The vertical screw conveyor- powder properties and Screw conveyor design

Alma Kurjak

Department of Chemical Engineering, Lund Institute of Technology,

P.O. Box 124, SE-221 00 Lund, Sweden January 2005

1 Abstract This Master Thesis is a study of how powder properties and screw conveyor design influence the flow properties of a vertical screw conveyor. Studies show that different powder properties like particle size, bulk density and particle shape have a large influence on screw capacity. Coarse powders will flow into the screw easier than fine powders. The screw capacity will also be higher if a dense powder is used. Particle with a round shape have lower internal friction that results in a greater screw capacity. It was also shown that the Hausner ratio, assessed from tapped and apparent density and angle of repose are effective methods to determine the free-flowing properties of the powder. Studies also show that the clearance and the free length of the intake have a big influence on screw capacity. No correlation between conveying length and conveyor capacity was found. Key words: vertical screw conveyor; powder properties; Hausner ratio; iron powders; flowability; screw design;

2 Introduction Different mixers can be uses for homogenisation of powders. One of them is the Orbiting screw mixer. Orbiting screw mixers work excellent and the only disadvantage is that they are expensive. One cheaper alternative is a centre screw mixer. It consists of a conical vessel and a screw that is surrounding by a tube. The products to be mixed are conveyed upwards by the screw positioned in a central mounted guiding pipe. The combination of the rotating screw and conical vessel results in an efficient mixing. The aim of this thesis was to identify different powder properties and screw design that influences the flow properties of the vertical screw conveyor.

3 Theory Different powder properties can be used to identify if powder is free flowing or not.

3.1 Particle size Particle size has influence on flowability of a powder. In general, fine particles with very high surface to volume ratios are more

cohesive than course particles. Particles larger than 250 µm are usually relatively free flowing, but as size falls below 100 µm powder become cohesive and flow problems are likely to occur. Powders having a particle size less than 10 µm are usually extremely cohesive. [1]

3.2 Bulk density Because powders normally flow under the influence of gravity, dense powders are generally less cohesive than less dense powders. [1]

3.3 Particle shape Particle shape has a large influence on flow properties. A group of spheres has minimum interparticle contact and generally optimal flow properties, whereas a group of flakes have a very high surface-to-volume ratio and poorer flow properties. [1]

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3.4 Hausner ratio The Hausner ratio is a measure of how compressible a powder is in relation to bulk density. It is derived from the quotient between tapped density (TD) and apparent density (AD)

ADTDratioHausner =

Values less then 1.25 indicate good flow whereas values greater than 1.25 indicate poor flow. [1]

3.5 Angle of repose The angle of repose given in table 2 may be used as a guide to flow performance. [1]

Table 1 Angle of repose as an indication of powder flow properties

ANGLE OF RESPONSE

TYPE OF FLOW

< 20 Excellent 20 – 30 Good 30-34 Passable > 40 Very poor

4 Materials

4.1 Powder Höganäs AB produces two different kinds of ferrous powders:

• sponge-iron powders, and • water-atomized (unalloyed and low-

alloyed) iron powders The external shapes of both particles are irregular and similar to one another. However, the sponge iron particle has as its name suggests a spongy internal structure and water-atomized is internally compact. In the Belgium plant, gas-atomized powders with almost perfect round shape are produced. In this work, nine different iron powders were used:

Sponge-iron powder: NC100.24, W40.24, MH300.29, SC100.29 and M1000

Water-atomised iron powder: AT40.29, ASC100.29 and ASC300.29

Gas-atomized iron powder: Fe6.8Si

In table 1, apparent density and flow of different iron powder are present. [2]

Table 2 Properties of some different iron powders

Powder AD (g/cm3)

Flow (s/50g)

Fe6.8Si 4.3 15 ASC100.29 3.0 25 NC100.24 2.4 31 AT40.29 3.1 29 W40.24 2.5 38

SC100.26 2.7 30 MH300.29 2.9 27 ASC300 2.9 25

4.2 Apparatus The measurements were performed with three different screws. The first screw was used for preliminary studies; just to see how screws work. With the second screw almost all experiments were done. The third screw was used to see if the results will be the same if a bigger screw conveyor is used, see figure 1.

Figure 1 Vertical screw conveyor

5 Methods

5.1 Methods to characterize powder

5.2 Size measurement To determine particle size, laser diffraction, Sympatec HELOS was used.

5.3 Bulk Density (Apparent density) Bulk density was determined by filling the powder through a standardized funnel into a small cup, levelling-off the surplus powder on top of the cup and dividing the weight of powder contained in the cup by the cup volume (25 cm3). [2]

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5.4 Particle Shape To determine the shape of different iron powders, electron microscope pictures were used.

5.5 Hausner Ratio The Hausner ratio is derived from the quotient between tapped density (TD) and apparent density (AD). Tapped density was measured using Stamp volume meter.

5.6 Angle of repose For measuring the drained angle of repose was used, se figure 2.

Figure 2 Measurement of drained angle of

repose

5.7 Flow Flow rate is the time in seconds, which an amount of 50 g dry powder needs to pass the aperture of standardised funnel se figure 3.

Figure 3 Aperture for measurement of

Flow and AD

5.8 Methods to characterize screw conveyor design

5.9 Clearance Tubes with different diameters were used to see which influence the clearance has on screw capacity.

5.10 The free length of intake Tubes with different lengths were used to see which influence the free length has on screw capacity.

5.11 Conveying length Tubes with different lengths but the same free length of intake were used to see which influence the conveying length has on screw capacity

6 Results and discussion

6.1 Powder properties Figure 4, shows how mass flow depends on screw velocity for different powders.

0

500

1000

1500

2000

3 6 9 12Velocity (rev/s)

Mas

s flo

w (g

/s)

ASC100.29

NC100.24

W40.24

ASC300

AT40.29

M1000

MH300.29

SC100.26

Fe.6.8Si

Figure 4 Mass flow for some different iron

powder

6.2 Size The screw capacity is greater for coarse powders than for fine due to the differences in flowability. Fine particles have large specific surface area and are more cohesive than coarse particles. If the powder is more cohesive, it is not free flowing. A coarse powder will be more free flowing than a finer powder and will thus flow more easily into the screw.

6.3 Aerated bulk density Bulk density is one important parameter for screw capacity since the screw has a fixed volume. The higher the bulk density of a powder the more mass of the powder can be introduced into the screw. It means that powders with o large bulk density will have a largest screw capacity than obtained with powders with a low bulk density.

6.4 Particle Shape If the shape of a particle is known then conclusions can be made of internal friction of the powder. In the following list powder with high internal friction is given first; MH300.29 > ASC300 > ASC100.29 > NC100.24 >

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AT40.29 > SC100.26 > W40.24 > M1000 > Fe6.8Si. This comparation is made on the particle shape appearanced from SEM picture. A group of spheres has low internal friction whereas a group of flakes has higher internal friction. Compared to powders with lower internal friction, powders with higher internal friction flow into the intake area of the screw conveyor with lower velocity. It implies that use of powders with low internal friction (round powder-Fe6,8Si) will result in greater screw capacity than obtained with powders with higher internal friction.

6.5 Hausner Ratio Iron powders have different Hausner Ratios, see table 3. Free flowing powders are less cohesive and have Hausner Ratio close to 1 while less free flowing powder have Hausner Ratio > 1.25. If a powder is free flowing, it will easier flow into the screw. Use of free-flowing powders will result in a greater screw capacity than obtained with not free-flowing powders

Table 3 Hausner Ratio and angle of repose for different iron powders

PULVER HAUSNER RATIO

ANGLE OF REPOSE °

Fe6.8Si 1.10 25.8 M1000 1.10 33.4 ASC100.29 1.27 34.6 W40.24 1.12 34.9 NC100.24 1.26 35.3 AT40.29 1.17 36.0 SC100.26 1.27 - MH300.29 1.28 47.2 ASC300 1.34 48.2

6.6 Angle of repose Iron powders have different angels of repose, see table 3. A comparison between angle of repose and screw capacity, figure 4, for different iron powders shows that powders with low angle of repose get a higher screw capacity than powders with a high angle of repose. It was expected because powders with a low angle of repose flow more easily into the screw.

6.7 Flow No correlation between flow and screw capacity was found. It seems as if this method is not sufficient to describe screw capacity.

6.8 Screw conveyor design

6.9 Clearance At a large clearance a back flow of bulk material opposite to the conveying direction occurs followed by reduction in conveyor capacity, see figure 5. However, if clearance is small milling and jamming can take place between screw and casing. Clearance is also necessary for smooth running of the conveyor. Therefore, it is important to find the smallest clearance at which no milling and jamming process takes place.

0

1000

2000

3000

4000

2 4 6 8 10 12Clearance (mm)

Mas

s flo

w (g

/s)

60 Hz

70 Hz

80 Hz

Figure 5 Mass flow for ASC100.29 at difference clearance

6.10 The free length of intake At a very low speed, it is enough to have a short free length of intake to obtain maximum output, se figure 6. As speed increases, the vortex formed in the screw limits the amount of powder that can enter the screw. To compensate for this, a larger free length of intake is necessary at higher speed to obtain maximum output.

0

100

200

300

400

500

600

700

800

900

1000

3 5 7 9Velocity (rev/s)

Mas

s flo

w A

SC10

0.29

(g/s

)

11

I = 6 cm

I = 12.5 cm

I = 17.5 cm

Figure 6 Mass flow at different length of intake

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6.11 Conveying length It is difficult to say which influence conveying length has on the conveyor capacity. It seems as if there is no correlation between conveying length and conveyor capacity, see figure 7. Probably conveying length has no influence on the capacity. Perhaps, the arrangement used in this work was not big enough to see any differences.

100

200

300

400

500

600

700

3 5 7 9Velocity (rev/s)

Mas

s flo

w A

SC10

0.29

g/s

11

Conveyor lenght 28.5 cm

Conveyor lenght 33.5 cm

Conveyor length 40 cm

Figure 7 Mass flow at different conveying

lengths

7 Conclusions Powders with coarse particles will flow into a screw easer than powder with fine particles. This results in a greater mass flow. The screw capacity will also be higher if dense powder is used. Round powder, have lower internal friction that results in a greater screw capacity. Hausner Ratio and angle of repose are most likely efficient methods to measure if powder is free flowing or not. The clearance and the free length of intake have a large influence on screw capacity. No correlation was found between conveying length and conveyor capacity.

8 Acknowledgments Special thanks to my supervisors Ingrid Eriksson at Höganäs AB and Anders Axelsson at the Department of Chemical Engineering, Lund Institute of Technology for guiding me through the whole work during my Master thesis. I would also like to thank all at Höganäs AB for useful help and advice on various problems.

9 Reference [1] Aulton Michael E.:

Pharmaceutics The Science of Dosage Form Design p.197-210 and 133-135

[2] http://www.diegm.uniud.it/fmiani/PMS CHOOL/CHAPT03.PDF

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