Smelting
Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron, copper, silver, tin, lead, and zinc. The reducing agent is commonly a fossil-fuel source of carbon, such as carbon monoxide from incomplete combustion of coke. In earlier times, charcoal served this purpose. The oxygen in the ore binds to carbon at high temperatures. This happens because the chemical potential energy of the bonds in carbon dioxide is lower than that of the bonds in the ore. Sulfide ores such as those commonly used to obtain copper, zinc, or lead are roasted before smelting. Roasting heats the ore in the presence of oxygen from air. This oxidizes the ore and liberates the sulfur as sulfur dioxide gas. Workers must make these compounds undergo a chemical reaction to extract the metal. Reduction is the final, high-temperature step in smelting. A reducing environment often provided by carbon monoxide pulls the final oxygen atoms from the raw metal. The carbon source acts as a chemical reactant to remove oxygen from the ore. As most ores are impure, it is often necessary to use flux, such as limestone or dolomite. This calcination reaction emits carbon dioxide.
Smelters can be classified into two types depending on their business model. Custom smelters treat ore on behalf of customers or buy ore for treatment. Integrated smelters depend directly on a specific mining operation and tend to be located next to a mine. Smelting most prominently takes place in a blast furnace to produce pig iron. Plants for the electrolytic reduction of aluminium are referred to as aluminium smelters. Electric phosphate smelting furnaces appeared in TVA chemical plants during 1942. Traditional reverberatory furnaces were about 40 meters long, 3 meters high, and 10 meters wide. Fuel was burned at one end to melt dry sulfide concentrates. These older furnaces 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. Some examples of bath smelters include the Noranda furnace, the Isasmelt furnace, and the Teniente reactor.
Of the seven metals known in antiquity, only gold regularly occurs in nature as a native metal. The others occur primarily as minerals. In the Old World, humans learned to smelt metals in prehistoric times, more than 8000 years ago. The earliest current evidence of copper smelting dates from between 5500 BC and 5000 BC. This evidence has been found in Pločnik and Belovode, Serbia. Campfires are about 200 degrees Celsius short of the temperature needed for smelting. Some propose that the first smelting of copper may have occurred in pottery kilns. Copper-tin bronzes were developed around 3500 BC 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%. The earliest known cast lead beads were thought to be in the Çatalhöyük site in Anatolia. They dated from about 6500 BC but recent research suggests they were not actually lead.
The earliest evidence for iron-making consists of small iron fragments with appropriate amounts of carbon admixture. These were found in Proto-Hittite layers at Kaman-Kalehöyük and dated to 2200, 2000 BC. Most early processes in Europe and Africa involved smelting iron ore in a bloomery. This kept the temperature low enough so that the iron does not melt. It produces a spongy mass of iron called a bloom. Workers must consolidate this bloom with a hammer to produce wrought iron. Some of the earliest evidence to date for the bloomery smelting of iron is found at Tell Hammeh, Jordan. From the medieval period, an indirect process began to replace the direct reduction in bloomeries. This used a blast furnace to make pig iron. In the 13th century during the High Middle Ages the blast furnace was introduced by China. China had been using it since as early as 200 BC during the Qin dynasty. Both bloomery and blast furnace processes are now obsolete. Instead, mild steel is produced from a Bessemer converter or by other means including smelting reduction processes such as the Corex Process.
Smelting has serious effects on the environment, producing wastewater and slag. It releases toxic metals such as copper, silver, iron, cobalt, and selenium into the atmosphere. Smelters also release gaseous sulfur dioxide, contributing to acid rain. The smelter in Flin Flon, Canada was one of the largest point sources of mercury in North America in the 20th century. Even after smelter releases were drastically reduced, landscape re-emission continued to be a major regional source of mercury. Lakes will likely receive mercury contamination from the smelter for decades. Air pollutants generated by aluminium smelters include carbonyl sulfide, hydrogen fluoride, polycyclic compounds, lead, nickel, manganese, and mercury. Copper smelter emissions include arsenic, beryllium, cadmium, chromium, lead, manganese, and nickel. Lead smelters typically emit arsenic, antimony, cadmium, and various lead compounds. Labourers working in the smelting industry have reported respiratory illnesses inhibiting their ability to perform physical tasks demanded by their jobs.
In the United States, the Environmental Protection Agency has published pollution control regulations for smelters. Air pollution standards exist under the Clean Air Act. Water pollution standards operate under the Clean Water Act. Treatment technologies include recycling of wastewater and settling basins. Clarifiers and filtration systems handle solids removal. Oil skimmers and filtration manage dissolved metals. Chemical precipitation and filtration remove organic pollutants. Carbon adsorption and biological oxidation treat complex organics. Wastewater pollutants discharged by iron and steel mills includes gasification products such as benzene, naphthalene, anthracene, cyanide, ammonia, phenols, and cresols. These combine with a range of more complex organic compounds known collectively as polycyclic aromatic hydrocarbons. Aluminum smelters typically generate fluoride, benzo(a)pyrene, antimony, and nickel. Lead smelters may discharge antimony, asbestos, cadmium, copper, and zinc.
Common questions
What is the definition of smelting and what reducing agent does it use?
Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. The reducing agent is commonly a fossil-fuel source of carbon, such as carbon monoxide from incomplete combustion of coke.
When did humans first learn to smelt metals in prehistoric times?
Humans learned to smelt metals in prehistoric times more than 8000 years ago. The earliest current evidence of copper smelting dates from between 5500 BC and 5000 BC found in Pločnik and Belovode, Serbia.
Where was the blast furnace introduced during the High Middle Ages?
The blast furnace was introduced by China in the 13th century during the High Middle Ages. China had been using it since as early as 200 BC during the Qin dynasty.
Which smelter in Canada became one of the largest point sources of mercury in North America in the 20th century?
The smelter in Flin Flon, Canada was one of the largest point sources of mercury in North America in the 20th century. Even after releases were drastically reduced, landscape re-emission continued to be a major regional source of mercury.
What environmental regulations govern air pollution standards for smelters in the United States?
Air pollution standards exist under the Clean Air Act published by the Environmental Protection Agency. Water pollution standards operate under the Clean Water Act with treatment technologies including recycling of wastewater and settling basins.