- Email: [email protected]
Effects of grinding parameters on product fineness in jet mill grinding
Minerals Engineering. Voi. 11, No. I1,
Pergamon 0892-687~(98)00094J--8
pp. 1089-1094, 1998 © 1998 Elsevier Science Ltd All fights reserved 0892-6875/98/$ ~ see front matter
TECHNICAL NOTE EFFECTS OF GRINDING PARAMETERS ON PRODUCT FINENESS IN JET MILL GRINDING
R. T U U N I L A a n d L. N Y S T R O M Lappeenranta University of Technology, Department of Chemical Technology, Box 20, 53851 Lappeenranta, Finland. E-mail: [email protected] (Received 17 September 1997; accepted 14 July 1998)
ABSTRACT
The influence of several grinding parameters on the product fineness in jet mill grinding was stud,led experimentally with a laboratory scale spiral type jet mill. The most significant variables in the jet mill grinding were feed rate, volumetric flow rate of grinding air and the height of an inside classification tube for grinding at constant pressure. Results showed that the height of the inner classification tube (vortex finder) is also a relevant variable in a spiral jet mill, and should be taken into account in optimising the process. The finest product size was obtained with smallest material feed rate, highest grinding air flow rate and with the shortest classificaIion tube. © 1998 Elsevier Science Ltd. All rights reserved Keywords Fine particle processing; grinding; particle size
INTRODUCTION Raw minerals naturally occur quite coarse-grained, so they have to be ground to a required fineness, determined, for ex;unple, by the needs of the paper or the paint industry. In these cases ultra fine grinding is needed as final processing. Ultra fine grinding is a unit operation process where particles are ground to a fineness where 8t)% of the particles are smaller than 1-10 i~n. Mills often used for ultra fine grinding are the attrition mill and the jet mill [1,2]. In cases where dry grinding, or a very narrow particle size distribution for the product, is needed, jet mills are mostly used. The jet mill is a stutic machine which does not have any grinding media. The milling component of the jet mill consists of a chamber with a nozzle or nozzles. The particles to be pulverised are accelerated by pressurised gas or steam jets, and the grinding effect is produced by interparticle collision or by impact against solid surfaces [3,4]. The aim of this work is to study the influence of the different grinding parameters of the spiral type jet mill on the product fineness to optimise the grinding process. Presented at Minerals Engineering '97, Santiago, Chile, July-August 1997
1089
1o9o
R. Tuunilaand L. Nystr6m EXPERIMENTAL
Equipment and materials
The mill used in this study was a pancake or spiral type jet mill where the nozzles are set in a ring at a certain angle and are supplied with compressed air from the outside. The mill used in the study and the flowsheet of the process are shown in Figure 1. The diameter of the cylindrical grinding chamber is 100 mm, the height of the chamber 15 mm and the diameter of the nozzles 1.5 ram. The mill outlet is in the centre to obtain separation by centrifugal force. The diameter of the inner classification tube (vortex finder) is 45.8 mm. feed air
grinding Air
feed
I/
,,,/ alr~
/ pmauet Fig. 1 The jet mill used and the process flowsheet.
The grinding tests were performed in a similar way with three different minerals limestone (Parfill limestone), FGD (Flue Gas Desulphurisation) gypsum from a power plant, and waste gypsum from a phosphoric acid plant (later phosphogypsum). Samples of the ground minerals were taken from the filter bag which was replaced with a new bag after every experiment. Both gypsum samples were quite moist (30% water) and therefore they were dried before grinding to prevent accumulation of the material in the grinding chamber. The mass-based particle size distributions of the feed materials and of the ground products were measured with a SediGraph 1500 particle size analyser. In the results median particle sizes (ds0) are used as meaningful values. The particle size distributions of the samples were analysed in a 0.2% NaHMP-water solution. For gypsum analysis the solutions were saturated with the gypsum to prevent the gypsum from dissolving in the water. Grinding tests The effects of feed rate, volumetric flow rates of grinding air and feed air, angle of nozzles, and height of inside classification tube on grinding were studied in laboratory tests. All tests were carded out with a constant air pressure of 450 kPa. The effect of feed rate was studied by grinding both gypsum types at three different rates: 1.2, 3.6, and 6.0 g/rain, with the other parameters constant (see Table 1). Three angles: 23 °, 33 °, and 43 °, were used in studying the influence of the angle of the nozzles with a feed rate of 4.0 g/rain. When studying the effect of the grinding air flow rate, four flow rates of compressed air (450 kPa): 2.2, 4.4, 6.6, and 8.8 m3/h, were used. The influence of the feed air flow rate was tested with three feed flow rates: 4.4, 5.5, and 6.6 m3/h. The influence of the height of the inner classification tube (ho was also tested with three different values, which were 5, 8, and 11 mm, the height of the tube referring to the a distance from the bottom of the chamber to the top of the tube (Figure 1). Grinding conditions in each test are given in Table 1. The influence of regrinding was also tested with every material by regrinding the mill product for every material twice. The same conditions were used, namely feed rate 4.0 g/rain (0.24 kg/h), volumetric flow
Product fineness in jet mill grinding
1091
rates of pressurised feed air 6.6 m3/hand grinding air 8.8 m~/h, at a pressure of 450 kPa, angle of nozzles 43 ° and height of classification tube 5 ram. TABLE 1 Experimental conditions at a pressure of 450 kPa
.
1.2, 3.6 and 6 g/min Feed rates Angle o f nozzles 43 ° 6.6 m3/h Volumetric flow rate of feed air Volumetric flow rate o f grinding air 8.8 m3/h Height o f classification tube 5mm
.
Angle o f nozzles 23 °, 33 °, and43 ° Feed rate 4 g/min 6.6 m3/h Volumetric flow rate & f e e d air Volumetric flow rate of grinding air 8.8 m3/h 5mm Height o f classification tube
.
Volumetric rates of feed air 4.4, 5.5, and 6.6 Nma/h 4 g/rain Feed rate 43 ° Angle o f nozzles Volumetric flow rate of grinding air 8.8 ma/h 5mm Height o f classification tube
.
Volumetric rates of grinding air Feed rate Angle o f nozzles Volumetric flow rate of feed air Height o f classification tube
2.2, 4.4, 6.6, and 8.8 m3/h 4 g/rain 43 ° 6.6 mS/h 5mm
.
Heights o f classification tube Feed rate Angle o f nozzles Volumetric flow rate of feed air Volumetric flow rate of grinding air
4 g/min 43 ° 6.6 m3/h 8.8 m3/h
5, 8, and 11 mm
RESULTS AND DISCUSSION
The results showed that the most significant parameters affecting jet mill grinding were material feed rate, grinding air flow nlte, and height of the classification tube. According to this study the angle of the nozzles and the flow rate of feed air were less significant parameters. The effects of these two parameters for FGD gypsum and for phosphogypsum are shown in Table 2, which also shows that there was no significant effect of the parameters on the product fineness. However, with both materials the smallest product sizes were achieved with the largest angle (43°), and this was chosen as the angle of the nozzles in further experiments. The choice of the highest angle is also justified by the results given by Ahlbus [5], who has found that an angle of 60° is most effective. The influence of the flow rate of grinding air was almost negligible, but because the lowest particle size for the phosphogypsum, and almost the lowest size for FGD gypsum, was
1092
R. Tuunila and L. Nystr6m
achieved with the highest tested feed air flow rate, of 6.6 m3/h (29.4 Nm3/h), this was chosen as the flow rate in further experiments. The highest volumetric flow rates of feed air caused some dusting problems outside the mill and therefore high flow rates were not used. The effects of the angle of the nozzles and the volumetric rate of grinding air on the product fineness in jet mill grinding at a pressure of 450 kPa
TABLE 2
FGD gypsum
phosphogypsum
(pm)
(pm)
36.1
32.1
Feed size Angle o f nozzles (0)
Product size (pm)
23 33 43
17.60 17.38 17.23
18.07 18.76 17.30
Volumetric rate o f feed air (m3/h)
Product size (pm)
4.4 5.5 6.6
17.20 17.36 17.23
16.63 16.92 16.30
The results showed that with every mineral a finer product was achieved when ground with a low feed rate and with a high grinding air flow rate, as can be seen from Figure 2. The effects are similar to those given by Yoon [6] and Ramanujam and Venkatesvarlu [9], hut differ from those of Mohanty and Narashnan [7], who found that there is an optimal feed rate for each mineral when the minimum product size is achieved. 14 12
m
_e O
m O. C
/
//
8 6
"X
./'
_a 4 E
• FGD gypsum
2
• Phosphogl~unl
0 0
2
I
I
4
6
Feed rate [g/min]
Fig.2
8
-
=
-
• FGD gyl~um
_
z
0
~
u
m
~
l'
I
2
4
6
8
10
Volumetric flow rate of gdnding air [ma/h]
Influence of the feed rate and grinding air (compressed) flow rate on the product fineness in jet mill grinding.
Product fineness in jet mill grinding
1093
The effect of grinding air flow rate, shown in Figure 2, can be interpreted as resulting from a higher velocity of particles, which increases the impact energy when the particles collide with each other, or with the wall of the grinding chamber. The height of the iLnner classification tube was also found to be a relevant parameter in the spiral type jet mill, and should altso be taken into account when optimising the grinding conditions. With every mineral a shorter tube led to a finer product, as can be seen from Figure 3.
20
18 E 16
f
,,.-41
.__..._.,,----
14 /
12 e o 10
/
Z
~8 e-
_m 6 "U O
• Phosphogypsum
--
4
• FGD gypsum
-
2
• Limestone
0
0
5
10
15
Height of intemal tube [ram]
Fig.3 Influence of the height of the classification tube on the product fineness in the jet mill. The effect of regrinding on the product fineness is given in Table 3, which shows that by regrinding with the same mill only a slightly finer product can be achieved. Because the incremental energy consumption of such a jet mill stage is quite high [8], the possibilities of obtaining a better product fineness by regrinding are not economie,'dly profitable. TABLE 3 Effect of regrinding on the product fineness
Median particle size / ~tm
Numberof gfindings 0 (feed) 1 2 3
FGDgypsum Phosphogyp. Limestone 36.1 14.5 12.9 10.9
32. l 15.0 12.7 12.5
23.6 14.3 10.6 10.3
CONCLUSIONS An experimental study on fine grinding of three minerals with the spiral type jet mill was carried out in the laboratory to inv(;stigate the effects of different grinding parameters and regrinding on the product fineness at constant pressure. The results were as follows:
1094
R. Tuunila and L. Nystr6m
1.
The height of the inside classification tube must also be taken into account in optimising the grinding conditions of the spiral type jet mill.
.
Other important parameters in jet mill grinding are the feed rate and the flow rate of grinding air, when grinding at constant pressure.
.
The importance of regrinding is so slight that it is not economically profitable to repeat grinding with the same mill.
REFERENCES .
2. 3. 4. . . ,
8.
9.
Prasher, C.L., Crushing and Grinding Handbook, Biddies Ltd, Chiehester (1987). Hixon, L.M., Select an effective size-reduction system, Chem.Eng.Prog., May, 36--44 (1991). Kukla, R.J., Understand your size-reduction system, Chem.Eng,Prog., May, 23-35 (1991). Vogel, A., The alpine fluidized bed opposed jet mill, Powder Handling & Processing, 3, 129-132 (1991). Ahlbus, F.E., Fluid energy grinding or jet mill grinding, Advanced Powder Technology, 3, 273-284 (1992). Yoon, S.H., Scale-up method for a horizontal-type jet mill, Advanced Powder Technology, 5, 53-59 (1994). Mohanty, B. & Narasiman, K.S., Fluid energy grinding, Powder TechnoL, 33, 135-141 (1982). Dotson, J.M., Extending the range of jet mills, Ind. Eng.Chem., 2(42), 62-65 (1962). Ramanujarn, M. & Venkanteswarlu, D., Studies in fluid energy grinding, Powder Technology, 3, 92-101 (1969/70).
C o r r e s p o n d e n c e on papers published in Minerals Engineering is invited, preferably by email to [email protected], or by Fax to +44-(0)1326-318352
Pergamon 0892-687~(98)00094J--8
pp. 1089-1094, 1998 © 1998 Elsevier Science Ltd All fights reserved 0892-6875/98/$ ~ see front matter
TECHNICAL NOTE EFFECTS OF GRINDING PARAMETERS ON PRODUCT FINENESS IN JET MILL GRINDING
R. T U U N I L A a n d L. N Y S T R O M Lappeenranta University of Technology, Department of Chemical Technology, Box 20, 53851 Lappeenranta, Finland. E-mail: [email protected] (Received 17 September 1997; accepted 14 July 1998)
ABSTRACT
The influence of several grinding parameters on the product fineness in jet mill grinding was stud,led experimentally with a laboratory scale spiral type jet mill. The most significant variables in the jet mill grinding were feed rate, volumetric flow rate of grinding air and the height of an inside classification tube for grinding at constant pressure. Results showed that the height of the inner classification tube (vortex finder) is also a relevant variable in a spiral jet mill, and should be taken into account in optimising the process. The finest product size was obtained with smallest material feed rate, highest grinding air flow rate and with the shortest classificaIion tube. © 1998 Elsevier Science Ltd. All rights reserved Keywords Fine particle processing; grinding; particle size
INTRODUCTION Raw minerals naturally occur quite coarse-grained, so they have to be ground to a required fineness, determined, for ex;unple, by the needs of the paper or the paint industry. In these cases ultra fine grinding is needed as final processing. Ultra fine grinding is a unit operation process where particles are ground to a fineness where 8t)% of the particles are smaller than 1-10 i~n. Mills often used for ultra fine grinding are the attrition mill and the jet mill [1,2]. In cases where dry grinding, or a very narrow particle size distribution for the product, is needed, jet mills are mostly used. The jet mill is a stutic machine which does not have any grinding media. The milling component of the jet mill consists of a chamber with a nozzle or nozzles. The particles to be pulverised are accelerated by pressurised gas or steam jets, and the grinding effect is produced by interparticle collision or by impact against solid surfaces [3,4]. The aim of this work is to study the influence of the different grinding parameters of the spiral type jet mill on the product fineness to optimise the grinding process. Presented at Minerals Engineering '97, Santiago, Chile, July-August 1997
1089
1o9o
R. Tuunilaand L. Nystr6m EXPERIMENTAL
Equipment and materials
The mill used in this study was a pancake or spiral type jet mill where the nozzles are set in a ring at a certain angle and are supplied with compressed air from the outside. The mill used in the study and the flowsheet of the process are shown in Figure 1. The diameter of the cylindrical grinding chamber is 100 mm, the height of the chamber 15 mm and the diameter of the nozzles 1.5 ram. The mill outlet is in the centre to obtain separation by centrifugal force. The diameter of the inner classification tube (vortex finder) is 45.8 mm. feed air
grinding Air
feed
I/
,,,/ alr~
/ pmauet Fig. 1 The jet mill used and the process flowsheet.
The grinding tests were performed in a similar way with three different minerals limestone (Parfill limestone), FGD (Flue Gas Desulphurisation) gypsum from a power plant, and waste gypsum from a phosphoric acid plant (later phosphogypsum). Samples of the ground minerals were taken from the filter bag which was replaced with a new bag after every experiment. Both gypsum samples were quite moist (30% water) and therefore they were dried before grinding to prevent accumulation of the material in the grinding chamber. The mass-based particle size distributions of the feed materials and of the ground products were measured with a SediGraph 1500 particle size analyser. In the results median particle sizes (ds0) are used as meaningful values. The particle size distributions of the samples were analysed in a 0.2% NaHMP-water solution. For gypsum analysis the solutions were saturated with the gypsum to prevent the gypsum from dissolving in the water. Grinding tests The effects of feed rate, volumetric flow rates of grinding air and feed air, angle of nozzles, and height of inside classification tube on grinding were studied in laboratory tests. All tests were carded out with a constant air pressure of 450 kPa. The effect of feed rate was studied by grinding both gypsum types at three different rates: 1.2, 3.6, and 6.0 g/rain, with the other parameters constant (see Table 1). Three angles: 23 °, 33 °, and 43 °, were used in studying the influence of the angle of the nozzles with a feed rate of 4.0 g/rain. When studying the effect of the grinding air flow rate, four flow rates of compressed air (450 kPa): 2.2, 4.4, 6.6, and 8.8 m3/h, were used. The influence of the feed air flow rate was tested with three feed flow rates: 4.4, 5.5, and 6.6 m3/h. The influence of the height of the inner classification tube (ho was also tested with three different values, which were 5, 8, and 11 mm, the height of the tube referring to the a distance from the bottom of the chamber to the top of the tube (Figure 1). Grinding conditions in each test are given in Table 1. The influence of regrinding was also tested with every material by regrinding the mill product for every material twice. The same conditions were used, namely feed rate 4.0 g/rain (0.24 kg/h), volumetric flow
Product fineness in jet mill grinding
1091
rates of pressurised feed air 6.6 m3/hand grinding air 8.8 m~/h, at a pressure of 450 kPa, angle of nozzles 43 ° and height of classification tube 5 ram. TABLE 1 Experimental conditions at a pressure of 450 kPa
.
1.2, 3.6 and 6 g/min Feed rates Angle o f nozzles 43 ° 6.6 m3/h Volumetric flow rate of feed air Volumetric flow rate o f grinding air 8.8 m3/h Height o f classification tube 5mm
.
Angle o f nozzles 23 °, 33 °, and43 ° Feed rate 4 g/min 6.6 m3/h Volumetric flow rate & f e e d air Volumetric flow rate of grinding air 8.8 m3/h 5mm Height o f classification tube
.
Volumetric rates of feed air 4.4, 5.5, and 6.6 Nma/h 4 g/rain Feed rate 43 ° Angle o f nozzles Volumetric flow rate of grinding air 8.8 ma/h 5mm Height o f classification tube
.
Volumetric rates of grinding air Feed rate Angle o f nozzles Volumetric flow rate of feed air Height o f classification tube
2.2, 4.4, 6.6, and 8.8 m3/h 4 g/rain 43 ° 6.6 mS/h 5mm
.
Heights o f classification tube Feed rate Angle o f nozzles Volumetric flow rate of feed air Volumetric flow rate of grinding air
4 g/min 43 ° 6.6 m3/h 8.8 m3/h
5, 8, and 11 mm
RESULTS AND DISCUSSION
The results showed that the most significant parameters affecting jet mill grinding were material feed rate, grinding air flow nlte, and height of the classification tube. According to this study the angle of the nozzles and the flow rate of feed air were less significant parameters. The effects of these two parameters for FGD gypsum and for phosphogypsum are shown in Table 2, which also shows that there was no significant effect of the parameters on the product fineness. However, with both materials the smallest product sizes were achieved with the largest angle (43°), and this was chosen as the angle of the nozzles in further experiments. The choice of the highest angle is also justified by the results given by Ahlbus [5], who has found that an angle of 60° is most effective. The influence of the flow rate of grinding air was almost negligible, but because the lowest particle size for the phosphogypsum, and almost the lowest size for FGD gypsum, was
1092
R. Tuunila and L. Nystr6m
achieved with the highest tested feed air flow rate, of 6.6 m3/h (29.4 Nm3/h), this was chosen as the flow rate in further experiments. The highest volumetric flow rates of feed air caused some dusting problems outside the mill and therefore high flow rates were not used. The effects of the angle of the nozzles and the volumetric rate of grinding air on the product fineness in jet mill grinding at a pressure of 450 kPa
TABLE 2
FGD gypsum
phosphogypsum
(pm)
(pm)
36.1
32.1
Feed size Angle o f nozzles (0)
Product size (pm)
23 33 43
17.60 17.38 17.23
18.07 18.76 17.30
Volumetric rate o f feed air (m3/h)
Product size (pm)
4.4 5.5 6.6
17.20 17.36 17.23
16.63 16.92 16.30
The results showed that with every mineral a finer product was achieved when ground with a low feed rate and with a high grinding air flow rate, as can be seen from Figure 2. The effects are similar to those given by Yoon [6] and Ramanujam and Venkatesvarlu [9], hut differ from those of Mohanty and Narashnan [7], who found that there is an optimal feed rate for each mineral when the minimum product size is achieved. 14 12
m
_e O
m O. C
/
//
8 6
"X
./'
_a 4 E
• FGD gypsum
2
• Phosphogl~unl
0 0
2
I
I
4
6
Feed rate [g/min]
Fig.2
8
-
=
-
• FGD gyl~um
_
z
0
~
u
m
~
l'
I
2
4
6
8
10
Volumetric flow rate of gdnding air [ma/h]
Influence of the feed rate and grinding air (compressed) flow rate on the product fineness in jet mill grinding.
Product fineness in jet mill grinding
1093
The effect of grinding air flow rate, shown in Figure 2, can be interpreted as resulting from a higher velocity of particles, which increases the impact energy when the particles collide with each other, or with the wall of the grinding chamber. The height of the iLnner classification tube was also found to be a relevant parameter in the spiral type jet mill, and should altso be taken into account when optimising the grinding conditions. With every mineral a shorter tube led to a finer product, as can be seen from Figure 3.
20
18 E 16
f
,,.-41
.__..._.,,----
14 /
12 e o 10
/
Z
~8 e-
_m 6 "U O
• Phosphogypsum
--
4
• FGD gypsum
-
2
• Limestone
0
0
5
10
15
Height of intemal tube [ram]
Fig.3 Influence of the height of the classification tube on the product fineness in the jet mill. The effect of regrinding on the product fineness is given in Table 3, which shows that by regrinding with the same mill only a slightly finer product can be achieved. Because the incremental energy consumption of such a jet mill stage is quite high [8], the possibilities of obtaining a better product fineness by regrinding are not economie,'dly profitable. TABLE 3 Effect of regrinding on the product fineness
Median particle size / ~tm
Numberof gfindings 0 (feed) 1 2 3
FGDgypsum Phosphogyp. Limestone 36.1 14.5 12.9 10.9
32. l 15.0 12.7 12.5
23.6 14.3 10.6 10.3
CONCLUSIONS An experimental study on fine grinding of three minerals with the spiral type jet mill was carried out in the laboratory to inv(;stigate the effects of different grinding parameters and regrinding on the product fineness at constant pressure. The results were as follows:
1094
R. Tuunila and L. Nystr6m
1.
The height of the inside classification tube must also be taken into account in optimising the grinding conditions of the spiral type jet mill.
.
Other important parameters in jet mill grinding are the feed rate and the flow rate of grinding air, when grinding at constant pressure.
.
The importance of regrinding is so slight that it is not economically profitable to repeat grinding with the same mill.
REFERENCES .
2. 3. 4. . . ,
8.
9.
Prasher, C.L., Crushing and Grinding Handbook, Biddies Ltd, Chiehester (1987). Hixon, L.M., Select an effective size-reduction system, Chem.Eng.Prog., May, 36--44 (1991). Kukla, R.J., Understand your size-reduction system, Chem.Eng,Prog., May, 23-35 (1991). Vogel, A., The alpine fluidized bed opposed jet mill, Powder Handling & Processing, 3, 129-132 (1991). Ahlbus, F.E., Fluid energy grinding or jet mill grinding, Advanced Powder Technology, 3, 273-284 (1992). Yoon, S.H., Scale-up method for a horizontal-type jet mill, Advanced Powder Technology, 5, 53-59 (1994). Mohanty, B. & Narasiman, K.S., Fluid energy grinding, Powder TechnoL, 33, 135-141 (1982). Dotson, J.M., Extending the range of jet mills, Ind. Eng.Chem., 2(42), 62-65 (1962). Ramanujarn, M. & Venkanteswarlu, D., Studies in fluid energy grinding, Powder Technology, 3, 92-101 (1969/70).
C o r r e s p o n d e n c e on papers published in Minerals Engineering is invited, preferably by email to [email protected], or by Fax to +44-(0)1326-318352