The Mineralogy and Production of Lead

The Mineralogy and Production of Lead

Wherever mineralized fluids have percolated through rocks, and Pb⁺⁺ ions have encountered S^-- ions, the highly insoluble mineral galena, PbS, has precipitated. Galena is a cubic crystal with the same structure as NaCl, with P replacing Na and S replacing Cl. A galena cube is shown at the right (image © Amethyst Galleries). It is a semiconductor, with the small band gap of 0.37V, so it possesses good conductivity at room temperature. This causes its shiny lead-gray metallic lustre, but it is not a metal, and is brittle. The "crystal" of a crystal set is a galena crystal. The "whisker" makes a point-contact diode with rectifying properties. Its specific gravity is 7.4-7.6, so it is nearly as dense as iron. Only exceptionally does it occur as large, well-formed crystals, usually cuboids, suitable for a mineral collection, much more often disseminated in small bits in rock and other minerals.

In the Mississippi valley, the limestone rocks have in many places been eaten away by ground waters, and the voids have been filled with minerals, among them galena. Galena gave its name to the town in northwest Illinois, and large deposits were found in southwestern Wisconsin. Southeastern Missouri also has large galena deposits, that are still mined, and the Tri-State area around Joplin, including parts of Missouri, Oklahoma and Kansas, was once one of the great lead-producing areas of the world. This Mississippi valley galena is unusual in not being associated with silver, which usually accompanies lead.

Leadville, Colorado is named after the lead that accompanied the silver that was once mined there. In England, Derbyshire, Cumberland and Cornwall all had extensive galena deposits that have been worked from Roman times on. The mining area of the Harz Mountains in Germany was notable for galena, which was found in the Erzgebirge as well. The western United States and Australia also have important galena deposits. Lead is not a rare metal, as mercury is, but is found at many locations.

Galena deposits may have been altered by water filtering down from above, oxidizing and enriching the ore. This supergene enrichment has changed the sulphide into anglesite, PbSO₄, and cerussite, PbCO₃. This occurred notably on the island of Anglesey, where there were early lead workings. These "oxide" ores are much easier to treat than the sulphide, and were worked out long ago. The only ore of lead that need be considered is galena.

The problem of winning lead is complicated by the low concentration of lead in the available ore, sometimes only a few percent, and by the presence of numerous impurities. To make the process clearer, we'll assume we have relatively pure galena, and not many impurities. First of all, because of the sulphur, carbon will not reduce lead from galena. The galena must first be roasted in air to oxidize the sulphur and change the galena to oxide. A typical reaction is 2PbS + 3O₂ → 2PbO + 2SO₂. Now we can add some raw ore to the results of roasting, and get 2PbO + PbS → 3Pb + SO₂ by further heating. The elemental lead then runs to the bottom of the furnace, where it can be tapped off and cast into pigs. This simply shows the general theory of the pyrometallurgy of lead. Ore of such purity and concentration to make this simple scheme works does not exist.

First of all, the ore must be concentrated to separate the lead ore from the zinc ore, for example. It turns out to be possible to do this with flotation. A substance is added that wets the zinc ore, allowing it to sink to the bottom, but does not wet the galena, allowing it to be caught up in a foam that floats on the surface of the water. The result is not just lead sulphide, but also the sulphides of copper, iron, zinc, antimony and arsenic. The enriched ore is then roasted in ovens to drive off as much of the sulphur as possible. The roasted ore must be ground and sintered to put it in the form of porous chunks that allow gases to pass through freely, and will not collapse into a thick, impervious layer in the blast furnace. Lead ores are such that these two operations are best combined into one simultaneous roasting-sintering process that produces a sinter ready for the lead blast furnace.

The second stage of smelting can take place in an ore hearth, or a larger blast furnace. The sintered ore is charged, with coke, limestone flux, and other additives depending on the impurities present. The products that accumulate at the bottom are lead, matte (containing iron and copper), speiss (containing iron and arsenic), and slag (containing the silicates, zinc, iron and calcium). Cold air is blown in at the bottom, and flue dust and gases come out the top. The lead bullion from Mississippi valley ores is called "soft" lead and is pure enough for most uses without further treatment. The other by-produts are treated to separate their valuable constituents. Zinc, incidentally, does not dissolve in molten lead, and can be added to extract impurities by differential solubility.

Lead is sold as soft lead, 99.90% pure, common lead (lead that has been desilvered), 99.85% pure, and corroding lead (for paint), 99.94% pure. Hard lead is alloyed with 6%-18% antimony, which increases the strength of the lead. The addition of only 1% Sb or 3% Sn increases the strength by 50%. Hard lead is used for battery plates. Terne plate is heavy sheet steel coated with a lead-tin alloy. 75-25 and 50-50 Pb-Sn alloys are used.