Production Good Practice
Crushing plant
Ideally, a quarry will sell everything that is extracted and processed. In the past, quarries produced a range of single-size
aggregate products up to 40 mm in size. However, the trend for highly specified aggregate has meant that products have become increasingly finer. Currently, many quarries do not produce significant quantities of aggregate coarser than 20 mm; it is not unusual for material coarser than 10 mm to be stockpiled and recrushed on demand. Decreasing the top size of aggregate produced has had an inverse effect on the proportion of
fines produced; a 40 mm top size results in 5-10% fines, 20 mm top size results in 15-20% fines and 10 mm top size results in 35-40% fines. This represents up to an eight-fold increase in fines production.
The amount of fines produced increases as material progresses from
primary to
secondary and subsequent stages. The amount of fines arising from the
primary crushing stage is strongly influenced by the
blasting process; if rock can be removed without blasting this will reduce the amount of fines produced. The amount of fines generated during blasting may be as high as 20%. Table 2 indicates the fines content generated at each stage of the crushing process; the proportion of fines produced varies with the type of rock and also the type of
crusher used.
| Production Technology |
Table 2 |
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| Quarry fines produced in hard rock aggregate operations |
| Production Stage |
Rock type |
Proportion of fines in the crusher product (weight %) |
| Primary crushing |
Igneous + Metamorphic
Limestone
Sandstone
|
3 - 6% (Jaw) to 10 - 15% (Gyratory)
6 - 7% (Jaw) to 20% (Impact)
1 - 2% (Jaw) to 15 - 20% (Jaw & Gyratory)
|
| Secondary crushing |
Igneous + Metamorphic
Limestone
Sandstone
|
0 - 23% (Cone)
15 - 25% (Cone) to <30% (Impact)
10 - 15% (Cone)
|
Tertiary crushing
(and subsequent stages)
|
Igneous + Metamorphic
Limestone
Sandstone
|
5 - 30% (Cone) to 40% (Impact)
<20% (Impact) to 40% (Hammer mill)
~15% (Cone) to 40% (Impact)
|
NB Fines = quarry fines; the proportion of quarry fines produced is attributed to specific crushers (given in brackets after the figure)
Jaw crusher good practice
Overview
A jaw crusher consists of two metal plates that crush material as they close together (for further details on jaw crushers
click here).
As a compression crusher, they generally produce the coarsest material; this is due to the preferential breakage of rocks along inherent lines of weakness. Jaw crushers are mainly used in primary crushing as a means of preparing rock for subsequent processing stages; they are rarely used as secondary crushers (where they are used to boost primary production capacity). Some sand and gravel plants use granulators (a squat form of jaw crusher) for crushing cobbles (Figs. 8, 9 & Photo 10).
Jaw crushers do not produce a large proportion of
quarry fines (material <4 mm) material; at a
closed side setting (CSS) of 40 mm a jaw crusher will produce less than 10% of quarry fines and at a CSS of 200 mm it produces less than 1%.
Key finding
Jaw crushers are mainly used in primary crushing where the amount of fines produced is typically less than 5%; any attempts to minimise fines production at this stage will have little effect on the total fines arising as most are produced in the secondary and tertiary stages.
Feed
Jaw crushers are routinely choke fed as this maximises production capacity and ensures that particles are uniformly broken. It promotes stone-on-stone crushing which breaks up flaky or slabby particles; this probably results in a higher proportion of fines than if operated under non-choke conditions. Reduction in fines produced could be achieved by trickle feeding material into the jaw crusher; however this would have an adverse effect on particle shape and also it would reduce throughput capacity. Ideally, the feed rate should not be switched from choke to non-choke, as this would have a knock-on effect on the down-stream secondary processing plant. In practice, many jaw crushers are fed in this intermittent fashion due to gaps in the delivery of feed material from the quarry. Jaw crusher feed is pre-screened using a grizzly -screen prior to crushing; this is in order to remove material finer than the closed side setting.
Key finding
In practice, many jaw crushers are not fed to their design capacity; this is because the subsequent processing plant does not have sufficient capacity to handle the volume of material that would be produced if the jaw crusher were working to capacity.
Figure 8. Jaw Crusher diagram
Figure 9. Detailed Jaw Crusher diagram
Crusher setting
Ideally, the
reduction ratio of a jaw crusher should be 6:1; this is calculated as the ratio between the particle size of the feed (the F80 is used, this is the particle size at which 80% is finer than the top size of the feed) and the particle-size of the product (the P80 is used, which is the size at which 80% is finer than the top size of the crushed product). The finer the
closed side setting (CSS) the greater the proportion of fines produced. The closed side setting of a jaw crusher is constrained by the need to maintain the
nip angle within a narrow range (typically 19 to 23o); too large an angle causes boiling in the crushing chamber (this is where the
jaw plates cannot grip onto the rock and it keeps slipping up and down).
Key finding
Increasing the CSS in an attempt to reduce the amount of fines produced may have the opposite effect; it would lead to a greater proportion of oversize material, which would need recrushing and would ultimately lead to a higher proportion of fines being produced.
The settings on a jaw crusher are more designed for producing material for
secondary crushing. The best particle shape is typically found in material that is approximately the same size as the closed side setting. Smaller sizes will contain a higher proportion of elongated particles because they have passed through the crusher without being touched. Larger sizes will also contain a higher proportion of elongated particles because long and narrow pieces have also passed through the crusher without being touched. This indicates that the closed side setting is best set to the size of the main product required to give the best results.
 Photo 10. Primary Jaw Crusher
Cone and gyratory crusher good practice
Overview
Cone crushers (Figs. 11, 12 & 13) and
gyratory crushers (Photo 15) consists of a cone that crushes material as it rotates within a crushing chamber.
For further detail on how cone crushers and gyratory crushers work, click here.
Cone crushers are used in
tertiary roles as an alternative to impact crushers where shape is an important requirement but the proportion of fines produced needs to be minimised. Even though the reduction in fines produced may only be a few percentage points, this would represent a significant volume of material in a large operation.
 Figure 11. Cone Crusher diagram
 Figure 12. Detailed Cone Crusher diagram
 Figure 13. Secondary Cone Crusher diagram
Key finding
Cone crushers are mainly used in secondary and tertiary roles, therefore attempts to minimise fines production will have a greater effect on the overall production of fines compared to attempts at minimising fines production by primary gyratory or jaw crushers.
Feed
Uniform distribution of
feed material around the cone crusher inlet is good practice as it allows production of a consistent product and consistent operation of the crusher.
Choke feeding is important for cone crushers as it maintains a good particle shape by facilitating an inter-particle crushing action; trickle feeding is not a sensible option as it increases the proportion of flaky material in the crusher product.
Key finding
Pre-screening of the feed to remove the fines, especially in tertiary crushing is good practice; it helps to avoid packing of material in the chamber and maintain an effective crushing action.
It is advisable to maintain approximately 10-15% of material finer than the
closed side setting in the feed to assist crushing action. Pre-screening to remove 6-10mm
aggregate from the feed should be avoided as void space in the chamber results in an increased proportion of flaky material in the product.
 Photo 15. Primary Gyratory Crusher
Crusher settings
The liner profiles are designed for a range of product sizes from extra coarse (EC) to extra fine (EF); the EF liner profile will result in the highest fines proportion for a given cone crusher.
Key finding
The finer the closed side setting the greater the proportion of fines produced.
Monitoring the crushing force, as registered through the load on the crusher motors and also the pressure on the hydraulic mantle adjustment mechanism, will give forewarning of crusher packing problems before they become too acute.
Impact crusher good practice
Overview
Impact crushers consist of a set of
hammers that crush material as they spin within a crushing chamber (for further details
click here). Impact crushers tend to be used where shape is a critical requirement and the feed material is not very abrasive. The crushing action of an impact crusher breaks a rock along natural
cleavage planes giving rise to good product quality in terms of shape. The quality of these products makes them ideal for use in highly specified roadstone and concrete aggregate applications (Figs. 16, 17, 18, 19 & Photos 14 & 20).
Key finding
Improvement in product particle shape comes at the price of producing excessive fines.
 Figure 16. HSI Crusher diagram
 Figure 17. Detailed HSI Crusher diagram
Feed arrangement
It is vitally important that the feed arrangement to an impact crusher ensures an even distribution of feed material across the full width of the rotor. This will allow for even distribution of energy into the feed material and uniform wear patterns ensuring consistent product gradation and power consumption.
 Figure 18. VSI Crusher diagram
Figure 19. Detailed VSI Crusher diagram
Crusher settings
Size reduction in an impact crusher relies on energy being imparted into the rock from the rotor. This initial impact is responsible for more than 60% of the crushing action with the remainder being made up of impact against an adjustable breaker bar and a small amount of inter-particle collision.
Key finding
Size reduction is directly proportional to the rotor speed; it largely dictates the amount of fines produced. Slower rotor speeds can be used as a means of reducing the amount of fines produced but may result in a product with a less desirable particle shape.
Slower rotor speeds are preferable as a means of minimising the amount of wear on crusher components.
 Photo 14. VSI at sand and gravel plant
Photo 20. Primary Impact Crusher
The product grading from an impact crusher will change throughout the life of the wearing parts particularly the impact hammers or blowbars. As the profile of the hammer changes with increasing wear, the product grading becomes coarser. Many modern impact crusher installations have a variable speed drive arrangement that allows an increase in the rotor speed to compensate for wear on the impact hammers.
Open discharge arrangements in impact crushers rely on retention of the rock within the crushing chamber. This is achieved by reducing the gap between the rotor hammers (crushing members) and the impact curtain.
Key finding
Decreasing the gap between the hammers and impact curtain increases particle retention in the chamber. This increases the size reduction ratio; however it also reduces throughput capacity and increases fines production.
Closed discharge arrangements rely on a series of grids to retain the material within the crushing chamber; these are generally not adjustable. Decreasing the size of the grid apertures has the effect of increasing the residence times of material in the crushing chamber. This increases the size
reduction ratio; however it also reduces throughput capacity and increases fines production.
Washing plant
Overview
Washing plant performance has continued to improve over recent years, however there has been little incentive to treat water as anything other than a freely available and cheap commodity (Photos 21, 22 & 23). The need to comply with the legal and regulatory framework has always been the principal driver behind the protection of the natural water environment; recent changes in regulation will have far-reaching consequences for the quarrying industry. For further details on Quarry fines waste
click here. For example, the recent implementation of time limited abstraction licences introduces the prospect that quarries may be forced to close due to failure to obtain a licence renewal. The risk to a business-as-usual approach should not be underestimated. Quarrying depends on long-term
planning permissions and it is on this basis that large-scale capital investments are made. This is now out of step with the new abstraction licencing regime, under which abstraction licences could be terminated within a much shorter period, effectively bringing the life of a quarry to a premature end.
Key finding
There have been only limited signs of investment in more sophisticated approaches that promote the minimisation of water use and consumption.
 Photo 21. Washing plant
Water is widely used in operations around a typical quarry site.
Water use can have a number of detrimental effects, such as consumption (loss of water volume) and
contamination, both of which ultimately may cause impacts on ground and surface water resources if not managed in an appropriate fashion. At other sites, water present on the site may not be used, but must still be removed in order to create conditions in which quarrying activities can proceed safely and efficiently. This may also create impacts on the water environment. The use of water efficient technology, water recycling and water
reuse (for example, through the use of settling ponds or
lagoons or other classification / filtration methods to remove contained solids) can all substantially reduce the overall consumption of water at a site.
 Photo 22. Washer barrel
 Photo 23. Washing plant cyclone
Washing plant good practice
Although water may be consumed on site in order to manage impacts associated with
quarry fines (for example, to suppress
dust), the most significant use at many sites is for the recovery of quarry fines from the aggregate in order to produce a clean product. Increasingly this is achieved in dedicated washing plants, which are designed to remove fine-grained particles and recover a clean aggregate product from
crushed rock or sand and
gravel.
Key finding
Washing plants represent the greatest opportunities for efficient water use.
Although a significant proportion of water used in a washing plant may be treated and recycled (either to the washing plant or for other uses around the site), there are still losses (such as moisture water in the product) that must be addressed through continuing inputs of additional water. In this respect, washing plant are water consumers as well as water users. While it is possible to reduce water use in the washing plant, it is important to balance such reductions with the associated loss in operational efficiency. It is important to focus on water consumption (that it, where water is lost and must be replaced). The most significant water losses occur outside of the washing plant during the treatment, discharge or recycling of the plant discharge. Ideally, water leaving the washing plant should be stripped of any
suspended solids (and other contaminating materials as appropriate) and recycled back to the plant in order to limit unnecessary
abstraction from surface water and groundwater resources. Options for cleaning the washing plant discharge include
silt lagoons and thickener / filter press systems.
From a water use perspective, thickener or filter press systems may be preferable to silt lagoons, as water losses (via evaporation, ground infiltration) are largely avoided, reducing the top-up water that is required (Photos 24, 25 & 26).
 Photo 24. Washing plant thickener
 Photo 25. Filter cake from a plate press
Where water is in short supply, operators should take steps to properly store, manage and recycle all available water. This may require the proactive capture and routing of surface run off to on-site water holding areas. Water storage areas (and also silt lagoons) should be located on ground with low permeability in order to minimise water losses into groundwater. If practical (from technical, economic and environmental perspectives), it may be possible to consider the use of dry or water efficient processes to recover
quarry fines. Dry recovery may not only minimise water use (thus reducing the environmental impact of mineral
extraction) but also may remove the need for settling ponds and lagoons and enable easier handling of fines (potentially encouraging their use in other applications).
Key finding
It is important to note that at present, the feasibility of using waterless methods has only been established for a very narrow range of quarried materials.
 Photo 26. Discharge into silt lagoon
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Crushing Plant
Good operational practice for compression crushers, ensure that:
- the crusher is correctly specified, including capacity, stroke and crusher cavity;
- product volumes and gradings are monitored throughout the life of the crusher;
- feed material is the correct size for the feed opening, well-graded and free of fines;
- feed and volume are sufficient to maintain
choke-feeding, even when feed is intermittent ;
- feed distribution is consistent to ensure an even loading on the crusher;
- crusher liners are correctly specified to ensure even wear and reduce packing;
- crusher setting is maintained to maximise product and balance total crusher loading.
Poor operational practice. Do not:
- trickle feed, as this may lead to product
flakiness and high, uneven wear;
- reduce the amount of oversize material in the feed;
- feed undersize as this may lead to uneven wear and packing;
- assume constant throughput and product grading, as feed and crusher settings may vary.
Maintenance. Monitor:
- crusher-drive motor loads, high amps may indicate mechanical or crusher liner problems;
- crusher run down times, shorter times may give early warning of mechanical problems;
- cooling water temperature and oil pressure/ flow/ temperature;
-
manganese spread; action may be required to avoid overstressing of machine frame or fouling of static members.
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