Field of the invention

[1] The present invention relates to an attritor mill of the type comprising a grinding container which forms a grinding chamber in its inside, and a rotor housed inside said container, wherein from the rotor a plurality of arms radially protrudes, which are arranged to agitate a plurality of grinding free bodies mixed with the material to be treated. The invention furthermore relates to the application of such an attritor mill to the treatment and recycling of scraps such as, for example, urban solid waste, food scraps or more generally the so-called "wet waste", remains from mowing or pruning, vegetal residues from agricultural activity, waste similar to the urban ones, dry organic fraction of the urban waste, scraps from mechanical selection of the urban waste, silicate or carbonate- based inorganic materials.

State of the art

[2] It is currently known to treat the urban waste by grinding them in ball or pebble-type mills provided with a big drum rotating about a horizontal axis. The drum is charged with the waste to be treated mixed with a proper quantity of steel pebbles which act as grinding bodies. By making the drum rotate on itself about a horizontal axis, the steel pebbles grind, crush and mince the waste. By moving the steel pebbles at very low average speeds, this first type of ball mills is convenient and efficient if large-sized; for example, the city of Leicester (UK) uses, for the treatment of urban waste, a big horizontal axis ball- type mill charged with about 42 tons of steel pebbles. A second limit of this type of mills is the humidity content of the organic waste after the treatment, which remains in any way too high in order for the waste to be used as fuel.

Ball or pebble-type mills, often called attritor mills, are by the way known. These mills are provided with a grinding chamber wherein a rotor, by rotating about a vertical axis, randomly strikes a mass of steel pebbles mixed with other material to be treated. The latter material is mainly ground as a result of the repeated and very frequent impacts, crushing and rubbing among or anyway against the pebbles. Attritor mills having the aforementioned features are themselves already known, for example from US3131875, US3339896, US3084876, US3670970. The author of the present invention noticed that it is not easy to adapt the known attritor mills to grinding or anyway to treating large amounts of materials, such as food scraps or waste with a high content of animal or vegetal tissues, relatively soft and humid or generally pasty solids, or relatively dry and brittle inorganic materials based for example on silicates or carbonates, making them operate with an efficiency acceptable in large mass productions and not only in small productions for laboratory activities or for relatively expensive products such as pigments. Perhaps, due to their relatively low electromagnetic efficiency, attritor mills do not seem to have been used so far in the massive treatment of urban waste or waste products.

[3] Hence, it is a first object of the invention to provide an attritor mill more efficient than the known ones, especially capable of grinding greater amounts of materials with lower power consumption compared to the known attritor mills.

A second object of the invention is providing a mill capable of treating urban solid waste, even organic, such as for example food scraps (the so-called "wet fraction " of the urban waste) , scraps from mowing or pruning, vegetal residues from agricultural activity, waste similar to the urban ones, dry organic fraction of the urban waste, suitable for being made as a small- sized plant.

[4] A third object of the invention is providing a mill capable of treating the aforementioned urban solid waste, by producing a drier output material, and thus more suitable for being directly used as fuel or anyway less in need of further drying or sterilization treatments in order to reduce its bacterial load.

[5] A fourth object of the invention is providing a process for treating urban solid waste, even organic, of the aforementioned type, more suitable for being implemented with relatively small-sized plants.

Summary of the invention

[6] In a first aspect of the invention, such objects are achieved through an attritor mill having the features of claim 1.

In a second aspect of the invention, the fourth object is achieved through a process for treating organic solid waste having the features according to claim 13. The dependent claims are directed to further optional features of the attritor mill and of the process according to the invention.

The advantages achievable through the present invention will be more apparent, to the person skilled in the field, from the following detailed description of an example of a particular embodiment not limitative, shown with reference to the following schematic figures .

List of figures

Figure 1 is a perspective view o fan attritor mill according to a particular first embodiment of the invention ;

Figure 2 is a first lateral view, partially sectional according to plane II-II, of the attritor mill of Figure 1 ;

Figure 3 is a top view, partially sectional according to plane III-III, of the attritor mill of Figure 1 ;

Figure 4 is a view according to a radial direction of a support protruding of the mill of Figure 1 ;

Figure 5 is a second lateral view, partially sectional according to plane II-II, of the part of attritor mill of Figure 1.

Detailed description [7] Figures 1-5 relate to an attritor mill, indicated with the overall reference 1, according to a particular embodiment of the invention.

The attritor mill 1 comprises a grinding container 3 which forms a grinding chamber 5 in its inside and a rotor 7 housed inside said chamber (Figure 2) . The grinding container 3 can be made up of, or any way can comprise a metal sheet shell, preferably in a cylindrical shape. The rotor 7 preferably comprises a pin or a central shaft 70. From the pin or central shaft 70, or more generally from the rotor 7 a plurality of arms 9, 9', 9" radially protrudes and the attritor mill is arranged to grind, crush, mince or micronize a material to be treated, such as for example urban solid waste, food scraps, waste with a high content of animal or vegetal tissues, remains from mowing or pruning, vegetal residues from agricultural activity, waste similar to the urban ones, dry organic fraction of the urban waste, by actuating the following steps:

- mixing the material to be treated with a plurality of grinding free bodies 11 in the grinding chamber 5;

agitating the material to be treated and the plurality of grinding free bodies 11 by making the rotor 7 rotate on itself so that its arms 9, 9', 9" strike the free grinding bodies 11 and at least a part of the latter bounce against the walls 13 of the grinding chamber, against other free grinding bodies 11, and/or strike the material to be treated. [8] The wording "free bodies" in the present description indicates bodies not directly fixed or anyway mechanically constrained to other parts of the mill 1. These free bodies are therefore free to move and to bounce in the grinding chamber, hampered in this substantially by the material to be treated only.

The grinding free bodies 11 can be for example spheres, pebbles, pellets, balls, cobbles, granules of materials such as steel, copper or other metals, metal materials, stones, glass. The grinding free bodies 11 can also be other bodies in a various and more angular shape, such as for example waste and metal scraps, nails, screws and bolts; however, these grinding free bodies as well, after a certain permanence in the grinding chamber, assume shapes even and even more rounded and more similar to peddles, cobbles or bullets.

[9] Preferably the mill 1 is arranged to rotate the rotor 7 on itself, during the normal operation, about a substantially vertical axis Al, or at least having an inclination with respect to the vertical not greater than about 45 ° .

[10] In order to increase the average height - both in time and in the number of the various grinding bodies 11 - at which the various free grinding bodies 11 fly during normal operation, and to reduce the amount of grinding bodies 11 which lie on the bottom of the grinding chamber 11, according to one aspect of the invention, on the bottom or anyway in the lower part of the grinding chamber (5) at least one lifting projection (19) is present, which is arranged to deviate mainly upwards the free grinding bodies (11) which substantially randomly hit the projection itself (19) (Figures 2, 3) .

Each of the lifting projection 19 can have for example the shape of an oblong rib which extends radially with respect to the rotation axis Al of the rotor, and defining some inclined faces - for example plane - 190, 192 on its longitudinal sides. Preferably the inclined faces 190, 192 have a global inclination a, with respect to the vertical or anyway to the rotation axis Al of the rotor 7, preferably comprised between 30° and 50°, more preferably comprised between 35° and 45° degrees, even more preferably comprised between 37° and 43° and for example equal to about 40-41° (Figure 4) . Alternatively the lifting projections can extend from the center outwards of the grinding chamber, even though not in radial directions.

Advantageously the arms of the rotor 9 which are more distant from the bottom of the grinding chamber 5 are substantially shorter than the arms 9', 9" which are closer to the bottom of the grinding chamber 5; in this way the rotating arms 9, 9', 9" tend to rise, and to maintain more raised, the mass of the free grinding bodies 11, thus increasing the efficiency of the grinding .

Preferably the length LB of the arms 9, 9', 9" of the rotor progressively reduces by moving away ideally from the bottom of the grinding chamber 5 (Figure 2); thus the ability of the mill 1 of rising and of maintaining raised the mass of the free grinding bodies 11 increases. The various arms 9, 9', 9" can be fixed to the rotor so as to substantially lie in two ideal mutually orthogonal planes.

The rotor 7 can be driven for example by a proper electric motor, not shown, arranged below the grinding chamber 5.

The attritor mill 10 is arranged to make the rotor 7 rotate on itself, for example through the aforementioned electric motor, at a speed preferably comprised between 500 and 1500 rev/min, and more preferably comprised between 800 and 1200 rev/min.

[11] Preferably the length LB of the arms 9, 9', 9" of the rotor progressively reduces according to a linear law by moving away ideally from the bottom of the grinding chamber 5.

Preferably the waste or other material to be treated are introduced into the grinding chamber from the top, for example through the supplying duct 15 which preferably is substantially vertical (Figure 1). However the wet fraction of the urban organic waste, which can contain up to 80% by weight of humidity, often tends to form a pasty mass which advances with a relative difficulty in possible ducts and does not lend itself to be introduced into the mill 1 by simple gravity. Therefore the attritor mill 1 is advantageously provided with a feeder (not shown) arranged to push the material to be treated into the grinding chamber 5. Such a feeder can comprise, or be made of a screw or Archimedean screw feeder, a piston, a hydraulic or mechanical pusher, a volumetric pump. Such a screw, Archimedean screw or hydraulic pusher can be housed for example in the same supplying duct 15.

[12] Each arm 9, 9', 9" can have for example substantially circular cross sections, and more in particular can be in the shape of a cylindrical rod.

[13] Through a proper choice of the rotational speed of the rotor 7 and of other operating parameters, such as for example the ratio between the amount of waste or other material to be treated and of grinding free bodies 11 present in the grinding chamber 5, it is possible to bring the temperature of the material to be treated and of the grinding bodies 11 in the chamber 5 at temperatures sufficiently hot, for example comprised between 60° and 100° degrees so as to quickly desiccate the material to be treated and reduce it, at the end of the treatment, in the form of granules or relatively dry powder that can be extracted from the grinding chamber 5 through one or more extraction grids 17, preferably located in the upper part of the grinding chamber .

[14] Advantageously the extraction grid or grids 17 have holes or meshes of dimensions such as to let the particles of treated material of the desired dimensions pass, and on the other hand to retain the free grinding bodies 11 inside the grinding chamber 5; preferably the holes or meshes of each of the extraction grids 17 have a diameter, or anyway a maximum length or width, equal to or less than 2 cm, more preferably comprised between 3 mm and 9 mm and even more preferably equal to 6 mm. Through holes or meshes of the extraction grids of about 6 mm treating organic and/or solid waste the particle size of the extracted waste ranges from 100 microns to 5 mm.

Preferably the attritor mill 1 comprises a suction system arranged to suction and extract the powders or other particles of treated material from the grinding chamber 5, and the suction system comprises a fan or other aspirator 21 located outside the grinding chamber itself (Figure 5) . To this end the grinding container 3 can be enclosed in an external casing 23 capable of assuring a proper pneumatic seal, and the inside of the casing 23 can be fluidically connected to the fan or other suction 21 through a collecting duct 25. The fan or other suction 21 is preferably motorized, for example driven by an electric motor.

[15] Advantageously the attritor mill 1 is provided with a grid cleaning system arranged to keep clean the at least one extraction grid 17 or at least to reduce its clogging. To this purpose the grid cleaning system comprises at least one nozzle 27 arranged to emit air jets towards the at least one extraction grid. Preferably the grid cleaning system comprises a plurality of nozzles 27. Preferably the attritor mill 1 is furthermore provided with a logic unit programmed or anyway arranged to activate in sequence, for example one at a time, the nozzles 27; the fewer nozzles are simultaneously activated, the less the flow set between the free grinding bodies 11 and the material to be treated breaks .

[16] As shown in Figures 2, 5 the rotor 7 can be fixed to the bottom of the grinding chamber 5 through a bearing inserted on the rotor itself. Close to the bottom of the grinding chamber the rotor advantageously comprises a protecting plate 29 arranged to protect the bearing from dusts, granules or other particles coming from the grinding chamber itself. The protecting plate can substantially be shaped more or less as a planar disc (Figures 2, 5) . The protecting plate 29 preferably lies in a plane substantially perpendicular to the rotation axis Al of the rotor itself, is integrally fixed to the rest of the rotor 7 and rotates together with it, and substantially covers at least half of the surface of the bottom or of the cross section of the grinding chamber. More preferably the protecting plate 29 has such an extension that the distance between its peripheral edge and the walls of the grinding chamber 5 be comprised between one and five times the diameter, or anyway a maximum length or width, of the above holes or meshes of each of the extraction grids 17; more preferably the distance between the peripheral edge of the protecting plate 29 and the walls of the grinding chamber 5 is comprised between 3 and 3,5 times the diameter, or anyway a maximum length or width, of the aforementioned holes or meshes of each of the extraction grids 17.

[17] Advantageously the peripheral edge of the protecting plate 29 is flared so that the distance between the edge of the plate and the walls of the grinding chamber 5, according to a radial direction with respect to the rotation axis Al of the rotor, increase by ideally moving towards the bottom of the grinding chamber, preventing the particles coming from the grinding chamber 5 from getting stuck between the edge of the protecting plate 19 and the walls of the grinding chamber 5 and permitting instead their outflow through proper drains downstream of the protecting plate 29 itself.

In order to increase the bounces of the free grinding bodies 11 upwards and to be more efficient, the at least one lifting projection 19 is advantageously obtained integrally on the protecting plate 29.

[18] Preferably the grinding chamber 5 has a substantially cylindrical shape.

The ratio between the height HC and the diameter DC of the cylindrical grinding chamber 5 is preferably greater than 0,8, more preferably greater than 1 and for example comprised between 0,8 and 2 or between 0,8 and 1,3.

[19] As already previously exposed, under proper operating conditions the mass of material to be treated and the grinding free bodies 11 heat also reaching high temperatures, that can advantageously be exploited to desiccate the material to be treated and favoring its transformation into dusty or granular material. To this end, advantageously the attritor mill 1 is provided with a control and adjusting system of the temperature arranged to detect the temperature in the grinding chamber during the functioning of the mill and to control it, preferably in real time, keeping it in a desired range of values, for example between 50-90 degrees C.

Besides proper sensors, the control and adjusting system of the temperature can comprise one or more of the following subsystems:

- a measuring device for controlling with an adequate precision the amount - or the capacity - of material to be treated that is fed into the grinding chamber 5;

- a cooling system of the grinding chamber that can be for example a water, air or other liquids or gaseous thermo-vectors cooling system;

a logic unit arranged to control and vary the rotation speed of the rotor 7, the aforementioned measuring device and/or cooling system.

[20] In a second aspect, the invention relates to a process for treating organic solid waste such as for example urban solid waste, food scraps or more generally the so-called "wet waste", remains from mowing or pruning, vegetal residues from agricultural activity, waste materials similar to the urban ones, dry organic fraction of the urban waste, comprising the following steps:

- providing an attritor mill 1 in its turn comprising a grinding container 3 which forms a grinding chamber 5 in its inside and a rotor 7 housed inside said container 3, wherein from the rotor 7 a plurality of arms 9, 9', 9" radially protrudes;

- mixing the organic solid waste with a plurality of grinding free bodies 11 in the grinding chamber 5;

- agitating the organic solid waste to be treated and the plurality of grinding free bodies 11 making the rotor 7 rotate on itself so that its arms 9, 9', 9" strike the free grinding bodies 11, and at least a part of the latter bounce against the walls of the grinding chamber 5 and other free grinding bodies, and/or strike the organic solid waste to be treated, so as to grind, crush and mince the latter. In general the use of an attritor mill 1 having peddles or other grinding free bodies agitated by a vertical axis rotor permits to efficiently treat urban organic waste of the previously exemplified types, desiccating and transforming them into a dry granular or dusty material or anyway with smaller humidity contents and a greater calorific power, and thus ready to be directly used as fuel. Thanks to the high average speeds of the free grinding bodies 11 during the functioning of the attritor mill 1, even being the latter substantially modest-sized, it is capable of treating high amounts of urban organic waste, having a humidity content up to about 80% by weight, quickly and with a good energetic and mechanical efficiency; for example, a single attritor mill 1 having a cylindrical grinding chamber with a diameter DC of about 64-65 cm and a height HC of about 82 cm is capable of treating, by reducing it into dust or dry granulate, an average capacity of 900 kg/hour of urban solid waste that is "wet" with a humidity content of about 80% by weight, against an electric energy consumption of about 20 kW/h. This makes the attritor mill 1 particularly suitable for making plants for treating waste or other materials, relatively small- sized, hence particularly suitable for being installed on islands, mountain zones or other places difficult to be reached or with scarce energetic availability, or again particularly suitable for being supplied with solar, aeolic energy or with other alternative energies .

Advantageously the organic solid waste to be treated are introduced in continuous into the grinding chamber 5 and after being treated they are extracted in continuous from the same grinding chamber 5.

[21] It is now described a possible functioning and use example of the previously described attritor mill 1.

The urban organic solid waste, or other material to be treated, possibly after preventive grosser screening or grinding, are introduced into the grinding chamber 5 through the supplying duct 15, for example through the aforementioned Archimedean screw or pusher and mixed with the free grinding bodies 11 and with the mass of waste - or other material to be treated - already present in the chamber 5 and already partially ground. As the rotor 7 quickly rotates on itself about the axis Al, the arms 9, 9', 9" repeatedly and randomly strike the free grinding bodies 11 more or less like baseball bats, throwing the bodies 11 one against the other, against the semi-ground mass of waste or against the walls of the grinding chamber 5. A good part of the mechanical energy provided by the rotor 7 directly or through the grinding bodies is converted into thermal energy that heats the chamber 5 and its content, in particular the free grinding bodies 11 and the waste - or other material to be treated - ground or semi- ground. As a consequence, the ground or semi-ground wastes, even if originally decomposed and putrefying, dry and desiccate, generally becoming more fragile and breaking up into granules, scales, dust, shavings or other particles substantially dry and anyway more volatile compared with the waste entering the chamber 5.

[22] Being more volatile, both due to the motion of the free grinding bodies 11 and to the suction from downstream - or of the push from upstream - of the fan 21, the dry particles of the waste tend to rise towards the upper part of the grinding chamber 5 until they exit through the extraction grids 17, which limit the maximum dimension of the exiting particles keeping the free grinding bodies 11 in the grinding chamber. Periodically, for example after a few days, the attritor mill 11 can be stopped and a user, for example through the door 28 can enter the grinding chamber 5, clean it from possible encrustations or dirt and extract the free grinding bodies (Figure 1) . The free grinding bodies 11 can thus be sieved eliminating those that have become too small due to wear and tear, afterwards they can be re-introduced into the chamber 5 possibly together with fresh free grinding bodies.

[23] From the previous description it is clear how the mill 10 is thus particularly suitable for operating in continuous, with a continuous inflow of waste or other material to be treated into the grinding chamber 5 and a continuous outflow of desiccated particles from the grids 17, requiring technologies relatively simple in order to automate the process. The aforementioned use of Archimedean screws or other feeders in continuous, and of the fan or other suction or blower 21 helps automating and making the functioning in continuous of the mill 1 possible. The use of the heat produced by the transformation of the mechanical power provided by the rotor 7 to desiccate or anyway to dry the organic solid waste helps increasing the energetic efficiency of the attritor mill and of the process for treating the organic solid waste according to the invention. Furthermore, the combined action of the temperature and of the repeated and violent impacts with the free grinding bodies 11 helps destroying a great part of the bacteria initially present in the waste, thus in particular destroying the resistant shells that protect such microorganisms, disinfecting and by the way making the product exiting from the mill biologically more stable . [24] The previously described examples of embodiments can undergo various changes and variations yet without coming out of the scope of protection of the present invention. For example an attritor . mill according to the invention can also operate discontinuously, for example by supplying the material to be treated and extracting the treated material according to the batch modality. The lifting projections 19 can be in the shape of not only rectilinear oblong ribs, but for example also curved. Moreover, all details are replaceable with technically equivalent elements. For example the used materials, as well as their dimensions, can be any according to the technical needs. It is to be intended that an expression such as "A comprises B, C, D" also comprises and describes the - particular case wherein "A consists of B, C, D" . The examples and lists of the possible variations of the present application are to be intended as non exhaustive lists.