Copper Smelting

Lore & Background

Copper was the first metal to be smelted. How the discovery came about is debated. Campfires are about 200 °C short of the temperature needed, so some propose that the first smelting of copper may have occurred in pottery kilns. The development of copper smelting in the Andes, which is believed to have occurred independently of the Old World, may have occurred in the same way. The earliest current evidence of copper smelting, dating from between 5500 BC and 5000 BC, has been found in Pločnik and Belovode, Serbia. A mace head found in Turkey and dated to 5000 BC, once thought to be the oldest evidence, now appears to be hammered, native copper. Copper–tin bronzes, harder, and more durable, were developed around 3500 BC, also in Asia Minor. At present-day the direct product of copper smelters is anode copper which has a purity ranging from 98.5 to 99.8%. Anode copper can then be electrorefined to produce cathode copper with a purity of 99.99%.

In Their Own Story

Sulfide ores such as those commonly used to obtain copper are roasted before smelting in order to convert the sulfides to oxides, which are more readily reduced to the metal. Roasting heats the ore in the presence of oxygen from air, oxidizing the ore and liberating the sulfur as sulfur dioxide gas. In recent centuries, reverberatory furnaces have been used to keep the charge being smelted separately from the fuel. Traditionally, they were used for the first step of smelting: forming two liquids, one an oxide slag containing most of the impurities, and the other a sulfide matte containing the valuable metal sulfide and some impurities. Such 'reverb' furnaces are today about 40 meters long, 3 meters high, and 10 meters wide. Fuel is burned at one end to melt the dry sulfide concentrates (usually after partial roasting) which are fed through openings in the roof of the furnace. The slag floats over the heavier matte and is removed and discarded or recycled. The sulfide matte is then sent to the converter. While reverberatory furnaces produced slags containing very little copper, they were relatively energy inefficient and off-gassed a low concentration of sulfur dioxide that was difficult to capture; a new generation of copper smelting technologies has supplanted them. More recent furnaces exploit bath smelting, top-jetting lance smelting, flash smelting, and blast furnaces. Flash smelters account for over 50% of the world's copper smelters.

Reader's Guide

Smelting involves more than just melting the metal out of its ore. Most ores comprise chemical compounds of the metal and other elements, such as oxygen (as an oxide), sulfur (as a sulfide), or carbon and oxygen together (as a carbonate). To extract the metal, workers must make these compounds undergo a chemical reaction. Smelting, therefore, consists of using suitable reducing substances that combine with those oxidizing elements to free the metal. In the case of sulfides and carbonates, a process called 'roasting' removes the unwanted carbon or sulfur, leaving an oxide, which is more suitable for reduction to metal. Roasting is usually carried out in an oxidizing environment. For sulfide ores, roasting results in replacement of sulfide, partly or completely, by oxide. Reduction is the final, high-temperature step in smelting, in which the oxide becomes the elemental metal. A reducing environment (often provided by carbon monoxide, made by incomplete combustion in an air-starved furnace) pulls the final oxygen atoms from the raw metal. The carbon source acts as a chemical reactant to remove oxygen from the ore, yielding the purified metal element as a product. Fluxes are materials added to the ore during smelting to catalyze the desired reactions and to chemically bind to unwanted impurities or reaction products. Calcium carbonate or calcium oxide in the form of lime are often used for this purpose, since they react with sulfur, phosphorus, and silicon impurities to allow them to be readily separated and discarded, in the form of slag.

Did You Know?

• Copper was the first metal to be smelted.
• The earliest current evidence of copper smelting dates from between 5500 BC and 5000 BC, found in Pločnik and Belovode, Serbia.
• Copper–tin bronzes were developed around 3500 BC in Asia Minor.
• The direct product of copper smelters is anode copper with a purity ranging from 98.5 to 99.8%.
• Flash smelters account for over 50% of the world's copper smelters.