The question of the high quality of lime is not a simple one. A soft reburned lime with a large specific surface area is more reactive in the causticizing process than a hard burned one with a small specific surface area [1]. A small specific surface area of the reburned lime means that the diameter of the lime grains is large, whereas a large specific surface area leads to a small diameter of the grains. The specific surface area of the lime affects the particle size of the lime mud produced in the causticizing process. A soft reburned lime leads to small lime mud particles and vice versa [1]. Since the separation of the lime mud and white liquor is more easily accomplished when the lime mud consists of large particles [3,4], a hard burned lime is preferable at this point. This is true whether the separation process is filtration or sedimentation [5]. It has been found [6] that the filter cake is easily washed if the diameter of the lime mud particles are small. Thus, when it comes to the causticizing reaction as well as to the lime mud washing, a large specific surface area of the reburned lime is desired. On the other hand, in the separation process of lime mud and white liquor, a small specific surface area is preferred. This requires an optimizing procedure where the specific surface area may serve as a measure of the property of the lime.What determines the specific surface area of the lime?
The present study has made important contributions to the understanding of the sintering mechanisms of calcined lime mud. The content of carbon dioxide in the atmosphere of the calcination reactor influences the specific surface area of the calcined lime mud, and is thus important for the quality of the lime. Based on the findings from this study and earlier laboratory experiments [14, 2], the difference in the specific surface area of lime from a fluidized bed and that of lime from a rotary kiln may be explained. The lime from the rotary kiln has a smaller specific surface area. In a rotary kiln, the lime bed moves slowly and the exchange of gas between the bed and the gas bulk is poor. Consequently, the carbon dioxide content in the gas in the bed is higher than in the gas bulk. In a fluidized bed, the mixing conditions are much better and, thus, the carbon dioxide content in the gas in the bed is smaller when compared with a rotary kiln. Thus, it may not only be the temperature but probably also the content of carbon dioxide in the atmosphere of the reacting beds that makes the difference.