Aluminum Recycling, 2nd Edition

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5

Beneficiation Technology

After mining, the next step in traditional metal extraction is minerals processing,

in which the valuable minerals are liberated from the gangue material in the ore.

Following this, the minerals are sorted into a separate stream, using differences

between their physical properties and those of the gangue. In some cases, chemical

or thermal processes may be used to remove impurities from the concentrated min-

erals. As a nal step, the concentrated minerals may be agglomerated to make them

easier to ship and easier to handle in subsequent smelting processes.

In Chapter 3, the mining (collecting) of scrap aluminum was described. Over

the next two chapters, the equivalent mineral processing (beneciation or upgrad-

ing) of this ore will be introduced. In this chapter, the techniques and equipment

used to separate scrap aluminum from contaminants will be introduced. This will

be followed by a description in Chapter 6 of how this technology is put into practice.

As with the processing of traditional minerals, the beneciation of scrap can be

divided into four types of unit process. These include comminution, sorting, thermal

processing, and agglomeration.

COMMINUTION (NIJKERK AND DALMIJN, 1998)

Aluminum scrap comes in a wide variety of size and conditions. Pieces can be as small

as the aluminum wires used in electronic equipment or as large as a jet airliner. Scrap

is sometimes liberated, free of any attached pieces of other materials. However, it is

often found bolted, welded, or otherwise attached to other parts. To be remelted, the

scrap will have to be separated from these other materials. The dismantling of auto-

mobiles and other large assemblies described in Chapter 3 accomplishes this to some

degree. However, in many cases, the value of the scrap recovered by dismantling is

insufcient to justify the expense of this labor-intensive process.

Many sorting processes require scrap pieces of a given size to be effective.

In addition, furnaces have maximum sizes of scrap that can be successfully fed.

Consistent particle size is also essential to the proper functioning of some sorting

devices. Some scrap already has the proper size for automated sorting, but most of

it does not. Comminution is the process by which oversize material is reduced to the

proper size for further processing. The comminution of naturally occurring miner-

als is usually done by crushing it into smaller rocks. However, metal scrap cannot

be turned into smaller separate pieces in this fashion. Instead, it must be torn apart.

Numerous devices have been invented for tearing apart metal scrap. Here, they will

be separated into three categories—shears, impact shredders, and rotary shredders.

Most of the shears in operation are either alligator or guillotine shears. Figure 5.1

shows a typical alligator shear (Fowler, 2003), so named because the hinged cut-

ting motion is similar to that of an alligator. Only the top portion of the jaw moves

68 Aluminum Recycling

during operation. The cutting blade is made of hardened tool steel and can be

resharpened when it gets dull. Sizes and power ratings vary, according to the size and

type of material to be cut. Most alligator shears use electric power, although gasoline

and diesel engines are available.

Alligator shears have been available since the early 1900s. Major changes in the

basic design include the following:

• The use of hydraulic rather than mechanical ywheel drives. Hydraulic

drives allow the machine to be stopped immediately to prevent damage to

the machine or injury to the operator, a feature not available in ywheel-

driven shears.

• The addition of a hold-down clamp to keep scrap pieces still while being

cut.

• The addition of guards around the machine to prevent injury to the opera-

tors. Alligator shears were notorious safety hazards before the introduction

of these guards, which are now mandatory in most countries.

One of the advantages of the alligator shear is its versatility (A-Ward, 2005). This is

particularly important in the processing of aluminum scrap, which takes a variety of

forms. Figure 5.2 shows a crane-mounted shear used for recovering scrap that cannot

be brought to a processing plant. Crane-mounted shears remove aluminum and other

scrap metals from aircrafts, ships, rail cars, and other large objects. Another special

type of alligator shear is the nibbling or metal cleaning shear used for detaching

small metal attachments such as bolts or collars. Metal-cleaning shears are espe-

cially popular for separating steel parts from cable scrap (American Recycler, 2003).

FIGURE 5.1 An alligator shear. (From Ohio Baler Co., http://www.ohiobaler.com/

alligatorshear.htm. With permission.)