OBJECTIVES:
1. To determine flow characteristics of a powder.
2. To determine the angle of repose and the factors that influence it.
INTRODUCTION:
The angle of repose of
a granular material is the steepest angle of descent or dip of the slope
relative to the horizontal plane when material on the slope face is on the
verge of sliding. When bulk granular materials are poured onto a horizontal
surface, a conical pile will form. The internal angle between the surface of
the pile and the horizontal surface is known as the angle of repose and is
related to the density, surface area , size and shapes of the particles,
and the coefficient of friction of the material.
Angle of repose of a granular material is
important in the design of equipment for the processing of particulate solids
and also be used in determining whether
a slope of a stockpile or uncompact gravel bank will likely collapse or
not. Thus, there are numerous methods for measuring angle of repose and each
produces slightly different results.
METHODOLOGY:
i) Apparatus and Materials:
- sand
- cylinder
- ruler
- water
1. 100g of sand is prepared and weighed by
using weighing balance.
2. The sand is placed in the cylinder with
the rubber base at the bottom.
3. The cylinder is removed and allowed the
sand to flow out and form a heap.
4. The height, slope and diameter of the heap
is measured.
5. The angle of repose is calculated.
6. The experiment is repeated by using other sand or with the addition of glidant/water.
RESULTS:
DISCUSSION:
Angle of repose is one of
the methods used to characterize the flow of a material. In this experiment,
angle of repose for different sizes of sand and with the presence of a glidant
is determined. The factors that affect the angle of repose are also studied.
From the
result obtained, in the absence of glidant, the angle of repose decreases as
the size of particles increases. This is because smaller particles have
dominant cohesive and adhesive forces as compared to particle weight, whereas
in bigger particles gravity plays a dominant role thus giving a smaller angle
of repose. Studies have shown that angle of repose is also gravity-dependent.
Cohesiveness of finer particles due to electrostatic forces causes difficulties
in flowing thus forming a steeper pile which shown by the 150 micron sand
having the highest angle (56.31°) compared to 500 and 850 micron sand. On the
other hand we can see that mixture of various sizes of sand give a high angle
of repose also (45.0°). When particles of different sizes and irregular shape
are mixed together, the mechanical interlocking of particles increases and
thereby increase the rolling friction. As a result a steeper pile with bigger
angle of repose is formed.
When 10%
concentration of magnesium stearate is added, the angle of repose increases.
Magnesium stearate is used as a glidant. It may also act as a lubricant in
reducing friction. Glidant functions in improving the flow of a material. From
the result, 850 micron sand has a good flow. The addition of a glidant should decrease
the angle of repose and enhances the flow of the materials. But however the
addition of 10% magnesium stearate in this experiment produces a product of vice
versa. This is because the rate of flow is improved by the addition of
magnesium stearate up to a limiting concentration of glidant. Above a certain
concentration (in this case 10%), the glidant will function to inhibit
flowability. Thus, a glidant will only work at a certain range of
concentrations.
For a
better result, it is advised to use a protractor instead of ruler as it reduces
parallax error and it measures an accurate angle. The glidant and the sand
should be mixed until an even distribution of mixture is obtained. This is to
ensure that the glidant function effectively. Besides, the lifting velocity of
the cylinder should just be moderate to avoid distraction to the material flow.
For a clean and tidy working environment, a paper should be placed before
starting the experiment so that the sand did not cluttered elsewhere.
Experimenter must also wear goggle, mask and lab coat all the time to protect
the eyes and nose from coming in contact with the sand.
QUESTIONS :
1.What
are the angle of repose for the sands that you use?
For sands without magnesium stearate, the angle of repose of sand (150 µm) is
56.31˚, sand (355 µm) is 41.99˚, sand (500 µm) is 40.36˚, sand (850 µm) is
38.66˚ and variety of sands is 45.00˚. As for 100g of sands with 10% magnesium
stearate, the angle of repose of sand (150 µm) is 57.99˚, sand (355 µm) is 45.00˚,
sand (500 µm) is 45.00˚, sand (850 µm) is 43.53˚ and for variety of sands is
48.99˚ .
2.
What are the factors that can influence the angle of repose of a powder?
One of the factors that influence the angle
of repose is the moisture. Moisture will increases the angle of repose of
sediments as a small amount of moisture between sand grains will bind them
together due to cohesion. However, too moisture can result in
particles moving freely over one another and therefore dramatically reduces the
angle of repose. For most dry sands, the
angle of repose increases with decreasing grain size, but usually lies between
about 40 and 45 o. Next is the shape and texture of the sand
particles. Angle of repose is greater for particles that are irregular, flatter
and more angular. Besides, the addition of glidant will decrease the angle of
repose of the material by reducing the cohesion force between sand particles.
3.
What other methods can you use to measure the angle of repose?
The other methods that can be used to
measure angle of repose is tilting box method which is suitable for
fine-grained, non-cohesive materials and contain individual particle size less
than 10 mm. The material is placed within a box with a transparent side to
observe the granular test material. The box is slowly tilted and is stopped
when the material begins to slide in bulk. Then, the angle of tilt is measured.
Besides, revolving cylinder method can also be used to measure angle of repose.
The material is placed within a cylinder with at least one transparent face.
The cylinder is rotated at a fixed speed and the observer observes the material
moving within the rotating cylinder. The granular material will assume a
certain angle as it flows within the rotating cylinder and the dynamic angle of
repose is obtained.
CONCLUSION
:
1. The angle of repose increases with
decreasing particle size of sand materials.
2. The angle of repose increases after
addition of 10% of glidant (magnesium stearate).
REFERENCES:
REFERENCES:
https://books.google.com.my/books?id=ZLI4i_iSfG8C&pg=PA16&lpg=PA16&dq=particle+size+analysis+ANGLE+OF+REPOSE&source=bl&ots=S1OOa_4Xri&sig=n3396yLA7vASfc1sqMoWjwoaReI&hl=en&sa=X&ei=2MGKVJeHI4iJuwS72YCgDQ&ved=0CEwQ6AEwBw#v=onepage&q=particle%20size%20analysis%20ANGLE%20OF%20REPOSE&f=false
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