Impurities in lead

The lead—copper phase diagram (1) is shown in Figure 9. Copper is an alloying element as well as an impurity in lead. The lead—copper system has a eutectic point at 0.06% copper and 326°C. In lead refining, the copper content can thus be reduced to about 0.08% merely by cooling. Further refining requites chemical treatment. The solubiUty of copper in lead decreases to about 0.005% at 0°C. 

Silicon. Sihcon [7740-21-3] added to copper forms alloys of high strength and toughness along with improved corrosion resistance, particularly in acidic media. Sihcon in small amounts can improve fluidity. Sihcon is a very harmful impurity in leaded tin bron2e alloys, however, because it contributes to lead sweat and unsoundness. 

Antimony appeared in all notes at a low level and was rubbed off both front and back, indicating its presence in the inks. If it was not added deliberately, it might have been present as an impurity in lead. Antimony frequently is alloyed with lead to increase its strength and to improve the electrical properties of lead plates in storage batteries and often is present in reclaimed lead. 

Instrumental parameters for the analysis of lead-based alloys are shown in Table 7 and results obtained by Hwang [8] for the analysis of impurities in lead solder are shown in Table 8. 

RESULTS OBTAINED [8] FOR THE ANALYSIS OF IMPURITIES IN LEAD SOLDER... 

Bismuth is a rather rare element in the earth s crust, but its oxides and sulfides appear at sufficient concentrations as impurities in lead and copper ores to make its recovery from these sources practical. Annual production of bismuth amounts to several million kilograms worldwide. Although elemental bismuth is a metal, its electrical conductivity is quite poor and it is relatively brittle. The major uses of bismuth arise from its low melting point (271.3°C) and the even lower melting points of its alloys, which range down to 47°C. These alloys are used as temperature sensors in fire detectors and automatic sprinkler systems because, in case of... 

Copper and Other Impurities in Lead. Lead (that used in storage batteries, for instance) is dissolved in nitric acid and the greater part of the lead precipitated by adding sulfuric acid. The supernatant is treated with citric acid and the pH adjusted to about 6 with ammonia. The polarograms show waves of copper at —0.2 V, bismuth at —0.5 V, and lead (the unprecipitated remainder) at —0.4 V. The wave of iron(III), if present, coincides with that of copper(II). 

As a rule, national battery standards stipulate only Pb purity grade of 99.99% without specifying the type and amount of allowable impurities. The specific infiuence of additives to and impurities in lead alloys has been in the focus of attention of many researchers [6—12]. Table 4.3 summarises tbe maximum allowable impurity levels for both primary and secondary lead for battery use [10]. Secondary lead comes from recycling batteries after purification. Lead of the purity grade presented in Table 4.3 can be used for the manufacture of leady oxide and lead alloys for both positive and negative grids. 

Presence of impurities in lead coolant is injurious for at least two reasons, namely ... 

---