Calcining zone

Stage 2, Calcination Zone When the magnesite reaches a temperature of about 750°C, the pressure of carbon dioxide produced by dissociation of magnesite equals the partial pressure of CO2 in the combustion atmosphere. As the magnesite progresses through the calcination zone, the temperature further rises and the surface layer of the ore begins to decompose. 

Stage 4, Sintering If all of the magnesite has decomposed to magnesium oxide before it leaves the calcination zone, then the process of sintering begins. For further discussion of sintering see Section 9.2.1. 

Stage 5, Cooling The calcined magnesite leaves the calcination zone and starts cooling. 

The double-inclined kiln incorporates two inclined sections in the calcining zone see Figure 6.13. The offset arches created by the inclines create a void on both sides of the kiln where fuel and preheated combustion air can be fired in combustion chambers. 

The multichamber shaft kiln is a variation of the double-inclined kiln. It consists of 4 or 6 alternating inclined sections in the calcining zone, which... 

The parallel-flow regenerative kiln consists of two interconnected vertical cylindrical shafts. Burden is charged alternatively to each shaft and drawn downward through a preheating heat exchange zone and into the calcining zone. 

Calcining zone In this zone, the remaining chemical bond water is released by dehydration of clay constituents and then vaporized and the raw meal is decarbonated. The liquid phase begins to form. 

Rates of calcination of different limestones under identical conditions can vary markedly (see section 15.3). For a given kiln output, a limestone with a slow calcination rate requires either a larger calcining zone, or higher temperatures in that zone, and due allowance should be made when designing the kiln and/or its refractory lining. 

From the viewpoint of thermal efficiency, however, wood is far from being an ideal fuel. Much of the volatile matter distils from the calcining zone and its calorific value is lost from the viewpoint of calcination (see section 16.7.4 for the discussion of high-grade and low-grade heat). Most of the volatiles emerge as smoke in the exhaust gases. 

Traditional shaft kilns operate continuously and are fired with fuel introduced into the calcining zone (see section 16.2.2). Various fuels have been used, including bituminous coal, producer gas, fuel oil and natural gas. 

In the case of bituminous coal, the relatively inexpensive smalls grade was widely used. The coal was introduced to the kiln at the top of the calcining zone, either by hand-shovelling or by mechanical projection, into a space created by a narrowing of the shaft (see section 16.4.2 for details). After charging, the kiln was drawn to cover the bed of coal with pre-heated limestone. 

In the cooling zone, a less refractory lining may be used (e.g., 35 to 40 % alumina), although many operators prefer to use the same refractories as in the calcining zone. This is particularly true in the case of mixed-feed kilns, where the base of the calcining zone is not well defined. 

The linings in the preheating and cooling zones have less arduous duties than that in the calcining zone. They are, therefore, generally simpler and use less expensive materials. 

High temperature re-carbonation can occur in lime kilns under abnormal conditions in which some kiln gases enter the cooling zone. It has been observed when re-circulating exhaust gases (to moderate calcining zone temperatures), when shaft kiln internal structures, such as arches, have failed and when crot-ching has occurred. 

Figure 16.L Cross-section of a vertical shaft kiln (a) preheating zone (b) calcining zone (c) cooling zone...

Such kilns gave increased outputs as well as considerably improved thermal efficiencies, as effective heat recovery occurred in the preheating and calcining zones. In addition, it was possible to produce a more consistent product by monitoring lime quality and adjusting the fuel input accordingly. 

The double-inclined kiln (design H in Table 16.1) is shown in Fig. 16.10. It is essentially rectangular in cross-section, but incorporates two inclined sections in the calcining zone. Opposite each inclined section, off-set arches create spaces into which fuel and preheated combustion air are fired via three combustion chambers. 

Batches of limestone are charged alternately to each shaft. The burden is drawn downwards through a preheating/regenerative heat exchange zone, past the fuel lances and into the calcining zone. From there the quicklime passes into the cooling zone. 

The design of burner is important for the efficient and reliable operation of the kiln. The flame should be of the correct length — too short and it causes excessive temperatures and refractory failure, too long and it does not transfer sufficient radiant heat in the calcining zone with the result that the back-end temperature rises and thermal efficiency decreases. The flame should not impinge on the refractory. Oxygen enrichment of the combustion air, and particularly of that... 

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