Reducing Raw Material Size | A Crucial Operation
A crusher is a machine designed to reduce large-sized rocks into smaller pieces. Crushers are used to reduce the size so that the designated machines can handle the material. Crushing is a process of transferring a force amplified by mechanical means through a material made of particles that bond together more strongly and resist deformation more than those in the material being crushed do.
Crushing devices hold the material between two parallel or tangent solid surfaces, and apply sufficient force to bring the surfaces together to generate enough energy within the material being crushed so that its particles separate from (fracturing), or change alignment in relation to (deformation), each other. The earliest crushers were handheld stones, where the weight of the stone provided a boost to muscle power, used against a stone anvil.
In the cement industry, a crusher plays a very important role in bringing down the size of various materials like limestone, iron ore, and coal etc., to a manageable level for further processing. The main objective of the crushing process is to reduce material size suitable to be moved on a conveyor, and suitable to be fed to a raw mill. To achieve this objective, more than one crushing stage might be necessary (primary, secondary, and tertiary).
The type of crusher to be used will depend on the kind of force applied to get the desired result. It can be classified as impact and hammer crusher if impact force is used. If shear force is used, then it can be a roller or sizer crusher. If compression force is used, it can be either a gyratory or jaw crusher. However, the type of crushers that suit a particular operation depends on several factors like:
- Feed size (top size, percentage of fines)
- Product size (ball mill: 30 - 50 mm, VRM: 3-5 per cent of roller diameter)
- Rock hardness, work index
- Abrasiveness (silica content)
- Stickiness (moisture, mineralogy)
- Throughput (tonnes per hour)
- Power consumption
Depending on the feed size and the product size, the stages of crushing can be primary, secondary and tertiary. The other properties to be borne in mind while selecting crushers are abrasiveness, stickiness and hardness of a material.
For deciding on the size of a crusher, one will have to go back from kiln capacity to the limestone requirement. Once the throughput of the kiln is known and if the plant is going to have a single shift or two shift operations at the quarry, the size of a crusher can be decided. On a few occasions, it has been experienced that availability of spare parts and maintainability of a machine also play an important role in the selection process.
There are mobile crushing plants in operation all around the world. It was thyssenkrupp that pioneered mobile crushing plants, making it possible to crush minerals directly at the site of extraction and synchronise the process to the rate of mining advance, into sizes ready for conveyor transport of the crushed material from the mining face to the processing plant. The crushing capacity of these machines has been around 500 to 5,000 tonnes/hour.
Mobile crushing plants are built ready-mounted on their own travel gear. Semi-mobile plants, on the other hand, are moved from one location to the next as a piggyback load on separate haul units. Each crushing plant comes ready to operate with its own feed hopper, feed conveyor, discharge conveyor and material transfer belt. The size and design of the crusher naturally depend upon the work that it is intended for.
There is also a concept of semi-mobile crushing plants. These are generally to be recommended when a change of location in long periods is required in order to optimise raw material transportation and operational costs of the quarry department. The cement industry is yet to try out these options.