Technology: Extraction and Crushing
Extraction
Overburden removal
Overburden in
hard rock quarries may include
soil, sub-soil,
unconsolidated younger rock formations, other contaminating rock bodies, and/or weathered materials. Unconsolidated overburden may be removed using excavators; wheeled loading shovels or hydraulic back-hoe excavators. Consolidated overburden may require
blasting before it can be removed.
Overburden removal at sand and gravel operations has a greater emphasis on the reuse of overburden material as part of the ongoing
restoration process. For further details on restoration
click here. Overburden usually consists of soil, peat or glacial till; the thickness can range from less than 1 m to about 15 m. Overburden to mineral ratios are also highly variable and, although it is commonly quoted that they should not exceed 2:1, higher ratios are worked where the mineral is of good quality.
Wherever possible, overburden material is directly reused in site restoration; where this is not possible it is transported to a suitable site specified in the
planning permission. Soils and sub-soils are carefully stripped using specially equipped hydraulic backhoe excavators and stored in special storage bunds. They require careful handling and storage if their physical structure and chemical characteristics are to be preserved. For this reason different layers within the soil are stored separately from each other so that they may be re-laid sequentially during restoration. Where possible soils will be directly placed onto previously worked out areas as part of progressive restoration.
Drilling and primary fragmentation
Drilling and
blasting is carried out to fracture the rock to enable mechanical excavation. For further details on
blasting,
click here. It is normally essential in hard rock quarries; holes are drilled behind the working face and filled with an explosive (Photos 27, 28 & 29). When detonated, the rock is broken into manageable fragments to be taken away for further
crushing and
processing. Drilling and blasting is rare at sand and gravel operations; it is sometimes required prior to the excavation of more consolidated materials.
The most common blasting agent is ANFO, a mixture of ammonium nitrate (fertiliser) and fuel oil. Laser profiling may be used to characterise the
quarry face before blasting so that the exact quantity of explosive is used (see Rock Blasting animation below). Blasting agents are often brought to site in separate form and mixed when inserted into the hole; often by specialist contractors. Use of blasting agents and explosives in quarries is regulated by the Quarries (Explosives) Regulations 1988. Poor blasting significantly increases production costs, it is therefore designed to obtain optimum
fragmentation reducing the need for secondary breakage without producing excessive
quarry fines (material finer than 4 mm).
 Photo 27. Before hard rock blast
 Photo 28. During hard rock blast
 Photo 29. After hard rock blast
Electronic
detonators allow blasts to be triggered sequentially to minimise vibration. Careful design of the delay between blasts (the blast sequence) mean that the pressure waves from each blast interfere with each other and reduce vibration. The amount of explosive used in a blast is carefully measured to reduce the risk of fly rock (pieces of material that are thrown clear of the blast site).
There is a trade-off between
blasting costs and crushing costs; if large rock pieces are produced by blasting the cost of crushing will be increased. Oversize rock slows the loading process and may exceed the maximum size set for the primary crusher, leading to reduced efficiency. It may also increase the maintenance costs for both
loading and crushing equipment. Blasting generally tends to be optimised according to handling and crusher efficiency criteria. A recent project Less Fines Production in Aggregate and Industrial Minerals Industry358 examined the potential for reducing the amount of fines resulting from blasting in quarries via the use of new blasting methodologies.
Through careful control of hole location, depth, detonation sequence and timing the blasting process can be optimised in terms of cost and minimising environmental and social impacts. The energy released when the explosive is detonated does not only go toward breaking the rock; some energy is lost via heat, sound (as noise), displacement and ground shaking (vibration). These are the main environmental and social impacts associated with blasting.
Key finding
Fragmentation on blasting is designed to produce material suitable for the primary crusher, but significant quarry fines may also be produced
In weaker, or inherently fractured, rock types, mechanical breaking (known as ripping) may be employed (Photos 30 & 31). A single tooth mounted at the rear of a powerful crawler-tractor is often used. Mechanical
rippers enable the selective
extraction of material and have less environmental impact than blasting. However, this is a slower process than blasting and may be uneconomical in high volume, large scale operations.
 Photo 30. Bulldozer ripper in quarry
 Photo 31. Bulldozer ripper close-up.
Further detailed information on drilling practices and options can be found at:
Further information on blasting is available at:
Secondary fragmentation
Depending on the primary
fragmentation method used, it may be necessary to reduce the size of a proportion of rock on the quarry floor before it can be efficiently and safely handled by the excavators and trucks and fed to the
crushers for processing. The most straightforward method is the use of a hydraulic breaker (also known as a hammer or pecker - Photo 32) or a drop ball (Photo 33). Secondary
blasting by placement of explosive on or within the oversize rock can also be used. To remain cost effective a quarry operator will try to reduce the amount of secondary breaking required with a good blasting sequence.
 Photo 32. Hydraulic breaker in quarry
 Photo 33. Drop ball in limestone quarry
Digging, loading and hauling
The methods employed for
digging and
loading are dependent on production rate, rock type and height of the pile of blasted material. Typically, either hydraulic backhoes (Photo 34) or hydraulic face shovel excavators (Photo 35) (which have a high bucket filling efficiency and can hold up to 10m3 in a single load) are used.
Hauling of the blasted rock to the processing plant is most often carried out using rigid dump trucks, with capacity ranging from 15 to 100 tonnes. Hauling is a major cost in quarrying operations and much attention is given to gradients and surfaces of access ramps and the distances travelled. A system of fixed
conveyors, which can operate on a much steeper gradient than dump trucks, is sometimes a cost-effective alternative (Photos 36 & 37). In many operations a mobile
primary crusher is used, allowing the machine to be brought up to the face, where it can be fed directly by the excavator.
 Photo 34. Loading with backhoe
 Photo 35. Loading with face shovel
 Photo 36. Sand and gravel conveyor
 Photo 37. Crushed rock conveyor
Sand and gravel
Sand and
gravel is either worked in wet or dry conditions. Wet quarries may be dewatered, where pumps are installed after the initial excavation to draw down the water table, and worked dry (Photo 38).
Dry working is the most efficient in terms of maximising extraction and it also enables more selective extraction. Where deposits exceed 5 m,
dragline excavators are extensively employed; these are robust and efficient at feeding
conveyor systems. Where deposits are thinner or more consolidated, hydraulic backhoes are used. Some very
unconsolidated deposits, such as dune sands or some glacial deposits may be
excavated directly from the face by wheeled front-end loaders. Wherever possible, conveyors are used for haulage in preference to dump trucks; field conveyors can be several kilometres long, and transport up to 1000 tonnes per hour.
 Photo 38. Sand and gravel extraction
In wet quarries, at depths of less than 10 m, long-boom draglines can be used, the main disadvantage of these being a high loss of fine sand (Photo 39). In deeper water, grab dredgers are used. Difficulty in maximising extraction in wet quarries comes from being unable to visually inspect progress in the working and identify those areas from which more extraction is possible. Given that initial capital investment in dredgers is high and the quarrying process is less efficient most operators prefer to work sites dry and will employ pumps to temporarily lower the water table when workings would otherwise be flooded.
 Photo 39. Sand and gravel dredging
In general, due to the small
particle size of extracted material, it is easier to use conveyors to move material from the extraction area to the processing plant at sand and gravel operations than at
hard rock quarries. However, hauling via trucks is still commonly used, with the choice of conveyor or truck being made based on economics and environmental / social considerations.
Marine-based sand and gravel is worked by
trail dredging (Photo 40), where a suction pipe is pulled across the sea bed at slow speed (between 1 and 3 knots). The passage of the pipe across the sea bed leaves a groove about 2.5 m wide and 0.25 m deep and allows relatively thin deposits to be worked, whilst the substrata below the deposit remain largely undisturbed. Deposits at water depths of up to 40 m can be worked. The largest dredgers can load at a rate of 2000 tonnes per hour. Water and solids are pumped into the hold, with water displaced by additional dredged material as dredging proceeds. Primary screening takes place on board the dredger but the main processing takes place at a land-based processing plant. Marine deposits are usually of high quality (with a low percentage of
fines) and can be landed directly into areas of high demand. Various techniques for unloading are used, but increasingly ships are self unloading, using pumps to unload. The high capital cost of these specialised dredgers drives the need for minimising the time spent away from the dredging sites.
 Photo 40. Marine dredging
Crushing plant technology
Crushing, a type of
comminution, is carried out to produce particles of a given size distribution and particle shape. The most common types used are
compression
crushers and
impact crushers . Many crushers also incorporate a component of
abrasion and
attrition which leads to the production of fine material. The physical and mechanical properties of a rock govern the way it breaks apart. Brittle minerals with
inelastic behaviour will fracture when subjected to sufficient stress; the presence of cracks or flaws in the crystalline matrix of the mineral will act to concentrate stress, resulting in crack propagation and ultimately fracturing. Some minerals display
elastic behaviour, whereby stress is absorbed by distortion of the crystal matrix without fracturing. Fracturing preferentially occurs along
cleavage planes,
grain boundaries,
laminations,
bedding planes,
foliation,
joints and other planes of weakness.
Compressive crushing produces material that consists of two distinct size ranges; coarse particles formed by
tensile fracturing and fine particles formed by compressive fracturing. Impact crushing produces material with a uniform particle shape and size. Crushing is usually performed dry and in several stages .
The energy used by crushing equipment causes distortion and fracturing, which creates new surfaces. The amount of energy used to create these new surfaces is approximately 2% of that used in the crushing process; the remainder is lost as sound, heat and vibration. Crushing of brittle material uses less energy than crushing of elastic material; the latter may change shape rather than fracture. Comminution theories used to determine the amount of energy required for crushing often assume brittle behaviour.
Crushers applying a steady continuous compressive stress, such as
roll crushers, which are not ideally suited to the minerals industry, consume the lowest energy per unit volume. Jaw,
gyratory and
cone crushers consume the most energy, and impact crushers are intermediate consumers. The
Bond Work Index is the commonest measurement of
grindability; typical values are shown in Table 3. The selection of a crusher is made by considering the type of material to be crushed, the
feed size, throughput, the required product size and quality, the product's commercial value, as well as the capital cost, power requirements, operational costs and environmental restrictions relating to the crusher. There is a direct correlation between the Bond Work Index and the capital investment required; the harder the rock the more crushing stages and/ or larger equipment is required. A crusher that is ideally suited technically may not be the best choice when economic factors are brought into play. Also, the performance of a crusher will be reliant upon the crushing plant which it is part of; therefore it is important that decisions regarding new equipment are made after crushing trials using the largest practical volumes of the material to be worked.
359
| Production + Process Technology |
Table 3 |
|
| Bond Work Index (Wi, kWh per tonne) |
| Mineral |
Work Index (Wi) |
Mineral |
Work Index (Wi) |
| Barite |
4.7 - 6.9 |
Glass |
3.4
|
| Basalt |
17.0 - 22.5 |
Granite |
15.1 - 16
|
| Cement clinker |
14.8 |
Limestone |
9.0 - 12.8
|
| Coal |
12.5 - 13.0 |
Mica |
148.0
|
| Dolomite |
9.0 - 12.4 |
Quartz |
13.6 - 14.1
|
| Feldspar |
12.8 |
Quartzite |
9.6
|
NB Values were taken from two sources
377,
378; some quarries have values that vary significantly from these.
Crusher throughput capacity is typically quoted in tonnes per hour (tph). The nature of crushing is essentially volumetric and capacity figures typically refer to rocks with a bulk
density of 1600 kilograms per cubic metre (kg/m3). The product yield is more important than the throughput capacity; 100 tph of crushed material with 30% oversize only results in 70 tph of product, whereas a crusher that produces 85 tph of crushed material with 5% oversize results in 80 tph of product.
Jaw crusher
A
jaw crusher consists of two plates inclined toward each other; the "swing"
jaw plate is pivoted such that it moves relative to the other fixed jaw plate (Figs. 8 & 9). The angle between the plates (the
nip angle) is typically 19o to 22o. The
swing jaw is powered by a flywheel and is braced with a
toggle plate; this controls the crusher product outlet and is a release mechanism for uncrushable material such as
tramp iron. Crushers with a
single-toggle plate design have a more direct connection to the flywheel which imparts a circular elliptical movement to the swing jaw; whereas crushers with a
double toggle plate design are connected to the flywheel via the toggle plates which imparts a back and forth motion to the swing jaw.
 Figure 8. Jaw Crusher diagram
 Figure 9. Detailed Jaw Crusher diagram
In most jaw crushers, the flywheel has substantial mass; this helps to maintain rotational inertia and evens out power requirements. The circular-elliptical movement of the swing jaw helps to pull feed material through the crushing chamber. The
jaw plates have manganese steel
liners, with a corrugated or other surface profile designed to optimise crushing; these liners can be reversed to ensure even wear or replaced when worn out. Wear tends to be focused at the bottom, where the gap between the jaw plates is the smallest. The product outlet size is defined by its
closed side setting (CCS) and
open side setting (OSS). The amount of movement between the plates (crusher throw) varies between 1 and 7 cm. A short throw is used for fine-grained, brittle,
hard rock and a longer throw for more coarse-grained, tough, elastic materials. Incorrect selection of the throw can lead to overheating of bearings, increased power consumption and reduced throughput. 350
Material is fed into the jaw crusher via a hopper; the opening width ranges from 0.8 to 2 m and the gape (gap between the crusher plates) from 0.25 to 1.2 m (Photo 10). Feed may be pre-screened to prevent undersize from entering the crushing chamber. This increases the capacity of the crusher, reduces the risk of product contamination and choking in the crusher.
 Photo 10. Primary Jaw Crusher
Key finding
The jaws are kept full (choke fed) to reduce jaw impact, reduce the amount of slabby material and minimise wear, especially where abrasive material is being crushed.
As feed material works its way down through the crushing chamber it is nipped and released several times as the plates move in and out (Jaw Crusher animation below). The
nip angle is kept within a close range to ensure that material does not slip and that maximum size reduction is achieved. The number of crushing actions is controlled by the rotational speed of the flywheel, the profile of the jaw plate liners and the feeding conditions. The particle-size of the product varies due to the state of wear on the crusher liners and toggle plates; the
CSS is monitored to avoid the product drifting out of its required size range.
Reduction ratio are typically in the range 7:1 to 8:1; this varies with ratios up to 10:1 for limestone and as low as 5:1 for hard rocks such as
granite.
Smaller jaw crushers have 100 kW or smaller motors and flywheel speeds up to 300 rpm, larger crushers have motors greater than 250 kW and flywheel speeds as low as 200 rpm. Each turn of the flywheel is equivalent to a complete crushing action i.e. full opening and closing of the swing jaw. Double-toggle crushers have an intermittent cycle, with crushing only taking place in the chamber during the forward stroke of the swing jaw. Single-toggle crushers have a continuous crushing cycle; when the upper feed inlet is opening the lower product outlet is closing; therefore crushing is always taking place at some point within the crusher.
The production capacity of a jaw crusher is directly proportional to the
CSS; increasing the CSS allows more material to be discharged through the outlet. A crusher with a feed opening of 1000 mm will produce 125 tph at a CSS of 70mm, whereas at a CSS of 200 mm it will produce four times as much. Increasing the
CSS also decreases the amount of
comminution that takes place; a 70 mm CSS will result in a product with approximately 40% finer than 40 mm, whereas when using a 200 mm CSS this is only 15%. Crushers with a larger gape and wider crusher plates have a higher production capacity for a given CSS, for example at a CSS of 175 mm a jaw crusher with a feed opening of 1000 mm will produce 300 tph, whereas a jaw crusher with a 2000 mm feed opening will produce up to three times as much.
Cone crusher
A
cone crushers consists of an inverted cone (the
bowl or concave) that sits over a conical head (Figures 11, 12 & 13). The feed inlet is at the apex of the crusher; the crushing chamber (or
cavity) tapers from the feed inlet to the product discharge outlet. The crushing surfaces are protected with high manganese steel
liners; the head liner is known as the
mantle. The head is seated on a vertical shaft that is driven by spiral bevel gears connected to a counter shaft. This causes the head to move in an elliptical path around the main shaft; viewed in cross-section the crushing action is similar to that of a jaw crusher. Unlike a jaw crusher, the rotational speed of the motor does not directly equate to the number of crushing actions; this is due to the gearing between the counter shaft and main shaft. A flywheel (sheave) is attached to the countershaft to maintain inertia. Hydraulic mechanisms control the crusher CSS; they also act as an emergency release mechanism for removal of uncrushable material such as tramp iron. The amount of movement between the head and the bowl is known as the eccentric throw (difference between the CSS and
OSS); increasing the throw will increase throughput capacity but may have an adverse effect on product particle shape.
 Figure 11. Cone Crusher diagram
 Figure 12. Detailed Cone Crusher diagram
 Figure 13. Secondary Cone Crusher diagram
It is recommended that material be fed into the crusher using a distributor to maintain an even distribution of well-graded material throughout the crushing chamber. The particle size of the feed should be controlled so that it readily enters the chamber and fine material is removed. Increasing the moisture content of the feed material can be used to avoid packing of material in the cavity; water flush can be used to increase the throughput capacity.
Key finding
Cone crushers rely on a full chamber (choke feed) to give the best results. Failure to choke feed results in lower quarry fines production, but also reduced capacity, poor product shape and uneven crusher liner wear.
Feed material works its way through the
cavity and is discharged through the outlet (Cone Crusher animation below). The gap between the head and concave becomes narrower toward the bottom; whereas the diameter of the cavity becomes wider. The cone crusher delivers a straightforward horizontal
impact; material is nipped and released approximately six times in the cavity. The cavity can be divided into annular crushing zones; the volume of these zones increases towards a choke point. The point of optimal crushing is typically at the base of the chamber. The reduction ratio is typically 6:1 to 8:1 for
secondary crushing and 2:1 to 3:1 for
tertiary crushing. Cone crushers operate a continuous cycle; crushing takes place in the chamber 100% of the time.
Key finding
Increasing the rotational speed of the crusher can increase the throughput capacity; but it may increase the residence time in the cavity which will have the effect of reducing throughput capacity and increasing fines production.
The production capacity of a
cone crusher is directly proportional to the CSS; increasing the CSS allows more material to be discharged. The size of the cavity is controlled by the use of different liners; cavity size ranges from extra fine (EF), fine (F), medium-fine (MF), medium (M), medium coarse (MC) coarse (C) and extra coarse (EC). As a rule, finer cavities result in finer products. There are two types of head used; the standard head and short head. Short head cone crushers are used for finer crushing. Some heads have a stepped profile that allows finer crushing, albeit at a lower throughput capacity. The profile of the liners can be monitored to ensure that efficient crushing is taking place; regular analysis of the crusher product will help to indicate when liners need to be replaced.
Cone crusher throughput capacity decreases with decreasing
CSS. A cone crusher with an Extra Coarse cavity (feed opening, 299 mm; CSS, 25 mm) will produce up to 380 tph whereas the same cone crusher with a Fine cavity (feed opening, 107 mm; CSS, 13 mm) will produce up to 185 tph;. However, less comminution takes place; a 13 mm CSS will result in a crusher product with 66% finer than 10 mm, whereas this is only 30% with a 25 mm CSS. Larger crushers have a higher production capacity for a given CSS due to the increased volumetric capacity within the crushing chamber.
Gyratory crusher
Gyratory crushers are similar to
cone crushers; they are frequently used in large-throughput
primary crushing roles (Photo 15 & Fig. 41). Typically, they have larger capacities (up to 8000 tph) compared to jaw crushers (up to 1500 tph). Gyratories operate at a slower speed than cone crushers, typically in the range 85 to 105 rpm. They do not require
feed mechanisms and are usually fed direct from the back of a dump truck. The available feed opening area of a gyratory crusher is approximately three times greater than that of a jaw crusher of a similar gape; the gyratory has a higher capacity.
 Photo 15. Primary Gyratory Crusher
The most commonly-used type is a supported-shaft gyratory crusher; the main shaft is suspended from an overhead spider. The angle of the mantle is steeper than that used in a cone crusher; because of this gyratory crushers have a smaller throw. This affects the flow of material and improves the particle shape as material is struck more frequently in the crusher
cavity. The
CSS is adjusted by raising or lowering the head.
 Figure 41. Detailed Gyratory Crusher diagram
It is desirable for gyratory crushers to be run under
choke feeding conditions; however this may not always be the case in practice as there are usually gaps between deliveries of feed material by dump truck. The particle-size of the product may vary over time due to the state of wear on the crusher
liners and concaves; the
CSS of the crusher is monitored to avoid the crusher product drifting out of its required size range.
Where gyratory crushers are used in
secondary,
tertiary and subsequent crushing roles they have similar operating principles to primary gyratory crushers. They tend to be more tolerant of variations in feed material and feed rate than cone crushers.
Roll crushers
Roll crushers have limited application in the quarrying industry, aside from their use in
limestone operations; this is largely because of their low throughput. They tend to be used for sedimentary, friable rock and wet or sticky materials that jaw or gyratory crushers struggle to process. These crushers use a combination of
impact,
shear and
compression to break up the feed material; they can be subdivided into sledging or slugger rolls, and crushing rolls.
Sledging or slugger rolls have either a single or double-roll construction. A single-roll sledging crusher consists of a heavy roll with rows of teeth (or picks) that are used to grip the rock and feed it into the crushing chamber. Facing the roll is a
breaker plate; the distance between this plate and the roll is the crusher setting. A two-roll slugger crusher consists of two toothed rolls. A mineral sizer is similar to a two-roll slugger but with more pronounced teeth that are staggered on the contra-rotating rolls; this helps to increase the capacity by allowing the passage of undersize material and enabling a relatively high
reduction ratio. Crushing rolls consist of double, contra-rotating rolls that act primarily as
compressive crushers; they are especially used for friable, non-abrasive material.
Impact crusher
In the past, impact crushers were mainly used for crushing non-abrasive materials, such as limestone. The development of
abrasion resistant wear-parts has increased their use for the crushing of abrasive rocks, such as
sandstone.
An impact crusher consists of a crushing chamber lined with impact plates or bars (known as
anvils or
breaker plate); within this chamber is a rotating shaft with fixed
rotors that support metal bars (known as
beaters,
impellors,
hammers or
blow bars). The shaft may be horizontal (
Horizontal Shaft Impact crusher (HSI); Figs. 16, 17 & Photo 20) or vertical (
Vertical Shaft Impact crusher (VSI); Figs. 18, 19 & Photo 14). HSI crushers are generally used for coarse (primary and
secondary) crushing; VSI crushers are used mainly for finer (secondary and tertiary) crushing. Some HSI crushers have double-rotors; the first for coarser material and the second for finer material. VSI crushers are either rock-on-metal or rock-on-rock; the latter type has lower wear on crusher components.
 Figure 16. HSI Crusher diagram
 Figure 17. Detailed HSI Crusher diagram
 Figure 18. VSI Crusher diagram
 Figure 19. Detailed VSI Crusher diagram
A
hammer mill is a form of
HSI crusher with pivoted hammers that are allowed to swing freely (swing-hammer) within the crushing chamber. They are typically used in secondary or tertiary roles and are commonly used in limestone operations.
The impact crusher shaft rotates at a speed that is inversely related to its rotor size. In VSI crushers, the smallest rotors are 0.3m in diameter with shaft rotations up to 5300 rpm; the largest rotors are 1.2m in diameter with shaft rotations as low as 800rpm. In HSI crushers, the smallest rotors are 0.4m in diameter and 0.6m wide with shaft rotations up to 1500 rpm; the largest rotors are 2m in diameter and 2.3m wide with shaft rotations as low as 280 rpm. The relative velocity at the end of the rotor (tip speed) is within the range 35 to 90m/s. Speeds of 30 to 40 m/s are used to produce all-in material and 60m/s for
manufactured sand. HSI crushers tend not to be operated above 60 m/s. VSI crushers that utilise rock-on-rock crushing operate at 55 to 70 m/s. Swing hammer mills operate up to 90 m/s.
 Photo 14. VSI at sand and gravel plant
 Photo 20. Primary Impact Crusher
Feed material is broken by direct contact with the
beaters, metal surfaces or feed material. It is fed tangentially into the path of the beaters, which deliver a series of sharp blows to the particles and accelerates them into the crushing chamber. The first contact causes the largest degree of
comminution; fractured material then passes from one
breaker plate to another, travelling along a
grinding path. The crushing chamber is usually lined with feed material; this minimises wear on the crusher parts.
VSI crushers incorporate a dual feeding system that separates the feed into two streams; one fed to the rotor (rotor feed), whereas the other is fed directly into the crushing chamber (crushing chamber feed). Different manufacturers refer to this as a hydracascade or bi-flow feed system. This increases the rock-on-rock interaction (semi-autogenous crushing) and reduces the wear on crusher parts. Unlike crushing by
compression, impact crushed material tends to have no residual stress, which is important for material used in construction. Also, it is useful for crushing more plastic material.
Key finding
In VSI crushers, decreasing the ratio of rotor feed to chamber feed will increase the particle-size of the product; reduce the fines content and the product size-distribution is narrower.
HSI crushers have a maximum feed size that ranges from 250 to 1900 mm and capacities ranging from 40 to over 1000 tph.
VSI crushers have a feed size that ranges from 20 to 76 mm and capacities that range from 3 to over 2000 tph. Optimising the rotor speed, breaker plates, crushing chamber feed and rotor diameters, can be used to control the reduction ratio and product grading.
Key finding
VSI crushers are used as a shaping machine to produce a good cubical shape with a low flakiness however many fines may also be produced.
The proportion of flaky material in aggregate can be reduced by up to 50% by using a VSI crusher. It can also be used to remove undesirable material such as thinly intercalated shale, which is pulverised, leaving the harder sandstone as a coarser aggregate.
|
|