Metal pelleting and cooling

Abstract

Abstract and SummaryPelleting is a process of conditioning, compacting, and extruding a finely divided material resulting in larger particle sizes with greater bulk density and improved flow characteristics in which dust is essentially eliminated. The process has a relatively short history beginning in 1929 with the conception and design of equipment using the die and roller principle. Conditioning is accomplished by the use of good quality steam added at a controlled rate to a regulated flow of material in a paddle type mixer integrally mounted on the pellet mill. This steam adds heat and moisture to enhance the pelleting characteristics of the material. Compaction is accomplished by the action of the rolls upon the die face which also supplies the force necessary to extrude the material through the die holes thus forming pellets. In the pelleting of oilseed meals the sizes of pellets are generally limited to 1/4 in, diameter for export with 1/4 in., 3/4 in., and 7/8 in. being quite common for animal feeding. Many times the 3/4 in. or 7/8 in. pellets are broken through crumbling rolls to form a coarse, granular product. A cooling and drying step is necessary following the pelleting process to remove heat and excess moisture to result in a stable product. This step is accomplished by the use of either a vertical or horizontal cooler by means of which ambient air is drawn through a moving bed of pellets. Exhaust air from this process must be discharged to atmosphere through an efficient cyclone collector to remove entrained dust and feed particles. Following the cooling step the pellets are usually screened to remove the particles smaller than the pellet, thought in the export trade this step may be eliminated. It is desirable to remove extraneous material from the feedstock ahead of the pellet mill in order to protect the dies and rolls. This is accomplished by magnets in the case of magnetic materials and high capacity centrifugal screeners for nonmagnetic materials. In the case of solvent extracted meals with a very low residual oil content, it is usually customary to blend back refinery residues to supply a small amount of oil to enhance pellet quality and equipment capacity. Various oilseed meals exhibit different pelleting characteristics resulting in a wide variation in pellet mill performance.