I am trying to determine suitable values of ball to powder ratio(BPR), diameter of the ball and milling time for a material.
There are certain general rules that apply to ball milling, for example:
- The finer the finished powder, the smaller the balls have to be: ball diameters should not be more than 1000 times larger than the particle size at the end of grinding.
- The coarser the starting powder, the larger the ball size should be: Ball diameters must not be less than 50 times larger than particles before grinding.
- The mass of the ball charge must be between 5 and 10 times the powder charge.
- Diameter of the rolling ball mill must be 20-30 times larger than the ball diameter.
- Grinding time is in a non-linear (quadratic or even logarithmic) dependence to reduce particle size. The longer the grinding process takes, the more inefficient it becomes.
The search for an optimal grinding parameter set is a complex and purely empirical task because the number of influencing factors is very large.
Dear Jagannadh Sripada
As suggested by Dr Vadim Verlotski , Mechanical alloying/milling is a complex process that depends on number of variables (eg. Milling time, speed, BPR, PCA, Milling media etc.) to achieve the desired product phase and/or microstructure. As you may aware, these variables are inter-dependent. For eg. if BPR is increased, then milling time taken to achieve a particular particle size/phase formation is less and vice versa. Even though it is suggested to have BPR of 10-20:1 it is also reported to vary from 1:1 to 200:1. Again the final properties depend on kinetic energy imparted during milling in terms of milling media and time.
Reference: Mechanical alloying and milling, C Suryanarayana
Anas
Dr. Vadim Verlotski Thank you for your answer. Is there any specific method to factor in the material specific properties such as flowbility of the powder, hardness and density of the material etc to determine the BPR and diameter of the balls?
Mr. Anas N S Yes I am aware of the variables associated with milling. My curiosity is that to achieve a specific particle size distribution, if the material properties of the precusor material will effect it's output? If so, I am interested in the methods and studies associated with it.
Dear Jagannadh,
It is clear that the grinding process has to be determined primarily by the material properties of the ground powder. Such properties as hardness, brittleness, impact resistance, chemical resistance to oxygen and water (when wet milling) are decisive for the choice of grinding balls (steel, hard metal or ceramic) and possible milling variants (wet or dry). Without knowing what material you want to grind, it is not possible to give a recomendation for the grinding process. To give specific advice about milling I need the following data:
- Powder material (detailed description if possible),
- Particle size required at the end of grinding,
- Required powder quantities (necessary for the practical selection of the milling device).
Regards
Vadim
Dear Jagannadh,
SPEX 8000 mill is a small laboratory mill with only one pair of balls. This grinder is suitable for a quick sample preparation of quite coarse powder with undefined grain size distribution, but in no case can you seriously optimize the milling process with this device. If you can prepare sufficiently good powder samples with your SPEX 8000, it is OK, but you cannot study the milling process with this mill. All the things we have discussed (BPR and others) apply to the grinding process in a classic ball mill and not to a "toy" laboratory device.
Regards
Vadim
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