LEAD Ore

hiny, blue-white soft metal when its surface is fresh. On exposure to the air, it becomes covered by a dull, gray layer of basic carbonate that adheres closely and protects it from further alteration. It resembles aluminium in this respect, which is protected by a dull, gray layer of oxide. Otherwise, lead would react rapidly with the oxygen and carbon dioxide in the air. When placed in sulphuric acid, lead is protected by a similar layer of PbSO4 that adheres strongly. For these reasons, lead is often used to sheathe cables for burial, to protect roofs from the atmosphere, and as tanks and pipes for sulphuric acid. Lead has many and varied uses in technology. This article mentions a very large number of uses of lead and its compounds, from cathedrals to crystal sets, from batteries to sailing ships.

Lead and its compounds were known and widely used in antiquity. Its metallurgy was well-developed even then. The uses of lead depended on its corrosion resistance, its softness and ease of working, and its low melting point. Metal pipes were made of lead, starting with a plate that was rolled into a cylinder and fusion welded down the seam. Buildings were roofed with sheets of lead, and glazed with lead mullions. Molten lead was poured into holes in stone to hold fasteners. Boxes of lead were used as protective containers, and as coffins. Lead was used in the metallurgy of other metals, as we shall see in some detail. Lead was, indeed, used in machines of all kinds, not as a structural material, but where its fusibility and workability were an advantage, and for many small items of daily use.

At the current time, the greatest quantity of lead is used in lead-acid storage batteries. Until recently, next in importance was the use in paint pigments. However, environmental considerations have largely removed lead from paints. Similarly, the once considerable usage of lead in tetraethyl lead antiknock compound has also disappeared for a similar reason. Lead is still used for sheathing cables, in bearing alloys, in artistic pigments and glazes, for decorative glass, in the chemical industry, and, of course, for bullets, which have always been in special demand in the United States.

This article gives some general information on lead, and discusses a few processes and devices in which lead has played a central role, both recently and historically. The banned uses of lead are also described, to satisfy curiosity that might otherwise be frustrated. As much of the curious lore of lead has been mentioned as I could find.
Properties of Lead

The chemical symbol for lead is Pb, from Latin plumbum. The name "lead" is cognate with the Dutch "lood." The high-German "Blei" has no echoes in English or Dutch. English, however, uses the word "plumber" for a worker in lead. The older term is preserved in the name "Ledbetter." Dutch still uses the word "loodgieter" for a plumber, which means "lead-pourer." In Greek, lead is molubdos, a name now used for molybdenum. This meant "lead pencil" in Greek, which describes the soft MoS2 well.

The atomic number of lead is Z = 82, and its stable isotopes are of mass numbers A = 204, 206, 207 and 208, of which the last is most abundant (52.3%). Each isotope is the end of a natural radioactive series, except for 204, which is the rarest at 1.48% abundance. The isotopic abundance, and so the atomic weight, varies in lead from different locations. The average atomic weight is 207.22. The series beginning with Th232 (half-life 1.39 x 1010 y) ends with Pb208. The series beginning with U238 (half-life 4.5 x 109 y), the most abundant natural isotope of uranium, ends with Pb206, and includes the natural isotope of radium, Ra226. The famous fissionable U235 (half-life 7.1 x 108 y) ends with Pb207. Because of the large number of electrons it contains, and its high density, lead is the most practical absorber of gamma rays.

The density of lead is 11.3437 g/cm3, higher than that of common metals like iron (7.86) and copper (8.933), but less than that of mercury (13.6), and not in the league with the really dense metals like gold (19.3), tungsten (19.3), platinum (21.45), iridium (22.42) or osmium (22.48), which are about twice as dense. Lead is, nevertheless, the densest of the common and inexpensive metals, and some uses depend on this property. The fishing sinker and the sailor's lead are humble examples. Lead crystallizes in the face-centered cubic structure, with lattice constant a = 0.494 nm.

The melting point of lead is 327.35°C, and the boiling point is 1515°C. The only common metals with lower melting points are tin and bismuth, while the boiling point is high enough to allow processes in liquid lead over a wide temperature range. The coefficient of linear expansion is 29.5 x 10-4 per °C. The bulk modulus is 0.44 x 106 Mbar. The heat conductivity is 0.081 cal/cm-s-°C, and the specific heat is 0.03046 cal/g-K (all properties are room-temperature values). The heat of fusion is 6.26 cal/g. The electrical resistivity is 20.648 μΩ-cm (compare to copper, 1.683). The hardness of pure lead is 1.5 on the Mohs scale (between talc and gypsum: it can be scratched by the fingernail), and its tensile strength is only 2000 psi. The Young's modulus is 2.56 x 106 psi. Its crystalline form is face-centered cubic, with lattice constant 0.4939 nm. Lead alloys considerably with bismuth and tin, to a smaller degree with antimony and silver. Lead anneals itself (rerystallizes) at room temperature, so cold work does not harden it.
The Chemistry of Lead

The electron configuration is 6s26p2 over a filled 5d subshell, similar to that of C, Si, Ge and Sn, the other elements in its column in the periodic table. The spectroscopic ground state is 3P0, and the resonance line is at 283.39 nm. Because of this, and the high boiling point, lead does not color the gas flame. The first ionization potential is 7.415V, second 15.04V, third 32.1V and fourth 38.97V. The lead spectrum is a good example of jj-coupling, which has effects even in the ground state.

Lead exhibits the oxidation state +2, corresponding to loss of the two p electrons, in most of its common compounds. In this oxidation state, lead is generally basic. The oxidation state +4 also occurs, and in it lead is more acidic. Lead is generally amphoteric, like aluminium, especially in the +4 state, like tin.

Lead forms a series of oxides, which are important compounds. The principal ones are PbO, a yellow oxide called litharge, and PbO2, a reddish-brown substance called lead dioxide, or, erroneously in technical practice, lead peroxide. It is, of course, not a peroxide. Orange-yellow Pb2O3 is PbO·PbO2, while red Pb3O4, red lead or minium, is 2PbO·PbO2. In addition to these, there is Pb2O, lead suboxide, a black, amorphous substance that is PbO·Pb. If metallic (liquid) lead is heated in air below 545°C, red lead forms; if above 545°C, litharge forms.

Basic lead carbonate, 2PbCO3·Pb(OH)2, when pure, is a brilliant white substance that makes an excellent paint pigment, called white lead. It reacts with H2S to produce black PbS, so should not be used in chemistry laboratories. It was generally made by the "Dutch Process" where perforated plates were treated with acetic acid, air and CO2. The lead for this process was called "corroding lead" in commerce, and had to be very low in antimony, which caused the white lead to darken.

The pigment "chrome yellow" was PbCrO4. Mixed with prussian blue, it made a very good green (this is not always the case, but it worked here). "Chrome red" was PbCrO4·PbO. Lead gave a full spectrum of bright colors, used by artists as well as by house painters. Red lead made a paint that protected iron and steel from corrosion; the famous Forth Bridge in Scotland is usually seen covered in red lead. The painters work continuously on it.

Colorless lead nitrate, Pb(NO3)2 is the most soluble lead compound. Lead acetate, Pb(CH3COO)2 is also soluble. It has a sweet taste, and for this reason is called "sugar of lead." When it was discovered, in Roman times, it was used in certain baked treats, which proved fatal. Any soluble lead salt dried on paper can offer an easy test for H2S. When moistened and waved around, it will turn black. Your nose, actually, is not a good detector, since the rotten-egg odor rapidly disappears by fatiguing your sense of smell. The halides of lead are rather insoluble, while the sulphate, carbonate and sulphide are quite insoluble. As we have mentioned, this insolubility is what protects lead from corrosion. Lead arsenate, Pb(AsO4)2 is a double poison, from the Pb and from the As. It was used for dusting cotton plants to fight the weevil.

In qualitative analysis, lead is identified by first adding dilute HCl to the sample, which precipitates the chlorides of mercury, silver and lead, the only insoluble chlorides of the common cations. After this precipitate is separated, it is treated with hot water (still in acid solution). Lead chloride is much more soluble in hot water than in cold, so it is separated by pouring off the supernatant. Then the addition of a little sulphuric acid will produce a precipitate of PbSO4, proving the presence of lead.

A powdered sample, mixed with a little Na2CO3 as flux on a charcoal block, and then heated in the reducing blowpipe flame (the yellow part), will make a drop of liquid Pb and some PbO. This is a mineralogist's test.

One of the most famous lead compounds was tetraethyl lead, Pb(C2H5)4. Here the lead has made four tetrahedral covalent bonds, like carbon in CH4, to fool the motor fuel into letting it dissolve. In the cylinder, the heat knocks the ethyl radicals off and the lead forms a cloud of PbO. The ethyl stops any explosion front in its tracks. Therefore, the fuel charge does not detonate and burns smoothly. This permitted the use of cheaper straight-chain hydrocarbons in motor fuel, instead of the branched and aromatic hydrocarbons of greater octane rating. The lead is only there to carry the ethylene into the combustion zone; it would evaporate if added to the fuel directly.

Tetraethyl lead was made by treating ethyl bromide or ethyl chloride with an alloy of sodium and lead, NaPb. The sodium grabbed the halogen, the lead latched on to the ethyl radical, and some lead was left over, which could be reused. Some extra ethyl bromide was added to the fuel, so that in the heat of the combustion, it would combine with the lead that had initially been released, and carry it out in the exhaust. Otherwise, the lead would stay around and foul the spark plugs and valves (which it did anyway). Some people thought the lead lubricated the valves and was necessary for the engine, but this is false.