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2003 International Ash Utilization Symposium, Center for Applied Energy Research, University of Kentucky, Paper #2. Copyright is held by the Authors.
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Development of a Robust Quality Control Procedure to
Measure Particle Size Distributions in Fly Ash
J H Potgieter1, S S Potgieter2, R A Kruger3 and J Krüger4
1.
Department of Chemical & Metallurgical Engineering, Technikon Pretoria, Private Bag X680,
Pretoria, 0001, South Africa
2.
Department of Chemistry & Physics, Technikon Pretoria, Private Bag X680, Pretoria, 0001,
South Africa
3.
Ash Resources, P O Box 3017, Randburg, 2125, South Africa
4.
135 William Drive, Silverton, 0184, South Africa
ABSTRACT
This paper describes an alternative quality control approach to laser diffractometry particle size
measurement. The investigation was prompted by the need to find a robust way to discriminate
between various particle size fractions obtained in a fly ash classifier plant by unskilled operators.
Different fly ash particle sizes were placed in glass jars of varying volumes and masses and rolled down
a fixed incline unto a horizontal plane. The effect of different incline angles, jar sizes and masses of
each fly ash particle size fraction on the distance traveled on the horizontal plane were investigated. It
was found that a unique relationship exists between the distance traveled by a specific mass of material
at a constant incline angle and the size fraction of the fly ash in a given flask/ jar. It was thus proven
that this approach could successfully render a robust plant quality control procedure.
INTRODUCTION
The rate of a reaction in any solid is governed by its available surface area. This is of particular
importance in powdery materials, and therefore the measurement of particle size and the subsequent
surface area due to it, forms a routine measurement in most industries producing and utilising such
materials. It is especially important for mineral admixtures like fly ash, silica fume and slag that are
used in the building and construction industry that their particle sizes be controlled closely, because it
can have a major influence on the physical and mechanical behaviour of the cement mortars and
concrete produced from it.
-2Fly ash is a byproduct of coal combustion in the power generating industry. South Africa currently
produces approximately 30 million tons of fly ash annually, of which only about 1.2 million tons are
consumed again. The majority of the utilised fraction goes into cement and concrete manufacturing,
while the very fine fractions are utilised as fillers for paint and polymer production. The separation of
the fine fraction of fly ash, which is called cenospheres, from the bulk unclassified portion obtained
from the electrostatic precipitators of the power station, is accomplished through the use of a series of
cyclones and results in a number of fractions with a narrow size range.
The preferred method of particle size analysis nowadays is through measurements on dedicated
instruments using laser diffraction methodology [1-5], as well as sedimentation technology. While
these methods yield very reliable results, they are expensive and require well-trained technicians to
ensure smooth and error-free operation. In an unsophisticated economy of a developing country where
one often has to rely on semi-skilled labourers to perform analyses, it is preferable to have a more
robust and cheaper way of measurement. If one does use sophisticated equipment, it is advisable to
have a back-up procedure to validate results or to use in cases of breakdown of automated equipment.
Sieving can be used for this purpose, but is not suitable with very small particle size materials and
becomes unreliable at particle sizes < 45 µm.
The major aim of this investigation was therefore to develop a robust method of particle size
measurement for use by semi-skilled operators to control particle size separations by classifiers/
cyclones in a fly ash plant. A further aim was to have a back-up analysis procedure available in cases
of malfunctioning of a laser diffractometer measuring apparatus. This paper will describe the approach
adopted to achieve these aims, the alternative equipment developed and the results obtained.
EXPERIMENTAL PROCEDURE
Materials:
Cenospheres with various particle size distributions were supplied by Ash Resources
(Pty) Ltd. after separation in a fly ash plant by a series of cyclones. The materials investigated are
noted in Table 1.
-3Table 1. Materials used in the investigation
Sample Description
Explanation
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
OPC
Cenospheres with an average particle size of 5 µm
Cenospheres with an average particle size of 15 µm
Classified fly ash with 85 % < 45 µm particles
Tailings from the ash dump
Ordinary Portland Cement
Equipment: A laser particle size analyser, Malvern Masterizer 2000, was used to measure the particle
size distributions of each material. A 1% Calgon solution was employed as a dispersing agent during
the measurements. The chemical analyses of the different fly ashes and OPC were done on a Bruker
SRS 3300 XRF spectrometer after fusion of the different materials with lithiumtetraborate flux [6].
Proposed alternative method: A rolling table with a partly adjustable incline piece was constructed
from hardboard and mounted on a steel frame. It was varnished with ordinary wood varnish to present
a smooth surface along which movement could take place. Glass jars of various sizes were filled with
different masses of the various materials and rolled from a fixed position on the inclined part down the
rolling table. The positions where the jars came to a stand still on the flat part of the table were
recorded and noted. Each measurement was performed three times and the values obtained were
averaged. It was found that the values measured rarely differed by more than 5 mm in each particular
case.
The constructed rolling table is shown in Fig. 1.
The influence of a number of variables on the distance that the jars rolled down from the inclined part
onto the horizontal level of the rolling table was investigated. These include:
(1)
(2)
(3)
(4)
(5)
The effect of the jar size
The effect of the sample mass in each jar size
The type of material
The effect of various particle sizes of the fly ash
The effect of the angle of inclination.
-4-
Fig. 1 Rolling table used in the investigation.
RESULTS AND DISCUSSION
The chemical composition of the various different fly ash fractions was remarkably constant, and the
mean composition thereof and of OPC are summarized in Table 2.
Table 2.
Main element composition (% m/m) of the different fly ash fractions and OPC used in the
investigation
Element as oxide
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
Fly ash
OPC
52.5 + 0.5
34.3 + 0.4
3.5 + 0.1
4.5 + 0.2
1.2 + 0.1
1.9 + 0.1
22.3
4.7
3.4
64.6
2.9
0.2
The particle size distributions of the various materials as measured by the Malvern instrument, are
shown in Figs. 2-6. The most important characteristics, such as the density of each material, the mean
particle diameter (D50), and maximum particle size of 90% of the particles (D90), which can be deduced
from the graphs, are presented in Table 3.
-5Table 3.
Important physical parameters of the various fly ash fractions and OPC used in the
investigation
Sample
D50
(µm)
D90
(µm)
Relative
density (g/cm3)
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
OPC
7.1
20.6
16.1
88.0
28.4
13.1
98.9
55.7
232.3
130.6
2.15
2.25
2.26
2.29
3.10
Fig. 2 Particle size distribution of Plasfill 5 cenospheres used .
Fig. 3 Particle size distribution of Plasfill 15 cenospheres used .
-6-
Fig. 4 Particle size distribution of Plasfill 5/45 cenospheres used .
Fig. 5 Particle size distribution of Plasfill 45/110 cenospheres used .
Fig. 6 Particle size distribution of OPC used .
-7From the curves shown in Figs. 2-6 and the data summarized in Table 3, the following conclusions can
be drawn:
(1) The relative density of the Plasfill products increase with increasing mean particle size.
(2) The cement has a higher relative density than any of the fly ash fractions.
(3) The mean particle size D50 of the cement is only smaller than that of the coarsest fly ash fraction,
Plasfill 45/110.
One of the first important observations made in this investigation, was that the compaction of the
material inside the jar had an effect on the distance that it rolled along the horizontal part of the table.
The difference in distances that the jar rolled when it was shaken after each roll, only at the start of the
rolling tests or only after every 5th roll down the incline, can be seen very clearly in the data
summarized in Tables 4 & 5.
Table 4.
The effect of shaking on the rolling distance of a jar down a 15° incline onto a horizontal
table.
Distance traveled (mm) when shaking
At the start
After each roll
Material
Volume of
jar (ml)
Mass of
material (g)
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
290
30
30
30
30
2035
775
806
748
2123
856
850
876
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
420
20
20
20
50
771
785
756
848
803
906
965
960
-8Table 5.
The effect of the frequency of shaking on the rolling distance of a jar down a 15° incline
onto a horizontal table.
Distance traveled (mm) when shaking
After each roll
After every 5th roll
Material
Volume of
jar (ml)
Mass of
material
(g)
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
156
10
30
30
30
1817
949
873
751
1679
843
826
721
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
290
20
20
20
20
2155
1418
1520
1333
1741
1331
1430
1260
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
420
40
40
40
40
1031
1235
1330
1275
935
1185
1255
1165
The results in table 4 and 5 indicate that frequency of shaking the material inside the jar has a very
definite effect on the distance that the jar travels along the horizontal plane after rolling down an
incline. This can be ascribed to compaction and/or agglomeration of the powdery material inside the
jar due to the rolling motion. It is obvious that when the jar is shaken at the start of each test, that a
larger distance is traveled than when it is only shaken at the beginning of each series of recordings or
only after the 5 times it has rolled down the incline. All subsequent tests were consequently carried out
by shaking the jar and its constituents every time before it was allowed to roll down the incline.
The effect of the sample mass and type of material on the distance that the various jars rolled down a
fixed incline of 14° is given in Table 6.
-9Table 6.
Effect of jar size and sample mass on the distance traveled along the horizontal table from a
14° incline.
Material
∞ =
Volume of
jar (ml)
Plasfill 5
Plasfill 15
Plasfill 45/110
OPC
156
Plasfill 5
Plasfill 15
Plasfill 45/110
OPC
290
Plasfill 5
Plasfill 15
Plasfill 45/110
OPC
420
10g
Distance traveled (mm) with masses of
20g
30g
40g
50g
1792
1706
1621
1500
∞
∞
930
820
800
650
595
585
-
1675
1600
1580
1530
1536
1180
1060
930
1407
-
-
1125
1100
1050
1015
980
915
902
885
910
775
750
895
860
796
730
jar rolled past the end of the table.
From the results summarized in Table 6, the following conclusions can be drawn:
(1)
Regardless of the jar size, the distance traveled along the horizontal plane decreases as the mass
of the particular material increases. There is an exception in this regard, as can be observed with
the extremely fine cenospheres Pozzfill 5. For the smallest jar size, the jar rolled past the end of
the table with increased sample mass. The only explanation for this behaviour is that the material
is so fine that it does not undergo significant compaction during the rolling motion and the greater
masses gave the jars increased kinetic energy that causes it to keep on rolling past the end point of
the table.
(2)
As the particle sizes of the materials increase, there is a decrease in the distance that the jar
travels before coming to rest on the horizontal plane of the table.
(3)
There is a large enough distance between the distances traveled by the various fly ash fractions in
the different jars to characterise them each uniquely.
-10(4)
The OPC caused the different sizes of jars to come to a stop at a shorter distance than similar
masses of the various fly ash fractions.
The effect of different inclination angles on the distance traveled by the various jars with their various
materials is given in Table 7.
Table 7.
The effect of inclination angle on distance traveled by jars on a horizontal rolling table.
Distance traveled (mm) from incline
of
14°
15°
16.3°
Material
Volume of
jar (ml)
Mass of
material
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
OPC
156
10g
30g
30g
30g
30g
1792
650
595
585
1817
2055
949
873
751
-
1055
805
900
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
OPC
290
20g
30g
30g
30g
30g
1675
1180
1060
930
2155
1418
1520
1333
-
2270
1580
1380
1225
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
OPC
420
40g
40g
40g
40g
40g
980
915
902
885
1031
1235
1330
1275
-
1325
1290
1360
1190
From the values given in Table 7, it can be seen that the distance traveled by any particular jar
containing a fixed mass of material increases with the angle of inclination.
Figures 7-12 show the relationships between the distances traveled by the jars along the horizontal
plane of the rolling angle from a fixed angle of inclination of 15 º and the ratios (mass of material:
mass of jar) and (mass of material: volume of jar) for 4 different materials used in the 3 different jar
sizes.
-11-
2
1.8
mass material/mass bottle
Distance (m)
Distance traveled (m)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
Type of material
Fig. 7 Relationship between distance traveled by a 156 ml jar and the ratio of mass of material to mass
of the jar from a 15 º incline.
2
1.8
Distance traveled (m)
1.6
mass materail/ volume bottle
Distance (m)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
Type of material
Fig. 8 Relationship between distance traveled by a 156 ml jar and the ratio of mass of material to
volume of the jar from a 15 º incline.
2.5
mass material/mass bottle
Distance (m)
Distance traveled (m)
2
1.5
1
0.5
0
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
Type of material
Fig. 9 Relationship between distance traveled by a 290 ml jar and the ratio of mass of material to mass
of the jar from a 15 º incline.
-122.5
mass material/ volume bottle
Distance travelled (m)
2
Distance (m)
1.5
1
0.5
0
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
Type of material
Fig. 10 Relationship between distance traveled by a 290 ml jar and the ratio of mass of material to
volume of the jar from a 15 º incline.
1.4
1.2
Distance traveled (m)
1
mass material/mass bottle
Distance (m)
0.8
0.6
0.4
0.2
0
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
Type of material
Fig. 11 Relationship between distance traveled by a 420 ml jar and the ratio of mass of material to mass
of the jar from a 15 º incline.
1.4
1.2
Distance traveled (m)
1
0.8
mass material/ volume bottle
Distance (m)
0.6
0.4
0.2
0
Plasfill 5
Plasfill 15
Plasfill 5/45
Plasfill 45/110
Type of material
Fig. 12 Relationship between distance traveled by a 420 ml jar and the ratio of mass of material to
volume of the jar from a 15 º incline.
-13-
It is clear from the figures that for each particular jar size, the forms of the curves of the ratios (i) mass
of material : mass of the jar and (ii) mass of material : volume of the jar are the same. However, they
are completely different for each of the jar sizes used in the investigation. It can furthermore be
observed that there is no single simple linear relationship between either of the ratios and the different
materials employed in anyone of the jar sizes used in the investigation. Resorting to any one of these 2
ratios as a way to discriminate between various particle size fractions of the fly ash is therefore not a
very sensitive and reliable and thus not recommended as a quality control measure.
CONCLUSIONS
From the investigation it can be concluded that there is a unique relationship between the distance
traveled along the horizontal plane of a rolling table by a jar of a particular volume with a fixed mass of
material and the specific particle size and distribution of the material. This was proven conclusively for
a variety of fly ashes with different particle size fractions, and it was also shown to differ from the
behaviour displayed by OPC. This approach can therefore be utilised by unskilled operators as a robust
quality control test/tool on a fly ash plant using cyclones/classifiers to separate the different size
fractions of cenospheres from unclassified fly ash.
REFERENCES
1.
P MORETON, Measuring particle size and zeta potential, Lab Africa, pp. 16-17 (1998)
2.
P K KOLAY and D N SINGH, Physical, chemical, mineralogical and thermal properties of
cenospheres from an ash lagoon, Cem & Concr. Res., 31, pp. 539-542 (2001)
3.
A RAWLE, Repeatable particle size analysis, World Cement, pp. 56-61 (June 1999)
4.
M BUMILLER and A MALCOLMSON, Particle size analysis: a review, World Cement,
pp. 50-53 (June 1998).
5.
M CYR and A TAGMIT - HAMAN, Particle size distribution of fine powders by laser
diffraction spectrometry. Case of cementitions materials, Materials & Structures, 34, pp. 342-
-14350 (2001).
6.
K. NORRISH and HUTTON, Standard XRF procedure for sample fusion & analysis, Geochim. &
Cosmochim. Acta, (1969)