (see Air pollution Lead compounds, industrial toxicology). 

The ziac concentrate is first roasted ia a fluid-bed roaster to convert the ziac sulfide to the oxide and a small amount of sulfate. Normally, roasting is carried out with an excess of oxygen below 1000°C so that comparatively Htfle cadmium is eliminated from the calciaed material ia this operation (3). Siace the advent of the Imperial Smelting Ziac Furnace, the preliminary roasting processes for ziac and ziac-lead concentrates result ia cadmium recovery as precipitates from solution or as cadmium—lead fume, respectively, as shown ia Figure 1. 

Type of dryer tions, extracts, milk, blood, waste liquors, rubber latex, etc. gents, calcium carbonate, bentonite, clay sbp, lead concentrates, etc. trifuged sobds, starch, etc. dry. Examples centrifuged precipitates, pigments, clay, cement. ores, potato strips, synthetic rubber. objects, rayon skeins, lumber. sheets. her sheets. 

Notes. (1) If the lead concentration is too high to be measured directly using the 283.3 nm resonance line then further dilution of the sample solution is necessary. 

A cyclic-voltammetric peak current of 12.5 pA was observed for the reversible reduction of a 1.5 mM lead solution using a 1.2 mm-diameter disk electrode and a 50 mV s 1 scan rate. Calculate the lead concentration that yields a peak current of 20.2 pA at 250 mV s 1. 

Example 3-5 Polarogram A was obtained for a lOmL lead-containing sample. The limiting current increased (to B) after adding 100 pL of a 0.10 m lead standard to the 10 ml sample. Calculate the original lead concentration in the sample. 

Extrapolation of the data obtained In this study to the situation world-wide leads to a tentative estimate that people living In about one third of the cities of the world may be exposed to air lead concentrations which are either marginal or unacceptable. The problem Is probably most acute In large urban areas In developing countries with dense automobile traffic. This problem could be easily mitigated through a world-wide effort to eliminate lead In fuels. 

Less Information Is available for the other three pollutants shown In the tables. In Industrial countries In North America and Europe air quality monitoring Indicates that NOj levels may Increase risk to 15-20 percent of the residents. Exceedances of the shortterm guideline for CO appear relatively common In the reporting cities. Such exposures are likely to occur In locations with high traffic densities. With regard to Pb levels, approximately 20% of the cities have annual average lead concentration levels which exceed the WHO guideline. 

Crecelius, E.A. Piper, D.Z., Particulate Lead Concentration Recorded... 

Atmospheric fluxes of lead in the United States rose steadily from the first decades of this century, reaching a maximum in the early 1970s (see Eisenrich et al., 1986 and references therein). Passage of the Clean Air Act of 1972 and its subsequent amendments resulted in dramatic reductions in atmospheric lead concentrations, although lead fluxes worldwide still remain 10-1000 times background levels (Settle et al, 1982 Settle and Patterson, 1982). 

Evenson, M. A. and Pendergast, D. D. "Rapid Ultramicro Direct Determination of Erythrocyte Lead Concentration by Atomic Absorption Spectrophotometry with Use of a Graphite Tube Furnace". Clin. Chem. (1974), 20, 163-171. 

Variation In Lead Concentration Along Single Hairs as Measured by Non-Flame Atomic Absorption Spectrophotometry". Nature (1972), 238, 162-163. 

Figure 5. Ispoleth map of kriging error of estimations of lead concentrations (pg/g) in soil (5).

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