Mobile Plant Crushing

The crushing of borrow pit material with a mobile crushing plant is becoming more popular for unsealed road material supply. This may be as a single crusher unit or, in the other extreme, a multi-stage crushing and screening plant. This can depend not only on what end-product is required and the economics of supply but also on the type of plant that is available at the time required. This operation permits high production rates (up to 400 tonne per hour) whilst also producing a consistent, graded product which is often easier to handle and place.

Further details regarding quarrying and aggregate production can be found in: <www.in.gov/dot/div/testing/manuals/aggregate/chapter 05.pdf>

Mobile crushing plants are commonly available either as self contained track-mounted, or semi- trailer modules.

There are six principal types of rock crushers used in the manufacture of road construction materials:

 jaw crushers  gyratory crushers  cone crushers  impact crushers  hammer mill crushers

All crushing relies on either compressing rock particles between two metal surfaces or by the high- speed impact on, or by, rock particles against hard surfaces. Depending on the type of material being crushed, there are some characteristics, such as particle shape and Atterberg Limits, which can be influenced by the inherent nature of the rock. The selection of the appropriate crusher type can modify these characteristics to some extent in the final product.

Jaw Crusher

The basic style of crusher is the jaw crusher (Figure 5.4). It consists of two hardened metal plates with a tapering gap between them. One metal plate is fixed (fixed jaw) and the other (swing jaw) oscillates, causing the taper to alternately open and close. In simple terms, the feed particles fall into the taper to the point where the open jaw separation matches their size; as the taper then closes, the particle is compressed and fractures. The broken particles then drop further down the taper either to be caught again or eventually fall through the gap at the bottom of the taper. The eventual maximum size of the material is controlled by the gap. Some particle-to-particle crushing occurs in the process; this is more likely to occur if the crushing chamber is kept full.

Source: Pennsylvania Crusher Corporation

Figure 5.4: Jaw crusher

Gyratory Crusher

The gyratory crusher (Figure 5.5) uses an eccentrically-mounted tapered spindle which rotates within an inverted static cone. The rotary oscillation of the spindle causes a progressive rotary closure of the gap between the cone and the spindle. The profile between the crushing surfaces is similar to that of the jaw crusher and the crushing process is similar.

Source: FLSmidth Minerals Source: Metso Corporation Figure 5.5: Gyratory crusher

Cone Crusher

Cone crushers (see Figure 5.6) operate in a somewhat similar fashion to the gyratory crusher. However, a significant difference is the shape of the crushing surfaces (cone and mantle) and the crushing chamber. The longer chamber shape and flatter lying orientation causes a higher degree of stone-to-stone contact which results in the production of finer particles by grinding action rather than breakage by direct particle compression. This type of compression crusher is considered to be more suited for the production of more material in the fine particle range as well as more equant-shaped particles. A variety of cone and mantle profiles are available to suit the properties of various rock types and perhaps modify their inherent crushing characteristics according to product requirements. Cone Feed Feed Discharge Sizing gap Discharge Eccentric rotation Cone Feed Feed Discharge Sizing gap Discharge Eccentric rotation

Source: Boral Australian Construction Materials Figure 5.6: Cone crusher

Impact Crushers

Impact crushers rely on the high-speed impact of rock particles against a hardened metal surface (Figure 5.7). This can be achieved either as a hammer (or bar) striking the rock particle or the particle, having been accelerated, striking a static anvil. According to the strength and structure of the particles, the impact causes fracture or (partial) pulverisation of the particle. Pulverisation tends to cause the rounding of particles with finer-sized material being the result of the breakdown. Impact crushers are particularly susceptible to abrasive material and can suffer high wear rates.

Source: Pennsylvania Crusher Corporation

Figure 5.7: Impact crusher

Hammer Mill Crushers

Hammer mill crushers consist of a series of hammers or bars attached to a rapidly-rotating horizontal shaft. Particles fed into the crusher are struck by the hammers and, consequently accelerated by the impact, will strike a static anvil. The impacts can cause breakage or pulverisation of the particles. Particle size control can be adjusted by controlling the size of a discharge aperture.

Vertical Shaft Impact (VSI) Crushers

VSI crushers consist of a rapidly-rotating, vertically-mounted rotor into which rock material is fed (Figure 5.8). The rotation accelerates the particles horizontally through discharge ports in the rotor causing them to impact against an anvil surface. With high-speed rotation, the rock discharge from one port will strike rock that has been discharged from a previous port, thus causing a high level of rock-to-rock impact. A modified version of the VSI can involve feed rock cascading through the impact zone; a similar end result will be achieved.

Source: Boral Australian Construction Materials

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STABILISATION OF UNSEALED ROADS