The following piece is an excerpt from an article written by Charles R. Harbison and presented at the 10th Annual Bulk Material Handling Seminar in 1977. It has since been edited and condensed for the purpose of this article.
Continued from Part 1
The techniques of balling, pelletizing, micro-pelletizing, conditioning, and granulation are all accomplished as a general rule, most efficiently with the same equipment. That is not to say, however, that a specific material my not be best in the pin mixer, which is frequently used for micropelletizing. In addition, conditioning, for some materials, may be best accomplished in the pug mill.
In general, however, the pelletizing equipment is used extensively in all five techniques by merely adjusting the available operational variables. There are four basic types of pelletizing devices; disc, cone, drum and deep disc.
Pelletizing results in a generally spherical shaped agglomerate, a pellet. Pelletization is caused by impacting a particular type of motion or agitation, normally rotation, tumbling and rolling to the powdered material during or subsequent to the addition and mixing with a liquid binder. All of the aforementioned pelletizing units produce the pellets in a somewhat similar manner.
Let’s take a look at what takes place in or on the machine and how the phenomena is achieved before explaining the difference in the types of pelletizing units.
Assume as a base condition; finely divided inert material with a distribution of particle size ranging from, say 40 mesh to minus one micron. Pure water will be the binding agent. The material need not be dry and indeed normally is not, but ideally, should have a few percent less moisture than the final pellet moisture for operational control. The material is fed to the unit at a steady controlled rate and the water is sprayed onto the material in the unit while the unit is rotating. Droplets of the water will collect several particles; the rotation will force impact and densify this loosely formed nuclei or seed. This densification forces water to the surface to pick up more particles. Continuing rotation causes the seed to recycle through the spray area to be re-wetted to pick-up still more particles and so on. (See fig. 1). The water will have been forced rather firmly into the minute interstices and nearly each and every particle will be coated with water.
This arrangement of the particles is not unlike that of a good concrete mix where all the spaces, interstices, between the larger particles are filled with smaller particles and the interstices between these are filled with yet smaller particles and so on until the final interstices are filled with water. (See fig.1). Surface tension and the capillary forces of the water is the major binding force that is holding the pellet together. There is a very thin layer of water between each particle, and this results in the pendular bonding system so ably described by the late Professor Rumpf. (See fig.2). Since we started with an inert material and pure water, the bond is caused solely by the water, should you dry the pellet, i.e., drive off the water the pellet will, lose strength accordingly. In practice, of course, the feed material is not totally inert and most often being made up of several materials, some of which in the presence of water will dissolve at least to a small extent or will react with one or more constituents and excellent strength may result. Drying the pellet will solidify the bond increasing the strength. The liquid of course can be anything to enhance the bond, if it is compatible with the end product. Binding liquid can be tailored to cause binding reactions or merely contribute adhesiveness. Inert relatively course material or materials of a very narrow particle range would necessarily depend entirely on the adhesiveness of the binder.
FIGURE 1 and FIGURE 2
FIGURE 3 and FIGURE 4
The water is sprayed onto this moving surface of material. When the material reaches a height it is no longer pulled or carried and gravity will cause it to roll down across the upcoming material. As described previously, the water droplet has formed the nuclei of the pellet with several particles, as it rolls downwardly, it is compacted, forcing the water to the surface thus enabling more particles to be picked up. As the rotation continues, this nuclei or seed is rewetted as it passes through the spray area thus contacting loose feed material to pick up additional material thus growing until reaching the end of the unit or growing to a size that causes it to discharge. Speed and angle of these units is so adjusted to discharge a maximum of “on size” pellets. Size control for the drum and cone is normally accomplished with a closed cycle screening system where over size is crushed and returned to the pelletizer with the undersize. Disc installations very often produce a product “on size” or with under and over size within acceptable limits. However, when close product size is specified, a disc system requires screening and recycling of the product discharge also. Feed material to the disc, drum and cone units have traditionally been wet, especially in iron ore pelletizing. The ore moisture content being at or just short of pellet moisture because feed preparation is a wet process. Dry pellet feed shows some advantages in certain situations and is often mandated by process. There has been very little theoretical work concluded for this application.
Alteration of what has been standard pelletizer geometry to gain a deeper bed of contained material in the pelletizer shows some indication of increasing dry feed productivity and can help minimize “dusting.” Dry feed is very often pre-conditioned prior to pelletizing in a standard circuit.
FEECO has been manufacturing agglomeration equipment such as pelletizers, rotary drum agglomerators, pin mixers, and paddle mixers since 1961. We also have the ability to run agglomeration feasibility tests in our laboratory testing and tolling facility. For more information on our agglomeration equipment, or agglomeration feasibility testing, contact us today!
You might like: