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The fundamentals of the comminution of metals in shredders of the swing-hammer type
Int. J. Miner. Process. 74S (2004) S385 – S393 www.elsevier.com/locate/ijminpro
The fundamentals of the comminution of metals in shredders of the swing-hammer type S. Sander, G. Schubert*, H.-G. J7ckel TU Bergakademie Freiberg, Institut fu¨r MVT/AT, Agricolastage 1, 09599 Freiberg, Germany
Abstract The comminution of metals (scraps) is carried out in order to obtain suitable fragment size distributions and/or to liberate the valuable components. In the case of thin-walled metals and metal scraps, respectively, shredders of the swing-hammer type are frequently used for the liberation and size reduction. The technically relevant equipment is introduced in this paper. Inside these shredders, the material is subject to complex stressing modes. On the basis of systematic investigations using a small-scale horizontal shaft shredder, four successive stages occurring during the process of the comminution of metal sheets are explained. D 2004 Elsevier B.V. All rights reserved. Keywords: comminution; swing-hammer shredders; metal scraps
1. Introduction The size reduction of automobile scrap and other types of light steel scrap as well as scraps from electronic appliances, lead-acid accumulators, nonferrous metals and metal chips is generally carried out by means of swing-hammer shredders. This is due to the complex stressing modes occurring inside this type of equipment, which are well suited for the comminution of thin-walled metal scraps. The objectives of the size reduction are to obtain suitable size distributions required by the subsequent processing steps, to increase the bulk density and to liberate the components of composites and assemblies (Schubert, 1997). * Corresponding author. Fax: +49 3731 39 2815. E-mail address: [email protected] (G. Schubert). 0301-7516/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2004.07.038
Some swing-hammer shredders typically utilised for the comminution of automobile scrap and other types of light steel scrap can be seen in Fig. 1. The input material is fed from the side, the mass flow is controlled by the power consumption. In the case of the types shown in (a) and (b), it is subsequently transported into the relatively narrow gap between the impacting tools and the lower part of housing, where it is subject to an intense deformation and comminution. The material, which has become sufficiently small, is discharged from the chamber of comminution by means of grates. The configuration of the discharge grates most common for the comminution of light steel scrap is shown in Fig. 1a (model Lindemann/Newell). Usually, one grate is placed above the rotor and in some cases a second one below it. The application of a
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Fig. 1. Swing-hammer shredders for the comminution of automobile scrap and other types of light steel scrap. (1) Rotor and impacting tools (swing-hammers); (2) anvil; (3) discharge grate; (4) device for discharging unshreddable items; (A) feed; (P) product.
vertical discharge grate is accompanied by a modified design of the upper part of the housing, which has to be narrower (Fig. 1b, model Thyssen HRT). According to the manufacturer, this results in a decrease of the specific energy consumption for the process. The so-called Kondirator (model Lindemann) shown in Fig. 1c is characterised by a reverse direction of rotation of the rotor. Thus, the feed is carried into the space above the rotor first. The unshreddable items can be discharged by means of the swivelling grate (4) before the material is transported into the narrow gap underneath the rotor. Therefore, this swing-hammer shredder is suited for the size reduction of light steel scrap having small portions of scrap from demolished steel constructions. In Fig. 2, the throughput of hammer crushers applied in mineral processing and of swing-hammer shredders used for the comminution of steel scrap can
be seen as a function of the drive power. It becomes obvious that the energy demand for the size reduction of scrap exceeds the one of minerals by about one order of magnitude. This is a result of the manifold energy consuming deformation processes occurring. Therefore, it has been of special interest to investigate the microprocesses and governing parameters of the comminution of metals.
2. Experimental For the investigations concerning the microprocesses taking place inside the swing-hammer shredders, a small-scale horizontal shaft shredder, a modified Izod pendulum and an impact apparatus were used (see Fig. 3). In order to make sure that the process of comminution could be examined sepa-
Fig. 2. Throughput vs. drive power for hammer crushers and swing-hammer shredders (Ho¨ffl, 1989; Schubert, 1984; Schubert, 1989).
S. Sander et al. / Int. J. Miner. Process. 74S (2004) S385–S393
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Fig. 3. Small-scale shredder (a), modified Izod pendulum (b) and impact apparatus (c). (1) Rotor with impacting tools (swing-hammers); (2) upper part of housing; (3) lower part of housing (flap, discharge in the case of batch tests); (4) anvil; (5) discharge grate (closed in the case of batch tests); (6) feed chute; (7) weights of impacting tool (50 kg); (8) impacting tool with strain gages; (9) lifting and breaking device for the impacting tool; (10) anvil construction; (11) compressor; (12) compressed air tank; (13) magnetic valve; (14) acceleration tube; (15) chock block; (16) impact chamber with velocity measurement (light barrier); (17) baffle plate (deflection 0. . .378).
rately from the kinetics of classifying taking place simultaneously, the discharge grates located above (5) and underneath the rotor of the small scale horizontal shaft shredder were replaced by steel plates. The lower part of housing is designed to act as a flap (3). After a time defined in advance, the material could instantaneously be discharged from the chamber of comminution by lowering this flap. Utilising a torque measurement shaft, the mechanical power draw was recorded as a function of shredding time. The area enclosed by the power-vs.-time-plot is equal to the energy consumption. As test material, test bodies of zinc sheet and other materials were
used. A detailed description of the modified Izod pendulum and the impact apparatus can be found in the work of Kirchner (2000). As a result of the intensive deformation of the material, the characterisation of the products of the comminution has shown to be difficult. The fragment size and fragment mass distributions as well as their median values were used to indicate the results of the comminution. It has to be paid attention to the fact that every fragment can be described by its three main dimensions a, b and c, whereby azbzc has to be fulfilled. The medium dimension b was fixed to be the characteristic for the fragment size. It is estimated
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Fig. 4. Measurement of the main dimensions a and b of a fragment by the determination of the best fitting ellipse of equal area (a) and estimation of the fragment volume by the calculation of the ellipsoid of rotation having the dimensions a ell and b ell (b).
by the dimension b ell of the best fitting ellipse of equal area of projection (Fig. 4a) (Sander et al., 2002). The determination of the increase in surface area can be carried out only by bending open the fragments and measuring the length of fractures and cracks. Thus, this method is limited to the products, which had been subject to short time shredding. Additionally, the degree of bending B and the degree of compaction K are determined in order to characterise the deformation of the fragments. B can be calculated according to B¼
Aa¨q APr : Aa¨q AK
m ; qM d
K¼
6m : qM paell b2ell
ð3Þ
Herein, m represents the fragment mass, a ell and b ell are the dimensions of the best fitting ellipsoid of rotation of equal area of projection (Fig. 4b) and q M is the material density.
ð1Þ
Therein, A Pr and A K represent the area of projection of the fragment and of the sphere of equal mass, respectively. The equivalent fragment area can be obtained by using Aa¨q ¼
in which m, q M and d are the fragment mass, the material density and the wall thickness of the test body. The degree of compaction can be calculated according to
ð2Þ
3. The microprocesses of the comminution of metals in shredders of the swing-hammer type On the basis of systematic investigations utilising test bodies of sheet metal, four successive stages could be distinguished during the process of comminution (Kirchner, 2000). The 3rd and 4th stage, however, are bound to longer residence times of the material inside the chamber of comminution. Thus,
S. Sander et al. / Int. J. Miner. Process. 74S (2004) S385–S393
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they can be missing in the case of swing-hammer shedders working continuously. 3.1. Stage 1 The 1st stage of the comminution comprises the tearing off of single fragments from the feed as a result of the combined action of tensile stress, bending and torsion. It takes place in the space close to the feed chute including the anvil. The comminution possibly taking place was examined by means of the modified Izod pendulum. It became clear that breakage can occur only if the specific section modulus W B,bez, which can be calculated for the geometry under investigation using WB;bez ¼
WB bd 2 ¼ ; w 6w
ð4Þ
exceeds a critical value. Therein, W B is the section modulus of the test body, b and d represent the width and wall thickness of the test body, respectively, and w is the clearance between impacting tool and anvil. At values lower than the critical one, the test body is only bent. In the case of the relatively wide clearances occurring in industrial scale shredders, a very high section modulus would be necessary in order to initiate breakage during the 1st stage. Pieces of scrap having values as high as W B must not be fed into the swing-hammer shredder for the sake of plant safety. However, from investigations using the small scale horizontal shaft shredder, it can be derived that the 1st stage of comminution can occur under certain conditions only. The feed has to be voluminous and has to contain openings. If it is clamped by the feeding device, the impacting tools can act into the material and tear off fragments. 3.2. Stage 2 Initially, the following 2nd stage of the comminution is characterised by an intense deformation (bending to compaction) of the comparatively large platy fragments, which leads to the formation of flaws. As a result of the combined action of tensile stress, bending and torsion, they propagate until breakage. This process becomes obvious if conducting batch tests at varying shredding times and,
Fig. 5. Fragment mass and fragment size vs. specific energy consumption (a) and degree of bending and degree of compaction vs. specific energy consumption (b) (Zinc; comminution in small scale horizontal shaft shredder; circumferential velocity: 50 m/s; test body size: (2002001) mm3).
therefore, at varying specific energy consumption (Fig. 5a). At the beginning of the stressing, the fragment size b 50 decreases whereas the fragment mass m 50 is hardly changed. Simultaneously, the degree of bending increases rapidly (Fig. 5b). This is due to the intense deformation of the material, which is not accompanied by comminution at first. As the formation of breakage sets in, the fragment mass starts to drop also. In contrast, the degree of bending almost remains on the same level. The increase of the degree of compaction K 50 is not a consequence of the deformation processes but is also caused by the reduction of the fragment size and fragment mass, respectively. The energy required by the deformation and subsequent comminution can only be provided if there is a high relative velocity between the impacting tools and the material. Therefore, the narrow gap
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Fig. 7. Specific energy consumption per unit increase in surface area vs. growth of surface area (Zinc; comminution in small scale horizontal shaft shredder; circumferential velocity: 50 m/s; test body size: (2002001) mm3).
Fig. 6. Specific energy consumption vs. shredding time (a) and fragment mass vs. specific energy consumption (b) (Zinc; comminution in small scale horizontal shaft shredder; test body size: (1001001) mm3).
The effect of the deformation on the process of comminution becomes clear looking at Fig. 7. In the case of test bodies not preliminarily stressed, a high amount of energy is required for the deformation. Thus, the specific energy consumption per unit increase in surface area is very high at the beginning. As the comminution proceeds, it levels off at a constant value of w A=6 J/mm2. The comminution of test bodies, which had been bent several times prior to feeding, consumes less specific energy per unit growth of surface area. 3.3. Stage 3
inside the lower part of housing plays a decisive role. The material is decelerated by the frequent contacts with the walls of the housing. They also can act as an abutment for short periods of time. This enables the intense bending to compaction of the fragments and is a precondition for the formation of the effective tensile stress in combination with bending and torsion required for the actual comminution. As can be seen in Fig. 6, the amount of energy required for the deformation preceding the comminution depends on the circumferential velocity. At equal shredding time, the specific energy consumption increases with rising value of v (a). Consequently, the products are finer. However, if regarding the fragment mass m 50 as a function of the specific energy consumption, it becomes obvious that the comminution at higher circumferential velocity requires less energy.
If the fragments are too small in order to form abutments inside the lower part of housing, impacts against the walls of the housing become crucial for a further comminution. They cause a proceeding deforTable 1 Specific energy required for breakage as a function of the material of the test bodies (each with four test bodies; test body size: (33331) mm3; impact velocity: 50 m/s) Material
Zinc St14 Aluminium
Number of impacts
Specific energy consumption in kJ/kg
Mean value
Standard deviation
Mean value
Standard deviation
64 252 450
7.8 56.2 97.8
76 320 550
8.7 67.1 112.3
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mation (bending to compaction) of the fragments, which results in the formation of flaws. As a consequence of internal deadlock and of locally varying deformations, tensile stress is formed inside the fragments. Thus, a gradual crack extension until breakage takes place. These processes can be attributed to the 3rd stage of comminution. The processes taking place inside the small scale horizontal shaft shredder can well be reproduced by means of the impact apparatus shown in Fig. 3c. Using platy test bodies, the specific energy required for breakage is a function of the material properties (Table 1). Fig. 9. Breakage probability vs. specific energy consumption with mass of the test bodies and fragments as a parameter (Zinc; comminution in impact apparatus; impact velocity: 50 m/s).
Fig. 8. Degree of bending vs. number of impacts (a) and degree of compaction vs. number of impacts (b) (Comminution in impact apparatus; test body size: (33331) mm3; impact velocity: 50 m/s).
These differences can be explained if considering the development of the degree of bending and degree of compaction as a function of the number of impacts (Fig. 8). In comparison to zinc, for the steel St14, a much higher number of impacts is necessary in order to obtain a considerable deformation (a). This is due to the higher material strength. Once the bending occurs, steel—as a result of its properties (high tensile strength and permanent strain after rupture)—tends to be compacted very strongly. At the same time the test body is obviously bent back and forth, which can be derived from the evolution of the degree of compaction (b). Aluminium on the other hand is immediately deformed and increasingly compacted. Fig. 9 shows the breakage probability distributions of test bodies and fragments selected from products obtained using the small scale horizontal shaft shredder as a function of the specific energy consumption. In the case of platy test bodies, it can be seen that the value of w m required in order to obtain an equal breakage probability decreases if the test bodies become heavier. Although still detectable, this effect is less evident if looking at the fragments, which had been taken from a product of a batch test. Therefore, it can be assumed that the preliminary stress obtained during the preceding stages of comminution is dominating the demand of energy of the 3rd stage.
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4. Conclusions
Fig. 10. Number of fragments having bN1 mm and portion of fragments having bb1 mm vs. specific energy consumption (a) and REM of fines resulting from abrasion (b) (Zinc; comminution in small scale horizontal shaft shredder; circumferential velocity: 50 m/s; test body size: (2002001) mm3).
3.4. Stage 4 The 4th stage of comminution is characterised by a further intense compaction of the fragments until they possess a spherical shape. Simultaneously, the formation of fines can be observed. The number of fragments having sizes of bN1 mm levels off for higher shredding times, whereas the portion of the fines with bb1 mm increases permanently (Fig. 10a). Thus, the formation of fines is the major mechanism of comminution within this stage. It is caused by the superficial wear of the stressed material by abrasion. The formed particles are subject to further intense deformation processes (Fig. 10b).
The comminution of metals and scrap in swinghammer shredders presupposes a sufficient deformation (bending to compaction) of the fragments, which results in the formation of flaws. The flaws propagate as a consequence of the combined action of tensile stress, bending and torsion until breakage is reached. This process is repeated several times for the formed fragments until they pass the discharge grate. By means of systematic investigations utilising platy test bodies, four successive stages could be observed during comminution: The 1st stage takes place in the space adjacent to the anvil and comprises the tearing off of fragments from the feed. However, its occurrence is restricted to voluminous material. During the 2nd stage, an intense deformation resulting in the formation of flaws in the spots where the material is bent is observed. The demand of energy for that is influenced by the circumferential velocity of the impacting tools. As a result of the combined action of tensile stress, bending and torsion the flaws propagate until breakage. The 3rd stage is characterised by further deformation and compaction of the fragments due to impacts. Breakage of the fragments occurs as a result of a gradual crack formation, which is caused by internal tensile stress. Thereby, the amount of energy consumed is dependent on the fragment mass and in particular on the stressing obtained during the preceding stages. The stage 4 leads to a further compaction of the fragments until they have the shape of spheres. Then, the major mechanism of comminution is the formation of fines as a result of abrasion taking place at the surface of the fragments.
Acknowledgement The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support.
References Hfffl, K., 1989. Zerkleinerungs- und Klassiermaschinen. Deutscher Verlag fqr Grundstoffindustrie, Leipzig. Kirchner, J., 2000. Mikroprozesse und Einfluggrfgen bei der Zerkleinerung der Schrotte und Metalle in Shreddern mit
S. Sander et al. / Int. J. Miner. Process. 74S (2004) S385–S393 horizontal angeordnetem Rotor, Dissertation, TU Bergakademie Freiberg. Sander, St., Schubert, G., Timmel, G., 2002. Powder Technology 122 (2–3), 177 – 187. Schubert, H., 1989. Aufbereitung fester mineralischer Rohstoffe vol. 1. Deutscher Verlag fqr Grundstoffindustrie, Leipzig.
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Schubert, G., 1984. Aufbereitung metallischer Sekund7rrohstoffe. Deutscher Verlag fqr Grundstoffindustrie, Auflage, Leipzig. Schubert, G., 1997. Zerkleinerungstechnik fqr das Recycling von Abf7llen und Schrotten. Freiberger Forschungshefte. A 840, 12 – 32.
The fundamentals of the comminution of metals in shredders of the swing-hammer type S. Sander, G. Schubert*, H.-G. J7ckel TU Bergakademie Freiberg, Institut fu¨r MVT/AT, Agricolastage 1, 09599 Freiberg, Germany
Abstract The comminution of metals (scraps) is carried out in order to obtain suitable fragment size distributions and/or to liberate the valuable components. In the case of thin-walled metals and metal scraps, respectively, shredders of the swing-hammer type are frequently used for the liberation and size reduction. The technically relevant equipment is introduced in this paper. Inside these shredders, the material is subject to complex stressing modes. On the basis of systematic investigations using a small-scale horizontal shaft shredder, four successive stages occurring during the process of the comminution of metal sheets are explained. D 2004 Elsevier B.V. All rights reserved. Keywords: comminution; swing-hammer shredders; metal scraps
1. Introduction The size reduction of automobile scrap and other types of light steel scrap as well as scraps from electronic appliances, lead-acid accumulators, nonferrous metals and metal chips is generally carried out by means of swing-hammer shredders. This is due to the complex stressing modes occurring inside this type of equipment, which are well suited for the comminution of thin-walled metal scraps. The objectives of the size reduction are to obtain suitable size distributions required by the subsequent processing steps, to increase the bulk density and to liberate the components of composites and assemblies (Schubert, 1997). * Corresponding author. Fax: +49 3731 39 2815. E-mail address: [email protected] (G. Schubert). 0301-7516/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2004.07.038
Some swing-hammer shredders typically utilised for the comminution of automobile scrap and other types of light steel scrap can be seen in Fig. 1. The input material is fed from the side, the mass flow is controlled by the power consumption. In the case of the types shown in (a) and (b), it is subsequently transported into the relatively narrow gap between the impacting tools and the lower part of housing, where it is subject to an intense deformation and comminution. The material, which has become sufficiently small, is discharged from the chamber of comminution by means of grates. The configuration of the discharge grates most common for the comminution of light steel scrap is shown in Fig. 1a (model Lindemann/Newell). Usually, one grate is placed above the rotor and in some cases a second one below it. The application of a
S386
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Fig. 1. Swing-hammer shredders for the comminution of automobile scrap and other types of light steel scrap. (1) Rotor and impacting tools (swing-hammers); (2) anvil; (3) discharge grate; (4) device for discharging unshreddable items; (A) feed; (P) product.
vertical discharge grate is accompanied by a modified design of the upper part of the housing, which has to be narrower (Fig. 1b, model Thyssen HRT). According to the manufacturer, this results in a decrease of the specific energy consumption for the process. The so-called Kondirator (model Lindemann) shown in Fig. 1c is characterised by a reverse direction of rotation of the rotor. Thus, the feed is carried into the space above the rotor first. The unshreddable items can be discharged by means of the swivelling grate (4) before the material is transported into the narrow gap underneath the rotor. Therefore, this swing-hammer shredder is suited for the size reduction of light steel scrap having small portions of scrap from demolished steel constructions. In Fig. 2, the throughput of hammer crushers applied in mineral processing and of swing-hammer shredders used for the comminution of steel scrap can
be seen as a function of the drive power. It becomes obvious that the energy demand for the size reduction of scrap exceeds the one of minerals by about one order of magnitude. This is a result of the manifold energy consuming deformation processes occurring. Therefore, it has been of special interest to investigate the microprocesses and governing parameters of the comminution of metals.
2. Experimental For the investigations concerning the microprocesses taking place inside the swing-hammer shredders, a small-scale horizontal shaft shredder, a modified Izod pendulum and an impact apparatus were used (see Fig. 3). In order to make sure that the process of comminution could be examined sepa-
Fig. 2. Throughput vs. drive power for hammer crushers and swing-hammer shredders (Ho¨ffl, 1989; Schubert, 1984; Schubert, 1989).
S. Sander et al. / Int. J. Miner. Process. 74S (2004) S385–S393
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Fig. 3. Small-scale shredder (a), modified Izod pendulum (b) and impact apparatus (c). (1) Rotor with impacting tools (swing-hammers); (2) upper part of housing; (3) lower part of housing (flap, discharge in the case of batch tests); (4) anvil; (5) discharge grate (closed in the case of batch tests); (6) feed chute; (7) weights of impacting tool (50 kg); (8) impacting tool with strain gages; (9) lifting and breaking device for the impacting tool; (10) anvil construction; (11) compressor; (12) compressed air tank; (13) magnetic valve; (14) acceleration tube; (15) chock block; (16) impact chamber with velocity measurement (light barrier); (17) baffle plate (deflection 0. . .378).
rately from the kinetics of classifying taking place simultaneously, the discharge grates located above (5) and underneath the rotor of the small scale horizontal shaft shredder were replaced by steel plates. The lower part of housing is designed to act as a flap (3). After a time defined in advance, the material could instantaneously be discharged from the chamber of comminution by lowering this flap. Utilising a torque measurement shaft, the mechanical power draw was recorded as a function of shredding time. The area enclosed by the power-vs.-time-plot is equal to the energy consumption. As test material, test bodies of zinc sheet and other materials were
used. A detailed description of the modified Izod pendulum and the impact apparatus can be found in the work of Kirchner (2000). As a result of the intensive deformation of the material, the characterisation of the products of the comminution has shown to be difficult. The fragment size and fragment mass distributions as well as their median values were used to indicate the results of the comminution. It has to be paid attention to the fact that every fragment can be described by its three main dimensions a, b and c, whereby azbzc has to be fulfilled. The medium dimension b was fixed to be the characteristic for the fragment size. It is estimated
S388
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Fig. 4. Measurement of the main dimensions a and b of a fragment by the determination of the best fitting ellipse of equal area (a) and estimation of the fragment volume by the calculation of the ellipsoid of rotation having the dimensions a ell and b ell (b).
by the dimension b ell of the best fitting ellipse of equal area of projection (Fig. 4a) (Sander et al., 2002). The determination of the increase in surface area can be carried out only by bending open the fragments and measuring the length of fractures and cracks. Thus, this method is limited to the products, which had been subject to short time shredding. Additionally, the degree of bending B and the degree of compaction K are determined in order to characterise the deformation of the fragments. B can be calculated according to B¼
Aa¨q APr : Aa¨q AK
m ; qM d
K¼
6m : qM paell b2ell
ð3Þ
Herein, m represents the fragment mass, a ell and b ell are the dimensions of the best fitting ellipsoid of rotation of equal area of projection (Fig. 4b) and q M is the material density.
ð1Þ
Therein, A Pr and A K represent the area of projection of the fragment and of the sphere of equal mass, respectively. The equivalent fragment area can be obtained by using Aa¨q ¼
in which m, q M and d are the fragment mass, the material density and the wall thickness of the test body. The degree of compaction can be calculated according to
ð2Þ
3. The microprocesses of the comminution of metals in shredders of the swing-hammer type On the basis of systematic investigations utilising test bodies of sheet metal, four successive stages could be distinguished during the process of comminution (Kirchner, 2000). The 3rd and 4th stage, however, are bound to longer residence times of the material inside the chamber of comminution. Thus,
S. Sander et al. / Int. J. Miner. Process. 74S (2004) S385–S393
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they can be missing in the case of swing-hammer shedders working continuously. 3.1. Stage 1 The 1st stage of the comminution comprises the tearing off of single fragments from the feed as a result of the combined action of tensile stress, bending and torsion. It takes place in the space close to the feed chute including the anvil. The comminution possibly taking place was examined by means of the modified Izod pendulum. It became clear that breakage can occur only if the specific section modulus W B,bez, which can be calculated for the geometry under investigation using WB;bez ¼
WB bd 2 ¼ ; w 6w
ð4Þ
exceeds a critical value. Therein, W B is the section modulus of the test body, b and d represent the width and wall thickness of the test body, respectively, and w is the clearance between impacting tool and anvil. At values lower than the critical one, the test body is only bent. In the case of the relatively wide clearances occurring in industrial scale shredders, a very high section modulus would be necessary in order to initiate breakage during the 1st stage. Pieces of scrap having values as high as W B must not be fed into the swing-hammer shredder for the sake of plant safety. However, from investigations using the small scale horizontal shaft shredder, it can be derived that the 1st stage of comminution can occur under certain conditions only. The feed has to be voluminous and has to contain openings. If it is clamped by the feeding device, the impacting tools can act into the material and tear off fragments. 3.2. Stage 2 Initially, the following 2nd stage of the comminution is characterised by an intense deformation (bending to compaction) of the comparatively large platy fragments, which leads to the formation of flaws. As a result of the combined action of tensile stress, bending and torsion, they propagate until breakage. This process becomes obvious if conducting batch tests at varying shredding times and,
Fig. 5. Fragment mass and fragment size vs. specific energy consumption (a) and degree of bending and degree of compaction vs. specific energy consumption (b) (Zinc; comminution in small scale horizontal shaft shredder; circumferential velocity: 50 m/s; test body size: (2002001) mm3).
therefore, at varying specific energy consumption (Fig. 5a). At the beginning of the stressing, the fragment size b 50 decreases whereas the fragment mass m 50 is hardly changed. Simultaneously, the degree of bending increases rapidly (Fig. 5b). This is due to the intense deformation of the material, which is not accompanied by comminution at first. As the formation of breakage sets in, the fragment mass starts to drop also. In contrast, the degree of bending almost remains on the same level. The increase of the degree of compaction K 50 is not a consequence of the deformation processes but is also caused by the reduction of the fragment size and fragment mass, respectively. The energy required by the deformation and subsequent comminution can only be provided if there is a high relative velocity between the impacting tools and the material. Therefore, the narrow gap
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Fig. 7. Specific energy consumption per unit increase in surface area vs. growth of surface area (Zinc; comminution in small scale horizontal shaft shredder; circumferential velocity: 50 m/s; test body size: (2002001) mm3).
Fig. 6. Specific energy consumption vs. shredding time (a) and fragment mass vs. specific energy consumption (b) (Zinc; comminution in small scale horizontal shaft shredder; test body size: (1001001) mm3).
The effect of the deformation on the process of comminution becomes clear looking at Fig. 7. In the case of test bodies not preliminarily stressed, a high amount of energy is required for the deformation. Thus, the specific energy consumption per unit increase in surface area is very high at the beginning. As the comminution proceeds, it levels off at a constant value of w A=6 J/mm2. The comminution of test bodies, which had been bent several times prior to feeding, consumes less specific energy per unit growth of surface area. 3.3. Stage 3
inside the lower part of housing plays a decisive role. The material is decelerated by the frequent contacts with the walls of the housing. They also can act as an abutment for short periods of time. This enables the intense bending to compaction of the fragments and is a precondition for the formation of the effective tensile stress in combination with bending and torsion required for the actual comminution. As can be seen in Fig. 6, the amount of energy required for the deformation preceding the comminution depends on the circumferential velocity. At equal shredding time, the specific energy consumption increases with rising value of v (a). Consequently, the products are finer. However, if regarding the fragment mass m 50 as a function of the specific energy consumption, it becomes obvious that the comminution at higher circumferential velocity requires less energy.
If the fragments are too small in order to form abutments inside the lower part of housing, impacts against the walls of the housing become crucial for a further comminution. They cause a proceeding deforTable 1 Specific energy required for breakage as a function of the material of the test bodies (each with four test bodies; test body size: (33331) mm3; impact velocity: 50 m/s) Material
Zinc St14 Aluminium
Number of impacts
Specific energy consumption in kJ/kg
Mean value
Standard deviation
Mean value
Standard deviation
64 252 450
7.8 56.2 97.8
76 320 550
8.7 67.1 112.3
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mation (bending to compaction) of the fragments, which results in the formation of flaws. As a consequence of internal deadlock and of locally varying deformations, tensile stress is formed inside the fragments. Thus, a gradual crack extension until breakage takes place. These processes can be attributed to the 3rd stage of comminution. The processes taking place inside the small scale horizontal shaft shredder can well be reproduced by means of the impact apparatus shown in Fig. 3c. Using platy test bodies, the specific energy required for breakage is a function of the material properties (Table 1). Fig. 9. Breakage probability vs. specific energy consumption with mass of the test bodies and fragments as a parameter (Zinc; comminution in impact apparatus; impact velocity: 50 m/s).
Fig. 8. Degree of bending vs. number of impacts (a) and degree of compaction vs. number of impacts (b) (Comminution in impact apparatus; test body size: (33331) mm3; impact velocity: 50 m/s).
These differences can be explained if considering the development of the degree of bending and degree of compaction as a function of the number of impacts (Fig. 8). In comparison to zinc, for the steel St14, a much higher number of impacts is necessary in order to obtain a considerable deformation (a). This is due to the higher material strength. Once the bending occurs, steel—as a result of its properties (high tensile strength and permanent strain after rupture)—tends to be compacted very strongly. At the same time the test body is obviously bent back and forth, which can be derived from the evolution of the degree of compaction (b). Aluminium on the other hand is immediately deformed and increasingly compacted. Fig. 9 shows the breakage probability distributions of test bodies and fragments selected from products obtained using the small scale horizontal shaft shredder as a function of the specific energy consumption. In the case of platy test bodies, it can be seen that the value of w m required in order to obtain an equal breakage probability decreases if the test bodies become heavier. Although still detectable, this effect is less evident if looking at the fragments, which had been taken from a product of a batch test. Therefore, it can be assumed that the preliminary stress obtained during the preceding stages of comminution is dominating the demand of energy of the 3rd stage.
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4. Conclusions
Fig. 10. Number of fragments having bN1 mm and portion of fragments having bb1 mm vs. specific energy consumption (a) and REM of fines resulting from abrasion (b) (Zinc; comminution in small scale horizontal shaft shredder; circumferential velocity: 50 m/s; test body size: (2002001) mm3).
3.4. Stage 4 The 4th stage of comminution is characterised by a further intense compaction of the fragments until they possess a spherical shape. Simultaneously, the formation of fines can be observed. The number of fragments having sizes of bN1 mm levels off for higher shredding times, whereas the portion of the fines with bb1 mm increases permanently (Fig. 10a). Thus, the formation of fines is the major mechanism of comminution within this stage. It is caused by the superficial wear of the stressed material by abrasion. The formed particles are subject to further intense deformation processes (Fig. 10b).
The comminution of metals and scrap in swinghammer shredders presupposes a sufficient deformation (bending to compaction) of the fragments, which results in the formation of flaws. The flaws propagate as a consequence of the combined action of tensile stress, bending and torsion until breakage is reached. This process is repeated several times for the formed fragments until they pass the discharge grate. By means of systematic investigations utilising platy test bodies, four successive stages could be observed during comminution: The 1st stage takes place in the space adjacent to the anvil and comprises the tearing off of fragments from the feed. However, its occurrence is restricted to voluminous material. During the 2nd stage, an intense deformation resulting in the formation of flaws in the spots where the material is bent is observed. The demand of energy for that is influenced by the circumferential velocity of the impacting tools. As a result of the combined action of tensile stress, bending and torsion the flaws propagate until breakage. The 3rd stage is characterised by further deformation and compaction of the fragments due to impacts. Breakage of the fragments occurs as a result of a gradual crack formation, which is caused by internal tensile stress. Thereby, the amount of energy consumed is dependent on the fragment mass and in particular on the stressing obtained during the preceding stages. The stage 4 leads to a further compaction of the fragments until they have the shape of spheres. Then, the major mechanism of comminution is the formation of fines as a result of abrasion taking place at the surface of the fragments.
Acknowledgement The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support.
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