Objectives
1.
To break solid particles into smaller particles
or powder.
2.
To grind the given substances
to a smaller size using a ball mill.
3.
To obtain the size distribution
of the initial and the final mixture by sieving.
4.
To analyze the results using
the available theories.
Introduction
The basic motive behind performing the
size-reduction is to make too large to be used solid
materials usable. It leads to an increase
in surface area per unit volume which enhances the rate
of the reaction by allowing more sites for
the reaction to take place. Moreover handling of
smaller size particles is much easier as
compared to that of bigger particles.
Size
reduction is major unit operation in industries handling particulate solid. The
industries like
mineral dressing, paint, pharmaceutical as
well as black powder handles large amount solid
materials which need to be grinded to fine
size.
In mineral processing, particles containing
valuable minerals must be broken
sufficiently fine to liberate valuable
minerals from waste constituents, so that they can
be easily separated by an appropriate
beneficiation method. In power plants, coals are
ground to increase fineness, which
increases particle surface area. This allows for a
proportional increase in the burnout rate.
Grinding is generally an inefficient
process and there are many factors that can affect
grinding performance. Grinding performance,
in terms of material breakage and power
consumption, has been studied with a wide
range of operating parameters such as mill speed,
charge filling, ball size and lifter type.
The presence of worn balls inside the charge is very evident
inside the mill, but their contribution and
their effects on milling kinetics; load behaviour and mill
power is not fully established.
Procedures
1.
300-500g of coarse salt was
weighed.
2.
Metal balls of various sizes
were inserted into the mill.
3.
Coarse salt was added into the
mill containing the metal balls.
4.
Milling process was carried out
at speed 5 for 10 minutes.
5.
After 10 minutes, coarse salt
was weighed again.
6.
Powder obtained was sieved
using the sieve nest.
7.
Graph of paricle size
distribution(histogram) was plotted using the method above.
Results and calculation
Initial weight of coarse salt: 339.9256g
Layer of sieve nest
|
Amount of salt collected(g)
|
1st layer( most coarse)
|
288.2349
|
2nd layer
|
15.7145
|
3rd layer
|
6.0543
|
4th layer
|
5.3691
|
5th layer( most fine)
|
4.1173
|
Final weight of salt collected:
288.2349+15.7145+6.0543+5.3691+4.1173
=319.4901g
Differences in the initial and final value:
339.9256-319.4901
=20.4355g
Results:
Group
|
Weight (g)
|
Final
weight
|
||
300µm
|
250µm
|
150µm
|
||
Group 1 &
group 2
Speed = 5 rev
Time = 10 minutes
|
12.0589 g
|
3.6538 g
|
5.0079 g
|
20.7206 g
|
Group 3 &
group 4
Speed = 5 rev
Time = 20 minutes
|
20.1958 g
|
0.1794 g
|
2.2832 g
|
23.3446 g
|
Group 5 & group 6
Speed = 3 rev
Time = 10 minutes
|
12.0507 g
|
1.3862 g
|
0.2967 g
|
13.7336 g
|
Group 7 &
group 8
Speed = 3 rev
Time = 20 minutes
|
11.1693 g
|
0.8602 g
|
0.4682 g
|
12.4977 g
|
Calculations:
Percentage frequency: group 1 & 2 Percentage frequency: group 3
& 4
Particle size, 300 µm = 12.0589g/20.7206 Particle
size, 300µm = 20.2832g/23.3446g
=
0.5819 x 100% = 0.8688 x 100%
= 58.19% = 86.88%
Particle size, 250µm = 3.6538
g/20.7206 Particle size, 250µm = 0.1794g/23.3446g
= 0.1763 x100% = 0.00768 x 100%
= 17.63%
= 0.77%
Particle size, 150µm = 5.0079
g/20.7206 Particle size, 150µm =2.2832g/23.3446g
= 0.2416 x100% =0.0978 x 100%
= 24.16% = 9.78%
Percentage frequency: group 5 & 6 Percentage frequency: group 7
& 8
Particle size, 300 µm =12.0507/13.7336 Particle
size, 300µm = 11.1693/12.4977
=
0.8774 x 100% = 0.8937 x 100%
= 87.74%
= 89.37%
Particle size, 250µm = 1.3862
/13.7336 Particle size, 250µm = 0.8602g /12.4977
= 0.1009 x100% = 0.0688 x 100%
= 10.09% = 6.88%
Particle size, 150µm =
0.296/13.7336 Particle size, 150µm =0.4682 g /12.4977 g
= 0.0215 x100% =0.0374 x 100%
=
2.15% = 3.74%
Discussion
It is interesting to note from the comparison of the particle size distribution plots that the nature of the material being crushed and the mode of crushing can influence the form of the product size distribution.
There are considerable problems in making
an ideal comparison between the results of ball milling. These problems mainly
from the experimental difficulties in being able to utilise the techniques of
ball milling size regimes. The reduction of size of particles in a step-wise
fashion is a tedious process.
Therefore, there are some precautions steps
to be applied such always use dry material in performing test to obtain the
best outcome. Avoid the loss of portions of sample in transferring into or out
of cylinder to get the most accurate value when weighing. Always check mass of
steel spheres periodically for loss due to wear to make sure that only the
particle tested is weighed without presence of any impurities which possibly
may be contributed by the steel spheres.
Questions
1. What are the factors affecting
the reductions of particle sizes (milling)?
Nature of raw materials, nature
of the finished product and material properties such as crack propagation,
toughness and surface hardness are the factors
2. What are the equipments used to
reduce particle size?
Cutting machines like
knife cutters, slitters, dicers. Crushers like jaw crushers, gyratory crushers.
Crushers are suitable to be used for coarse and fine size reduction. Grinders
like hammer mills, rolling-compression mills, tumbling mills. Grinders are used
generally for intermediate and fine size reduction. Ultrafine grinders like
hammer mills, agitated mills and fluid-energy mills.
3. What are the factors influence
the choosing of equipment in particle size reduction?
The selection of equipment
depends on characteristic of product to be processed, particle size
distribution required, the objective of particle size reduction or end-use of
the resulting product.
References
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