study of crushing and grinding
TRANSCRIPT
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Summary
The objectives of this experiment were to calculate the power required for size reduction, to
perform screen analysis of the product and to calculate the mean particle size. For this purpose
both crushing and grinding were done for brick, whereas only crushing was done for concrete.KWh reading (in terms of rev) was recorded from the energy meter both at empty state of the
crusher and during crushing of concrete form which experimental power requirement for size
reduction was calculated. Theoretical power required was calculated by applying Bonds law
for concrete. Sieves of different mesh size were used for screen analysis of both brick &
concrete particles. A shaker was provided for that purpose, which ensures a better screen
analysis within a short period of time. Two graphs (one for concrete and another for brick) had
been plotted showing cumulative distribution plot for screen analysis. The experimental power
consumption was 0.57886 kWh and theoretical power consumption was 0.028197 kWh. Linear
mean diameter of concrete and brick were 0.12294 mm and 0.1883 mm respectively. The
possible discrepancies in this experiment is discussed in discussion section.
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Introduction
The objective of crushing and grinding operations is size reduction of particles. Size reduction
is usually carried out in order to increase the surface area because, in most reactions involving
solid particles, the rate is directly proportional to the area of contact with a second phase. Solids
may be reduced in size by a number of methods. Compression or crushing is generally used for
reduction of hard solids to coarse size. Impact gives coarse medium, or fine sizes. Attrition or
rubbing yields fine products. Cutting is used to give definite sizes.
In general, the terms crushing and grinding are used to signify the subdividing of large solid
particles to smaller particles. In the food processing industry, a large number of food products
are subjected to size reduction. Roller mills are used to grind wheat and rye to flour and corn.
Soybeans are rolled, pressed and ground to produce oil and flour. Hammer mills are often used
to produce potato flour, tapioca and other flours. Sugar is ground to a finer product. Since size
reduction has important industrial, especially in chemical industries so the study of size
reduction equipment is necessary.
Grinding operations are found in many industries like cement industries. Limestone, marble,
gypsum, and dolomite are ground to use as fillers in paper, paint and rubber. Raw materials for
the cement industry, such as lime, alumina and silica are ground on a very large scale.
Solids may be reduced in size by a number of methods. Compression or crushing is generally
used for reduction of hard solids to coarse size. Impact gives coarse medium, or fine sizes.
Attrition or rubbing yields fine products. Cutting is used to give definite sizes.
In this experiment, both crushing and grinding were done for brick, whereas only crushing was
done for concrete. The products were sieved and screen analyses were performed. Theoretical
and experimental power requirements were calculated. It was found that they were not very
close to each-other.
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Experimental Setup
Feed
Crushing
chamber
Flywheel
Electric
motor
Plates
Figure 1: Schematic diagram of a jaw crusher.
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Mill
Rotating bed
Support
Electric motor
Figure 3: Schematic diagram of a pebble mill.
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Table 2: Observed data for screen analysis of brick.
No. of Observation Mesh No. Screen Aperture,
mm
Retained Mass of
Brick, kg1 6 3.327 1.000
2 8 2.362 0.600
3 10 1.651 0.534
4 14 1.168 0.0361
5 16 0.991 0.0192
6 20 0.833 0.0163
7 28 0.589 0.0094
8 35 0.417 0.0031
9 48 0.295 0.0030
10 65 0.208 0.0509
11 80 0.175 0.1182
12 100 0.147 0.0732
13 150 0.104 0.0099
14 Residue -- 0.0046
Total bricks without mesh no. 6 screen = 0.9973 kg
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Calculated data
Table 3: Calculation of linear mean diameter for brick particle.
SizeRange
MassFractio
n,
x
C.M.F Avg.Dia.
d
mm
d2
mm2
x/d
mm-1x/d2
mm-2
CMF of sample
Smalle
r than
Size
noted
Larger
than
Size
noted
+6 - - - - - - 1 0
-6+8 0.6016 0.60162 2.8445 8.091180 0.211504 0.07435 0.39837 0.601624
-8+10 0.0535 0.65516 2.0065 4.026042 0.026685 0.01329 0.34483 0.655168
-10+14 0.0361 0.69136 1.4095 1.986690 0.025681 0.01822 0.30863 0.691366
-14+16 0.0192 0.71061 1.0795 1.165320 0.017834 0.01652 0.28938 0.710618
-16+20 0.0163 0.72696 0.912 0.831744 0.017921 0.01965 0.27303 0.726962
-20+28 0.0094 0.73638 0.711 0.505521 0.013256 0.01864 0.26361 0.736388
-28+35 0.0031 0.73949 0.503 0.253009 0.006179 0.01228 0.26050 0.739496
-35+48 0.0030 0.74250 0.356 0.126736 0.008449 0.02373 0.25749 0.742504
-48+65 0.0510 0.79354 0.2515 0.063252 0.202933 0.80689 0.20645 0.793542
-65+80 0.1185 0.91206 0.1915 0.036672 0.618903 3.23187 0.08793 0.912062
-80+100 0.0733 0.98546 0.161 0.025921 0.455889 2.83161 0.01453 0.985460
-100+150 0.0099 0.99538 0.1255 0.015750 0.079098 0.63026 0.00461 0.995387
-150 0.0046 1 0.052 0.002704 0.088701 1.7057 0 1
-- x=1.0 -- -- -- x/d=1.77 x/d2=9.40 -- --
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Sample calculation
Experimental power calculation
Mass of crushed concrete = 1.75 kg
Time required for one revolutions at empty state of crusher =27.09
Time required forone revolutions for crushing of concrete = 18.87 sec
100 revolutions is associated with 1 KWh power
Thus, 1 revolution is associated with 0.01 KWh power
Experimental power required for crushing the concrete = 3600)09.27
1.0
87.18
1.0(
= 0.57886 KWh
Theoretical Power Calculation
Mass flow rate,.
m = 3600)14.90787.18
75.1(
tons/hr = 0.3680 tons/hr
Work index of concrete (dry crushing of cement clinker), Wi= 13.45
(Ref: McCabe, Smith, 6th ed, page: 967)
80% of feed passes a average aperture size, Dpa=2
26.67+18.85mm = 22.76 mm
80% of product passes a mesh size, Dpb= 19.3 mm (From Figure: 04)
Theoretical power required for crushing 1.75 kg of concrete
P = )76.22
1
3.19
1(45.133162.03680.0 kW = 0.028197 KWh
Ratio of experimental power required to theoretical power required = 0.57886: 0.028197
= 20.53: 1
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Linear mean diameter calculation
Linear mean diameter for concrete 12294.04.978
0.612
2
i
i
i
i
d
x
d
x
mm
Linear mean diameter for brick 1883.09.40
1.77
2
i
i
i
i
d
x
d
x
mm
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Graphical Representation
Figure 4: Cumulative mass fraction vs. average particle diameter graph for concrete.
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25
Cumu
lativemassfratction
Average particle diameter, d mm
Smaller than
size noted
Greater thansize noted
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Figure 5: Cumulative mass fraction vs. average particle diameter graph for brick.
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5
Cumulativemassfractio
n
Average particle diameter, d mm
Smaller than
size noted
Larger than
size noted
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Results and Discussions
Experimental values
Power required for crushing 1.75 kg concrete = 0.57886 kWh
Linear mean particle diameter of concrete = 0.12294 mm
Linear mean particle diameter of brick = 0.1883 mm
Theoretical values
Power required for crushing 1.81 kg concrete = 0.028197 kWh
From the obtained result it was found that there was huge deviation between experimental and
theoretical power required for crushing 1.75 kg concrete. This huge deviation might be
occurred because extra energy was consumed in jaw crusher for producing huge noise, certain
amount of heat, vibration and friction among the moving parts. These all things reduced the
efficiency of the jaw crusher. Belts joining the motor armature and wheel might had some
looseness generated from friction of long time using. Thus it caused the crusher to consume
extra energy during the loaded condition than vacant condition. The motor used in the jawcrusher itself was not highly efficient. It had also consumed certain extra amount of energy
during crushing for its low efficiency. Power required for crushing was recorded from energy
meter for only one observation. Several observation should be taken to get more accurate value.
The measurement of linear particle diameter was not fully accurate. There was loss of particle
mass during shaking and fine particles were suspended in air which might cause error in results.
A single standard series of screen was not used rather both the Tyler standard screens and the
American standard screens were used. That led to a great erroneous result. The wire meshes
were age-old and rusty. Erosion might change the screen apertures to a certain limit and this
can affect our results. There was clogging of small particles in the wire mesh. Separating them
was difficult. We had to count this error. The shaker was out of order and did not prove any
good. Moreover, some particle was lost during the experiment on the floor and the atmosphere.
All that have been seen after performing the experiment and calculations is that, the experiment
could be done under much more carefulness if the discrepancies could be avoided.
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Conclusion
Chemical engineers meet particulate solids in carrying out many industrial operations where
crushing and grinding is a part of any process. Though crushing and grinding is very much
inefficient process from the energy consideration, it has large industrial application. In this
experiment concrete and brick were crushed and brick was grinded further. Concrete and brick
both were dry. Power required for crush of concrete calculated which proved the in-efficiencies
of the process. However, the experiment gives us practical knowledge about industrial crushing
and grinding.