Introduction
Sizers are a type of crusher used for the reduction of a variety of materials, including coal, iron ore, gold, nickel, bauxite, salt, phosphate and more. These machines can be used in all stages of the crushing process, from primary crushing through tertiary crushing (if required). They are sized and selected based on the type of material to be crushed, infeed size and desired product size.
Sizers feature two rotating roll shafts equipped with teeth that help to pull the material being crushed between the rolls for sizing. Primary and Secondary Sizers crush the material between the inward rotating roll bodies, while Tertiary Sizers typically crush material between the outward rotating roll bodies and the adjustable crushing combs installed into the side plates. Tertiary Sizers can also crush effectively and efficiently inward as well.
The way the material is fed into the Sizer is extremely important. Improper feeding can increase component wear and hinder operation, potentially causing nuisance stalls. For optimum operation and minimal maintenance, Sizers should be fed according to the following best practices:
- Feed material should be distributed evenly and across the full length of the rolls
- Material should be fed centrally between each roll
- Avoid choke feeding the Sizer
- If using a Feeder, it is advisable that the Feeder width matches the length of the crushing rolls
- Screening ahead of the Sizer
1. Feed material evenly and across full length of the Sizer rolls
Feeding material evenly across the full length of the rolls ensures even wear rates on components and wear plates, avoiding premature changeout of teeth in a high wear area, and allows for optimum throughput.
2. Material should be fed centrally between each roll
In addition to feeding evenly across the full length of the Sizer rolls, feeding the material centrally between each roll reduces the risk of uneven component wear and optimizes throughput.
3. Avoid choke feeding
Sizers do not like to be choke fed, as it can cause higher wear on the teeth, surging of material through the machine onto the conveyor belt below (causing spillage) and nuisance stalls from having too much material in the hopper, which can affect throughput. Avoid choke feeding by incorporating a mechanical feeder into the crushing plant design. Feeders allow a steady control of material flow to the downstream circuit.
4. Ensure the Feeder width matches the Sizer length
If using a Feeder to deliver material to the Sizer, ensure the width of the Feeder matches the length of the Sizer Rolls. The material delivered from the Feeder should also be perpendicular to the Sizer rolls. This will ensure an even distribution of the material across the length of the rolls.
Additionally, feeding the material onto the first roll on a Primary Sizer closest to the Feeder between the 10 and 2 o’clock position will allow for some of the finer material to pass down the sides without the need to be processed through the middle.
While Feeders can be placed over the end of the Sizer (so that the feed is in line with the rolls), this configuration can cause high concentrated wear at one end of the machine along with the possibility of pre-mature stalling due to the high concentration of throughput in one area, limiting the full volumetric capacity of the Sizer.
5. Screening before sizing
Screening before the Sizer has many advantages, including:
- Greatly reducing wear rates within the Sizer by removing the undersized material
- Reducing overall fines generation
Best methods for feeding sizers
On occasion, Sizers can be fed either directly from an excavator or similar piece of equipment, or they can be fed automatically with Feeders, conveyors or Screens.
The best method for feeding sizers depends largely on the amount of material being processed and the crushing stage in which the sizer is located.
For example, Primary Sizers in coal applications are usually fed by a Vibrating Feeder, Apron Feeders or Drag Chain Feeders; Secondary Sizers are usually fed by Primary Sizer discharge conveyors; and Tertiary Sizers are usually fed by Secondary Sizer discharge conveyors, roller screens, Vibrating Screens or banana screens.
Mineral Sizers processing large tonnages are typically fed by an Apron Feeder, while Mineral Sizers processing low tonnages can be fed directly with an excavator or similar machine.
In most cases, feeding with a Feeder or Screen is preferable, as conveyors are typically not as wide as the Sizer, which does not allow for even feed distribution across the length of the rolls. When using conveyors for feeding Sizers, an adjustable spreader or deflector plate must be installed within the chute work between the conveyor head and the Sizer to achieve the even and central feed distribution required.
Likewise, feeding a Sizer directly from an excavator or similar may be more cost-effective up-front, but direct feeding can cause the higher teeth wear, material surges onto the conveyor (causing spillage) and nuisance stalls from having too much material in the hopper that can result from choke feeding.
Regardless of the equipment used for feeding Sizers, following the five best practices outlined above for feeding sizers will ensure optimum throughput and long wear life for the sizer’s components.
