Ring removal in rotary kilns used by the pulp and paper industry

Ring formation occurs in the rotary kilns used by the pulp and paper industry to regenerate the lime needed to produce white liquor. The main rings are: burning zone rings, mid-kiln rings, dust rings and/or mud balls, and mud rings. While the exact mechanisms of ring formation are not completely understood, frequent changes inthe production rate and the mud solids, high soda content in the mud, and high levels of dust are believed to play a major role in ring formation. The presence of rings can restrict the movement of material through the kiln. If undetected, this can result in overheating of the refractory lining. In the worst case, the presence of rings can lead to unscheduled downtime to clean out the ring and replace damaged refractory bricks. Depending on the severity, the cost of repairs and lost production can be in excess of US$3 million for a single event. Even if the root cause of ring formation is known, from a practical standpoint, it can be difficult to eliminate rings. In mills where ring formation is a recurring problem, mechanical ring removal systems should be considered. Thispaper describes the use of ring removal systems as a cost effective way to mitigate the problems associated with ring formation.