AN INVESTIGATION INTO TRAMP IRON DETECTION AND SEPARATION

V. Mason and S. R . Reichard Sugar Research Institute, Mackay, Queensland, Australia

This paper describes the work undertaken to evaluate the use of electro- magnetic separators for the collection of tram iron, and the use of eddy current detectors to detect the presence of metallic objects on a high speed conveyor belt. The tests showed that magnetic separators could operate very efficiently if the material was presented to them in the correct way. The eddy current detectors were found to lack sufficient discrimina- tion of the mass of the tramp iron to be useful to detect all types of tramp iron and it is recommended that their use be limited to the detection of very large pieces of tramp iron that would cause significant damage.

INTRODUCTION

The first stage of processing sugar cane in a raw sugar factory is to pass the cane through a "shredder". During the early 1 970's, it was shown that tramp iron coming into the raw sugar factory with the cane supply was the prime cause of failure in the hammer tips (Mason, Martin I ) . Consequently, in Australia large electromagnetic tramp iron separators begans to be fitted immediately prior to the shredder. The work described in t-his paper was undertaken to investigate the separation ability of large electromagnets with a supplementary investigation of the use of metal detectors. The magnets were evaluated for use in conjunction with a high speed rubber belt conveyor or as part of a cane chute delivery system with the cane in more or less free fall. The metal detectors were only examined in conjunction with a conveyor belt.

MAGNETIC SEPARATION

Tests were conducted with two magnets, an Eriez 2 400 series chute magnet and a Mecal TMG-24-5 magnet. The magnetic field of both magnets was exami- ned in-situ in the sugar mill. Their size and magnetic field characteristics were very similar so that only the Eriez magnet was used for separation tests in an experimental rig and the results presented below are primarily for this mag- net. The Eriez magnet has a face 1 7 2 0 mm high and 2 230 mm wide with a wedge protruding 100 mm from the magnet face designed to provide shelter for any tramp iron adhering to the face of the magnet (see Fig. 2, for example).

Characteristics of the Magnetic Field

Figures 1 and 2 illustrate the magnetic field of the magnet. Figures l (a ) and (b) show the magnetic field strength up the face of the magnet, while Fig. l(c)

0 500 1 000 1 500 2 000 2 500

Magnitude of magnetic flux (tesla x 1 04)

Dlstance up front of magnet (mm)

0 500 1 000 1 500 2 000 2 500

Magnitude of magnetlc flux (tesla x 1 04)

(b)

Figure 1 (a) and (b). Magnetic f ~ e l d strenght in front of the magnet. (a) i n the plane throught the vertical centreline; (b) in the ver t~ca l plane 650 mm to r ~ g h t of c e n t r e h e .

(ww) 1auBew 40 a3e4 ssome uo!i!sod

shows the field across the width of the magnet at approximately the position of the strongest field strength. These figures show only the magnitude of the magnetic 1 flux density. The direction of that flux is shown in Fig. 2. If a piece of magnetic material was able to move freely under the action of magnetic forces alone, then Fig. 2 indicates the direction of travel.

Magnet

IY - C - q /

/ / I \ \ - - / , / I \ \ - - - / I - C

, / I \ \ \ - . - ,

I

/ / 1 I \ $ \ - / / I \ \ - 0 / / \ \ \ . \

0 0 1 / I \ \ \ \ (b)

Figure 2. Direction of the magnettc f ~ e l d : (a) in a plane through the ~ ' e r t ~ c a l cen t re l~ne : (b) in the horizontal plane 400 mm up the face of the magnet .

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Although the magnetic flux density is relatively easy to measure, it does not 'five an indication of the magnet's ability to attract tramp iron. In fact, a unifor- me magnetic field, however strong, will not exert any force on a small piece of magnetic material placed in it. The force on a piece of material is proportional to t l e product of the magnetic flux density (B) and the rate of change of that flux density away from the face of the magnet (dB/dx). The constant of proportionali- ty depends on the magnetic properties of the material as well as its volume and shape. A sphere of a given volume will be attracted less strongly than any other object made of the same material and having the same volume, but a relatively larger surface area than the sphere. Thus spheres and other compact shapes are the most difficult objects to attract to a magnet. Nevertheless, the quantity B (dB/dx) gives a useful indication of a magnet's separating power. Eriez Magne- tic Pty. Ltd. call this quantity the "force index" of the magnet. Ideally, it should be calculated along the path a piece of magnetic material would follow on its way to the magnet. In Fig. 3 this has been done for the Eriez and Mecal magnets along the line in the central vertical plane where the flux density is directly towards the face of the magnet. A comparison of the curves shows that the field strength and force indices for the two magnets are very similar at these particular

1 I I - - - - -v

- - Force index / - -

-

- - - - - - - - - - -

\ L - Eriez magnet - 600 mm from bottom O. I

- - -- - Mecal magnet - 700 mm from bottom

I I I

0 290 400 600 800

Distance from face of magnet (mrn)

Figure 3. Field strcngth and force ~ n d e x curves on the magnet centreline.

positions. It will be noticed that the field strength and for& indices decay appro- ximately, exponentially, with distance from the magnet and hence plot as straight lines on the semi-log paper used for Fig. 3. However, the main conclusion from this portion of the investigation is that the proportion of the face of the magnet over which the magnetic attraction is large and directly towards the face of the magnet is quite restricted. Thus in the vertical direction, the magnetic attraction is strong and towards the magnet over only about a third of the face [see Figs. l ( a ) and 2(a)] while in the horizontal direction the effective width is about 60 per cent of the physical width.

Experimental Investigation of Magnetic Separation

The only definitive way to determine with certainty a magnet's ability to attract tramp iron is to experimentally determine the conditions under which it will collect that iron. Hence a test rig was constructed so that the magnet could be held in a variety of positions relative to a conveyor belt. The 1,07 m wide conveyor belt was run at either slow speed (about 0,2 mls) to simulate the typical sugar mill slow speed cane supply system or at high speed (about 2,15 m/s). Billets of cane could be placed over the tramp iron on the belt to test the effect this would have. The standard burden depth used for the high speed belt was 250 mm.

The "tramp iron" specimens used in these experiments are listed in Table 1. The short lengths of cylindrical bar (objects 1 and 5) were the most difficult of the nine objects to attract to the magnet because they are nearest to spherical shape.

Table I. Steel Tests Objects

Object Number

Description of Object Mass

25 mm dia, mild steel bar 30 mm long 100 g 0,75 inch Whitworth bolt 57 mm long 265 g 1,O inch Whitworth bolt 89 mm long 480 g 1,25 inch Whitwortb bolt 89 mm long with nut 1,06 kg 82 mm dia, mild steel bar 100 mm long 4,37 kg 82 mm dia, mild steel bar 185 mm long 7,82 kg 132 mm O . D . 69 mm I.D. hollow bar 308 mm long 24,15 kg 610 mm length of rail line 18,03 kg 360 mm dia. dished, two hoie rail wheel 40,05 kg

Free Fall Attachment Boundaries

One essential requirement when designing a magnetic separator layout is to know how close to the magnet a piece of tramp iron must be to ensure that it is caught by the magnet. One such indication is obtained by dropping a small piece of tramp iron from a stationary position in front of the magnet to determine the position from which the magnet just holds the iron. These positions then define an "attachment boundari" in front of the magnet. These boundaries were deter- mined for the Eriez magnet with its face vertical and sloping back at 15" and 30" from the vertical using the small short cylindrical steel specimen (object 1, Table 1).

97 1

The boundaries were determined both in the vertical centreline plane and in a plane 900 mm to one side of the centreline plane. The resulting boundaries for magnet angles of zero and 30 degrees from the vertical are shown in Figs. 4(a) and (b). They show:

1. that the boundaries are much further from the magnet on the vertical centreli- ne plane than towards the sides of the magnet, and

, Attachment boundary

\

Magnet

Attachment boundary

at 900 mm to left of centreline

Distance from face of magnet (mm)

(a)

Figure 4 (a). Attachment boundaries in front of the magnet: (a) magnet face vertical.

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Attachment boundary

at magnet centreline

Attachment boundary

I I at 900 mm to left of 1 centreline

I1 "1 I I 1 I I 1

, -800 -400 0 400 r

D~stance in front of magnet (mm) I 1 1

(b)

I Figure 4 (b). Attachment boundar~es In front of the magnet: (b) magnet face t~ l ted back 30 degrees

" from vertical.

1

t

2. that the boundaries are further from the magnet as the magnet face is leant backwards. This effect is most pronounced at the centre, but is present to a lesser extent towards the sides of the magnet.

Several additional comments are required concerning these boundaries:

3 . The attachment boundaries would move further from the face of the magnet for most common shapes of tramp iron since they would be more easily attracted to the magnet than the short steel cylinders.

4. The attachment boundaries would move closer to the magnet when billet cane or other material on the belt was present to hinder the motion of the tramp iron towards the magnet. Unfortunately, the magnitude of this effect could not be determined experimentally.

5 . Experiments with a 40 kg rail wheel (object 9), with the magnet face sloping back at 30" to the vertical, showed that it would be held by the magnet even if it was at the side of the magnet, SO long as it was sliding down the face of the magnet. When the wheel was at one side of the magnet, any tendency for the wheel to bounce on the face of the magnet, reduced the magnet's ability to retain it.

6. All objects dropped at one side of the magnet, moved towards the centre of the magnet and were held on the lower central area of the magnet.

Experiments With a High Speed Conveyor

Four different beltlseparator configurations were investigated as described be. low. In each case when a cane burden was placed on the belt, it had a central depth of about 250 mm and the tramp iron pieces were placed under the belt. 11 should be noted that an inherent advantage of using a troughed rubber belt is thal the material concentrates towards the centre of the belt and is, therefore, directec mainly towards the centre of the magnet where the magnetic effect is greatest.

1) Belt Discharge Material Directed at the Face of Magnet

If the magnet was arranged so that the discharge from the head of the higk speed belt hit the face of the magnet and the magnet was placed reasonably ~ 1 0 s ~ to the discharge point, then there was found to be a very high probability of a1 tramp iron adhering to the magnet. In the tests, the magnet face was eithel vertical or sloping back 20 degrees from the vertical, and the discharge materia hit the face of the magnet between 600 mm and 1 400 mm above the bottom edge of the magnet. The maximum gap between the nearest point on the belt and tht face of the magnet was 1 060 mm. Under these conditions, all tramp iron objectl were held on the magnet in every test. Since this configuration appears to givf very efficient collection, complete details of the tests are given in Appendix A , I appears that the success of this configuration relies on two aspects:

1. All material is being projected directly towards the face of the magnet, 2. When the material hits the face of the magnet, it is forced to break up an(

rearrange itself. In doing so, it gives the tramp iron a chance to work its way through the material and adhere to the magnet.

Note that if the feed material was allowed to slide down a feed chute whicl directed it onto the face of the magnet, then the effective cane feed configuratiol

is the same as the high speed belt configuration described above. The tramp iron collection efficiency should, therefore, be just as high although the penalty paid is the increased complexity of the required material feed system.

2) Magnet Used as Bottom Floor of Belt Discharge Chute

In this configuration, the material leaving the belt head pulley was directed into a chute, the floor of which was the face of the magnet. The angle of the face of the magnet was such that the face was as near as possible tangential to the flow of material at the point of impact with the magnet. Test rig limitations allowed

. only one test configuration to be evaluated but this gave complete collection of the iron. It is, therefore, felt that this is another magnetlbelt configuration that is worthy of consideration as long as:

1. the material being carried by the belt is in a thin, loose, free moving form so that the tramp iron can move throug it to the face of the magnet, and

2, the material is not moving over the face of the magnet at too high a speed. This implies that the magnet should be as close to the head pulley as possible and that the face of the magnet should not be too steep.

(Appendix B gives details of the test configuration evaluated.)

3) Magnet Over the Discharge From the Bell

The magnet was placed over the belt material as it left the head of the head pulley of the high speed belt. The material subjected to the influence of the magnet was largely in free flight, although the top of the face of the magnet was over the head pulley to a limited extent.

This was not a successful configuration. Those arrangements which most near- ly collected all the iron required the magnet to be close to the head pulley. As a result, a large piece of tramp iron might possibly have jammed between the magnet and the head pulley. In no configuration was it possible to collect all the small 25 mm diameter steel specimen (item 1, Table 1) through the mat of cane. However, it must be mentioned that the magnet used was designed, prima- rily, for use in a chute and it is possible that a magnet designed to have a magnetic field suitable for use in this configuration would have greater separation efficiency.

4) Magnet Suspended Over the Belt

In this arrangement, the idlers were removed from a section of the belt to flatten its profile and the magnet held over the belt. Even with the magnet touching the top of the cane burden on the belt, it was not possible to pull the smaller tramp iron pieces through the mat of cane onto the magnet. Unless the material on the belt offers very little resistance to the movement of the iron through it, then this is a totally unacceptable magnet configuration.

Experiments With a Slow Speed Conveyor

In the Australian sugaL industry the usual cane feed conveyor to the shredder is srow moving with up t o a metre depth of cane on .it. A slowly rotating set of

I

kicker knives at the head of this conveyor then kicks the cane at &ady rate into the shredder feed chute. The simplest place to put a magnet on most existing mills is on the wall of the feed chute facing the head of the conveyor. With this situation, most of the tramp iron will be right on the aonveyor surface and hence will drop off the end of the conveyor under gravity. From a tramp iron collection view point, this situaton is, therefore, similar to the free fall attachment boundary problem discussed earlier. For satisfactory operation, the head pulley should, therefore, be quite close to the magnet. However, the slow speed conveyors normally used in sugar factories have stell slat boards which retain tramp iron as they pass round the head pulley. If the head pulley is close to the magnet, this not only makes induced magnetism greater but also tends to cause problems with long thin items (e.g. item 6, Table 1) which can jam between magnet and head pulley because such items try to align themselves with the magnetic field between the magnet and conveyor boards. The practical solution to this problem is to fit, either a stationary scraper board, or a revolving kicker drum just under the head pulley to dislodge iron adhering to the board and project it towards the face of the magnet. Of these two options, the rotating kicker was easily the most successful and enabled clearance distances between the head pulley or kicker and magnet face of up to 900 mm, while still retaining all tramp iron on the magnet.

Details of the tests, with and without the rotary kicker, are given in Appen- dix C.

METAL DETECTORS

The use of a metal detector is an alternative approach to the protection of a sugar mill from the effects of tramp iron. The main disadvantage with their use is that they do not remove the offending material from the cane supply. The actual removal would have to be manual. The resultant expenditure of time and manpo- wer could be quite considerable and tould only be justified if the material detected and removed would have caused and expensive amountof damage. Hence, the object could be detected, but rather towards the detection of pieces weighing 200 g or more.

A survey of readily available detector units was undertaken. As a result it was decided that a unit with a single detection coil located under the high speed belt was probably adequate for the sugar industry's needs and a typical unit of this type was made available for the investigation.

Detector Evaluation

Field Strength (Sensitivity) Pattern

Since the detector under test only had one sensing loop the response of the unit had to be related to the strength of the electromagnetic field around the loop. Plotting a map of the field strength was, therefore, chosen in preference to the tedium of exploring the trip sensitivity at various locations within the detection region. The field was measured at various locations using a small ferrite rod aerial 12 mm diameter per 12,5 mm long wound with a coil of 2,42 mH connected in parallel with a tuning capacitor of 0,08 F. The amplitude of the rms voltage of the 11,5 kHz detector signal was measured using a millivoltmeter. The ferrite

search coil was mounted on a small lenght of dry timber as handle. Each measu- rement was taken with the search coil rotated until a maximum reading for that location was obtained and, therefore, represents the absolute value of the field strenght at any given location. i

The map of the field strengths thus measured is plotted in Fig. 5 . The field can be seen to be very evenly distributed over the width of the belt, thus giving a uniform sensitivity across the width. The sensitivity is greatest close to the belt (and the detector loop) and decreases towards the top of the burden region. If a second detector coil were fitted above the burden, that region could be covered with increased sensitivity.

Tripping Levels Relative to Size, Shape and Type of Tramp Metal

Checks with a piece of tramp iron showed that its detection (i.e. tripping) sensitivity was directly related to the field strength for a given shape and size of objetc. Table 2 shows the maximum trip distance above the belt and the field strength at that point for five solid steel cylinders of similar shape.

Table 2. Relationship Between Maximum Trip Distance and Field Strength for Different Sizes of Steel Cylinders

Cylinder D~mensions Maximum Trip Distance Field

Cylinder Above Belt Strenght

Number Diameter Lenght (mm)

(mm) (mm)

( m v )

In practice, the effect of shape, attitude and material also has a major effect on its detectability. Table 3 shows the trip distance above the belt for a variety of pieces of tram iron. The fact that a cast iron pulley (2,2 kg) is detected at about the same distance as a flat metal drink can (64 g) illustrates the difficulties of "tuning" a system for a specific size of tramp metal. Detectors are, therefore, likely to become a nuisance by causing unwanted alarms from large but flimsy metal objects.

The final major finding of the investigation was the sensitivity of the detector to electrical interference. The unit was found to trip:

1. if a small hand tool was switched on and off in the immediate vicinity of the unit;

2. if the unit was left switched on , but unattended overnight (this occurred on five out of six nights, the cause being unknown);

3. kif lightning strikes occurred within visible range of the tesdt site.

There is, therefore, no doubt that such detectors would require the same precautions in power supply installation as solid stage logic equipment, compu- ters, etc.

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Table 3. Results of Tests With Different Shapes and Types of Tramp Metal

Dimensions Maximum Trip Tramp Metal Diameter Length Distance

Mild steel cylinder (solid) Mild steel piece of pipe Brass cylinder (solid) Cast iron pulley' Aluminium pulley Steel cylinder (on end) (solid) Steel drink can (on end) Steel drink casn (lying flat) Aluminium can (lying flat) Aluminium can (on end) Stell drink can hammered flat Mild steel nut (hole vertical), (1" Whit) Mild steel cylinder (solid, flat down)

CONCLUSIONS

These tests have shown that tramp iron of all shapes and sizes can be successfu- lly separated from a cane supply, particularly if a high speed belt is used to deliver the cane to the magnet. However, with metal detectors the situation is not so good, since their ability to discriminate the mass of a piece of tramp iron is quite poor. Their use must, therefore, be relegated to the detection of large pieces of tramp iron that could cause major damage to equipment. This being so, the ideal position for a metal detector would be after a tramp iron magnet as an additional assurance that no large pieces of tramp metal (whether of magnetic or non- magnetic material) had passed the magnet.

REFERENCES

1. Martin, L. G. (1977): Magnetic Collection of Tramp Iron. Proc. Qld. Soc. Sugar Cane Technol.,

44th Conference, pp. 251-254. 2. Mason, V. (1977): Implications of Recent Investigations of Sheredder Hammer Tip Materials.

Proc. Qld. Soc. Sugar Cane Technol., 44th Conference, pp. 255-259.

APPENDIX A. HIGH SPEED BELT DISCHARGE DIRECTED AT THE FACE OF THE MAGNET

Figure A1 shows the magnetlbelt configuration used in these tests, while Table A1 list the individual test results.

Figure A l . Magnetlbelt configuration (belt speed approximately 2.1 5 mls).

Table AI. Test Results for Configuration Shown in Figure A 1

d h With Items Not Held Free Fall Impact Point

m a Cane on Magnet ' on Face of Magnet

965 1 620 965 90" No ' None 600 mm above bottom 1 025 1 620 1 025 90" No (5) Tip of wedge

910 1200 910 90" No (11, (5) Bottom of magnet 990 1200 990 90" No (11, (5) Missing bottom of magnet

1 100 1200 1 100 90" No 1, 5, 9 Missing bottom of magnet 360 2.170 1 060 70" No None 1400 mm above bottom 360 2 170 1060 70" Yes None 1400 mm above bottom 130 1980 780 70" No None 1500 mm above bottom 130 1980 780 70" Yes None 1500 mm above bottom 180 1 600 600 70" No None 1200 mm above bottom 180 1 600 600 70" Yes None 1200 mm above bottom 360 1570 860 70" No None 1000 mm above bottom 360 1 570 860 70" Yes None 1000 mm above bottom 340 1200 710 70" No None 800 mm above bottom 340 1 200 710 70" Yes None 800 mm above bottom

' S e e Table 1 for item identification numbers. Brackets indicate that some items adhered to magnet.

APPENDIX B. MAGNET USED AS FLOOR OF CHUTE AT DISCHARGE FROM A HIGH SPEED BELT

Figure B1 shows the test configuration used in these tests, while Table B1 tabulates the results.

Figure B1. Magnethell configurat~on (belt speed approx~mately 2.15 m/s).

Table Bl Test Results for C o n f ~ g u r a t ~ o n Shown In F ~ g u r e B1

W ~ t h Items, Not Held h d a Comments

Cane on Magnet '

480 500 60" No None R I ~ l i m ~ t a t ~ o n s prevented inves- tlgatlon of other posltlon.

480 500 60" Yes None

' See Table 1 for Item ~ d e n t ~ f i c a t ~ o n numbers. Brackets ~ n d ~ c a t e that some Items adhered to magnet

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' ' > - . a, i ,,,. C 1 f, 3 , .

F* J

APPENDIX C. TRIALS WITH A SLOW SPEED BELT I

Figure C1. ~ a ~ n k t l b e l t configuration (slow speed belt]

Table C1. Test Results for Configuration Shown in Figure C1

The most significant results from the tests with a slow speed belt (simulating a typical sugar mill slat conveyor) are presented here.

d h m a With Items Not Held Can,e on Magnet '

I

Comments

400 1 200 400 90" No 500 1 200 500 90" No 500 1 200 500 90" Yes 600 1 200 600 90" No 600 1 200 600 90" Yes 700 1 2 0 0 700 90" No

400 1 600 790 75" No 495 1 600 890 75" No 495 1 600 890 75" Yes

None Distance m ~mprac t~ca l ly small

(1). (5) 1 1. 5, (6) 1. 5 1 2. (3). ( 4 , D ~ s t a n c e d defln~tely too 5 . 6. 7. (9) great 1. (5). (6) 1. (5) 1. 2. 5. 6 1. 2. 3. 4. 5 . Distance d definitely too 6 . 7. (9) great (1). 5 1. 5 . (7). 9 1. 5 . 9

' See Table 1 for item identification numbers. Brackets indicate that some items adhered to magnet.

Figure C2. Magnet belt configuration with a kicker roll (slow speed belt).

Table C2. Test Results for Configuration Shown in Figure C2

With Items Not Held d h m d' m' a

Cane b n Magnet ' Comments

1 140 1 850 1 140 1 000 800 90" No (1). (51, (9) Marginal 885 1600 885 765 565 90" No None

1000 1600 1000 880 680 90" No None 980 1 300 980 880 680 ,90" No Most Items Too low and too far away

' See Table 1 for item identification numbers. Brackets indicate that some items adhered to magnet. Zero belt speed. Items released manually from position close to belt just above kicker roller.

RECHERCHE SUR LA DETECTION ET LA SEPARATION DES DEBRIS DE FER

V. Mason et S. R. Reichard Institut de recherches sucrikres, Mackay, Queensland, Australie

L'exposC rksume les travaux realisks pour evaluer l'emploi des skpara- teurs Clectromagn6tiques destines a retenir les debris de fer, et l'utilisation des dktecteurs,,,h courant de Foucault por deceler la presence d'objets mktalliques dans un conducteur grande vitesse. Les tests ont montre que les skparateurs magnktiques peuvent s'avkrer trks efficaces si la matikre

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leur est presentee correctement. Les dktecteurs 2 courant de Foucault ne sont pas ti msme de deceler toutes sortes de debris. Les auteurs recomrnan- dent donc de limiter leur utilisation a la detection de debris assez grands qui causeraient des dommages importants.

INVESTIGACI~N SOBRE LA DETECCION Y SEPARACI~N DE FRAGMENTOS DE HIERRO

V. Mason y S. R. Reichard Instituto de Investigaciones Azucareras, Mackay, Queensland, Australia

RESUMEN

La presente ponencia describe el trabajo realizado con el objetivo de evaluar el uso de separadores electrornagn6ticos para atrapar pedazos de hierro y la utilizacion de detectores por corriente parasita para descubrir la presencia de objetos metdlicos sobre un conductor de alta velocidad. Los resultados de las pruebas mostraron que 10s separadores rnagn6ticos son capaces de funcionar con gran eficacia si el material se les presenta en la forma correcta. En cuanto a 10s detectores por corriente parasita, se lleg6 a la conclusi6n de que 10s misrnos carecen ~ d e la suficiente capacidad de discriminaci6n frente a la rnasa de hierro para ser considerados tiles en la detection de todos 10s tipos de hierro, por lo cual su uso queda limitado a la detecci6n de 10s pedazos muy grandes, que pudieran ocasionar daiios de consideraci6n.