What is Crushing?
Crushing is the first mechanical stage in the process of comminution in which the main objective is the liberation of the valuable minerals from the gangue. It is generally a dry operation and is usually performed in two or three stages.
What are the stages of Crushing and why?
For the purpose of mineral extraction when ore is excavated from earth the size of run of-mine ore can vary a lot between fines powders to big boulders of 1.5 m across. Moreover the feed can also be intermittent type in the form of dumpers feeding the crushing plant. The crusher at this stage is called primary crusher the main purpose of which is to give continuous feed to the plant down the line in a size convenient for further handling in the process plant.
In the next stage further crushing may be required so as to ensure proper releasing of mineral from gangue. The crusher in this stage is called secondary crusher.
There can be also another stage of crushing called tertiary crushing depending on the initial size of the run of mine ore and the final product size requirement.
Different stages are required as total size reduction in one single crusher is not possible efficiently. Moreover smaller sizes of material tend to jam the flow of ore in the crusher and unless there is sufficient space for the crushed ore to expand in volume there will be choking of the crusher. Hence vibrating screens are deployed in between different stages of the crushers so that only those oversize ore can be fed to the crusher whose sizes need to be reduced. This also helps in reducing the capacity requirement of the crusher as total run of mine ore need not be fed to the crusher. Also there will be closed circuit operation in case of secondary and tertiary crushers so as to avoid passing of any oversize material to the final stage. Primary crushing is done in open circuit only.
What are the types of Crushers?
There are two main types of primary crushers in metalliferous operation- jaw and gyratory crushers. There is also some limited use of impact crushers as primary crushers.
The bulk of secondary and tertiary crushing of metalliferous ores is performed by cone crushers. Sometimes crushing rolls and hammer mills are also used for secondary crushing.
What is a Jaw Crusher and how it works?
The distinctive feature of jaw crusher is the presence of two plates which open and shut like animal jaws. The jaws are set at an acute angle to each other, and one jaw is pivoted so that it swings relative to the other fixed jaw. Material fed into the jaws is alternately nipped and released to fall further into the crushing chamber. Eventually it falls from the discharge aperture. Time must be given for the rock broken at each “bite” to fall to a new position before being nipped again. The ore falls until it is arrested. The swing jaw closes on it, quickly at first and then more slowly with increasing power towards the end of the stroke. The fragments now fall to a new arrest point.
All jaw crushers have a heavy flywheel attached to the drive, which is necessary to store energy on the idling half of the stroke and deliver it on the crushing half. Since the jaw crusher works on half-cycle only, it is limited in capacity for its weight and size. Due to its alternate loading and release of stress, it must be very rugged and needs strong foundations to accommodate the vibrations. The discharge size of material from the crusher is controlled by the set, which is the maximum opening of the jaws at the discharge end. This can be adjusted by using toggle plates of the required length. The speed of jaw crushers varies inversely with the size. The main criterion in determining the optimum speed is that particles must be given sufficient time to move down the crusher throat into a new position before being nipped again.
What is a Gyratory Crusher and how it works?
The gyratory crusher consists essentially of a long spindle, carrying a hard steel conical grinding element, the head, seated in an eccentric sleeve. The spindle is suspended from a “spider” and, as it rotates, it sweeps out a conical path within the fixed crushing chamber, or shell, due to the gyratory action of the eccentric. The spindle is free to turn on its axis in the eccentric sleeve, so that during crushing the lumps are compressed between the rotating head and the top shell segments, and abrasive action in a horizontal direction is negligible. As in the jaw crusher, maximum movement of the head occurs near the discharge.
At any cross-section there are in effect two sets of jaws opening and shutting like jaw crushers. In fact, the gyratory crusher can be regarded as an infinitely large number of jaw crushers each of infinitely small width. Since the gyratory crushes on full cycle, it has a much higher capacity than a jaw crusher of the same gape, and is usually favoured in plants handling very large throughputs. Large gyratory crushers often dispense with expensive feeding mechanisms and are often fed direct from trucks. They can be operated satisfactorily with the head buried in feed. Choked crushing is encouraged to some extent so that the rock-to-rock crushing produced in the primaries reduces the rock-to-steel crushing required in the secondary crushing stage, thus reducing steel consumption. Operating crushers under choke feeding conditions gives more even mantle wear and longer life. Gyratory crushers are mostly used in surface crushing plants.
What is a Cone Crusher and how it works?
The cone crusher is a modified version of gyratory crusher. The essential difference is that the shorter spindle of the cone crusher is not suspended, as in the gyratory, but is supported in a curved, universal bearing below the gyratory head or cone. Power is transmitted from the source to the countershaft through a V-belt or direct drive. The countershaft has a bevel pinion pressed and keyed to it, and drives the gear on the eccentric assembly. The eccentric has a tapered, offset bore and provides the means whereby the head and main shaft follow an eccentric path during each cycle of rotation.
Unlike a gyratory crusher, which is identified by the dimensions of the feed opening and the mantle diameter, a cone crusher is rated by the diameter of the cone lining. There are two types of cone crushers: the Standard for normal secondary crushing and the Short-head for fine, or tertiary duty. They difference is mainly in the shape of their crushing chambers.
The Standard cone has “stepped” liners which allow a coarser feed than in the Short-head. The Short-head has a steeper head angle than the Standard, which helps to prevent choking from the much finer material being handled. It also has a narrower feed opening and a longer parallel section at the discharge. The distributing plate on the top of the cone helps to centralise the feed, distributing it at a uniform rate to the entire crushing chamber.
An important feature of the crusher is that the bowl is held down either by an annular arrangement of springs or by a hydraulic mechanism. These allow the bowl to yield if “tramp” material enters the crushing chamber, so permitting the offending object to pass. If the springs are continually “on the work”, as may happen with ores containing many tough particles, oversize material will be allowed to escape from the crusher. This is one of the reasons for using closed-circuit crushing in the final stages.
What are other types of Crushers?
Besides the above common types of crushers there are other varieties of crushers, which are mentioned below for academic interest only.
Gyradisc crusher – This is a specialised form of cone crusher, used for producing very fine material, and such crushers have found application in the quarrying industry for the production of large quantities of sand at economic cost.
Rhodax crusher – This is a specialised form of a cone crusher, referred to as an inertial cone crusher. Developed by the FCB Research Centre in France, the Rhodax crusher is claimed to offer process advantages over conventional cone crushers and is based on inter-particle compression crushing.
Roll crusher – These are used in mills where handling of friable, sticky, frozen, and less abrasive feeds, such as limestone, coal, chalk, gypsum, phosphate, and soft iron ores is being carried out. Jaw and gyratory crushers have a tendency to choke near the discharge when crushing these types of friable rocks.
Unlike jaw and gyratory crushers, where reduction is progressive by repeated pressure as the material passes down to the discharge point, the crushing process in rolls is one of single pressure.
Impact crushers – In these crushers, comminution is by impact rather than by compression. Sharp blows are applied at high speed to free-falling rock. The moving parts are beaters, which transfer some of their kinetic energy to the ore particles on contacting them. The internal stresses created in the particles are often large enough to cause them to shatter. These forces are increased by causing the particles to impact upon an anvil or breaker plate.
What is Grinding?
Grinding is the last stage in the process of comminution; in this stage the particles are reduced in size by a combination of impact and abrasion, either dry or in suspension in water. It is performed in rotating cylindrical steel vessels which contain a charge of loose crushing bodies – the grinding medium- which is free to move inside the mill, thus comminuting the ore particles. This is necessary for getting proper size of the ore so that the mineral can be separated form it economically. Grinding is the most energy consuming process, hence its use need to be properly justified.
What are the methods of Grinding?
According to the ways by which motion is imparted to the charge, grinding mills are generally classified into two types: tumbling mills and stirred mills.
In tumbling mills the mill shell is rotated and motion is imparted to the charge via the mill shell. The grinding medium may be steel rods, balls, or rock itself. Tumbling mills are typically employed in the mineral industry for coarse-grinding processes, in which particles between 5 and 250 mm are reduced in size to between 40 and 300 µm.
In stirred mills the mill shell is stationary and motion is imparted to the charge by the movement of an internal stirrer. Fine grinding media inside the mill are agitated or rotated by a stirrer, which typically comprises a central shaft to which are attached pins or discs of various designs. Stirred mills find application in fine (15-40 µm) and ultra-fine ( < 15 µm) grinding.
What is a Tumbling mill and how it works?
Tumbling mills are of three basic types: rod, ball, and autogenous. Structurally, each type of mill consists of a horizontal cylindrical shell, provided with renewable wearing liners and a charge of grinding medium. The drum is supported so as to rotate on its axis on hollow trunnions attached to the end walls. The diameter of the mill determines the pressure that can be exerted by the medium on the ore particles and larger the feed size the larger needs to be the mill diameter. The length of the mill, in conjunction with the diameter, determines the volume, and hence the capacity of the mill. The feed material is usually fed to the mill continuously through one end trunnion, the ground product leaving via the other trunnion, although in certain applications the product may leave the mill through a number of ports spaced around the periphery of the shell. All types of mill can be used for wet or dry grinding by modification of feed and discharge equipment.
In rod mill the tumbling media is rods. The distinctive feature of a rod mill is that the length of the cylindrical shell is between 1.5 and 2.5 times its diameter, this is necessary to prevent the rods from turning within the shell and becoming wedge.
In ball mill the tumbling media is steel balls. Since balls have a greater surface area per unit weight than rods, they are better suited for fine finishing.
In autogenous grinding mills the tumbling media is the ore itself. In semi-autogenous grinding mills steel balls are utilised in addition to the natural grinding medium.
The main advantages of autogenous or semi autogenous mills are their lower capital cost, the ability to treat a wide range of ore type including sticky and clayey feeds, relatively simple flow sheets, the large size of available equipment, lower manpower requirements, and reduced grinding media expense.
What is a Stirred mill and how it works?
In contrast to tumbling mills where motion is imparted to the charge via the rotational mill shell, in stirred mills motion is imparted to the charge by the movement of an internal stirrer while the shells are stationary. The stirred mills can be vertical type as in Tower mill or they can be horizontal as in IsaMill. In tower mill steel balls or pebbles are placed in a vertical grinding chamber in which an internal screw flight provides medium of agitation. In IsaMill there are rotating grinding discs mounted on a shaft which is coupled to a motor and gearbox. The grinding discs agitate the media and ore particles.