Stone quarry process line
The primary crusher is fed via a chute and vibrating feeder . The base of the feeder is made of steel “grizzly” bars and it is here that the first screening operation is actually done. Fine material and dust produced by the blast, along with any remaining subsoil or weathered rock from the top of the quarry face, drops through the bars onto a separate conveyor belt and onto a stockpile. This screened material is called scalpings and is used as rock fill.
Primary crushing is usually by a jaw crusher consisting of a heavy metal plate which moves backwards and forwards against a fixed plate (these are the “jaws”). The moving plate is kept in motion and given its crushing energy by a large flywheel. The crusher is wider at the top than at the bottom. Rock from the quarry face is fed into the top of the crusher and crushed rock falls out of the bottom of the jaws. The size of the crushed stone which passes through the jaws is partly governed by the gap set at the bottom of the jaws, though larger size rocks can pass through if the rock being crushed is slabby or elongate in shape. Large scale gyratory crushers can also be used.
The output from the primary crusher is conveyed onto the primary stockpile from which the secondary crusher is fed. There is a screen house just after the secondary crusher which screens out small size crushed stone and dust onto blinding stockpiles. The larger sized stones pass through to the final crushing stages where they are fed through a series of cone crushers and screens. The output from the final cone crushers is conveyed to a screen house where large multiple deck screens sort the crushed stone into the required aggregate sizes.
Secondary, tertiary and quaternary crushers are generally gyratory, or cone, crushers. These operate on the principle of a steel mantle mounted on an eccentric bearing and vertical shaft assembly. Rotation of the eccentric assembly makes the mantle gyrate within a static outer concave. There is a gap between the mantle and the concave. The shape of the gap is tapered towards the base. As the mantle gyrates inside the concave, the gap between it and the concave at any one point opens and closes on each gyration, this produces the required crushing action. Stone is fed in at the top and crushed product falls out from the bottom of the cone. The mantle can be raised or lowered within the concave, allowing the gap, and therefore the size of the crushed product, to be varied to a limited degree. If the crusher is jammed by a stray bit of steel,
e.g., a digger bucket tooth, the mantle automatically moves down to clear the obstruction.
Each stage of crushing produces progressively smaller sized stones. In order to produce a usable end-product, the crushed rock has to be screened into various size categories. Crushed and screened rock is called aggregate. Screening is carried out at various stages in the crushing process. Screens are basically box frames into which sheets of screen meshes of the required apertures are inserted, clamped and tensioned. Screens are usually “multi-deck”, i.e., two or more screen meshes are stacked vertically within the screen frame. The whole screen is coupled to its support frame by springs or resilient rubber mountings. Screens are made to vibrate by a rotating transverse shaft. The shaft is machined to be unbalanced, and when driven by an electric motor by v-belts, the required vibratory motion to agitate the aggregate is imparted. Screen decks are mounted at an angle so that the aggregate moves down them. Aggregate is fed onto the high end of the top deck and the vibration causes the aggregate to jiggle down the screen until it either drops through a mesh aperture or falls off the end of a deck. The aggregate is then sorted or ‘screened’ according to the mesh sizes fitted, from large aperture mesh at the top, to small aperture mesh at the bottom.
Final screens are typically mounted in a screen house over large bins or hoppers into which the different sizes or grades of aggregate are fed. The hoppers are raised on legs so that trucks can drive under them to be loaded. Material is continually drawn from the storage bins for immediate use (e.g. in a coating plant) or for transfer, either by dump truck or conveyor, to storage stockpiles in the quarry