Kiln Connection / Refractories

KILN CONNECTION: Repairing and Protecting Kiln Linings

Columnist Ralph Ruark describes a method of repairing and protecting kiln linings.

Figure 1. Shrinkage and gaps expose the metal kiln shell to the hot gases within the kiln.

Every now and then, a new product comes along that solves a traditional kiln maintenance problem. I recently discovered one that might be useful for manufacturers with fiber-lined kilns.

RCF Basics

Refractory ceramic fiber (RCF) linings have been used for many years in kilns and ovens of all types. The advantages of lightweight linings are significant, particularly in the case of periodic kilns. Because the RCF material is so lightweight, it has very low thermal mass. Less heat is stored in the refractory lining when the kiln is heated, and less heat must be removed during the cooling cycle, which shortens firing cycles and saves energy.

Unfortunately, nothing is perfect. While the traditional RCF has a composition of roughly 50% Al2O3/50% SiO2, the fiber lining components begin to shrink with time and temperature. Regardless of how the fiber was formed-blankets, folded modules or layered blocks-shrinkage cracks gradually occur, and gaps appear between the refractory modules. Packing these gaps with fiber material is helpful, but it is hard work; eventually, the packing falls out (as seen in Figure 1). This exposes the metal kiln shell to the hot gases within the kiln and damages the kiln casing.

Possible Solutions

In some kilns, vacuum-cast refractory fiber board is placed over “wallpaper” fiber. The lining is held tight to the kiln shell using an alumina spike, which is attached to the external kiln casing, while the board and blanket combination is secured at the hot face by a ceramic washer. This is often the case with dirt-sensitive fired products, such as glazed ware, that might be spoiled by loose fibers. Occasionally, the alumina spikes break from thermal and mechanical shock, causing the boards to sag, break, and fall off.

Replacing the broken alumina spikes can be time consuming. Although the metal cup holding the spike can be removed and replaced by a variety of methods, the process requires hours of downtime. Because it is so difficult, the required maintenance is often not done until the metal kiln casing is damaged.

Ceramic Bolts

A new solution solves these problems in a simple and effective way. In essence, ceramic bolts* screw directly into the existing fiber. Older, devitrified linings with gaps and holes can be easily covered with either of two kinds of bolts, in combination with fiber board or blanket. The ceramic bolts can be provided with optional washers to provide a secure mounting for fiber or vacuum-cast boards. Bolts are driven right into the old lining with a cordless drill and a socket; no pilot hole is necessary. The bolts are capable of holding 30 lbs or more, so there is no issue with them pulling out, even in badly deteriorated linings. The bolts are of two styles: a lag bolt and an auger design. The auger design is preferred when the underlying fiber is badly deteriorated.

The bolts can also be applied to insulating firebrick to seal cracks and heat leaks with a veneer of fiber. The installation of a new fiber board can be accomplished in a few minutes, rather than several hours, and fiber patches using blanket can be completed very quickly.

*Developed by Stellar Canada Inc. (Handled in the U.S. by Cardinal Refractories in Ohio).

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