Summary
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SUMMARY OF GOOD PRACTICE: PRODUCTION AND PROCESS TECHNOLOGY
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| Good practice for extraction |
The extraction, or quarrying, of material is by drilling and blasting, or simply mechanical excavation, followed by haulage to the processing plant by truck or conveyor.
- Overburden should be reused in site restoration; preferably direct placement where possible.
- Blasting is required for hard rock excavation; wherever possible alternatives (such as mechanical ripping or direct excavation) should be used.
- Blasting should be designed to obtain the optimum fragmentation to reduce the need for secondary breakage but without producing excessive fines.
- Unnecessary noise and vibration associated with blasting can be minimised by controlling drill hole parameters, sequential blast designs and optimising the amount of explosives used
- Sand and gravel quarries are either worked wet or dry, but dry extraction is the most efficient as it allows greater resource recovery and more selective extraction.
- Hauling of extracted rock is a significant production cost; the use of conveyors is a cost-effective alternative to trucks.
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| Good practice for production plant |
The production of aggregate and other quarry products is carried out by crushing, washing, screening and classification. The current UK market requirement for finer aggregate products has resulted in higher fines production across the quarrying industry so quarry fines minimisation has become an increasing priority for operators.
- Production plant should be designed to meet product specifications; plant design should start with the products in mind rather than the as-quarried rock to avoid unnecessarily complicated processing and fines production.
- Crushing should be carried out in several stages with small size reduction ratios; the number of stages should be optimised to limit fines production.
- The amount of energy required to crush rock is known as the grindability or Bond Work Index; this can be used to select the most appropriate crusher.
- It is generally accepted that compression crushing produces less fines than impact crushing; to minimise fines, avoid crushing processes that have major components of attrition and abrasion.
- Attempts to minimise fines production should be focused on the later stages of production; primary crushing typically produces less than 10% fines whereas secondary and tertiary crushing produces up to 40% fines.
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| Good practice for crushing equipment: jaw crushers |
Jaw crushers are typically used as a primary crushing stage and are one of the quarrying industry workhorses.
- Ideally, the closed side setting (CSS) should be set to give a size reduction of less than 6:1; smaller settings will produce more fines than larger settings.
- Choke feeding is preferable; this helps to reduce impact and wear on the jaw plates, minimises the top size and reduces the production of flaky material.
- Lowering the crusher speed will reduce the amount of fines produced; the trade-off will be a correspondingly lower crusher throughput capacity.
- Optimum selection of the jaw crusher throw is important as it maximises the amount of energy used in crushing; short throws are used for fine-grained, brittle, hard rock (such as basalt and limestone) and longer throws for coarser-grained, tough, more-elastic rocks (such as granite and sandstone).
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| Good practice for crushing equipment: gyratory and cone crushers |
Gyratory crushers are typically used in primary crushing stages; whereas cone crushers are typically used in secondary and tertiary crushing stages.
- Ideally, the closed side setting (CSS) should be set to give a size reduction of less than 6:1; lower reductions may cause boiling in the crushing chamber and it should be remembered that smaller settings will produce more fines than larger settings.
- Material should be uniformly distributed as it is fed into the crusher.
- Choke feeding is preferable, typically the cone crusher is buried in feed material; this helps to reduce impact and wear on the crusher liners, improves throughput capacity, minimises top size and reduces the production of flaky material.
- Gyratory crushers tend to be more tolerant of variations in feed material and feed rate than cone crushers.
- Increasing the crusher speed will improve the particle shape and increase the throughput capacity; the trade-off is an increase in the crushing chamber residence time and higher fines production.
- Regular sampling and particle-size analysis of crusher product is useful as it indicates when liners need to be replaced.
- Cone crushers should be considered as an alternative to impact crushers especially where good (cubical) particle shape and fines minimisation are required.
- Monitoring the load on the crusher motors and hydraulic mantle adjustment system will forewarn of crusher packing problems.
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| Good practice for crushing equipment: impact crushers |
Impact crushers are typically used as a final crushing stage to reduce the proportion of flaky material; they produce a higher proportion of fines than compression crushers.
- Feed material should be introduced into the crusher across the full width of the rotor to ensure uniform product gradings and uniform wear in the crusher.
- Higher rotor speeds will increase the size reduction ratio and fines production; slower speeds will reduce fines but also results in a poor product shape.
- Increasing the proportion of rotor feed to crushing chamber feed in VSI crushers will reduce the fines produced; the grading of the crusher product will also be coarser.
- Increasing the gap between the rotor and impact surfaces will reduce the crushing chamber retention time, reduce size reduction ratio and lower fines production.
- Screening and recirculation of oversize material will improve the aggregate particle shape (more cubical), however closed circuit crushing increases fines production.
- Impact crushers should be replaced by cone crushers where good (cubical) particle shape and fines minimisation are required.
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| Good practice for washing plant |
Washing is carried out to produce clean sand and gravel, or recover stone from clay-rich scalpings at crushed rock quarries; aim to remove material finer than 0.063 mm.
- All water should be recycled and reused wherever possible.
- Water consumption can be minimised by water-efficient technology; thickeners and filter presses enable the reuse of process water, and minimise the need for lagoons.
- Lower quality, recovered water may be acceptable for use in less-critical applications such as dust suppression.
- Demonstration of water efficient practice is a requirement for the renewal of time-limited abstraction licences; the adoption of waterless fines removal technology would remove the need for silt lagoons and enable the easier handling of fines.
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| Good practice for dry fines recovery |
Dry fines recovery is carried out by dry screening and/ or air classification. Feed material requires drying, this is an energy-intensive process.
- Low moisture contents are required for efficient dry fines recovery; for the removal of fines (<0.063 mm) the moisture content should be 0.1 to 0.5 wt%.
- Centrifugal air classifiers are recommended for removal of fines; efficiency of fines removal relies on a controlled feed, stable airflow and multiple classification stages.
- Screening should be optimised to ensure efficiency; inefficient screening results in poor quality products, increased load on crushers and over-crushing.
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