Shaft-type kilns

Traditional shaft-type kilns Conventional long wet, dry rotary kilns Fluidized-bed type kilns... 

High-temperature rotary drum lime and cement kilns are of similar configuration to rotary drum furnaces and dryers discussed in section 4.2, except that they are of higher temperature construction and longer. This is a very specialized field. (See Perry The Rotary Cement Kiln, reference 64.) A shaft-type lime kiln is shown in figure 1.11. 

Three different types of furnaces are generally in use for calcination. The shaft furnace is considered to be the most suited for calcining coarse limestone. Furnaces of the rotary kiln type are used for handling materials of mixed particle sizes and lumps which disintegrate during the process. Calcination can be carried out in a fluidized bed-reactor for materials of small and uniform particle size. These furnaces are usually fired with gas, oil or coke in some cases electric heating is resorted to. 

Batch Furnaces This type of furnace is employed mainly for the heat treatment of metals and for the drying and calcination of ceramic articles. In the chemical process industry, batch furnaces may be used for the same purposes as batch-tray and truck dryers when the drying or process temperature exceeds 600 K (620°F). They are employed also for small-batch calcinations, thermal decompositions, and other chemical reactions which, on a larger scale, are performed in rotary kilns, hearth furnaces, and shaft furnaces. 

Different types of reactors are utilized for a wide variety of pyrolysis applications, including processing of waste plastics. The worldwide waste plastic pyrolysis systems utilize the fixed-bed designs of vertical shaft reactors and dual fluidized-bed, rotary kiln and multiple hearth reactor systems. The type of reactor used is chiefly based on material to be pyrolyzed and expected products from the pyrolysis. Stainless steel shaking type batch autoclave and stainless steel micro tubular reactors have also been used extensively [14]. Fluidized-bed reactors have been extensively used in producing raw petrochemicals from the pyrolysis of waste plastics [22, 24]. 

In addition to the different shaft kiln types, rotary kilns are often used for lime burning. This enables good quality quicklime to be obtained with cheap fuels. Smaller limestone particles (10 to 60 mm) are utilized. 

The most popular type of gasification furnace in Europe is the vertical shaft gasifier, used in the Andco-Torrax, Saarberg Fernvarme and Pyrogas process. Pyrolysis processes are often conducted in an indirectly heated rotary kiln reactor, e.g. in the Kiener, GMU or Krauss-Maffei process. Fluidized bed reactors are used at the universities of Hamburg, Eindhoven and Brussels and thus seem more popular in academic than in industrial spheres. 

The production of chemical-grade magnesia or light-burned MgO requires careful control of the calcination temperature to achieve the required specific surface area of the finished product. A furnace well suited to this requirement is the multiple-hearths Herreshoff-type. For the production of dead-burned magnesia typically shaft or rotary kilns are employed. See Chapter 5 for furnaces used in MgO production. 

Three aspects are common to all shaft kiln designs, namely charging, drawing of the ore, and combustion. A shaft kiln is essentially a vertical refractory lined cylinder or ellipse. The ore is charged in at the top of the furnace, along with, in some cases, a solid fuel such as coke or anthracite coal. Other fuels such as natural gas and oil can also be employed. There are a number of different variants of shaft kilns, such as the mixed feed, traditional type and modern basic design, annular, parallel-flow regenerative, double inclined, and multichamber. 

Fig. 6.8-11 Diagrams of the three major furnace types used for the hardening of iron ore pellets a) shaft, b) traveling grate, c) grate-kiln D, Drying F, firing (sintering) C, cooling...

Limestones that are relatively soft, are subject to thermal degradation, or produce a soft lime may not be suitable for calcining in shaft kilns. They may, however, be calcined in certain types of rotary or other kilns (see section 16.4.11),... 

It is understood that some producers of dolomite, who operate shaft kilns to make particularly soft-burned dolomite, inject steam into the combustion gases to moderate kiln temperatures. The resulting lime has a higher reactivity, and is particularly suitable for the production of Type S hydrate [15.1]. 

G. Chenxiang, The structural characteristics and rationality of China new type ordinary lime shaft kiln , Proc. International Lime Congress, Berlin, 1994. 

Annular shaft kiln is a type of shaft kiln in which the burden occupies an annulus between a central column and the shell. 

Shaft-kiln clinker typically contains Type 111 belite formed after conversion from Type 1 (Ono, 1995). 

The most common adsorbant used is granular or powdered activated carbon. This material, which is available from almost all forms of organic carbon-containing matter, is a microcrystalline nongraphite form of carbon. The production of activated carbon can be achieved by use of rotary kilns, hearth furnaces, or furnaces of the vertical shaft or fluidised bed type, and each is suitable for the generation of different pore size and the source of carbon. The pore volume and size are influenced by both the carbon source and method of production. The adsorption properties are directly related to the pore volume, pore size distribution and the nature of the functional groups on the surface of the carbon. Activation is achieved chemically, by treatment by dehydration with zinc chloride or phosphoric acid, or by treatment with steam, hot carbon dioxide or a mixture of both. The activated carbon is available in three basic forms, powder, granules or as cylindrical or spherical pellets. For solvent recovery systems the carbon is usually obtained from either wood charcoal, petroleum residues or coconut shells and is often used in the form of pellets. 

Calcium aluminate cement with a lower or intermediate AI2O3 content cannot be produced in rotary or shaft kilns of the type common in the manufacture of Portland clinker. The temperature range between incipient melting and complete fusion of the raw mixes is too narrow to permit successful clinkerization, in which a melt and solid phases must coexist. Moreover, the viscosity of calcium aluminate (-ferrite) melts is significantly lower than that of the calcium silicate-aluminate-ferrite melts formed when Portland clinker is produced. 

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