CARBON BLACK
Overview:
Carbon black is an inorganic, colloidal material produced by the incomplete combustion of natural gas or other petroleum products. It may also be known as lamp black, furnace black, acetylene black, channel black, thermal black or gas black depending on the process that produces it. Carbon black is manufactured by some form of combustion that allows the flame to impinge on a cool surface, leaving a black residue. This residue is then collected as carbon black.
In appearance, carbon black may be an intensely black amorphous powder or finely divided pellets. It is insoluble in water and solvents. It has a bulk density is 1.8-2.1 g/cm3 and a high surface-area-to-volume ratio. It is used as a black pigment for inks and paints, and in the manufacture of tires, rubber and plastic products, among other uses.
Characteristics and Challenges:
As a safety concern, prolonged exposure to carbon black should be avoided. Inhalation of the dust may cause respiratory problems; carbon black may cause drying of the skin with repeated contact. There is some evidence to suggest that carbon black is a carcinogen.
Carbon black should be stored in a clean, dry space away from exposure to high temperatures, open flames, and strongly aromatic chemicals. Since carbon black will adsorb moisture and chemical vapors, it should be stored in well-sealed containers.1
Some carbon black grades may have trace concentrations of carbon monoxide adsorbed onto its particle surfaces. Confined space entry procedures should include testing for oxygen, carbon monoxide, and other toxic gases, as appropriate.
Carbon black is difficult to handle; it cakes and packs easily and tends to fluidize, meaning the fine carbon black particles take on characteristics of a liquid. As a result, carbon black fines have a tendency to form deposits in the conveying line. Carbon black is insoluble with water and solvents. Manufacturers must remain vigilant to prevent spills and the cleanup that would follow. Intensive and costly dry vacuuming, with
appropriate filtration, is the preferred method for removing carbon black surface dust and cleaning spills.
If the carbon black arrives at the processing facility in bulk bags, the frames used to discharge the bags should be equipped with additional accessories to reduce the need for operator intervention when emptying the carbon black from the bag. This includes features such as spring-loaded frames that will elongate and stretch the bags as they empty and lighten, making them rigid and removing any pockets of carbon black cornered in the bags. Bag activating devices are usually effective in dislodging collections of carbon black, promoting a better flow. Some of these bag activating devices also serve as an airtight seal between the bulk bag and the receiving hopper.
After the bulk bag has been connected to the receiving hopper and the carbon black begins to flow, the air inside the hopper is forced out. Unless this displaced air passes through a filter, airborne dust particles can escape into the surrounding atmosphere, creating a potentially hazardous situation. A dust collector mounted on the discharger frame will contain the carbon black inside the conveyance system. Carbon black dust trapped in the filter media can be returned to the receiving hopper by a pneumatic pulse through the filter.
If the carbon black is being added into the processing stream from smaller (>50 lb./22.76 kg.) bags, a bag-breaking station with a dust hood, filtration devices and a pneumatic pulse cleaner is the recommended solution to support the manual unloading of the material. If empty bag disposal is an issue, a dust-tight bag compacting system may be necessary.
With its poor flow characteristics, carbon black will tend to "rat hole" or bridge inhoppers, completely blocking the flow of the material, causing a shutdown while it is removed and elevating the risk of dusting or spillage. Feed hoppers should be designed with proper geometry and may need to incorporate devices such as air fluidizers or vibrators to dislodge carbon black from hopper walls or mechanical agitators to promote flow.
If a flexible screw is being used to convey the material, it is important to use a screw that matches carbon black's characteristics and other application requirements. Generally a screw with a wider, flatter surface than the standard-sized screw is necessary for this type of material.
Should your carbon black application feature the loading the material into bulk bags, the bag capacity will be maximized by use of a vibratory densification deck to level the carbon black as it fills the bag and by the application of load cells to attain the desired weight. Seals and other dust-containment devices will ensure a dust-tight operation.
Flexicon Applications:
Flexicon's BEV-CON™ screw conveyor is highly effective in moving carbon black. Flexicon systems are also helping clients reduce waste and improve plant cleanliness in their carbon black applications.
Containment of the carbon black particles is the other key element to success in conveying this material. Fine carbon black dust may be small enough to penetrate electrical boxes and other electrical devices, possibly creating electrical hazards resulting in equipment failure.
To learn how one manufacturer discharges carbon black from bulk bags and conveys it to a mixer while containing dust, see this case history article on Kirkhill-TA Co.
Related Articles on Carbon Black:
1 International Carbon Black Association
Sources: Where noted. All other information courtesy Flexicon Corp.