indirect smelting

Indirect smelting is a metallurgical process used to extract metals from their ores, particularly lead. It involves heating the ore concentrate, often mixed with fluxing materials like limestone or iron ore, to high temperatures (around 1,400°C or 2,550°F) in a moving grate furnace. This process, called roasting, removes most of the sulfur and agglomerates fine particles. The sulfur combusts to sulfur dioxide, which can be recovered as a byproduct. The resulting material, called sinter, is then loaded into a blast furnace with coke fuel. Air is blown in to combust the coke, producing carbon monoxide that reacts with metallic oxides in the sinter to yield molten metal. Nonmetallic impurities form a slag with the flux. The molten lead, known as base bullion, is then further refined to remove remaining impurities. This method replaced older indirect smelting techniques in the 1970s, offering advantages like reduced gas and dust volumes.12