study of crushing and grinding

8/12/2019 Study of Crushing and Grinding

1/17


2/17

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.


3/17

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.


4/17

Experimental Setup

Feed

Crushing

chamber

Flywheel

Electric

motor

Plates

Figure 1: Schematic diagram of a jaw crusher.


5/17


6/17

Mill

Rotating bed

Support

Electric motor

Figure 3: Schematic diagram of a pebble mill.


7/17


8/17

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


9/17

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 -- --


10/17


11/17

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


12/17

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


13/17


14/17

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


15/17

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


16/17

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.


17/17

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.

study of crushing and grinding

Documents