Page 175 - Materials Chemistry, Second Edition

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146 Waste Management Practices: Municipal, Hazardous, and Industrial
In the blast furnace, molten iron is produced. Workable iron ores tend to be rich in iron (III)
oxide (Fe O ). Recovering the iron by removing oxygen from the ore is an early processing step.
3
2
Industrial iron production involves reducing iron (III) oxide in the blast furnace. Most of the iron
(III) oxide is reduced using carbon monoxide as follows:
Fe O 3CO → 2Fe(I) 3CO 2 (6.5)
2
3
Not all the iron (III) oxide is reduced by carbon monoxide, however. A fraction of the iron is
reduced directly using carbon as the oxidizing agent:

Fe O 3C → 2Fe(I) 3CO (6.6)
2
3
In the actual steelmaking process, iron is converted into steel by forcing oxygen through the molten
metal from the furnace. This oxidizes the impurities in the molten metal.
In steelmaking there are three basic types of furnaces: the open hearth, the basic oxygen fur-
nace, and the electric arc furnace. The open-hearth furnace has declined in popularity in recent years
as it is relatively slow in preparing a batch of steel compared with the other furnace types. The pri-
mary feedstock in the basic oxygen furnace is molten pig iron, produced in a blast furnace from iron
ore (hematite and magnetite), limestone, and coke. Molten pig iron can be combined with steel
scrap. The electric arc furnace operates almost exclusively on steel scrap and is discussed below.
A schematic of the steelmaking process is shown in Figure 6.5.

6.10.2 FERROUS RECYCLING
Among all the materials recycled worldwide, iron and steel represent the greatest tonnages. Iron has
been manufactured for thousands of years and scrap was recycled even in the earliest times of pro-
duction. Today, the scrap recycling industry processes an average of 50 million tons of scrap iron
and steel annually (ISRI, 1993b).
Recovery of ferrous metals from appliances (“white goods”) was estimated at 2.3 million tons
of the total ferrous in appliances in 1997. Overall recovery of ferrous metals from durable goods
(large and small appliances, furniture, and tires) was estimated to be 29.3% (3.7 million tons) in
1999 (Table 6.5). Steel food cans and other cans were recovered at a rate of 56.1% (1.5 million
tons). Approximately 170,000 tons of other steel packaging, mostly steel barrels and drums, was
recovered for recycling in 1999 (U.S. EPA, 2001).
Processors buy ferrous scrap from a number of sources including municipalities, demolition oper-
ations, automobile dismantlers, shipyards, and industrial plants (Figure 6.6). Steel cans, also known
as “tin cans” due to the presence of a corrosion-resistant tin coating, are recovered along with other
consumer items at the curbside or an MRF. Cans are often commingled with nonferrous containers
and must be separated magnetically. Afterwards they are compacted and shipped to a de-tinning facil-
ity. In the de-tinning plant, the cans are shredded and the material is again passed through a magnetic
separator to remove aluminum, often from bi-metal cans and other nonferrous metals. The clean steel
is then de-tinned either by heating in a kiln to volatilize the tin, or by reaction with sodium hydrox-
ide and an oxidizing agent. Tin is recovered by electrolysis and formed into ingots (Tchobanoglous
et al., 1993). This process allows for the production of both high-quality tin and steel. The chemically
de-tinned steel is used for the production of new steel. Tin cans de-tinned by heating are not suitable
in steelmaking, however, as the heat causes some of the tin to diffuse into the steel and occur as an
impurity. In some applications impurities from tin will not interfere with the production of new steel,
and the de-tinning process may be skipped altogether (Rhyner et al., 1995).
White goods are large, bulky appliances such as washing machines, refrigerators, freezers, and
stoves (Figure 6.7). The annual discard rate is over 12 million tons (Table 6.5) (U.S. EPA, 2001).
Appliances contain large amounts of ferrous along with copper and aluminum. Before baling or