Tips to maximize crushing efficiency

Crushing efficiency is key to production. Photo by Kevin Yanik.

Efficiency can be defined by the ratio of a machine’s work to the energy supplied to the machine.

To apply what this means to your crusher, operations produce the exact sizes in the reduction process that their market demands. In the past, quarries produced a range of single-size aggregate products – up to 40 mm in size.

Still, the trend for highly specified aggregate means products are increasingly finer.

Currently, many quarries do not produce significant quantities of aggregate coarser than 20 mm. It is not unusual, though, for material coarser than 10 mm to be stockpiled for further crushing.

To keep your site crushing efficiently and reduce the number of fines produced, here are some tips and best practices you can follow:

Feeding jaw crushers

Jaw crusher considerations

An annual radial clearance check of the main frame bearing should be recorded to track bearing wear.
The minimum discharge setting should not exceed full load amps.
Do not crush oversize material against the barrel of the jaw. That area is not considered part of the crushing chamber.
Periodic oil sampling of the bearings is advised when oil lube is used.
The discharge conveyor and under-crusher hopper should be wider than the discharge width of the crusher.
Sufficient tension must be placed on the toggle plate to ensure proper alignment.

Fun fact

In practice, many jaw crushers are not fed to their designed 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 was working to capacity.

Jaw crushers are routinely choke fed, as this maximizes production capacity and ensures particles are uniformly broken. This promotes stone-on-stone crushing, breaking up flaky or slabby particles.

If you seek fewer fines, trickle feeding material into the jaw crusher could achieve this. But this would have an adverse effect on particle shape, and it also reduces throughput capacity, hindering the crusher’s efficiency.

Ideally, the feed rate should not be switched from choke to non-choke, as this can cause problems downstream at the 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.

Additionally, jaw crusher feed should be pre-screened using a grizzly screen prior to crushing to remove material finer than the closed-side setting.

Setting jaw crushers

Ideally, the reduction ratio of a jaw crusher should be 6-to-1. There are different ways to calculate reduction ratio, but the best way is something called the P80 factor.

The reduction ratio is then calculated by comparing the input feed size passing 80 percent versus the discharge size that passes 80 percent. The finer the closed-side setting, the greater the proportion of fines produced.

The closed-side setting of a jaw crusher helps determine the nip angle within a chamber, typically 19 to 23 degrees. Too large of 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, avoiding being crushed. The nip angle gets flatter as the machine is set tighter.

The settings on a jaw crusher are designed to produce material ideal for secondary crushing. The best particle shape is typically found in material that is about 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 may also contain a higher proportion of elongated particles because they are further from the closed-side setting. This can cause bridging issues in downstream machines.

The bottom line is that the best particle shape and performance comes from a choke-fed chamber.

Feeding cone and gyratory crushers

Cone and gyratory crusher considerations

Make sure the cone crusher does not get ring bounce or chatter.
Variable flow controlled by level sensors will ensure consistent feeding and maintain a full crushing chamber.
Low oil temperature should prevent operation of the unit.
High oil temperature should stop the crusher feed and allow the unit to cool. After the machine has cooled, further investigation of the high-temperature condition is needed before the machine is operated again.
No oil flow should prevent operation of the unit. If the condition occurs during operation, the feed should be stopped immediately – with the crusher stopped 30 seconds later.
Gradations of cone feed and crusher discharge must be monitored as a crushing/reduction performance indicator and as a liner wear indicator. This should be done at least once a month, and more often if required at the site.
Daily inspections below the crusher should be made to ensure no bridging, plugging or foreign material are under the crusher.

Did you know?

It is critical that a cone-type crusher be choke fed to produce the best product shape and quality. It is not as important in a jaw, as material is not generally stockpiled after the jaw. Because the cone is part of the secondary and tertiary stations, particle shape assisted by a choke-fed chamber is important because finished products are created in these stages.

Uniform distribution of feed material around the cone crusher inlet is good practice. This allows production of a consistent product, as well as efficient, reliable operation of the crusher.

Choke feeding is important for cone crushers because it maintains a good particle shape by facilitating an inter-particle crushing action. Trickle feeding is not the best option because it increases the proportion of flaky material in the crusher product, hindering its efficiency.

It is a good rule to maintain about 10 to 15 percent of material finer than the closed-side setting in the feed to assist crushing action. More than 10 to 15 percent will likely cause ring bounce due to the pressures in the chamber.

Setting cone and gyratory crushers

The liner profiles are designed for a range of product sizes, from extra coarse to extra fine. The extra fine liner profile will result in the highest fines proportion for a given cone crusher.

It’s important to find the right liner for the feed gradation and desired product. If the liner is too large, feed material will drop too far in the chamber before being crushed. Too fine of a liner will prevent material from entering the chamber at all.

Monitoring the crushing force as registered through the load on the crusher motors, as well as the pressure on the hydraulic mantle adjustment mechanism, will give forewarning of crusher packing problems before they affect your efficiency.

Also, the finer the closed-side setting, the greater the proportion of fines produced. The finer setting lowers throughput volume.

Try to match the closed-side setting of the crusher to the top size of the product to be produced. If closing the circuit at 1 in. to produce a 1-in.-minus product, set the crusher at or near 1 in. – or slightly below.

Feeding impact crushers

Size reduction in an impact crusher relies on energy being conveyed into the rock from the rotor, and it begins with your feed.

The initial impact is responsible for more than 60 percent of the crushing action, with the remainder made up of impact against an adjustable breaker bar and a small amount of inter-particle collision.

This is why 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.

Setting impact crushers

Impact crusher considerations

Primary impactors should be of a solid rotor type to reduce maintenance costs.
Impactors should have a connected horsepower sufficient to ensure 110 percent utilization. The crusher applications, speed and setting are the factors affecting this.
Impactors should be consistently fed based on the amperage drawn.
Impactors should be fed evenly over the full width of the rotor.
Precise calculations are needed to achieve the correct material penetration into the rotor.
The efficiency of a secondary or tertiary horizontal shaft impactor decreases with the mass and size of the material. The smaller the size, the less effective the crushing.

Did you know?

An even feed will ensure the impactor wears uniformly and continues to meet product specifications and ultimate efficiency.

Size reduction and, ultimately, the crusher setting are directly proportional to the rotor speed. This largely dictates how many fines are produced.

Slower rotor speeds can be used as a means of reducing fines but may result in a product with more oversize or return than is desired. Slower rotor speeds are preferable as a means of minimizing the wear on crusher components, as well as for achieving less fines production and optimal product size.

The product grading from an impact crusher will change throughout the life of the wear parts, particularly the impact hammers or blow bars. As the profile of the hammer changes with increased 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.

In many impact crushers, a third curtain or crushing chamber can be added to increase reduction in every pass through the machine. This can be important in finer product applications where the third chamber can provide the desired output gradation. A third chamber that increases the reduction will also increase the power needs and, normally, the wear cost.

One tip to consider: Decreasing the gap between the hammers and impact curtain increases particle retention in the chamber. This increases the size reduction ratio, but it also reduces efficiency throughput capacity and increases fines production.

Takeaways

Follow the steps outlined in this article to achieve the best crushing efficiency for jaw, cone, gyratory and impact crushers and to ultimately increase profits and reduce fines production. By taking these steps, you’re reducing the amount fines produced and adding dollars to your pocket.

Information for this article courtesy of McLanahan Corp.