Pyrometallurgy
Encyclopedia
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 alloy
s, suitable as feed for further processing.
Examples of elements extracted by pyrometallurgical processes include the oxides of less reactive elements like Fe
, Cu
, Zn
, Chromium
, Tin
, Manganese
.
Pyrometallurgical processes are generally grouped into one or more of the following categories:
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."
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 dryer
s, 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.
Calcination processes are carried out in a variety of furnaces, including shaft furnaces, rotary kiln
s, and fluidized bed reactor
s.
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."
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.
The term, 'refining' can also refer to certain electrolytic
processes. Accordingly, some kinds of pyrometallurgical refining are referred to as 'fire refining'.
Extractive metallurgy
Extractive metallurgy is the study of the processes used in the separation and concentration of raw materials. The field is an applied science, covering all aspects of the physical and chemical processes used to produce mineral-containing and metallic materials, sometimes for direct use as a...
. 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 alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
s, suitable as feed for further processing.
Examples of elements extracted by pyrometallurgical processes include the oxides of less reactive elements like Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, Cu
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, Zn
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, Chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, Tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, Manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
.
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."
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 dryer
Rotary dryer
The rotary dryer is a type of industrial dryer employed to reduce or minimize the liquid moisture content of the material it is handling by bringing it into direct contact with a heated gas. The dryer is made up of a large, rotating cylindrical tube, usually supported by concrete columns or steel...
s, fluidized bed dryers, and flash dryers.
Another type of drying, called spray drying
Spray drying
Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some...
, 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
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 kiln
Rotary kiln
A Rotary kiln is a pyroprocessing device used to raise materials to a high temperature in a continuous process. Materials produced using rotary kilns include:* Cement* Lime* Refractories* Metakaolin* Titanium dioxide* Alumina* Vermiculite...
s, and fluidized bed reactor
Fluidized bed reactor
A fluidized bed reactor is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid is passed through a granular solid material at high enough velocities to suspend the solid and cause it to behave as though it were a fluid...
s.
Roasting
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
Smelting involves thermal reactions in which at least one product is a molten phase.Metal oxides can then be smelted
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
by heating with coke
Coke (fuel)
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.- History :...
or charcoal
Charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...
(forms of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
), a reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
that liberates the oxygen as carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
leaving a refined mineral. Concern about the production of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
is only a recent worry, following the identification of the enhanced greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
.
Carbonate ores are also smelted with charcoal, but are sometimes need to be calcined first.
Other materials may need to be added as flux
Flux (metallurgy)
In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
, aiding the melting of the oxide ores and assisting in the formation of a slag
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
, as the flux reacts with impurities, such as silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
compounds.
Smelting
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
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
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 electrolytic
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
processes. Accordingly, some kinds of pyrometallurgical refining are referred to as 'fire refining'.
External links
- U.S. Patent 5616168 Hydrometallurgical processing of impurity streams generated during the pyrometallurgy of copper