Aggregate refers to a variety of natural or recycled manufactured materials that form a blend, with particle sizes ranging from a few tenths of an inch (a few millimeters) to a few inches (several centimeters) in diameter. The most common materials are sand and gravel extracted from old or current stream flows and crushed rock or recycled materials, all of which are used in multiple applications, including concrete, road base and soil conditioning. Types of aggregate equipment include cage mills, hammer mills and impactors. Jaw crushers, roll crushers and gyratory crushers are in this category as well. Aggregate equipment performs size reduction, size classification, transportation and blending work.
Size reduction of the feed material is the main purpose of aggregate equipment. The material is reduced in size by either impact or compression forces. Impact forces are applied over a short interval while compression forces are applied more uniformly and more slowly. Impact force equipment is used more frequently in the initial stages of size reduction. Compression force equipment is used at all stages in size reduction.
Impact aggregate equipment includes hammer mills, impactors and cage mills. In a hammer mill, the material is fed into a chamber with high-speed rotating hammer heads that collide with the material, shattering it. As the particles scatter, they impact stationary breaker bars on the outside walls of the chamber before being discharged out the bottom. Impactors slam a piston either vertically or horizontally into the material to be crushed. These are usually batch-mode machines and are used with the largest sizes of feed.
In cage mills, the material is fed into the center of the vertically mounted disk-shaped machine and hurled against the sides of a spinning cage. The cage walls are made of massive bars spaced closely together. As the material particles fly from one side to the other, impacting both the bars and other particles, they break apart until some of the particles are small enough to pass between the bars. Once outside the cage, the material smashes against breaker bars on walls of the chamber. The material, not having enough kinetic energy to resist gravity, falls out of the bottom, is screened, and large chunks are recycled into the feed stream of the cage mill.
Compression crushers are comprised of a variety of aggregate equipment. This category includes jaw crushers that work similar to common nutcrackers. Roll crushers compress the material between two rolls rotating inward against each other. Cone or gyratory crushers compress the feed material between two conical surfaces. The cone shape enables the force to be consistently maintained, even as the particle size becomes smaller.
Associated aggregate equipment includes classifiers that separate the particle sizes of the exit streams from the size reduction equipment. These may be shaken screens or may consist of perforated conveyors. Conveyors are very important at aggregate sites and are often mobile units that can link different size reduction pieces of equipment together to make a process stream. Blending may be accomplished through successive passes of the input feeds through hoppers with static blades in the interior or by large rotating blades in bins.