Page 177 - Materials Chemistry, Second Edition

164 3 Metals

not sufficient to completely melt the iron, so a spongy mass consisting of iron and
silicates are formed. Through heat/hammering treatments, the silicates mix into the
iron lattice, creating wrought iron. This form of iron was used exclusively by early
blacksmiths, since the heating of wrought iron yields a malleable, bendable, and
extremely easy compound to work with.
Most modern applications for metallic iron are steel related, exploiting its high
hardness, ductility and tensile strength. Figure 3.3 shows a flowchart for the various

procedures that are used for modern steelmaking. The first step uses a blast fur-
[3]
nace that is comprised of a massive, refractory-lined steel column wherein
pelletized iron ore, charcoal, and calcium sources (from limestone and dolomite)

are poured into the top, and a large jet of pre-heated (ca. 1,050 C) air is blown in
from the bottom. As mixing of the components occurs at various temperature
regimes, the various oxides present in the ore are reduced to metallic iron. From
the coolest – hottest portions of the blast oven, corresponding to the highest – lowest
regions, respectively, the following oxides are reduced (Figure 3.4):

1. 500–600 C: Hematite (Fe 2 O 3 )

2. 600–900 C: Magnetite (Fe 3 O 4 )

3. 900–1,100 C: Wustite (FeO)

4. >1,100 C : FeO 0.5
Since iron ore is largely comprised of aluminosilicate minerals, a byproduct is also
formed within the blast furnace, known as slag (ca. 30–40 wt.% SiO 2 , 5–10 wt.%
Al 2 O 3 , 35–45 wt.% CaO, 5–15 wt.% MgO, and 5–10 wt.% CaS).
It should be noted that it takes 6–8 h for the native iron ore to descend toward the
bottom of the blast furnace, but only ca. 8 s for the pre-heated air to reach the top of
the furnace. Oftentimes, a fused solid known as sinter is also added to the blast
furnace, which is comprised of fine particulates of iron ore, coke, limestone and
other steel plant waste materials that contain iron. The reducing agent within the
blast furnace (coke) is comprised of 90–93% carbon, and is formed by heating coal
to remove the volatile components such as oil and tar. The coke is ignited at the
bottom of the blast furnace immediately upon contact with the air blast; since there is
excess carbon in the furnace, the active reducing species is CO rather than CO 2 .
The molten iron collects at the bottom of the blast furnace and is cooled into a
form known as pig iron. This form of iron is comprised of ca. 93.5–95% Fe, 0.3–
0.9 T Si, 0.025–0.05% S, 0.55–0.75% Mn, 0.03–0.09% P, 0.02–0.06% Ti, and 4.1–
5% C. Due to the relatively high carbon concentration, this form of iron is too brittle
and hard for any structural applications. It should be noted that re-heating pig iron
with slag and hammering to remove most of the carbon will yield wrought iron.
However, it is often most useful to convert this form into high-strength steel.
While impurities such as silicon, calcium, aluminum, and magnesium were largely
removed in the slag, phosphorus and sulfur impurities remain behind, and are
dissolved into the molten iron.
The next step involved in steelmaking is reducing the carbon concentration of the
pig iron. The molten iron formed in the blast furnace is transported to the basic
oxygen furnace (BOF) via transfer ladles (Figure 3.5a). Lime (CaO) is added