A number of paper manufacturers soon became aware of the savings that could be achieved by recovering the lime. Beginning in the ’20’s, efforts were directed toward lime recovery installations. Today, a proper Rotary Lime Kiln Operation is an integral part of all modern pulp mills.
How a Rotary Lime Kiln Works
Kiln Feed and Feeders
High quality lime can be produced at a uniform rate only if the kiln is fed at a constant rate with lime sludge of constant composition. In some cases the feed to the kiln consists of a pumpable sludge containing 55 to 65% water. However, in most installations a drum filter or a centrifuge is installed just ahead of the kiln to reduce the moisture content to 35 to 50%. This cake is fed to the kiln through a screw conveyor, which is water-jacketed for that portion extending into the kiln feed end connection.
A ferris wheel slurry feeder and a surge tank located ahead of the filter or centrifuge will insure a constant rate of feed to the kiln. The surge tank should be provided with efficient means of agitation and dilution control to maintain uniform consistency of the feed to the ferris wheel slurry feeder. The overflow from the feeder is returned to the surge tank. The surge tank should have a capacity of 1 to 2 hours’ kiln feed.
Proper Rotary Lime Kiln Combustion
The uniformity of the kiln product, as well as kiln operation as a whole, depends first on controlled feed and second on proper combustion control. Good control of both feed and combustion will establish and maintain a definite temperature gradient throughout the length of the kiln. Any fluctuations in combustion or kiln feed will cause corresponding changes in this temperature gradient, with the result that the lime will not be uniformly calcined.
A constant draft at the firing hood is necessary for good combustion control. An automatic draft controller should be installed. This controller will regulate a damper in the exhaust system to maintain a constant draft at the firing hood. The usual draft at the firing hood will range to 0.05 in. water column.
The fuel should be burned with about 5 to 10% excess air to insure complete combustion. Too much air, as well as too little air, wastes fuel. Periodic Orsat analysis of the exhaust gases should be made to determine whether the correct draft is being maintained at the firing hood for proper combustion conditions.
The burner should be adjusted so that the flame is confined to a long, narrow cone with no impingement on the brick lining, since impingement will greatly reduce brick life.