Powder Technology 286 (2015) 610–615
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Powder Technology
journal homepage: www.elsevier.com/locate/powtec
Utilization of grinding aids in dry horizontal stirred milling
Okay Altun a,⁎, Hakan Benzer a, Alper Toprak a, Udo Enderle b
a
b
Hacettepe University, Mining Engineering Department, 06800 Beytepe, Ankara, Turkey
Netzsch Feinmahltechnik, Sedanstraße 70, 95100 Selb, Germany
a r t i c l e
i n f o
Article history:
Received 27 June 2015
Received in revised form 30 August 2015
Accepted 1 September 2015
Available online 4 September 2015
Keywords:
Stirred mill
Dry grinding
Fine grinding
Grinding aid
Cement
a b s t r a c t
Within the context of the study 3 types of grinding aids, which were glycol-based, TEA-based and TIPA-based,
were tested in dry stirred milling of cement. The objective was to evaluate their influences on mill performance
and cement quality then find out the most proper type with its dosage rate. The results showed that use of grinding aid improved the grinding performance of the mill as finer size distribution than that of no grinding aid was
obtained at the same milling condition. The tests at the same dosage rate indicated that, TIPA-based grinding aid
had the lowest specific energy utilization with 16.8 kWh/t compared to that of glycol (17.9 kWh/t ) and TEAbased aids (19.75 kWh/t) respectively. The product size distributions showed that TIPA-based aid had finer product with d50 of 10.54 μm compared to other types. Cement quality tests, which were assessed by measuring the
compressive strength, implied that TIPA had the highest improvement in 28-days strength with 18.8% at 700 g/t
where 8.9% and 5.6% increase were achieved for glycol-based and TEA-based grinding aids respectively. As a conclusion of the study, TIPA based grinding aid was found to be the most effective one for dry stirred mill application
of cement due to its provided operational benefits and quality improvements.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Dry energy intensive processes, such as cement manufacturing,
demand efficient grinding technologies to reduce the energy costs
where inefficient ball milling technology is still in use. Development of
stirred media mills improved the energy utilization in fine grinding
area when compared to conventional ball milling. There have been
studies compared the energy utilizations of both technologies, which
reported that stirred milling is more energy efficient [1,2]. This can be
attributed to having higher media filling ratio, agitating the fine media
at higher tip speeds hence having higher power utilization per unit
volume [3].
When cement grinding operation is considered, there is a ball mill
and air separator operated in closed circuit to achieve required fineness.
Within the circuit, high amount of material is circulated back to the mill
owing to its coarseness. This coarse product can be ground with an efficient milling technology, such as stirred milling, hence the production
rate of the circuit could be increased. Some other applications could be
the use of the stirred mill on electro filter product and final product
streams with the aim of improving the quality (ultimate strength) of
the finished product as well as energy efficiency [4]. Pilot scale dry horizontal stirred mill was developed for this purpose with the partnership
of Netzsch-Feinmahltechnik GmbH. The mill was tested in cement
⁎ Corresponding author.
E-mail address: okyaltun@hacettepe.edu.tr (O. Altun).
http://dx.doi.org/10.1016/j.powtec.2015.09.001
0032-5910/© 2015 Elsevier B.V. All rights reserved.
grinding area at different operating conditions and design features
[4,5]. Moreover, tests were performed at different types and dosage
rates of grinding aids, which was investigated within the scope of
this paper. Grinding aids have been used in cement grinding applications for many years and literature reports energy saving operations
when they are utilized [6,7]. They are mostly organic compounds and
commonly consist of glycols, alkanolamines and phenols [8].
The high polarity in their chemical functioning groups of –OH, –NH2,
–COOR, –SO3–, causes the tendency to adsorb on electrostatic surfaces
from fractured covalent bonds of Ca-O, Al-O, and Si-O, and to resist
the agglomeration [9]. The selection of the grinding aids depends on
several parameters e.g., the benefits on overall specific energy consumption, cement strength, surface area development etc. and Toprak
et al. [6], in their studies compared several types of grinding aids and
concluded that each of them has varied influences on the process and
product quality.
Up to date, the grinding aids have been used in stirred milling applications as well. Zheng et al. [10] tested various types of them in limestone grinding and concluded that the surface area of the product
could be doubled with the use of proper grinding aid. Choi et al. [11]
in their stirred mill test works showed that it was possible to reduce
the utilized energy by 37% with the use of grinding aids. Within the context of the study, 3 types of grinding aids (glycol, TEA and TIPA) were
tested and the influences on mill performance and product quality
were discussed by considering the differences in the agglomeration energies and adsorption mechanisms reported in the literature. The study
O. Altun et al. / Powder Technology 286 (2015) 610–615
611
Fig. 2. Discharge end of the mill with the product separator.
Table 2
The milling conditions of the initial test studies.
Fig. 1. Dry horizontal stirred mill (1-feed hopper, 2-control panel, 3-grinding chamber, 4-product outlet).
is thought to be beneficial for the development of proper grinding aids
particularly for dry horizontal stirred mill employed in cement grinding.
2. Materials and methods
2.1. Description of the mill setup
The photograph of the mill used in this study is depicted in Fig. 1
where the key components are also illustrated such as control panel,
feed unit (feed hopper and rotary valve), grinding chamber and product
discharge. In addition, Table 1 gives the technical data of the mill.
The rotary valve mounted under the feed hopper adjusts the mill
feed rate, which is measured from the product outlet by cutting the material flow. A torque sensor installed measures the power draw of the
mill, which displays on the control panel instantly. In grinding operation
of the mill, air is supplied from the feed inlet in order to improve material transportation towards the discharge end. The ground particles
leave the mill from the product outlet where a product separator exists
with the aim of retaining the media inside. Product separator is a cage
having openings smaller than the bead size and attached to the discharge end of the mill directly as illustrated in Fig. 2.
The temperature is important parameter that should be controlled in
cement manufacturing. Therefore, the measurements were undertaken
via infrared or non-contact thermometer from the outside of the mill
discharge end. Such information was used to identify if the product of
the stirred mill was in acceptable temperature limits [12].
Media Type
steel
Media size (mm)
Stirrer design
Stirrer speed (m/s)
Media filling (%)
Feed rate (kg/h)
4
disc
5.42
60
400
in the hopper before being ground. The tests were performed at different dosage rates, which were 500–700–1000 g/t, and at the same milling conditions given in Table 2, so that the variations coming from the
operational changes were minimized.
This study assumed that the mill feed and product samples had the
same amount of grinding aids that means the given dosage rate was
fully adsorbed by the material. Therefore, no measurements were
undertaken on how much of the grinding aids were absorbed by the
product.
2.3. Material characterization
The feed sample together with the test products were characterized
regarding to their size distribution curves, specific surface area and cement properties. In this context, Sympatec laser sizer (Germany) having
size measurement range between 1.8 μm and 500 μm was utilized in dry
mode then the whole distribution from top size down to 1.8 μm was determined. The size distribution curves were used to determine the slope
and the mean size parameters. The slope, which was expressed by n parameter in RRBS equation, is an important parameter in cement
strength development [14] and within the study the influences of
stirred milling were discussed. Specific surface area measurements
were undertaken via Blaine method [15]. Moreover, cement strength
at different ages (7 days and 28 days) and water demands of cement
mortar were determined at the cement plant by applying the standard
procedures [16].
2.2. Grinding aids
Although various types of grinding aids are utilized by the industry
[6,13] within the scope of the study, 3 types of them were tested for
dry stirred milling which were; TEA, TIPA and Glycol-based ones. The
grinding aids tested were in liquid form and mixed with feed material
Table 1
Technical specifications of the mill.
Effective diameter (cm)
26.4
Effective length (cm)
Motor power (kW)
Maximum feed rate (kg/h)
Stirrer tip speed (m/s)
Maximum air flow rate (L/h)
75.0
18
500
1.08 - 9.76
1000
Fig. 3. Feed material characteristics.
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O. Altun et al. / Powder Technology 286 (2015) 610–615
Table 3
Chemical assay of the cement processed in the mill.
Element
%
CaO
SiO2
Al2O3
Fe2O3
MgO
SO3
K2O
Na2O
Loss of ignition
63.27
19.68
4.94
3.38
2.03
3.07
0.77
0.52
2.32
Fig. 5. The size distributions obtained from glycol-based grinding aid tests.
Table 4
Change in process data with chemical dosage.
Chemical
dosage (g/t)
Specific energy
(kWh/t)
Material amount inside
the mill (kg)
Mean product size,
d50 (μm)
0
500
700
1000
21.15
20.18
19.55
19.31
17.1
12.2
11.2
11.3
14.64
14.07
13.71
13.65
The sample used in grinding tests was collected from the finished
product stream of a cement grinding circuit when producing CEM I
42.5R type cement. Fig. 3 illustrates the feed characteristics in terms of
the cumulative particle size distribution curve by mass, temperature
(°C), mean size (d50,3), slope, cement strength and water% parameters.
Additionally, Table 3 gives the chemical assay of the cement processed
in the mill.
3. Results and discussions
As can be followed from Table 4, the product of the mill gets finer in
case of using grinding aids. This can be attributed to the fluidization of
bulk material inside the mill that lead the particles already in desired
fineness to be transported through the discharge. It can also be concluded that the change in product mean size is dosage dependent. Similar
conclusion was drawn by Hasegawa et al. [18] who noted an increase
in specific surface area of the product with the amount of additives
used. Another observation is the decrease in mill load. There is a considerable difference between grinding aid and no aid conditions, which
provided a reduction in specific energy consumption by 7.5% from 0 to
700 g/t. The results also proved that further increase in dosage rate
(1000 g/t) is not beneficial in terms of performance improvement. The
improvement in mill transportation can also be expressed by calculating
the retention time of the material. Since the tests were performed at the
same feed rate (400 kg/h), retention time of the material reduced from
154 seconds (0 g/t) to 101 seconds (700 g/t), showing the mill conditions were improved. Nair and Paramasivam [19] also observed increase
in the flowability of the material in their batch ball milling studies when
grinding aid was utilized.
3.1. Measuring the effects of grinding aids on material transportation
As reported in the literature, introducing grinding aids into the milling equipment makes the transportation easier since the material
becomes fluidized owing to neutralizing the surface charges of the particles [6,17]. This study aimed to reveal the influences of grinding aids
on the mill inside and discharge by conducting systematic test studies
when TIPA-based chemical was used. In this context, 4 tests at changing
dosage rates were carried out initially then the product samples were
collected when the steady state conditions were established. In addition, the mill was crashed stopped for each of the condition and chamber was removed to weigh the material amount inside. The test results
are summarized in Table 4.
3.2. Comparison of the grinding aid performances
3.2.1. The tests with glycol-based grinding aid
Fig. 4 illustrates the trends of specific energy, mean size, Blaine numbers while Fig. 5 depicts the size distribution curves obtained from the
tests with glycol-based grinding aid. As can be understood from Fig. 4,
the condition without grinding aid (no aid) is the least efficient one
among the tests since higher specific energy is consumed and relatively
coarser distribution is obtained. The tests also showed that dosage rates
of 500 g/t and 700 g/t had similar performance data i.e., energy consumption, product mean size and specific surface area. Both conditions
provided reduction in specific energy consumption by 14.5% with respect to no aid condition, which had 20.96 kWh/t. Further increase in
dosage rate (1000 g/t) contributed additional energy saving with 3.3%
(700 g/t-17.9 kWh/t, 1000 g/t-17.3 kWh/t). The size distributions illustrated in Fig. 5 imply that, no aid condition has coarser and wider distribution compared to other tests. Additionally it is seen that, changing the
dosage rates has no considerable influence on the mean size and slope
Table 5
Cement properties of glycol-based grinding aid test products.
Fig. 4. Grinding results of glycol-based grinding aid tests.
Feed
No aid
500 g/t
700 g/t
1000 g/t
7 days strength
(MPa)
28 days strength
(MPa)
Water demand %
40.8
41.1
41.4
41.8
43.1
49.6
51.2
52.1
52.3
54.0
28.2
27.2
29.2
29.2
29.4
O. Altun et al. / Powder Technology 286 (2015) 610–615
Fig. 6. Grinding results of TEA-based grinding aid tests.
613
Fig. 8. Grinding results of TIPA-based grinding aid tests.
illustrating the process performance implies that neither the product
fineness nor the specific energy consumption parameters dependent
on the variation in grinding aid dosage since each test was performed
at 19.7 kWh/t. However they are all more efficient than no aid condition
where the specific energy utilization was 20.96 kWh/t. The particle size
distributions illustrated in Fig. 7 show that the slope and the mean sizes
of grinding aid tests are the same and different from no aid condition
where the distribution was coarser and wider. Finally, the temperature
measured from outside of the discharge end section was 93 °C, which
was similar to that of glycol-based grinding aid test.
The cement properties given in Table 6 indicate that the variation in
dosage rate has no influence on both the strength and water demand
properties. However, all the results are higher than no aid condition.
The maximum 28-days strength was obtained at 1000 g/t, which
corresponded to 5.6% increase compared to that of feed material.
Fig. 7. The size distributions obtained from TEA-based grinding aid tests.
of the product size distributions. Throughout the test studies, the temperature measured from outside of the discharge end section was
94 °C that was well below 120 °C where dehydration of gypsum occurs.
From cement properties point of view, the variations in compressive
strength and water demand were investigated. The results given in
Table 5 imply that, grinding operation has an influence on cement properties and additionally, it is dependent on the grinding aid and its adjusted dosage rate. It also indicates that the highest increase in
28 days strength was observed at the dosage rate of 1000 g/t where it
was improved by 8.87% with respect to that of feed (non-ground).
When the water demands are compared, it is found that the grinding
aids have higher per cent values than feed and no aid test. Having less
n value (slope) in size distribution curve (Fig. 5) compared to other
products is thought to be effective on obtaining decreased water
demand [14].
3.2.3. The tests with triisopropanolamine (TIPA)-based grinding aid
Figs. 8 and 9 show the variations in process trends of TIPA aid with
the dosage rate. It is understood from the results that no aid condition
has higher specific energy utilization and it decreases substantially up
to 1000 g/t. That is, from 500 g/t to 700 g/t, energy consumption is reduced by 4.5% (from 17.6 to 16.8 kWh/t) however further increase
(1000 g/t) has no contribution. The particle size distributions imply
that, the mean sizes together with the slope of the curves are the
same for all of the test products and parallel to each other except the
no aid case where the curve is wider and coarser. Moreover, the temperature was 92 °C that was similar to the previous measurements.
Regarding to the cement properties, the strength and water demand
measurements were undertaken. As can be seen from Table 7, the cement strength at all ages increased with respect to the feed material
as a result of grinding process. The results indicated that the highest increase in 28 days was observed at 700 g/t with 18.8% where the dosage
3.2.2. The tests with triethanolamine (TEA)-based grinding aid
While testing the TEA-based grinding aid the same methodology as
the glycol-based one was applied. Initially, the influences on the process
performance and product size distribution curves were evaluated. Fig. 6
Table 6
Cement properties of TEA-based grinding aid test products.
Feed
No aid
500 g/t
700 g/t
1000 g/t
7 days strength
(MPa)
28 days strength
(MPa)
Water demand %
40.8
41.1
43.5
43.6
44.8
49.6
51.2
52.2
52.1
52.4
28.2
27.2
28.4
28.4
28.4
Fig. 9. The size distributions obtained from TIPA-based grinding aid tests.
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O. Altun et al. / Powder Technology 286 (2015) 610–615
Table 7
Cement properties of TIPA-based grinding aid test products.
Feed
No aid
500 g/t
700 g/t
1000 g/t
7 days strength
(MPa)
28 days strength
(MPa)
Water demand %
40.8
41.1
44.2
46.3
46.6
49.6
51.2
55.4
58.9
58.5
28.2
27.2
28.2
28.2
28.6
rates of 500 g/t and 1000 g/t increased the strength by 11.69% and
17.94% respectively. The improved results of the grinding aids compared to no aid condition proved that the use of TIPA-based aid is beneficial on quality improvement of cement.
Fig. 11. The change in 28-days strength development of cement with grinding aids.
3.2.4. Comparison in terms of process performance
As presented in the previous chapters, the specific energy utilization
of the mill, the specific surface area and the mean size parameters were
considered while evaluating the influences of the grinding aids. Within
the chapter the test results are illustrated in Fig. 10 comparatively.
Fig. 10 summarizes that, TIPA-based grinding aid has more pronounced effect on mill performance among the grinding aids tested. In
terms of product mean size, its operation has the finer product with
d50 of 10.54 μm (Fig. 10a, at 1000 g/t). Regarding to the specific energy
consumption, its operation utilized 6.4% and 15% less energy than
glycol-based and TEA-based grinding aids at 700 g/t respectively
(Fig. 10b). The results presented in Fig. 10 prove that using TIPA based
aid resulted in obtaining less specific energy consumption with finer
product size compared to glycol based and TEA based aids. The difference in energy utilizations between the grinding aids can be attributed
to change in dispersion of the powder inside the mill that is a function of
the fluidity of the cement powder. As the particles are dispersed, the energy is transferred more efficiently thus the performance is improved.
Sverak et al. [13] in their laboratory scale test studies with Portland cement clinker concluded that the mill had different grinding results depending on the type of grinding aid used. Jolicoeur et al. [20] in their
studies tested 5 different grinding aids and found out the fluidity
index with their sieving technique. They concluded that, the calculated
fluidity index of glycol type aid was two times higher than TEA that supports the conclusions drawn within the study. In the literature, several
studies regarding to TEA, TIPA comparison can be found that supports
the outcomes of the dry stirred milling study. Mishra et al. [21] studied
on adsorption mechanisms of grinding aids on clinker surface. In their
studies, agglomeration energies of C3S phase, on which different types
of grinding aids were adsorbed, were calculated. They concluded that
C3S surface without aid had the highest agglomeration energy and
then TEA and TIPA came respectively. That is, TIPA aid disperses the particles more effectively hence improved grinding performance is obtained. Finally, Katsioti et al. [22] compared the effects of TIPA and TEA type
grinding aids on grindability of clinker. The study showed that, TIPA and
TEA increased the grindability of clinker by 26% and 14% respectively. In
other words, using TIPA based aid improves the energy efficiency of a
mill.
3.2.5. Comparison in terms of product quality
Fig. 11 illustrates the influences of grinding aid types on ultimate
strength development (28-days) of cement. The strength results were
compared with reference or feed sample. It is seen from the figure
that, the tests with grinding aids have higher strength values than no
aid condition. This can be attributed to difference in product size distributions since no aid sample is coarser than the test products.
It is also understood from the figure that, TIPA-based grinding aid
has the highest development with 17.94% at 1000 g/t and 18.75% at
700 g/t among the grinding aids tested. On the other hand, it was
observed that TEA-based chemical has the least influence on strength
development. Within the stirred mill application, TIPA-based aid was
found to be more beneficial compared to TEA and glycol-based ones.
The influences of different types of grinding aids on strength development have been reported in the literature [6,22,23]. Toprak et al. [3] in
their studies on cement grinding circuits indicated that the 28-days
strength of cement could be improved by 2% to 4% with the use of grinding aids. In particular, TEA-based grinding aid was found to be the most
beneficial one when the economy of the given circuit was considered.
Gartner and Myers [23] studied the effects of TEA and TIPA on hydration
of cement. They concluded that TIPA grinding aid had more pronounced
effect on early strength development owing to increased hydration of
C4AF. The mechanism was proposed to be that TIPA remained in solution and formed iron complexes that affected the strength. Katsioti
et al. [22] in their studies compared the effects of TEA and TIPA on
strength development and showed that TIPA improved the strength
significantly.
Fig. 10. The variation of mill performance with grinding aids.
O. Altun et al. / Powder Technology 286 (2015) 610–615
4. Conclusions
Within the scope of the study different types of grinding aids, glycolbased, TEA-based and TIPA based, were tested then the influences on
dry stirred mill performance and cement properties were discussed.
The test studies implied that the use of grinding aids lead to having
finer product size distribution and decreased specific energy consumption when compared to no aid condition. Among the grinding aids tested, TIPA-based grinding aid was the most effective one at all the dosage
rates. The highest difference was observed at 700 g/t, where the mill utilized 16.8 kWh/t of specific energy, which was 6.4% and 15% less than
glycol-based and TEA-based grinding aids used at the same dosage
rate. In addition to influences on the specific energy consumption of
the mill, TIPA-based grinding aid produced finer product with d50 of
10.58 μm. The variations in the performances of grinding aids can be attributed to the change in the fluidity of the powder since different grinding aids have different influences as reported in the literature. Finally,
the cement strength measurements were undertaken for the test products. It was understood from the results that TIPA-based grinding aid
had the highest development in 28-day strength among the aids tested.
At 1000 g/t, the strength increased by 17.94% and at 700 g/t it increased
by 18.75% with respect to reference or feed sample. The assessments
concluded that TIPA-based grinding aid was the most suitable type for
the dry horizontal stirred mill operation as higher mill performance
and product quality are obtained.
Acknowledgements
Authors appreciate contributions of Assoc. Prof. Abdullah Obut
(Hacettepe University) for his assistance in writing, SET Italcementi
Ankara Cement Plant for the assistance in laboratory studies and
Hacettepe University Research Foundation Unit (Project No: 013 T06
604 006) for the financial support.
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