Processing of metal from ore

Date: 2015-10-07; view: 458.

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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.