Processing of metal from ore

The principle ores of lead are galena (PbS), anglesite (PbSO₄), and cerussite (PbCO₃). Most ores contain less than 10% lead, and ores containing as little as 3% lead can be economically exploited. Ores are crushed and concentrated by froth flotation typically to 70% or more. Sulfide ores are roasted, producing both metallic lead and a mixture with sulfates and silicates of lead and other metals contained in the ore.

Lead that has not been converted to metallic form in the roasting process is reduced in a coke-fired blast furnace. This converts much of the remaining lead to its metallic form. The slag that separates as a result of this process contains concentrations of copper, zinc, cadmium, and bismuth that can be recovered economically, as well as up to 15% concentration of unreduced lead.

Metallic lead that results from the roasting and blast furnace processes still contains significant contaminants of arsenic, antimony, bismuth, zinc, copper, silver, and gold. The melt is treated with air, steam, and sulfur, which oxidizes the contaminants except silver, gold, and bismuth. The oxidized contaminants are removed by drossing, where they float to the top and are skimmed off.

Most lead ores contain significant concentrations of silver, resulting in the smelted metal also containing silver as a contaminant. Metallic silver as well as gold is removed and recovered economically by means of the Parkes process.

Desilvered lead is freed of bismuth by treating it with metallic calcium or magnesium, which forms a bismuth dross that can be skimmed off.

Very pure lead can be obtained by processing smelted lead electolytically by means of the Bette process. The process uses anodes of impure lead and cathodes of pure lead in an electrolyte of silica fluoride.

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Descriptive chemistry

Various oxidized forms of lead are easily reduced to the metal. An example is heating PbO with mild organic reducing agents such as glucose. A mixture of the oxide and the sulfide heated together without any reducing agent will also form the metal.

2PbO + PbS   →   3 Pb + SO₂

Metallic lead is attacked only superficially by air, forming a thin layer of oxide that protects it from further oxidation. The metal is not attacked by sulfuric or hydrochloric acids. It does, however, dissolve in nitric acid with the evolution of nitric oxide gas to form dissolved Pb(NO₃)₂.

3 Pb + 8 H⁺ + 8 NO₃^–   →   3 Pb²⁺ + 6 NO₃^– + 2 NO + 4H₂O

When heated with nitrates of alkali metals, metallic lead oxidizes to form PbO (also known as litharge), leaving the corresponding alkali nitrite. PbO is representative of lead's II oxidation state. It is soluble in nitric and acetic acids, from which solutions it is possible to precipitate halide, sulfate, chromate, carbonate (PbCO₃), and basic carbonate (Pb₃(OH)₂(CO₃)₂) salts of lead. The sulfide can also be precipitated from acetate solutions. These salts are all poorly soluble in water. Among the halides, the iodide is less soluble than the bromide, which, in turn, is less soluble than the chloride.

The II oxide is also soluble in alkali hydroxide solutions to form the corresponding plumbite salt.

PbO + 2OH^– + H₂O   →   Pb(OH)₄^2–

Chlorination of plumbite solutions causes the formation of lead's IV oxidation state.

Pb(OH)₄^2– + Cl₂   →   PbO₂ + 2 Cl^– + 2 H₂O

Lead dioxide is representative of the IV state, and is a powerful oxidizing agent. The chloride of this oxidation state is formed only with difficulty and decomposes readily into the II chloride and chlorine gas. The bromide and iodide of IV lead are not known to exist. Lead dioxide dissolves in alkali hydroxide solutions to form the corresponding plumbates.

PbO₂ + 2 OH^– + 2 H₂O   →   Pb(OH)₆^2–

Lead also has an oxide that is a hybrid between the II and IV oxidation states. Red lead (also called minium) is Pb₃O₄.

Lead readily forms an equimolar alloy with sodium metal that reacts with alkyl halides to form organometallic compounds of lead such as tetraethyl lead.

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