Production efficiency in a high-volume foundry depends in large part on the readiness of melting furnaces to provide the needed molten metal.

Representative permanent-mold aluminum foundry melting furnace

Furnace Cleaning and Maintenance

General Aluminum/Metalloy, a manufacturer of permanent- and semipermanent-mold aluminum castings for the automotive, industrial and commercial markets, has a wide range of melting furnaces in various facilities having melting capacities ranging from 800 to 6000 lb/hr (360 to 2,730 kg/hr). Extensive furnace refractory deterioration due to dross build-up on the refractory and penetration into the refractory required daily cleaning and also a frequent need to make major repairs or to completely reline furnaces. The company was looking for ways to keep its melting furnaces at peak operating efficiency by extending furnace life, thus minimizing downtime and saving furnace maintenance costs.

The company's normal maintenance schedule requires each furnace to be cleaned every day. This was difficult in the past using a variety of conventional high-alumina melting-furnace refractories from a number of refractory manufacturers. Furnace walls, floors and doors were scraped using metal implements to remove the aluminum-oxide dross buildup, which inevitably occurs when air comes into contact with molten aluminum. The tenacity of the dross on the refractory required an extremely aggressive cleaning process, which often removed part of the refractory as well. This was particularly true for the doors, which required relining every two months. In addition, the dross penetrated conventional high-alumina refractories to such a degree that it could not always be removed during regular night shift cleaning operations. Oxide growths the size of basketballs were not uncommon at the metal line area, according to the company.

As aluminum oxide penetrates and builds up on the walls of a furnace, it gradually reduces furnace capacity, and can even deform the walls to such a degree that major repairs are required. In areas of the furnace where the oxide buildup is bad, the insulating properties of the refractory is reduced, which reduces heat retention in the melt bath and makes the furnace work harder to hold a batch of metal at a given temperature. To address these problems, furnaces occasionally were shut down for a more thorough cleaning. However, much of the refractory material would come off with the oxide, requiring costly repairs to the lining. Eventually, a complete furnace relining was necessary, which put the furnace out of commission for three weeks. The relining operation required the work of five persons at a cost of $175,000.

Aluminum melting furnace lined with high-alumina refractory showing dross build-up and refractory deterioration

Solving the Problem

Because there was no significant improvement in the performance of various high-alumina refractories, General Aluminum tried a different approach by going to Stellar Materials Inc.'s Thermbond, a modified phosphate-bonded material. Thermbond is a family of patented engineered refractories consisting of a two-part system including dry formulation and liquid activator. The materials are supplied as premeasured components, which are added together to form a unique bonded refractory. The key advantage of this material in aluminum-foundry applications is said to be its natural nonwetting characteristics to aluminum without the use of additives. By comparison, other refractories typically use additives to achieve nonwetting characteristics, which eventually oxidize out of the products causing them to lose their effectiveness. The inherent nonwetting characteristics of Therm-bond means that oxides can easily be removed during night shift cleaning operations without damaging the underlying refractory, ultimately resulting in a longer lining life.

The phosphate-bonded material initially was tested on the doors of some smaller furnaces. According to the facilities manager, the doors have a blade that is submerged into the metal bath, and over time, the blades deteriorate and fall off into the metal. Using high-alumina refractory on the doors, the company had to reline the 4 ft wide by 8 ft high (1.2 by 2.4 m) doors every two months. The new refractory material lasted from 6 to 12 months. The success of the new material in extending door life on smaller furnaces led General Aluminum to install the material in all of its furnaces as they come due for relining. To date, the company has applied Thermbond to 15 furnaces. General Aluminum also specifies that newly purchased furnaces be lined with the phosphate-bonded refractory.

General Aluminum claims to have gained a number of advantages from its use of this refractory, starting with installation. Thermbond's initial high strength reduces installation time and allows furnaces to return to service faster. A high compressive strength is achieved for the refractory within an hour of casting it. Also, the refractory typically cures within a few hours after it is applied without the use of external heat. In most applications, the refractory can be put into service immediately after bake-out. Fewer structural forms are required because there is minimal hydraulic pressure on the forms.

Daily cleaning is another area where the new refractory has shown superior performance to that of the previous material. Much less dross adheres to the walls of the furnace, and removing the dross that does form is easier. The company no longer spends several labor hours per furnace on cleaning. And because dross does not stick to the refractory, the performance of each furnace is improved. Furnace capacity is maintained at the required level for a longer period of time and heat retention in the melt bath is better according to the company.

The primary failure mode of the high-alumina refractory used in the past was for the refractory to break off at the location of dross contact. By comparison, because the dross deposits come off the phosphate-bonded refractory so easily during daily cleaning, the company expects that furnace refractory life will improve significantly. General Aluminum started to reline its furnaces using Thermbond four years ago and says that none have needed relining since. The walls are still as smooth and flat as when the refractory was installed.

Another benefit of the material is that it bonds extremely well to itself and to other refractories so it can easily be repaired or veneered to extend furnace service life. The company used the refractory to repair the sidewall of a furnace lined with the previous refractory; an area of the furnace was going bad due to mechanical wear from cleaning at the metal line. The bad section was cut out and repaired using a cast Thermbond patch, which still is operating after 2-1/2 years.

Aluminum melting furnace lined with modified phosphate-bonded Thermbond refractory showing general absence of dross build-up and absence of deterioration from slag/metal/refractory reaction

Less Maintenance Saves Money

In the four years since General Aluminum started installing the phosphate-bonded refractory it in its furnaces, the company has reduced routine cleaning and maintenance costs and minimized the need for repairs. More significantly, the company has avoided numerous complete furnace relines, saving $175,000 on each one. The oldest installations of the new refractory have already shown double the life of the previous material and are still in excellent condition. If each of the 15 relined furnaces doubles its service life, General Aluminum could see a savings in over $2 million in maintenance costs. IH

For more information: David Mintz is marketing director, Stellar Materials Inc., 100 E Linton Blvd., Tower B, Suite 500, Delray, FL 33483, 561-000-0000; fax: 561-330-9355; e-mail: david.mintz; Internet: