electrostatic technologies for materials recovery in high-intensity electric fields romanian...
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Recycling materials through electricity.TRANSCRIPT
Electrostatic technologies for materials recovery in high-intensity
electric fields
ADRIAN SAMUILA, ALEXANDRU IUGA, ROMAN MORAR, VASILE NEAMTU, LUCIAN DASCALESCU∗
The paper synthesises the authors’ original research in the development of two innovative electrostatic technologies: non-ferrous materials recovery in high-intensity electric fields and plastics triboelectrostatic separation. Both technologies are of great importance in materials recovery from industrial wastes, in particular from Wastes of Electric and Electronic Equipment (WEEE). At the same time, the authors point out the actions to be taken to integrate the activities of the High-Intensity Electric Fields Laboratory (HIEFL) of Cluj-Napoca in a research network whose objective is to elaborate solution for the treatment of solid industrial wastes. The extended references include 30 significant articles published by the HIEFL in scientific and technical journals with high impact factor.
Key words: electrostatic separation, corona discharge, high intensity electric field, materials recovery
1. Introduction
The European Union policy in relation to the environment and sustainable development
calls for significant changes in current patterns of development, production, consumption
and promotes the reduction of wasteful consumption of natural resources and the prevention
of pollution. The waste electrical and electronic equipment represents one of the target
areas to be regulated, in view of the application of the principles of prevention, recovery and
safe disposal.
In this respect, the electrostatic technologies for materials recovery using high-intensity
electric fields represent innovative solutions, contributing to pollution control and resources
conservation [1 - 13], [14 - 44].
The paper presents the authors point of view on the following topics: 1. How could
industry take advantage of the novel electrostatic technologies for solid waste recycling? 2.
What are the hot research topics in this field? 3. Why a research network in electrostatic
technologies is imperatively necessary?
The corona-electrostatic separation is employed in conducting and non-conducting
materials recovery from granular mixtures, such as chopped electric wires wastes [41],
minerals [30] and non-ferrous foundry wastes.
Adrian SAMUILA, Alexandru IUGA, Roman MORAR, Vasile NEAMTU - High-Intensity Electric Fields Laboratory, ELMA Center, Technical University, 15 C-Daicoviciu St., 400020 Cluj-Napoca, Romania, Phone +40264401429, email: [email protected]; Lucian DASCALESCU - Electronics and Electrostatics Research Unit, LAII-ESIP, UPRES-EA 1219, University Institute of Technology, 4 avenue de Varsovie, 16021 Angoulême Cedex, France, Phone +330545673245, Fax +330545673249, Email: [email protected]
2.1. Corona discharge
The electrostatic separation make use of corona discharge (Figure 1) for charging the
particles by ions bombardment.
Figure 1. Corona discharge produced by a multiple-needle electrode connected to a positive DC high voltage supply.
The corona effect is obtained in a strong non-uniform electric field generated by reduced
radii of curvature of metallic electrodes (Figure 2), when the high voltage exceeds a critical
value named corona inception voltage.
Figure 2. Dual corona electrodes: wire type (left), fixed pitch needles (middle), and variable pitch needles (right) used for generating high-intensity electric field.
A dual corona electrode consists of one or more ionizing elements (metallic wires or
needles) attached to a metallic tubular support. The High-Intensity Electric Fields Laboratory
of Cluj-Napoca developed different models of such electrodes [18], [23], [29].
The distribution of the electric field is important in evaluating of the electric forces
controlling particle movement in electrostatic separators. Commercial software offers
adequate solutions for the numerical analysis of electrostatic fields (Figure 3).
Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu 3
Figure 3. Equal-potential lines of the electrostatic field in a blade-cylinder configuration computed with Surface Charge Simulation Program at a high voltage level below the corona inception voltage.
The analysis of the corona electric field is a much more difficult problem. Solutions are
obtained only in a few simple electrode configurations [27], [40], [43].
2.2 Insulation-metal separation
The roll-type corona separator is the standard equipment for the selective sorting of
granular mixtures, based on differences in superficial electric conductivities [4]. The space
charge generated by corona effect produces the charging of the insulating particles, such as
PVC, PE, and resin. This charge q is slowly transferred to the metallic roll so that the
insulating particles are pinned and maintained attached to the grounded roll electrode
(Figure 4) due to the electric image force [13]:
2 0
• ε 0 –electric permittivity of free space.
Figure 4. Pinning effect produced by the electric image force acting on insulating particles after charging by ion bombardment while passing through a positive corona discharge.
4 Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu
Conductive particles are not affected by the corona field; they are charged by
electrostatic induction in contact with the grounded roll and are attracted to the high-voltage
electrode. Some electrode configurations are particularly effective in controlling particle
trajectories (Figure 5); thus, a second corona electrode increases the pinning effect and the
custom-designed electrostatic electrode deviates the conductive particles.
The insulation/metal corona-electrostatic separation is now a mature technology [4], [5],
[37], [41]. Research may contribute to optimization [24], [33] and robust design of
electroseparation process [17].
Figure 5. Trajectories of insulating (left) and conductive (right) particles in a roll-type corona-electrostatic separator.
The laboratory and pilot plant experiments performed on chopped electric wire wastes
pointed out that from a granular mixture containing 60% copper and 40% PVC, in a three
stages corona-electrostatic separation, final concentrates of PVC and copper of more than
99.8 % purity may be obtained.
Metals and plastics recovery from wastes is one of most important corona-electrostatic
technologies. The ELSIM insulation/metal electrostatic separators developed by authors and
Electromures Company have been successfully used by cables manufactures from
Bucharest and Targu Mures.
2.3. Muscovite mica recovery
The electric field forces are used to extract feldspar from pegmatite, a complex of
minerals containing variable quantities of quartz and muscovite mica, too [30]. The shape
difference between flat mica flakes and isometric granules of feldspar and quartz (Figure 6)
enables the successful separation of these minerals.
The laboratory experiments have been carried out on samples containing about 50%
mica, 25% feldspar, 15% quartz, and 10% other minerals, with grain sizes ranging between
(0.16–0.4) mm. The corona field is generated between a wire type electrode, connected to a
DC high-voltage supply, and a grounded metallic roll electrode. The latter rotates and carries
the material to be separated through the corona field zone. Thus, all the constituents of the
pegmatite ore are charged by ion bombardment. The electric image force is greater for flat
particles of mica and smaller for isometric granules of quartz and feldspar. As a
consequence, mica flakes are better pinned on the roll surface, rotate with it and are
Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu 5
removed by the brush. Feldspar and quartz granules are thrown away from the roll surface
by the centrifugal force soon after they leave the corona field zone.
Figure 6. Operating parameters of a roll-type corona-field separator for the recovery of mica from a mineral containing also feldspar & quartz; R = 100 mm, n = 150 rev/min,
tungsten wire Ø=0.2 mm, s = 100 mm, α = 75°, U = - 25 kV, electric heater 400 W, adjustable [30].
Drying of the granular material prior to separation (with an over-tray electric heater) is
necessary for removing the superficial moisture from the particles.
2.4. Brass recovery
The high-intensity electric field may be used for brass recovery from non-ferrous foundry
wastes. Laboratory experiments were performed on samples containing more than 60%
brass (Figure 7), in the corona electrostatic field generated by two active electrodes: one
corona (similar to wire type presented in Figure 3) and the other electrostatic (tubular type,
25 mm in diameter).
Figure 7. Main constituents of brass dross granular samples; 1-slag; 2-quartz; 3-refractory material; 4-brass cuttings; 5-brass drops; 6-brass granules with nonmetallic inclusions.
3. Triboelectrostatic recovery of plastics
Plastics recovery by triboelectrostatic separation has two essential stages:charging of
the granular mixture components with opposite polarity chargesq in the tribocharging device
and selective sorting of plastics granules based on the electric force F = qE in the
electrostatic field E generated by two plate electrodes (Figure 8). The design of the
triboelectrostatic separator TESS, developed by the High-Intensity Electric Fields
Laboratory, was established having in mind the criteria of competitiveness, functionality, and
versatility (different tribocharging devices and electrodes). The maximum voltage between
electrodes is 150 kV.
The triboelectric series (Figure 9) arrange the materials based on their work function.
Figure 8. Triboelectrostatic separator TESS for plastics; 1-fluidised bad tribocharging device; 2-turboblower; 3-air velocity regulator; 4-electrostatic separator inlet;
5-plate electrodes 1000x200 mm; 6-positioning panel; 7-dielectric splitter [19].
MINUS PLUS
PVC PP PET Al
Figure 9. Experimental triboelectric series for PVC and PET granules in contact with the PP and aluminum chambers of the fluidized bed tribocharging device.
Materials with lower work function (for example PET) loose easier electrons and
positively charge in contact with a material positioned to the left in the triboelectric series (for
example PP and PVC).
The development of efficient tribocharging devices is a prerequisite for the success of
any plastics recovery technology. The electrostatic laboratories from Cluj-Napoca and
Angoulême developed several fluidized bed (Figure 8), vibrating and rotating cylinder
(Figure 10) tribocharging devices.
Figure 10. Tribocharging device with rotating cylinder.
Figure 11 presents the PET/PVC separation results [19] using the experimental setup in
Figure 8 equipped with a fluidized bed tribocharging device.
Figure 11. Quality of PET concentrates obtained by electrostatic separation of the 50% PET & 50% PVC samples, after tribocharging in aluminum and polypropylene chamber.
4.1. Extreme-gap corona electrodes
The standard corona field separation of granular mixtures is performed at 50 mm inter-
electrodes gaps (Figure 13 and 14). In some applications, reduced gaps (25 mm) may be
advantageous, as accompanied by reduced corona-inception voltage and simplified
constructive solutions [18]. In other applications, large gaps (150 mm) present the
advantage of extending the field zone corresponding to particle charging by ion
bombardment and diminishing the number of spark discharges when the conducting
particles are passing through the field [18].
Figure 13. Wire corona electrodes positioned at small, standard and large gaps with respect to the grounded roll electrode.
Figure 14. The current-voltage characteristics of the corona discharge in large gaps electrodes configurations.
Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu 9
4.2. Optimizing the operation of corona separators using computer-assisted experimental design technique
The corona electrode position is a key factor for efficient operation of corona separator,
as it influences the particles charging and the exerted forces in electric field [23]. The radial
position s1, angular position α1 and the high-voltage level supplying the corona electrode U
are the parameters considered in optimizing the electrostatic separation process (Figure 15).
Based one the results obtained for experimental points indicated in Figure 15, the
response surfaces are plotted using an optimizing program, as shown in Figure 16. The
optimal position of the corona electrode corresponds to the minimum quantity of middling
(M) generated in the separation process.
Figure 15. The experimental domain defined by 3 variables: high-voltage level U, angular α1 and radial s1 position of the corona electrode, in view of separation process optimisation.
Figure 16. Response surfaces determined with MODDE program for optimizing separation process, based on “minimum quantity of middling (M)” criterion.
mediu
Excellence in electrostatics research also implies the design of specific application
equipment. Thus, the High-Intensity Electric Fields Laboratory developed four laboratory and
pilot plant research separators, one of them being the modular unit ELSMOD displayed in
Figure 17.
Figure 17. Active zone of the corona-electrostatic pilot-plant separator ELSMOD.
The main feature of the research separator is the possibility to modify a great number of
parameters in order to model a multi-factorial separation process.
The recognized competence of Cluj-Napoca research team in the R&D of electrostatic
separation equipment made possible the cooperation with CARPCO Inc., Jacksonville,
Florida, in developing of multifunctional separator model EHTP 111-15 (Figure 18).
Figure 18. Research electrostatic separator developed by Carpco, Inc. U.S.A., in cooperation with High Intensity Electric Fields Laboratory Cluj-Napoca; 1-high-voltage supply; 2-control panel;
3-feeder; 4-high-voltage electrodes; 5-grounded rotating roll electrode; 6-collecting boxes.
11
The research pilot plant & laboratory equipment in Figure 18 may be configured as a
corona electrostatic roll-type separator or plate-type electrostatic separator.
Four collecting compartments and three adjustable splitters assure flexibility in collecting
of separation products and adaptability to the plate-type electrostatic separator. The
electronic DC high-voltage supply is reversible (+/-) and continuously-adjustable (0 – 40) kV.
The material is introduced in the active zone of the separator assured by a vibratory feeder
with hopper or a velocity feeder. The neutralizing electrode is supplied from a high-voltage
transformer, continuously-adjustable from 0 to 12 kV, AC.
4.4. Robust control of the electrostatic separation process
A robust electrostatic separation process is characterized by insensitiveness of quantity
and quality of separation products to the variation of uncontrolled parameters. The objective
of robust control tests is determining the operation parameters, for example high-voltage
level U and roll electrode speed n, so that noise variables, such as granules size 2r and
copper content Cu[%] of the feeding material exert a minimum influence on recovery and
purity of separation products (Figure 19).
The robustness of the corona-electrostatic separation process was analyzed using
Taguchi method [17], [24]. In industrial applications of corona-electrostatic separation, the
robust control of the process avoids important reprocessing costs and the diminution of
product quality.
Figure 19. Control variables U, n and noise factors 2r, Cu[%] considered in the robustness analysis of the copper / polyvinyl chloride electrostatic separation process.
4.5. Perspectives research in high-intensity electric fields technologies
Electrostatics has a solid tradition in Romanian universities [4], [5], [8] and research
institutes [7], [11]. Maintaining and developing both fundamental and applied research in this
mediu
field require a critical mass of financial and human resources, as well as an active
cooperation with experts from different areas.
The research in electrostatic eco-technologies should be oriented in the following
directions:
• Study of fundamental phenomena in electrostatics, such as particle charging, fine
particle behavior in high intensity electric fields, corona discharge, superficial
phenomena.
• Modeling of electrostatic processes in order to shorten the time laps from laboratory
studies to industry application, and reduce the cost of new equipments and
technologies.
• Integration of electrostatic eco-technologies in the recovery flow-sheet of useful
components from electric and electronic wastes and plastic packages.
• Design of high performance equipment to increase the productivity and quality of the
recovered materials.
• Development of research devices and instruments to study the behavior of fine
particles (micronics size) in electric field.
• Research and development of devices for selective tribocharging of multi-component
granular mixtures, in the presence of uncontrolled factors (additives, material ageing,
surface state).
atmosphere, state of imponderability, vacuum).
Re-enforcement of electrostatic research is possible only by participation of Romanian
laboratories in the Seventh Frame Program of the European Union. Complexity and difficulty
of the electrostatic research require the setup of an international network including several
excellence research centers with complementary competences, but also equipment
manufacturers and electrostatic technologies users.
Acknowledgements
The authors acknowledge the contribution of their present and former PhD students Eng.
Marius Blajan, Eng. Laur Calin, Dr.Eng. Adrian Mihalcioiu to the experiments described in
this paper. Most of the work was funded by the Romanian Education and Research Ministry
and some industrial partners.
References
[1] Directive 2002/96EC of the European Parliament and of the Council of 27 January 2003 on Waste Electrical and Electronic Equipment (WEEE).
[2] J.E. McCarthy – „Recycling computers and electronic equipment and regulatory approaches of e- Waste” . Report for Congress, Congressional Research Service, The Library of Congress, 2002.
[3] H. R. Manouchehri, K. Hanumantha Rao, K.S.E. Forssberg - “Review of electrical separation methods. Part 1: Fundamental aspect, Minerals and Metallurgical Processing, 17 (1), 23-36 (2000). Part 2: “Practical considerations”, 17(3), 139-166 (2000).
[4] L. Dascalescu, A. Iuga, R. Morar - “Electrosepararea materialelor granulare”, In: “Electrotehnologii. Volumul 1. Electrostatica”, F.T. Tanasescu, M. Bologa,. R. Cramariuc (Editori), Bucuresti, Ed. Academiei Romane, 1999, pp. 88-116.
13
[5] L. Dascalescu, R. Morar, A. Iuga, A. Samuila, Michaela Mihailescu – „Electrostatic technologies for the recycling of non-ferrous metals and plastics from wastes”, In: "The Modern Problems of Electrostatics with Applications in Environment Protection”, I.I. Inculet, F.T. Tanasescu and R. Cramariuc (Eds.), NATO Science Series, Dordrecht, Kluwer Academic Publishers, 1999, pp. 77-89.
[6] Y. Higashiyama, K. Asano – “Recent progress in electrostatic separation technology”, Particulate Science and Technology, 16, 77-90 (1998).
[7] I.I. Inculet, G.S.P. Castle, J.D. Braun – “Electrostatic separation of plastics for recycling”, Particulate Science and Technology, 16 , 91-100 (1998).
[8] G. Dragan – “Tehnica tensiunilor inalte” vol. 1, Bucuresti, Editura Tehnica, 1996. [9] A. Iuga, Al., L. Dascalescu, R. Morar, I. Csorvassy, V. Neamtu – “Corona-electrostatic separators
for recovery of waste non-ferrous metals”, Journal of Electrostatics, 23, 235-243 (1989). [10] L. Dascalescu - “An Introduction to Ionized Gases: Theory and Applications”, Toyohashi University
of Technology, 1993. [11] I.I. Inculet - “Electrostatic Mineral Separation”, New York, John Wiley, 1984. [12] N.F. Olofinski – “Elektricheskie methody obogashcheniya”, Moskva, Nedra, 1977. [13] J. E. Lawver, W. P. Dyrenforth - “Electrostatic Separation”, In: Electrostatics and its Applications,
A.D. Moore (Ed), New York, John Willey, 1973, pp. 221-249.
Selected authors publications
[14] R. Morar, I. O. Muntean, I. Cuglean, I. Almasan - “Tehnologii de depoluare a mediului”, Cluj- Napoca, Editura Dacia, 2004.
[15] A. Samuila, L. Dascalescu – “Comportarea materialelor granulare in camp electric”, Cluj-Napoca, Editura Mediamira, 1999.
[16] V. Neamu - “Separarea materialelor granulare”, Cluj-Napoca, Editura Casa Cartii de Stiinta, 1998. [17] L. Dascalescu, A. Samuila, A. Mihalcioiu, S. Bente, A. Tilmatine – “ Robust design of electrostatic
separation processes”, IEEE Transactions on Industry Applications, 41 (3), 715-720 (2005). [18] A. Samuila, M. Blajan, R. Beleca, M. Huzau, R. Morar, L. Dascalescu, A. Iuga – “Modeling of wire
corona electrode operation in electrostatic separation at small and large gaps”, Journal of Electrostatics, 63 (6-10), 955-960 (2005).
[19] A. Iuga, L. Calin, V. Neamtu, A. Mihalcioiu, L. Dascalescu – “Tribocharging of plastics granulates in a fluidized bed device”, Journal of Electrostatics, 63 (6-10), 937-942 (2005).
[20] L. Dascalescu, A. Urs, S. Bente, M. Huzau, A. Samuila – “Charging of mm-size insulating particles in vibratory devices”, Journal of Electrostatics, 63 (6-10), 705-710 (2005).
[21] A. Bendaoud, L. Dascalescu, M. Blajan, A. Samuila, A. Stochita, P.V. Notingher – “Corona charging of granular layers of insulating particles at the surface of a grounded electrode”, Journal of Electrostatics, 63 (6-10), 643-647 (2005).
[22] A. Mihalcioiu, L. Dascalescu, S. Das, K. Medles, R. Munteanu – “Virtual instrument for statistic control of powder tribocharging processes”, Journal of Electrostatics, 63 (6-10), 565-570 (2005).
[23] A. Samuila, A. Urs, A. Iuga, R. Morar, F. Aman, L. Dascalescu – “Optimization of corona electrode position in roll-type electrostatic separators”, IEEE Transactions on Industry Applications, 41 (2), 527-534 (2005).
[24] L. Dascalescu, A. Mihalcioiu, A. Tilmatine, Michaela Mihailescu, A. Iuga, A. Samuila – “Electrostatic separation processes. A linear-interaction optimization model using Taguchi’s experimental design technique”, IEEE Industry Applications Magazine, 10 (6), 19-25 (2004).
[25] A. Iuga, Simona Vlad, Michaela Mihailescu, L. Dascalescu – “ A laboratory plate/screen-type electrostatic separator for granular mixtures: Design, engineering, and application”, Particulate Science and Technology, 22 (3), 275-283 (2004).
[26] A. Tilmatine, S. Flazi, S. K. Medles, Y. Ramdani, L. Dascalescu – “ Electrostatic separation: mechanical uses and recycling of industrial wastes”, Journal of Electrostatics, 61 (1), 21-30 (2004).
[27] A. Caron, A., L. Dascalescu – “Numerical modeling of combined corona-electrostatic fields”, Journal of Electrostatics, 61 (1), 43-55 (2004).
[28] A. Urs, A. Samuila, A. Mihalcioiu, L. Dascalescu – “Charging and discharging of insulating particles on the surface of a grounded electrode”, IEEE Transactions on Industry Applications, 40 (2), 437-441 (2004).
[29] F. Aman, R. Morar, R. Köhnlechner, A. Samuila, L. Dascalescu, L. – “ High-voltage electrode position: A key factor of electrostatic separation efficiency”, IEEE Trans. on Industry Applications , 40 (3), 905-910 (2004).
[30] A. Iuga, I. Cuglesan, A. Samuila, M. Blajan, D. Vadan, L. Dascalescu – “Electrostatic separation of muscovite mica from feldspathic pegmatites”, IEEE Trans. on Industry Applications, 40 (2), 422- 429 (2004).
[31] A. Urs, C. Dragos, A. Samuila, A. Iuga, L. Dascalescu – “Electrostatic Sizing of Abrasive Particles Using a Free-Fall Corona Separator”, Particulate Science and Technology, 22 (1), 85-92 (2004).
mediu
[32] A. Samuila, A. Mihalcioiu, A. Urs, L. Dascalescu – “Unipolar charging and contact discharging of insulating particles on the surface of a grounded electrode”, Institute of Physics Conference Series, 178, 65-69 (2004).
[33] L. Dascalescu, A. Tilmatine, F. Aman, Michaela Mihailescu – “Optimization of electrostatic separation processes using response surface modelling”, IEEE Trans. on Industry Applications, 40 (1), 53-59 (2004).
[34] L. Dascalescu, A. Samuila, Michaela Mihailescu, A. Iuga, R. Köhnlechner – “ Expert system applications for electrostatic separation processes”, Institute of Physics Conference Series, 178, 355-360 (2004).
[35] Simona Vlad, A. Iuga, L. Dascalescu – “Numerical computation of conducting particle trajectories in plate-type electrostatic separators”, IEEE Transactions on Industry Applications, 39 (1), 66-71 (2003).
[36] Michaela Mihailescu, A. Samuila, A. Urs, R. Morar, A. Iuga, L. Dascalescu – “ Computer-assisted experimental design for the optimization of electrostatic separation processes”, IEEE Transactions on Industry Applications, 38 (5), 1174-1181 (2002).
[37] L. Dascalescu – “Electrostatic separation of plastics from industrial wastes. A review”, Journal of Electrostatics, Japan, 25, 282-288 (2001).
[38] Simona Vlad, A. Urs, A. Iuga, L. Dascalescu – “Premises for the numerical computation of conducting particle trajectories in plate-type electrostatic separators”, Journal of Electrostatics, 51- 52 (1-4), 259-265 (2001).
[39] D. Rafiroiu, I. Suarasan, R. Morar, P. Atten, L. Dascalescu – “Corona inception in typical electrode configurations for electrostatic processes applications”, IEEE Transactions on Industry Applications, 37 (3), 766-771 (2001).
[40] D. Rafiroiu, C. Munteanu, R. Morar, A. Meroth, P. Atten, L. Dascalescu – “Computation of the electric field in wire electrode arrangements for electrostatic processes applications”, Journal of Electrostatics, 51-52 (1-4), 571-77 (2001).
[41] A. Iuga, R. Morar, A. Samuila, L. Dascalescu – “Electrostatic separation of metals and plastics from granular industrial wastes’, IEE Proceedings: Science, Measurement and Technology, 148 (2), 47-54 (2001).
[42] L. Dascalescu, Simona Vlad, A. Iuga, P.L. Levin – “Electrostatics of conductive particles in contact with a plate electrode affected by a non-uniform electric field”. Journal of Physics D: Applied Physics, 34 (1), 60-67 (2001).
[43] D. Rafiroiu, R. Morar, P. Atten, L. Dascalescu – “Premises for the mathematical modeling of the combined corona-electrostatic field of roll-type separators”, IEEE Transactions on Industry Applications, 36 (5), 1260-1266 (2000).
[44] Simona Vlad, Michaela Mihailescu, D. Rafiroiu, A. Iuga, L. Dascalescu – “ Numerical analysis of the electric field in plate-type electrostatic separators”, Journal of Electrostatics, 48 (3-4), 217-229 (2000).
ADRIAN SAMUILA, ALEXANDRU IUGA, ROMAN MORAR, VASILE NEAMTU, LUCIAN DASCALESCU∗
The paper synthesises the authors’ original research in the development of two innovative electrostatic technologies: non-ferrous materials recovery in high-intensity electric fields and plastics triboelectrostatic separation. Both technologies are of great importance in materials recovery from industrial wastes, in particular from Wastes of Electric and Electronic Equipment (WEEE). At the same time, the authors point out the actions to be taken to integrate the activities of the High-Intensity Electric Fields Laboratory (HIEFL) of Cluj-Napoca in a research network whose objective is to elaborate solution for the treatment of solid industrial wastes. The extended references include 30 significant articles published by the HIEFL in scientific and technical journals with high impact factor.
Key words: electrostatic separation, corona discharge, high intensity electric field, materials recovery
1. Introduction
The European Union policy in relation to the environment and sustainable development
calls for significant changes in current patterns of development, production, consumption
and promotes the reduction of wasteful consumption of natural resources and the prevention
of pollution. The waste electrical and electronic equipment represents one of the target
areas to be regulated, in view of the application of the principles of prevention, recovery and
safe disposal.
In this respect, the electrostatic technologies for materials recovery using high-intensity
electric fields represent innovative solutions, contributing to pollution control and resources
conservation [1 - 13], [14 - 44].
The paper presents the authors point of view on the following topics: 1. How could
industry take advantage of the novel electrostatic technologies for solid waste recycling? 2.
What are the hot research topics in this field? 3. Why a research network in electrostatic
technologies is imperatively necessary?
The corona-electrostatic separation is employed in conducting and non-conducting
materials recovery from granular mixtures, such as chopped electric wires wastes [41],
minerals [30] and non-ferrous foundry wastes.
Adrian SAMUILA, Alexandru IUGA, Roman MORAR, Vasile NEAMTU - High-Intensity Electric Fields Laboratory, ELMA Center, Technical University, 15 C-Daicoviciu St., 400020 Cluj-Napoca, Romania, Phone +40264401429, email: [email protected]; Lucian DASCALESCU - Electronics and Electrostatics Research Unit, LAII-ESIP, UPRES-EA 1219, University Institute of Technology, 4 avenue de Varsovie, 16021 Angoulême Cedex, France, Phone +330545673245, Fax +330545673249, Email: [email protected]
2.1. Corona discharge
The electrostatic separation make use of corona discharge (Figure 1) for charging the
particles by ions bombardment.
Figure 1. Corona discharge produced by a multiple-needle electrode connected to a positive DC high voltage supply.
The corona effect is obtained in a strong non-uniform electric field generated by reduced
radii of curvature of metallic electrodes (Figure 2), when the high voltage exceeds a critical
value named corona inception voltage.
Figure 2. Dual corona electrodes: wire type (left), fixed pitch needles (middle), and variable pitch needles (right) used for generating high-intensity electric field.
A dual corona electrode consists of one or more ionizing elements (metallic wires or
needles) attached to a metallic tubular support. The High-Intensity Electric Fields Laboratory
of Cluj-Napoca developed different models of such electrodes [18], [23], [29].
The distribution of the electric field is important in evaluating of the electric forces
controlling particle movement in electrostatic separators. Commercial software offers
adequate solutions for the numerical analysis of electrostatic fields (Figure 3).
Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu 3
Figure 3. Equal-potential lines of the electrostatic field in a blade-cylinder configuration computed with Surface Charge Simulation Program at a high voltage level below the corona inception voltage.
The analysis of the corona electric field is a much more difficult problem. Solutions are
obtained only in a few simple electrode configurations [27], [40], [43].
2.2 Insulation-metal separation
The roll-type corona separator is the standard equipment for the selective sorting of
granular mixtures, based on differences in superficial electric conductivities [4]. The space
charge generated by corona effect produces the charging of the insulating particles, such as
PVC, PE, and resin. This charge q is slowly transferred to the metallic roll so that the
insulating particles are pinned and maintained attached to the grounded roll electrode
(Figure 4) due to the electric image force [13]:
2 0
• ε 0 –electric permittivity of free space.
Figure 4. Pinning effect produced by the electric image force acting on insulating particles after charging by ion bombardment while passing through a positive corona discharge.
4 Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu
Conductive particles are not affected by the corona field; they are charged by
electrostatic induction in contact with the grounded roll and are attracted to the high-voltage
electrode. Some electrode configurations are particularly effective in controlling particle
trajectories (Figure 5); thus, a second corona electrode increases the pinning effect and the
custom-designed electrostatic electrode deviates the conductive particles.
The insulation/metal corona-electrostatic separation is now a mature technology [4], [5],
[37], [41]. Research may contribute to optimization [24], [33] and robust design of
electroseparation process [17].
Figure 5. Trajectories of insulating (left) and conductive (right) particles in a roll-type corona-electrostatic separator.
The laboratory and pilot plant experiments performed on chopped electric wire wastes
pointed out that from a granular mixture containing 60% copper and 40% PVC, in a three
stages corona-electrostatic separation, final concentrates of PVC and copper of more than
99.8 % purity may be obtained.
Metals and plastics recovery from wastes is one of most important corona-electrostatic
technologies. The ELSIM insulation/metal electrostatic separators developed by authors and
Electromures Company have been successfully used by cables manufactures from
Bucharest and Targu Mures.
2.3. Muscovite mica recovery
The electric field forces are used to extract feldspar from pegmatite, a complex of
minerals containing variable quantities of quartz and muscovite mica, too [30]. The shape
difference between flat mica flakes and isometric granules of feldspar and quartz (Figure 6)
enables the successful separation of these minerals.
The laboratory experiments have been carried out on samples containing about 50%
mica, 25% feldspar, 15% quartz, and 10% other minerals, with grain sizes ranging between
(0.16–0.4) mm. The corona field is generated between a wire type electrode, connected to a
DC high-voltage supply, and a grounded metallic roll electrode. The latter rotates and carries
the material to be separated through the corona field zone. Thus, all the constituents of the
pegmatite ore are charged by ion bombardment. The electric image force is greater for flat
particles of mica and smaller for isometric granules of quartz and feldspar. As a
consequence, mica flakes are better pinned on the roll surface, rotate with it and are
Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu 5
removed by the brush. Feldspar and quartz granules are thrown away from the roll surface
by the centrifugal force soon after they leave the corona field zone.
Figure 6. Operating parameters of a roll-type corona-field separator for the recovery of mica from a mineral containing also feldspar & quartz; R = 100 mm, n = 150 rev/min,
tungsten wire Ø=0.2 mm, s = 100 mm, α = 75°, U = - 25 kV, electric heater 400 W, adjustable [30].
Drying of the granular material prior to separation (with an over-tray electric heater) is
necessary for removing the superficial moisture from the particles.
2.4. Brass recovery
The high-intensity electric field may be used for brass recovery from non-ferrous foundry
wastes. Laboratory experiments were performed on samples containing more than 60%
brass (Figure 7), in the corona electrostatic field generated by two active electrodes: one
corona (similar to wire type presented in Figure 3) and the other electrostatic (tubular type,
25 mm in diameter).
Figure 7. Main constituents of brass dross granular samples; 1-slag; 2-quartz; 3-refractory material; 4-brass cuttings; 5-brass drops; 6-brass granules with nonmetallic inclusions.
3. Triboelectrostatic recovery of plastics
Plastics recovery by triboelectrostatic separation has two essential stages:charging of
the granular mixture components with opposite polarity chargesq in the tribocharging device
and selective sorting of plastics granules based on the electric force F = qE in the
electrostatic field E generated by two plate electrodes (Figure 8). The design of the
triboelectrostatic separator TESS, developed by the High-Intensity Electric Fields
Laboratory, was established having in mind the criteria of competitiveness, functionality, and
versatility (different tribocharging devices and electrodes). The maximum voltage between
electrodes is 150 kV.
The triboelectric series (Figure 9) arrange the materials based on their work function.
Figure 8. Triboelectrostatic separator TESS for plastics; 1-fluidised bad tribocharging device; 2-turboblower; 3-air velocity regulator; 4-electrostatic separator inlet;
5-plate electrodes 1000x200 mm; 6-positioning panel; 7-dielectric splitter [19].
MINUS PLUS
PVC PP PET Al
Figure 9. Experimental triboelectric series for PVC and PET granules in contact with the PP and aluminum chambers of the fluidized bed tribocharging device.
Materials with lower work function (for example PET) loose easier electrons and
positively charge in contact with a material positioned to the left in the triboelectric series (for
example PP and PVC).
The development of efficient tribocharging devices is a prerequisite for the success of
any plastics recovery technology. The electrostatic laboratories from Cluj-Napoca and
Angoulême developed several fluidized bed (Figure 8), vibrating and rotating cylinder
(Figure 10) tribocharging devices.
Figure 10. Tribocharging device with rotating cylinder.
Figure 11 presents the PET/PVC separation results [19] using the experimental setup in
Figure 8 equipped with a fluidized bed tribocharging device.
Figure 11. Quality of PET concentrates obtained by electrostatic separation of the 50% PET & 50% PVC samples, after tribocharging in aluminum and polypropylene chamber.
4.1. Extreme-gap corona electrodes
The standard corona field separation of granular mixtures is performed at 50 mm inter-
electrodes gaps (Figure 13 and 14). In some applications, reduced gaps (25 mm) may be
advantageous, as accompanied by reduced corona-inception voltage and simplified
constructive solutions [18]. In other applications, large gaps (150 mm) present the
advantage of extending the field zone corresponding to particle charging by ion
bombardment and diminishing the number of spark discharges when the conducting
particles are passing through the field [18].
Figure 13. Wire corona electrodes positioned at small, standard and large gaps with respect to the grounded roll electrode.
Figure 14. The current-voltage characteristics of the corona discharge in large gaps electrodes configurations.
Simpozionul Impactul AQ-ului comunitar asupra echipamentelor i tehnologiilor de mediu 9
4.2. Optimizing the operation of corona separators using computer-assisted experimental design technique
The corona electrode position is a key factor for efficient operation of corona separator,
as it influences the particles charging and the exerted forces in electric field [23]. The radial
position s1, angular position α1 and the high-voltage level supplying the corona electrode U
are the parameters considered in optimizing the electrostatic separation process (Figure 15).
Based one the results obtained for experimental points indicated in Figure 15, the
response surfaces are plotted using an optimizing program, as shown in Figure 16. The
optimal position of the corona electrode corresponds to the minimum quantity of middling
(M) generated in the separation process.
Figure 15. The experimental domain defined by 3 variables: high-voltage level U, angular α1 and radial s1 position of the corona electrode, in view of separation process optimisation.
Figure 16. Response surfaces determined with MODDE program for optimizing separation process, based on “minimum quantity of middling (M)” criterion.
mediu
Excellence in electrostatics research also implies the design of specific application
equipment. Thus, the High-Intensity Electric Fields Laboratory developed four laboratory and
pilot plant research separators, one of them being the modular unit ELSMOD displayed in
Figure 17.
Figure 17. Active zone of the corona-electrostatic pilot-plant separator ELSMOD.
The main feature of the research separator is the possibility to modify a great number of
parameters in order to model a multi-factorial separation process.
The recognized competence of Cluj-Napoca research team in the R&D of electrostatic
separation equipment made possible the cooperation with CARPCO Inc., Jacksonville,
Florida, in developing of multifunctional separator model EHTP 111-15 (Figure 18).
Figure 18. Research electrostatic separator developed by Carpco, Inc. U.S.A., in cooperation with High Intensity Electric Fields Laboratory Cluj-Napoca; 1-high-voltage supply; 2-control panel;
3-feeder; 4-high-voltage electrodes; 5-grounded rotating roll electrode; 6-collecting boxes.
11
The research pilot plant & laboratory equipment in Figure 18 may be configured as a
corona electrostatic roll-type separator or plate-type electrostatic separator.
Four collecting compartments and three adjustable splitters assure flexibility in collecting
of separation products and adaptability to the plate-type electrostatic separator. The
electronic DC high-voltage supply is reversible (+/-) and continuously-adjustable (0 – 40) kV.
The material is introduced in the active zone of the separator assured by a vibratory feeder
with hopper or a velocity feeder. The neutralizing electrode is supplied from a high-voltage
transformer, continuously-adjustable from 0 to 12 kV, AC.
4.4. Robust control of the electrostatic separation process
A robust electrostatic separation process is characterized by insensitiveness of quantity
and quality of separation products to the variation of uncontrolled parameters. The objective
of robust control tests is determining the operation parameters, for example high-voltage
level U and roll electrode speed n, so that noise variables, such as granules size 2r and
copper content Cu[%] of the feeding material exert a minimum influence on recovery and
purity of separation products (Figure 19).
The robustness of the corona-electrostatic separation process was analyzed using
Taguchi method [17], [24]. In industrial applications of corona-electrostatic separation, the
robust control of the process avoids important reprocessing costs and the diminution of
product quality.
Figure 19. Control variables U, n and noise factors 2r, Cu[%] considered in the robustness analysis of the copper / polyvinyl chloride electrostatic separation process.
4.5. Perspectives research in high-intensity electric fields technologies
Electrostatics has a solid tradition in Romanian universities [4], [5], [8] and research
institutes [7], [11]. Maintaining and developing both fundamental and applied research in this
mediu
field require a critical mass of financial and human resources, as well as an active
cooperation with experts from different areas.
The research in electrostatic eco-technologies should be oriented in the following
directions:
• Study of fundamental phenomena in electrostatics, such as particle charging, fine
particle behavior in high intensity electric fields, corona discharge, superficial
phenomena.
• Modeling of electrostatic processes in order to shorten the time laps from laboratory
studies to industry application, and reduce the cost of new equipments and
technologies.
• Integration of electrostatic eco-technologies in the recovery flow-sheet of useful
components from electric and electronic wastes and plastic packages.
• Design of high performance equipment to increase the productivity and quality of the
recovered materials.
• Development of research devices and instruments to study the behavior of fine
particles (micronics size) in electric field.
• Research and development of devices for selective tribocharging of multi-component
granular mixtures, in the presence of uncontrolled factors (additives, material ageing,
surface state).
atmosphere, state of imponderability, vacuum).
Re-enforcement of electrostatic research is possible only by participation of Romanian
laboratories in the Seventh Frame Program of the European Union. Complexity and difficulty
of the electrostatic research require the setup of an international network including several
excellence research centers with complementary competences, but also equipment
manufacturers and electrostatic technologies users.
Acknowledgements
The authors acknowledge the contribution of their present and former PhD students Eng.
Marius Blajan, Eng. Laur Calin, Dr.Eng. Adrian Mihalcioiu to the experiments described in
this paper. Most of the work was funded by the Romanian Education and Research Ministry
and some industrial partners.
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