2.2.4 High-purity aluminum
For most applications,the purity of aluminum as it comes from the potroom (i.e., up to
99.9%) is adequate. High-purity aluminum of at least 99.97% aluminum content is necessary
for certain special purposes (e.g. reflectors or electrolytic capacitors); for such applications,
the potroom metal has to be further refined in an additional process. Less than
one percent of the total volume of primary metal undergoes this second stage of refining.
High-purity aluminum is produced by so-called three-layer electrolysis (the Hoopes cell).
This cell, in contrast to the two layers of the Hall-Héroult process, operates with three
liquid layers. The lowest layer, called anode metal, receives the input of normal primary
aluminum to which has been added about 30% copper to increase the specific gravity to
3.4–3.7. The second layer is the molten electrolyte with a specific gravity of about 2.7–2.8,
and the uppermost layer is the separated high-purity liquid aluminum with a specific
gravity of 2.3. The cell has a siphon chamber for loading the anode metal.
Aluminum produced in this way is 99.99% pure. Higher purities of up to 99.9999% (“sixnines”
aluminum) can be obtained using one or two additional zone-refining operations.
Zone-refining traps impurities in a molten zone that moves gradually from one end to
the other of a specially prepared ingot. Lesser purities in the range 99.97–99.98% are today
produced in limited quantities by fractional crystallization. Here, any impurities that
form an eutectic system with aluminum can be concentrated in the liquid melt, which
can be separated from the primary crystals of aluminum, and the purer aluminum separated
once it has crystallized out.
Another way of producing 99.97–99.98% purity is simply to mix higher-purity metal with
that of a lower purity. The organic electrolysis which was formerly used for making the
highest-purity metal is seldom if ever used because the highly inflammable electrolyte
requires extreme safety measures and no economic advantage over zone-refining.
2.3 The Production of Secondary Aluminum
A used aluminum part, whether extrusion, sheet or plate, forging or casting, or a used
finished product such as a can or a cast wheel, can be efficiently remelted and reconverted
via the appropriate fabrication route—ingot casting, extruding, rolling or die-casting—
into a new usable form. The resulting material loss by surface oxidation, called melt
loss, varies from a few tenths of one percent in the case of clean, uncoated, massive castings
or forgings to as much as 10% for light-gage coated packaging scrap. Melt loss depends
very much on the type of feedstock: its shape and gage and the type and thickness
of lacquer or other form of coating—all of these factors have a major influence on the
amount of metal “lost.” Melt loss also depends very much on the chosen method of melting.
The economics of recycling, together with improved techniques of scrap preparation and
melting provided higher yields and thus led to the further development of the secondary
aluminum industry. This field grew rapidly during the 1950s, with the blossoming of the
nonmilitary uses of aluminum; today, it meets 35% of the total aluminum metal demand