The following piece is an excerpt from an article was 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.
Feed material preparation is quite critical to successful pelletizing. Proper premixing of feed when there is a multiplicity of materials and especially if these materials are of different particle size range and or bulk density is also very important. To the extent possible mixing and handling immediately prior to feeding onto the pelletizer should also de-aerate the material and stabilize the feed bulk density.
Feed material particle size distribution is very important, as explained previously and especially to the drum and cone, and to a lesser extent the disc. The top size, (i.e., largest fraction) is critical and limited in all units. It is my experience that top particle size appears to be a function of material depth in the nucleation area of the unit but let’s leave that study for a later time.
The pelletizing drum, also known as a rotary drum agglomerator, is used frequently in the high tonnage applications such as iron ore pelletizing. The drum has been also used quite frequently in fertilizer applications and equipped with special “internals” in detergent manufacture. The cone has not been nearly as popular. The disc has had the most wide spread acceptance throughout the broadest range of industries. The wide range of unit sizes and the fact that the geometry of the disc units has a natural tendency to classify thus making control somewhat easier, makes them an ideal solution for various materials. The deep discs, that is, units with depth/diameter ratio of .5 or greater, appear to have a higher tolerance for a higher percentage of coarse material. This advantage, however, is offset by the difficulty with the deep units to make the smaller size pellets.
Pelletizing units are universally sensitive to fluctuations in the feed, i.e., bulk density, particle size distribution temperature and feed rate (gravimetric) variation. In fact, maintenance of pelletizing rate and product quality is close to impossible without optimizing the constancy of feed rate. One needs but little experience in troubleshooting pelletizing systems to realize that making constant the material feed rate is the source of most pelletizer system problems. Experience has proven that inconsistent pelletizer feed rate is the most frequent source of trouble in maintaining constant and consistent pellet quality and obviously production rates.
Pellet size is often critical especially if the pellet is the end form of the product. Size may be just as critical however, when the pellet is an intermediate processing step. Maintaining constant pellet diameter throughout the length of the production run is not easy; it should be expected that periodic adjustments will be made. To achieve a pellet production of a given pellet size means “tuning” the available variables. The nature of the feed material itself dictates these adjustments. The adjustments are: material feed rate, water feed rate, spray nozzle size and location, machine angle, and speed. Retention time in a machine is a function of machine angle and machine speed and feed rate. Material feed rate has narrow limits when maintaining close control over product size and quality and is dictated by machine size and geometry. Water rate also has narrow limits dictated either by the nature of the material or subsequent drying or process requirement. Machine angle and speed adjustment are largely dictated by machine geometry, “fine tuning” of speed and angle is gained through operator experience with the material being pelletized. This kind of experience and the ability to see subtle changes taking place in or on the machine give credence to the “art” involved in pelletizing.
The different units have different operating characteristics, some obvious and diverse, others but nuances. All operating differences and variables, however, are very dependent on the nature of “idiosyncrasy” of the material being agglomerated.
It is obvious that binder characteristics will add requirements and considerations of their own. It is not practical to go into operational detail here that is best left for in situ circumstances. It is also true that when working with particulate materials, the only practical general rule is that general rules are totally unreliable.
Pellet physical quality is quite obviously as critical as production rate and size control. Achieving physical quality involves some of the same adjustments to achieve and control pellet size. Adjusting retention time to alter reroll time for formed pellets has the greatest influence of pellet quality of all the machine adjustments. Changing chemistry (i.e., the use of stronger binders) of course would have the most profound effect on pellet quality. Unfortunately, this also increases the product cost. Adjusting feed particle size when practical or possible is often the surest way to achieve the desired quality at maximum production rates.
Pellet quality is first measured on the “green ball” or fresh pellet in terms of crush strength, number of drops preceding breakage from a measured height, and a tumble or abrasion index. These tests are often repeated after a given time of natural or ambient drying or specified time and temperature forced drying. There are tests dictated by the use to which the pellets are put, such as, determination of pellet abrasion resistance, porosity, and high temperature swelling. There may be others as required by special circumstances.
Care should be exercised when specifying pellet quality. For example, specifying 60 pound crush strength when actually 10 pound crush is adequate can be specifying impractical costs or a parameter not possible for the material under consideration.
Though this paper has really been but an introduction, elementary and unsophisticated, a reference for general information, a starting place for further study, there is a wealth of more detailed information available.
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!
To download this paper in its entirety, see Material Size Control Through Agglomeration