Charging the Cast Furnace
Charging the furnace is the first step in the casting
process. The cast house is equipped with the following in order to
prepare a charge, which is a mixture of raw materials that are
melted down to make an alloy. The equipment is as follows:
Aluminum Casting Equipment
- Pre-heat oven:
used to pre-heat prime
(prime is 99.9% pure aluminum) before it is added to the
furnace. The heat comes from the recuperator and does not require
the use of additional gas. Pre-heating is done to remove water
from the prime. The presence of water when the metal is placed in
the furnace will cause an explosion as the water rapidly
Hot air from furnace flows over a series of tubes where the air
is heated which in turn feeds the burners for the furnace and in
turn reduces gas consumption.
- Front-end Loader/ Fork Truck:
the front-end loader is a lift with a large bucket that is used
to add scrap to the furnace when preparing a charge. The fork
truck is used to add prime ingot into the side doors of the
furnaces as well as remove dross and stir the mix.
used to melt down aluminum and cast logs. The furnaces are open
well reverb. Open well means there is a well opening in the front
of the furnace where is scrap added. The definition of reverb is
when the flame does not melt the aluminum but the heat from the
walls and ceiling of the furnace. The following diagram will
illustrate the above definitions.
The furnaces are powered by natural gas 95-98% of the time. When
there is a gas curtailment, propane gas is used. The furnaces are
made and lined with 18"- 24" refractory, which is a heat
resistant material that comes in two forms: brick and a castable
Aluminum Casting Materials
There are two types of aluminum logs, primary and secondary.
Primary consists of pure aluminum and secondary consists of prime
and scrap. There are three components used when charging a furnace
to make secondary aluminum: prime, scrap, and hardeners. Bonnell
Aluminum's Carthage, Tennessee and Newnan, Georgia casting
facilities manufacture secondary logs. However, all billets/logs
are cast within the Aluminum Association's specifications.
99.9% pure aluminum. Prime comes in three forms: T-bars, tub
sows, and pig.
- T-Bar and Tub Sows weigh more than fifty pounds
- Pig is prime that weighs less than 50 pounds
- Prime is also categorized by the iron content for
example, 10/20 prime tells us there is a maximum of .10% silicon
and .20% iron.
material purchased from outside sources and that which is
generated from within the plant.
Elements, which are added to a bath of aluminum to increase
strength and give the final product the characteristics, desired
such as finish, strength, and grain refinement. The elements are
as follows: Silicon (Si), Iron (Fe), Copper (Cu), Manganese (Mn),
Magnesium (Mg), Chromium (Cr), Zinc (Zn), Titanium (Ti), and Boron
(Bo). Silicon, iron, copper, manganese, magnesium, chromium, and
zinc are used to increase strength and to improve finish. Titanium
and Boron are used for grain refinement which is a reduction in
the size of the grains, creates a more consistent grain, and
Preparing the Bath
The first step in charging the furnace is adding the appropriate
amount of alloying agents: scrap, prime, and hardeners. A charge
is a mixture of raw materials that are melted down and
mixed to make an alloy. Prime and scrap are first added to the
furnace. The alloy being cast determines the quantity of scrap and
prime to be used to charge the furnace. Prime and scrap are
preheated to remove moisture, which would cause an explosion.
To calculate how much prime needs to be added to the furnace you
must first know how much of a heal was left in the furnace from
the previous drop. A heal
is the remaining metal left in the furnace after one heat
has been completed. A drop
is the act of pulling the pin on the furnace and allowing
the aluminum to flow into the pan until the formation of logs is
complete. The formula for calculating how many pounds of prime to
Prime (lbs) = [Charge (lbs) - Heal (lbs)] *
[Target % of Prime]
Once the prime has melted, scrap is put into the furnace until the
furnace is full and allowed to melt. The melting
point of aluminum
is 1215 degrees Fahrenheit.
is the removal of hydrogen from molten metal by bubbling a
mixture of gasses up through the melt. Flux
is a substance that promotes fusion, especially of metals
or minerals. Fluxing
causes impurities, such as alkaline, sodium, and lithium,
(which cause the material to have a bad finish), to rise to the
surface of the bath. Once degassing is complete a sample is taken
and analyzed for proper chemical content.
is a mixture of aluminum oxides and non-metallic material,
which float to the surface of molten aluminum. Dross is produced
whenever aluminum is added to the furnace that has been painted,
anodized, or dirty. Dross is skimmed off of the top of the molten
aluminum into dross pots. Dross is cooled with argon gas to
eliminates the oxygen in the mixture and prevent thermiting. Thermiting
is the temperature at which aluminum will burn up. The dross is
recycled to recover the aluminum from within it.
Samples are taken and analyzed using a spectrometer. A spectrometer
determines the concentration of elements in an alloy by comparing
the relative intensity of the wavelengths of radiation produced
when a sample is struck with an electric spark. Hardening agents,
such as silicone and magnesium, are added to bring the
concentration of the alloying agents up to the specification of
the alloy being cast.
Making a Cast
The temperature of the metal must be between 1300-1340 degrees
Fahrenheit, the analysis must be within the ranges specified, and
degassing must be complete to begin a drop. The casting process
used at Bonnell Aluminum is called direct
because the cooling of the log is taking place in the mold
itself where water is running through the mold. After all
criteria are met the drop can begin. Before tapping the furnace
safety equipment must be put on. Tapping
is the process of pulling the plug pin out of the furnace
so the aluminum can flow into the trough and pan.