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Carbon black [C.A.S. NO. 1333-86-4]
is virtually pure elemental carbon in the form
of colloidal particles that are produced by incomplete
combustion or thermal decomposition of gaseous
or liquid hydrocarbons under controlled conditions.
Its physical appearance is that of a black, finely
divided pellet or powder. Its use in tires, rubber
and plastic products, printing inks and coatings
is related to properties of specific surface area,
particle size and structure, conductivity and
color. Carbon black is also in the top 50 industrial
chemicals manufactured worldwide, based on annual
tonnage. Current worldwide production is about
18 billion pounds per year [8.1 million metric
tons]. Approximately 90% of carbon black is used
in rubber applications, 9% as a pigment, and the
remaining 1% as an essential ingredient in hundreds
of diverse applications.
Modern carbon black products
are direct descendants of early "lamp blacks"
first produced by the Chinese over 3,500 years
ago. These early lamp blacks were not very pure
and differed greatly in their chemical composition
from current carbon blacks. Since the mid-1970s,
most carbon black has been produced by the oil
furnace process, which is most often referred
to as furnace black.
Production
Two carbon black manufacturing processes (furnace
black and thermal black) produce nearly all of
the world's carbon blacks, with the furnace black
process being the most common. The furnace black
process uses heavy aromatic oils as feedstock.
The production furnace uses a closed reactor to
atomize the feedstock oil under carefully controlled
conditions (primarily temperature and pressure).
The primary feedstock is introduced into a hot
gas stream (achieved by burning a secondary feedstock,
e.g., natural gas or oil) where it vaporizes and
then pyrolyzes in the vapor phase to form microscopic
carbon particles. In most furnace reactors, the
reaction rate is controlled by steam or water
sprays. The carbon black produced is conveyed
through the reactor, cooled, and collected in
bag filters in a continuous process. Residual
gas, or tail gas, from a furnace reactor includes
a variety of gases such as carbon monoxide and
hydrogen. Most furnace black plants use a portion
of this residual gas to produce heat, steam, or
electric power.
The thermal black process uses
natural gas, consisting primarily of methane or
heavy aromatic oils, as feedstock material. The
process uses a pair of furnaces that alternate
approximately every five minutes between preheating
and carbon black production. The natural gas is
injected into the hot refractory lined furnace,
and, in the absence of air, the heat from the
refractory material decomposes the natural gas
into carbon black and hydrogen. The aerosol material
stream is quenched with water sprays and filtered
in a bag house. The exiting carbon black may be
further processed to remove impurities, pelletized,
screened, and then packaged for shipment. The
hydrogen off-gas is burned in air to preheat the
second furnace.
Physical & Chemical
Properties
Carbon black is not soot or black carbon, which
are the two most common, generic terms applied
to various unwanted carbonaceous by-products resulting
from the incomplete combustion of carbon-containing
materials, such as oil, fuel oils or gasoline,
coal, paper, rubber, plastics and waste material.
Soot and black carbon also contain large quantities
of dichloromethane- and toluene extractable materials,
and can exhibit an ash content of 50% or more.
Carbon black is chemically
and physically distinct from soot and black carbon,
with most types containing greater than 97% elemental
carbon arranged as aciniform (grape-like cluster)
particulate. On the contrary, typically less than
60% of the total particle mass of soot or black
carbon is composed of carbon, depending on the
source and characteristics of the particles (shape,
size, and heterogeneity). In the case of commercial
carbon blacks, organic contaminants such as polycyclic
aromatic hydrocarbons (PAHs) can only be extracted
under very rigorous laboratory analytical procedures
(soxhlet extraction using organic solvents and
high temperatures). These extracts, though they
may be similar to those derived from soot, are
unique, however, because carbon black extracts
exist only in extremely small quantities. Water
and body fluids are ineffective in removing PAHs
from the surface of carbon black and, therefore,
they are not considered to be biologically available.
Two other commercial carbonaceous products often
confused with carbon black are activated carbon
and bone black. Each is produced by processes
different from commercial carbon black and possesses
unique physical and chemical properties.
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