Crushing and Screening Equipment

Note: This highly informative and elaborate paper was written by the Author many years back and holds valuable information. Readers should update themselves with the latest developments that might have occurred after the presentation of this paper.

The world has caught speed. Fast construction is the new mantra. A company that finishes its projects in a time bound manner soon enters the top bracket. Speedy construction activities have correspondingly hiked the demand for raw materials; aggregates being the major material amongst them. As a result, crushing and screening equipment is more in demand than ever and has moved from the secondary list to primary list of equipments.

Crushing and screening equipment parameters have direct relation with the rocks to be crushed. Rocks vary in characteristics. Some are very hard to break. Some tend to produce flaky material. All types of rocks, may be granite, basalt, quartzite or limestone have their uses and need crushers for breaking the boulders to smaller sizes and then further reduction of broken stone to the desired size.

Type of Crushers: Crushers to which the rock is first fed can be termed as Primary Crushers. Next ones that reduce the size of broken stone further can be called secondary crushers. And if the required size of aggregate has not been received, another crusher deployed to further reduce the size of stone will be called Tertiary crusher. In rare cases, a quaternary crusher may follow a tertiary crusher. And a complete crushing set up having all the three types of crushers thereby converting the rock to the desired sized aggregate at one location is called Crushing plant.

Crushing equipment: Size of the stone to be crushed and the method of crushing decide the type of crushing equipment to be used. The crushing equipment can be classified as follows:

Compression or Jaw Crushers.
Hammer or Impact Crushers.
Roller Crushers.
Pressure or Cone Crushers.

Compression or Jaw crushers: These crush the stones brought from quarries by compressing them. These crushers are installed at the quarry itself to reduce the size of stones brought from the rocks and sending them down to other crushers for further crushing to size. Mostly, a conveyor controls the rate of feeding the rock to the crusher. Jaws are used to crush the large stones to smaller stones and production of stone dust is very less. One of the jaws is stationary while the other swings to crush the stone at a rate of a few swings every second. Material used for the jaws has to be extremely tough. Mostly cast steel is used for this purpose and jaws are made fully reversible. Higher is the crushing speed and crushing chamber size, better is the efficiency and output of the crusher.

Hammer or Impact Crushers: These crushers are best suited to crush hard materials like Granite. In this type of crushers, unlike the jaws used in compression crushers, the rock stones are thrown by a high speed rotor against a crushing chamber or rock box to reduce them to smaller sizes. A hopper and a feed tube are used to feed the rock to the centre of the rotor. Now-a-days, Vertical Shaft Impactor (VSI) crushers are preferred as these can crush hard materials and produce aggregates and crushed sand. Horizontal shaft impactors (HSI) are suitable for soft materials like limestone only. VSI crushers act as ideal tertiary crushers. These crushers produce significant quantity of fine dust. Their main advantage is that these don’t produce flaky material and resultant particles are cubical in shape. VSI crushers can be easily transported and installed and need comparatively low investment.

Roller Crushers: Roller crushers are not suitable for installation at stone quarries. These can further crush the stones already reduced in size by the compression crushers and hammer crushers. These crushers contain roller shafts which keep rotating towards each other and thereby crushing the material being fed in between.

Pressure or cone crushers: Pressure crushers are also suitable for further reducing the size of already crushed stone. These can act as secondary and tertiary crushers. Crushing of stones is done by applying pressure to them between a stationary surface (not jaw) and an eccentric rotating spindle. Their crushing surfaces are conical in shape. Their production capacity is very high. These discharge the crushed stone direct to the ground through free fall. Heavy cast steel is used to build the shell. Feed opening size is normally kept as 75 mm to 200 mm.

Capacity of Crushing equipment: Crushing equipment can be of mini, normal or giant size. A mini crusher can produce as less as 10 tonnes of aggregate per hour, a normal crusher will produce 150 tonnes per hour and a giant sized crusher can produce even 1100 tonnes of aggregate every hour. Choice of crusher depends upon the output required, type of rock and size of the aggregate to be produced. A 50”x38” Jaw crusher will accept a feed size up to 800 mm. With a motor of 160 Kw and speed of 200 rpm, it may produce 600 tonnes per hour. A 36”x24” Jaw crusher will accept a feed size up to 500 mm and give an output of 200 TPH with a motor of 75 Kw and speed of 275 rpm.

Efficiency of crushing equipment: For best efficiency of crushing equipment, care should be taken that no time gets lost in acceptance of the stone to be crushed. For this, the stone brought from the quarries must be able to pass through the opening of the crushing equipment. Otherwise, it will have to be taken back and reduced in size.

Computerized control: Crushing equipment loaded with operation software need minimum manual operations. This helps in cutting down their operation costs. The operator can change the crushing program from the control panel to get the desired output of aggregates. Some control panels are touch screen type making the operation job much easier.

Maintenance aspects: As the crushers crush stones, these need to be very tough equipment. Repeated maintenance problems may badly affect their efficiency. Above all, their crushing components have to be tough ones to avoid quick wear during crushing and handling of stones. In Jaw crushers, cast steel jaws should be such that could be reversed and used so that the part that has come under maximum wearing shifts position and another part is now under maximum load.

Hammer crushers require lesser maintenance than compression crushers because of less wear and tear. These are energy efficient also thus per unit cost of their use is lesser. In these crushers, care has to be taken that the stones fed in to this type of crushers are dry. Wet stones cling to the hammers and reduce their efficiency.

Roller crushers too require less overall maintenance but uniform wear and tear of rollers keeps taking place. It is therefore important to decide the diameter of rollers depending upon the size of the stone being fed to them. If the size of stones fed to these crushers is large, the roller surfaces are changed from smooth ones to undulated ones.

Equipment manufacturers: A number of manufacturers in India are producing a variety of crushing equipment. Puzzolana Machinery Fabricators of Hyderabad are producing stationary as well as mobile crushing plants. Newa Engineers produce Jaw Crushers and Cone crushers with special feature of jaw crushers mounted on wheels. Their wheel mounted mobile jaw crushers can give an output of 600 tonnes per hour and are suitable to act as primary crushers for any type of rocks. Built with German technology, Apollo crushers produced by Gujarat Apollo Industries Limited are of Jaw type and Impact type. These crushers have a high reduction ratio and sometimes produce aggregates of size ready for use. Chinese companies like Shanghai Shibang Machinery Limited has also brought Impact and Cone crushers to India. Taurian engineers of Mumbai produce crushers under Jumbo, Rhino and Platinum brands with output up to 500 tonnes per hour. While Rhino and Jumbo are Jaw Crushers, Platinum is a Cone Crusher. Metso Minerals is producing primary crushers of Jaw, Impact and Cone type. This company with its branches in over 100 countries has come a long way on crusher equipment front. Sandvik produces impact crushers with an output of 300 to 350 tonnes per hour. Proman has built a wide range of VSI crushers with power varying from simple 50 HP to as high as 1500 HP.

Nawa crushing plants: Nawa Engineers’s crushing plants include a 200-400 tonne per hour Jaw Crusher of 44”X32” as primary crusher, a long head roller bearing cone crusher as secondary crusher and a 400-500 tonne per hour capacity tertiary crusher for highway projects and for commercial production of aggregates. Another crushing plant by Nawa is a mobile one on 3 axle wheel chassis with 48”X36” Jaw Crusher of 600 tonne per hour capacity as primary crusher, twin cone mobile, 2 axle chassis long head and short head cone crushers as secondary crushers and twin drive mobile VSI 2 axle chassis tertiary crushers.

Nawamobile 1209J: Produced by Nawa Engineers, Hyderabad, this is a mobile Jaw Crusher as is clear from its name. It has a feed size of 800 mm, is mounted on wheels with suspension chassis having 3 axles and can be easily moved from one place to other like a trailer. Its output is as high as 600 tonne per hour.

Fintec 1440: Produced by Sandvik, Fintec 1440 is an Impact crusher having output up to 350 tonnes per hour. Weighing 50 tonnes and 15.5 metres in length, it can be moved from site to site and uses a 330 KW CAT diesel engine for its output.

Nawapactor VSI crusher: VSI crushers are today becoming a favorite investment as these produce concrete aggregate as well as crushed sand and thus bring good returns. Nawapactor VSI crushers are being produced in 6 models with output varying from as little as 40 TPH to as large as 800 TPH.

Terex Finlay J1160 Crusher: Recently launched in India by North Ireland company Terex Finlay, this Jaw Crusher has an advanced PLC Control and monitoring system to operate the crusher without error. It is powered by a Caterpillar C9 engine of 300 HP and has a very heavy duty chassis. Weighing around 37 tonnes, J1160 has a high output and reduction ratio.

Screening equipment: All crushers need screening equipment for proper segregation of crushed material. A crushing plant is complete in itself when it includes screening equipment located at appropriate locations among primary, secondary and tertiary crushers. Mobility and automation of crushers and screens through computerized control are now considered essential features of crushing and screening equipment. To segregate all crushed materials of various types of rocks, may be granite, basalt, quartzite or limestone, screens are essentially required.

Location of screens: In a crushing plant, the screens are normally located in the secondary unit to receive the material crushed by the primary crusher and supplied by the conveyor belts. The screen segregates the 50 mm, 20 mm, 10 mm, below 10 mm and above 50 mm aggregates and piles them separately except above 50 mm material which is sent to secondary crusher through another conveyor belt for further crushing and re-screening.

Screening plant for natural stones: Sometimes, natural stone is preferred for use as aggregate instead of crushed stone because of roundness, lesser surface area and lesser consumption of cement. In such cases, only screening plants are installed at the quarry and crushing plant is spared. The boulders and gravel is fed over the screens which segregate 50 mm down, 20 mm down, 10 mm down and 5 mm down materials while that above 50 mm is stockpiled separately for sending to an impact crusher.

Screening plant components: A screening plant has the following essential components—

Feed Hopper
Feed Box
Screens and decks
Driving equipment
Vibrating equipment
Belt conveyors

Feed Hopper: Screening plant hopper receives the material to be screened. It is a heavy duty component that is made adjustable to suit the length and the loading height. Generally its capacity is kept around 10 cubic metres. Fixed grizzly is provided in it to sidetrack oversized materials. The inclination of grizzly can be altered as suitable through a hydraulic cylinder arrangement provided for this purpose.

Feed box: The feed box transfers the material to the screens with minimum load impact to the mesh and by evenly spreading the feed over the full length and breadth of the screen to achieve maximum screening efficiency. Generally, it is provided with rubber lining to absorb the impact of the feed.

Screens and decks: The Screen structure has to be heavy duty and extremely strong to receive the stone aggregate. The wire mesh should preferably be of hardened steel. Earlier, only woven wire or perforated plate screens were used in screen meshes. Now, polyurethane screens have come under use because of their flexibility and better resistance to abrasion. These prove cheaper than woven wire screens or perforated plates which are normally, 6 to 8 mm thick. Decks are provided to hold the screens in position.

Driving equipment: The driving motor is mounted on top or side of a screen. Mounting of the motor at this location helps in its easy maintenance along with that of eccentric shafts, lubricating system, pulleys, weights, belts and bearings. Capacity of the motor depends upon the number of decks and size of the screen. A 3 deck machine with screen size of 1.5 mX5.0 m and screening area of 7.5 sq metres will require a motor of 20 KW while the same equipment with 2 decks may require 15 KW capacity motor. Number of decks increases the required capacity of the motor. A 4-deck screening machine with screening area of 7.5 sq metres will require a 25 KW motor. A 3 deck screen with 15 sq metre area will need a motor of 40 KW. The speed of the motor is kept variable to match the required output.

Vibrating equipment: Eccentric weights are provided in the screening plant to rotate in linear or circular direction to produce vibrations in the screen. The amplitude of vibrations depends on the number of these weights and can be increased or decreased by adding or removing the weights.

Belt conveyors: A screening plant has a charging conveyor, a top conveyor, side conveyors and the discharging conveyor. The charging conveyor transfers the load from the feed hopper to the top conveyor. The charging conveyor should be well supported over rollers for smooth running and carriage of load. Top conveyor is generally trough shaped and hydraulically operated. It transfers the material to vibrating screen. The side conveyors are normally ribbed type. This helps them carry the material at greater inclinations. These are foldable type for easy transportation. The discharge conveyor is foldable through a hydraulic system and receives the screened material from the screen decks.

Vibrating screens: Vibrating screens have now eliminated the rotary screens from the construction scene as these are more efficient and versatile than rotary screens. Vibrating screens are inclined screens with multiple decks of different mesh opening sizes. The size of openings is kept as 50 mm, 20 mm, 10 mm and below.

Selection of crushing and screening equipment: While selecting crushing and screening equipment, work out the following data:

Type of rock to be crushed.
Size of aggregate to be produced.
Daily output required.
Whether the crusher has to be installed permanently or moved from site to site.
Maximum investment that can be made.

Now, look for the following points:

Whether the crushing and screening equipment has the reduction ratio to meet the size requirements.
Whether it is suitable for type of rock to be crushed.
It has least power or fuel consumption.
It has least maintenance requirements.
Equipment warranty for sufficient period is available.
Whether the crushing equipment is stationary or mobile type. If mobile equipment is to be purchased, the equipment should be easy to move and with least track costs.
Whether it is easy to re-assemble.
Whether it provides more than required output. (So that loss of output due to unforeseen closures could be accommodated)
Whether its cost falls within the estimate.
Whether the material used for jaws or hammers or rollers or cones is durable and resists wear and tear.
Maximum automation, minimum manual operations.
Least vibrations, shock and noise during operation.
Nip angle for a smooth flow of material.
Least requirement of lubricants. Some crushers need 300 litres of lubricants while others may need just 70 litres. Time interval for replacement also varies.
Easy availability of spares and service.
Minimum screen area and maximum screen mesh replacement interval.
It has minimum breakdown track record. There is easy availability of mesh and spares.

Mobile screening plants: Track mounted mobile screening plants can be reckoned as the latest screening equipment. These can scalp the heavy over burden and unsuitable material at the quarry itself and can separate out sticky material. Almost custom made screening plants can be had these days. Number of decks may be varied as per one’s requirement. Decks can be horizontal or inclined and stroke option can be chosen. Versatility lies in their use for washing, dry screening or wet screening. Mobile screening plants with PLC controlled operation and not needing any concrete foundations are the preferred choice of the users because of their ability to move from one site to the other. If the mobile screening plants have their own power source and don’t need an external link, these become the ideal choice for the users.

Mesh change facility: Screening machines should have easy mesh change facility as mesh is the component that may need a change after some time. For this ample space should be available between the decks. Further, the fixing arrangement should be simple for easy removal and replacement. Clamp plates for fixing the screens should be easily removable or made loose and should allow removal of screen from feed end or discharge end as suitable to the user. Once the screen mesh is removed and new one is fixed, mesh tensioning system should be able to tighten it fully. Still more important is that the tensioning machines should be able to tighten the mesh without causing any ridges in it. The mesh should remain fully as per desired shape. This is possible only if the tensioning machines cause uniform stress all over the area of screen mesh.

Extec S-7 screen: Extec’s S series screens have added S-7 screen to their fleet. It is a triple deck screen designated as the ultimate and inspirational screener based on double screen concept. It has a large capacity, longest tail conveyor, longest side conveyors and high speed belts to produce aggregate at high efficiency and with good grading. Its screening area has two individual screen boxes incorporating 3 decks in each box. Each screen box has a separate drive and each can be angled independently. Even the vibrations, amplitude and speed of each screen box can be separately adjusted. Screen boxes can be adjusted to 20 different adjusting angles. This arrangement results in massive screening area without affecting the cleanliness of final products. The screener can give 5 products thus an extra side conveyor has been added to it. Longest side and tail conveyors result in great stockpiling capacity and the side conveyors feed directly in to the cone crusher where required. Extec has patented its screen box lift mechanism. This mechanism helps in easy change of mesh whenever required. The running of belts has been made faster, again aiming at high production of aggregates. To provide a stable but mobile base to all this arrangement, the tracks have been increased in length and the chassis has been made stronger.

In Extec S-7, the primary box has 3 decks, each of 10 feet X 5 feet size. The secondary box has top and middle decks of 10 feet X 5 feet each and the bottom deck of 8 feet X 5 feet. Thus total top deck area is 20 feet X 5 feet, middle deck area of 20 feet X 5 feet and bottom deck area of 18 feet X 5 feet.