(B) Thermal-oxidative decomposition (incomplete combustion)

Here, the energy required for carbon black formation is provided by combustion of fuel or part of the feedstock by carefully controlling the air deficiency and the flow conditions in the reactor. Two principal processes can be distinguished, depending on the type of gas flow involved. The furnace black process produces carbon black in a turbulent flow environment of a closed reactor system with appropriately designed flow. The feedstock is injected in the hot off-gas providing the energy for the thermal decomposition of the feedstock from the combustion of fuel or feedstock in the combustion zone of the reactor. The gas black process (and historic channel black process) is operated with a diffusion flame generated by combusting oil or natural gas feedstock in a reactor system being open to the outer air atmosphere. Normally, the decomposition products are burned as soon as they reach the outer zone of the diffusion flame containing sufficient oxygen. If a cooled object (typically water-cooled rollers) is put inside the flame where the oxygen diffusion rate is lower than the rate of the decomposition reaction and thus carbon particles are formed causing the flame to glow, the combustion is reduced, and carbon particles are deposited at the cooled surface.

The thermal-oxidative process is by far more important than the thermal decomposition process in the absence of oxygen and accounts for more than 98% of carbon black production worldwide. Also the traditional lampblack process carried out in a closed system falls in this category.

The carbon black formation seems, although not yet fully elucidated, to follow a basic formation mechanism consisting of different steps for the carbon black growth which essentially applies for all types of manufacturing processes (Bansal and Donnet 1993a; Taylor 1997; Bourrat 2000):

  • 1. Vaporization of the feedstock (in the case of an oil feedstock) and fragmentation of the hydrocarbon molecules down to C1 or C2 units by the energy generated in the process.
  • 2. Precipitation of nuclei or growth centers for the primary carbon particles from recombining fragments.
  • 3. Growth of the nuclei to primary particles by deposition of carbon around the growth centers.
  • 4. Possible adhesion of the primary particles and fusion to complex aggregates by deposition of pyrolytic carbon onto the aggregate surface (secondary particle growth).
  • 5. Agglomeration of the aggregates (cyclic growth) may occur for some carbon black types.

This mechanism explains the carbon black morphology and determines the primary carbon black properties including primary particle size, porosity, aggregate shape, aggregate size, as well as surface morphology. From these primary properties, secondary properties such as carbon black structure (void volume), specific surface area, surface energy, volatile content, electrical and thermal conductivity, and compressibility can be derived. The carbon black morphology and surface chemistry are controlled by the manufacturing process and process conditions.

 
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