Kiln rings

Horizontal ring kilns (Fig. 16.4), such as the Hoffmann and de Wilt kilns consisted of about 20 interconnected chambers, some of which preheated the limestone with kiln gases, some calcined the limestone using fuel added through trap doors in the roof of the kiln and others served as the cooling zone. After a suitable... 

Figure 16.4. Diagram of a ring kiln (by courtesy of Intermediate Technology fhiblications)...

The concept of tunnel kilns was similar to that of ring kilns. They consisted of a linear, or circular tunnel with fixed preheating, calcining and cooling zones, through which plates or trucks, loaded with limestone, were drawn. 

The temperature in the hottest part of the kiln is closely controlled using automatic equipment and a radiation pyrometer and generally is kept at about 1100—1150°C (see Temperature measurement). Time of passage is about four hours, varying with the kiln mix being used. The rate of oxidation increases with temperature. However, the maximum temperature is limited by the tendency of the calcine to become sticky and form rings or balls in the kiln, by... 

Mechanical problems thermal expansion, concentric stress cracks, displacements, deformation of the kiln shell, formation of grooves, forces originating from retaining rings... 

Bartha, P., Using magnesia-spinel bricks to prevent the formation of rings in rotary cement kilns , World Cement, 1990 98-100. 

The rotary kiln is simple in concept, but difficult to operate in practice. The rotary kiln is a refractory lined, steel cylinder mounted horizontally on trunions and riding rings. It is pitched slightly toward the discharge end to facilitate material flow through the kiln. The kiln is fed from the high end and can be fed either whole tires or TDF chips. It can be fired internally or heated externally. 

Minor or trace components derived from raw materials, fuel, refractories or other plant materials, or added deliberately, can affect the reactions of clinker formation, or the properties of the product, or both. Their effects can be beneficial or harmful. Beneficial effects include acceleration of the clinkering reactions or lowering of the temperature at which they occur, or increase in the reactivity of the product leading to more rapid strength development. Harmful effects include decrease in alite content, volatilization in the kiln with consequent formation of kiln rings or other deposits, decrease in the durability of concrete made with the cement, or the introduction of poisonous elements. Some elements have beneficial effects at low concentrations and harmful ones at higher concentrations. Bucchi (B33,B28) reviewed some of the effects on the manufacturing process. 

Chloride contents must be kept low to avoid corrosion of steel in reinforced concrete (Section 12.3) and formation of kiln rings and preheater deposits. Contents below 0.02% are preferred, though higher ones can be acceptable if a sufficient proportion of the kiln gases is bypassed or in less energy-efficient (e.g. wet process) plants. 

Mined clay with 22 percent moisture is broken up into pieces of less than 5 cm (2 in) in a rotary impact mill without screen, and fed to a rotary gas-fired kiln for drying (see Fig. 20-58). The moisture content is then 8 to 10 percent, and this material is fed to a mill, usually a Raymond ring-roll mill with an internal whizzer classifier or a pan mill. Hot gases introduced to the mill complete the drying while the material is being pulverized to the required fineness. 

Gypsum When gypsum is calcined in rotary kilns, it is first crushed and screened After calcining it is pulverized. Tube mills are usually used. These impart plasticity and workability. Occasionally such calcined gypsum is passed through ring-roller mills ahead of the tube mills. 

The residence time of material in the rotary kiln is determined based on the kiln slope, the speed of rotation, kiln diameter, and by the presence of any internal restrictions either by design or due to ring formation in operation. The residence time, x expressed in minutes, can be calculated from the formula first given by the U.S. Bureau of Mine ... 

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