Ozone layer and ozone hole debate Categories: Ecological resources; environment, conservation, and resource management; social, economic, and political issues Ozone, a form of the elemen
Trang 1According to their chemical compositions, there
are seven types of oxides: A2O, AO, A2O3, ABO3,
AB2O4, AO2, and AmBnO2(m+n), where A and B are
met-als and m and n are integers Cuprite (CuO2), an
im-portant ore of copper, belongs to type A2O Both
periclase (MgO) and tenorite (CuO) belong to type
AO Corundum (Al2O3), hematite, and ilmenite
be-long to type A2O3 Corundum can be utilized as an
abrasive and a gemstone Hematite is an important
ore of iron Ilmenite is an ore of titanium Perovskite
(CaTiO3) belongs to type ABO3 Spinel, chrysoberyl,
and gahnite (ZnAl2O4) belong to type AB2O4
Pyrolusite, cassiterite, and rutile (TiO2) belong to
type AO2 Pyrolusite is an ore of manganese
Cassiter-ite is an important source of tin Rutile is an ore of
tita-nium Columbite-tantalite (4[(Fe,Mn)(Nb,Ta)2O6])
belongs to type AmBnO2(m+n)
Besides chemical compositions, the structural,
op-tical, and physical properties of oxides are studied
Structural information can be revealed by X-ray
dif-fraction Optical properties include color appearance,
reflection, and transmission Physical properties
in-clude density, mechanical strength, and thermal
ca-pacitance
Some of the oxides are distributed throughout the
world, while others are limited to a few regions For
example, magnetite can be found in the United
States; hematite can be found in the United States,
Venezuela, Brazil, Canada, and Australia; and
cassiter-ite can be found in Malaysia, Bolivia, and other
coun-tries
Xingwu Wang
See also: Aluminum; Beryllium; Copper; Igneous
processes, rocks, and mineral deposits; Iron;
Manga-nese; Oxygen; Pegmatites; Quartz; Sand and gravel;
Silicon; Tin; Titanium
Oxygen
Category: Mineral and other nonliving resources
Where Found
Oxygen is the most abundant element in the Earth’s
crust (46.6 percent by weight), occurring mainly as
oxides and silicates of metals The earth’s waters are
85.8 percent oxygen by weight, and the atmosphere is
23.0 percent oxygen The combined weight of oxygen
in the crust, hydrosphere, and atmosphere is about 50 percent
Primary Uses
In addition to its importance in the combustion of food for energy by living organisms, oxygen has many commercial applications It is used in the iron and steel industry, in rocket propulsion, in chemical synthesis, and to hasten the aerobic digestion of sewage solids Technical Definition
Oxygen (abbreviated O), atomic number 8, belongs
to Group VI of the periodic table of the elements Its chemical properties are somewhat similar to those of sulfur It has an average molecular weight of 15.9994 and six naturally occurring isotopes, three of which are radioactive with half-lives on the order of seconds and minutes At ordinary temperatures, oxygen is a colorless, odorless gas Its liquid form is pale blue Ox-ygen melts at−218° Celsius and boils at −183° Celsius Oxygen can form compounds with all other elements except the low-atomic-weight elements of the helium family
Description, Distribution, and Forms The total content of oxygen in the Earth’s air, crust, and oceans is approximately 50 percent by weight In chemically combined form, it is found in water and in the clays and minerals of the lithosphere Despite the fact that it is an active element, forming oxides easily
by the process of combustion, elemental oxygen makes
up about 23 percent of the atmosphere Dissolved gas-eous oxygen is found in the waters of the Earth, where
it provides for the respiration of most marine animals and for the gradual oxidation of waste materials in lakes and rivers
Elemental oxygen is found in three allotropic forms: the ordinary diatomic molecule found in the atmo-sphere (O2), ozone (O3), and the unstable, nonmag-netic, and rare pale blue O4form, which decomposes easily to O2 Unstable atomic oxygen is a short-lived species that results from the absorption of ultraviolet radiation by ozone in the upper atmosphere or from electrical discharges
The solvent properties of water are attributable to the great difference in the strength of attraction for the bonding electrons between hydrogen and oxy-gen, which makes the resulting molecule very polar The H2O molecules are attracted to both cations and anions, surrounding them by the attraction of the
Trang 2negative oxygen or the positive hydrogen,
respec-tively Water also dissociates slightly into H+ and OH−
ions These processes allow water to form hydrates
with, and to react with, many compounds
History
Most chemists agree that the discovery of oxygen was
made independently by Carl Scheele in Sweden and
Joseph Priestley in England at about the same time In
1774, Priestley heated mercuric oxide and collected
the liberated gas over water He showed that the
“dephlogisticated air” (oxygen) was capable of
sup-porting burning and was respirable Scheele prepared
oxygen in 1771-1772 by heating various carbonates
and oxides Although his experiments were performed
earlier than those of Priestley, the latter published
his results first The great French chemist
Antoine-Laurent Lavoisier was the first to recognize that oxygen
is an element, and he was able to explain the
combus-tion process correctly This explanacombus-tion revolucombus-tion-
revolution-ized the field of chemistry and provided the stimulus
for the discovery of many new elements
Obtaining Oxygen
For many years the only means of obtaining oxygen
was by the fractional distillation of liquid air A
varia-tion of this basic process is still used when high-purity
oxygen is needed In 1971, an ambient temperature
process was introduced by the Linde Division of Union
Carbide Corporation The process uses a pressure
cy-cle in which “molecular sieves” are used to selectively
absorb nitrogen from the air The resulting product
contains about 95 percent oxygen and about 5
per-cent argon and is economically preferable in
situa-tions where the argon will not interfere
Uses of Oxygen
The greatest consumers of oxygen are the steel,
chem-ical, and missile industries The oldest use of oxygen
is in the welding of steel by means of a hot
acetylene-oxygen torch Thicknesses of steel of up to 0.6 meter
can be cut by a high-pressure oxygen stream after
heating with an acetylene torch An oxygen stream
passed through molten iron can remove carbon
im-purities by means of combustion to carbon dioxide
In the chemical industry, oxygen is used for the
production of hydrogen from natural gas or
“synthe-sis gas”:
CH4+ 0.5 O2→ CO + H2
Other important industrial processes are the manu-facture of hydrogen peroxide, sodium peroxide, eth-ylene oxide, and aceteth-ylene
Large rockets are propelled from their launch pads
by the combustion of a fuel similar to kerosene The fuel and oxygen are kept in liquid form in separate tanks until ignition (In some rockets the second stage
is propelled by the combustion of hydrogen.) Oxygen has limited but important uses in the health-care industry in the treatment of pneumonia, emphysema, and some heart problems Hyperbaric chambers provide high-pressure, oxygen-rich atmo-spheres for the treatment of both carbon monoxide poisoning and decompression sickness (“the bends”)
Grace A Banks
Further Reading
Ardon, Michael Oxygen: Elementary Forms and Hydro-gen Peroxide New York: W A Benjamin, 1965 Gilbert, Daniel L., ed Oxygen and Living Processes: An Interdisciplinary Approach New York: Springer, 1981.
Greenwood, N N., and A Earnshaw “Oxygen.” In
Chemistry of the Elements 2d ed Boston:
Butterworth-Heinemann, 1997
Hayaishi, O., ed Molecular Oxygen in Biology: Topics
in Molecular Oxygen Research New York: American
Elsevier, 1974
Jackson, Joe A World on Fire: A Heretic, an Aristocrat, and the Race to Discover Oxygen New York: Viking,
2005
Lane, Nick Oxygen: The Molecule That Made the World.
New York: Oxford University Press, 2002
Lewis, Bernard, and Guenther von Elbe Combustion, Flames, and Explosions of Gases 3d ed Orlando, Fla.:
Academic Press, 1987
Massey, A G “Group 16: The Chalcogens—Oxygen, Sulfur, Selenium, Tellurium, and Polonium.” In
Main Group Chemistry 2d ed New York: Wiley, 2000 Scott, Gerald Atmospheric Oxidation and Antioxidants.
New York: Elsevier, 1965
Weeks, Mary Elvira Discovery of the Elements 7th ed.
New material added by Henry M Leicester Easton, Pa.: Journal of Chemical Education, 1968
Web Site Universal Industrial Gases, Inc
Oxygen (O2) Properties, Uses and Applications: Oxygen Gas and Liquid Oxygen
http://www.uigi.com/oxygen.html
Trang 3See also: Atmosphere; Fuel cells; Minerals, structure
and physical properties of; Oxides; Ozone layer and
ozone hole debate; Water
Ozone layer and ozone hole debate
Categories: Ecological resources; environment,
conservation, and resource management; social,
economic, and political issues
Ozone, a form of the element oxygen, forms naturally
in the stratosphere and provides the Earth with a filter
from ultraviolet radiation Some human activities
cause a decrease in the amount of ozone present, an
ef-fect that has been described as a hole in (more correctly
a “thinning” of ) the ozone layer.
Background
Ozone is a highly reactive form of oxygen It is
com-posed of three oxygen atoms in a molecule (O3)
rather than the more usual two atoms (O2) Ozone
is formed from diatomic oxygen where high energy
is present Near the Earth, ozone forms
in high-temperature combustion processes,
such as in automobile engines and in
elec-trical sparks In the stratosphere it forms
be-cause of high-energy ultraviolet radiation
Once formed, ozone is quick to react with
other molecules Near the Earth there are
many molecules with which to react, and
the ozone concentration remains low In
the stratosphere there are few molecules
present, so the ozone concentration builds
up and forms what is termed the ozone
layer Ozone also disappears naturally by
de-composing to ordinary oxygen, so there is
a natural limit to the concentration that
ac-cumulates, and a steady state occurs The
ozone layer is actually quite diffuse, and the
ozone concentration is never very high
Description, Distribution, and
Concentrations
Since the mid-1950’s, measurements of
ozone concentrations in the atmosphere
have been made regularly In the early
1970’s, analysis of the measurements
sug-gested that something was causing a
reduc-tion in the concentrareduc-tion of ozone in the strato-sphere, particularly in the region over the South Pole Continued measurements confirmed a similar lower-ing over the North Pole area and a spreadlower-ing of the ef-fect over a larger area Laboratory experiments show that molecular fragments containing unpaired elec-trons are effective in speeding the decomposition of ozone This catalytic effect is particularly strong in the presence of small ice crystals, which are present in the stratosphere in the polar regions in winter
Chlorofluorocarbons Chlorofluorocarbons (CFCs) are a class of chemicals that have found wide use as propellants in aerosol cans, cleaning solvents for electronic circuit boards, and working fluids in air-conditioning and refrigera-tion The stability of these molecules is a prime factor
in their utility, but this property also allows the mole-cules to drift into the stratosphere when they are re-leased Most other escaping molecules react or are washed out by precipitation before they gain much height in the atmosphere In the stratosphere, CFCs decompose by irradiation and form molecular frag-ments to which ozone is sensitive CFCs are not the
The Antarctic hole in the ozone layer from 2000 data provided by the Total Ozone Mapping Spectrometer earth probe (UPI/Landov)
Trang 4only artificial cause of ozone depletion, but they have
been recognized as a major contributor Much of what
is known about the way the ozone layer forms and
de-composes comes from the work of Paul J Crutzen,
Mario J Molina, and F Sherwood Rowland, who
re-ceived the 1995 Nobel Prize in Chemistry for their
work on this subject
The Importance of Ozone
Ozone is decomposed when the energy available in
part of the ultraviolet region of the spectrum is
ab-sorbed by the molecule When the energy is used in
such a fashion, it is no longer present in the sunlight
that comes through the stratosphere to the Earth
This type of energy, if it does make it to the Earth, is
capable of causing the reaction of other molecules,
including those of biological importance The
evi-dence is overwhelming that the primary cause of
nonmelanoma skin cancers is chronic long-term
ex-posure to ultraviolet light Australia has the highest
incidence of skin cancer in the world Other human
interactions may lead to melanoma skin cancers and
cataracts Increased ultraviolet levels also cause cellular
modifications in plants, including food crops, which
may lead to their death Of particular concern is the
in-hibition of photosynthesis in the phytoplankton that
forms the base of the ocean food chain The ozone
layer acts as a filter to limit the Earth’s exposure to
high-energy light With a diminishing level of filtering,
one would expect that there would be a global increase
in the effects of overexposure to ultraviolet radiation
The Ozone Debate
Some scientists contend that ozone depletion is a part
of a natural cycle related to sunspot activity
Knowl-edge of what has happened in the distant past is
cir-cumstantial and not easy to interpret, but most
scien-tists agree that human activities play a significant role
in the current decrease in the ozone layer In terms
of the human contribution, CFCs have received the
major attention, and their production was severely
limited by international agreement in the 1987
Mon-treal Protocol and later revisions CFCs are no longer
used for propellants, and their role as cleaners is
all but over However, their use as refrigerant fluids
continues while economically viable, safe substitutes
are being sought People in developed countries have
become extremely dependent on air-conditioning
(nearly all large buildings are designed to be
air-conditioned rather than open to the outside) The
search for substitutes has proved difficult, with eco-nomic, safety, and environmental concerns all plac-ing limits on what is acceptable
Part of the controversy concerning banning CFCs
is based on ethical considerations Developed coun-tries utilized CFCs to gain their positions; should they then prohibit the use of CFCs in developing coun-tries? Should these countries not be allowed to reap the same advantages as others even if there is an envi-ronmental price to be paid? There are no easy, satis-factory answers to such questions
International Day for the Preservation of the Ozone Layer
In 1985, the Vienna Convention was signed by twenty-two countries Two years later, the Montreal Protocol was signed on September 16, a day which has been designated by the United Nations as International Day for the Preservation of the Ozone Layer The theme for the day in 2008 was “Montreal Protocol: Global Partnership for Global Benefits.” On Interna-tional Day 2008, the World Meteorological Organiza-tion (WMO) released several statements on ozone and ozone-related matters, including the following by Ban Ki-moon, the secretary general of the United Na-tions
After decades of chemical attack, it may take an-other fifty years or so for the ozone layer to recover fully As the Montreal Protocol has taught us, when we degrade our environment too far, nursing it back to health tends to be a long journey, not a quick fix
According to WMO, the 2008 Antarctic ozone hole was larger than the one of 2007 The observed changes
in the stratosphere could delay the expected recovery
of the ozone layer It is therefore vital that all member states with stratospheric measurement programs con-tinue to support and enhance these measurements Routine ozone measurements in all parts of the world, using surface-based spectrophotometers, balloon-borne sensors, aircraft, and satellites, have been made by the National Meteorological and Hy-drological Services of WMO members and partners worldwide since the 1950’s In the 1980’s, compre-hensive measurements started under coordination of the WMO Global Atmosphere Watch (GAW) These
measurements have been critical to the series of Scien-tific Assessments of Ozone Depletion published since the
mid-1980’s by WMO and the Ozone Secretariat of
Trang 5the United Nations Environment Programme,
docu-menting progress made under the Vienna
Conven-tion for the ProtecConven-tion of the Ozone Layer (signed in
1985 by twenty-two countries) The most recent of
these assessments came out in the spring of 2007 The
work on the following ozone science assessment
be-gan in the middle of 2009
The Montreal Protocol on Substances That
De-plete the Ozone Layer underpins efforts to combat
depletion of the Earth’s fragile protective shield It
also contributes to combating climate change,
be-cause many of the chemicals controlled under the
treaty also contribute to global warming By phasing
out CFCs and deciding to accelerate a freeze and
phase-out of hydrochlorofluorocarbons (HCFCs), the
treaty has provided two benefits at once The U.N
secretary-general expressed the hope that
“Govern-ments will look at such results and feel empowered to
act across a wide range of environmental challenges,
and not only in prosperous times.”
In August, 2008, WMO released its first of the 2008
series biweekly Antarctic Ozone Bulletin on the current
state of stratospheric ozone in the Antarctic These
bulletins use provisional data from the WMO/GAW
stations operated within or near the Antarctic, where
the most regular and dramatic decreases in ozone occur
According to the 2008 bulletin, the vortex was
more circular than at the same time in 2007 The
me-teorological conditions observed indicate that the
2008 ozone hole was smaller than that of 2006 but
larger than that of 2007
The Antarctic ozone hole reached its maximum
in-tensity in late September/early October In 2008, the
ozone hole appeared relatively late On September
13, 2008, the ozone hole covered an area of 27 million
square kilometers The maximum area reached in
2007 was 25 million square kilometers WMO and the
scientific community continue to make ozone
obser-vations from the ground, from balloons, and from
sat-ellites, together with meteorological data, to keep a
close eye on the ozone development and depletion
Ozone Depletion and Climate Change
Many scientists are increasingly aware of the possible
links between ozone depletion and climate change
According to many studies, increased atmospheric
concentrations of greenhouse gases (GHGs) may lead
to warmer temperatures in the troposphere and at the
Earth’s surface However, in the stratosphere, at
alti-tudes where we find the ozone layer, there will be a
cooling effect A cooling of the stratosphere in winter over the latter decades of the twentieth century and the first decade of the twenty-first century has indeed been observed, both in the Arctic and in the Antarc-tic Lower temperatures enhance the chemical reac-tions that destroy ozone At the same time, the amount
of water vapor in the stratosphere has increased at the rate of about 1 percent per year A wetter and colder stratosphere means more polar stratospheric clouds, which may lead to more severe ozone loss in both po-lar regions
Together with the International Council for Sci-ence (ICSU), WMO coordinated the International Polar Year 2007-2008 Thousands of scientists collabo-rated to increase understanding of processes that take place in polar regions, including those of strato-spheric ozone and ultraviolet radiation In February,
2009, WMO and ICSU celebrated the closure of the International Polar Year in Geneva and released WMO’s State of Polar Research
Kenneth H Brown, updated by W J Maunder
Further Reading
Andersen, Stephen O., and K Madhava Sarma Pro-tecting the Ozone Layer: The United Nations History.
Edited by Lani Sinclair Sterling, Va.: Earthscan, 2002
Asimov, Issac What’s Happening to the Ozone Layer
Mil-waukee, Wis.: Gareth Stevens, 1993
Booth, Nicholas How Soon Is Now? The Truth About the Ozone Hole New York: Simon & Schuster, 1994 Christie, Maureen Ozone Layer: A Philosophy of Science Perspective New York: Cambridge University Press,
2001
Dessler, Andrew The Chemistry and Physics of Strato-spheric Ozone New York: Academic Press, 2000 McElroy, Michael B The Atmospheric Environment: Ef-fects of Human Activity Princeton, N.J.: Princeton
University Press, 2002
Parker, Larry, and Wayne A Morrissey Stratospheric Ozone Depletion New York: Novinka Books, 2003 Parson, Edward A Protecting the Ozone Layer: Science and Strategy New York: Oxford University Press, 2003 Reid, Stephen J Ozone and Climate Change: A Beginner’s Guide Amsterdam: Gordon and Breach, 2000 Roan, Sharon Ozone Crisis: The Fifteen-Year Evolution of
a Sudden Global Emergency New York: Wiley, 1989 Somerville, Richard C J “The Ozone Hole.” In The For-giving Air: Understanding Environmental Change 2d
ed Boston: American Meteorological Society, 2008
Trang 6Zerefos, Christos, Georgios Contopoulos, and
Greg-ory Skalkeas, eds Twenty Years of Ozone Decline:
Pro-ceedings of the Symposium for the Twentieth Anniversary
of the Montreal Protocol New York: Springer, 2009.
Web Sites
National Oceanic and Atmospheric
Administration
The Ozone Layer
http://www.oar.noaa.gov/climate/
t_ozonelayer.html
U.S Environmental Protection Agency Ozone Layer Depletion
http://www.epa.gov/ozone/strathome.html See also: Aerial photography; Agenda 21; Air pollu-tion and air pollupollu-tion control; Antarctic treaties; At-mosphere; Biosphere; Clean Air Act; Climate Change and Sustainable Energy Act; Earth Summit; Gore, Al; Greenhouse gases and global climate change; Indus-trial Revolution and indusIndus-trialization; Kyoto Proto-col; Landsat satellites and satellite technologies; Mon-treal Protocol; Oxygen; United Nations Framework Convention on Climate Change
Trang 7Paper
Category: Products from resources
The pulp and paper industry produces a wide variety
of primary products, including newsprint, printing
and writing papers, packaging and industrial papers,
corrugated containers, gray and bleached boxboards,
bags, dissolving pulps, and wood pulp All pulping
processes involve tremendous amounts of water and
timber.
Background
Before the invention of paper, written words were
pre-served on fabric in the form of scrolls The Chinese
are credited with inventing paper around 105 c.e
Historians note that this date was chosen somewhat
subjectively, as early experiments in the process of papermaking probably stretched over a long period
of time before the process was perfected No records exist that indicate how the Chinese first made paper, but it is believed that this early paper was made by pouring fibrous pulp onto flat cloth-covered molds, then drying it—essentially the same way paper is pro-duced today Once the pulp had dried, an interlock-ing matrix of fibers created the paper Early forms of paper were not as well processed as modern paper products In fact, early forms of paper had more in common with the fabrics they replaced than with modern paper They were coarse in nature, but they did lie flat This quality made it possible for the first real books to be produced
Over the following five hundred years, the Chinese papermaking process slowly spread throughout Asia,
In this 1936 photograph, factory workers add pulp to a machine as part of the papermaking process (SSPL via Getty Images)
Trang 8from Vietnam and Tibet to Korea and eventually to
Japan in the sixth century The Japanese refined
the process and continued to produce high-quality
paper varieties for centuries The process moved west
through Nepal and India Several papermaking
de-vices were captured by Islamic warriors, thus moving
the technology further west through the Muslim world
It went to Baghdad into Egypt and across North
Af-rica The technology finally entered Europe in the
twelfth century when the Moors invaded Spain and
Portugal
In 1456, the German printer Johannes Gutenberg
successfully printed a Bible on his movable-type press,
making it possible for the written word to move out to
a much larger population Industrial papermaking
and printing grew from this point
The Fourdrinier Machine
The first major improvement in papermaking was
dipping the molds directly into the fibrous pulp (the
exact date of this improvement is unknown) Dipping
the molds allowed artisans to produce a greater
quan-tity of high-quality paper
Paper was made by hand until the early nineteenth
century, when the Fourdrinier brothers, Henry and
Sealy, introduced the first machine designed
specifi-cally for the manufacture of paper The Fourdrinier
brothers were the financiers of the first modern
pa-permaking machine, which was designed by Nicholas
Louis Robert in Essonnes, France Robert received a
patent for the continuous papermaking machine in
1799 Unable to afford the cost of development and
implementation of his machine, Robert and his
part-ner, Saint-Léger Didot (who often claimed the
contin-uous papermaking machine was of his own
inven-tion), sent Didot’s brother-in-law, John Gamble, to
England to find financial backing A British patent
was awarded in October, 1801 The first continuous
paper machine was installed and made operational in
Hertfordshire, England, in 1803 The next year,
an-other machine followed Robert sold the rights to his
invention to the Fourdrinier brothers in England
The principle of Robert’s machine was to construct
the paper on an extensive woven-wire cloth that
re-tained the matted fibers while allowing the excess
water to drain through—this same principle holds
with all modern papermaking machines
In the United States, the first documented
paper-making machine was installed in 1817, in Brandywine
Creek, Delaware, by the Thomas Gilpin Mills This
machine differed from the Fourdrinier device in that
it was a cylindrical mold The first Fourdrinier device was installed in the United States in 1827
Production of Pulp and Paper Paper production has changed significantly since the early industrial days and even the boom manufactur-ing years of the 1960’s and 1970’s The recyclmanufactur-ing of paper products has become commonplace, as have government-mandated levels of postconsumer fiber content A single sheet of paper could contain fibers from hundreds of different trees around the world These fibers travel thousands of kilometers from the forest to the office printer While recycling technolo-gies have greatly improved in the twenty-first century, there is still only a 10 percent chance that the com-mon paper used in personal printers contains post-consumer recycled fibers On average, office employ-ees in the United States use almost ten thousand sheets of paper, roughly 12 kilograms of paper per person per year In 2005, the average North American created 302 kilograms of paper waste per year com-pared to 231 kilograms for citizens of high-income countries other than the United States and Canada,
or 39 kilograms for citizens of middle-income tries, or 4 kilograms for citizens of low-income coun-tries
The manufacturing of paper and paperboard in-volves the production and conversion of pulp from some fibrous furnish “Furnish” is any blend of fibrous materials (such as timber, wood chips, or recycled pa-per) used to produce pulp Wood is the most com-monly used furnish—roughly 95 percent of all pulp and paper manufacturers use wood in some form The second most widely used form of furnish is sec-ondary fibers from either mill waste or postconsumer fibers, such as newsprint and corrugated boxes The usage of secondary fibers grows as consumer and commercial demand increases for products made from recycled paper
Pulp Production The production of pulp once involved the breaking down of homogeneous furnish feedstock into its fi-bers, often bleaching to increase the whiteness of the paper fibers, and mixing with water to produce a slurry In August, 1998, the Environmental Protection Agency (EPA) passed a regulation called the cluster rule This rule requires the pulp industry to stop the use of bleaches in paper production and imposes the
Trang 9use of chlorine-free colorants instead These chlorine
dioxide derivatives are created from sodium chlorate
instead of chlorine A totally chlorine-free future is
be-ing sought by the EPA for paper production in the
United States and other countries
There are four types of pulping processes:
chemi-cal, semichemichemi-cal, mechanichemi-cal, and secondary fiber
pulping Chemical pulping includes the kraft (sulfate)
process, soda pulping, sulfite pulping, and neutral
sul-fite chemical pulping Mechanical pulping includes
chemi-mechanical, thermo-mechanical,
chemi-thermo-mechanical, refiner mechanical pulping, and stone
groundwood pulping The type of pulping process
affects the durability, appearance, and intended use
of the resulting paper product Regardless of the
pulping method employed, pulping is “dirty.” During
the pulping stage of production, nuisance odors may
be released into the air, and dioxins from kraft
chemi-cal bleaching may be released into wastewater Thus
the pulping process is a major concern to the EPA in
the United States and equivalent agencies in Europe
Chemical pulping liberates the fibers from the
fur-nish by dissolving the lignin bonds, which hold the
cellulose fibers together, by cooking wood chips in
liq-uid chemical solutions at extremely high
tempera-tures and pressures Kraft pulping is by far the
domi-nant form of chemical (and nonchemical) pulping
because of its early development in the 1800’s, its
abil-ity to use nearly every species of wood as furnish, and
the fact that its resulting pulps are markedly stronger
than those of other chemical processes However,
chemical pulp yields are roughly 45 to 50 percent In
other words, roughly 50 percent of the furnish is
con-verted into pulp
Semichemical pulping produces very stiff pulp and
is used mainly for corrugated containers The
semi-chemical process consists of the partial digesting of
hardwood furnish in a diluted chemical solution
be-fore it is mechanically refined to separate the fibers
from the weakened furnish Pulp yields range
be-tween 55 percent and 90 percent, depending on the
process employed
Mechanical pulping processes involve the
reduc-tion of furnish to fiber by either beating or grinding
This is the oldest known method of releasing the
cel-lulose fibers from wood furnish The pulp yields are
high, up to 95 percent, especially when compared
with chemical pulping yields of 45 to 50 percent
How-ever, the mechanically produced pulp is of low strength
and quality Thus, mechanical pulp is often combined
with chemical pulp to increase both its strength and quality
Finally, secondary fiber pulping relies on recov-ered (recycled) papers as furnish Typically, second-ary fibers are presorted and preprocessed before they are sold to a pulp and paper mill If the recovered pa-pers have not been preprocessed, then they must first
be treated to remove common contaminants, such as adhesives, coatings, inks, and dense plastic chips The most common technique of secondary fiber pulping involves mixing the recycled furnish in a large con-tainer of water, which is sometimes heated Pulping chemicals may be added to induce the dissolution of paper or paperboard The mix is then stirred by a ro-tor to produce the pulp
Pulping processes involve tremendous amounts of water, and most require large amounts of timber Of all the wood harvested globally for industrial pur-poses, 42 percent goes into the production of paper Latin America is a growing supplier of harvested wood for paper manufacturing Furthermore the in-ternational Organization of Economic Cooperation and Development indicates that paper and pulp in-dustries are the largest consumers of water of all the major industrial sectors The papermaking process generates large amounts of air and water pollutants, especially during the pulping stage It ranks third be-hind the chemical and steel industries in greenhouse emissions In 2000, the world’s largest producers of paper pulp were the United States, at 57,002 metric tons, and Canada, at 26,411 metric tons, followed
by China, Finland, Sweden, Japan, Brazil, Russia, In-donesia, and Chile
Manufacturing Paper There are two general steps in the process of making paper and paperboard: wet-end operations and dry-end operations During the wet-dry-end operations, pro-cessed pulp is transformed into a paper product via a paper machine, the most common of which is the Fourdrinier paper machine
Pulp slurry (more than 90 percent water at the start) is deposited on a rapidly moving wire mesh for removal of the water by gravity, vacuum chambers, and vacuum rolls After vacuum rolling, a continuous sheet is left, which is then pressed between a progres-sion of rollers to extract any additional water and to compress the fibers The sheet is then ready for dry-end operations During this stage, the sheet enters a drying area, where the paper fibers start to bond as
Trang 10they are compressed by steam-heated rollers The
sheets are then pressed between massive rollers to
re-duce paper thickness and to prore-duce a smooth
sur-face After a smooth thin sheet of paper is produced,
coatings may be applied to improve the color, luster,
printing detail, and brilliance Finally, the paper prod-uct is spooled for storage
From there, the process of bringing the consumer
a standard 8.5-inch-by-11-inch sheet of paper involves nothing more than loading the spool of oversized
pa-Pulp Processes
Dissolving kraft Highly bleached and purified kraft process wood pulp, suitable for
conversion into products such as rayon, viscose, acetate, and cellophane
Bleached paper-grade kraft
and soda, unbleached
kraft
Bleached or unbleached kraft process wood pulp, usually converted into paperboard, coarse papers, tissue papers, and fine papers such as business, writing, and printing papers
Dissolving sulfite Highly bleached and purified sulfite process wood pulp, suitable for
conversion into products such as rayon, viscose, acetate, and cellophane
Paper-grade sulfite Sulfite process wood pulp with or without bleaching, used for
products such as tissue papers, fine papers, and newsprint
Semichemical Pulp processed by chemical pressure and (sometimes) mechanical
forces with or without bleaching, used for corrugating medium (for cardboard), paper, and paperboard
Mechanical pulp Pulp manufacture by stone groundwood, mechanical refiner,
thermochemical, chemi-mechanical, or chemi-thermomechanical means for newsprint, coarse papers, tissue, molded fiber products, and fine papers
Nonwood chemical pulp Production of pulp from textiles (e.g., rags), cotton linters, flax,
hemp, tobacco, and abaca to make cigarette wrap papers and other specialty products
Secondary fiber deink Pulps from waste papers or paperboard using a chemical or solvent
process to remove contaminants (such as inks, coatings, and pigments), used to produce fine, tissue, and newsprint papers Secondary fiber non-deink Pulp production from waste papers or paperboard without deinking
processes to produce tissue, paperboard, molded products, and construction papers
Fine and lightweight papers
from purchased pulp
Paper production from purchased market pulp or secondary fibers
to make clay-coated printing, uncoated free sheet, cotton fiber writing, and lightweight electrical papers
Tissue, filter, nonwoven, and
paperboard from purchased
pulp
Paper production from purchased market pulp to make paperboard, tissue papers, filter papers, nonwoven items, and any other
products other than fine and lightweight papers
Source: U.S Environmental Protection Agency Development Document for Proposed Effluent Limitations Guidelines and Standards for the Pulp, Paper, and Paperboard Point Source Category, October, 1993.