Information about carbon black
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Application of carbon black
While carbon black was exclusively used as a pigment until the beginning of this century, its use as an active filler in rubber was the starting point for a new rapidly expanding application. In automobile tyre production, it was found that treads filled with carbon black had a markedly higher abrasion resistance than those filled with zinc oxide. This discovery, together with increasing use of motor vehicles, was the basis for the present importance of carbon black as a filler in rubber. Today, at least 35 different grades of carbon black are used as fillers in rubber, and about 80 grades are used in pigments or special applications [47, InfoMil, 2002]. Table 1 shows the fields of application of carbon black.
Table 1: Breakdown of total carbon black sales according to the fields of application [47, InfoMil, 2002]
Approximately 90 % of all carbon black sales are to the rubber industry [47, InfoMil, 2002]. However, while the major portion of carbon black is sold to the tyre industry, carbon black is also used by the automotive and the rubber industries in general as a major component in the production of mechanical rubber goods. Carbon black production is, therefore, largely dependent on developments in the automotive industry.
The remaining 10 % of carbon black sales are to the non-rubber industry. Carbon black is used by the printing industry (for pigment blacks in printing inks) and by the plastic industry. These sectors consume roughly one-third each of the total pigment black sales. A further important application, especially for the higher priced, fine particle size carbon blacks, is in the production of black coatings/paints, which accounts for approximately 9 % of production. This is followed by the paper industry, accounting for approximately 4 %. Other non-rubber areas are, for example, the manufacture of electrodes and the reduction of metal oxides. Together, these applications have a share of about 21 % of total non-rubber carbon black sales.
About 90 % of the carbon black produced is used by the rubber industry as a reinforcing filler in tyres, tubes, conveyor belts, cables, rubber profiles, and other rubber goods. Furnace blacks are predominantly used in rubber processing. Fine particle size carbon blacks (reinforcing blacks) are used for the production of rubber mixtures with high abrasion resistance (e.g. tyre treads). Coarser carbon blacks (semi-reinforcing blacks) are used in rubber mixtures requiring low heat build-up and resistance to permanent deformation during dynamic stress (e.g. carcase compounds, equipment mountings, and seals). Extremely coarse carbon blacks (non-reinforcing blacks) are incorporated into mixtures with high elasticity and good extrusion properties.
Quantitatively, the pigment blacks are substantially less important than the rubber blacks. They are used for the manufacture of printing inks, colouring plastics, fibres, lacquers, coatings, and paper. Oxidised carbon blacks are frequently used in the printing ink and coating industry. While high-colour gas blacks are still predominant in lacquers and coatings, furnace blacks are becoming more and more important in plastics, coatings, and printing inks. Besides their two main uses as reinforcing fillers and pigments, small amounts of carbon blacks are used by the electrical industry to manufacture dry cells, electrodes, and carbon brushes. Special blacks are used to give plastics antistatic or electrical conduction properties. Another application is the UV stabilisation of polyolefins.
Applied processes and techniques
The term ‘carbon black’ is used for a group of well-defined, industrially manufactured products, which are produced under carefully controlled conditions. The physico-chemical properties of each grade of carbon black are kept within narrow specifications. Carbon black is a form of highly dispersed elemental carbon with extremely small particles. Depending on the raw materials and production processes, carbon black also contains chemically bound hydrogen, oxygen, nitrogen, and sulphur.
Due to its excellent pigmentation properties, especially its light stability and universal insolubility, carbon black has been used as a black pigment since early times. It was produced for this purpose by burning oils, fats, or resinous materials. The flame was either quenched on a cool surface ‘impingement black’, or cooled in special stacks ‘lamp black’ where the carbon black was deposited. Today, both methods are still used in the various production processes of carbon black.
The ‘channel black process’, a process for making impingement blacks using natural gas as a raw material, has been used in the United States since the end of the 19th century. This process has now been abandoned because of economic and environmental considerations. A similar process for the production of impingement blacks, the ‘gas black process’, is still used today.
The increasing demand for carbon black led to new production processes. The most important process today is the ‘furnace black process’. Developed in the United States in the 1930s and substantially improved in the 1950s, it is a continuous process, which allows the production of a variety of carbon black grades under carefully controlled conditions. Nearly all rubber grades and a significant part of pigment-grade carbon blacks are now manufactured by the furnace black process. Nevertheless, other processes, such as ‘gas black’,‘lamp black’, ‘thermal black’, and ‘acetylene black’ processes, are still used for the production of specialities. These processes are further elaborated in sections "Heating".
Mixtures of gaseous or liquid hydrocarbons, which can be vaporised, represent the raw materials preferable for the industrial production of carbon black. Since aliphatic hydrocarbons give lower yields than aromatic hydrocarbons, the latter are primarily used. Unsubstituted polynuclear compounds with 3 – 4 rings give the best yield [47, InfoMil, 2002].
The materials rich in these compounds are certain fractions of coal tar oils and petrochemical oils from petroleum refining or the production of ethylene from naphtha (aromatic concentrates and pyrolysis oils). These aromatic oils, which are mixtures of a variety of substances, are the most important feedstock today. Oil on a petrochemical basis is predominant. The aromatic portion of a typical petrochemical oil consists of 10 – 15 % monocyclic, 50 – 60 % bicyclic, 25 – 35 % tricyclic, and 5 – 10 % tetracyclic aromatics [47, InfoMil, 2002].
Important characteristics determining the quality of a feedstock are the C/H ratio as determined by elemental analysis and the Bureau of Mines Correlation Index (BMCI), which is calculated from the density and the mid-boiling point or from the density and the viscosity.
Both the C/H ratio and BMCI values give some information on the aromaticity and, therefore, the expected yield. Further characteristics are viscosity, pour point, temperature of solidification, alkali metal content (due to its influence on the carbon black structure), and sulphur content.
Natural gas, which was previously the predominant feedstock for the production of channel blacks, has lost its importance for economic reasons. However, natural gas is still the most important secondary feedstock in the furnace black process, although other gases and oils are used in some cases. The term ‘secondary feedstock’ is used for easier distinction between the primary feedstock as the main carbon source for the carbon black. In the rest of this chapter, a practical distinction will be made between primary feedstock and secondary feedstock. The term ‘fuel’ will be then reserved for non-reactor related combustion processes. In several patents, recycled tail-gas, in combination with oxygen or oxygen-enriched air, has also been proposed as a secondary feedstock, but has not gained any commercial importance. Moreover, acetylene, due to its high price, is used only as a feedstock for the production of highly specialised conductivity blacks (e.g. used in dry cell batteries).
Sulphur content in the feedstock used in the production of carbon black is of key importance for the assessment of the environmental impact of the European carbon black plants [47, InfoMil, 2002].
Best available technologies (BATs)
1. Use low sulphur feedstock. The use of low sulphur primary feedstock with a sulphur content in the range of 0.5 - 1.5 % as a yearly average. The corresponding BAT specific emission level is 10 – 50 kg SOX (as SO2) per tonne of rubber grade carbon black produced, as a yearly average. These levels are achieved assuming that the secondary feedstock is natural gas. Other liquid or gaseous hydrocarbons can be used as well. In the production of speciality grade carbon black (high surface pigment blacks), higher emission levels are expected.
2. Preheat air required in the process to save energy. Ensure that air required in the process is preheated in heat exchangers by the hot gases (containing carbon black) leaving the furnace black reactor.
3. Maintain optimum operational parameters in the carbon black collecting system. Maintain optimum operation of a high performance bag filter to ensure high carbon black collection efficiency and minimum product losses of the residual carbon black in the filtered tail-gas.
4. Utilise the energy content of the tail-gas. For new plants, this aspect should be considered prior to the selection of the location for the carbon black plant, as this gives the highest potential for energy recovery. Possible marketable products are power, steam, hot water and the tail-gas itself. The combustion of the remaining tail-gas without energy recovery, as is the case in a flare, can only be considered when all the possibilities of economically viable recovery of energy are exhausted.
5. Apply primary deNOX techniques to reduce the NOX content in flue-gas originating from tail-gas combustion in energy producing systems. New plants: The emission levels associated with BAT are <0.6 g NOX/Nm3 as an hourly average at 3 % O2 during normal production. Higher NOX emissions can be expected during carbon black grade changes.
Existing plants: The associated emission levels are in the range of 0.6 – 1.0 g NOX/Nm3 as an hourly average at 3 % O2 during normal production. Higher NOX emissions can be expected during carbon black grade changes. The NOX emissions from flares should be kept as low as possible by proper design and operation.
6. Apply fabric filters for the air conveying system, vent collection system and dryer purge gas. For the low temperature air conveying and vent collector systems, associated emission levels are 10 to 30 mg/Nm3 as a half-hour average. For the dryer purge filter, associated emission levels are <20 to 30 mg/Nm3 as a half-hour average. The emissions are not related to a specific oxygen content. It should be noted, for all filters, that the lower level of the emission range is more difficult to achieve consistently when finer grades of carbon black are dealt with.
7. Recycle off-spec carbon black back into the process. Off-spec carbon black can be recycled back into the process to a certain extent. This can be done by admixing small quantities of off-spec carbon black with regular carbon black. Product specifications finally determine the total amount of off-spec carbon black that can be reprocessed.
8. Water recycling. Investigate the possibility of recycling rinsing water and, if possible, storm-water in the process, if it does not affect product quality. The collected rinsing water and (part of) the storm-water can be used after filtration as a source of process water.
Source: European Commission, Large Volume Inorganic Chemicals - Solids and Others industry, August 2007, p. 205-207
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