Aluminum Recycling, 2nd Edition

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9

Fossil-Fuel Furnaces

The reverberatory furnace used to melt aluminum scrap has numerous designs and

sizes, and new designs appear on a regular basis. However, new furnaces are expen-

sive, and so many older designs are still widely used. The goal of furnace design is

consistent, however: to generate the highest melting capacity per unit volume, while

maximizing thermal efciency to reduce fuel costs. Other considerations include the

ability of the furnace to handle nely divided scrap without excessive melt loss, the

ability to process coated scrap if prior decoating is unavailable, minimizing mainte-

nance costs, and ease of charging and tapping.

There are a number of ways to classify furnace designs. This chapter will sepa-

rate them into single-chamber and multiple-chamber types. Additional sections at

the back will describe two specialized types of gas-red furnace: the rotary and

crucible furnace. For more complete description of furnace designs, the reader is

encouraged to read the review by Schmitz (2006).

SINGLE-CHAMBER DESIGNS

(KEARNEY, 1988; SCHAEFER FURNACES, 2008)

The wet-hearth furnace described in the previous edition of this book has become

largely obsolete, for reasons described by Newman (2010). As a result, the most com-

mon type of single-chamber furnace is the dry-hearth furnace shown in Figure 9.1.

This furnace features a sloping hearth on the right onto which solid scrap is placed

for initial heating. As the metal melts, it drains down the hearth into the bath on the

left, leaving other metallic materials behind (Boeckenhauer and Kaczmarczyk, 2008;

Kennedy, 2001). The scrap also dries during the heating process, reducing the pos-

sibility of explosions and also reducing the potential for melt loss from the interaction

between the metal and water vapor. When the molten metal reaches the desired tem-

perature, it is either pumped out or tapped from a hole in the side. Burners are mounted

in the opposite end of the furnace from the charge door; mounting the ue near the

charge door gives hot combustion gases more chance to transfer energy to the melt as

they head toward the ue. Burners can be roof or side mounted; for larger-capacity fur-

naces, regenerative burners or the use of preheated air is advantageous (Valder, 2011).

Without the use of stirring or other melting aids, melt rates of 200 kg/h-m

2

are typical.

Metal removal from dry-hearth and other furnaces is accomplished one of three

ways. The use of a hydraulic pump is the oldest (Brooks, 1970) and is used mostly

for furnaces that are not integrated into a melting/holding/casting line. However,

the pumps require maintenance and periodic replacement, and safety is a concern.

Electromagnetic pumps were described in the previous chapter as a means of stirring

the melt; these can also be used to transfer molten metal from the furnace and are

increasingly used for that purpose (EMP Technologies, 2007; Starczewski, 2012).