Primary Uses Iron and its principal alloy, steel, are widely used in tools, machines, and structures.. It is used in three main products: wrought iron, steel, and cast iron.. Even when b
Trang 1duction reached 81.6 million metric tons, a 3.6
per-cent rise, while livestock, poultry, and dairy output
in-creased by 7 percent Livestock production inin-creased
to 10.2 million metric tons during 2007 While
agri-cultural programs aimed at modernization have raised
the production levels, problems remain; these
in-clude poor weather conditions, outdated equipment
and farming techniques, and shortages of viable seed
and water Moreover, the combined burdens of
gov-ernment subsidies and price controls in the food
sector remain burdensome to Iran’s economy
Wheat, Iran’s most important crop, is grown mainly
in the western and northwestern regions of the
coun-try From 1999 to 2004, wheat imports in the Middle
East began to contract, especially in Iran In 2007,
Iran was self-sufficient in wheat production and
be-came a net exporter of wheat for the first time
How-ever, this gain was short-lived after poor weather
con-ditions in the second half of 2008 damaged Iran’s
wheat crop, resulting in the need to import a
mini-mum of 1.8 million metric tons of wheat, increasing
Iran’s budget deficit Acording to the U.N Food and
Agriculture Organization (FAO), Iran had to lower its
wheat production forecasts from (13.6 to 11) metric
tons and significantly reduce its wheat exports In the
past, Kazakhstan was able to meet Iran’s demand for
wheat, but it too had problems with its wheat crop
in 2008, and Iran had to rely on wheat exporters such
as the European Union, Canada, Australia, and the
United States Despite U.S trade sanctions, in early
2009, Iran spent $96 million on imports from the
United States—including wheat, soybeans, and
medi-cal supplies Previously, rice, the major crop cultivated
in the Caspian Sea region, did not meet domestic
needs and resulted in substantial imports In 2008,
Iran imported 19 percent of its foodstuffs and other
consumer goods
Cynthia F Racer
Further Reading
Axworthy, Michael A History of Iran: Empire of the Mind.
New York: Basic Books, 2008
Hyne, Norman J Nontechnical Guide to Petroleum
Geol-ogy, Exploration, Drilling, and Production 2d ed.
Tulsa, Okla.: PennWell Books, 2001
Louër, L Transnational Shia Politics: Religious and
Politi-cal Networks in the Gulf New York: Columbia
Univer-sity Press, 2008
Sagar, Abbuj D “Wealth, Responsibility, and Equity:
Exploring an Allocation Framework for Global
GHG Emissions.” Climatic Change 45, nos 3/4
(June, 2000): 511-527
See also: Agricultural products; Agriculture indus-try; Copper; Iron; Nuclear energy; Oil and natural gas reser voirs; Organization of Arab Petroleum Ex-porting Countries; Steel
Iron
Category: Mineral and other nonliving resources Where Found
Iron is one of the most abundant metals in the world, constituting 35 percent of the entire planet and 5 per-cent of the Earth’s crust It combines with other ele-ments in hundreds of minerals, the most important of which are hematite and magnetite Australia, Brazil, China, India, and Russia have been the top five pro-ducers of iron ore
Primary Uses Iron and its principal alloy, steel, are widely used in tools, machines, and structures Historically, discover-ies and inventions involving the many uses of iron have been crucially important Iron is also essential to biological metabolism
Technical Definition Iron is a chemical element (symbol Fe, from the Latin
ferrum) and a metal of the transition Group VIII on
the periodic table Its atomic number is 26 and its atomic weight 55.487 Iron’s melting point is 1,535° Celsius, its boiling point 3,000° Celsius, and its density 7.86 grams per cubic centimeter
Description, Distribution, and Forms Iron is the cheapest and most widely used metal in the world It is used in three main products: wrought iron, steel, and cast iron Although each is approximately
95 percent iron and is produced with the same fuel, each has vastly different properties, arising from dif-ferent production methods Wrought iron, contain-ing negligible amounts of carbon, has a meltcontain-ing point
so high that it was not achieved by humans until the nineteenth century When hot, wrought iron can be forged and welded, and even when it is cold it is duc-tile—capable of being shaped and hammered Steel
Trang 2contains 0.25 to 1.25 percent carbon, with a lower
melting point than wrought iron It can be forged
when hot and is extremely hard when quenched
(cooled quickly by plunging into water or another
cooling medium) Cast iron, with approximately 2 to
4.5 percent carbon, is easily melted and poured into
molds When cool it is soft and easily machined, but it
is brittle and does not withstand tension forces well
The principal iron ores are hematite, magnetite,
li-monite, pyrite, siderite, and taconite Hematite and
magnetite are the richest and most common ores
They are known as iron oxides because they are
com-pounds of iron and oxygen
Hematite (Fe2O3) can contain as much as 70
per-cent iron but usually contains closer to 25 perper-cent
Significant deposits are found near Lake Superior,
and in Alabama, Australia, Belgium, and Sweden It
may appear in colors ranging from black to dark red
and may occur as shiny crystals, grains of rock, or
loose particles
Magnetite (Fe3O4) is a black magnetic material
of-ten called black sand Limonite (2Fe2O33H2O), or
brown hematite, is a hydrated variety of hematite; it is
also called bog-iron ore It can contain as much as 60
percent iron ore and is yellowish to brown in color It
is found in Australia, France, Germany, the former
So-viet Union, Spain, and the United States
Pyrite (FeS2), also called fool’s gold because of its shiny yellowish surface, is about half sulfur Siderite (FeCO3) is a gray-brown carbonate ore that was once found in large deposits in Great Britain and Germany Taconite is a hard rock that contains specks or bands
of either hematite or magnetite
History Iron was probably discovered accidentally in the late Bronze Age when it was found in the ashes of fires that had been built on top of red iron ore Artifacts of iron weapons and tools have been found in Egypt (includ-ing the Great Pyramid of Giza) dat(includ-ing to 2900 b.c.e Iron has probably been made on a regular basis since at least 1000 b.c.e The Chinese had independently de-veloped their own furnaces and techniques for produc-ing cast iron by the sixth century b.c.e The Romans acquired ironworking technology from the Greeks and spread it throughout northern Europe Because iron ore was readily available throughout the Near East and Europe, iron was less expensive than copper and bronze, the “metals of aristocracy.” As a result, it was used to make many everyday tools and utensils, earning its later nickname, “the democratic metal.” Through the Middle Ages, the common method of producing iron was the bloomery method A bloom-ery may have been as simple as a circular hollow in the ground, several meters deep and several me-ters across The iron ore was heated in a bed of burning charcoal within this hollow, often with the use of bellows to increase the fire’s tempera-ture As the heat reached about 800° Celsius (nor-mally the highest temperature attainable in early bloomeries), the oxygen in the ore separated from the iron and combined with carbon to form slag The iron changed to a pasty mass called the
“bloom.” The operator removed the bloom when
he judged it was ready and alternately hammered and reheated it to remove the slag and to consoli-date the iron The final product was wrought iron, produced at temperatures below iron’s melting point, a process referred to as the “direct” method Sometimes the iron would accidentally melt in the bloomery; this was undesirable, because pro-longed exposure allowed the iron to absorb car-bon from the charcoal, creating cast iron Be-cause of its lack of ductility and low resistance to abrasion, cast iron was unsuitable for working into tools and weapons and was therefore consid-ered worthless
Iron and Steel: World Production, 2008
Metric Tons
South Korea 31,000,000 55,000,000
United Kingdom 11,000,000 14,000,000
United States 36,000,000 94,000,000
Other countries 138,000,000 312,000,000
Source: Data from the U.S Geological Survey, Mineral Commodity
Summaries, 2009 U.S Government Printing Office, 2009.
Trang 3The major limitation of the bloomery was its low
volume of output per unit of labor Even when
bloom-ery technology had fully matured, a large bloom
might weigh only 90 kilograms, and the annual
out-put of that bloomery would probably have been less
than 20 metric tons of wrought iron In an effort to
in-crease output, the blast furnace was developed (by
building up the walls of the bloomery, according to
some sources) This new technology was so successful
that by the middle of the sixteenth century the blast
furnace had replaced the bloomery as the prevalent
method of iron production
Early blast furnaces stood about 4.5 meters high,
later reaching 10 meters or more (The use of coke—
made by heating coal in an airtight container to drive
out gases and tar—as a fuel, beginning in the early
1700’s, allowed taller furnaces, since it did not crush
as easily as charcoal and could be stacked higher.)
The interior cavity widened as it descended from the
top opening for about two-thirds of the furnace’s
height At that point the cavity began to narrow,
cul-minating in a chamber at the very bottom of the
fur-nace, called the crucible
The structure of the furnace created a chimney
ef-fect, drafting air through it to accelerate combustion;
waterwheel-powered bellows usually supplemented
the draft The ore, charcoal, and limestone (a flux)
were dumped into the blast furnace from above As
the ore melted and the level of raw materials dropped,
more would be added on top of them In this way, it
was possible to keep a furnace in continuous
opera-tion for months at a time As the ore slowly worked its
way toward the crucible, it was exposed for a
pro-longed period to heat, which melted it (at about
1,400° Celsius), and carbon, which it absorbed The
molten iron collected in the crucible, and the slag,
floating on top of the iron, was pulled off through side
openings The end product was a large volume of
mol-ten iron with a high carbon conmol-tent—cast iron
The molten iron could be tapped directly from the
crucible Some of it would be poured into oblong
molds pressed into damp sand These molds were
usu-ally laid out with several smaller molds attached at
right angles to the largest mold, reminding the
iron-workers of a sow and suckling pigs—hence the term
“pig iron.” The pig iron would later be converted to
wrought iron at a forge The molten iron might also
have been cast directly into molds for stove and
fire-place parts, pots and pans, cannons, cannon balls,
and many other products In the nineteenth century,
cast iron was also used for machine parts, railroad tracks, and structural elements By that time cast iron had found many uses, and the demand for iron prod-ucts increased dramatically
A blast furnace could produce, typically, 180 metric tons of iron per year—a tenfold increase over the bloomeries In producing a larger output for less la-bor, however, a trade-off was necessary: the addition
of another step in the process To create wrought iron—the most desirable iron product until the late nineteenth century—from the cast iron coming from the blast furnace, the carbon had to be removed This was done in a refinery hearth in which the bloom was heated indirectly without coming in contact with the fuel In this way, the carbon already present burned off, and no additional carbon was absorbed from the fuel Despite this added step, the blast furnace pro-duced a much larger volume of iron, and for less labor, than previous methods had As a result, the de-velopment of the blast furnace was the key to making iron products much more common beginning in the fifteenth century
Even with the blast furnace, the production of good wrought iron was limited by the use of coke Coke introduced more impurities to the cast iron than charcoal had, making it more difficult to pro-duce high-quality wrought iron In 1784, an English-man, Henry Cort, devised a new process to address this problem Known as the “puddling process,” it be-gan by heating the pig iron in a coke-fired rever-beratory furnace (one in which the heat was reflected off the roof of the furnace in order to keep the iron from coming in contact with the coke) Workers stirred the molten metal to expose more of it to the air, thus burning off carbon As the carbon content decreased, the melting temperature increased, and the metal gradually stiffened, separating it from the more liquid slag When the process was complete, workers gath-ered the low-carbon iron in a “puddle ball” and shaped
it in a rolling mill Thanks to Cort’s puddling process, wrought iron became an important factor in the In-dustrial Revolution Its dominance of the iron market lasted until the 1860’s, when steel production began
on a large scale via the Bessemer process
Obtaining Iron Ore
An ore’s quality for commercial purposes depends on several factors While a pure ore may contain as much
as 70 percent iron, ores are seldom found in their pure state It is more realistic to expect a 50 to 60
Trang 4per-cent iron content At less than 30 perper-cent, an ore is
probably uneconomical Other factors in
determin-ing an ore’s quality include the amount of
constitu-ents such as silicon and phosphorus in the ore, the
geographical location of the ore, and the ease with
which it can be extracted and processed
In prehistoric times iron ore was probably gath-ered from meteorites, high-grade outcroppings, and other sources that required little or no work to ex-tract As the demand increased and those sources were exhausted, mining techniques had to be devel-oped to extract iron ore from the Earth
Data from the U.S Geological Survey, U.S Government Printing Office, 2009.
12,000,000 12,000,000
110,000,000 42,000,000 27,000,000 80,000,000 54,000,000 20,000,000 50,000,000
Metric Tons
900,000,000 750,000,000
600,000,000 450,000,000
300,000,000 150,000,000
Venezuela
Sweden
South Africa
Russia
Mexico
Mauritania
Ukraine
United States
Other countries
Note: Estimates for China are for crude ore, other estimates are for usable ore.
330,000,000
390,000,000
35,000,000
770,000,000
200,000,000
32,000,000 26,000,000
India
China
Canada
Brazil
Australia
Iran
Kazakhstan
Iron Ore: World Mine Production, 2008
Trang 5Most iron ore is obtained either by the open-pit
mining process or by hard-rock shaft mining
Open-pit mining is employed when the ore is lying near the
surface Large machinery removes the overlying soil
and rocks (called overburden) to expose the ore It
is then broken up with explosives and loaded onto
a transportation system (usually large earth-moving
trucks) by huge power shovels As the process
contin-ues, the equipment digs deep into the Earth, creating
a large pit often several square kilometers in area and
150 meters or more deep Most of the world’s iron ore
is mined in this way
Ore that lies deep below the surface is removed via
the more traditional hard-rock shaft mining A shaft is
sunk near the deposit from which tunnels and
addi-tional shafts branch out into the deposit Shaft mining
is much more expensive and dangerous than open-pit
mining and is normally used only for very high-grade
ore that cannot be reached in any other way
All ores must be processed before being sent to the
blast furnace; the ore’s quality and iron content
deter-mine the degree and type of processing needed At
a minimum, ore must be crushed, screened, and
washed prior to reducing in a blast furnace
In the screening process, ore is separated into
lumps that are large enough to be put into the blast
furnace (7 to 25 millimeters across) and smaller
par-ticles called fines Fines are not suitable for use in a
blast furnace because the particles will pack together
and hinder the efficient flow of hot gases To
cor-rect this, a process called sintering is used to make
larger particles out of the fines Sintering begins by
moistening the fines to make particles stick together
Coke is then added to the mixture After passing
un-der burners, the coke ignites, heating the fines until
they fuse into larger particles suitable for use in the
blast furnace
As the best ore deposits become exhausted (or
be-come uneconomical to mine because of their
inacces-sibility), methods of upgrading low-quality ore become
necessary Collectively, these processes are known as
beneficiation The first step in beneficiation is to
con-centrate the ore by one of several techniques The
general objective is to concentrate the iron and
re-move the silica Most techniques rely on the
differ-ence between the density of iron and that of the
surrounding rock to separate the two materials Ore
might be leached and dried, pulverized and floated in
a mixture of oil, agglomerated into larger particles, or
separated magnetically Concentrating the ore by these
techniques reduces both the shipping costs and the amount of waste at the blast furnace plant
After beneficiation, the concentrated ore is a very fine powder that would not work properly in a blast furnace Since the concentrate is too small even for sintering, the pelletizing process is used In pelletiz-ing, the concentrate is moistened and tumbled in a drum or on an inclined disk, and the resulting balls of ore are fired to a temperature of about 1,300° Celsius
to dry and harden them These pellets are usually about 10 to 15 millimeters across and are then ready for the blast furnace
Although the exact chemical processes have been fully understood beginning only during the twentieth century, the goal of iron making has always been to re-lease oxygen from its chemical bond with iron The blast furnace is the most efficient and common way to
do this Modern blast furnaces work on the same prin-ciples as those developed in the fifteenth century, but they are larger and have benefited from centuries of re-finement to the design, materials, and process A mod-ern blast furnace may be as much as 30 meters tall and
10 meters in diameter Because of improvements in materials, a blast furnace may stay in continuous oper-ation for two years, requiring maintenance only when its brick lining wears out Some of the most important advances involve the use of mathematical modeling and supercomputers to provide more accurate and timely control over the process The output of a mod-ern furnace may exceed 10 million kilograms per day
A modern blast furnace has five readily identifiable sections; from the top down they are: throat, stack, barrel, bosh, and hearth (or crucible) The ore, coke, and limestone (collectively called the charge) enter the furnace through the throat The distribution and timing of the charge is carefully monitored at all times
to ensure proper operation The throat opens onto the stack, which resembles a cone with the top cut off The stack widens as it descends because the tempera-ture of the charge increases as it works its way down the furnace, causing the charge to expand The next section, the barrel, is a short, straight section that con-nects the stack to the bosh, a shorter, upside-down ver-sion of the stack The bosh narrows as it descends be-cause the iron is beginning to liquefy and compact by the time the charge reaches the bosh At the bottom
of the bosh are nozzles called tuyeres through which blast air is blown into the furnace The air coming through the tuyeres has been preheated to about 1,000° Celsius or higher, and oxygen is sometimes
Trang 6added to it This hot air causes the coke in the charge
to burn The oxygen in the air combines with carbon
from the coke to create carbon monoxide gas, which
in turn removes the oxygen from the ore The
burn-ing coke also produces temperatures up to 3,000°
Cel-sius to melt the iron The liquid metal collects in the
bottom section, called the hearth or crucible Just as
in earlier furnaces, the slag floats on the molten iron,
and workers periodically pull it off through openings
in the side of the furnace
Several direct reduction processes (in which the
temperature never exceeds iron’s melting point) were
developed in the twentieth century but are used only
in special circumstances The basic process relies on
hot gases to reduce the iron ore in a way roughly
anal-ogous to the process of the earlier bloomeries Since
the iron is never completely melted, slag never forms,
and the final product contains impurities that must
be removed during the steelmaking process Direct
reduction furnaces can be built more quickly and
cheaply than blast furnaces, and they produce less
pollution The disadvantages are that they
re-quire a supply of cheap natural gas and the iron
ore must be processed to a very high grade
Uses of Iron Ore
The vast majority of iron produced in blast
fur-naces is converted to steel The remainder is cast
as pig iron and later converted to either cast iron
or wrought iron At a foundry, the pig iron is
melted to a liquid state in a cupola (a small
ver-sion of a blast furnace) and then cast in molds
(some of them are still made with damp sand) to
make machine parts, pipes, engine blocks, and
thousands of other items Wrought iron is now
made in limited quantities Its production begins
by melting pig iron and removing impurities The
molten iron is then poured over a silicate slag and
formed into blooms which can then be shaped
into products
Iron is used in a vast range of special-purpose
alloys developed for commercial applications The
major classifications of these alloys are discussed
below only in broad outline; within each
group-ing there remains an enormous variety because of
the wide range of special needs
Magnetic alloys are either retentive (hard) or
nonretentive (soft) of magnetism The hard
al-loys remain magnetized after the application of a
magnetic field, thus creating a permanent
mag-net One family of hard alloys contains cobalt and mo-lybdenum (less than 20 percent of each), while an-other contains aluminum, nickel, cobalt, copper, and titanium Once magnetized they are used in such ap-plications as speaker magnets, electrical meters, and switchboard instruments because of the constancy of their magnetic field and their resistance to demagne-tization The soft alloys also fall into two families: those with nickel and those with aluminum The nickel alloys are used in communications and electric power equipment, while those containing aluminum are used to carry alternating current
High-temperature alloys, used in high-temperature environments such as turbine blades in gas turbines and superchargers, are generally referred to as either iron-based, cobalt-based, or nickel-based They are formulated to retain their chemical identity, physical identity, and the strength required to perform their intended function, all at extreme, high temperatures The most common electrical-resistance alloys are best known as heating elements in toasters, radiant
Service centers
& distributors 24%
Construction 20%
Transportation 13.5%
Other &
undistributed 40%
Containers 2.5%
Source:
Historical Statistics for Mineral and Material Commodities in the United States
U.S Geological Survey, 2005, iron and steel statistics, in
T D Kelly and G R Matos, comps.,
, U.S Geological Survey Data Series 140 Available online at http://pubs.usgs.gov/ds/2005/140/.
U.S End Uses of Iron and Steel
Trang 7heaters, water heaters, and so on They usually
con-tain nickel (as much as 60 percent), chromium
(ap-proximately 20 percent), and sometimes aluminum
(approximately 5 percent) Alloys without the nickel
have higher resistivity and lower density and are used
in potentiometers, rheostats, and similar applications
Corrosion-resistant alloys are designed to resist
corrosion from liquids and gases other than air or
oxy-gen and usually contain varying amounts of nickel
and chromium along with combinations of
molybde-num, copper, cobalt, tungsten, and silicon No one
al-loy is capable of resisting the effects of all corrosive
agents, so each is tailored to its intended purpose
The powdered iron technique employs iron that
has been finely ground and mixed with metals or
non-metals to form the desired alloys After a binder is
added, the mixture is pressed to the desired shape in a
mold This process has the advantages of precise
con-trol over the makeup of the alloy and the ability to
form iron pieces to precise dimensions with little or
no working required afterward
Iron is important to almost every organism and is
used in a variety of ways It is involved in oxygen
trans-port, electron transfer, oxidation reactions, and
re-duction reactions Iron is a constituent of human
blood Some iron compounds have medical uses,
such as stimulating the appetite, treating anemia,
co-agulating blood, and stimulating healing
Brian J Nichelson
Further Reading
Dennis, W H Foundations of Iron and Steel Metallurgy.
New York: Elsevier, 1967
Gordon, Robert B American Iron, 1607-1900
Balti-more: Johns Hopkins University Press, 1996
Greenwood, N N., and A Earnshaw “Iron,
Ruthe-nium, and Osmium.” In Chemistry of the Elements 2d
ed Boston: Butterworth-Heinemann, 1997
Harris, J R The British Iron Industry, 1700-1850.
Basingstoke, England: Macmillan Education, 1988
Hillstrom, Kevin, and Laurie Collier Hillstrom, eds
Iron and Steel Vol 1 in The Industrial Revolution in
America Santa Barbara, Calif.: ABC-CLIO, 2005.
Krebs, Robert E The History and Use of Our Earth’s
Chemical Elements: A Reference Guide Illustrations by
Rae Déjur 2d ed Westport, Conn.: Greenwood
Press, 2006
Lewis, W David Sloss Furnaces and the Rise of the
Bir-mingham District: An Industrial Epic Tuscaloosa:
Uni-versity of Alabama Press, 1994
Moniz, B J Metallurgy 4th ed Homewood, Ill.:
Ameri-can Technical Publishers, 2007
Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews
http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng.htm
U.S Geological Survey Iron and Steel: Statistics and Information http://minerals.usgs.gov/minerals/pubs/
commodity/iron_&_steel U.S Geological Survey Iron and Steel Scrap: Statistics and Information http://minerals.usgs.gov/minerals/pubs/
commodity/iron_&_steel_scrap U.S Geological Survey
Iron and Steel Slag: Statistics and Information http://minerals.usgs.gov/minerals/pubs/
commodity/iron_&_steel_slag/index.html#mcs U.S Geological Survey
Iron Ore: Statistics and Information http://minerals.usgs.gov/minerals/pubs/
commodity/iron_ore See also: Alloys; Australia; Belgium; Bessemer pro-cess; Brazil; China; Coal; France; Germany; India; In-dustrial Revolution and inIn-dustrialization; Metals and metallurgy; Mineral resource use, early history of; Open-pit mining; Russia; Steel; Steel industry; United States
Irrigation
Category: Obtaining and using resources
Because agriculture is basic to human existence, irri-gation has been practiced since prehistoric times Es-sentially, irrigation is the application of water to soil to overcome soil moisture deficiency so that crops can have adequate water supply for optimal food produc-tion Irrigation is essential to sustained large-scale food production.
Trang 8Irrigation systems were important to many ancient
civilizations They were the basis of life in the ancient
civilizations of Egypt, India, China, and Mesopotamia
(modern day Iraq) Some irrigation works in the Nile
Valley that date back to around 3000 b.c.e still play an
important role in Egyptian agriculture In the United
States, the first irrigation systems were developed by
American Indians, and traces of ancient water
distri-bution systems, made up of canals, were still visible at
the beginning of the twenty-first century
Scope and Land Requirements
In 1977, the Food and Agriculture Organization
(FAO) of the United Nations estimated that the total
global land area under irrigation was 223 million
hectares By 2000, about 270 million hectares were
ir-rigated worldwide In the United States, more than 20
million hectares are irrigated for crop production
Some form of irrigation is practiced in every country
in the world Although irrigation results in increased
food production, it is extremely water intensive For
example, to grow 1 metric ton of grain (adequate for
50 percent of an average person’s supply for five years
and six months) requires as much
as 1,700 cubic kilometers of water per person per year In the United States, 40 percent of total freshwater withdrawals is for irrigation The value of irrigation is that it greatly in-creases agricultural productivity For example, in 1979, the FAO reported that although irrigated agriculture represented only about 13 percent
of global arable land (agricultural land that, when properly prepared for agriculture, will produce enough crops to be economically efficient), the value of crop production from ir-rigated land was 34 percent of the global total production
For irrigation to be economically viable, the land in consideration must
be able to produce enough crops to justify the investment in irrigation works The land must be arable and irrigable; that is, sufficient water for irrigation must exist Soil suitable for irrigation farming has the following attributes The soil must have a rea-sonably high water-holding capacity and be readily penetrable by water; the rate of infiltration (percola-tion) should be low enough to avoid excessive loss of water through deep percolation beyond the root zone
of the crops The soil must also be deep enough to allow root development and permit drainage of the soil, and it must be free of harmful (toxic) salts and chemicals—especially those that tend to bond to soil and reach dangerously high concentrations Finally, it must have an adequate supply of plant nutrients Land slopes should permit irrigation without ex-cessive runoff accompanied by high erosion rates The land should be located in an area where irriga-tion is feasible without excessive pumping or convey-ance costs Generally, the land should permit the planting of more than one type of crop so that the investment in irrigation works can be utilized year-round, and ideally should allow the flexibility of plant-ing more economically viable crop types should eco-nomic conditions dictate such changes
Types of Irrigation Systems Generally, irrigation systems can be classified as non-pressurized systems (also known as gravity or surface
An example of a field irrigation machine (©Edward Homan/Dreamstime.com)
Trang 9systems) and pressurized systems Historically,
non-pressurized systems, in which water was flooded onto
the soil surface via open channels, were the first to be
constructed In fact, nonpressurized systems preceded
pressurized ones by thousands of years
Nonpres-surized systems include canals, open channels, and
pipes that are not flowing full Pressurized systems
in-clude all types of sprinkler systems and low-pressure
nozzle systems
There are five basic methods of implementing
ir-rigation systems: flooding, furrow irir-rigation,
subir-rigation, trickle irsubir-rigation, and sprinkling Several
subcategories exist within these five basic categories
Flooding systems include wild flooding, controlled
flooding, check flooding, and basin flooding
applica-tions In all cases the irrigated area is flooded with
water The degree to which flooding is controlled or
administered differentiates the types of flooding For
example, in wild flooding there is not much control or
preparation of the irrigated land In contrast, check
flooding is accomplished by admitting water into
rela-tively level plots surrounded by levees In check
flood-ing the check (area surrounded by levees) is filled
with water at a fairly rapid rate and the water is allowed
to infiltrate into the soil
Furrow irrigation is used for row crops—hence the
name (a furrow is a narrow ditch between rows of
plants) In this method evaporation losses are
mini-mized and only about 20 to 50 percent of the area is
wetted during irrigation, in contrast to flooding
irri-gation In sprinkler application water is sprinkled on
the irrigated land The sprinkling is possible because
the water is delivered under pressure Sprinkler
sys-tems provide a means for irrigation in areas where the
topography does not permit irrigation by surface
methods
Subirrigation methods are useful in areas where
there is permeable soil in the root zone and a high
water table In this method irrigation water is applied
below the ground surface to keep the water table high
enough so that water from the capillary fringe is
avail-able to crops Subirrigation has the advantages of
minimizing evaporation loss and requiring minimal
field preparation In trickle (or drip) irrigation a
plas-tic pipe with perforations is laid along the ground at
the base of a row of crops The water issuing from the
perforations is designed to trickle Excellent control
is achieved, and evaporation and deep percolation
are minimized
Emmanuel U Nzewi
Further Reading
Albiac, Jose, and Ariel Dinar, eds The Management of Water Quality and Irrigation Technologies Sterling,
Va.: Earthscan, 2009
Cuenca, Richard H Irrigation System Design: An Engi-neering Approach Englewood Cliffs, N.J.: Prentice
Hall, 1989
Heng, L K., P Moutonnet, and M Smith Review of World Water Resources by Country Rome: Food and
Agriculture Organization of the United Nations, 2003
Linsley, Ray K., et al Water Resources Engineering 4th
ed New York: McGraw-Hill, 1992
Morgan, Robert M Water and the Land: A History of American Irrigation Fairfax, Va.: The Irrigation
As-sociation, 1993
Postel, Sandra Pillar of Sand: Can the Irrigation Miracle Last? New York: W W Norton, 1999.
Shortle, James S., and Ronald C Griffin, eds Irrigated Agriculture and the Environment Northampton,
Mass.: Edward Elgar, 2001
Zimmerman, Josef D Irrigation New York: Wiley, 1966.
Web Site U.S Geological Survey Irrigation Water Use http://ga.water.usgs.gov/edu/wuir.html See also: Dams; Hydrology and the hydrologic cycle; Streams and rivers; Water; Water rights; Water supply systems
Isotopes, radioactive
Category: Mineral and other nonliving resources Where Found
All the known elements have at least one radioactive isotope, either natural or artificially produced There-fore, the radionuclides are found in the Earth’s crust,
in its surface waters, and in the atmosphere
Primary Uses Radioisotopes are used in many areas of science and industry as tracers or as radiation sources They pro-vide fuel for the nuclear generation of electricity and have found both diagnostic and therapeutic uses in medicine
Trang 10Technical Definition
Radioactive isotopes are unstable nuclides that decay
ultimately to stable nuclides by emission of alpha,
beta, gamma, or proton radiation, by K capture, or by
nuclear fission
Description, Distribution, and Forms
Alpha, beta, and gamma radiation are the three types
of naturally occurring radioactivity; they result in the
transmutation of one chemical nucleus to another
Al-pha decay is the ejection from the nucleus of a particle
equivalent in size to a helium nucleus The daughter
nucleus has an atomic number (Z) two less than that
of the parent and a mass number (A) four less than
the parent The equation below represents the
emis-sion of an alpha particle from a polonium nucleus to
produce an isotope of lead (a gamma ray is also
emit-ted in rare cases)
210
Po → 206
Pb +4He +γ
Beta decay results from the change within the
nu-cleus of a neutron into a proton Z increases by one,
while A is unchanged The equation below illustrates
beta emission by phosphorus to become sulfur
32
P → β–+32S
In gamma decay, electromagnetic radiation is
emit-ted as a nucleus drops to lower states from exciemit-ted
states It is the nuclear equivalent of atomic line
spec-tra that show wavelengths of visible light emitted by
at-oms when electrons drop from higher to lower energy
levels Nuclear fission is an extremely important
pro-cess by which isotopes of the heavy elements such as
uranium 235 capture a neutron and then split into
fragments
235
U +1n → 140
Ba +94Kr + 2 neutrons
The neutrons produced are captured by other nuclei,
which in turn fission, producing a chain reaction
This is the process that resulted in the first atomic
bomb and is now used in nuclear plants to produce
electric power
History
The story of radioactivity begins with Wilhelm Conrad
Röntgen’s work with cathode-ray tubes Roentgen
al-lowed cathode rays to impinge on various metal
sur-faces and observed that highly penetrating radiations,
which he called X rays, were produced He noted
simi-larities between the X rays and sunlight in that both could expose a photographic plate and could cause certain metals and salts to fluoresce
This fluorescence was of interest to Antoine-Henri Becquerel, who discovered by accident that crystals of uranium salt left on a photographic plate in a drawer produced an intense silhouette of the crystals Al-though his understanding of the phenomenon was limited at the time, what Becquerel had observed was the effect of uranium radioactivity
Marie and Pierre Curie pursued the study of this phenomenon with other minerals They worked to isolate and characterize the substances responsible and were able to isolate and purify samples of polo-nium and radium Other scientists worked at the same time to characterize the radiations emitted In 1903, Ernest Rutherford and Frederick Soddy proposed that the radiations were associated with the chemical changes that radiation produced, and they character-ized three types of radiation: alpha (α), beta (β), and gamma (γ) rays
Obtaining Radioisotopes The use of nuclear fission to produce energy is based
on a principle formulated by Albert Einstein, E = mc2
E is energy, m refers to mass, and c is a constant equal
to 3.0 × 108m/c The complete conversion of one gram
of matter per second would produce energy at the rate of nine trillion watts
The main particles contained in the nucleus of an atom are protons and neutrons The mass of a given nucleus is less than the sum of the masses of the con-stituent protons and neutrons This mass defect has been converted, according to the equation above, to energy (binding energy) in the process of forming the nucleus The separation of the nucleus into its constituent particles would require replacement of this energy The binding energy per nucleon is a mea-sure of the stability of a particular nucleus Those nu-clei having mass numbers between 60 and 80 have the highest binding energy per nucleon and are there-fore the most stable A large nucleus such as uranium can split into fragments with sizes in the 60 to 80 mass range When this happens, the excess binding energy
is released
Uses of Radioisotopes Radioisotopes are used in a number of ways in the fields of chemistry and biology Radioimmunoassay (RIA) is a type of isotopic dilution study in which