to spherical pellets: the majority

to spherical pellets: the majority of carbon blacks are processed this way,
especially grades for the rubber industry. For pigments, blacks are densified and
packaged as carbon black powder, or are dry-pelletized.
The gas black (channel black) process, based on gas feedstock, was originally
developed in the USA using natural gas, but is now obsolete. As natural gas
was not readily available in Europe at the time (in the mid-1930s), a similar
process using coal tar distillates was developed by Degussa, known as the gas
black process, and this is still operated. Gas is burned in large numbers of
small flames, impinging on water-cooled rollers, depositing carbon black on the
rollers and also in the air. The black from the two streams is combined.
The process is flexible, producing particle sizes from 10 to 30 jiim, but the
structure cannot readily be controlled. The carbon blacks resulting are highly
rated as pigments for their efficient dispersion properties and high colour
depth. The blacks react as distinctly acidic in an aqueous solution, due to
surface groups containing oxygen, which can be increased by oxidative aftertreatment,
making grades that are particularly useful in coatings and printing
inks.
The lampblack process is one of the oldest commercial processes, in which the
fuel is burned in a pan beneath a fume hood lined with refractory bricks and
cooled: process gases containing carbon black are cooled and filtered. The
process is used today for standard grades of pigment and rubber black: they have
coarse primary particles and broad distribution of sizes from 60 to over 200 jim.
Despite development of alternatives, it has not yet been possible to replace these
grades in their specific applications.
7.2.1.2 Thermal decomposition processes
The thermal black process is discontinuous, using natural gas or higher
hydrocarbons and oils. A unit consists of two reactors one of which is heated in
cycles of 5-8 minutes with fuel and air, while the other is heated with fuel
without air, giving alternating heating and decomposition cycles. Actual
formation of the carbon black occurs more slowly in the absence of oxygen and at
a decreasing temperature, giving large, solid particles with a coarse particle size,
ranging from 120 to 200 jim or from 300 to 500 |im, depending on whether the
natural gas is diluted with inert gases.
The acetylene black process relies on the exothermic decomposition of
acetylene into carbon and hydrogen at elevated temperatures, the heat released
sustaining the reaction. Acetylene blacks have average particles sizes in the 30 -
40 jLim range, but the particle shape is more branched than the spherical shape of
thermal blacks.
7.2.1.3 Effect of particle size and structure on properties of carbon blacks
The size of a particle and its shape (structure) have a fundamental effect on the
properties of the particular type of carbon black, particularly the colour strength,
dispersability, and electrical conductivity. The basic effect of higher/lower
particle size and structure on the properties of carbon black is illustrated in
Table 7.8.