GRINDING AND PARTICLE SIZE ANALYSIS

PRACTICAL 1
TITLE: GRINDING AND PARTICLE SIZE ANALYSIS
AIM: TO DETERMINE THE EFFICIENCY OF A BALL MILL
APPARATUS AND THEIR USES:
500g of crushed limestone.
A ball mill with steel balls: it is a powered machine used for grinding the crushed mineral. It reduces the limestone to about 10-300 micron.
A generator: it powers the ball mill and the weighing machine.
Analytical weighing machine: used for taking the mass of the particles from sieves of different size.
Laboratory sieve: it is used for classifying the particles into different sizes. We have sieves with screen size 2000,1180, 600, 425, 300, 212, 63, 38, pan.
A hand brush for clearing the ground particles on the inner walls of the ball mill.

PROCEDURE FOR THE EXPERIMENT:
The already crushed limestone is fed into the ball mill for further liberation. The ball mill grinds it for 10minutes, powered by a standby generator.
After 10 minutes, the ground limestone is poured out and the walls of the ball mill, is cleared with the aid of the hand brush.
The ground limestone is placed on the weighing machine where a mass of 420g which is our feed size is obtained.
The ground limestone of mass 420g is fed into the sieves with sizes arranged in the order of 2000,1180,600,425,300,212,63,38,pan. The pan is shaking so that the particles will move gradually down to the pan; duration about 20-30 minutes.
The particles left on each sieve (over size), is poured out and weighed so as to obtain the weight retained.



RESULT:
The table shows the weight of the particles retained on each sieve.
Particle size (micron) Weight retained(g)
2000 5.98
1180 25.34
600 83.58
425 44.02
300 36.39
212 31.19
63 161.51
38 27.70
PAN 3.10

CALCULATIONS
The information below shows the weight retained, % weight retained, cumulative weight % passing and cumulative weight % retained.

particle size(micron) weight retained %
 weight retained Cumulative
 weight % passing Cumulative
 weight % retained
2000 5.98 1.42 98.58 1.42
1180 25.34 6.03 92.54 7.45
600 83.58 19.90 72.64 27.35
425 44.02 10.48 62.16 37.83
300 36.39 8.66 53.50 46.49
212 31.19 7.43 46.07 53.92
63 161.51 38.45 7.62 92.37
38 27.7 6.60 1.02 98.97
pan 3.1 0.74 0.28 99.71

The graphical representation of the relationship between cumulative weight% retained and particle size is shown below.

QUESTION 2:
From the graph, P80= 770      , P50=250      ,
Reduction ratio (R)=  =  = 2.34
Feed= 3000 microns,   product = 105 microns,   throughput =250tph,
Work index = 13.2KWh/t
Power = throughput (t/h) * work input (KWh/t); but we don’t have our work input. From Bond’s equation W= 10wi, where W = work input, Wi = work index.
Putting the values of the given parameters, the above equation becomes
W = 10*13.2
W =10.47KWh/t
Thus, power = 250*10.47
=2617.5KW.
OBSERVATION:
It was observed that the sum of weight retained in each laboratory sieve was less than the initial feed size (420g), this was because some of the particles were lost as dust during shaking of the sieve and some particles were stuck to the sieve after shaking.
PRECAUTIONS
I ensured that the ball mill was closed and tightened properly to avoid steel balls from falling during grinding.
I ensured that the analytical weighing balance was closed when I was taking the weight of particles from each sieve size to obtain the relative weight of the particles and avoid taking mass of air too.
I ensured proper electrical connection of the ball mil to the generator
I ensured that sieves were properly arranged in increase order of coarseness


CONCLUSION
From the graph;
P75 = 630 microns
P50 = 250 microns
P25 = 140 microns
Efficiency of the ball mill, E = P75-P252P50 *100
Efficiency of the ball mill, E = 630-1402*250 *100=98%
Efficiency of the ball mill, E= 98%
From the calculations above, it can be concluded that the ball mill is not 100% efficient.

PRACTICAL 2
Part 1– Rock Cycle:
 
Part II:  Mineral and Rock Identification:
Sample 1: GRAPHITE

Sample Graphite
Hardness Mohrs: 1 – 2
Streak Colour Black
Specific Gravity 2.1 – 2.3
Luster Metallic

Sample 2:

Sample Pyrite
Hardness Mohrs: 6 – 6.5
Streak Colour Greenish black to Brownish black
Specific Gravity 4.9 – 5.2
Luster Metallic


Sample 3:
Sample Feldspar
Hardness Mohrs: 6.0 – 6.5
Streak Colour White
Specific Gravity 2.55 – 2.76
Luster Vitreous



Sample 4:Made up of a course grained igneous rock



Sample Diorite
Hardness Mohrs: 6 – 7
Streak Colour Black
Specific Gravity 2.8 – 3
Luster Shiny


Sample 5: This sample contains gravel sized particles (circled).

Sample Conglomerate  
Hardness Mohrs: 2 – 3
Streak Colour White
Specific Gravity 2.86 – 2.88
Luster Dull






Sample 6:
Course Grained.
Dominant Minerals: Amphibole, Sodium- and Calcium-rich plagioclase feldspar.


Sample Gabbro
Hardness Mohr: 7
Streak Colour Black
Specific Gravity 2.86 – 2.87
Luster N.A

Part III –  Give it Some Thought.
Referring to the accompanying photos of five minerals, determine which of these specimens exhibit a metallic luster and which have a nonmetallic luster.


METALLIC LUSTER NON METALLIC LUSTER
E A
C B
D

If the number of protons in a neutral atom is 92 and its mass number is 238:
Uranium.
92 electrons.
146 neutrons.


COOLING HISTORY
Aphanitic texture: aphanitic rocks form from lava which crystallize rapidly on the earth surface. Because it makes contact with the atmosphere, they cool quickly so the minerals will not have time to form large crystals( they form fine grains) the individual crystals are not distinguishable with the naked eye.
Porphyritic texture: develop when conditions during cooling change relatively quickly. The  earlier formed minerals, form slowly and remain as large crystals called phenocrysts, while the fine grained matrix is referred to as the groundmass. A Porphyritic texture indicates two stages cooling: slow, then fast.
phaneritic texture: it describes coarse grained rocks. They are characteristic of intrusive(plutonic) rocks, and have crystals that can be seen with the unaided eyes. Indicates slow cooling history
Vesicular texture: it is characterized by a rock being pitted with many cavities called vesicles. The cooling is rapid and because of that, trapped may escape from the lava.



REFERENCES
http://www.indiana.edu/%7Egeol105/images/gaia_chapter_5/igneous_rock_textures.htm
E.J. Tarbuck and F.K. Lutgens, Earth An introduction to physical geology. seventh edition, Prentice Hall, 2002.
http://www.minsocam.org/msa/collectors_corner/id/mineral_id_keyi3.html
http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html
http://hyperphysics.phy-astr.gsu.edu/hbase/geophys/texture.html