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Quarries are investing in equipment to increase production and improve the quality of their products, such as vertical shaft impact crushers (VSI). This type of crusher works with autogenous comminution of the material to improve the shape and size of the aggregates.

Course and fine aggregates represent 60 to 80% of the volumetric composition of a Portland cement concrete, 30% being occupied by fine aggregate. Globally, the employment of crushed fine aggregates by concrete batching plant has been growing. Factors such as constancy within the granulometric properties, low level of impurity and smaller environmental impact, when compared to fine aggregates coming from rivers and excavations, have been contributing to this growth. However, despite the advantages mentioned, crushed aggregates present disadvantages related to the shape of the grains.

With the objective of improving this deficiency and increasing the consumer’s market, the companies that produce crushed aggregates (quarries) have invested in technology and new equipment. The main highlight has been the use of more modern and efficient crushers, trying to correct the shape of the grains, the granulometric composition and even the quality of the fine aggregates. Within this context, there is the increasing use of the vertical shaft impact crushers (VSI).

The principle of comminution of the VSI type crushers is based on the acceleration of the particles through a central rotor that rotates in high velocity. The grains enter the central part of the rotor and are thrown by the centrifugal force to the sides of the equipment, at velocities that may go up to 90 m/sec (approximately 320 km/hr), possibly colliding with the bed formed by the accumulated aggregates in the carcass of the crusher, or in the aggregates that fall in the form of a lateral cascade. The reduction of the particles occurs due to the collision of the rock against rock, generating a greater quantity of fine particles and an aggregate with a more cubic shape when compared to the aggregates obtained in the compression cone crushers. Normally the VSI crushers are employed in quarries as tertiary and quaternary crushers, for the exclusive production of crushed fine aggregate or for the reduction of the dimension of a material that eventually presents small commercial value.

VSI crushers generally utilize a high speed spinning rotor at the center of the crushing chamber and an outer impact surface of either abrasive resistant metal anvils or crushed rock. Utilizing cast metal surfaces 'anvils' is traditionally referred to as a "shoe and anvil VSI". Utilizing crushed rock on the outer walls of the crusher for new rock to be crushed against, known as “autogenous” crushing, is traditionally referred to as "rock on rock VSI".

The principal uses of VSI are in quarries and gravel pits to produce products for roads and asphalt, ballast, manufactured sand for concrete and aggregates for use in the construction industry. But VSIs are also used for the recycling of building material, namely concrete and asphalt and the processing of steel slag. VSI crushers are a crucial part of making industrial sand, which is then used to manufacture products such as abrasives, ceramics, glass, paint, sealants, and more.

Preferred for more Cubical shape aggregates

The VSI crushers utilize velocity rather than surface force as the predominant force to break rock. In its natural state, rock has a jagged and uneven surface. Applying surface force (pressure) as in compression crushers’ results in unpredictable and typically non-cubical resulting particles. Utilizing velocity rather than surface force allows the breaking force to be applied evenly both across the surface of the rock as well as through the mass of the rock. Rock, regardless of size, has natural fissures (faults) throughout its structure. As rock is 'thrown' by a VSI rotor against a solid anvil, it fractures and breaks along these fissures. As a result, the product resulting from VSI crushing is generally of a consistent cubical shape such as that required by modern superpave highway asphalt applications. Final particle size can be controlled by 1) the velocity at which the rock is thrown against the anvil and 2) the distance between the end of the rotor and the impact point on the anvil.

Best suited for RAP crushing

Crushing and screening RAP is a recycling application, which requires very different end product needs than virgin aggregate and even concrete crushing applications. Jaw and cone crushers are highly effective in crushing hard rock. RAP, however, is not hard rock, and the unique characteristics of RAP crushing can actually cause cone crushers to negatively impact product output.

As cone crushers crush and re-crush the RAP to break it apart, the asphalt can be literally stripped from the RAP, creating white rock, which will require the extra step and cost of recoating.

A jaw crusher in a RAP circuit is used for primary reduction, prepping it for the cone and requiring at least two crushers, and in some instances, a vertical shaft impactor (VSI) to make a specified product. A vertical shaft impactor in most RAP applications typically requires only one crusher. Two may be necessary where very high capacity or special products are required.

Cone crushers can work fairly well until heat is added into the equation, but as heat and pressure build, the RAP can become a hardened material, so compressed that it can become uncrushable, resulting in what is called “Bonne Float,” almost metal-to-metal compaction. The forces that can be created are directed down through the bearings and can destroy them, requiring extensive and expensive down time.

The VSI crusher breaks the interstitial lattice of the RAP’s binding matrix and creates a beneficiation of the original virgin aggregate—simply put, knocking off the arrowheads and cubing up the product. The result is a minimum of white rock and fines produced by the VSI, and a more structurally sound cubical product. Cone crushers tend to “pancake” the RAP and do little to reduce the elongated aggregate, rather than produce the cubical product required for highest quality recycling of RAP.

As the VSI separates the binding matrix, the material is aired out and moisture is released, making for a drier material going into the hot mix. Drier material saves energy as it eliminates the need for more fuel in the burner.