Lead Ore Smelting
Any of several minerals from which Lead is extracted. The primary ore is galena or Lead sulphite PbS. This is unstable, and on prolonged exposure to the atmosphere it oxidizes into the minerals cerussite PbCO3 and anglesite PbSO4. Lead ores are usually associated with other metals, particularly silver which can be mined at the same time – and zinc, which can cause problems during smelting.
The Sulfide Mineral, Galena, a Lead sulfide, PbS, is the most important ore mineral of Lead. It can contain 86% Lead. Its metallic, Lead-gray cubic crystals (isometric system) and cubic, perfectly cleavable masses are distinctive and characteristic. Hardness is 2.5, streak Lead gray, and specific gravity 7.4-7.6. Galena is a widespread mineral deposited by hydrothermal solutions as large, irregular masses in dolomitize limestone and in zones of contact metamorphism and as veins in volcanic rocks. It often contains enough silver to be mined as a silver ore.
Most commercial deposits of Lead ore are in the form of veins, where hot fluids have leached the ore from cooling igneous masses and deposited it in cracks in the surrounding country rock, and in thermal metamorphic zones, where the heat of igneous intrusions has altered the minerals of surrounding rocks. Lead is mined in over 40 countries, but half of the world's output comes from the USA, Canada, Russia, Kazakhstan, Uzbekistan, Canada, and Australia.
Ore is recovered by blasting then dumping, followed by crushing and finally hoisting to the surface for treatment. In mining, the ore is extracted by drilling or blasting and then crushed and ground. The ore is then treated using extractive metallurgy. The Froth flotation process separates the Lead and other minerals from the waste rock (tailings) to form a concentrate. The concentrate, which can range from 50% to 60% Lead, is dried and then treated using pyrometallurgy. The concentrate is sintered before being smelted in to produce a 97% Lead concentrate. The Lead is then cooled in stages which cause the lighter impurities (dross) to rise to the surface where they can be removed. The molten Lead bullion is then refined by additional smelting with air being passed over the Lead to form a slag layer containing any remaining impurities and producing 99.9% pure Lead.
Manufacturing Technique For Lead Ore
The Lead was separated from the waste rock by crushing and washing, and then smelted into a molten state and poured into moulds to set into "pigs" or bars of Lead. The material brought out of the mine was called 'bouse', and was tipped into bays known as 'bouse steads' or ' bouse teems' for the first stage of processing. Women and children hand-crushed the bouse on a dressing-floor with small flat hammer-like tools called buckers. At a later stage, water-powered mills with crushing rollers took over this laborious task. The large overshot waterwheel at Killhope Lead Mining Museum was used to power a crushing mill. After crushing into small pieces, the heavy Lead ore was sorted from the waste in a variety of processes using water. Various stages of processing were used involving equipment called jiggers, buddles and dolly tubs. All these used the tendency of the heavy Lead ore to settle more quickly in water than the lighter waste.