Abrasive Conditions, Severity of Lime Kiln
Abrasion In Refractory
Evaluating the Severity of Abrasive Conditions in an Industrial Furnace
What are the conditions that cause abrasive wear? Obviously, there must be a motion of particles over a refractory surface. The abrasive particles must be forced against the surface. The force may be gravity or a high-velocity gas stream carrying dust. The force can also be the weight of the limestone charge in a rotary kiln. The particles likely will be harder than the refractory surface, but that is not always the case. And larger particles, like the limestone feed to a rotary kiln, will cut faster and deeper than finer particles.
An everyday example of abrasive wear is the removal of a rust spot from the body of a vehicle with a sanding disc. We can select alumina sandpaper for the abrasive medium on the disc. Alumina is also called corundum and has a Mohs hardness of 9. Remember steel has a Mohs hardness of 4. If you are trying to remove a lot of rust, use a coarse grade of sandpaper. Now equations for abrasion have been developed, and two of the more important parameters include the force applied and the velocity of the abrasive particles. When sanding, the more force applied to the disc, the greater the amount of rust removed. The faster the disc is rotating, that is, the higher the angular velocity, rust is removed more quickly.
A simple rating index can be devised to describe the severity of the abrasion. Do not confuse this rating system with the cc-loss result of an abrasion test. A high abrasion severity index for an application or condition will require a LOW cc-loss refractory. In rating the application, a + sign indicates the condition is present. A ++ indicates a more extreme condition. A ½ indicates the condition is present but mild. A zero indicates the condition is not a significant factor for abrasion.
Adding the score can make a rough approximation of the abrasion severity. A severity index score of 2 or below suggests abrasion is fairly low. A score of 3 suggests the abrasion is moderate and can be handled by a regular refractory with a C-704 test result in the range of 10 cc’s loss. A severity index score of 4 suggests a severe condition, and using a high strength material, like a low-cement castable, with a C-704 test result of 7 cc’s-loss or less should be considered. A severity index of 5 or higher is indicative of an extreme condition, and an extreme-service refractory should be used.
This can be applied to an industrial application, using the rotary lime kiln process as an example.
Abrasion in a Lime Kiln Transfer Chute Refractory Lining
The lime industry uses a refractory-lined vessel called a stone preheater to improve kiln efficiency. The transfer chute is the breeching between the stone preheater, up and above, and the rotary kiln, below. During operation, rams or pushers shove the limestone off the preheater floor causing the stone to fall through the hopper onto the refractory in the transfer chute, as shown by the gray dots represent the stone in this sketch below. The gas flow is represented by the red arrows.
The first condition for abrasion, motion, which is the partially calcined limestone falling off the preheater floor into the hopper. There is also some abrasive dust entrained in the gas flow. Force is present because falling limestone is being accelerated by gravity during its fall through the hopper. In fact, the force is pretty strong, as not only is there an abrasive action, there is also an impact from the falling charge. Is the limestone harder than the refractory surface? Actually not, as limestone, or calcite, is 3 on a Mohs scale. But the particle size is very large.
Motion, force, and particle size are present, but hardness is not. Because the force is so intense, it rates a double plus. This application has a severity index of 4. The conclusion is that the abrasion conditions are severe, and a high strength low-cement castable, at a minimum, is required. One might consider an extreme abrasion resistant castable for the lining.
Abrasion in Lime Kiln Nose Ring
At the other end of the lime kiln, the nose ring conditions appear milder. We have motion, of course, the rotation of the kiln. Because the lime has a lower density than limestone, the force is given a ½ value. There are definitely coarse particles, though. Overall, the severity index is a 2-1/2, moderately abrasive. 70% alumina brick discharge dams are common in the lime industry, and 70% brick has medium abrasion resistance with a C-704 result of about 10-cc’s, consistent with a severity index of 2-1/2.
Abrasion in a Long Kiln Feed End
A long lime kiln feed end is a difficult application. The motion is the rotation of the kiln, of course. The force is the weight of the stone charge, and perhaps, the impingement from the stone sliding out of the feed pipe, rating two + signs. The stone, calcite, is actually softer than the refractory, but the particle size is very coarse, also getting a ++ rating. The severity index rating is a 5, meaning that extreme service materials are appropriate. This includes phosphate-bonded high alumina brick and extreme service castables. If there are internal heat exchangers in the feed end, the motion factor gets a ++ rating, and the application rates a 6 needing an extreme service refractory material.