Pyrometallurgy

Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals. Pyrometallurgical treatment may produce saleable products such as pure metals, or intermediate compounds or alloys, suitable as feed for further processing.

Pyrometallurgical processes are generally grouped into one or more of the following categories:

• Drying
• Calcining
• Roasting
• Smelting
• Refining

Most pyrometallurgical processes require energy input to sustain the temperature at which the process takes place. The energy is usually provided in the form of fossil fuel combustion, exothermic reaction of the material, or from electrical heat. When enough material is present in the feed to sustain the process temperature solely by exothermic reaction (i.e. without the addition of fuel or electrical heat), the process is said to be "autogenous."

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Drying

Drying is thermal removal of liquid moisture (not chemically bound) from a material. Drying is usually accomplished by contacting the moist solids with hot combustion gases generated by burning fossil fuels. In some cases, heat for drying can be provided by hot air or inert gas that has been indirectly heated. The amount of heat required for a given drying operation corresponds to the heat required to vaporize the liquid moisture, the heat required to raise the temperature of the products (dry solids and water vapor) to the final drying temperature, and heat required to offset radiant heat losses.

Usually the drying temperature is set at a nominal value above the boiling point of water, often about 120°C. In special cases, such as in the drying of certain water-soluble salts, higher drying temperatures are required. In salt drying, the feed moisture is saturated with dissolved salts, which alters the boiling point and requires higher drying temperatures.

Drying of moist solids is carried out in several types of industrial dryers, including rotary dryers, fluidized bed dryers, and flash dryers.

Another type of drying, called spray drying, is carried out when the material to be dried is completely dissolved in aqueous solution. The solution is sprayed (usually through a specially designed nozzle) into a heated chamber and as the water is evaporated, solids crystallize. The water vapor is exhausted from the dryer, and dry solids are collected, usually in a conical section of the dryer. Solid material produced from a spray dryer often has special particle size and shape characteristics, which may be controlled by the concentration of dissolved material in the solution, and the design of the atomizing spray nozzle.

Calcining

Main article: Calcination

Calcining is thermal decomposition of a material. Examples include decomposition of hydrates such as ferric hydroxide to ferric oxide and water vapor, or decomposition of calcium carbonate to calcium oxide and carbon dioxide and or of iron carbonate to iron oxide. Calcination processes are carried out in a variety of furnaces, including shaft furnaces, rotary kilns, and fluidized bed reactors.

Roasting

Main article: Roasting (metallurgy)

Roasting consists of thermal gas-solid reactions, which can include oxidation, reduction, chlorination, sulfation, and pyrohydrolysis.

The most common example of roasting is the oxidation of metal sulfide ores. The metal sulfide is heated in the presence of air to a temperature that allows the oxygen in the air to react with the sulfide to form sulfur dioxide gas and solid metal oxide. The solid product from roasting is often called "calcine." In sulfide roasting, if the temperature and gas conditions are such that the sulfide feed is completely oxidized, the process is known as "dead roasting." Sometimes, as in the case of pre-treating reverberatory or electric smelting furnace feed, the roasting process is performed with less than the required amount of oxygen to fully oxidize the feed. In this case, the process is called "partial roasting," because the sulfur is only partially removed. Finally, if the temperature and gas conditions are controlled such that the sulfides in the feed react to form metal sulfates instead of metal oxides, the process is known as "sulfation roasting." Sometimes, temperature and gas conditions can be maintained such that a mixed sulfide feed (for instance a feed containing both copper sulfide and iron sulfide) reacts such that one metal forms a sulfate and the other forms an oxide, the process is known as "selective roasting" or "selective sulfation."

Smelting

Main article: Smelting

Smelting involves thermal reactions in which at least one product is a molten phase.

Metal oxides can then be smelted by heating with coke or charcoal (forms of carbon), a reducing agent that liberates the oxygen as carbon dioxide leaving a refined mineral. Concern about the production of carbon dioxide is only a recent worry, following the identification of the enhanced greenhouse effect.

Carbonate ores are also smelted with charcoal, but are sometimes need to be calcined first.

Other materials may need to be added as flux, aiding the melting of the oxide ores and assisting in the formation of a slag, as the flux reacts with impurities, such as silicon compounds.

Smelting usually takes place at a temperature above the melting point of the metal, but processes vary considerably according to the ore involved and other matters.

Refining

Main article: Refining (metallurgy)

Refining is the removal of impurities from materials by a thermal process. This covers a wide range of processes, involving different kinds of furnace or other plant.

The term, 'refining' can also refer to certain electolytic processes. Accordingly, some kinds of pyrometallurgical refining are referred to as 'fire refining'.

Environmental aspects

The gases produced by pyrometallurgical processes often present air pollution problems. In the early part of the 20th century, gaseous pollution from pyrometallurgical processes was largely uncontrolled. For example, waste gases from roasting near Queenstown, Tasmania over the years killed off all the vegetation, which then allowed all the top soil to erode. The result was a dramatic and unnatural change to the surrounding terrain. Dust or "fume" from pyrometallurgical processes presented serious hazards related to the health and safety for workers in, and residents near, pyrometallurgical plants. These metallurgical dusts often contained arsenic, cadmuim, mercury, lead, zinc, and other heavy metals. At the same time, dust losses from metallurgical plants also represented loss of potentially valuable product. Dust capture technologies were developed such as baghouses and electrostatic precipitators. Gas treatment techniques were also developed, such as wet gas scrubbing. Additionally, as in the case of sulfur dioxide, techniques were developed for production of sulfuric acid from waste gases.

See also

• reverberatory furnace
• Blast Furnace