When maximum and minimum ball sizes are known, it is necessary to define the proportions in which different sizes will constitute the load.
Loads can be characterized by two parameters, each one of them contributes in a different way to the grinding process:
- Unitary weight — This parameter is considered in the first chamber of the ball mill. Higher the value higher is the impact. It contributes to impact grinding.
- Specific surface — This parameter that must be considered in the second chamber of the ball mill. Higher the value higher is the friction grinding and higher surface generation. It contributes to friction grinding.
Average unitary weight (i) of grinding media in the load is defined as
Average specific surface (o) of grinding objects in the load is defined as
An equilibrium load is the one that is designed to keep both specific surface and unitary weight constant during operation.
For makeup loading of the mill these two factors are taken into consideration and accordingly sizes are loaded.
Example:
Suppose a mill is loaded with different size media with different weight. To find the “ i ” and “o” the calculation is done like this
Total load in the chamber = 30 Mt.
Ball details are as under
Mt of balls is calculated as per percentage used to total media
Individual weight of a ball is calculated
No of balls is calculated with weight of different sizes by ball weight
Piece weight is calculated by diving total gm of load by total no of balls
Surface produced in chamber is calculated taking media load as per the table
Surface produced by each ball size multiplied by number of balls
Surface of a ball=
Total surface (m2) for particular size ball is
Sum of total surface is the surface area produced in a chamber.
The standard for piece weight for
- Raw-meal 800 to 1000 grams.
- Closed circuit Cement mill
- 1st chamber 1200 to 1500gm.
- 2nd chamber 50 to 100gm.
- Open circuit cement mill
- 1st chamber 1200 to 1800gm.
- 2nd chamber 30 to 80gm.
- Coal 200 to 300gm.
