Page 151 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications
The use of iron and steel as structural materials has been one of the most im-
portant modern technological developments. Primitive ferrous tools first appeared
about 4000 to 3000 B.C. They were made from meteoritic iron, obtained from mete-
orites that had struck the earth. True ironvvorking began in Asia Minor in about
1100 B.C. and signaled the advent of the Iron Age. Invention of the blast furnace in
about 1340 A.D. made possible the production of large quantities of high-quality
iron and steel. (See Table I.2.)

5.2 Production of Iron and Steel

5.2.l Raw Materials
The three basic materials used in iron- and steelmaking are iron ore, limestone, and
coke. Although it does not occur in a free state in nature, iron is one of the most
abundant elements in the World, making up about 5% of the earth’s crust (in the
form of various ores). The principal iron ores are taconite (a black flintlike rock),
/vematite (an iron-oxide mineral), and limonite (an iron oxide containing water).
After it is mined, the ore is crushed into fine particles, the impurities are removed by
various means (such as magnetic separation), and the ore is formed into pellets,
balls, or briquettes using water and various binders. Typically, pellets are about
65% pure iron and about 25 mm in diameter. The concentrated iron ore is referred
to as benejqciated (as are other concentrated ores). Some iron-rich ores are used di-
rectly, without pelletizing.
Coke is obtained from special grades of bituminous coal (a soft coal rich in
volatile hydrocarbons and tars) that are heated in vertical ovens to temperatures of
up to 1150°C and then cooled with Water in quenching towers. Coke has several
functions in steelmaking, including (a) generating the high level of heat required for
the chemical reactions in ironmaking to take place and (b) producing carbon
monoxide (a reducing gas, meaning that it removes oxygen), which is then used to
reduce iron oxide to iron. The chemical by-products of coke are used in the synthe-
sis of plastics and of chemical compounds. The gases evolved during the conversion
of coal to coke are used as fuel for plant operations.
The function of limestone (calcium carbonate) is to remove impurities from
the molten iron. The limestone reacts chemically with impurities, acting like a flux
(meaning to flow as a fluid) that causes the impurities to melt at a low tempera-
ture. The limestone combines with the impurities and forms a slag (Which is light),
floats over the molten metal, and, subsequently, is removed. Dolomite (an ore of
calcium magnesium carbonate) also is used as a flux. The slag is used later in mak-
ing cement, fertilizers, glass, building materials, rock-Wool insulation, and road
ballast.

5.2.2 lronmaking
The three ravv materials described previously are dumped into the top of a blast fur-
nace (Fig. 5.1), an operation called charging the furnace. A blast furnace is basically
a large steel cylinder lined with refractory (heat-resistant) brick; it has the height of
about a 10-storey building. The charge mixture is melted in a reaction at 1650°C,
with the air preheated to about 1100°C and blasted into the furnace (hence the term
“blast furnace”) through nozzles (called tuyeres). Although a number of reactions
may take place, the basic reaction is that of oxygen with carbon to produce carbon
monoxide, vvhich, in turn, reacts with the iron oxide and reduces it to iron.