Practical 1

Practical 1: Ball milling

Objective: To learn how to analyze the particle size and the techniques to reduce the particle size of coarse salt into fine powders with the aid of ball mill and to determine the factors affecting the particle size reduction.

Introduction:

Particle size reduction can be achieved by five main methods which are cutting method, compression method, impact method, attrition method, and combined impact and attrition method. Ball milling is a method which combines both impact and attrition of particles in a series of cascade reaction to produce particle size reduction. A ball mill is a type of grinding mill which is used to grind or mix metals or raw materials for further processing. A ball mill is made up of a hollow cylinder mounted in a position that enables it to be rotated on its horizontal longitudinal axis. Mills contain balls with many different diameters and size reduction occurs by impact of the particles with the balls and attrition. The grinding balls roll halfway around the bowls and then are thrown across the bowls, impacting on the opposite walls at high speed. Various materials are placed into the mill drum and rotated with the mined materials that are to be crushed. Balls of ceramic, small rocks or balls made from stainless steel are used for the purpose of crushing and grinding. The internal device of the ball mill grinds material into powder like substances and can rotate continuously for optimal grinding and refinery production. The operation of reducing particle size is important to aid efficient processing of solid particles by facilitating powder mixing or the production of suspension, exposing cells prior to extraction or reducing the bulk volume of material to improve transportation efficiency.

In this practical, we were given coarse salt. Then, we were required to grind the coarse salt by using ball milling machine. After that, we were required to determine the particle size distribution through sieving process.

Materials and apparatus:

300g coarse salts

Metal balls with different diameters

Ball mill

Sieve nest

Experimental Procedures:

1. 300g of coarse salt was weighed.

2. Stainless steel balls which consist of various sizes were put into the mill.

3. The coarse salt was put into the mill.

4. The milling process was started for around 15-20 minutes with the suitable speed of rotation.

5. After the process was completed, the products from the milling process were weighed again.

6. The products obtained were sieved using sieve nest.

7. A graph of the particle size distribution (histogram) was plotted.

Results:

Speed 5, 10 minutes

Diameter of sieve aperture (µm)	Weight (g)
300 µm	5.0079
250 µm	3.6538
150 µm	12.0589

Speed 5, 20 minutes

Diameter of sieve aperture (µm)	Weight (g)
300 µm	20.2832
250 µm	0.1794
150 µm	2.8825

Speed 3, 10 minutes

Diameter of sieve aperture (µm)	Weight (g)
300 µm	0.2967
250 µm	1.3862
150 µm	12.0507

Speed 3, 20 minutes

Diameter of sieve aperture (µm)	Weight (g)
300 µm	0.4682
250 µm	0.8602
150 µm	11.1693

Questions:

1. What are the factors which can influence the particle size reduction?

One of the factors that can affect the particle size reduction is the amount of material in a mill which is the feed. When there is too much feed, cushioning effect will be produced; when there is too little feed, it may results in loss of efficiency and abrasive wear of the mill parts. Another factor is the speed of rotation. At low angular velocities, the ball move with drum until the gravity force exceeds the frictional force of the bed on the drum, and the balls slide back to the base of the drum. The sequence is repeated, very little relative movements of balls are produced so size reduction in this case is also minimum. At high angular velocities, the balls are thrown out onto the mill wall by centrifugal force and no size reduction occurs. Diameters of the metal balls also affect the size reduction. The large balls will break down the coarse feed material while the small balls help to reduce the void between the balls in order to produce fine products. Moisture content and surface hardness also plays a role in particle size reduction.

2. What apparatus can be used to reduce the particle size?

The equipments that can be used to reduce particle size are cutter mill, mortar and pestle, end-runner mill, edge-runner mill, hammer mill, vibration mill, roller mill, ball mill and fluid energy mill.

3. What is the factor that can affect the selection of apparatus to reduce the particle size?

Selection of size reduction mills are carried out based on the particle properties such as hardness, toughness, the size reduction range required, the available of equipment and the cost of size reduction.

Conclusion:

Ball mill is the example of combination method which produce size reduction by both impact and attrition of particles. Optimum particle size reduction can be achieved by taking all the factors that will enhance the process into consideration. Besides that, choosing the appropriate machine to produce particles with desired characteristics is also important in particle size reduction.