Aluminum Recycling, 2nd Edition

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Introduction

The industrial-scale production and use of aluminum metal is less than 150 years

old. Yet in that time, the industry has grown until it is second only to the iron and

steel industry among metal producers. The growth in aluminum usage was particu-

larly rapid in the years following World War II, and every sector of the industry can

point to products that were never produced from aluminum a generation ago but are

now primarily manufactured from an aluminum alloy. Beverage cans, sports equip-

ment, electrical buswork, window frames—all are now produced from aluminum,

along with thousands of other products.

Books on the production of aluminum metal have previously focused on its recov-

ery from naturally occurring raw materials. The principal natural ore for aluminum

is bauxite, a mineral consisting primarily of hydrated aluminum oxides. Aluminum

is recovered from bauxite by a selective leaching sequence known as the Bayer pro-

cess (Damgaard et al., 2009; OECD, 2010), which dissolves most of the aluminum

while leaving impurities behind. The aluminum is recovered from the leach solution

by precipitating it as aluminum hydroxide. The hydroxide is then dried and calcined

to generate puried alumina. The calcined alumina is in turn fed to electrolytic cells

containing a molten salt electrolyte based on cryolite (Na

3

AlF

6

). The alumina dis-

solves in the cryolite and is electrolyzed to generate molten aluminum metal and

carbon dioxide gas. This process has been the sole approach for producing primary

aluminum metal since the late 1800s and will likely continue in this role for decades

to come.

However, in the last 50 years, an increasingly large fraction of the world’s alumi-

num supply has come from a different ore source. This ore is the aluminum scrap

recovered from industrial waste and discarded postconsumer items. The treatment

of this scrap to produce new aluminum metal and alloys is known as recycling, and

metal produced this way is frequently termed secondary. Figure 1.1 shows the frac-

tion of world aluminum production from primary and secondary sources (OECD,

2010); about one-third of the aluminum produced in the world is now obtained from

secondary sources, and in some countries the percentage is much higher. As a result,

the extraction of aluminum by recycling is now a topic worth describing.

The processes used for recycling aluminum scrap are very much different from

those used to produce primary metal but in many ways follow the same general

sequence. This sequence begins with mining the ore, followed by mineral processing

and thermal pretreatment, and then a melting step. The metal is then rened, cast

into ingots or billets or rolled into sheets, and sent to customers. Aluminum recyclers

also face similar challenges to the producers of primary aluminum: the need to pro-

duce a consistent alloy with the required chemistry, minimize energy usage, reduce

the amount of waste generated, and manufacture the highest-quality product at the

lowest possible cost from raw materials of uncertain chemistry and condition.