Aluminum Recycling, 2nd Edition

199

12

Metal Refining and

Purification

As Chapter 4 pointed out, aluminum alloys produced by remelting scrap are less

valuable than alloys with the same composition produced from primary metal. The

reason for this is concern over the purity of the recycled metal alloy, which is often

inferior to that of primary. The development of rening technology for molten alu-

minum is designed to eliminate this deciency, which will allow recycled metal to

compete with the primary in more applications. Much of the technology used for

rening molten aluminum has been introduced only in the last 30 years, and what

was once a minor footnote in an aluminum production owsheet is now a major

consideration.

The choice of rening strategy and technology used by scrap remelters depends

on several factors—the type of scrap being remelted, the type of furnace used, the

type of product being generated, and, most importantly, the needs of customers.

As a result, there is no universal rening technology. A complicating factor is the

blending of scrap with primary metal in melting furnaces, which also combines the

rening concerns of the two types of metal. Because of this, much of the technology

used for rening remelted aluminum scrap is also used to purify molten primary

metal. Since the basic principles are the same, a discussion of rening technology

for secondary aluminum is essentially a discussion of molten aluminum rening in

general.

The following discussion is a brief introduction to the theory and equipment used

for molten aluminum rening. Enough in-depth information exists on this technol-

ogy alone to write an entire book about it. Those wishing to learn more about alumi-

num rening are encouraged to consult the references at the end of this chapter, in

particular the review by Zhang et al. (2011).

COMMON IMPURITIES IN MOLTEN ALUMINUM

Table 12.1 lists the most common impurities in molten aluminum (Waite, 2002) and

compares their concentration in primary and secondary metal. The impurities can be

divided into three classes—hydrogen, reactive metals (including magnesium), and

inclusions.

Hydrogen: As previously discussed, dissolved hydrogen in molten aluminum is

obtained from a reaction between water vapor and the molten aluminum (Foseco,

2011; Fruehan and Anyalebechi, 2008):

3H

2

O + 2Al = Al

2

O

3

+ 6H (12.1)

200 Aluminum Recycling

The magnesium in remelted alloy scrap also reacts with water vapor:

H

2

O + Mg = MgO + 2H (12.2)

Both reactions are highly favored thermodynamically and limited only by the forma-

tion of an oxide skin on the melt surface, which prevents contact between the water

vapor and the molten metal. Factors that encourage these reactions and increase the

dissolved hydrogen content of the metal include the following:

• A higher vapor pressure of water vapor in the atmosphere (i.e., higher

humidity), which drives the reactions to the right (Foseco, 2011)

• Metal turbulence, which destroys the oxide skin and allows the reactions

to continue

• Wet or damp charge materials, which make water vapor directly available

to the melt

In addition to natural humidity, water vapor in the products of combustion (POC)

from fossil-fuel red furnaces is a source of hydrogen (Enright, 2007; Foseco, 2011).

As a result, extended exposure to POC in transfer ladles can raise dissolved hydro-

gen content still further. Some alloying elements (Cu, Fe, Si, Zn) raise the activity

coefcient of dissolved hydrogen in molten aluminum, decreasing its solubility

(Fruehan and Anyalebechi, 2008); others (Li, Mg, Zr) lower the activity coefcient

and raise the solubility. Sigworth et al. (2008) have shown that 2000-series alloys

(see Chapter 2) have a hydrogen solubility up to 30% lower than that of pure alumi-

num, and 5000 series can dissolve hydrogen at levels 50% or higher than the pure

metal. As Table 12.1 shows, the hydrogen content of remelted scrap is usually higher

than that of primary metal. This is caused mostly by water in the scrap and by the

use of fossil fuels for remelting it (Pyrotek, 2006).

TABLE 12.1

Common Impurities in Primary and Secondary Molten

Aluminum

Impurity

Concentration in

Primary Metal

Concentration in

Secondary Metal

Hydrogen 0.1–0.3 wppm 0.4–0.6 wppm

Inclusions (PoDFA scale) >1 mm

2

/kg (Al

4

C

3

) 0.5 < mm

2

/kg < 5.0 (Al

2

O

3

,

MgO, MgAl

2

O

4

, Al

4

C

3

, TiB

2

)

Alkali Sodium 30–150 ppm < 10 ppm

Calcium 2–5 ppm 5–40 ppm

Lithium 0–20 ppm < 1 ppm

Source: Waite, P., Technical perspective on molten aluminum processing, in Light

Metals 2002, Schneider, W., Ed., TMS-AIME, Warrendale, PA, 2002.