Limestone Dissociation Calcination

When limestone (calcium carbonate) is heated, it dissociates into quicklime and carbon dioxide in a process known as calcination. The calcination reaction follows. 

We assume that the activities of the solid phases are equal to unity. For the gaseous phase, the activity is a function of the dissociation pressure therefore the partial pressure of CO2 at equilibrium with CaCOa is (Themelis, 1995) 

HSC calculations show that decomposition will proceed at about 900°C. An alternative means of attaining decomposition at a lower temperature than 900° C is to decrease the bulk concentration of CO2 in the reactor by providing a flow of air, or any other gas with the exception of CO2, over the decomposing limestone. The task is to select the appropriate reactor with the least heat transfer resistance so as the reactor temperature is well above the dissociation temperature necessary to accomplish the reaction. We will examine the appropriate heat transfer resistances later. The heat of reaction can be estimated from Table 10.1. 

Magnesium carbonate, which is found in some quantities with limestone deposits, also dissociates as according to the reaction 

For the purpose of feedstock quality classification, mineral deposits that contain a significant amount of MgCOj (5-39 percent) are called dolomitic limestone, while those containing 40 percent are known as dolomite. 

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When a dense calcitic limestone is calcined at temperatures close to 900 °C, the volume of individual particles increases very slightly until about 60 % of the limestone has dissociated, and then decreases slightly to about 98 % of the original volume at the point when calcination is complete [15.8]. 

For the many applications of quicklime, active lime is the preferred product, hence careful control of the dissociation (calcination) process is necessary, bearing in mind that the time required for complete calcination depends on factors such as kiln temperature, stone size, and porosity of feed material. The dissociation of limestone above the decomposition temperature is a heterogeneous reaction (Figure 10.3). 

Like chalk, limestone is largely calcium carbonate and dissociates reversibly to lime and carbon dioxide at sufficiently high temperatures (calcination) ... 

In contrast with the high vacuum studies used to investigate the mechanism of CaCOj decomposition, the dissociation rates of large limestone particles (2 to 7 mm) were studied [29] under near isothermal conditions, as in a commercial calcining process. Kinetic data fitted the contracting volume equation with apparent values of of about 150 kJ moI between 973 and 1173 K. Predictably, CO2 decreased... 

The surface temperature rose to above 900 °C within 5 min. and then rose progressively to 1000 °C after 60 min., and reached the furnace temperature ter 67 min., by which time the particle was fully calcined. Calcination of the outer part of the sphere kept the temperature 5 mm below the surface below 900 °C until after 45 min., when the reaction interface reached it. The temperature at that point then rose towards the surface temperature. The calculated proportion of the CaCOs that had dissociated and the calculated radius of the limestone core are also shown in Fig. 15.3. 

Calcination is the heating of a substance so that a physical, or chemical change occurs. In the case of limestone, it refers to the dissociation of calcium and magnesium carbonates. 

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