Saturn Corp. processes 60 ton/day of aluminum in a 40-ton aluminum melting furnace to feed two casting lines at its Spring Hill, Tenn. auto plant. To charge molten aluminum into the furnace three times each day, the bath is drained about 15 in. lower than normal, and then it rises again. As the metal bath surface moves down and up the refractory belly band, oxide builds up on the refractory. "The real problem isn't the oxide you can see," said Hal Shiarla, Plant Engineer, "but the unseen portions of it that penetrate into the lining. Penetration displaces the lining material, so when the surface oxide is removed, the penetrated portions go with it, leaving pits in the refractory." This required frequent lining maintenance to prevent a production stoppage due to a furnace failure.
- Prevent/minimize oxide buildup on furnace belly band
- Prevent/minimize pitting of refractory from oxide penetration into belly band
- Eliminate refractory flaking
- Reduce frequency of refractory repair
- Avoid production shutdown due to furnace refractory failure
The company made a change from the original brick sidewall lining to a castable refractory from Plibrico Co., formulated for long life and resistance to oxide penetration. This was poured up to the metal line, and a plastic version rammed in above. The monolithic lining worked better than bricks, but pitting of the belly-band area after about nine months of nightly cleaning required repairs. Pitted areas were temporarily patched by hot-gunning, which usually took four to eight hours, followed by 10 hours of bake out, costing some productivity.
During an annual two-week summer shutdown, the pitted belly-band area (~12 in. above and below the normal operating metal line) was dug out and replaced with another variation of rammed plastic refractory formulated for higher strength at elevated temperatures. However, pitting reappeared after four months, requiring hot-gunned patching. During a year-end shutdown (about six months after installing the rammed plastic) the entire belly-band again was torn out and replaced with the fresh material.
The sequence of patching at four months and replacement at six months was repeated three times over the following 18 months. When the third replacement was due, Shiarla decided to rebuild the furnace using castable lining from top to bottom, but eventually, hot gunning was required to patch the belly band.
Because no existing formulation was solving Saturn's problem, Plibrico developed a new castable having special additives to provide a much more durable oxide barrier for the most severe molten aluminum environments. Shiarla agreed to try it on the melter's upper walls, replacing the belly band and everything above it. The new formulation suffered zero oxide penetration after three years of service, but nightly cleaning gradually caused belly-band wear from flaking due to thermal cycling. A refined formulation (Plicast AL-SHIELD 80 KK) was developed to withstand a wider temperature range. The installation "still looks like it did the day we put it in," said Shiarla. "Aside from the main benefits of being able to stay on our regular maintenance schedule, and not suffering any unplanned downtime for repairs between the scheduled shutdowns, the repairs we haven't had to make for the last three years have probably saved us $60,000 or more," he added.
Unplanned repairs have been all but eliminated by the new refractory formulation. Aluminum oxide still builds up on the walls and is removed each night. But the absence of penetration allows removing the oxide cleanly, and the refractory's improved resistance to thermal shock has ended the flaking.
For more information: Saturn Corp. (931-486-6410; e-mail: halshiarla@ gm.com; Plibrico Co. (tel: 773-549-7014; Internet: www.plibrico.com).