Practical 1 - Ball milling

Objective:

a)      To determine the technique of using ball milling.

b)      To determine the technique of sieving using sieve nest.

c)       To reduce the particle size of coarse salt into fine salt by using ball milling.

d)      To determine the particle size distribution of the coarse salt.

e)      To determine the factors affecting the process of reduction of size of particle.

Introduction:

          Ball mill is widely used for grinding ores and rocks in metal and non-metal mine. The main bearing of mill with cylinder diameter of 900-2700mm adopts the sphere tile bearing. The simple working principle of the ball mill can be explained as below:

          Reducing the speed by motor through speed reducer and gearwheel can drive the ball mills rotary par. The grinding steel balls are placed inside the cylinder. The material to grind is fed to the cylinder. The ball miller is started with suitable speed. The materials will be crushed into smaller size.

          In this experiment, coarse salt will be placed into the ball mil and with the aid of metal balls of different sizes the coarse salt will then be grinned into fine powder where the size particle of salt is determined by sieving using sieve nest. Particle size distribution then can be determined by plotting a graph.

Materials and Apparatus:

1.       Weight balance

2.       Different size of steel ball

3.       400 g of coarse salt

4.       Ball mill

5.       Sieve nest

Procedure:

1.       300-500 g of coarse salt is weighed.

2.       2. Steel balls of varying sizes are inserted into the mill.

3.       The weighed salt is placed inside the mill.

4.       The milling process is began for 15-20 minutes using a suitable speed setting.

5.       Upon completion, the crushed salt is weighed.

6.       The crushed is sieved using a sieve nest.

7.       A histogram of particle size distribution obtained is plotted after sieving.

  

Results: 

Weight (g) 300μm	Weight (g) 250μm	Weight (g) 150μm	Final weight (g)
21.389	2.1087	1.7108	25.3449

·         Percentage frequency

Particle size, 300μm	21.3890 g 25.3449 g X 100%=84.39 %
Particle size, 250μm	2.1087 g 25.3449 g X 100%=8.32%
Particle size, 150μm	1.7108 g 25.3449 g X 100% =6.75%

Discussion:

          The coarse salt can be broken down into smaller fine salt by using the milling process. Then, the fine salt will undergo the sieving process to assess varying particle sizes in order to construct the particle size analysis. By theory, the particle size is gradually decreasing in size with the increasing of the speed of the ball mill. The longer the time used for milling process, the smaller also is the particle. The size of the salt is more dominant.

          Based on the histogram plotted, the particle size distribution of coarse salts was observed. From the histogram shown in the result, the highest amount of salt will located at the particle size of 300μm at 84.39% , follow by 8.32% of 250μm and 6.75% of150 μm of salt in the decreasing percentage of frequency.

          There is a precaution in this experiment. Should ensure that the salt are in minimal moisture as the existence of water will increase the cohesive force among the salt particle which cause them to join together and form a larger participle or the salt will dissolve in the water. As a result, this will affect milling processes.

          There are a few errors occur which cause the result. For example, the inconsistency of the force exerted during the sieving which cause the irregular process of sieving occur. Hence, ensure consistent force when carrying out sieving. As a conclusion, after undergone milling and sieving process, the size of the coarse salt has the highest percentage of frequency that located at the particle size of 300μm.

Questions:

1.       What are the factors that need to be considered in choosing the suitable size reducing equipment?

·         There are many factors that need to be considered in choosing the suitable size reducing equipment which are the type of particle, financial aspect, efficiency of the equipment and availability of the equipment.

·         Firstly, the type of particle is the main factor that need to be consider in choosing a suitable equipment to reduce its size. For example, to reduce the size of particle in the emulsion, the equipment used is not suitable to reduce the particle size in powder.

·         Next, is the financial aspect. The equipment that is chosen should be cost and environmental friendly that is affordable. For example, for the reduction of size in small scale, the using of a larger and more expensive equipment is not necessary.

·         Then, the efficiency of the equipment. The equipment that can reduce the size of particle in a shorter time should choose.

·         Lastly, is the availability of the equipment. The equipment that is easily available in the market should be chosen.

2.       What other equipment can be used in size reduction of solid materials?

·         Surface grinder removes large amounts of material and grinds flat surfaces with noted spiral grind marks. It also can be used to make and sharpen,burn-outs, metal stamping die sets, flat shear blades, fixture bases or any flat and parallel surfaces.

·         The Silverson rotor/stator workhead can achieve rapid and uniform milling of both solid and semisolid materials to a low micron size in a single operation.

·         Crushers may be used to reduce the size, or change the form, of waste materials so they can be more easily disposed of or recycled, or to reduce the size of a solid mix of raw materials.

3.       What are the factors that can influence the size reduction process of a ball mill?

·         There are two main factors that can influence the size reduction process of a ball mill which are the particle and the ball mill used.

·         Firstly, is the factor of the particle. The hardness, slipperiness, melting point and moisture content can affect the size reduction process of a ball mill. If the particle is very hard , then it require larger force or more difficult to break it into smaller size.

·         Meanwhile, for the particle with high water content or slipperiness, it will posses a higher cohesive among the particles and hence the particles ill then join together and form a larger particle which is more difficult to break into smaller size.

·         Then, for the melting point, it is mainly affected by environmental temperature. I f the surrounding temperature is high and as well as the particle possess a lower melting point, then the particle will melt easily.

·         Next, is the factor of the ball mill that has being used. The higher the speed of this instrument, the particle can break into smaller size.

·         The time of grinding can also affect the size reduction process of a ball mill. The longer the time, the smaller is the size of particles.

Conclusion:

The large particle size of the coarse salt can be reduced by using ball milling. The particle size distribution of the particle can be determined from sieving after plotting the distribution graph.

Many factors will affect the reduction of the size of the particles by using ball mill which are mainly the particle factor ( for examples, hardness, slipperiness, melting point and moisture content ) and the equipment factor ( for examples, speed and the duration of time).

References:

1.       Second edition Pharmaceutics the Science of Dosage Form Design, M.E. Aulton.

2.       http://en.m.wikipedia.org/wiki/Ball_mill

3.    http://www.irjponline.com/admin/php/uploads/1934_pdf.pdf