PRACTICAL I : BALL MILLING

TITLE  
Ball Milling

OBJECTIVES
a.)   To reduce the particle size of coarse salt into fine powders by using ball milling.
b.) To determine the factors affecting the process of reduction of size of particle.
c.)  To determine the factors influencing the selection of apparatus which reduce the size of particle.

INTRODUCTION

A ball mill is an example of a comminution method which produces size reduction by both impact and attrition of particles. Ball mills consist of a hollow cylinder mounted such that it can be rotated on its horizontal longitudinal axis. Cylinder diameters can be greater than 3 m, although much smaller sizes are used pharmaceutically. The cylinder contains balls that occupy 30-50% of the total volume, ball size being dependent on feed and mill size. Mills usually contain balls with many different diameters owing to self-attrition, and this helps to improve the product as the large balls tend to break down the coarse feed materials and the smaller balls help to form the fine product by reducing void spaces between balls.

In this experiment, coarse salt will be placed into the ball mill. With the aid of metal balls of different sizes, coarse salt will then be comminuting into fine powder where the size particle of salt is determined using sieve nest .Sieve analysis is a method used to determine the particle size distribution of  a granular material and in this experiment, the granular material is coarse salt.

MATERIALS AND APPARATUS

400g coarse salt

Metal balls with different diameters

 

Ball mill

           

sieve nest

      

PROCEDURES

1. 369.3g of salt was weighed.
2.   Steel balls with various sizes were placed into the mill.
3. 369.3g of salt was then added into the mill.
4. Milling process was carried out for 10-20 minutes with suitable speed of rotation.
5. After the milling process was completed, salt powder was weighed.
6. The salt powder obtained was sieved by using sieve analysis (sieve nest).
7. Particle size distribution graph (histogram) was plotted after sieving.

RESULTS

Weight of salt before milling = 369.3g

Weight of salt after milling = 358.5g

Group	Sieve diameter (micrometer)	Particle Size Range (micrometer)	Weight (g)	Time (minutes)	Speed
1 & 2	150	150<x≤250	5.0079	10	5
	250	250<x≤300	3.6538
	300	300<x≤500	12.0589
3 & 4	150	150<x≤250	2.8825	20	5
	250	250<x≤300	0.1794
	300	300<x≤500	20.2832
5 & 6	150	150<x≤250	0.2967	10	3
	250	250<x≤300	1.3862
	300	300<x≤500	12.0507
7 & 8	150	150<x≤250	0.4682	20	3
	250	250<x≤300	0.8602
	300	300<x≤500	11.1693

Discussion:

At the beginning, there are about 369.3g of coarse salt was being used in ball milling, however, from the histogram, it is obvious that every group have large amount of salt that have particle size of 300≤ X≤500µm. This means that there is much more salt has weight greater than 500 µm. This shows that the process of ‘ball milling’ is not effective in reducing the size particle of coarse salt. This is because there are some errors we had did during the experiment even though we take into account all the factors which will enhance the size reduction process. 

There are some errors can be found during experiment. This is because every group has different result as there are different students to handle each experiment according to the factors.  Next, some of the salt was still left in the sieve nest and couldn’t took it out since it was stick at there, therefore the weight of the salt was inaccurate.

There are several factors that will affect the size reduction process. The first factor is the speed of rotation while operating the ball mill. At high angular velocities, the balls are thrown onto the mill wall by centrifugal force and no size reduction occurs. At low angular velocities, the balls move with drum until the gravity force exceeds the frictional force of the bed on the drum, then it will slide back to the base of the drum. The sequence is repeated, very little relative movements of balls are produced thus size reduction is also less. At two-thirds of the critical angular velocity where centrifuging occurs, a cascading action is produced. Balls are lifted on the rising side of the drum until their dynamic angle of repose is exceeded. At this point they will fall or roll back to the base of the drum in a cascade across the diameter of the mill. By this means, the maximum size reduction achieved. The second factor is the time taken for milling process. The milling efficiency decreases with increased milling time as the particles become smaller. It is assumed to subside completely when the grain size reaches a critical value. This is a consequence of the force applied to the slurry as two milling balls approach one another, causing a slurry flow away from the balls prior to collision, as seen in Figure 3. The smaller the particle, the more likely it is to be caught in the slurry flow.

Figure 3: A schematic illustration of the slurry flow and forces applied to a particle between two approaching milling balls.

                                  

The third factor is the types of material used. The exact type of bowl and balls used depend on the type of material being ground. For example, very hard samples might require tungsten carbide balls in steel bowls. Therefore, the ball mills that we used might not be suitable for grinding coarse salt. But for typical use, agate is a good choice. The fourth factor is the size of grinding media. The large metal balls will break down the coarse material while the small balls will help to reduce the void between the balls in order to produce fine products. The size of the metal balls we used during this experiment may be are not suitable, therefore the grinding of coarse salt into fine powder is not effective. So, the metal balls used should be variable in diameters and sizes in order to obtain finely grinded powder.

There are many apparatus that can be used to reduce size particles, including cutter mill, vibration mill, fluid energy mill, end-runner, edge-runner mills, hammer mill and roller mill, pestle and mortar. There are some factors that will affect the choosing of apparatus for particle size reduction. Usually, the selection of the size reduction mills is based on the particle properties such as the hardness of the product, the extend of size reduction required, the availability of equipment and the cost of size reduction.

CONCLUSION

Particle size can be reduced by the use of ball milling. However, from the result that we had, we know that the ball milling process is not effective as much more salt is greater than 500 µm. We should take into account the factors which will affect the reduction of particle size to increase the reduction of particle size. Selecting the appropriate machine to produce particles with desired characteristics is also important in particle size reduction.

REFERENCE

Aulton, M.E. 2002. Pharmaceutics: The Science of Dosage form Design. Edinburgh Churchill Livingstone.