Lightweight bubble alumina brick has outperformed dense firebrick in three high-temperature batch kilns at Thermal Ceramics UK.

When the DFC production facility for Thermal Ceramics UK (TC UK) was being relocated in 2001, new kilns for the high-temperature firing of dense refractory ceramic shapes were required to replace the existing circa 1950s kilns that had been updated over the years as production needs changed. The old refractory brick-lined kilns were plagued with poor temperature control and high maintenance costs from the frequent major refractory repairs required. Their energy efficiency was also poor, which had become a major problem with quickly rising fuel costs. A UK kiln builder, Jack Beaumont Kilns of Stoke-on-Trent (now Drayton Beaumont Kilns Ltd.), was contracted to supply new kilns as part of a total facility relocation package.

Many kiln designs were submitted by UK and European kiln manufacturers, and a variety of refractory linings were considered. The plant decided on three new batch kilns of the following types and sizes:

• A 12-cubic-meter two-car batch kiln
• A 4.5-cubic-meter fixed-hearth swing-door kiln
• A 1-cubic-meter elevator kiln

These kilns are used to fire an extensive range of alumina, mullite, magnesia and zirconia composition refractory components, including crucibles, tubes, saggars, and a variety of special shapes. All three kilns are gas-fired, with the two large kilns exhausted through the loading deck. These fast-fire units operate with 60 to 85 cycles per year and have soak temperatures in the range of 2550 to 2900°F.

In selecting the kiln linings, major consideration was given to energy efficiency (TC UK wanted a low-mass kiln to reduce heat-up energy and cold-to-cold cycle times), low maintenance costs (especially the ability to withstand the stresses of quicker cooling), kiln temperature uniformity over the firing cycle to help minimize rejects from the firing process, and a lightweight lining to simplify the structure of the kiln metal work and thus help minimize the equipment cost.

The Bubble Alumina Solution

Dense brick linings, similar to those used in the old kilns at TC UK, were rejected for many reasons, including their poor energy efficiency and high weight. Lightweight fiber systems capable of handling the 2912°F kiln cycles were next examined, but they are limited to high alumina fiber products, which provide little upper temperature limit safety margin and were perceived to be very expensive.

A mullite-bonded bubble alumina brick* (77% Al₂O₃) was eventually chosen for the hot face kiln linings. The brick has a 3250°F maximum service use limit, which gave an exceptional safety margin and the possibility of using the kilns to higher temperatures in the future. This medium-

density brick, which is designed to provide excellent high temperature strength and good thermal shock resistance, features insulation thermal conductivity and heat storage values less than half those of conventional dense alumina firebrick.

*Insalcor® brick, supplied by Thermal Ceramics.

More than 21,000 straight, arch and tongue-and-groove bubble alumina bricks were used in the construction of the three kiln hot face linings and flues. The bricks are tied back to the structure with high alumina anchor bricks** and stainless steel anchors. The lining construction was simple, lightweight and factory space efficient. This has allowed the 12-cubic-meter batch kiln to be integrated into the department layout. The low-mass bubble alumina brick was also used on the kiln cars, which enabled the company to use a common car handling system for the adjacent lower temperature fiber- and brick-lined kilns.

**Cerox®, supplied by Thermal Ceramics.

Maintenance and Energy Savings

The three high-temperature batch kilns lined with the bubble alumina brick have been operating successfully for three years with more than 200 cycles each on average. No significant maintenance has been undertaken on the bubble alumina brick linings in any of the fast-fire kilns, and all of the wall and roof linings are still in excellent condition. The bubble alumina brick has performed better than the dense brick construction of the previous kilns, which usually required extensive rebuilding of the structure every two years. A lower temperature conventional brick kiln of similar construction installed at the same time has recently required brickwork repair of damaged wall and door sections.

The low density of the bubble alumina product has also contributed to a significant reduction in energy use during the high-temperature kiln cycles. The exact amount is somewhat difficult to assess, as the new kilns benefited from multi-zone control, pressure control by exhaust dampers, and modern burners and programmable control systems. But on a like-for-like comparison vs. the old kilns, TC UK has measured a total savings of 42% of energy. It is believed that the bubble alumina brick accounts for the bulk of this amount, as the mass of the kilns has been reduced substantially. Less heat-up energy is needed for these batch kilns, with less gas required to heat the brickwork. The sidewall cold face temperatures are down as well, due to the superior insulation provided by the bubble alumina bricks. Combined with the minimal maintenance requirements, these benefits have enabled the plant to achieve substantial reductions in overall operational costs.

For more information about bubble alumina firebrick, contact Thermal Ceramics, P.O. Box 923, Dept. 167, Augusta, GA; (706) 796-4200;

e-mail tceramics@thermalceramics.com; or visit www.thermalceramics.com.