IT202100015629A1 - MILL FOR ULTRA-FINE GRINDING OF MATERIALS - Google Patents

Description

MILL FOR ULTRA-FINE GRINDING OF MATERIALS

TECHNICAL FIELD

The present invention relates to the sector of mills for ultrafine grinding of materials, for example minerals, organic waste, municipal solid waste or waste comprising halogenated polymers.

STATE OF ART

The use of ultrafine grinding mills ? known in various fields, and finds application in research laboratories and industrial plants. By way of example, ultra-fine grinding, or ultra-grinding, is a process used in the mining sector, in the pharmaceutical sector, in powder metallurgy, in the treatment of municipal solid waste or polymeric waste, even when these include halogenated polymers such as for example polyvinyl chloride (PVC) or polytetrafluoroethylene (PTFE). By ultrafine grinding is meant here and below the grinding of the material until obtaining a powder with particle size lower than 50 µm, typically lower than 10 µm.

In the following discussion, unless otherwise specified, with the term ?mill? it means a machine specifically suitable for obtaining the ultra-fine grinding of the material, regardless of the type and use. Will the machinery intended for coarse crushing be called more? specific, e.g. knife mill, hammer mill, jaw mill, etc.

In ultrafine grinding, the mill acts as a mechanochemical reactor and is able to determine modifications of the molecular structure of the treated material through the application of mechanical energy. In traditional procedures and more? widely known, the energy used to induce some specific chemical reactions ? supplied by energy in the form of thermal, light, or radio frequency electromagnetic radiation. In more times recent ? emerged that also? mechanical energy ? equally suitable for activating chemical reactions. For example, the application of ultrasound (?Sonochemistry?) in polymerization experiments generates products that cannot be made using usual forms of activation, such as UV and thermal light radiation. At the same time, the mechanical action of very high frequency vibrations, aimed at the individual molecules, is able to produce the breaking of even complex chemical bonds.

One of the effects in the induction of reactions by means of mechanical energy? the macroscopic destruction of the material. When heterogeneous particles are made to react in a mechanochemical reactor (for example a mill) various phenomena are obtained, such as for example reduction, oxidation or solid state transfer of ions. As an example, the use of ultrafine grinding ? was designed for the disposal of PVC waste. In the PVC structure, the chlorine is linked to the polymer chain through very strong covalent bonds. Traditionally, the only action used to detach chlorine atoms is? the thermal action of dereticulation, which occurs between 300?C and 500?C. The study of mechanochemistry has revealed that the same result can also be obtained by ultrafine grinding, for example in ball mills. In these grindings the molecular chains are gradually damaged, the chlorine is pushed out of the damaged structure and acts as a radical, conjugating with the cations available in the mass being treated. For example, in the mechanochemical tests on PVC carried out by various authors, yes ? obtained the formation of calcium chloride starting from the reaction between calcium carbonate and chlorine radical.

Another effect that ? possible to obtain with a mill for ultrafine grinding ? that of making quartz amorphous, for example in order to obtain silica (SiO2) for the production of glass.

Other effects that are commonly obtained with ultrafine grinding mills are the extraction of water and the elimination of the bacterial load from the treated mass. Such effects are particularly useful, for example in the treatment of municipal solid waste.

The mills of the known type suitable for obtaining the ultra-fine grinding of the material can take on different shapes but usually have in common a structure which includes a container (commonly called jar) inside which defined a grinding volume. A percentage of the grinding volume ? occupied by grinding elements, typically spherical in shape, for example made of steel, tungsten carbide, alumina or zirconia. To proceed with the grinding, the material to be treated is introduced into the grinding volume, together with the balls. When the jar? winding is closed and set in motion. The movement imprinted on the jar can vary according to the types of mill, for example, the jar pu? be put in rotation with respect to an eccentric axis, pu? be shaken with a reciprocating motion, etc. The effect obtained in these mills of known type? that inside the jar high-energy collisions occur continuously between the mass of the material to be treated and the same members of the mill, ie? the spheres and the walls of the jar.

These mills, while widely appreciated, are not free from defects.

First of all, the application of the grinding movement requires a high expenditure of energy because, together with the mass to be ground and the balls, the entire jar must also be set in motion. Furthermore, to obtain the desired effect, the movement to which the jar must be subjected cannot be a smooth and balanced rotation, but must be an eccentric rotation or a shaking reciprocating motion. These movements, in addition to the energy necessary to obtain them, imply strong stresses on the support structure, on the transmission members and on the joints of the mill.

Again, the way in which ? obtained ultrafine grinding in the mills described above is mainly based on impact grinding and only partially on friction. In other words, the material is crushed by the instantaneous and localized application of high compressive forces generated by the impacts. As known to the skilled person, the compression is not ? an efficient mechanism to obtain the failure of a material, since? the compression itself initially tends to compact the material itself, which ? therefore able to resist relatively effectively even in the presence of structural or microstructural damage. For this reason, in mills of the known type, only a small part of the mechanical energy introduced into the system is absorbed by the crushing of the material (grinding energy), while the main part is transformed into heat. For this reason, mills of the known type require a dedicated cooling system and present reliability problems, due to the significant wear phenomena to which they are quickly subjected.

In light of the energy considerations listed above, yes? calculated that in ball mills of the known type, the effective grinding energy represents only 1% of the total mechanical energy introduced into the system. How can the expert person? well understand, an efficiency of 1% represents a notable obstacle to the diffusion of mills of the known type on an industrial scale.

Furthermore, the fact that for the grinding the impacts are mainly exploited, which lead to the yielding of the material due to compression efforts, instead of? the friction, which leads to the yielding of the material due to shear stresses, implies that long or very long residence times of the material inside the mill are necessary. For example, the Applicant has found that the amorphization of a mass of quartz in a laboratory ball mill required a good 400 hours of residence time. The expert person can well understand that a procedure that takes times so? long, hardly pu? overcome an interest confined to laboratory research to arrive at having an industrial interest.

Finally, the fact that the entire jar is set in motion during grinding makes it impossible to feed the material continuously to be ground, and relegates mills of the known type to discontinuous, or batch, or batch operation of material.

? therefore felt the need for a mill for ultrafine grinding that has the suitable characteristics to allow its use in the industrial field.

OBJECTS AND SUMMARY OF THE INVENTION

Purpose of the present invention? therefore that of overcoming the drawbacks highlighted above in relation to the prior art.

In particular, a task of the present invention ? that of making available a mill for ultra-fine grinding which has improved efficiency compared to known mills.

Furthermore, a task of the present invention ? to make available a mill for ultrafine grinding that has a? reliability? improved over known mills. In particular, a mill that is not subject to significant wear phenomena in a short time.

Again, a task of the present invention ? that of making available a mill for ultra-fine grinding which allows reduced residence times with respect to known mills.

Finally, a task of the present invention ? that of making available a mill for ultra-fine grinding which, in addition to introducing further advantages, also maintains the advantages already available. obtained from mills of the known type.

These and other objects and tasks of the present invention are achieved by means of a mill in accordance with claim 1. Further characteristics are defined in the dependent claims. All the attached claims form an integral part of the present description.

The invention relates to a mill for ultra-fine grinding of material, comprising a supporting structure, a jar fixed with respect to the supporting structure, a grinding unit included inside the jar. In the mill of invention:

- the jar defines a grinding volume and a principal axis X;

- the grinding crew ? mounted so as to be able to rotate around the main axis X;

- does the grinding crew include a plurality? of grinding units arranged symmetrically with respect to the main axis X; And

- each grinding group comprises a plurality? n of hollow cylinders having respective axes parallel to the main axis X.

Furthermore, in each grinding group:

- the hollow cylinders are inserted one inside the other,

- the hollow cylinders are freely movable with respect to each other, - the hollow cylinder more? external has greater cross section and less height between the hollow cylinders of the grinding unit, e

- each hollow cylinder has a smaller cross-section and a greater height than the surrounding hollow cylinder.

Thanks to this structure, the functioning of the mill according to the invention is mainly based on friction and the consequent failure of the material due to shear stresses. This grinding mechanism ? clearly more efficient with respect to the one based on shocks and therefore on the yielding of the material due to compressive stresses which is used in mills of the known type.

In some embodiments, the grinding equipment comprises a main shaft disposed along the main axis X and two discs spaced apart from each other in the axial direction, wherein at least one of the two discs is axially spaced apart. fixedly splined on the main shaft.

This solution is particularly efficient from a structural point of view for the grinding unit.

Preferably at least one of the discs comprises a plurality of aerodynamic fins, suitable for generating a pressure difference between the two sides of the disc when the disc itself? put into rotation.

The difference in pressure generated by the aerodynamic fins creates an air flow which facilitates the passage of the material through the openings of the disc itself, making it more efficient. the entrance of the material to grind and the exit of the ground material are simple. Furthermore, the same air flow allows the grinding equipment to be cooled during operation of the mill.

Preferably for each grinding group ? an axle has been provided, mounted on at least one of the two discs, which constrains the grinding unit to rotate around a secondary axis Y, fixed with respect to the grinding unit and rotating within the grinding volume around the main axis X.

Preferably each hollow cylinder comprises a cylindrical body along which ? prepared a through hole and the axis of the cylindrical body and the axis of the through hole are parallel to each other.

These solutions for the grinding units and for the hollow cylinders are particularly efficient from the structural point of view.

Preferably, in each grinding group, in at least one hollow cylinder, the axis of the cylindrical body and the axis of the through hole are distinct from each other.

The predisposition of an offset cylindrical body, on equal terms? of mass, it increases the energy transmitted to the material being ground.

Preferably in a grinding group, the hollow cylinders have a mass equal to each other or decreasing from the hollow cylinder more? inside towards the hollow cylinder pi? external.

Keep the mass the same or reduce it from the hollow cylinder pi? inside towards the hollow cylinder pi? external is particularly advantageous because? limits the efforts transmitted by the grinding group on the respective axle.

Preferably when the hollow cylinder pi? outside of a grinding group reaches its position radially more? outside, the outer sidewall of the hollow cylinder rests on the inner sidewall of the jar.

The sliding that is obtained thanks to the contact between the two surfaces, allows to obtain the grinding by friction, that is? thanks to the application of shear stresses.

Preferably, the jar also comprises a discharge duct configured to allow the ground material to flow out and in which, upstream of the discharge duct ? a control valve is provided to open/close the exhaust duct.

The provision of the control valve allows you to adjust the residence time of the material within the grinding volume.

Preferably, the mill also comprises, downstream of the grinding volume, a filter configured to separate the particles of material exceeding a predetermined granulometry from the flow of ground material.

For example, the filter allows particles of material that are too large for the desired uses to be recirculated inside the mill, so that undergo a new grinding process.

Further characteristics, objects and advantages of the present invention will appear more clearly from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will come described hereinafter with reference to some examples, provided for explanatory and non-limiting purposes, and illustrated in the attached drawings. These drawings illustrate different aspects and embodiments of the present invention and, where appropriate, reference numerals illustrating similar structures, components, materials and/or elements in different figures are indicated by like reference numerals. Furthermore, for clarity of illustration, some references may not be repeated in all the figures.

Figure 1 ? a schematic side elevation view of a mill in accordance with the invention;

figures 2.a and 2.b are two possible schematic views of the section made along line II-II of figure 1, in accordance with two different embodiments of the invention;

figure 3 ? a schematic vertical sectional view of a mill in accordance with an embodiment of the invention;

figure 4 ? a schematic exploded view of the section of figure 3;

figure 5 ? a schematic plan view of a lid for a mill jar in accordance with the invention;

figure 6 ? a schematic plan view of a finned disc for a mill jar in accordance with the invention;

figure 7 ? a schematic isometric view of a mill jar according to an embodiment of the invention, in which certain elements are shown in transparency;

the figure 8 ? a schematic vertical sectional view of a grinding assembly similar to those shown in Figure 7;

figure 9 ? an exploded isometric view of a mill in accordance with the invention, in which some elements are shown in semi-transparency;

figure 10 ? an axonometric view of the elements of the mill of figure 9 in disassembled configuration;

figure 11 ? a schematic view of the friction grinding mechanism upon which the mill of the invention is based; And

figure 12 ? an enlarged view of the detail indicated with XII in figure 11.

DETAILED DESCRIPTION OF THE INVENTION

While the invention? susceptible to various modifications and alternative constructions, some preferred embodiments are shown in the drawings and will be described in detail below. It must be understood, however, that there is no no intention of limiting the invention to the specific embodiment illustrated, but, on the contrary, the invention is intended to cover all modifications, alternative constructions, and equivalents which fall within the scope of the invention as defined in the claims.

The description addresses in detail the aspects and the technical characteristics peculiar to the invention, while the aspects and the technical characteristics in themselves? known can only be hinted at. For these aspects, what is stated above with reference to the prior art remains valid.

The use of ?for example?, ?etc.?, ?or? indicates non-exclusive alternatives without limitation unless otherwise indicated. The use of ?includes? and ?include? means ?includes or includes, but not limited to? unless otherwise indicated.

The mill of the invention ? intended to operate in the presence of? acceleration of gravity? g. Based on the acceleration of gravity? g the vertical direction and the horizontal directions are defined unequivocally. Similarly, with respect to the acceleration of gravity? g, the terms ?high?, ?superior?, ?above? and similar with respect to the terms ?low?, ?inferior?, ?below? and similar.

The mill of the invention defines a main axis of rotation with respect to which the terms ?axial?, ?radial?, ?circumferential? and ?ring road?. Furthermore, the mill of the invention ? configured to generate a flow of material, which enters the whole or coarsely ground mill and exits the ultrafinely ground mill. With respect to the material flow, the terms ?upstream?, ?before? and the like as opposed to the terms ?downstream?, ?after? and similar.

The invention relates to a mill 20 for ultra-fine grinding of material 22. The mill 20 comprises a supporting structure 24, a jar 26 fixed with respect to the supporting structure 24, a grinding unit 28 included inside the jar 26, in which:

- the jar 26 defines a grinding volume V and a principal axis X; - the grinding crew 28 ? mounted so as to be able to rotate around the main axis X;

- the grinding unit 28 comprises a plurality? of grinding units 30 arranged symmetrically with respect to the main axis X; - each grinding group 30 comprises a plurality? n of hollow cylinders 32 having respective axes parallel to the main axis X; and wherein - in each grinding group 30:

- the hollow cylinders 32 are inserted one inside the other,

- the hollow cylinders 32 are freely movable with respect to each other,

- the hollow cylinder pi? outer 32n has a larger cross section and smaller height between the hollow cylinders 32 of the grinding unit 30, and

- each hollow cylinder 32i has a smaller cross-section and greater height than the hollow cylinder 32i+1 which surrounds it.

Here and below the hollow cylinders 32 are identified by the reference numeral 32 when considered as a whole. In the case in which a precise hollow cylinder 32 must be identified within a grinding group 30, an index i is added to the reference number 32 which varies from 1 to n, where 1 indicates the hollow cylinder most? internal. Similarly, the index i is used to identify other characteristics of the i-th hollow cylinder.

Preferably the jar 26 has the shape of a rotational solid and is axially closed by an upper lid 34 and by a lower basin 36. Preferably the jar 26 has the shape of a right cylinder with a circular section. In any case, the overall shape of the jar 26 unambiguously defines the main axis X. Preferably the jar 26 further comprises a loading hopper 38, mounted on the upper cover 34, configured to allow the loading and introduction of the material 22 to be to grind in the grinding volume V. Preferably, the jar 26 also comprises a discharge duct 40, for example formed in the lower basin 36, configured to allow the outflow of the ground material 22, in the form of an ultra-fine powder.

The grinding crew 28 ? rotatably mounted within the grinding volume V defined by the jar 26; preferably the grinding unit 28 comprises a main shaft 42 disposed along the main axis X. In accordance with some embodiments of the invention, the grinding unit 28 comprises two discs spaced apart from each other in the axial direction . Preferably the upper disc 44 ? located near of the upper lid 34 of the jar 26, while the lower disk 46 is located near of the lower basin 36 of the jar 26. At least one of the two discs 44, 46 ? keyed in a fixed way on the main shaft 42. The main shaft 42 can? be put into rotation through an engine 48 that will come? better described after you.

Between the two discs 44, 46 are included the grinding units 30, which in plan view are arranged according to radial symmetry or rotational symmetry with respect to the main axis X. In accordance with the various possible embodiments, the grinding units 30 can be present in a different number of specimens. For example, in the embodiment of figure 2.a three grinding groups 30 are schematically represented, in the embodiment of figure 2.b four grinding groups 30 are schematically represented, while in the embodiments of figures 3, 4, 7, 9 and 10 schematically represent two grinding groups 30.

Preferably for each grinding group 30 ? provided a pin 50 mounted on at least one of the two disks 44, 46, preferably on both disks 44, 46 (see for example figure 8.a). In some embodiments the pins 50 are mounted on the discs 44, 46 so that they can rotate about their axis, while in other embodiments the pins 50 are mounted on the discs 44, 46 in a rigid manner. As an alternative to a single pin 50 which extends from one disc to the other, can two semi-pins 50 be provided? separated, each constrained to a single disc 44, 46 (see for example figure 8.b).

Preferably both the main shaft 42 and the pins 50 are fixed to both discs 44, 46. This structural solution allows to obtain a particularly strong and rigid grinding unit 28, particularly suitable therefore for withstanding the stresses deriving from the grinding.

Advantageously, the discs 44, 46 comprise at least one opening 52 configured to allow the passage of the material 22 undergoing the grinding treatment. Preferably each disc 44, 46 comprises at least two openings 52 configured to allow the passage of the material 22, where the two openings 52 in plan view are arranged according to a radial symmetry or rotational symmetry with respect to the main axis X. In particular, the upper disc 44 includes one or more openings 52 configured to allow the passage of the material 22 to be ground, i.e. of the whole or coarsely ground material 22. Respectively, the lower disk 46 comprises one or more? openings 52 configured to allow the passage of the ground material 22, i.e. of the material 22 in the form of an ultrafine powder.

In accordance with some embodiments, at least one of the discs 44, 46 comprises a plurality of discs. of aerodynamic fins 54, suitable for generating a pressure difference between the two sides of the disc 44, 46 when the disc itself is? put into rotation. Preferably both discs 44, 46 comprise a plurality of of aerodynamic fins 54.

Preferably the aerodynamic fins 54 are arranged so as to create a higher pressure zone upstream of the disc 44, 46 and a lower pressure zone downstream of the disc 44, 46. In this way, when a disc 44, 46 comprising the fins aerodynamics 54 ? in rotation, the pressure difference creates an air flow which facilitates the passage of the material 22 through the openings 52 of the disc itself. In particular, if the upper disk 44 includes the aerodynamic fins 54, the rotation of the upper disk 44 creates an air flow which facilitates the entry of the material 22 to be ground, from the loading hopper 38 up to the inside of the grinding volume V. Respectively, if the lower disc 46 includes the aerodynamic fins 54, the rotation of the lower disc 46 creates an air flow which facilitates the exit of the ground material 22 from inside the grinding volume V to the discharge duct 40 How can the expert person? well understand, if both discs 44, 46 include the aerodynamic fins 54, the air flow that is created obtains the optimal result to facilitate the passage of the material 22.

The grinding units 30 are described in greater detail below. As mentioned above, each grinding unit 30 comprises a plurality of of hollow cylinders 32. Preferably each hollow cylinder 32 comprises a cylindrical body along which ? prepared a through hole; the axis of the cylindrical body and the axis of the through hole are parallel to each other. According to some embodiments, in at least one hollow cylinder 32 for each grinding group 30, the axis of the cylindrical body and the axis of the through hole, although parallel, they are distinct from each other. For the purposes of this discussion, if it is necessary to identify a single axis for the entire hollow cylinder 32, one can consider that the axis to consider is that of the cylindrical body. Of course, nothing would change if it were considered the axis of the through hole, since? these axes are always parallel to each other and are often coincident.

because are the hollow cylinders 32 inserted one inside the other, ? is it possible to identify a hollow cylinder pi? internal 321, a hollow cylinder pi? external 32n and possibly one or more? intermediate hollow cylinders 32i. The hollow cylinder extension 321 ? constrained to rotate around a secondary axis Y, fixed with respect to the grinding unit 28 and therefore rotatable within the grinding volume V around the main axis X. The secondary axis Y ? defined by the pin 50, or possibly by at least one semi-pin 50?, constrained to at least one disk 44, 46. The secondary axis Y ? preferably parallel to the main axis X. In other words, for each grinding group 30 ? a pin 50 is arranged which constrains the grinding group 30 to rotate around the secondary axis Y.

Some characteristics of the hollow cylinders 32 of a single grinding group 30 are described below. Considering the hollow cylinders from the most? internal 321 at the pi? outside 32n, height h (or axial extension) decreases; therefore the relation hi > hi+1 holds.

Furthermore, still considering the hollow cylinders from the pi? internal 321 at the pi? outside 32n, the cross section increases. Pi? in particular, each hollow cylinder 32 has its own external diameter Di (that is, the diameter of the cylindrical body) and its own internal diameter Di (that is, the diameter of the through hole). How can the expert person? well understand, so that? are the hollow cylinders 32 freely movable with respect to each other, ? It is necessary that the outside diameter Di of a given hollow cylinder 32i be smaller than the inside diameter di+1 of the surrounding hollow cylinder 32i+1. As far as the hollow cylinders 32 are concerned, the relation di < Di < di+1 < Di+1 is therefore valid.

Finally, the hollow cylinders 32 of a grinding group 30 can have mass m equal to each other or can have a decreasing mass from the hollow cylinder more? inside 321 towards the hollow cylinder pi? external 32n. Is the relation mi ? mi+1.

As mentioned above, each hollow cylinder 32 defines its own geometric axis and the axes of the hollow cylinders 32 are all parallel to the main axis X. This characteristic is? defined with respect to an equilibrium configuration, such as for example those schematically represented in figures 7 and 8. How can an expert person? well understand, the fact that the hollow cylinders 32 are freely movable one with respect to the other can? allow them to assume transitory configurations in which not all geometric axes are necessarily parallel to each other and/or to the main axis X. L?eventuality? that such transient configurations occur has no effect on the present description.

Preferably, all grinding units 30 have the same overall mass. In this way, when the hollow cylinders 32 of each grinding unit 30 assume the same arrangement with respect to the main axis X, the center of mass of the grinding unit 28 is? included in the main axis X and the grinding equipment 28 as a whole ? balanced with respect to a rotational movement around the main X axis. How can the expert person? well understand, the balance in the distribution of masses can not? naturally understand the mass of material 22 to be treated, which pu? introduce some imbalance in the rotational movement around the main axis X. However, also the mass of the material 22 tends, in the presence of the field of forces present inside the grinding volume V, to distribute itself in a statistically uniform way.

The configuration in which the hollow cylinders 32 of each grinding unit 30 assume the same arrangement with respect to the main axis X is obtained spontaneously when the grinding unit 28 is set in rotation, since it? the centrifugal force pushes each hollow cylinder 32 to the radially most? external among those that are allowed (see for example figures 8.a and 8.b). In this way the rotation of the grinding unit 28 is ideally balanced and does not introduce significant stresses in the direction perpendicular to the main axis X.

Advantageously, when the hollow cylinder pi? outside 32n of a grinding group 30 reaches its position radially more? typically due to centrifugal force, the outer sidewall of the hollow cylinder 32n rests on the inner sidewall of the jar 26. See, for example, FIGS. 8, 11 and 12. external 32n to the pi? interior 321, further contacts are obtained between the respective side walls. Pi? in particular, the outer side wall of a hollow cylinder 32i rests on the inner side wall of the hollow cylinder 32i+1 which surrounds it.

The contact surfaces defined by the jar 26 and the hollow cylinders 32 are the grinding surfaces of the mill 20 of the invention. In particular, the contacts described above are sliding type contacts and the speed? sliding increases moving from the hollow cylinder pi? inside 321 towards the outside. The speed? of creeping ? maximum in the contact between the outer side wall of the hollow cylinder pi? external 32n and the internal lateral wall of the jar 26, which ? stop. The material 22 which is found between two surfaces is subjected to high shear stresses which cause its structural yielding.

In some embodiments of the invention, both the jar 26 and the hollow cylinders 32 are made of steel. Preferably, the surfaces of the hollow cylinders 32 are hardened, for example by carburizing, carbonitriding or nitriding. Preferably, the inner surface of the jar 26 is also hardened, for example by carburizing, carbonitriding or nitriding.

Preferably the mill 20 comprises a motor 48 adapted to rotate the grinding unit 28, to keep it rotating for the necessary time, and to supply the mechanical energy necessary to obtain the desired grinding. The 48 engine can? be connected to the main shaft 42 directly or via a transmission unit. Preferably the 48 engine? an electric motor controlled by an inverter that allows you to modulate the speed? and the power supplied, according to the specific needs.

Preferably, the lower bowl 36, placed to close the jar 26 downstream of the lower disc 46, defines a discharge duct 40 which allows the ground material 22 to exit the jar 26. Preferably upstream of the discharge duct 40? a control valve 56 is provided which is suitable for opening/closing the discharge duct 40. The control valve 56 can? for example be a guillotine valve or another valve suitable for operating in the presence of ultra-fine powders.

In accordance with some embodiments, downstream of the grinding volume V, for example downstream of the discharge duct 40, ? provided a filter 58, for example a bag filter or a cyclone. The 58 filter? configured to separate particles exceeding a predetermined particle size from the main powder stream. These particles can be conveyed to a different destination than the main flow of the ultrafine powder, or they can be recirculated inside the mill 20 to be subjected to a further grinding process.

In accordance with some embodiments, downstream of the discharge duct 40 can an aspirator (not shown) be set up, configured to facilitate the flow of ultra-fine powder out of the mill 20.

Briefly, the operation of an exemplary embodiment of the mill 20 of the invention is described.

Preferably the material 22 to be ground is pre-treated in a coarse grinding machinery, such as for example a hammer mill or a knife mill. The material 22 to be ground is therefore prepared in the form of fragments, flakes, confetti or the like, having homogeneous dimensions. This preventive coarse grinding allows you to speed up and regularize the ultra-fine grinding process.

The material 22 to grind cos? treated is placed in the loading hopper 38. By gravity? and/or due to the suction generated by the rotation of the discs 44, 46 with the aerodynamic fins 54, the material 22 enters the grinding volume V.

While the material 22 resides in the grinding volume V, it is constantly involved in the sliding which is generated between the grinding surfaces, i.e. the inner surface of the jar 26 and the surfaces of the hollow cylinders 32 in rotation. The sliding at high speed? which ? subjected to the material 22, cause it to yield due to shear stresses. The rotation at high speed? of the grinding equipment 28 keeps the material 22 in agitation within the grinding volume V, which ? statistically involved in a large number of sweeps.

When the material 22 enters between two hollow cylinders 32, it is not trapped therein, but the fact that the outer hollow cylinder 32i has a lower height than the inner hollow cylinder 32i-1 allows the material 22 to exit easily.

When the grinding process ? considered terminated, or for the achievement of a predetermined granulometry or because? ? once the predefined residence time has elapsed, the material 22 is removed from the grinding volume V through the discharge duct 40.

To control the residence time of the material 22 in the grinding volume V pu? the control valve 56 which closes the discharge duct 40 can be used. The imposition of a residence time higher than that strictly necessary for ultra-fine grinding can be useful, for example, to guarantee the complete extraction of water from the mass of material 22 or to obtain some chemical reactions, such as for example mechanical calcination.

At the outlet of the grinding volume V is the material 22 ? now reduced to an ultrafine powder and pu? be made to slide easily by means of a flow of air. For example, can the air flow generated by the discs 44, 46 comprising the aerodynamic fins 54, or an air flow generated by an external aspirator can be exploited.

As mentioned above, the operation of the mill 20 of the invention is mainly based on friction and the consequent failure of the material 22 due to shear stresses. How can the expert person? well understand, this grinding mechanism ? clearly more efficient with respect to the one based on impacts and therefore on the yielding of the material 22 due to compression efforts. For this reason the mill 20 of the invention (based on friction) obtains clearly better performances than the mills 20 of the known type (based on shocks).

In the light of the studies conducted by the Applicant, in the mill 20 of the invention the effective grinding energy represents no less than 10% of the total mechanical energy introduced into the system. How can the expert person? well understand, although the absolute figure of 10% does not seem particularly satisfactory, ? it should be remembered that it must be compared with an average efficiency of about 1% of ball mills of the known type. The efficiency of the mill 20 of the invention therefore represents a surprising improvement.

The increased grinding efficiency first of all determines decidedly shorter residence times. faster than those necessary with known types of mills. By way of example, the Applicant compared the residence times necessary to obtain the amorphization of a given mass of quartz. How already? mentioned in the introductory part of the present discussion, in a known type of ball mill this result required a good 400 hours of residence time. In a mill 20 in accordance with the invention, the same result required only 30 minutes of residence time. The expert person can well understand what advantage can derive from a similar reduction in residence times for the purpose of using the mill 20 of the invention in the industrial field.

In the mill 20 of the invention, the increase in grinding efficiency and the reduction in residence times cause a drastic decrease in the heat produced during operation. In accordance with what has been experienced by the Applicant, is it not? necessary to install a cooling system. For the disposal of the little heat produced pu? the flow of air generated by the aerodynamic vanes 54 mounted on the discs 44, 46 is sufficient.

Depending on the? employment which ? intended for each mill 20 of the invention, different overall measurements and installed powers can be defined. As an example, one of the 20 pi? small among those made in accordance with the invention, typically intended for laboratory use, can? have an overall diameter of the jar 26 included within 15 cm and pu? have an installed power of about 3 kW. The operating regime of such a relatively fast mill 20 can around 3000 rpm. Such a mill 20 pu? have the ability? to grind approximately 20 kg/hour of material 22. At the opposite extreme, one of the mills 20 more? large among those made in accordance with the invention, intended for industrial use, pu? have an overall diameter of the jar 26 between about 80 cm and 100 cm and can have an installed power between approximately 40 kW and 50 kW. The operating regime of such a relatively slow mill 20 can around 1500 rpm. Such a mill 20 pu? have the ability? to grind approximately 1000 kg/hour of material 22.

The dimensions and powers shown above are only indicative. How can the expert person? well understand, different specific intended uses may require slightly different configurations. For example, a mill 20 specifically intended for the treatment of chlorinated plastics requires lower powers than a mill 20 of the same size specifically intended for the treatment of crystalline quartz.

In accordance with the results obtained by the Applicant, although? is possible, it is not particularly advantageous to make mills 20 of the invention which are more? big than what?. In fact, industrial-scale grinding processes are typically carried out in series, starting with coarse and low-cost machinery to arrive at the use of ultra-fine grinding mills only at the end of the process. By way of example, for the industrial grinding of materials which have a predominantly fragile behavior (such as for example minerals, inert materials and the like), initially hammer mills or jaw mills can be used. On the other hand, for the industrial grinding of materials which have a predominantly plastic behavior (such as for example polymers), initially knife mills can be used.

In accordance with the results obtained by the Applicant, even if it is simply necessary to increase the mass of material 22 to be treated in the unit? of time, the preparation in parallel of pi? mills 20 represents a preferable choice with respect to the arrangement of a single large size mill 20. In fact, this choice allows for greater flexibility? of use.

The fact that in the mill 20 of the invention the jar 26 remains stationary makes it possible to feed the material 22 to be ground even during grinding. Such a possibility? allows the user to operate the mill 20 of the invention continuously, leaving this? free the flow of the material 22, or in a semi-continuous way, managing the residence time of the material 22, for example through the control valve 56 located upstream of the discharge duct 40. Of course, if it were preferable for the user, the mill 20 of? the invention can? also work discontinuously, or batch, like mills of the known type.

How can the expert person? well understand, there are several factors which make the mill 20 of the invention less subject to wear and therefore more? reliable compared to a known type of mill. Among these factors we can highlight the fact that the jar 26 remains stationary during the grinding, that the movement of the grinding unit 28 is ideally balanced, and that less heat is developed during the grinding.

The Applicant has verified that the mill 20 of the invention can be advantageously employed for various uses. For example, can be used for the disposal of PVC, since? ? able to obtain in a very efficient way the dereticulation by mechanical means, detaching the chlorine atoms from the polymeric chain. Again, the same mill 20 ? suitable for producing extremely fine dry powders starting from even wet organic materials, for example for energy purposes or as semi-finished products for the chemical industry. Furthermore, the mill 20 of the invention can? be used for the treatment of hazardous organic materials, since? it ? capable of breaking molecular bonds and bringing substances to their ground state.

Finally, yes? seen that the mill 20 of? invention ? particularly advantageous in the field of powder metallurgy, in particular for the process called mechanical alloying. This procedure, in itself? known, ? a solid state and powder processing technique which involves, in a mill 20, the repeated cold welding, fracturing and rewelding of powder particles mixed together, in order to produce a homogeneous material 22. Also in this field, the mill 20 of the invention allows the production times to be considerably speeded up.

How can the expert person? well understand, the invention overcomes the drawbacks highlighted above in relation to the prior art.

In particular, the invention provides a mill 20 for ultrafine grinding which has an improved efficiency compared to known mills.

Furthermore, the invention provides a mill 20 for ultrafine grinding which has a reliability improved over known mills. In particular, a 20 mill that is not ? subject to significant wear phenomena in a short time.

Furthermore, the invention makes available a mill 20 for ultrafine grinding which allows reduced residence times with respect to known mills.

Finally, the invention makes available a mill 20 for ultra-fine grinding which, in addition to introducing further advantages, also maintains the advantages already present in the invention. obtained from mills of the known type.

In conclusion, all the details can be replaced by other technically equivalent elements; the features described in relation to a specific embodiment can also be used in the other embodiments; the materials used, as well as? the contingent shapes and dimensions may be any according to the specific implementation requirements without thereby departing from the scope of protection of the following claims.

Claims (10)

1. Mill (20) for ultra-fine grinding of material (22), comprising a supporting structure (24), a jar (26) fixed with respect to the supporting structure (24), a grinding unit (28) included inside the jar (26), wherein: - the jar (26) defines a grinding volume V and a principal axis X; - the grinding crew (28) ? mounted so as to be able to rotate around the main axis X; - the grinding equipment (28) comprises a plurality? of grinding groups (30) arranged symmetrically with respect to the main axis X; - each grinding group (30) comprises a plurality? n of hollow cylinders (32) having respective axes parallel to the main axis X; and wherein in each grinding group (30): - the hollow cylinders (32) are inserted one inside the other, - the hollow cylinders (32) are freely movable with respect to each other, - the hollow cylinder more? outside (32n) has a greater cross section and lower height than the hollow cylinders (32) of the grinding unit (30), - each hollow cylinder (32i) has a smaller cross section and greater height than the hollow cylinder (32i+1) which surrounds it. 2. Mill (20) according to claim 1, wherein the grinding equipment (28) comprises a main shaft (42) disposed along the main axis X and two discs (44, 46) spaced apart from each other ?other in the axial direction, in which at least one of the two disks (44, 46) ? fixedly splined on the main shaft (42). 3. Mill (20) according to claim 2, wherein at least one of the discs (44, 46) comprises a plurality of of aerodynamic fins (54), configured to generate a pressure difference between the two sides of the disc (44, 46) when the disc itself ? put into rotation. 4. Mill (20) according to claim 2 or 3, wherein for each grinding group (30) ? a pin (50) is provided, mounted on at least one of the two discs (44, 46), which constrains the grinding unit (30) to rotate around a secondary axis Y, fixed with respect to the grinding unit (28) and rotatable within the grinding volume V around the main axis X. 5. Mulino (20) in agreement with one or more? of the preceding claims, wherein each hollow cylinder (32) comprises a cylindrical body along which ? prepared a through hole; and in which the axis of the cylindrical body and the axis of the through hole are parallel to each other. 6. Mill (20) according to the preceding claim wherein, in each grinding unit (30), in at least one hollow cylinder (32) the axis of the cylindrical body and the axis of the through hole are distinct from each other. 7. Mulino (20) in agreement with one or more? of the preceding claims wherein, in a grinding unit (30), the hollow cylinders (32) have a mass equal to each other or decreasing from the hollow cylinder more? inside (321) towards the hollow cylinder pi? external (32n). 8. Mulino (20) in agreement with one or more? of the previous claims, in which when the hollow cylinder is more? outside (32n) of a grinding group (30) reaches its position radially more? outside, the outer sidewall of the hollow cylinder (32n) rests on the inner sidewall of the jar (26). 9. Mulino (20) in agreement with one or more? of the preceding claims, wherein the jar (26) further comprises a discharge duct (40) configured to allow the ground material (22) to flow out and wherein, upstream of the discharge duct (40) ? provided a control valve (56) suitable for opening/closing the exhaust duct (40). 10. Mulino (20) in agreement with one or more? of the preceding claims, further comprising, downstream of the grinding volume V, a filter (58) configured to separate from the flow of ground material (22) the particles of material (22) which exceed a predetermined granulometry.