Aggregate equipment includes various machines used to reduce, grade, convey, and mix materials such as sand, gravel, and crushed rock. Impact and compression forces are used to reduce material size. Types of equipment include hammer mills, impactors, cage mills, jaw crushers, roller crushers, and gyratory crushers. Classifiers and conveyors are also important in the process.
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 mined from old or current stream flows and crushed rock or recycled materials, all used in a variety of applications including concrete, roadbed and soil conditioning. Types of aggregate equipment include cage mills, hammer mills, and impactors. Jaw crushers, roller crushers and gyratory crushers also fall into this category. The aggregate equipment performs size reduction, size grading, conveying and mixing work.
Reducing the size of the feed material is the main purpose of the aggregate equipment. The material is reduced in size by impact or compressive forces. Impact forces are applied over a short range while compressive forces are applied more evenly and slower. Impact force equipment is used most frequently in the initial stages of size reduction. Compression force equipment is used in all stages of size reduction.
Impact aggregate equipment includes hammer mills, impactors, and cage mills. In a hammer mill, material is fed into a chamber with high-speed rotating hammer heads which collide with the material, crushing it. As the particles disperse, they strike the fixed breaker bars on the outer walls of the chamber before being discharged to the bottom. Impactors slam a piston vertically or horizontally into the material to be crushed. These are usually batch mode machines and are used with the largest feed sizes.
In cage mills, material is fed into the center of the vertically mounted disc-shaped machine and hurled against the sides of a rotating cage. The walls of the cage are made of massive bars spaced apart from each other. As particles of material fly from side to side, impacting both the bars and other particles, they break apart until some particles are small enough to pass between the bars. Once out of the cage, the material slams into the breaker bars on the chamber walls. The material, not having sufficient kinetic energy to resist gravity, falls to the bottom, is screened and large chunks are recycled back into the cage mill feed stream.
Compression crushers consist of a variety of aggregate equipment. This category includes jaw crushers that function similar to common nutcrackers. Roller crushers compress material between two rollers that rotate inward against each other. Cone or gyratory crushers compress the feed material between two conical surfaces. The cone shape allows the force to be maintained constantly, even as the particle size becomes smaller.
Associated aggregate equipment includes classifiers that separate the particle size of the output streams from size reduction equipment. These can be shaken screens or can consist of perforated conveyors. Conveyors are very important at aggregate sites and are often mobile units that can link different size reduction equipment together to create a process flow. Mixing can be accomplished by successive passes of the incoming feeds through hoppers with static blades inside or by large rotating blades in containers.
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