Crushing and conveying
At the Martha Mine rock is crushed and then transported from the open pit to the processing plant and waste rock embankments by an overland conveyor.
The crushing system at the Martha Mine open pit consists of a combination of two types of crushers, a jaw crusher and two Stamler feeder breakers. The jaw crusher is capable of crushing material with a strength of over 150 megapascals (MPa). By way of comparison, concrete has a strength of 20-30 MPa. The Stamlers deal with the softer material. The crushers are located below ground level, in a slot to minimise noise effects.
The jaw crusher
The jaw crusher operates by the mechanism of two large plates of steel moving towards each other on a cyclic basis to break rock from a large size to a smaller size. The plates are positioned so that the gap at the top is larger than the gap at the bottom.
Material is loaded into the crusher bin (1) by trucks or loaders. Prior to entering the jaw crusher the rock moves across a grizzly (2). The grizzly has a series of slots in it allowing the smaller pieces of rock to fall through (3). The remaining oversized material passes through the jaw crusher (4) and is sized down to less than 200mm. An electromagnet (5) removes tramp steel from the old workings. The material is transported (6) to the main overland conveyor via a transfer station.
The Stamler feeder breakers
The Stamler feeder breakers are used to break moderately hard and soft material. A rotating drum covered with replaceable carbide-tipped picks serves to break the rocks, and feed the material along the conveyor underneath.
Material is loaded into the crusher bins (1) by trucks or loaders.The material travels along a conveyor underneath the rotating drum (2). The picks fracture any material that is larger than the gap between the drum and the belt. An electomagnet (3) removes tramp steel from the old workings. The material is transported (4) to the main overland conveyor via a transfer station.
The conveying system
There are a number of conveyors required to ensure that following crushing, rock is transported to its correct destination. The largest conveyor on site (the overland conveyor) transports the ore from the open pit to the processing plant, and the waste rock to the waste rock embankments.
Transfer stations move rock from one conveyor to the next. For example a transfer station is located close to the processing plant, so that ore can be directed from the main overland conveyor to the processing plant, and waste can be directed to the waste rock embankments. Similarly a transfer station is sited at the surface facilities area in the open pit to transfer rock from the crusher conveyor to the main overland conveyor.
Often the rock may contain steel and timber from the old workings. Electromagnets exist at a number of points around the crushers and transfer stations to pick up steel, which can damage the conveyors. The wood and steel removed is referred to as tramp material, and is transported off site by truck. When rock contains a large proportion of timber and steel, it is hand sorted.
The main overland conveyor
After the rock is crushed, the main overland conveyor (CV2) separately transports the ore and waste rock 1.7 km to the transfer station adjacent to the processing plant, and waste rock is transferred to another conveyor (CV8) that takes it an additional kilometre to the waste rock embankments. Radial stackers enable different types of material to be blended or stockpiled separately as required. At the waste rock embankments, the material can also be loaded directly onto trucks. A 120 tonne truck can be filled in less than one minute.
The main overland conveyor has a capacity of 6,000 tonnes per hour. This capacity makes it one of the largest mining conveyors in Australasia.
It has a width of 1.35m and travels at a speed of 19 km/h. The belt is 20mm thick, constructed of rubber with steel cords, and has a mass when empty of 45 kg/m.
The conveyor is controlled by six specialised industry computers (PLCs) that are linked together by a fibre optic cable. These computers control the conveyor drives which need to be synchronized so that they work together, particularly when the conveyor is stopped and started. Various weighing and metal detecting devices also exist along the belt.
A number of counterweights at the drive end of the transfer stations keep the correct tension on the belt. These range from a mass of eight to 50 tonnes.
Water is used to clean the belts. This water is continually recycled and topped up as necessary via conveyor belt wash ponds. It is necessary to clean the belt, using high pressure sprays, to stop mud building up on the chains and pulleys and so prevent the belt from slipping.
A closed circuit television system allows the operator, located at the jaw crusher, to monitor what is happening along the crushing/conveying system, and take action as necessary. Cameras are set up to observe chutes, bins, hoppers and stockpiles, and the information is relayed back to the operator.