Mineral Crushing machinery and crushing operation

Sinonine know that as the ore crushing is performed in stages, crushing may be divided into primary, secondary, tertiary and quaternary stages based on the particle size. Correspondingly, the crushers can be classified into five groups according to the size of the product they produce.

---- Primary Crushers: Jaw crusher, Gyratory crusher.

----Secondary Crushers: Reduction gyratory, Cone crusher, Rolls crusher.

---- Tertiary Crushers: Short-head cone crusher.

----Fine Crushers: Impact crushers.

----Special Crushers: Bradford Breaker, Toothed Roll crusher.

Lumps of run-of-mine ore usually of 1 m size is reduced to 100–200 mm size in heavy duty primary crushers. The usual size of feed to secondary crushers is 600 mm and the product is usually 10–100 mm size. In tertiary crushers, particles of 250 mm size are reduced to 3–25 mm size.

Fine crushers reduce the coarse particle to fine, even to 200 mesh in some cases. Special crushers are designed for specific ores, for example, rotary breaker and toothed rolls crusher for coal and gravity stamps for gold ore milling.

1 JAW CRUSHERS

Jaw crushers consist of two jaw plates set at an acute angle, called angle of nip, to each other which forms a crushing chamber. One jaw is fixed and kept vertical, the other jaw is a movable or swing jaw and is moved to approach and recede alternately from the fixed jaw. Motion to the swing jaw is transmitted by pitman working on an eccentric and toggles. The material is fed between the jaws and is alternately nipped

and crushed. Single and Double Toggle Jaw crushers are shown in Figure1.

Figure 1 (a) Single toggle jaw crusher; (b) Double toggle jaw crusher.

Figure 2 Types of jaw crushers.

Swing jaw is pivoted at the top, it has a fixed receiving area and variable discharge area and is known as a Blake crusher. If the swing jaw is pivoted at the bottom, it has a fixed discharge area and variable receiving area and is known as a Dodge crusher. If the swing jaw is pivoted at an intermediate position, it has both variable receiving and discharge areas and is known as a Universal crusher.

In a Blake crusher, the distance between the two jaw plates at the feed opening is known as gape. The distance between the two jaw plates at the discharge opening is known as set. The minimum distance is called closed set and maximum distance is called open set. The maximum amplitude of swing of the jaw is known as throw. In

the mining industry, Jaw crushers are used to crush the run-of-mine ore to a size suitable for transportation. The reduction ratio of jaw crushers varies from 4 to 7.

2 GYRATORY AND CONE CRUSHERS

Gyratory and Cone crushers are of similar in construction and working. They consist of two vertical truncated conical shells of which the outer hallow conical shell is stationary and the inner solid conical shell is made to gyrate. In a gyratory crusher the inner conical shell is pointing up and outer conical shell is pointing down. The

reduction ratio varies from 3 to 10. Figure 3 shows the cut section of a gyratory crusher.

Reduction gyratory crusher is the modification of gyratory which has straight or curved heads and concaves and used for secondary crushing. The fine reduction gyratory crusher can also be used for tertiary crushing.

Figure 3 Cut section of  gyratory grusher.

(A) Gyratory; (B) Straight head and concave reduction gyratory; (C) Curved head and

concave reduction gyratory; (D) Standard cone; (E) Short-head cone; (F) Gyrasphere

 Figure4 Types of crushing chambers.

The Telsmith gyrasphere is another type having a spherical steel head and used for tertiary crushing. Figure 4 shows crushing chambers of all types of gyratory and cone crushers.

3 ROLL CRUSHERS

Roll Crusher (Figure 5) consists of two smooth heavy horizontal cylinders revolving towards each other and the feed material is nipped between the rolls and pulled downward through the rolls by friction. The distinguished feature of a roll crusher is that the material is crushed one time only whilst it is passing through the crushing chamber. Due to this fact, the reduction ratio of a roll crusher varies from 2 to 4, the lowest among all the crushers. Production of fines is minimum. They can handle friable, dry, wet, sticky, frozen, and less abrasive feeds well.

 Figure 5 Roll crusher.

Smooth-surfaced roll crushers are generally used for fine crushing whereas corrugated or toothed roll crushers are used for coarse crushing of soft materials. Single toothed roll crusher and Double toothed roll crusher are the two types of toothed roll crushers used for crushing coal.

4 IMPACT CRUSHERS

Impact crushers reduce the particles by impact forces applied through sharp blows of fixed or free swinging hammers revolving about central rotor at high speed to the free falling particles against stationary surfaces. They are used for relatively soft, friable, and sticky ores such as phosphates, limestone, clay, graphite and coal. Hammer mill (Figure 6) is one type of impact crusher.

Figure 6  Hammer mill

5 BRADFORD BREAKER

 Bradford breaker is a typical machine resembling cylindrical trommel screen in operation. It consists of a slightly inclined cylindrical chamber with a perforated wall (Figure7) and rotates about its axis at a low rpm. It is extensively used for primary crushing of run-of-mine coal.

Figure 7 Bradford breaker

The coal particles fed into the cylinder are lifted by longitudinal lifters within the cylinder and broken by falling and striking the coal below as the cylinder is rotated. The particles when broken to the required size quickly fall through the perforations thus produce few fines. Unbroken particles are discharged from the end of the breaker.

6 HIGH PRESSURE GRINDING ROLLS

In a roll crusher, the force of compression and friction makes the particles to crush. In High Pressure Grinding Rolls (Figure 8), rolls are subjected to high pressure so that comminution takes place by compressive forces as well as by inter-particle breakage. The force applied to the crushing zone is controlled by a hydro-pneumatic springs. As the product size from HPGR is fine, it can replace the conventional secondary and tertiary crushers.

Figure 8 High pressure grinding rolls

7 CRUSHING OPERATION

Crushers are usually operated dry. When the material fed to the crusher is at a slow rate, the individual particles are crushed freely. The crushed product is quickly removed from the crushing zone. This type of crushing, known as free crushing, avoids the production of excessive fines by limiting the number of contacts.

When the material fed to the crusher is at a high rate, the crusher is choked and it prevents the complete discharge of crushed product. This results in crushing between the ore particle and the crushing surface as well as between the ore particles. This type of operation increases the amount of fines produced. This type of choked feeding is preferred in some cases as it reduces the reduction stages.

Usually crushing is performed for any ore in two or three stages depending on the size of the feed particles and the size of the product particles required.

8 OPEN CIRCUIT AND CLOSED CIRCUIT OPERATIONS

Usually each stage of size reduction is followed by a screen which forms a circuit. Crushing may be conducted in open or closed circuit. Figure 9 shows the typical open and closed circuit operations:

In an open circuit crushing operation, the feed material is reduced by one crusher. The product of this crusher is screened and only oversize material is crushed by another crusher of small size as the throughput (the quantity of material crushed in a given time) is less. The crushed product from the second crusher and undersize material from the screen together form the final product.

In a closed circuit crushing operation, the oversize material from the screen is fed back to the same crusher. The quantity of the oversize material fed back to the crusher is called as circulating load. The undersize material from the screen is the required final product. Initially the quantity of final product produced is less than the quantity of the feed material. As the operation proceeds further the quantity of the final product gradually increases and will be equal to the quantity of the feed material after some time. After attaining this equilibrium condition, the quantity of the final product is always equal to the quantity of the feed material and the circulating load is constant.

The circulating load is expressed as a percentage of the quantity of feed material. It is to be noted that the throughput of the crusher is more than that of the crusher used in open circuit crushing operation and equal to the quantity of feed material plus the quantity of oversize material fed back from the screen.

Figure 9 Crushing circuits.

It is obvious that two size reduction machines are used in an open circuit operation whereas only one machine is employed in a closed circuit operation to get the final product. The objectives of employing a closed circuit operation are to minimize the production of fines and to reduce the energy consumption by avoiding size reduction of already reduced particles to the required size.

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