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Crushers 

 

Summary

This review looks at the basic concepts of crushers, including their types, principle, construction and usage. It also tells the recent developments in the crushers manufacturing and provide links to different simulation packages available, university and research centers and important books and crusher suppliers.

INTRODUCTION

Crushing is the first step of mineral processing where the ore/rocks from the mine site is fed to mechanical equipment in order to reduce the size for subsequent stages and thus liberate the valuable mineral from the gangue. Mostly this stage is dry and can be performed in various stages (2-4).

The crushers are used to reduce the size of the ROM ore (as large as 1.5m) to a size that can be fed to ball mill (as small s 4-6mm). Depending upon the requirement there could be a primary, secondary, tertiary and quaternary stage. In most of the operations primary stage is performed at the mine site (surface or underground). Primary crushers are commonly designed to operate 75% of the available time, mainly because of interruptions caused by insufficient crusher feed and by mechanical delays in the crusher (Lewis et al., 1976).

A flowsheet for a crushing plant is shown in following figure:

As shown in the figure the crushers can be operated in open circuit or in closed circuit, mostly secondary and tertiary crushers are operated in closed circuit.

TYPES OF CRUSHERS

PRIMARY CRUSHERS
These are heavy duty machines used to reduce the size of ROM ore to a size manageable by secondary crushers or AG/SAG mills. They always operated in open circuit. Jaw and Gyratory crushers are two most important types of primary crushers but sometimes impact crushers are also used if the ore is soft.

Jaw Crusher
The jaw crusher reduces the size by the compression force when the rock is pressed against the two plates. It consists of a set of vertical jaws, one jaw fixed and the other moved back and forth relative to it by a cam or pitman mechanism mounted such that the stationary jaw provide the breaking surface and the moving jaw exert the compressive force. The rock remains in the jaws until it is small enough to pass through the gap at the bottom of the jaws. The speed of jaw crushers varies inversely with the size, and usually lies in the range of 100-350rev/min. 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. The jaws are usually constructed from cast steel and are fitted with replaceable liners, made from manganese steel, or "Ni-hard", a Ni-Cr alloyed cast iron. All jaw crushers are rated according to their receiving areas, i.e. the width of the plates and the gape, which is the distance between the jaws at the feed opening. For example, an 1830X1220mm crusher has a width of 1830 mm and a gape of 1220 mm. Also see this link: http://www.mine-engineer.com/mining/jaw_crusher.htm

Gyratory Crusher

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, normally between 85 and 150rev/min, it sweeps out a conical path within the fixed crushing chamber, or shell, due to the gyratory action of the eccentric.

The outer shells are usually constructed from heavy steel casting or welded steel plate and are fitted with reinforced alloyed white cast-iron (Ni-hard) liners.

SECONDARY AND TERTIARY CRUSHERS

Cone Crusher
A cone crusher is similar in operation to a gyratory crusher, with less steepness in the crushing chamber and more of a parallel zone between crushing zones. The essential difference is in the short spindle, which is not suspended, but instead supported, in a curved universal bearing located below the gyratory head or "cone". As rock enters the top of the cone crusher, it becomes wedged and squeezed between the mantle and the bowl liner or concave. Large pieces of ore are broken once, and then fall to a lower position (because they are now smaller) where they are broken again. This process continues until the pieces are small enough to fall through the narrow opening at the bottom of the crusher.

Roll Crusher
A roll crusher crushes using compression, with two rolls rotating about a shaft, towards the gap between the rolls. The gap between the rolls is set to the size of product desired, with the realization that the largest feed particle can only be 4 times the gap dimension.

The particles are drawn into the gap between the rolls by their rotating motion and a friction angle formed between the rolls and the particle, called the nip angle. Some major advantages of roll crushers are they give a very fine product size distribution and they produce very little dust or fines.

Coal is probably the largest user of roll crushers, currently, though. Coal plants will use roll crushers, either single roll or double roll, as primary crushers, reducing the ROM coal. Usually, these crushers will have teeth or raised forms on the face of the roll.

Impact Crusher
Impact crushers use impact rather than pressure to crush material. The material is contained within a cage, with openings of the desired size on the bottom, end, or side to allow pulverized material to escape. This type of crusher is usually used with soft material such as coal, seeds, or soft metallic ores.

Hammer Mill
A hammer mill crushes material that is friable, by impacting it against a rotating hammer. Then the material is forced against a rugged solid plate called a "breaker plate" which further degrades the particle size. Finally, the material is forced over a discharge grate by the hammers, where crushed finer particles drop through the discharge grate and larger particles travel around for another crushing cycle, until they fall through the discharge grid.

The hammers are made from manganese steel or, more recently, nodular cast iron, containing chromium carbide, which is extremely abrasion resistant. The breaker plates are made of the same material. The hammers can weigh over 100kg and can work on feed up to 20 cm. The speed of the rotor varies between 500 and 3000rev/min.

Rotary Coal Breaker
This breaker reduces the coal size by impact and also screens out the oversize particles. The breaker consists of a cylindrical drum fitted with perforated plates. The drum rotates at speeds of between 12 and 18rpm. All of the large-size rocks and any tramp material pass along the breaker, being harder or otherwise more resilient to breakdown than the coal, and are subsequently discharged. The coal being softer gets broken and due to reduced size passes through the openings and collected as undersize.

There are many other different kind of crushers developed depending upon the ore characteristics and energy requirements such as Gyradisc and Rhodax crushers (special form of cone crushers), Rotary impact mill, Tidco Barmac Crusher, reversible hammer mill etc.

Key variables for a crusher is its open and closed side settings, operating gap for roll crushers, particle nipping angle, resident time for a particle in the crushing chamber and feed size of the particle.

SELECTION OF THE CRUSHERS

The principal design parameters that drive crushing plant selection and configuration include:
  • Production requirements
  • Ore characteristics
  • Project location
  • Operational considerations
  • Climatic conditions
  • Capital cost
  • Safety and environment
  • Life of mine/expansion plans
  • Maintenance requirements

Including these considerations there are some technical data that are required for the sizing of a crushing unit that are:

Crushing Work Index test that determine the bond crushing work index that along with the Bond's Third Theory of Comminution is used to calculate the net power requirement of crusher sizing. It can also be used to determine the required open size settings (jaw crushers and gyratory crushers) or closed sized settings (cone crushers) for a given product size. Abrasion Index test that determines the liner wear in a crushing unit.

RECENT DEVELOPMENTS

The principle operating mechanism is still the same as it was earlier. The developments are related to the design of bigger crushers and such as hydraulic cone crusher made by Sandvic Sweden, cone crushers on Symons series made by Metso group Finland, the inertia cone crusher made by Sino and Russia in co-operation, miler crushers made by KHD group and Krupp Polysius Germany, vertical impact crushers of Nordberg barmac B series made by Metso group Finland.

Other recent developments are related to the design of more efficient liners such as the rubber liners and magnetic liners.

MODELLING AND SIMULATION SOFTWARE

Different software that are used to design and model a crusher and then predicting its output are:
  1. JKSimMet (multipurpose simulating software)
  2. Metso Minerals Co. has developed a crushing plant simulator named BRUNO.
  3. Sandvik Rock Processing AB, Sweden introduced a simulator PlantDesigner® for optimization and design of crushing and screening plant circuits.
  4. The University of Utah, USA has developed a simulator MODSIM.
  5. A GoldSim model was constructed to simulate the performance of a mine's crusher system.

BOOKS AND MAGAZINES

The most important book for understanding the basics of comminution is Mineral Processing Technology by B. A. Wills. Other important books are:

In addition, the following conferences on comminution, crushing and milling publish proceedings which are the most useful source for getting the recent developments in this field. The conferences include:

Courses on crushers from EduMine include:

  • The Mill Operating Resource - 1: Crusher Types and Operation
  • Grinding 2 - Unit Operations: Cone Crusher Principles of Operation
  • Gyratory Crushing - Fundamental, Unit Operations and Circuits
  • An Introduction to Mining and Mineral Processing, Part 3 - Mineral Processing Methods
  • UNIVERSITY AND RESEARCH

    The Julius Kruttschnitt Mineral Research Center in Queensland, Australia, is one of the leaders in comminution research. Other university and research center includes:

    There are many more universities and organizations involve in the development of advance comminution methods and simulation software. Most of the research is based on development of an effective flowsheet and energy efficiency.

    SUPPLIERS

    There are many suppliers around the globe. Some of the major manufacturers of crushers are:

    A lot many suppliers from china are emerging out providing wide variety of reliable crushers at a very cheap price. For more information on crusher suppliers, you can search for crushers in the Suppliers section of InfoMine or click here.

    CONSULTANTS

    There are numerous crusher consultants in the world that provide information regarding the sizing and selection of the crushers and services. Most of the time the manufacturer and the suppliers provide their own consulting services. However there are some liaison and free consultants also, some of them are:

    REFERENCES

    Lewis, F.M., Coburn, J.L., and Bhappu, R.B. (1976). Comminution: A guide to size-reduction system design. Min. Engg., 28(Sept.), 29.

    Wills, B.A. (2006), Mineral Processing Technology, Elsevier Science & Technology Books, 110-117

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