MOLTENI, Danilo (Via Giovanni XXIII 49, Manerbio, I-25025, IT)
1. Separator for ferromagnetic materials including a conveyor belt (1) that forms a closed loop around a magnetic roller (2) and at least one motorized return roller (3), said conveyor belt (1) being not wound directly on said magnetic roller (2) but on an idle tube (4) of non-magnetic metal inside which the magnetic roller (2) is arranged and with respect to which it can slip, characterized in that it includes means for adjusting the position of the magnetic roller (2) such that the distance of the latter from the conveyor belt (1) progressively decreases between the angular position where the conveyor belt (1) comes in contact with said idle tube (4) and the angular position where the conveyor belt (1) moves away from the idle tube (4).
2. Separator according to claim 1, characterized in that it includes means for controlling the angular velocity of the magnetic roller (2) in a range between 0% and 200% of the angular velocity of the belt (1).
3. Separator according to claim 2, characterized in that the means for controlling the angular velocity of the magnetic roller (2) consist of a motor- reducer (11).
4. Separator according to claim 2, characterized in that the means for controlling the angular velocity of the magnetic roller (2) consist of a clutch keyed on the shaft (6) of the magnetic roller (2).
5. Separator according to any of the preceding claims, characterized in that the magnetic roller (2) is supported at the end portions of its shaft (6) by bearings (5) housed in eccentric supports (7) that can be rotated and then locked in the desired position by jaws (8).
6. Separator according to the preceding claim, characterized in that the idle tube (4) is supported at its ends by annular supports (10) mounted on bearings (9) arranged on the external surface of the eccentric supports (7).
7. Method for operating a separator for ferromagnetic materials including a conveyor belt (1) that forms a closed loop around a magnetic roller (2) and at least one motorized return roller (3), said belt (1) being wound on an idle tube (4) of non-magnetic metal inside which said magnetic roller (2) is arranged and with respect to which it can slip, means being provided for adjusting the position of the magnetic roller (2), said method comprising the steps of:
a) positioning the magnetic roller (2) such that the distance of the latter from the belt (1) progressively decreases between the angular position where the belt (1) comes in contact with said idle tube (4) and the angular position where the belt (1) moves away from the idle tube (4), and
b) operating the motorized return roller (3) such that the belt (1) moves at a desired speed (Vn).
8. Method for operating a separator for ferromagnetic materials according to the preceding claim, characterized in that it further includes the step of:
c) rotating the magnetic roller (2) at an angular velocity comprised in a range between 0% and 200% of the angular velocity of the belt (1).
The present invention relates to machines for separating materials according 5 to their magnetic properties, and in particular to a separator with an eccentric
magnetic roller that is adjustable in position and speed.
It is known that a magnetic separator is designed to extract from a flow of mixed materials all those parts having magnetic permeability, so as to separate them from the rest of the inert material. A typical separator for ferromagnetic 10 materials essentially consists of a magnetic pulley, acting as driving roller, which
draws a belt that conveys a mix of materials, the belt being closed in a loop around an idle return roller.
Magnetic pulleys with different magnetic field gradient suitable to separate materials with high or low magnetic permeability are used to select the material. 15 With a low field gradient only materials with high magnetic permeability are
attracted, whereas with a high field gradient both high magnetic permeability and low magnetic permeability materials are attracted.
A drawback of known separators, in particular those with high field gradient pulley, is that the material attracted by the corresponding polarities remains 20 attached to those polarities until the conveyor belt moves away from the roller thus causing the detachment of the attracted material in a very small area. As a consequence, both low magnetic permeability and high magnetic permeability materials fall in the same area and have to be subsequently sorted.
Another drawback stems from the fact that the magnetic materials bring 25 along a portion of the inert material, since the latter remains pinched between the
inductor (the alternate polarities of the roller) and the induced (the attracted magnetic material). Therefore also in this case a further working is required to increase the quality of the selected material.
Another type of magnetic separator is the eddy current separator that is used 30 to separate non-magnetic yet electrically conductive materials such as aluminum, copper, brass, etc. In this case there is provided a magnetic roller that rotates at high speed inside a non-magnetic tube around which the conveyor belt is wound.
The rotational speed of the roller must be very high (at least 1500 rpm) to induce in the conductive materials the eddy currents that in turn due to the fast variation of the magnetic field cause a repulsion of said materials that are thus separated from the mix. Moreover, in order to achieve the maximum operational efficiency the gap between the magnetic roller and the non-magnetic tube must be as small as possible, and this can cause overheating problems due to the high relative rotational speed between the two elements.
An example of a typical eddy current separator is found in DE 3817003 CI that discloses a separator in which, in order to adjust the effective range of the alternating magnetic field generated by the magnetic roller, the position of the axis of rotation of the magnetic roller is adjusted within the quadrant of the expulsion region of the material to be sorted, i.e. in the first 90° of contact between the conveyor belt and the non-magnetic tube.
The applicant has already devised a separator for ferromagnetic materials, described in WO 2005/120714, in which the return roller acts as driving roller for the belt that is wound around an idle tube of non-magnetic material inside which a magnetic roller can rotate at a speed different from the tube speed. The structure of such a separator is thus similar to the structure of an eddy current separator but in this case the means controlling the speed of the magnetic roller rotate it in a completely different speed range, since the angular velocity of the magnetic roller is comprised in a range between 1% and 200% of the angular velocity of the belt and in any case is different from 100% of the angular velocity of the belt.
The control of the roller speed with respect to the belt speed allows to obtain a relative slip that greatly reduces the pinch effect and therefore the probability of bringing inert material along with the magnetic material. Furthermore, the controlled slip allows also to obtain an immediate selection of the materials having different magnetic permeability, by opening them fan-like in the fall area with a progressive release of materials of increasing permeability.
This separator is a great improvement over prior art separators having a conventional structure, however it still has the drawback of being unable to precisely sort out materials with little differences in magnetic permeability. In other words, the adjustment of the relative slip only is insufficient to achieve an effective separation of materials with very close magneticity. Moreover, the range of the values of magnetic permeability within which the separator can operate is limited by said adjustment of the relative slip.
Therefore the object of the present invention is to provide a separator that is free from the above-mentioned drawbacks. This object is achieved by means of a separator having a structure similar to that described in WO 2005/120714 but further comprising means for adjusting the position of the magnetic roller such that the distance of the latter from the belt progressively decreases between the angular position where the belt comes in contact with the idle tube and the angular position where the belt moves away from the idle tube, so as to obtain a progressive increase of the magnetic field and of the consequent field gradient.
The main advantage of this separator comes from the fact that the control of the eccentricity of the magnetic roller allows to optimize the separation of more or less magnetic materials even when the differences in magneticity are very small.
A second great advantage of said control is the possibility of achieving said optimization even at different speeds of the conveyor belt and therefore for different flow rates of the material to be sorted.
A third important advantage stems from the several possible combinations of the three operating parameters of the machine: eccentricity of the magnetic roller, speed of the magnetic roller and speed of the conveyor belt. These combinations allow the present separator to effectively treat a wide range of products with very different magnetic permeability, from materials with medium- high magnetic permeability (car shredding, ashes, etc.) to materials with low magnetic permeability such as the materials used in the mining sector for the extraction and concentration of hematite (Fe 2 0 3 ).
Further advantages and characteristics of the separator according to the present invention will be clear to those skilled in the art from the following detailed description of an embodiment thereof, with reference to the annexed drawings wherein: Fig.l is a diagrammatic longitudinal sectional view showing the general structure of the separator;
Fig.2 is a diagrammatic longitudinal sectional view showing how the forces act on the material to be sorted and the resulting effect of separation and selection of the material achieved by the present separator;
Fig.3 is a partially sectional front view of the arrangement of the magnetic roller inside the idle tube;
Fig.4 is a side view of the elements of Fig.3; and
Fig.5 is a diagrammatic perspective view of the magnetic roller.
Referring to figs.l and 2, there is seen that a magnetic separator according to the present invention conventionally includes a conveyor belt 1 that forms a closed loop around a magnetic roller 2 and a return roller 3 to convey a mix of materials. In said mix the magnetic properties of the materials have been graphically indicated as follows: the star for low magnetic permeability material, the circle for medium magnetic permeability material, and the triangle for high magnetic permeability material.
As previously mentioned, this separator for ferromagnetic materials has a structure similar to the separator disclosed in WO 2005/120714: belt 1 is not driven by roller 2 but by the return roller 3 that is motorized, and it is not wound directly on roller 2 but on an idle tube 4 of non-magnetic metal (e.g. stainless steel) inside which roller 2 is arranged.
The novel aspect of the present invention is given by the presence of means for adjusting the position of the magnetic roller 2 inside the idle tube 4 such that belt 1 comes progressively closer to the surface of the magnetic roller 2 during its path along the 180° arc in which it is in contact with the idle tube 4.
The forces acting on the material to be sorted are illustrated in the diagram of Fig.2 as follows:
Fc = centrifugal force
Fa = attraction force
Fg = gravity force
R = resultant Among the three forces above that combine into resultant R, two are constant along the arc of contact of belt 1 with tube 4 whereas the third one progressively increases. In fact, the gravity force Fg is obviously constant in magnitude and direction, since it depends only on the mass of the material, and the centrifugal force Fc is always perpendicular to the surface of tube 4 oriented outwards as direction and it is constant in magnitude since it depends only on the speed Vn of belt 1.
On the contrary, the magnetic attraction force Fa is always perpendicular to the surface of tube 4 oriented inwards as direction but varies in magnitude along the path since it is directly proportional to the product of the magnetic field intensity by the magnetic field gradient. It should be noted that said product is not the magnetic attraction force, since the latter also depends on the intrinsic magnetic induction in the attracted material which in turn depends also on the shape factor of the piece, but it is the portion that depends only on the parameters of the separator and not on those of the treated material. Therefore with the progressive decrease of the distance between the surface of the magnetic roller 2 and the surface of belt 1 said product can increase from a few tens of thousands of Oe 2 /cm to over 20 millions of OeVcm, whereby the separator is capable to effectively operate on adjacent materials in a very wide range of magnetic permeability.
It should be noted that Fa depends also on other factors, namely the relative slip between the magnetic roller 2 and belt 1 and the shape factor of the single piece of ferromagnetic material, but once all the three operating parameters of the separator (Vn, position and speed of the magnetic roller 2) are set the variation of resultant R acting on each piece of material depends only on the position that the piece has reached along its path around the magnetic roller 2.
In the example illustrated in Fig. 2 there is seen that for the low magnetic permeability material indicated by the star the centrifugal force is predominant and rapidly causes it to move away from belt 1 along a tangent after an arc of about 45°, for the medium magnetic permeability material indicated by the circle the centrifugal force overcomes the magnetic attraction force after an arc of about 90° and the material falls almost vertically, and for the high magnetic permeability material indicated by the triangle the magnetic attraction force is so strong that it overcomes the centrifugal force and the gravity force, whereby the material moves away from belt 1 only after an arc of about 180°.
As illustrated in figures 3 and 4, the magnetic roller 2 is supported at the end portions of its shaft 6 by bearings 5 housed in eccentric supports 7 that can be rotated and then locked in the desired position by jaws 8. In this way, the reference axes A-A of the eccentric supports 7 can be rotated ±90° with respect to the vertical to determine the result of the magnetic action of roller 2 in the operating region according to the diagram of Fig.2.
The idle tube 4 of non-magnetic metal is in turn supported at its ends by annular supports 10 mounted on bearings 9 arranged on the external surface of the eccentric supports 7. These bearings 9 therefore support the idle tube 4 and allow it to rotate on its fixed axis, driven by belt 1, independently of the magnetic roller 2 whose speed is controlled by a motor-reducer 11, or the like, such that the angular velocity of the magnetic roller 2 is comprised between 0% and 200% of the angular velocity of belt 1. In the case when the angular velocity of the magnetic roller 2 is the same as that of the idle tube 4 and therefore of belt 1 there will be no relative slip, whereas if said angular velocity is different from 100% the difference in velocity results in a relative slip between roller 2 and tube 4.
The aim of this difference is that of obtaining two surfaces with a relative slip and therefore two different speeds whereby the attracted material, during the path defined by the 180° of tangency to the magnetic area, due to the backing or advancing of the magnetic polarities tends to rotate backward or forward with respect to the travel direction of the belt. This effect on the attracted material favors the above-mentioned progressive release of materials with increasing permeability, with a fan-like detachment that leads them to fall into distinct fall areas, without the pinch effect caused by materials with higher magnetic permeability affecting the fall area.
It should be noted that although the preferred embodiment provides the use of motor-reducer 11 to control the speed of roller 2, said speed can also be controlled (though over a smaller speed range) simply by means of a clutch keyed on shaft 6 of roller 2. In fact, in the absence of motor-reducer 11, the passage itself of ferromagnetic material on belt 1 tends to draw into rotation roller 2 that being idle only has the rotational friction of bearings 5, once the initial inertia is overcome.
This is obviously possible only if the mix of materials to be sorted has a sufficient concentration of ferromagnetic material, whereas if the concentration is low or the present material has low magnetic permeability roller 2 could be totally void of drive or clutch means since the friction of bearings 5 and/or its inertia is sufficient to keep its speed below the speed of belt 1.
Clearly in these two instances the speed of roller 2 can only be lower than that of belt 1, but in general also with the motor-reducer 11 is it preferable to rotate roller 2 at a speed lower than belt 1 even if the motor driving can allow it to rotate at a higher speed whenever this is useful for a more effective selection of the materials.
Regardless of the type of roller 2 used (motor-driven, clutched or idle), it is preferably made up of a ferromagnetic cylinder 12 whose external surface carries longitudinal rows of parallelepipedal permanent magnets made of rare earths (e.g. Iron-Boron-Neodymium) arranged with alternate North-South polarities in the circumferential direction as illustrated in Fig.5.
In another possible embodiment, not illustrated, the magnets are arranged in a chequered pattern, i.e. with the North-South polarities alternating also in the longitudinal direction in a same row, but this is not preferred since the variation of the field gradient also in the longitudinal direction may render the separation of the material along the width of belt 1 less homogeneous.
It should be noted that although in the illustrated example the eccentricity of the magnetic roller 2 with respect to the idle tube 4 is only in the vertical direction, it would also be possible to provide an eccentricity in the horizontal direction. Moreover, although the rows of magnets are in general arranged along the whole circumference of the ferromagnetic cylinder 12, the magnetic roller 2 could also stand still for the separation of materials with high and very high magnetic permeability whereby the rows of magnets could even be arranged only along an arc of 180°-210°.
In practice, the separator according to the present invention can treat a mix of ferromagnetic materials obtained from the above-mentioned known separator disclosed in WO 2005/120714 and through a correct trimming of its operating parameters it can achieve a precise separation of said materials. In fact, the present separator can remove from the belt along a natural trajectory the material with lower magnetic permeability such as rust and inerts full of iron powder (star), then it drops almost vertically the pieces of ferromagnetic steel physically conglobated in inert pieces (circle) that would end up polluting the marketable steel and finally, in the recovery region, the marketable steel (triangle) for which the magnetic attraction force prevails in the sum of forces.
Thanks to this novel separator the percentage of recovery and concentration of marketable steel is very high, indicatively greater than 90%, and for this type of material the magnetic roller 2 preferably stands still whereby the motor-reducer 11 may be absent and the magnets may even cover a limited arc of cylinder 12, e.g. 210° as mentioned above. Moreover, the variation of the product of the magnetic field intensity by the magnetic field gradient is preferably comprised between 50.000 Oe7cm and 6.000.000 Oe cm.
At the opposite end, the same separator can operate to separate and concentrate hematite (Fe 2 0 3 ) which in nature is found in the form of loam concentrated at 60÷70% mixed with 25÷30% of silicon oxide (Si0 2 ), the rest being aluminium oxide (A1 2 0 3 ), manganese oxide and other oxides. To make hematite usable for the production of cast iron without employing blast furnaces it is necessary to reach a concentration around 90%, which is presently obtained starting from very rare and expensive minerals in which hematite is already present at 75÷80% and employing large magnetic separators (e.g. the "Ferrous Wheel" models from Eriez of Erie, PA-USA) which require the use of water to separate the concentrated hematite from the rest of the material.
The present invention allows to overcome said limitations of known separators and to perform a dry operation starting from loam with a lower concentration, thus making the operation simpler and cheaper. In this case the magnetic roller 2 is completely covered with magnets and is rotated at an angular velocity that can be identical with or very close to the angular velocity of belt 1. The magnetic attraction force in this case varies in the range around 3÷23 * 10 6 OeVcm and therefore is capable of acting on materials with low and very low magnetic permeability.
In the light of the above it is clear that the materials with medium-high magnetic permeability are preferably separated at an angular velocity of the magnetic roller 2 close to 0% of the speed of belt 1 , whereas the materials with low-very low magnetic permeability are preferably separated at an angular velocity of the magnetic roller 2 close to 100% of the speed of belt 1. As a consequence, the materials with an intermediate magnetic permeability are preferably separated at an angular velocity of the magnetic roller 2 proportionally intermediate between 0% and 100% of the speed of belt 1, but it is possible to foresee the use of speeds >100% up to a maximum of 200% in some particular cases.
It is clear that the above-described and illustrated embodiments of the magnetic separator according to the invention are just examples susceptible of various modifications. In particular, roller 2 is preferably of the permanent magnets type and it can be made with magnets of different nature and with different magnetic circuits, but it could also be of the electromagnetic type.
Similarly, belt 1 , tube 4 and the driving roller 3 can be modified according to specific manufacturing needs, and more than one return roller can be provided depending on the shape and/or length of belt 1.