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.
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.
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 crushers 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.
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.
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.
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.
Its 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.
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.
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.
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.
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, youre reducing the amount fines produced and adding dollars to your pocket.
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