Different size reduction equipments can be used to obtain products of desired sizes. For example Crushers are used for coarse reduction of large quantities of solids. Grinders are used for intermediate and fine crushing. Ultrafine grinders give even more fine crushing.
To understand the concept of open and closed circuit grinding better, we first need to have the knowledge of types of feeding. There are two ways of feeding the material to the equipment :
- Free Feeding
- It is the feeding of material at a comparatively low rate so that the product can readily escape.
- Residence time of particles in machine is small.
- Production of appreciable quantities of excessive under size material is avoided.
- Less electrical power is consumed.
- Feed rate is slow.
- More manpower required.
2. Choke Feeding
- Machine is kept full of material.
- Discharge of products is impeded.
- Residence time of material is more.
- Less manpower required.
- More electrical power consumed.
- Used when crushing is to be done in one go.
- Excessive under size product is obtained due to higher degree of crushing.
- Used when less quantity of material is to be reduced in size.
Now, after having the idea about types of feeding, we shall talk about Open and Closed circuit grinding.
1. Open Circuit Grinding
- The plant is operated in Choke feeding mode.
- Material is passed only once through the equipment. After size reduction, products are directly sent to another unit without any recycle of over sized particles back to the equipment.
- One time feed - one time output.
- Therefore it is also called 'Once Through' type of machine.
- Used if the output obtained has a satisfactory particle size.
- Suitable for lower reduction ratio.
- Simple in operation.
Advantages :
- Less manpower required to operate the process.
- The operation involves only one-time input and one-time output. So, it is simple to operate.
- The product obtained is already of desired size range. So, no classifier required for further separation. So, Lower capital cost required.
Disadvantages :
- Low production rate.
- Higher energy consumption.
2. Closed Circuit Grinding
- Multiple time crushing is done.
- It is a sequential process in which feed is processed multiple times.
- Initially material is fed to crusher, after size reduction the products are sent to classifier for separation and then the over sized particles are sent back to the crusher as feed.
- It gives better output i.e particles of uniform size.
- Applicable to higher reduction ratio.
- Relatively complex in operation.
Advantages :- Low power requirement.
- Operated in free feeding mode.
- Higher capacity.
- No excessive grinding of particles.
- Comparatively negligible amount of coarse particles present in final output.
Disadvantages :- High capital cost.
- More manpower requirement.
- Slow feed rate.
- The plant is operated in Choke feeding mode.
- Material is passed only once through the equipment. After size reduction, products are directly sent to another unit without any recycle of over sized particles back to the equipment.
- One time feed - one time output.
- Therefore it is also called 'Once Through' type of machine.
- Used if the output obtained has a satisfactory particle size.
- Suitable for lower reduction ratio.
- Simple in operation.
Advantages :
- Less manpower required to operate the process.
- The operation involves only one-time input and one-time output. So, it is simple to operate.
- The product obtained is already of desired size range. So, no classifier required for further separation. So, Lower capital cost required.
Disadvantages :
- Low production rate.
- Higher energy consumption.
2. Closed Circuit Grinding
- Multiple time crushing is done.
- It is a sequential process in which feed is processed multiple times.
- Initially material is fed to crusher, after size reduction the products are sent to classifier for separation and then the over sized particles are sent back to the crusher as feed.
- It gives better output i.e particles of uniform size.
- Applicable to higher reduction ratio.
- Relatively complex in operation.
Advantages :
- Low power requirement.
- Operated in free feeding mode.
- Higher capacity.
- No excessive grinding of particles.
- Comparatively negligible amount of coarse particles present in final output.
Disadvantages :
- High capital cost.
- More manpower requirement.
- Slow feed rate.