Melting aluminum in a reverberatory furnace involves heating the charge to melting (approx. 1220F, or 660C) and holding at molten metal temperatures to 1290F (700C). A typical furnace is heated by burners located above the molten bath line. Temperatures in the upper part of the furnace-above the molten metal bath-range from 2010 to 2370F (1100 to 1300C).
Refractories for aluminum melting and holding furnaces must withstand mechanical abuse from charging, from thermal shock due to cyclic heating and from complex forces on the refractories when molten metal penetrates their surface. Penetration can destroy a lining in any furnace.
Generally, the severity of these forces increases sharply with increasing furnace size and varies widely with furnace practice. For example, mechanical shock and abrasion will likely be severe where large quantities of cold scrap are dumped into the furnace. Furnaces are either direct charged with solid materials through the main door or through doors on one side wall or are charged by placing scrap into a well containing molten aluminum.
Refractory selection considerations
The refractory lining is the heart of a furnace. It is the most important factor in furnace design after the "workability" of the furnace (that is, factors such as access for charging, cleaning and repair). Not only does the refractory lining need to contain the molten aluminum, but it also needs to withstand mechanical abuse and provide for minimum thermal wall losses.
Two areas of metal contact (the hearth and lower sidewall) suffer the most mechanical abuse and metal penetration. In the hearth, metal penetration can cause the hearth to heave up and away from the furnace bottom as a result of metal creeping into joints and cracks until it finally works its way under the top course of brick or a hot face lining.
In sidewalls, metal penetration usually concentrates at the metal line where molten aluminum and aluminum metal oxide, or alumina (which forms from a reaction between the furnace atmosphere and fluxes) react with the refractory and create a severe problem. Penetration at the metal line contributes to thermal spalling of the refractory and causes considerable damage when deposits that cling to the wall are removed during a cleaning operation.
These problems present challenges to the furnace designer to select the refractories that will provide optimal performance. The choices of refractories for the application include brick, plastic and low-cement cast (which has excellent nonwetting properties for hot face linings). Cost usually is the determining factor.
After selecting the lining material, it is very important to choose the proper thickness and back-up insulation to establish a freeze plane of the molten aluminum. The freeze plane is the point where the molten aluminum solidifies and will not penetrate the lining any farther. The best freeze plane location is in the hot face lining to reduce the possibility of the floor heaving.
The back-up material for the hot face lining is important, especially below the metal line. This material not only needs to be crushable to allow for the hot face lining expansion, but also it must have molten-metal tightness. The thickness of the crushable material also is important. For example, if it's too thin, the expansion of the lining could split the casing or heave the floor.
When a furnace is shut down, cooling and shrinking of the lining causes it to pull away from the crushable and create a gap between the hot face and back-up insulation, which could cause a lining failure at restart. Most of the lining shrinkage is caused by the contraction of the aluminum in cracks of the lining. Pure aluminum has a shrinkage greater than 6% as it freezes.
When designing a refractory lining, its dry-out time (or curing time in the case of plastic refractory) should be considered based on the heat available. That is, can the burner system be used, or are external burners required. Even if the burners are capable of drying out the furnace, some well melters will need external burners in the well, especially if the well is lined with a plastic. If the dryout is not done correctly, plastic linings could sheet off some of the material or slip. With castable, the material could explode if heated too quickly.
Refractory design and brick-lining installation
In designing a brick lining for an aluminum furnace, all walls should have a header course at the hot face, and the thickness should be of 9, 13-1/2or 18 in. etc. even brick, with a 3 in. series brick to reduce the total number of joints.
All walls, wall corners, wall-to-wall connections and arches should be bonded using a minimum 2-1/4 in. bond. When installing a brick lining, the walls should be level, straight and plumb, with minimum mortar joint thickness and tapped in place using a leather mallet for a molten metal-tight wall. For very tall brick walls, an anchor brick should be spaced in the upper walls so the wall will not bow in and collapse into the furnace. The brick lining of a well melter should have a Rowlock course to cap off the well so molten metal will not destroy the backup lining.
Brick arches should be a minimum of 9 in. thick and bonded. Also, the skew and backup of the arch are very important. If an arch is close to a wall, the soft block (crushable) should be eliminated and hard cast should be installed to backup the skew so the arch will not collapse.
The design of the division wall between the main chamber and well is very important because it is a "hot wall" that is hot on both sides. Wall thickness should be a minimum of 22-1/2 in. thick on the lower wall. The furnace steel casing should allow for the larger expansion of this wall.
For plastic linings, it is necessary to pay close attention to the wall anchor spacing and shelf brackets to support the uncured plastic. During plastic lining installation, ramming should be done parallel to the hot face and the blocks broken into smaller slabs, which should be bonded with each course. Anchors should be tapped in place so there are no voids around them. After ramming is complete, the hot face should be trimmed to the refractory anchor face, but the surface should not be smoothed out. In the upper wall only, construction joints should be cut into the hot face 1-1/2 in. deep and on 24 in. centers. Holes 1/8 in. in diameter and about 4 in. deep on 12 in. centers should be made, and then the surface should be sealed with water glass. As with brick linings, the design of plastic linings must take into account expansion of the lining.
Low-cement cast linings
A low-cement lining should be anchored to the casing only in the upper walls using a stainless steel stud projecting from the block back-up surface and a stainless steel "V" anchor welded to the stud. The placement of the cast should be done with a very large mixer for a more consistent pour. The wall-to-floor connections will be different than a brick or plastic lining to form a staggered joint.
Cast and plastic roofs
A monolith roof should be supported using refractory anchors on approximately 12 in. centers. Also, the roof should have control joints about 8 ft square to control cracking of the roof during cool down of the furnace. The roof-to-wall closure is very important and should be designed so the wall will be able to rise and not destroy the roof.
The first consideration is how the door is operated and then what type of refractory lining to use. The most common door operators are electric motor and gear box and air cylinder. The location of the operators should be such as to keep them away from the heat of the furnace when the door is opened. To withstand mechanical abuse, a lightweight (90 - 120 lb/ft3), high-temperature (2800 to 3000F, or 1540 to 1650C) cast should be used in doors to save weight, and "V" anchors should be spaced closer than in walls. Fiber linings are being selected for this application in some instances to save weight. However, the amount of mechanical abuse that lining must withstand should be considered before selecting fiber linings for doors. In a direct-charged melter, mechanical abuse on a fiber door usually is a result of the scrap falling on the door or molten aluminum splashing on it.