Comminution is the process by which mined ore is reduced in size to make for easier processing. The strict definition is ‘the action of taking a material, specifically mineral ore, and reducing it to minute fragments or particles’. This is typically achieved in several stages of a detailed and professional mining operation.
Some of the earliest versions of crushers were large stones used by humans to repeatedly hammer rocks placed upon an anvil (a hard stationary surface); the products made from this hammering action were transported by pack animals or humans in sacks. Historically, mining tasks were incredibly labour intensive, and the produced mined products were correspondingly expensive. The only tools available to breakdown ore were a drill bit, sledgehammer, or a pickaxe (and the will power of a few tireless workers). The majority of ore sizing and crushing operations were completed by hand until halfway through the 19th century. At this time, water powered trip hammers started assisting miners; it was roughly the beginning of the industrial revolution.
During the industrial revolution, commercial mining started seeing the use of explosives such as gunpowder at the heart of many mining operations; this mining method is known as ‘blasting‘ and it led to ever larger quantities of rock and minerals being liberated. Steam shovels were the next tool to revolutionise the mining industry. Over time, larger machines and more advanced mining techniques started making it possible to liberate significantly larger pieces of ore.
The demand for mined minerals and other mined by-products has not reduced over the past 150 years. To ensure supply could meet demand, many different crushing and conveying machines were invented. Without simultaneous advances within the fields of comminution and conveying, it would not have been possible to mine and convey materials safely and efficiently (even modern conveyors cannot convey single blocks of material weighing many tonnes).
Almost all quarry and mining operations today make use of crushers to reduce the size of larger materials; loose (smaller) sized materials do not typically require a crushing stage. When mining harder rock, jaw crushers, cone crushers, and/or gyratory crushers are usually employed.
Purpose of Crushers
A crusher is a machine designed to reduce the size of large rocks to smaller rocks, gravel, sand, or rock dust; this is essential for efficient transport of the product via conveyors etc. Crushing is the first of many stages that lead to separation of the mineral(s) from the waste (gangue) material. Waste material can be discarded or recycled allowing the mineral rich product to be further processed at the main plant.
Various types of crusher and mineral separator may be employed depending upon the throughput, hardness, and properties of the mineral being processed. In all cases, the crushing stage is essentially achieved by transferring a mechanically amplified force (via mechanical advantage) to a material, to breakdown the bonds which hold the material together.
Crushing is achieved by passing the feed between two solid surfaces, then by applying sufficient force to bring the surfaces together so that the molecules of the material being crushed are separated from (fracture), or, change alignment in relation to (deform), each other.
Crushers are commonly classified by the degree to which they fragment the starting material, with primary and secondary crushers handling coarse materials, and tertiary and quaternary crushers reducing particles to finer gradations. Each crusher is designed to work with a certain maximum size of raw material, and often delivers its output to a screening machine (screener) which sorts and directs the product for further processing. In many cases, initial crushing stages are followed by further milling stages (if the materials need to be further reduced); see our ball mill article for further details.
There are three common crushers used at mining and processing plants, these designs are:
- Gyratory Crusher
- Jaw Crusher
- Cone Crusher
Crusher Types (jaw, cone, gyratory)
Typically, the initial crushing stage is completed using either gyratory crushers or jaw crushers. It is often the case that there will be only one crusher installed, and this will be referred to as the ‘primary crusher’.
Cone crushers are more often used for 2nd, 3rd & 4th stage crushing steps (although not always).
Jaw Crusher Components
The main components of a typical jaw crusher are:
- Fixed Jaw
- Moving Jaw
- Jaw Liners
- Main Shaft (Pitman Assembly)
- Drive Pulley & Flywheel
- Toggle Plate(s) & Tension Rods/Assembly
Jaw Crusher with Single Toggle Plate Design
Jaw Crusher Classification
Jaw crushers are classified based on the position of the pivoting swing jaw. There are three main jaw crusher types:
- Blake crusher – the swing jaw is fixed at the lower position.
- Dodge crusher – the swing jaw is fixed at the upper position.
- Universal crusher – the swing jaw is fixed at an intermediate position.
The Blake crusher was first patented by Eli Whitney Blake in 1858 and it is the most common type of jaw crusher employed today. The Blake type jaw crusher has a fixed feed area and a variable discharge area. Blake type crushers come in various sizes and are commonly used for primary and secondary crushing roles.
How Jaw Crushers Are Sized
Jaw crushers are sized based upon the dimensions of the top feed inlet (gape) or the dimensions of the jaws. For example, a 32 x 54 sized jaw crusher will measure 32 inches from the movable to stationary jaw (when measured at the top i.e. the opening), and each jaw will have a 54-inch width. If a jaw crusher is rated by jaw plate size, a suitable rating maybe 600 x 400, which indicates a 600 mm by 400 mm jaw plate dimension. Sizes may be given in imperial (inches etc.) or metric (millimetre etc.).
How Jaw Crushers Work
A jaw crusher uses compressive force for breaking ore pieces. Compressive force is applied by two jaws (dies), one of the crusher jaws is stationary (fixed jaw) whilst the other is moving (swing jaw).
Material is fed into the top feed opening (gape) and gradually moves downwards towards the lower discharge outlet. As the materials passes towards the outlet, it is crushed between the stationary and moving jaws. The v-shaped area between the two jaws is referred to as the ‘crushing chamber’. Because the space between the two jaws becomes narrower towards the discharge outlet, the material size is progressively reduced.
The gap between the jaws at the discharge outlet dictates the material output size. A typical jaw crusher will have a crushing ratio of between 6:1 to 8:1 i.e. the feed material size is reduced by a factor of 6 or 8.
Jaw Crusher Operating Principle