Dusting in a lime kiln - Characterization of the material and unit operations

Licentiate thesis, 2019

The present thesis describes the background of, and gives some points of orientation for, the phenomenon of dusting in the burner zone of a lime kiln. The aim of this study is twofold: to increase knowledge of the factors that influence the degree of dusting and also to investigate the effect a dusting lime kiln has on the white liquor preparation process. The thesis covers parts of the historical development in the recovery process, earlier findings on dusting in the literature, the impact of dusting on the causticizing process and an explanation for the cause of dusting along with actions to avoid, or contain, it.

A Kraft pulp mill utilizes a rotary lime kiln to regenerate CaO from CaCO3 by reburning. The CaO is used to regenerate the cooking chemicals for the digester. A lime kiln is around 70-100 meters long and around 2-4 meters in diameter. The kiln is typically equipped with product coolers at the 
discharge end where hot reburned lime meets incoming secondary air in a counter-current heatexchange process.

Dusting in a lime kiln occurs when fine material chaotically swirls at the discharge end of the kiln. This is caused when the velocity of upward moving air through the discharge exceeds the terminal velocity of the material that drops back into the kiln from the product coolers. The reason for material
dropping back is that some material remains in the cooler opening and is not transported further into the cooler, as intended. Both the terminal velocity and the tendency to get stuck in the product cooler inlet are connected to the size of the reburned lime. If the lime is powdery fines, dust will be formed
in the burner zone. If the lime forms nodules, which is intended, dusting will not occur. 
A dusting kiln produces a fine particulate reburned lime that is very hard burned and has a low specific surface area. When this lime is slaked, the slaking and causticizing reaction is slower and might lead either to a lower yield of effective alkali or to a slower temperature rise in the slaker, and potential overliming. It is found that when dusting occurs in a kiln, the amount of material inside the kiln increases notably even though the feed rate of lime mud has not increased.
Furthermore, during dusting conditions, the reburned lime mud contains a very large fraction of fine material. A possible explanation for the increased load could be that the fine material is present in a large part of the kiln. The fine material is likely subjected to stronger friction than the nodules. This in
turn will lead to a slower material transport through the kiln and thus an increased amount of material inside the kiln.

Through mill trials, it is found that removing the recirculated material from the electrostatic precipitator for a couple of hours has a positive effect on bringing a dusting kiln back to a clear one.