Introduction:
Powder
can be defined as solid particles that either have the same or different
chemical composition. It is mainly used in production of tablet and capsule.
Every powder has its own flowability. Flowability is the capability of a loose
particulate solid to move by flow. Angle of repose is a term used to describe
the maximum angle, measured upwards from the horizontal, at which a pile of a
particular granular material will remain stable without any of the material
sliding downward. It is useful in designing storage and transportation
machinery for granular materials as it can give an engineer insight into the
appropriate size and shape of such devices. The angle of repose for a
particular powder can be used to determine the flowability and the flow
characteristics of a powder. The value is in the range of 0° to 90°. There are
various methods available to measure this value. In this experiment, we are
given sand of various sizes and to determine the angle of repose and the
factors that may influence it.
Objective:
1.
To determine the angle of repose of various
sizes of powder.
2.
To determine the factors that may influence the
angle of repose.
Apparatus:
1.
Cylinder
2.
Rubber
stopper
3.
Newspaper
4.
Weigh balance
5.
Spatula
6.
Weighing boats
Materials:
1.
100g of 150 micron sand mixture
2.
100g of 355 micron sand mixture
3.
100g of 500 micron sand mixture
4.
100g of 850 micron sand mixture
5.
100g of various sizes sand mixture
6.
Magnesium stearate
Procedure:
1.
100g of 150 micron sand mixture is prepared.
2.
The sand is placed in a cylinder with a rubber
base at the bottom.
3.
The cylinder is then removed and the sand is
allowed to flow out and heap is formed.
4.
The height, slope and diameter of the heap is
measured.
5.
The angle of repose is then calculated.
6.
The experiment is repeated using other sand or
with addition of glidant (magnesium stearate).
Results:
1. Sand
without Glidant
Size of
particles of sand (micron)
|
Height (cm)
|
Width (cm)
|
Angle of
repose
|
150
|
2.2
|
2.4
|
42.510
|
355
|
2.1
|
2.4
|
41.190
|
500
|
1.9
|
2.4
|
38.370
|
850
|
1.8
|
2.4
|
36.870
|
Various
Size
|
2.0
|
2.4
|
39.810
|
2. Sand is
mixed with Glidant
Size of
particles of sand (micron)
|
Height (cm)
|
Width (cm)
|
Angle of
repose
|
150
|
2.1
|
2.4
|
41.190
|
355
|
2.0
|
2.4
|
39.810
|
500
|
1.9
|
2.4
|
38.370
|
850
|
1.7
|
2.4
|
35.310
|
Various
Size
|
1.9
|
2.4
|
38.370
|
Discussion:
In this
experiment, we had measured an angle of repose of 150 mic, 355mic, 500mic,
850mic and various sizes of sands with and without the presence of glidant.
Magnesium Sterate is used as a glidants.
The angle
of repose (α) was calculated, from the following equation:
Height =
height of the cone of sand
The results show that with the
addition of glidants, the angle of repose will increase. This is because a
glidant is a substance that is added to a powder to improve its flow ability by
reducing friction between particles. The magnesium compounds works by mopping
up the excess moisture keeping the granules dry and free flowing. But there are
several other factors that will affect the effectiveness of glidants. Firstly,
the flow of the granules depends on the shape and size of the particles of the
glidants and the granules. Secondly, a glidant will only work at a certain
range of concentrations. Above a certain concentration, the glidant will in
fact function to inhibit flow ability.
Conclusion:
The larger
and more irregular the granular material grains, the higher the angle of
repose.
The sands
angle of repose decrease with the addition of glidants.
Questions:
1. What is the angle of repose for each materials?
Size of particles of sand (micron)
|
Angle of repose
(Without Glidant)
|
Angle of repose
(With Glidant)
|
150
|
42.510
|
41.190
|
355
|
41.190
|
39.810
|
500
|
38.370
|
38.370
|
850
|
36.870
|
35.310
|
Various Size
|
39.810
|
38.370
|
2. What other factor that will influence angle of repose for the materials?
Firstly, internal factor that will influence angle of the repose of the sand that we use is the particle size, coarser particles have high angles of repose than fine particles. The next factor is the particle shape. Another factor is cohesiveness, fine particles may reveal cohesiveness owing to spherical particles having a greater tendency to roll. Water content also affects the cohesiveness of particles. If water is added to particles such as sand, water coating the grains would tend to bind them together by its surface tension, giving rise to greater internal cohesion, and therefore shear strength. Thus, moist sand has a much higher angle of repose than dry sand.
Secondly, the external factors which is the presence of other components such as glidants. Next, the moisture of the sand. Angle of repose of loose dry powder increases by compacting as well as by introducing by moisture. Moist sand has a much higher angle of repose than dry sand. The last factors is the methods of measurement, Ledge and erater method give higher angle of repose than from the heap formation methods.
3. What other method can be used to calculate the angle of repose for the powder?
Ledge Method
Where the powder is initially charged into a rectangular box.A slot at the base of one vertical wall is closed by a board.The closure board is then removed to allow the material to flow slowly through the narrow slot.The angle with the horizontal plane of the surface of the powder equilibrium when the flow stops is calculated as the angle of repose.
Tilting box method
This method is appropriate for fine-grained, non-cohesive materials, with individual particle size less than 10 mm. The material is placed within a box with a transparent side to observe the granular test material. It should initially be level and parallel to the base of the box. The box is slowly tilted at a rate of approximately 0.3 degrees/second. Tilting is stopped when the material begins to slide in bulk, and the angle of the tilt is measured.
Fixed funnel method
The material is poured through a funnel to form a cone. The tip of the funnel should be held close to the growing cone and slowly raised as the pile grows, to minimize the impact of falling particles. Stop pouring the material when the pile reaches a predetermined height or the base a predetermined width. Rather than attempt to measure the angle of the resulting cone directly, divide the height by half the width of the base of the cone. The inverse tangent of this ratio is the angle of repose.
Revolving /Rotating cylinder method
Where a sealed hollow cylinder half full of powder surface is rotated until the powder surface shows its maximum angle with the horizontal.The material is placed within a cylinder with at least one transparent face. The cylinder is rotated at a fixed speed and the observer watches the material moving within the rotating cylinder. The effect is similar to watching clothes tumble over one another in a slowly rotating clothes dryer. The granular material will assume a certain angle as it flows within the rotating cylinder. This method is recommended for obtaining the dynamic angle of repose, and may vary from the static angle of repose measured by other methods. When describing the angle of repose for a substance, always specify the method used.
Crater Method /Discharge Method
Where circular tube is placed vertically on a plate with an orifice in the center.The height of the remaining powder against the wall of the tube is measured at eight equidistant points around the circumference to determine the angle of repose.
Dynamic Angle of Repose
Determine in the apparatus consisting of a drum with a roughened internal surface that is half filled with powder and slowly rotated around its horizontal axis.Within a certain range of rotation speeds (usually from 2.5 to 6 rpm) the surface of the powder in the drum comes to a sufficient steady condition.The maximum angle of bed inclination just before slump occurs is designated as the dynamic angle of repose.
References:
