Mineral Facts and Problems: 1960 Edition Page: 31
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a minimum aluminum content of 99.00 percent.
Digits 2 to 8 in the first position designate
copper, manganese, silicon, magnesium and
silicon, zinc, and other element, respectively; 9
is an unused series.
If the alloy has a minimum aluminum con-
tent of 99.00 percent, the second digit indicates
special control on impurities and the last two
digits indicate the minimum aluminum content
figures to the right of the decimal point. Thus,
1030 indicates 99.30 percent minimum alumi-
num content without special control on in-
When the first digit is 2 through 8, indicat-
ing a major alloying ingredient, the second
digit indicates alloying modifications and the
last two digits identify the alloy, usually by
the commercial code number previously used to
designate the same alloy. Thus, 2014, formerly
alloy 14S, contains copper as the major alloy-
ing ingredient with no alloy modification.
Additional letters, O and F, and letters H
and T with numbers, may follow the alloy
digits and designate the temper of the alloy.
No uniform system has been adopted by the
industry for designating casting alloys. These
alloys may be designated by American Society
for Testing Materials numbers, U.S. Federal
Specifications, Society of Automotive Engi-
neers, commercial, and individual company
PRIMARY ALUMINUM (16, 17, 19)
Primary aluminum is produced exclusively
by the electrolysis of alumina in a molten
cryolite bath. Purity of the alumina used
must be greater than 99.0 percent. The elec-
trolytic reduction cell consists of a carbon-
lined box containing a pad of molten alumi-
num which serves as the cathode, a carbon
anode, and an electrolyte of molten cryolite
in which the alumina is dissolved. Small
quantities of aluminum fluoride are used in the
electrolyte to combine with Na2O, an impurity
in alumina, to form artificial cryolite, and the
melting point of the bath is lowered by the
addition of small quantities of fluorspar. The
anode may be either one large block of carbon
or a number of small blocks of carbon. The
alumina is reduced to aluminum at the cathode
and carbon is oxidized to carbon dioxide at the
anode. The overall reaction may be written:
A1203,+2C - 2Al+CO2+CO
The carbon lining, which contains the molten
aluminum, is built into a steel shell. Typical
dimensions of the shell are 20 feet long, 10 feet
wide, and 3 feet deep. It is lined first with
refractory insulation. The insulation is then
lined with carbon blocks, joined by carbon
cement, or a monolithic lining is prepared by
baking carbon paste in place. The carbon lin-
ing usually must be replaced after 3 to 4 years.
The carbon anodes are consumed at a rate
of about 1 inch of vertical length per 24 hours
of operation and the method of replacement
is important in regard to overall costs. There
are two methods of replacing anode carbon
through: (1) Soderberg continuous anodes and
(2) prebaked anodes.
In the Soderberg anode a reinforced rectan-
gular steel shell, open at the top and bottom,
is suspended above the furnace. Carbon paste
or briquets of coke and pitch are added period-
ically as the anode is consumed. Current en-
ters the anode through rows of pins inserted
into the carbon mass either vertically or hori-
zontally. The heat of the bath and the heat
resulting from the electrical resistance of the
carbon bake the paste or melt and bake the
briquets so that at a point approximately 20
inches above the bath the carbon becomes a
hard monolithic mass. The steel pins, in addi-
tion to conducting current, support the carbon
mass. As the carbon descends through the
hopper and is consumed, the lowest pins are
periodically withdrawn and replaced at higher
levels in the anode box. The optimum anode-
cathode distance is maintained by raising or
lowering, simultaneously, the pins which are
baked into the lower part of the carbon block.
Inasmuch as the pad of molten metal builds
up at approximately the same rate as the anode
is consumed this adjustment is usually made
only when metal is withdrawn from the cell.
In the prebaked anode system sets of 16 to
24 prebaked carbon blocks are used. The size
of the blocks varies from plant to plant; in one
plant the blocks are 20 inches wide, 31 inches
long, 12 inches high, and weigh about 400
pounds. The blocks are supported in the bath
by steel stubs or rods which conduct the cur-
rent to the carbon. These stubs are inserted
into a hole in the top of the finished block and
anchored in place by casting molten iron
around the stub. The blocks are raised or
lowered separately to maintain proper position
with respect to the bath and are replaced in-
dividually as they are consumed (7).
The Soderberg system requires less labor
and, with the exception of moving the steel
pins, is a continuous method of feeding anode
carbon. The prebaked system results in better
electrical efficiency in the reduction cell but re-
quires fabricating and rodding facilities not
used in the Soderberg system.
The molten bath or electrolyte may be as
deep as 14 inches, but the anode is usually only
2 inches from the pad of molten aluminum.
The resistance of the bath is sufficient to main-
tain an optimum operating temperature of
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United States. Bureau of Mines. Mineral Facts and Problems: 1960 Edition, report, 1960; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc38790/m1/39/: accessed July 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.