John Flynn, of Terex Jaques, explains that quarries have numerous factors to consider when selecting cone crushers to create quality aggregate products.
Successful application of a cone crusher within a crushing circuit is measured by the amount of material passing through the cone, the power draw of the machine, the size distribution of the products coming out of the circuit and the shape of the product. The goal is to efficiently and economically produce the target products, conforming with the required specifications.
The mechanical factors that affect the production rate and quality of the material processed by a cone crusher include:
- Cone head diameter.
- Crushing chamber slope (angle).
- Cone head stroke.
- Gyrating speed.
- Manganese liner profile.
- Closed side setting (CSS).
- Crushing force, monitored as operating pressure.
- Applied power.
- Feed control.
In any crushing operation, physical characteristics of the material being processed affect the output product. The material characteristics that affect the crushing process include:
- Compressive strength.
- Bulk density.
- Feed gradation.
- Moisture content.
A measure of the size reduction achieved for a particular crusher application is the reduction ratio. A cone crusher in a secondary crushing application will typically work with a 3.5:1 to 5:1 reduction ratio. Tertiary cone crusher configurations typically work with a reduction ratio of 2.5:1 to 4:1.
The reduction ratio is defined as the ratio of the feed size for which 80 per cent will pass (F80), divided by the product size for which 80 per cent will pass (P80). The reduction ratio for tough, high strength, damp material is restricted to the low end of the application ranges whereas a soft, low strength, dry material can be successfully crushed at the higher end of the reduction ratio.
LONG STROKE ADVANTAGES
The newer, longer stroke, high powered machines of today will outperform the machines in common use 25 years ago. A large stroke provides a greater cross-sectional area for material to pass through the crushing chamber in a given amount of time. The result is that the longer the stroke, the greater the volume of material that can be processed through a given size machine.
The influence of crusher speed, or gyrations of the cone head per minute, is not as well defined as stroke. Depending on the crushing stroke, the CSS and the crushing chamber profile, the effect of increased speed can either increase or decrease the production rate of the crusher. For any combination of these, a different “sweet spot” in speed will produce maximum throughput for a given feed.
In general, a coarse crusher, such as a secondary cone in open circuit, should be run at the low end of the speed range. As the crushing becomes finer, an increase in speed has been found to be beneficial, especially when shape is a factor. Therefore, it is recommended that tertiary crushers that are operated in closed circuit and at the higher end of the speed range. It needs to be understood that the faster the crusher runs, the faster the manganese wears. The life of other mechanical components will also be reduced. Therefore, the optimum speed for any application is the slowest speed that produces the desired production rate, gradation and shape.
LIMITATIONS ON CSS
Minimum closed side setting for any cone crusher is that setting just before the factory recommended limit of operating pressure is reached. This is the point at which the hydraulic relief system will act to open the CSS. Minimum CSS may be greater or smaller than published settings based on the conditions and crushing characteristics of the material being processed. Generally, clay or other plastic material in the crusher feed must be eliminated to prevent the formation of compacted material or “pancakes”, which are non-crushable and will cause activation of the tramp iron relief system.
PRODUCING QUALITY PRODUCT
The breakage of rock in a compression crusher can result in a percentage of flat or elongated product. However, most construction specification rock products require a cubical product. The cubicity of the cone crusher product can be improved with the proper circuit design, screen selection and crusher operating parameters.
TEN RULES FOR CUBICAL PRODUCT SHAPE
When a cubical product shape is required, the following process controls when correctly applied to the crushing circuit will minimize the flat and elongated particles generated inside the cone crusher:
1. Keep the crusher choke-fed.
Choke feed means to keep the head covered with at least 150mm of feed material. Conditions that help keep the crusher choked include surge bins, bin level sensors, adjustable speed feeders, and automation.
2. Stable, continuous feed.
The gradation of the feed material should be continuous from the maximum to minimum particle size. Do not allow “holes” in the feed gradation (gap graded material). Use a well graded feed.
3. Keep some material smaller than CSS in feed.
Keep up to 15 per cent of feed below the CSS to encourage rock on rock attrition crushing. The small particles in the feed fill the voids between the larger particles which increase the density and promote attrition crushing, thereby improving the product shape.
While it is beneficial to retain some feed below the CSS, it is advisable to screen fines (-5mm) out of the feed to minimise compaction and avoid a tramping condition where the crusher will relieve under high pressure.
4. Restrict final stage reduction ratio to 3:1.
Feed size should be restricted so a high ratio of reduction is not attempted in one pass.
5. Distribute the feed evenly.
Crusher feed should be evenly distributed around the centre of the crushing chamber. Avoid feed segregation where the coarse new feed is on one side of the feed opening and the finer recirculated feed is on the opposite side of the feed opening.
6. Setting should be close to desired product size.
Studies have shown that the most cubical shape is achieved for those particle sizes that are close to the CSS. As the particle size gets larger or smaller than the CSS, the cubicity deteriorates.
7. Use an appropriate crushing chamber for the application.
The most efficient crushing chamber promotes continual crushing of material as it travels through the chamber. If the liners are too coarse, the material falls too far into the chamber which can lead to packing, increasing crushing pressure, power draw and inducing a bowl float or tramping condition. If the liners are too fine, the feed opening will restrict the larger lump sizes from entering the crushing chamber, resulting in reduced throughput, poor choke condition, decreased power draw and elongated product. In some cone crusher designs the feed opening will reduce as the liners wear.
8. Operate the crusher in closed circuit.
Operating a secondary cone crusher in closed circuit limits the material top size for processing in the tertiary circuit and allows the near size particles to fill the voids between the larger particles to induce attrition crushing and break flaky elongated particles. Operating a tertiary cone crusher in a closed circuit provides consistent input gradation and improves the shape by attrition crushing.
9. Operate the crusher at the correct speed for the application.
Running the crusher too fast can generate excessive fines because the material cannot fall as far with each revolution of the eccentric. The material becomes over-crushed and generates excessive crusher dust. Running the crusher too slow will coarsen the output and not allow the attrition rock on rock crushing action that leads to a cubical product.
10. Use the correct flow sheet.
Carefully analyse the complete flow circuit to balance the production. For best shape, do not mix secondary with tertiary/quaternary products.
GUIDE FOR CRUSHING CIRCUIT DESIGN
In any crushing circuit, it is good common sense to get the material to the product piles as quick as possible. This will reduce the wear and tear on the equipment and can lead to increased efficiency and capacity of the circuit because the finished product is not taking up room on the screens, conveyors, and in the crushers.
The size of the raw feed or blasted material and the specification of the final products will dictate the number of crushing stages and screens required. If there is also a shape specification, additional controls and crushing stages may be required. A low reduction ratio application may be able to get by with a jaw/cone/ screen circuit (two-stage crushing). A higher reduction ratio application may require a jaw/ screen/cone/screen/cone circuit (three-stage crushing). A large reduction ratio application may require four or more stages of crushing.
The crusher breaks down the rock to a smaller size. The screen is the “cash register” of the crushing circuit in that it determines the final gradation of each product pile. In general, it is good practice to oversize the screen to account for changes in environmental conditions that can affect
the overall performance of the crushing circuit.
CONE CRUSHER TYPES
Cone crushers can be categorised into three main design types. With floating bowl and screw bowl cone crushers the upper frame raises to open CSS or relieve crushing pressure. The third type – the spider-bearing cone crushers – incorporate a shaft supported by a hydraulic piston which controls CSS and crushing pressure. Each machine type has its own features and advantages and each is best suited to particular applications. Terex, through the TC cone, Cedarapids MVP, TG cone and Jaques Gyracone, includes each of these main cone crusher types within its equipment range to ensure it can provide the best option across all crushing applications.
John Flynn is the Australia and New Zealand business manager for Terex Jaques. Visit terex.com/mps/en-aus