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Analogue experimental study on centrifugal-air blast granulation for molten slag
Applied Thermal Engineering xxx (2014) 1e8
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Research paper
Analogue experimental study on centrifugal-air blast granulation for molten slag Xun Zhu a, b, *, Hui Zhang b, Yu Tan b, Hong Wang a, b, Qiang Liao a, b a b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing 400030, China Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2014 Received in revised form 15 October 2014 Accepted 15 November 2014 Available online xxx
Blast furnace slag is a by-product in iron and steel production process which has a high yield with extremely high discharge temperature. Aiming at energy and water saving as well as emission reduction, dry granulation technique appears to be a good application for the treatment of blast furnace slag. In this study, a granulation technique combining a high-speed rotating cup with air blast is proposed. The performance of this design was investigated by adopting a mixture of rosin and paraffin wax as the analogue of blast furnace slag. The effects of rotating speed of the atomizer, liquid flow rate and blast air flow rate on particle size, particle mass distribution and fiber mass fraction were studied. The effect of the function of air blast on the granulation performance was particularly discussed. The results showed that at a higher rotating speed and a smaller liquid flow rate, smaller particles can be easily obtained, yet the fiber mass fraction also increases. However, the increasing blast air leads to the increase of particle size and fiber mass fraction. For the operating conditions tested in this study, over 60% of total mass of particles fall within the size range of 0.5e1 mm, which means that the present system has a good performance in centrifugal granulation. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Molten slag Centrifugal granulation Air blast Particle Fiber
1. Introduction Blast furnace (BF) slag, which is discharged at temperature about 1500 C, is the main by-product in iron-making process. In the past few decades, iron and steel technology has seen tremendous development and production of iron has increased as a result, for example, the iron production in China reached about 750 million tonnes in 2013 [1]. For a slag generation rate of about 0.3 tonne per tonne liquid pig iron, 220 million tonnes of BF slag were produced. Furthermore, considering approximate 1770 MJ per tonne, the total energy carried by BF slag amounted to 13 million tonnes standard coal, which is about 13% of the energy consumption in a typical blast furnace process. This amount of heat is regarded as the last portion that has not been recovered in the steel and iron making industry. Therefore, heat recovery from BF slag is an important and rewarding task.
* Corresponding author. Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China. Tel.: þ86 023 65102474. E-mail address: [email protected] (X. Zhu).
BF slag is rich in CaO, SiO2, Al2O3, MgO, which is similar to the components of Portland cement. BF slag shows different inherent solid structures depending on the cooling processes. If a high temperature liquid slag is cooled very fast, it would not be crystallized and finally becomes glassy phase with high cementation activity, which can be a high value product, especially for substitution of Portland cement. By contrast, the value of crystallization slag formed by slow cooling is rather limited. Water quenching is widely employed to obtain glassy phase slag. This method refers to rapid cooling of molten slag in water, which prevents the crystallization of slag and breaks the slag into small particles by thermal stress. The water quenching method can recycle slag material but it fails to recover the waste heat of slag. Moreover, this method has several drawbacks such as consuming large amount of water, polluting the soil, water and air, thus it is not environmentally friendly. For the purposes of water conservation, pollution mitigation and energy saving, a variety of heat recovery technologies through dry granulation have been proposed in recent years. Among these technologies, the centrifugal granulation technique, which was proposed by Pickering and his colleagues in 1985 [2,3], has a good application prospect because of its advantages of compact
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Please cite this article in press as: X. Zhu, et al., Analogue experimental study on centrifugal-air blast granulation for molten slag, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.11.077
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structure, less energy-consuming, water saving and easy operation. In their experiment, slag was granulated to particles with an average diameter of 2 mm and then the particles were cooled by air to reach a glassy content of 95%. For the centrifugal granulation method, its technical process can be described as follows. High-temperature liquid slag is directly poured into a high-speed container (cup/disk/cylinder) and then the slag is radially projected outwards and subsequently broken into droplets. At the same time air is blown to cool the droplets. This process produces fine solid particles and hot air, realizing effective heat recovery and reuse of material. It is quite clear that good slag granulation is the most important requirement for this technique. This has attracted attention of researchers to further study on the centrifugal granulation, e.g., the work presented by Mizuochi et al. [4]. They examined the influence of cup speed, cup shape and slag viscosity on the granulation and their results showed that higher cup speed led to smaller slag particles. The size of the slag particles decreased obviously when the speed was in a range of 600e1800 rpm. However, the influence of speed appeared to be slight when it was over 1800 rpm. This finding also indicated that lower slag viscosity led to smaller particles and the cup shape had little influence on granulation. Akiyama et al. [5,6] explored the mechanism of granulation using a rotating disk. It was found that the liquid slag was firstly extended into a film on the disk, and then was ejected radially as plenty ligaments, and eventually, these ligaments were broken up into particles. A mathematical simulation of this experiment was developed by Purwanto et al. [7,8]. Through their mathematical model, the film thickness, the ligaments number and the particles diameter were predicted. The temperature distribution of a single particle was also analyzed. Xie et al. [9e11] developed a novel disc design to produce fine granules without the formation of slag wool. Their results indicated that more than 90% of the products were less than 1.5 mm in diameter. These slag samples are being further assessed for their suitability for cement applications. Recently, Liu et al. [12,13] carried out experiments on slag granulation by rotating cup atomizer (RCA) and they found that larger cup edge resulted into smaller average particle diameter when rotational speed was below 1000 rpm. The effect of cup size declined when rotational speed exceeded 1000 rpm. Meanwhile, Liu et al. [13], Yu et al. [14] and Min et al. [15] carried out a series of experiments with different simulant materials to explore the mechanism of slag granulation. The mixture of rosin and paraffin was proved to be an ideal simulacrum. Yang et al. [16] found that fibers, which were not conducive to subsequent use, were also produced accompanying with particles, and the mass fraction of such fibers enhanced along with the increasing rotating speed, indicating the existence of an optimum rotating speed. Furthermore, Kashiwaya et al. [17,18] did the slag granulation experiment using a rotary cylinder with several rows of nozzles, where the slag was granulated by being squeezed out from the nozzles. They investigated the impact of nozzle size on the particle diameter and the results showed that smaller nozzle size led to smaller particles, but particle diameter was not well correlated with nozzle size. The mechanisms of granulation need to be investigated further. To date, the mechanism of slag granulation has not been clearly explored, resulting in most studies on slag centrifugal granulation failing to out of laboratory yet. In the present study, the performance of a simplex centrifugal granulation and a centrifugal-air blast granulation was experimentally investigated using an analogue medium. The effects of rotating speed of the rotor, liquid flow rate and gas flow rate on particle size, particle mass distribution and fiber mass fraction were discussed.
2. Experiments and working medium 2.1. Experimental apparatus and procedure The schematic of experimental apparatus for centrifugal granulation of BF slag and other working media is shown in Fig. 1a. A furnace equipped with a crucible, a controlling stick and a liquid outlet at the bottom was employed to melt the working medium and to release the molten liquid medium at a preset flow rate through a connected tube. A rotating cup was designed with 126 mm in diameter, 40 mm in height to granulate the released molten medium. The rotating cup was driven by a motor with stepless speed adjusting. A chamber with a collecting disk was designed to collect the granulated particles. To investigate the influence of introduction of air blast on the granulation performance, a wind ring was specially designed with small holes around the circumference and was assembled with the rotating cup, as shown in Fig. 1b. Air was blasted upwards from the wind holes to meet the working medium out from the rotating cup. Furthermore, a window was set for visualization of the granulation process and a camera was employed to capture the phenomena. For the experiment on simplex centrifugal granulation, the working medium was firstly heated to a preset temperature and then, the molten medium was poured into the rotating cup that was preheated to the same preset temperature in advance to granulate the liquid medium. For the experiment on granulation with air blast, the blast air was sent through the holes on the wind ring prior to the release of liquid medium from the furnace. After the experiments, the collected particles were screened by a standard sieve and the mass in each individual size range was measured by an electronic balance. The average diameter of the particles was then calculated by Ref. [19].
da ¼
n X
di qi
(1)
i
where di is the average diameter of particles in each individual size range based on the adjacent standard screen size; qi is the mass percentage of each individual size range which represents the selection probability of the particles in each size range. The uncertainty in the average particles diameter is about 7%. 2.2. Working medium A mixture of rosin and paraffin with a mass ratio of 4 to 1 was chosen as the analogue to simulate the BF slag. The physical property parameters of the analogue and the experimental conditions were determined by the similarity theory. For the centrifugal granulation process, the diameter of produced particles can be determined by the experimental correlation [20]:
d ¼ 1:6ðReÞ0:26 ðOhÞ0:38 ðWeÞ0:42 ; R
(2)
where three dimensionless numbers, Reynold's number Re, Ohnesorge number Oh and Weber number We, can be calculated as
Re ¼
4rQ pmR
m Oh ¼ pffiffiffiffiffiffiffiffi rsR We ¼
ru2 R3 : s
(3)
(4)
(5)
Please cite this article in press as: X. Zhu, et al., Analogue experimental study on centrifugal-air blast granulation for molten slag, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.11.077
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3
Fig. 1. The schematic of analogue experimental apparatus.
In Eqs. (2)e(5), d is the diameter of granulated particles; r, m and s are the density, viscosity and surface tension of material in liquid
phase, respectively; Q is the liquid flow rate; R is the radius of rotary cup; u is the rotate speed of rotary cup. Based on the similarity theory, two physical processes can be considered as similar when the dimensionless numbers of two physical processes are equal to each other, respectively. In the analogue experiments, the analogue and slag shared the same granulation system, that is, Ra ¼ Rs (subscript ‘a’ and ‘s’ represents the parameter of the analogue and the slag, respectively). Thus, the physical property parameters of the analogue were determined through Oha ¼ Ohs since Oh is only related to the physical properties of the working medium for the same cup diameter. Based on the data of slag [7,15 and 21], it was calculated that the physical properties of the analogue at 108 C met the requirement of the experiments, as shown in Table 1. Thereafter, the operational conditions in the analogue experiments, including liquid flow rate and rotating speed, were determined through enabling the equality of Res and Rea, Wes and Wea, respectively. It was derived that Qs ¼ 2.1Qa and us ¼ 2.07/ua and thus, the operational conditions were fixed as shown in Table 2. 3. Results and discussion 3.1. Granulation process The centrifugal granulation processes recorded by camera at different operational conditions are displayed in Fig. 2(aed), where two liquid flow rates (4.6 g s1 and 8.5 g s1) and three rotating speeds (600, 850 and 1200 rpm) were tested. It can be seen that, during the process at the same liquid flow rate, clearly Table 1 Physical property parameters of analogue and BF slag. Parameter
r (kg/m ) m (Pa$s) s (N/m) 3
a b c
Inaba et al. [21]. Purwanto et al. [7]. Yi et al. [15].
BF slag (at 1500 C) a
2590 0.7b 0.478a
patterned ligaments were formed firstly at the lip of the cup with stable flying trajectories when the rotating speed was low, cf. Fig. 2a. Each ligament was pulled and stretched outwards by a head drop, which had a larger diameter than the ligament. After a certain time the head drop was broken away from the tip of the ligament, and this departure created a disturbance to the remaining ligament, which resulted in a series of breaking up of the ligaments into particles finally. The granulation process can be attributed to combined effects of inertia force, shear stress, centrifugal force and surface tension. The dominating centrifugal force over surface tension results in the formation of ligaments and final particles detachment. Thereafter, as the rotating speed increased, the number of ligaments significantly increased. The stable flying of the ligaments then became fluctuation, especially at the end portion of the ligament, leading to the final particle detachment. It was noted that the adjacent ligaments began to merge and form a sheeted liquid film, cf. Fig. 2b. When the rotating speed further increased, a whole liquid film, instead of sheeted film, was formed with enlarged outer diameter and intense fluctuation at the edge as well. Particle formation and detachment occurred at the outer edge of the film, and granulation almost fully processed owing to significant disturbance and strengthened shear stress, cf. Fig. 2c. It can be understood that the increased rotating speed enhances the effect of the centrifugal force, hence destabilizing the movement of the ligaments. The fluctuation in the ligaments provides the chance for adjacent ligaments to meet and merge, resulting in the formation of a liquid film. Furthermore, the increased rotating speed also accelerates the movement of the liquid medium and makes the liquid film on the cup thinner, thus giving rise to the extension of liquid film with larger out diameter. It should be pointed out that some ligaments were observed to solidify and became fibers prior to the formation of particles due to fast cooling. The formation of solidified fibers was inevitable during the granulation process, especially under the case with high rotating speed.
Analogue (at 108 C) 976 0.126 0.042c
Table 2 Operational conditions of analogue. Working medium
Q(g s1)
u (rpm)
Analogue BF slag
4e7 8e15
400e1500 800e3100
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Fig. 2. Breakup phenomenon of centrifugal granulation.
Similarly, when the liquid flow rate was increased at the same rotating speed, larger but relatively smooth liquid film with increasing ligament number was observed, cf. Fig. 2b and d. This also resulted into larger particles produced at the end. This is because that the increased liquid flow rate not only enhances the extension of the film but also increases film thickness, therefore, it strengthens the stability of the liquid film. Fig. 3 shows an image of the centrifugal-air blast granulation at liquid flow rate of 4.6 g s1 and rotating speed of 600 rpm. Compared with the simplex centrifugal granulation process shown in Fig. 2a, the granulating process for the centrifugal-air blast case had the same mode of ligament formation and breakup. However, the ligaments in this case appeared to be less organized due to the force by the perpendicularly incoming air blast. This led to a transition of the envelope shape of the flying liquid medium's trajectory from an approximate umbrella-shaped surface to a cone-shaped form. Furthermore, when the granulating was in the mode of film
Fig. 3. A snapshot of centrifugal air blast granulation.
formation and breakup, the liquid film could not be maintained in its original form once the blast air was introduced. The liquid film was torn into many fibrous pieces and then floccus droplets by the impingement from blast air.
3.2. Average diameter of particles Fig. 4 shows the variation of particle size with the rotating speed at liquid flow rate of 4 g s1 and 7 g s1, respectively. For the case of flow rate at 4g$s1, it was observed that particles size varied over a wider range of about 0.3e2 mm at lower rotating speed, while the size range decreased to about 0.2e1.5 mm at higher rotating speed, cf. Fig. 4a. The increase in the rotating speed gave rise to the decrease in the average particle size from 0.87 mm to 0.7 mm. For the case with flow rate of 7 g s1, as shown in Fig. 4b, similarly, both the particle size range as well as the average particle size slightly decreased with increasing rotating speed. The difference between the maximum and the minimum average diameter was 0.27 mm for this case. Compared with both cases, as shown in Fig. 4c, higher liquid flow rate gave rise to larger average particle diameter at lower rotating speed, while similar average particle diameter was obtained at higher rotating speed. Fig. 5 shows the variation of average particle diameter versus liquid flow rate at rotating speed of 400 rpm and 1200 rpm. It shows that the increase of liquid flow rate led to the increase of average particle diameter. However, the influence gradually diminished at higher rotating speed. Over the liquid flow rate tested in Fig. 5, the average particle diameter at lower rotating speed was always larger than that at higher rotating speed. Fig. 6 shows the variation of average particle diameter versus the flow rate of the blast air at the rotating speed of 400 rpm and 1500 rpm, respectively. The liquid mass flow rate for this case was 7g s1. Fig. 6 indicates that the average particle diameter increased with the introduction of air blast for all rotating speeds. With increasing blast air flow rate, the average particle diameter gradually increased at low blast air flow rates, while it turned to quick
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Fig. 4. Variation of particle size versus rotating speed.
increase at high air flow rates. The turning point was at 40 m3 h1 for rotating speed of 400 rpm, while it was at 60 m3 h1 for 1500 rpm. For all blast air flow rates tested, the average particle diameter at lower rotating speed was always larger than that at the higher rotating speed.
Fig. 5. Variation of average particle diameter versus liquid flow rate.
3.3. Mass fraction distribution Fig. 7 shows the distribution of mass fraction of the produced particles at different rotating speeds for liquid flow rate of 4 g s1 and 7 g s1, respectively. It can be seen that the majority of the
Fig. 6. Variation of average particle diameter versus blast air flow rate.
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Fig. 7. Particles mass fraction distribution at different rotating speeds.
particles size populated in the range of 0.5e1 mm for all rotating speeds and liquid flow rates tested. The particle mass fraction for this size range was about 60e70%. As shown in Fig. 7a, at liquid flow rate 4 g s1, the particle mass fraction decreased with increasing rotating speed for particle size larger than 1 mm. For particle size in the range of 0.5e1 mm, the mass fraction dropped first and then turned to increase with increasing rotating speed. It reached a minimum mass fraction at rotating speed of 500 rpm. In contrast, the particle mass fraction reached a maximum value at the rotating speed of 700 rpm in the size range of 0e0.5 mm. Similar variations were also found for the case with liquid flow rate of 7 g s1, as shown in Fig. 7b. A difference was that the minimum mass fraction was reached at rotating speed of 700 rpm in the size range of 0.5e1 mm, while the mass fraction increased with increasing rotating speed to reach the maximum value at the rotating speed of 1500 rpm in the size range of 0e0.5 mm. The mass fraction distribution of the produced particles at different liquid mass flow rates is shown in Fig. 8 for the cases with rotating speed of 400 rpm and 1200 rpm, respectively. The results indicated that the trend of liquid mass flow rate and rotating speed on particle size was opposite for both cases. It also can be seen that the concentration rate of particle size reached the maximum value in the size range of 0.5e1 mm when the liquid mass flow rate was 5.5 g s1 for both rotating speeds. The effect of blast air flow rate on particles mass fraction distribution is shown in Fig. 9 for rotating speeds of 400 rpm and
1500 rpm at liquid flow rate of 7 g s1. Similarly, the majority of the particles still populated in the size range of 0.5e1 mm. One can see that higher blast air flow rate increased the particles mass fraction for particle size larger than 1 mm at the rotating speed of 400 rpm, as shown in Fig. 9a. However, the particle mass fraction monotonously decreased with increasing blast air flow rate in the size range of 0.5e1 mm, while it was almost independent of the blast air flow rate in the size range of 0e0.5 mm. For the case with higher rotating speed, cf. Fig. 9b, the majority of the particles still populated in the size range of 0.5e1 mm. However, compared with the case with rotating speed of 400 rpm, the mass fraction was increased for particle size larger than 1 mm, and was reduced in the size range of 0.5e1 mm. 3.4. Fiber mass fraction As mentioned in 3.1, in addition to particles, fibers were inevitably generated during the granulation process. For the application scenario of the BF slag, these fibers are not desirable for subsequent usage of the slag. Therefore, the mass fraction of the fibers should be minimized during the granulation process. For this reason, the fiber mass fraction in the experiments was measured and investigated. The fiber fraction is defined as the ratio of fiber mass to the total mass of the working medium used during the experiment. Fig. 10 shows the influence of rotating speed on the fiber mass fraction at different liquid flow rates (4 g s1 and 7 g s1). It is noted
Fig. 8. Particles mass distribution at different liquid flow rates.
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Fig. 9. The effect of blast air flow rate on the particles mass fraction distribution.
that the fiber mass fraction increased with increasing rotating speed under liquid flow rate of 7 g s1. This is because that the high rotating speed made the liquid film thinner on the disk and then diminished the ligament diameter as well. The increased specific surface area resulted in effective cooling so that the ligaments were solidified too fast before they could be broken into particles. As mentioned above, high rotating speed gave rise to the increment in the ligament numbers. Both aspects caused the increase of fiber mass fraction. However, for liquid flow rate of 4 g s1, the fiber mass fraction dropped after it experienced a quick increase when the rotating speed was increased over 1000 rpm. This is likely attributed to the fully processing granulation under a film status at higher rotating speed. Fig. 11 shows the influence of liquid flow rate on fiber mass fraction. It shows that the fiber mass fraction decreased with the increase of liquid mass flow rate. This is because the detached ligament cannot be fully compensated by the incoming flow when the flow rate was low, especially under high rotating speed. The resulted thin ligaments were then subjected to a high cooling rate, leading to the premature solidification of the ligaments, hence higher fiber mass fraction. Nevertheless, increasing liquid flow rate
postponed the solidification and then facilitated the particle formation and detachment, hence lower fiber mass fraction. Fig. 12 shows the effect of blast air flow rate on fiber mass fraction at rotating speed of 400 rpm and 1500 rpm. It can be found that the fiber mass fraction increased under the blast air operating conditions. At the rotating speed of 400 rpm, the fiber mass fraction increased slightly when the blast air flow rate was low. However, the fiber mass fraction started to increase significantly when the blast air flow rate was increased to over 40 m3 h1. At the rotating speed of 1500 rpm, the fiber mass fraction decreased slightly first and then remarkably rose for blast air flow rate beyond 20 m3 h1. As expected, the fiber mass fraction at a higher rotating speed was always higher than that at a lower rotating speed. Furthermore, it also indicated that the fiber mass fraction increased notably under the film formation situation owing to the impinging effect by the blast air.
Fig. 10. Effect of rotating speed on fiber mass fraction.
Fig. 11. Effect of liquid flow rate on fiber mass fraction.
3.5. Analysis on effect of air blast The effect of air blast on liquid medium in the granulation process is mainly manifested in two aspects. One aspect is air blast's
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accounted for approximately 60% of the total mass of all particles. With the effect of air blast, the average particle diameter increased. Increasing the blast air flow rate caused the average particle diameter to increase gradually. (3) The fiber fraction increased with the increase in rotating speed and decreased with the increase of liquid flow rate. With the presence of air blast, the fiber fraction increased with increasing blast air flow rate. Experimental results indicated that the cooling effect of blast air plays a dominant role in the granulation process. Acknowledgements The authors acknowledge the support from the National Key Basic Research Project of China (973 program, No. 2012CB720403). References Fig. 12. Effect of blast air flow rate on fiber mass fraction.
disturbance effect. The air blast leads to higher relative velocity between air and liquid ligaments (or liquid film), which results in significant, intense disturbance and strengthened shear stress on the ligaments. Therefore, followed the theory of blast atomization [22], the diameter of particles fragmented from liquid ligament definitely decreases with increasing blast air flow rate and decreasing ligament diameter. The other aspect is air blast's cooling effect. The introduction of blast air enhances the cooling rate of the particles and thus results in quick drop of particles temperature, hence increasing the viscosity of liquid medium even solidified the liquid medium. According to Eq. (2), the increase in the liquid medium viscosity leads to larger particle diameter. It is noted that both aspects act against each other on the final particle diameter during a granulation process. Obviously, the experimental results of the present study indicate that the cooling effect plays a dominant role. 4. Conclusions In this study, a granulation technique combining a high-speed rotating cup with air blast was investigated by adopting a mixture of rosin and paraffin wax as the analogue of blast furnace slag. The effects of rotating speed, liquid flow rate and blast air flow rate on particle size, particle mass distribution and fiber mass fraction were studied. The main findings from this study are summarized as follows. (1) The experimental results exhibited the evolution of the granulation processes under various operational conditions. For the operating conditions tested, the diameter of most particles was below 2.5 mm. (2) The average particle diameter decreased with increasing rotating speed and decreasing liquid flow rate. The mass fraction of the particles that had a size of 0.5e1 mm
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
Analogue experimental study on centrifugal-air blast granulation for molten slag Xun Zhu a, b, *, Hui Zhang b, Yu Tan b, Hong Wang a, b, Qiang Liao a, b a b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing 400030, China Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2014 Received in revised form 15 October 2014 Accepted 15 November 2014 Available online xxx
Blast furnace slag is a by-product in iron and steel production process which has a high yield with extremely high discharge temperature. Aiming at energy and water saving as well as emission reduction, dry granulation technique appears to be a good application for the treatment of blast furnace slag. In this study, a granulation technique combining a high-speed rotating cup with air blast is proposed. The performance of this design was investigated by adopting a mixture of rosin and paraffin wax as the analogue of blast furnace slag. The effects of rotating speed of the atomizer, liquid flow rate and blast air flow rate on particle size, particle mass distribution and fiber mass fraction were studied. The effect of the function of air blast on the granulation performance was particularly discussed. The results showed that at a higher rotating speed and a smaller liquid flow rate, smaller particles can be easily obtained, yet the fiber mass fraction also increases. However, the increasing blast air leads to the increase of particle size and fiber mass fraction. For the operating conditions tested in this study, over 60% of total mass of particles fall within the size range of 0.5e1 mm, which means that the present system has a good performance in centrifugal granulation. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Molten slag Centrifugal granulation Air blast Particle Fiber
1. Introduction Blast furnace (BF) slag, which is discharged at temperature about 1500 C, is the main by-product in iron-making process. In the past few decades, iron and steel technology has seen tremendous development and production of iron has increased as a result, for example, the iron production in China reached about 750 million tonnes in 2013 [1]. For a slag generation rate of about 0.3 tonne per tonne liquid pig iron, 220 million tonnes of BF slag were produced. Furthermore, considering approximate 1770 MJ per tonne, the total energy carried by BF slag amounted to 13 million tonnes standard coal, which is about 13% of the energy consumption in a typical blast furnace process. This amount of heat is regarded as the last portion that has not been recovered in the steel and iron making industry. Therefore, heat recovery from BF slag is an important and rewarding task.
* Corresponding author. Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China. Tel.: þ86 023 65102474. E-mail address: [email protected] (X. Zhu).
BF slag is rich in CaO, SiO2, Al2O3, MgO, which is similar to the components of Portland cement. BF slag shows different inherent solid structures depending on the cooling processes. If a high temperature liquid slag is cooled very fast, it would not be crystallized and finally becomes glassy phase with high cementation activity, which can be a high value product, especially for substitution of Portland cement. By contrast, the value of crystallization slag formed by slow cooling is rather limited. Water quenching is widely employed to obtain glassy phase slag. This method refers to rapid cooling of molten slag in water, which prevents the crystallization of slag and breaks the slag into small particles by thermal stress. The water quenching method can recycle slag material but it fails to recover the waste heat of slag. Moreover, this method has several drawbacks such as consuming large amount of water, polluting the soil, water and air, thus it is not environmentally friendly. For the purposes of water conservation, pollution mitigation and energy saving, a variety of heat recovery technologies through dry granulation have been proposed in recent years. Among these technologies, the centrifugal granulation technique, which was proposed by Pickering and his colleagues in 1985 [2,3], has a good application prospect because of its advantages of compact
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structure, less energy-consuming, water saving and easy operation. In their experiment, slag was granulated to particles with an average diameter of 2 mm and then the particles were cooled by air to reach a glassy content of 95%. For the centrifugal granulation method, its technical process can be described as follows. High-temperature liquid slag is directly poured into a high-speed container (cup/disk/cylinder) and then the slag is radially projected outwards and subsequently broken into droplets. At the same time air is blown to cool the droplets. This process produces fine solid particles and hot air, realizing effective heat recovery and reuse of material. It is quite clear that good slag granulation is the most important requirement for this technique. This has attracted attention of researchers to further study on the centrifugal granulation, e.g., the work presented by Mizuochi et al. [4]. They examined the influence of cup speed, cup shape and slag viscosity on the granulation and their results showed that higher cup speed led to smaller slag particles. The size of the slag particles decreased obviously when the speed was in a range of 600e1800 rpm. However, the influence of speed appeared to be slight when it was over 1800 rpm. This finding also indicated that lower slag viscosity led to smaller particles and the cup shape had little influence on granulation. Akiyama et al. [5,6] explored the mechanism of granulation using a rotating disk. It was found that the liquid slag was firstly extended into a film on the disk, and then was ejected radially as plenty ligaments, and eventually, these ligaments were broken up into particles. A mathematical simulation of this experiment was developed by Purwanto et al. [7,8]. Through their mathematical model, the film thickness, the ligaments number and the particles diameter were predicted. The temperature distribution of a single particle was also analyzed. Xie et al. [9e11] developed a novel disc design to produce fine granules without the formation of slag wool. Their results indicated that more than 90% of the products were less than 1.5 mm in diameter. These slag samples are being further assessed for their suitability for cement applications. Recently, Liu et al. [12,13] carried out experiments on slag granulation by rotating cup atomizer (RCA) and they found that larger cup edge resulted into smaller average particle diameter when rotational speed was below 1000 rpm. The effect of cup size declined when rotational speed exceeded 1000 rpm. Meanwhile, Liu et al. [13], Yu et al. [14] and Min et al. [15] carried out a series of experiments with different simulant materials to explore the mechanism of slag granulation. The mixture of rosin and paraffin was proved to be an ideal simulacrum. Yang et al. [16] found that fibers, which were not conducive to subsequent use, were also produced accompanying with particles, and the mass fraction of such fibers enhanced along with the increasing rotating speed, indicating the existence of an optimum rotating speed. Furthermore, Kashiwaya et al. [17,18] did the slag granulation experiment using a rotary cylinder with several rows of nozzles, where the slag was granulated by being squeezed out from the nozzles. They investigated the impact of nozzle size on the particle diameter and the results showed that smaller nozzle size led to smaller particles, but particle diameter was not well correlated with nozzle size. The mechanisms of granulation need to be investigated further. To date, the mechanism of slag granulation has not been clearly explored, resulting in most studies on slag centrifugal granulation failing to out of laboratory yet. In the present study, the performance of a simplex centrifugal granulation and a centrifugal-air blast granulation was experimentally investigated using an analogue medium. The effects of rotating speed of the rotor, liquid flow rate and gas flow rate on particle size, particle mass distribution and fiber mass fraction were discussed.
2. Experiments and working medium 2.1. Experimental apparatus and procedure The schematic of experimental apparatus for centrifugal granulation of BF slag and other working media is shown in Fig. 1a. A furnace equipped with a crucible, a controlling stick and a liquid outlet at the bottom was employed to melt the working medium and to release the molten liquid medium at a preset flow rate through a connected tube. A rotating cup was designed with 126 mm in diameter, 40 mm in height to granulate the released molten medium. The rotating cup was driven by a motor with stepless speed adjusting. A chamber with a collecting disk was designed to collect the granulated particles. To investigate the influence of introduction of air blast on the granulation performance, a wind ring was specially designed with small holes around the circumference and was assembled with the rotating cup, as shown in Fig. 1b. Air was blasted upwards from the wind holes to meet the working medium out from the rotating cup. Furthermore, a window was set for visualization of the granulation process and a camera was employed to capture the phenomena. For the experiment on simplex centrifugal granulation, the working medium was firstly heated to a preset temperature and then, the molten medium was poured into the rotating cup that was preheated to the same preset temperature in advance to granulate the liquid medium. For the experiment on granulation with air blast, the blast air was sent through the holes on the wind ring prior to the release of liquid medium from the furnace. After the experiments, the collected particles were screened by a standard sieve and the mass in each individual size range was measured by an electronic balance. The average diameter of the particles was then calculated by Ref. [19].
da ¼
n X
di qi
(1)
i
where di is the average diameter of particles in each individual size range based on the adjacent standard screen size; qi is the mass percentage of each individual size range which represents the selection probability of the particles in each size range. The uncertainty in the average particles diameter is about 7%. 2.2. Working medium A mixture of rosin and paraffin with a mass ratio of 4 to 1 was chosen as the analogue to simulate the BF slag. The physical property parameters of the analogue and the experimental conditions were determined by the similarity theory. For the centrifugal granulation process, the diameter of produced particles can be determined by the experimental correlation [20]:
d ¼ 1:6ðReÞ0:26 ðOhÞ0:38 ðWeÞ0:42 ; R
(2)
where three dimensionless numbers, Reynold's number Re, Ohnesorge number Oh and Weber number We, can be calculated as
Re ¼
4rQ pmR
m Oh ¼ pffiffiffiffiffiffiffiffi rsR We ¼
ru2 R3 : s
(3)
(4)
(5)
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Fig. 1. The schematic of analogue experimental apparatus.
In Eqs. (2)e(5), d is the diameter of granulated particles; r, m and s are the density, viscosity and surface tension of material in liquid
phase, respectively; Q is the liquid flow rate; R is the radius of rotary cup; u is the rotate speed of rotary cup. Based on the similarity theory, two physical processes can be considered as similar when the dimensionless numbers of two physical processes are equal to each other, respectively. In the analogue experiments, the analogue and slag shared the same granulation system, that is, Ra ¼ Rs (subscript ‘a’ and ‘s’ represents the parameter of the analogue and the slag, respectively). Thus, the physical property parameters of the analogue were determined through Oha ¼ Ohs since Oh is only related to the physical properties of the working medium for the same cup diameter. Based on the data of slag [7,15 and 21], it was calculated that the physical properties of the analogue at 108 C met the requirement of the experiments, as shown in Table 1. Thereafter, the operational conditions in the analogue experiments, including liquid flow rate and rotating speed, were determined through enabling the equality of Res and Rea, Wes and Wea, respectively. It was derived that Qs ¼ 2.1Qa and us ¼ 2.07/ua and thus, the operational conditions were fixed as shown in Table 2. 3. Results and discussion 3.1. Granulation process The centrifugal granulation processes recorded by camera at different operational conditions are displayed in Fig. 2(aed), where two liquid flow rates (4.6 g s1 and 8.5 g s1) and three rotating speeds (600, 850 and 1200 rpm) were tested. It can be seen that, during the process at the same liquid flow rate, clearly Table 1 Physical property parameters of analogue and BF slag. Parameter
r (kg/m ) m (Pa$s) s (N/m) 3
a b c
Inaba et al. [21]. Purwanto et al. [7]. Yi et al. [15].
BF slag (at 1500 C) a
2590 0.7b 0.478a
patterned ligaments were formed firstly at the lip of the cup with stable flying trajectories when the rotating speed was low, cf. Fig. 2a. Each ligament was pulled and stretched outwards by a head drop, which had a larger diameter than the ligament. After a certain time the head drop was broken away from the tip of the ligament, and this departure created a disturbance to the remaining ligament, which resulted in a series of breaking up of the ligaments into particles finally. The granulation process can be attributed to combined effects of inertia force, shear stress, centrifugal force and surface tension. The dominating centrifugal force over surface tension results in the formation of ligaments and final particles detachment. Thereafter, as the rotating speed increased, the number of ligaments significantly increased. The stable flying of the ligaments then became fluctuation, especially at the end portion of the ligament, leading to the final particle detachment. It was noted that the adjacent ligaments began to merge and form a sheeted liquid film, cf. Fig. 2b. When the rotating speed further increased, a whole liquid film, instead of sheeted film, was formed with enlarged outer diameter and intense fluctuation at the edge as well. Particle formation and detachment occurred at the outer edge of the film, and granulation almost fully processed owing to significant disturbance and strengthened shear stress, cf. Fig. 2c. It can be understood that the increased rotating speed enhances the effect of the centrifugal force, hence destabilizing the movement of the ligaments. The fluctuation in the ligaments provides the chance for adjacent ligaments to meet and merge, resulting in the formation of a liquid film. Furthermore, the increased rotating speed also accelerates the movement of the liquid medium and makes the liquid film on the cup thinner, thus giving rise to the extension of liquid film with larger out diameter. It should be pointed out that some ligaments were observed to solidify and became fibers prior to the formation of particles due to fast cooling. The formation of solidified fibers was inevitable during the granulation process, especially under the case with high rotating speed.
Analogue (at 108 C) 976 0.126 0.042c
Table 2 Operational conditions of analogue. Working medium
Q(g s1)
u (rpm)
Analogue BF slag
4e7 8e15
400e1500 800e3100
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Fig. 2. Breakup phenomenon of centrifugal granulation.
Similarly, when the liquid flow rate was increased at the same rotating speed, larger but relatively smooth liquid film with increasing ligament number was observed, cf. Fig. 2b and d. This also resulted into larger particles produced at the end. This is because that the increased liquid flow rate not only enhances the extension of the film but also increases film thickness, therefore, it strengthens the stability of the liquid film. Fig. 3 shows an image of the centrifugal-air blast granulation at liquid flow rate of 4.6 g s1 and rotating speed of 600 rpm. Compared with the simplex centrifugal granulation process shown in Fig. 2a, the granulating process for the centrifugal-air blast case had the same mode of ligament formation and breakup. However, the ligaments in this case appeared to be less organized due to the force by the perpendicularly incoming air blast. This led to a transition of the envelope shape of the flying liquid medium's trajectory from an approximate umbrella-shaped surface to a cone-shaped form. Furthermore, when the granulating was in the mode of film
Fig. 3. A snapshot of centrifugal air blast granulation.
formation and breakup, the liquid film could not be maintained in its original form once the blast air was introduced. The liquid film was torn into many fibrous pieces and then floccus droplets by the impingement from blast air.
3.2. Average diameter of particles Fig. 4 shows the variation of particle size with the rotating speed at liquid flow rate of 4 g s1 and 7 g s1, respectively. For the case of flow rate at 4g$s1, it was observed that particles size varied over a wider range of about 0.3e2 mm at lower rotating speed, while the size range decreased to about 0.2e1.5 mm at higher rotating speed, cf. Fig. 4a. The increase in the rotating speed gave rise to the decrease in the average particle size from 0.87 mm to 0.7 mm. For the case with flow rate of 7 g s1, as shown in Fig. 4b, similarly, both the particle size range as well as the average particle size slightly decreased with increasing rotating speed. The difference between the maximum and the minimum average diameter was 0.27 mm for this case. Compared with both cases, as shown in Fig. 4c, higher liquid flow rate gave rise to larger average particle diameter at lower rotating speed, while similar average particle diameter was obtained at higher rotating speed. Fig. 5 shows the variation of average particle diameter versus liquid flow rate at rotating speed of 400 rpm and 1200 rpm. It shows that the increase of liquid flow rate led to the increase of average particle diameter. However, the influence gradually diminished at higher rotating speed. Over the liquid flow rate tested in Fig. 5, the average particle diameter at lower rotating speed was always larger than that at higher rotating speed. Fig. 6 shows the variation of average particle diameter versus the flow rate of the blast air at the rotating speed of 400 rpm and 1500 rpm, respectively. The liquid mass flow rate for this case was 7g s1. Fig. 6 indicates that the average particle diameter increased with the introduction of air blast for all rotating speeds. With increasing blast air flow rate, the average particle diameter gradually increased at low blast air flow rates, while it turned to quick
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Fig. 4. Variation of particle size versus rotating speed.
increase at high air flow rates. The turning point was at 40 m3 h1 for rotating speed of 400 rpm, while it was at 60 m3 h1 for 1500 rpm. For all blast air flow rates tested, the average particle diameter at lower rotating speed was always larger than that at the higher rotating speed.
Fig. 5. Variation of average particle diameter versus liquid flow rate.
3.3. Mass fraction distribution Fig. 7 shows the distribution of mass fraction of the produced particles at different rotating speeds for liquid flow rate of 4 g s1 and 7 g s1, respectively. It can be seen that the majority of the
Fig. 6. Variation of average particle diameter versus blast air flow rate.
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Fig. 7. Particles mass fraction distribution at different rotating speeds.
particles size populated in the range of 0.5e1 mm for all rotating speeds and liquid flow rates tested. The particle mass fraction for this size range was about 60e70%. As shown in Fig. 7a, at liquid flow rate 4 g s1, the particle mass fraction decreased with increasing rotating speed for particle size larger than 1 mm. For particle size in the range of 0.5e1 mm, the mass fraction dropped first and then turned to increase with increasing rotating speed. It reached a minimum mass fraction at rotating speed of 500 rpm. In contrast, the particle mass fraction reached a maximum value at the rotating speed of 700 rpm in the size range of 0e0.5 mm. Similar variations were also found for the case with liquid flow rate of 7 g s1, as shown in Fig. 7b. A difference was that the minimum mass fraction was reached at rotating speed of 700 rpm in the size range of 0.5e1 mm, while the mass fraction increased with increasing rotating speed to reach the maximum value at the rotating speed of 1500 rpm in the size range of 0e0.5 mm. The mass fraction distribution of the produced particles at different liquid mass flow rates is shown in Fig. 8 for the cases with rotating speed of 400 rpm and 1200 rpm, respectively. The results indicated that the trend of liquid mass flow rate and rotating speed on particle size was opposite for both cases. It also can be seen that the concentration rate of particle size reached the maximum value in the size range of 0.5e1 mm when the liquid mass flow rate was 5.5 g s1 for both rotating speeds. The effect of blast air flow rate on particles mass fraction distribution is shown in Fig. 9 for rotating speeds of 400 rpm and
1500 rpm at liquid flow rate of 7 g s1. Similarly, the majority of the particles still populated in the size range of 0.5e1 mm. One can see that higher blast air flow rate increased the particles mass fraction for particle size larger than 1 mm at the rotating speed of 400 rpm, as shown in Fig. 9a. However, the particle mass fraction monotonously decreased with increasing blast air flow rate in the size range of 0.5e1 mm, while it was almost independent of the blast air flow rate in the size range of 0e0.5 mm. For the case with higher rotating speed, cf. Fig. 9b, the majority of the particles still populated in the size range of 0.5e1 mm. However, compared with the case with rotating speed of 400 rpm, the mass fraction was increased for particle size larger than 1 mm, and was reduced in the size range of 0.5e1 mm. 3.4. Fiber mass fraction As mentioned in 3.1, in addition to particles, fibers were inevitably generated during the granulation process. For the application scenario of the BF slag, these fibers are not desirable for subsequent usage of the slag. Therefore, the mass fraction of the fibers should be minimized during the granulation process. For this reason, the fiber mass fraction in the experiments was measured and investigated. The fiber fraction is defined as the ratio of fiber mass to the total mass of the working medium used during the experiment. Fig. 10 shows the influence of rotating speed on the fiber mass fraction at different liquid flow rates (4 g s1 and 7 g s1). It is noted
Fig. 8. Particles mass distribution at different liquid flow rates.
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Fig. 9. The effect of blast air flow rate on the particles mass fraction distribution.
that the fiber mass fraction increased with increasing rotating speed under liquid flow rate of 7 g s1. This is because that the high rotating speed made the liquid film thinner on the disk and then diminished the ligament diameter as well. The increased specific surface area resulted in effective cooling so that the ligaments were solidified too fast before they could be broken into particles. As mentioned above, high rotating speed gave rise to the increment in the ligament numbers. Both aspects caused the increase of fiber mass fraction. However, for liquid flow rate of 4 g s1, the fiber mass fraction dropped after it experienced a quick increase when the rotating speed was increased over 1000 rpm. This is likely attributed to the fully processing granulation under a film status at higher rotating speed. Fig. 11 shows the influence of liquid flow rate on fiber mass fraction. It shows that the fiber mass fraction decreased with the increase of liquid mass flow rate. This is because the detached ligament cannot be fully compensated by the incoming flow when the flow rate was low, especially under high rotating speed. The resulted thin ligaments were then subjected to a high cooling rate, leading to the premature solidification of the ligaments, hence higher fiber mass fraction. Nevertheless, increasing liquid flow rate
postponed the solidification and then facilitated the particle formation and detachment, hence lower fiber mass fraction. Fig. 12 shows the effect of blast air flow rate on fiber mass fraction at rotating speed of 400 rpm and 1500 rpm. It can be found that the fiber mass fraction increased under the blast air operating conditions. At the rotating speed of 400 rpm, the fiber mass fraction increased slightly when the blast air flow rate was low. However, the fiber mass fraction started to increase significantly when the blast air flow rate was increased to over 40 m3 h1. At the rotating speed of 1500 rpm, the fiber mass fraction decreased slightly first and then remarkably rose for blast air flow rate beyond 20 m3 h1. As expected, the fiber mass fraction at a higher rotating speed was always higher than that at a lower rotating speed. Furthermore, it also indicated that the fiber mass fraction increased notably under the film formation situation owing to the impinging effect by the blast air.
Fig. 10. Effect of rotating speed on fiber mass fraction.
Fig. 11. Effect of liquid flow rate on fiber mass fraction.
3.5. Analysis on effect of air blast The effect of air blast on liquid medium in the granulation process is mainly manifested in two aspects. One aspect is air blast's
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accounted for approximately 60% of the total mass of all particles. With the effect of air blast, the average particle diameter increased. Increasing the blast air flow rate caused the average particle diameter to increase gradually. (3) The fiber fraction increased with the increase in rotating speed and decreased with the increase of liquid flow rate. With the presence of air blast, the fiber fraction increased with increasing blast air flow rate. Experimental results indicated that the cooling effect of blast air plays a dominant role in the granulation process. Acknowledgements The authors acknowledge the support from the National Key Basic Research Project of China (973 program, No. 2012CB720403). References Fig. 12. Effect of blast air flow rate on fiber mass fraction.
disturbance effect. The air blast leads to higher relative velocity between air and liquid ligaments (or liquid film), which results in significant, intense disturbance and strengthened shear stress on the ligaments. Therefore, followed the theory of blast atomization [22], the diameter of particles fragmented from liquid ligament definitely decreases with increasing blast air flow rate and decreasing ligament diameter. The other aspect is air blast's cooling effect. The introduction of blast air enhances the cooling rate of the particles and thus results in quick drop of particles temperature, hence increasing the viscosity of liquid medium even solidified the liquid medium. According to Eq. (2), the increase in the liquid medium viscosity leads to larger particle diameter. It is noted that both aspects act against each other on the final particle diameter during a granulation process. Obviously, the experimental results of the present study indicate that the cooling effect plays a dominant role. 4. Conclusions In this study, a granulation technique combining a high-speed rotating cup with air blast was investigated by adopting a mixture of rosin and paraffin wax as the analogue of blast furnace slag. The effects of rotating speed, liquid flow rate and blast air flow rate on particle size, particle mass distribution and fiber mass fraction were studied. The main findings from this study are summarized as follows. (1) The experimental results exhibited the evolution of the granulation processes under various operational conditions. For the operating conditions tested, the diameter of most particles was below 2.5 mm. (2) The average particle diameter decreased with increasing rotating speed and decreasing liquid flow rate. The mass fraction of the particles that had a size of 0.5e1 mm
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