A rotary kiln is a tube used in the mining industry for mixing and heating minerals. It is typically made of steel and can be modified for various designs. Fuel is limited to high energy fuels and controls include temperature zones and rotational speeds. A variant is used for recycling aluminum cans.
A rotary kiln, also called a rotary kiln, is a long tube used as a plug-type flow-through reactor in the mining industry. The mineral feed is introduced into the reactor portion and is carried along the length of the pipe as the pipe slowly rotates about its central axis. The materials are mixed and heated. The original designs were applied to the calcination treatment in the manufacture of Portland cement and the extraction of mercury from ores. These reactors are frequently used for the recovery of precious metals.
A typical rotary kiln consists of a single sheet of steel wrapped into a cylinder with a diameter of about 3 feet to 6 feet (0.9 m to 1.8 m) and a length of about 30 feet to 60 feet (9.1 m to 18.3 m) . In a common drive configuration, the tube is rolled over smooth stainless steel rings which rest on two rotating stainless steel rolls. Usually, there are multiple heat zones. Heat can be applied by exhausting the hot gas from the fuel, or the fuel can be fired directly into the tube. The transformed product is unloaded after being cooled in a refrigeration section where the heat is recovered and sent back to the combustion air.
Screw conveyors or simple threaded rods can be used in a separate rotary kiln design where more mixing is desired. Another setup is a reverberant design where the material is shaken along the tube. The rotary kiln design is modified by a tilting mechanism that allows liquid metal to flow out of the reactor. This design is very useful in the extraction of metals from ore or recycled materials, such as copper, gold and mercury.
Fuel is limited to high energy fuels, such as oil, gas or fluidized coal as high temperatures are required. The high energy requirement to melt metals also explains the use of direct contact with the combustion flame in some designs. The flue gases are generally passed against the flow of the material. Controls include forward speeds, temperature zones, and rotational speeds. Many companies now supply complete systems of standard design or custom designed for applications.
A variant of the rotary kiln is a batch process that consists of a rotating vessel titled on its side. Heat is supplied through the free space above the liquid. When the metal is molten, the vessel is tilted and the metal is poured into the moulds. This is a common design for recycling aluminum cans, as cans are mostly aluminum and organic materials that can be burned during the melting process.
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