Mineral Facts and Problems: 1960 Edition Page: 32
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MINERAL FACTS AND PROBLEMS, ANNIVERSARY EDITION
9500 to 1,0000 C. At this temperature the
alumina dissolved in the bath varies from 6 to
10 percent. When the alumina concentration
drops to 2 percent, the electrical resistance of
the cell increases sharply and the voltage drop
across the cell increases from 5 volts to 30 or
40 volts. The phenomenon is known as the
"anode effect." As soon as this condition oc-
curs, the crust of frozen cryolite on top of the
bath is broken and more alumina is added to
the cell which then returns to normal operating
Every 1 to 3 days the molten aluminum is
removed from the cell by aspiration. Cast iron
pots with airtight lids and downward sloping
spouts are used. The molten metal is blended
with other batches and then may be fluxed,
alloyed, cast, or if the fabricating facility is
nearby, transported molten to the fabricating
Plants constructed since 1950 have much
larger cells than were used in older plants.
These cells utilize currents from 65,000 to 125,-
000 amperes, with the majority of plants hav-
ing 80,000- to 100,000-ampere cells. The volt-
age drop across a single cell is 4.5 to 5.0 volts
and across a potline 600 to 800 volts.
The larger cells require less manpower per
pound of aluminum produced, but special
problems are encountered in cells designed to
use 100,000 and more amperes. The large cur-
rents cause powerful magnetic fields in the
metal and bath, resulting in violent agitation.
This agitation causes a fog of aluminum to be
dispersed in the bath which increases the pos-
sibility of reversing the reduction reaction.
Another detrimental effect is that the molten
metal piles up toward the negative leads caus-
ing a variation in the anode-cathode spacing.
Moveover, localized thermal effects may distort
the carbon lining (4).
Current efficiency varies from 85 to 90 per-
cent; losses are principally due to physical
losses of metal and reoxidation of aluminum by
carbon dioxide. Owing to the electrical resist-
ance the voltage efficiency is only 40 percent
with heat being lost by radiation, and in the
exhaust gases, in tapped metal, and in elec-
trodes removed from the cell. As a result the
overall energy efficiency is about 35 percent.
About 7.5 to 8.5 kw.-hr. is required in the cell
per pound of aluminum metal produced. Total
plant requirements are less than 10 kw.-hr. per
Table 5 summarizes the raw materials used
in producing one pound of aluminum.
To prevent pollution of the atmosphere,
gases from the cells are passed through a col-
lection system which removes alumina, carbon,
TABLE 5.-Raw materials and energy required to produce
one pound of aluminum
Alumina P-------------ound ...__ 1. 93
Cryolite-------------------do.. 0. 02-0. 03
Aluminum fluoride-_ - - - do - - - - - 0. 02-0. 03
Fluorspar- ---....------------ do------- 0. 003
Anode carbon -------------- do -- --- 0. 55
Cathode carbon_ - - do....--- 0. 02
Electrical energy----.... Kilowatt- 7. 5-8. 5
The relatively low tensile strength of com-
mercially pure aluminum limits its use in
heavy durable products. However, by alloy-
ing, cold rolling, or heat treating, aluminum
alloys with strengths approaching those of
mild steels have been developed. There are
more than 100 commercially-available alumi-
num alloys and several new alloys usually are
developed each year. The alloys offers a wide
variety of combinations of mechanical
strength, ductility, electrical conductivity, and
corrosion resistance. A small addition of man-
ganese increases the strength of wrought alu-
minimum. Magnesium and silicon, together or
separately, give aluminum alloys with good
corrosion-resistance and strengths. Copper
and zinc are used to produce alloys with high
strength-to-weight ratios. Small quantities of
other metals such as nickel, chromium, tita-
nium, and tin are sometimes added for grain
refinement or to impart special characteristics.
The production of super-purity aluminum
requires a second electrolytic step in which a
cell, known as the Hoopes cell, with three mol-
ten layers is used. The bottom layer is the
anode, a relatively impure aluminum alloyed
with copper to increase its density. The inter-
mediate layer in the conventional Hoopes cell
is molten cryolite with barium fluoride added
to give it a density greater than that of pure
aluminum. The top layer, which serves as the
cathode, is molten super-purity aluminum.
Direct smelting of aluminum-silicon alloys
from clay has been investigated. High-purity
clay is used to minimize contamination of the
alloy by iron and titanium. An electric fur-
nace with graphite electrodes has been used
with a carbon reductant, which may be coke,
charcoal, sawdust, hogged fuel, or mixtures of
these materials. At operating temperatures
pure aluminum would volatilize or react with
oxides of carbon. This is prevented by the ad-
dition of silicon which alloys with the alumi-
num and promotes absorption o^ the aluminum
Methods for recovering commercial grade
aluminum from Al-Si alloys have been investi-
gated. Experimental procedures have included
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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/m1/40/: accessed March 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.