1. What is calcination?
Calcination is the process of heating the material at an appropriate temperature lower than the melting point to decompose it and remove volatile substances such as crystal water, carbon dioxide or sulfur trioxide.
For example, limestone loses carbon dioxide after calcination to generate quicklime; aluminum hydroxide is dehydrated to generate alumina; basic titanium sulfate loses water and sulfur trioxide to generate titanium dioxide, etc.
The main physical and chemical changes that occur during the calcination process are:
(1) Thermal decomposition; (2) Recrystallization to obtain a certain crystal shape, crystal size, pore structure and specific surface area; (3) Proper sintering of crystallites to improve mechanical strength.
Calcination is a process in which natural compounds or artificial compounds undergo thermal dissociation or crystal form transformation. During calcination, compounds are thermally dissociated to form compounds with simpler components or undergo crystal transformation.
The calcination operation can be used to directly process the mineral raw materials to meet the subsequent process requirements, and can also be used for the post-processing of chemical beneficiation to produce chemical concentrates to meet the requirements of users for products.
The materials that need to be calcined are: cement, kaolin, limestone, etc.
2. Rotary kiln of calcining equipment
Almost all non-metallic materials are prepared by calcination process, which requires the use of non-mineral thermal calcination equipment that generates and utilizes high temperature.
Rotary kiln refers to a rotary calcining kiln (commonly known as a rotary kiln), which is a steel cylinder with a refractory lining and belongs to the building materials equipment category.
Rotary kiln classification
In the internal heat rotary kiln, the heat source is in direct contact with the material in the rotating kiln body for heat transfer; while in the external heat rotary kiln, the heat source is outside the kiln body, not in direct contact with the material in the kiln, and the heat is transferred through the kiln body wall. The material passed to the kiln.
The heating methods of external heating rotary kiln include electric heating, fuel oil heating, gas heating and coal heating.
The advantages of externally heated rotary kiln: it is easy to control the heating process, easy to ensure product quality, less exhaust pollution at the kiln tail, and easy to recover the solvent in the material.
In the internal heating rotary kiln, in order to make sufficient heat exchange between the material and the high-temperature hot flue gas and the kiln body wall, the filling rate of the material in the kiln is usually more than 15%, and the highest can reach more than 25%.
Analysis of Heat Transfer Process of Rotary Kiln
Schematic diagram of the heat transfer process of the kiln body cross-section of the external heat rotary kiln
Q0 is the total heat carried by the heat source outside the kiln, W;
Qw-g is the heat provided by the inner wall of the kiln to the gas in the kiln, W;
Qw-m is the heat provided to the particles by the inner wall of the kiln, W;
Qs is the heat dissipation between the outer wall of the kiln and the outside world, W.
In view of the heat transfer characteristics of the external heat rotary kiln, all the heat required by the external heat rotary kiln is transferred to the material in the kiln through the kiln body wall. The material particles will be distributed in a thin layer on the inner wall of the rotating kiln, especially for larger particles that are approximately spherical. When the kiln body rotates, the particles close to the wall surface will roll down and slide along the kiln wall, and the material layer will not have obvious fixation. layer and active layer.
Because all the heat required by the materials in the kiln is provided by the kiln body wall, in order to achieve the ideal heat transfer effect of the external heat rotary kiln, the kiln body speed is usually low to ensure sufficient contact time between the wall surface and the material. At this time, due to the thin material layer, the interaction between particles is weak.
Schematic diagram of muffle rotary kiln
1-feeding mechanism; 2-rotating drum; 3-combustion chamber; 4-chain drive wheel; 5-cooling box
Schematic diagram of straight flame chamber rotary kiln
1-kiln body; 2-kiln head trolley; 3-hot smoke chamber; 4-cooling chamber; 5-kiln head blower; 6-dust collection chamber; 7-chimney
The rotary kiln equipment (rotary kiln) is a cylindrical object with a certain slope, the slope is 3~3.5%, and the rotation of the kiln promotes the stirring of the materials in the rotary kiln (rotary kiln), so that the materials are mixed and contacted with each other. to react. The coal injection combustion at the kiln head produces a lot of heat, and the heat is transferred to the material by means of flame radiation, hot gas convection, and kiln brick (kiln skin) conduction. The material moves forward in the kiln depending on the inclination of the kiln cylinder and the rotation of the kiln.
The rotary kiln is mainly composed of a cylinder body, a rotating device, a supporting device, a blocking wheel device, and a kiln head sealing device.
It is pre-rolled from steel plates of different thicknesses. The cylinder is inlaid with refractory materials. Several tires are set outside the cylinder. The cylinder has a certain slope and is located on the supporting wheel corresponding to the tires.
The wheel belt is a sturdy large steel ring set on the cylinder to support the full gravity of the rotary kiln (including kiln bricks and materials); the weight of the wheel belt varies from 20 tons to 100 tons; the wall thickness near the wheel belt increases , the purpose is to reduce the deformation caused by the pressure of the supporting roller.
The idler is supported by bearings.
The hydraulic retaining wheel is a roller that moves around the longitudinal axis and is installed on the plane of the kiln tail pulley belt near the kiln head side.
Function: timely point out the unreasonable running position of the kiln body on the supporting roller, and limit or control the axial movement of the kiln body.
Purpose: To ensure the rotation of the kiln and to adjust the number of revolutions of the kiln.
The rotary kiln is operated under negative pressure, and there are gaps where the cylinder body is connected with the kiln head cover and the smoke chamber. In order to prevent air leakage, a sealing device must be provided, otherwise air leakage and material leakage will occur.
Combustion material in rotary kiln
The industry that uses the most rotary kilns is cement; secondly, there are a lot of rotary kilns used in ferrous and non-ferrous metallurgy. Iron, aluminum, copper, zinc, tin, nickel, tungsten, chromium and other metals use rotary kilns to process ores and intermediates. Calcining, and calcining active lime as a steelmaking solvent and dolomite for various purposes (can be used as slag-making agent for converter steelmaking, metal magnesium raw materials, refractory raw materials, etc.), calcined magnesia, aluminum-magnesium spinel, bauxite , hard clay and other refractory materials.
Due to the different calcined materials, the firing temperature of the rotary kiln is also different.
3. Development of rotary kiln
The application of rotary kiln originated in cement production. In 1824, the British cement worker J Asp invented the intermittently operated earth kiln; in 1883, German Dietz invented the continuous operation of the multi-layer shaft kiln; After the rotary kiln was patented in the United Kingdom and the United States, it was put into production and soon achieved considerable economic benefits. Later, the rotary kiln was widely used in many industrial fields, and became more and more important in these productions, becoming the core equipment of the corresponding enterprises.
The domestic rotary kiln system started relatively late, and the research and testing began in the early 1970s. In the 1980s, foreign advanced technology and equipment were introduced.