Lead Refining by Pyrometallurgy
Lead is one of the oldest metals known, being mentioned in the Bible and used by the Romans. It is one of the most common elements found in the Earths crust, comprising an estimated 0.014% of the total. Despite its widespread occurrence in nature, metallic lead is relatively rare and is found mainly in small mineral deposits. Lead is primarily obtained through the smelting and refining of ores.
Pyrometallurgical methods of refining lead involve processes such as cupelling, fluxing and slagging. Cupelling is the most common method used for extracting lead and can be divided into two parts. The smelting part involves melting the lead ore and burning off impurities, leaving the lead in a molten state. The refining part involves further purification of the metal, normally by cupelling and sulfidation.
Cupelling was the commercial process for refining lead until about 1950 and is still used for smelting for industrial uses today. It involves melting the lead ore in a furnace and allowing it to sink below a slag layer that contains the impurities. The molten lead then runs off into a collector and is refined by further melting. The lead is heated in a second furnace and flux, such as limestone and soda ash, is added. The added flux helps to form a slag layer that traps and holds the undesirable substances. This slag is then skimmed off and discarded.
Fluxing is another method used to refine lead. The process involves melting the lead ore in a furnace, adding fluxes to increase the solubility of the impurities, and then cooling and skimming off the impurities. Fluxes are substances, such as limestone and soda ash, that lower the melting point of impurities so that they dissolve easily into the lead. The flux helps to dissociate the impurities from the lead, allowing them to be skimmed off the top.
Slagging is another method used to refine lead. The process involves melting the lead ore in a furnace, adding siliceous materials to react with the impurities, and then skimming off the slag. Slags are materials with a lower density than lead, such as iron ore or quartz. The added slag material forms a layer on top of the molten lead, trapping the unwanted impurities and allowing them to be removed.
These are just a few of the pyrometallurgical methods used to refine lead. While the process of obtaining and refining lead is not as simple as other metals such as gold or silver, it has been used for thousands of years and continues to be an important part of industry today. The methods described above are just a few of the many that exist.
