News Date:2015-09-14
Once we have our preliminaryrotary kiln size, we can start to think about the details of the rotary kiln internals. In the case of a direct fired rotary kiln, in which the hot stream of gas flows directly through the interior of the rotary kiln, refractory is usually needed.
Arguably one of the most critical components of a direct fired rotary kiln, the refractory is what protects the carbon steel shell from the high temperatures within. A quality refractory is of the utmost importance, and many options are available, depending on the needs of the rotary kiln.
Typically, there are two kinds of refractories for lining a rotary kiln: castable, and brick. Each kind of refractory has its advantages and disadvantages. The choice of refractory is dependent on the rotary kiln temperature, material chemistry, and how abrasive the material is.
Castable refractory comes in a powder form and is mixed with water onsite. Before the mixture can be put in place, anchors are installed. These y-shaped anchors are similar to rebar in cement; they help give the castable lining its strength. Once these anchors are in place, the cement-like mixture is pumped into the lining of the rotary kiln, and allowed to cure for several days. Castable refractory has a similar material cost to brick. However, brick installation is much more labor intensive, as each brick is individually installed. This makes the overall cost of a brick lining more expensive than castable. The disadvantage to using castable refractory in a rotary kiln is that it is very susceptible to installation problems. When castable refractory is installed very well, it can nearly match the quality of brick. But if the castable is installed incorrectly, there can be a considerable difference in quality, and the life of the refractory can be severely compromised.
Besides lower overall cost, the advantage to using castable refractory in a rotary kiln is that it is usually easily patched when a problem is encountered. Down time is typically minimal, because the problem area can just be cut out and new refractory can be poured back in the cavity.
The second type of refractory we will look at is brick refractory. Refractory quality is measured by the percentage of alumina that it contains. Alumina gives the refractory its durability in terms of temperature resistance and strength. Brick is fired in a furnace under tightly controlled conditions that allow it to achieve better properties than a similar composition castable. Brick refractory is slightly more expensive than castable, but it does not require anchors, and it’s quality is superior. Castable is basically brick that has not been to the furnace yet. When processing a highly abrasive material, brick refractory is advisable most of the time, as castable does not have the durability to stand up against abrasive materials as well as brick. There is a downside to brick, however. Brick refractory is held in place much like the principle of a roman arch: bricks are held in place by the pressure of the other bricks pushing against each other. When a problem is encountered, typically the failed brick needs to be replaced, but when one brick is relying on the bricks around it to hold it in place, often one cannot replace just one brick, and whole sections of the refractory need to be replaced. Unlike castable refractory, the repair of a fail in brick refractory is much more involved.
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