Coal from run of mine (ROM), open cut or underground mines needs to be reduced to transportable and marketable product sizes.
This process, known as comminution, can be accomplished with a variety of crushing equipment.
Consideration of the application details such as desired product size, capacity, HGI (coal), percentage of rock and the hardness of rock in the feed are some of the factors that dictate crushing equipment selection for individual applications.
As many of the available types of crushers cross over application lines, other considerations such as space restrictions, mining methods and capital and operational costs will also figure in the final decision-making process.
The objective is to achieve the required size reduction while maximising process output and minimising operating costs and fines generation.
Types of crushing equipment
Size reduction takes place in stages due to the limited size reduction methodologies of individual crushing equipment.
The number of stages required for a coal mine is determined by the size of the feed and the required product output.
Feeder breakers are primarily used for primary crushing, whereas rotary breakers, DDC-sizers and rolls crushers can be used for all three stages.
The ideal crushing ratios for rolls crushers in coal are
- Primary <4:1
- Secondary <2.5:1
- Tertiary <3:1
More recently, larger diameter tertiary and secondary DDC-Sizers have been developed with a more robust tooth profile.
These machines are suitable for the ever-increasing tonnages required by mining companies. These larger diameter crushers have a greater nip angle and therefore can provide a greater crushing ratio. This has encouraged the use of two-stage crushing applications.
A two-stage crushing system is best employed when tonnages are relatively low, the coal and rock is soft and capital expenditure is limited.
Two-stage crushing systems operate best when a grizzly is installed on top of the receival hopper, or when the size of the ROM coal will not exceed 800 millimetres top size.
This allows the feeder breaker to easily crush to a nominal 150–200 millimetres, which then only requires a 4:1 reduction ratio for the secondary DDC-sizer to process to a nominal 50 millimetres.
If this cannot occur it may lead to some material sitting on top of the crusher, which is then unable to be crushed immediately.
This scenario can often generate more fines and increase the wear on the crusher segments.
Ideally, the crusher will need to grab and crush the top size coal immediately.
Two-stage crushing system: Feeder breaker and secondary sizer
If a two-stage system is employed it is recommended to install a screen before the secondary sizer. This will have a three-fold effect on the operation of the application:
- A screen will remove most on size and undersize material which does not need further crushing. This will lead to a better-quality product with reduced fines
- It will reduce the size of the secondary crusher
- It will reduce the wear rates on the crushers segments.
However, a screen is an additional capital cost.
Two-stage crushing system with screen
A three-stage crushing system is the best option for higher tonnages of hard coal and rock. Although a grizzly is beneficial, it may not be necessary in all cases.
For softer coal, a feeder Breaker can accept a 1200 millimetres lump of coal and reduce it to 350-400 millimetres in the first stage, followed by the secondary crusher with a reduction from 400 to 150 millimetres.
Finally, the tertiary crusher has a typical 3:1 ratio reduction from 150 to 50 millimetres. Again, a screen located before the tertiary crusher can be beneficial.
Three-stage crushing system: Feeder breaker, secondary sizer, screen and tertiary sizer
The three-stage crushing system will be more expensive to purchase than a two-stage system. However, it results in a better quality of coal because the crushing ratios are always < 3:1.
Couple this with screening before the tertiary crusher, and you have the ideal system; tonnages can be maintained, and fines production and crushing element wear rates will be kept to a minimum.
Brad Anstess is application and product specialist – coal at McLanahan Corporation.