Engineered Refractory Material Saves Aluminum Foundry $40,000/Furnace/Yr.

The Mahle Group, a 1400-employee, permanent mold aluminum foundry in Morristown, Tennessee, was experiencing aluminum oxide problems in the reverberatory furnaces used to recycle chips produced during metal-cutting operations. These furnaces have a 20,000-lb capacity and produce 2000 lb of metal/hr. Aluminum oxide is generated in furnaces of this type at the surface of the molten metal where it contacts the air.

In the past, Mahle used conventional high-alumina phosphate-bonded ceramic refractories. However, the aluminum oxide penetrated this refractory to such a degree that it could not be removed during regular night shift cleaning operations.

As aluminum oxide penetrates and builds up on the walls of a furnace, it gradually reduces the furnace's capacity and pushes the walls apart and the roof up, threatening to destroy the furnace. To keep this from happening, foundry personnel had to shut down the furnace every 6 months for a week. A three-person crew used jackhammers to remove the built-up oxide, and, as a result, much of the refractory material came off with the oxide, forcing the crew to add $5000 worth of repair materials to the walls of the furnace. This operation could be done only once for a given refractory installation. At the 1-year point, the refractory was in such poor shape that it had to be completely removed. This cost $21,000 for materials and occupied a crew of five people for another 2 weeks.

Aluminum oxide also presents quality issues. It has a tendency to form inclusions that would contaminate the finished parts if they were not removed. As a result, Mahle uses large aluminum balls and flake in a large crucible at the reverberatory furnaces to filter out oxides. In addition, the molten aluminum is periodically flushed with nitrogen to flush the oxides out of the filter bed so they can be skimmed off the top. These methods sufficed to eliminate any danger of oxide contamination, but they added considerable expense to foundry operations.

In 1996, Mahle discovered a product called Thermbond from Stellar Materials, Inc., Delray, Florida. It is a family of two-part engineered refractories that utilize a dry formulation and liquid activator. These materials are supplied as pre-measured components that are added together to form a bonded refractory. The key advantage of this material in aluminum foundry applications is that it is completely and naturally non-wetting to aluminum without the use of additives. This feature means that oxides can be removed with ease from refractory linings during nightly cleaning operations without damaging the underlying refractory.

Another advantage of Thermbond is that its high early strength reduces installation time and allows furnaces to return to service faster. It reaches a high compressive strength within an hour of casting and typically cures within a few hours after it is applied with no outside heat. In many applications, this refractory can be put into service immediately using an aggressive bake-out schedule. Less structural forms are required since there is minimum hydraulic pressure on the forms. The refractories bond well to themselves and to other refractories so they easily can be used to repair or veneer existing linings for extended service.

Mahle decided to try the new refractory on one reverberatory furnace used to recycle chips. The material cost $25,000, 25% more than the conventional refractory used in the past. Installation, including removal of the previous refractory, took 1 week. The aluminum oxide buildup was considerably less than in the past and the buildup that was there came off easily during night shift cleaning operations. The night shift foundry worker simply sprayed a mild flux on the refractory walls, then used a rake-like tool built internally from pipes on the end of a forklift to loosen up the aluminum oxide. This process easily removed all of the built-up material without damaging the refractory, leaving the walls in like-new shape.

With the buildup being removed every day, the refractory was in perfect condition at the end of its first 6 months. Mahle was so pleased that it replaced two more reverberatory furnaces with the new refractory. At the 1-year stage, all of the furnaces that had been lined with Thermbond still were in excellent condition. The foundry canceled the refractory replacement that would have normally occurred at that stage and installed the refractory on two more reverberatory furnaces. Two years after the initial installation, all of the refractories still are in excellent condition.

The new refractory material has significantly reduced the costs of Mahle's foundry operations. Since using Thermbond, a cost of $40,000/furnace/yr worth of maintenance that was required in the past to remove aluminum oxide buildup has been eliminated. The oldest lining has lasted 2 years and still is going strong. Foundry managers say that it will go at least 2 more years with a little patch work on some high-wear areas. They are so pleased with the performance of the new material that they plan to begin installing it in wet bath and open-hearth primary furnaces in the near future.