Patent Application
20220266252
The present invention relates to a process to s-prepare, starting from heterogeneous matrices of plastic and fragile materials, ground materials with maximum desired particle size, as well as a device optimized to implement such process.
In particular, as a way of example and without this 10 limiting the aims of the invention, it is to be said that the process and the device, object of the invention, can be optimized to obtain a ground material with maximum particle size lower than 3 mm and weighted average of the obtained particles 15 lower than 1 mm, with high productivity per hour and with specific reduced energy consumption, starting from heterogeneous matrices of plastic and fragile materials.
At the state of the art, the problem of grinding heterogeneous materials in order to reduce their average sizes is well known. In particular, owing to the respective technical problems linked to grinding, it is known that materials are divided in 25 fragile materials (with high Young's modulus) and plastic and fibrous materials (with low Young's modulus.
As it is well known, the Young's modulus describes the behavior of a material subject to deformation. Generally, it is well known that, by subjecting a material to an ever increasing stress, the deformation of the same is proportional to the stress at first (field of linear elastic deformations), and in the following, beyond a threshold dependent on the material examined (yield threshold), the deformation ends to be proportional to the stress and increases more than proportionally with respect to the same (field of plastic deformations), up to break the material.
The constant of proportionality between stress and deformation (Young's modulus), the yield threshold value and the deformation quantity that the material can absorb after yield and before being broken are specific features of each material.
For what pertains to the grinding problem, generally it is known that materials with high Young's modulus (with values typically between 104 MPa and 105 MPa and higher) are subjected to little deformations also after high stresses, and generally reach the material breaking at stress values little higher than the yield threshold. These materials are commonly indicated as “fragile” materials. Since, as it is known the deformation work needed to reach material breaking is proportional to the area described by the curve of stress-deformation points between zero stress and breaking, and since in case of fragile materials such area is reduced, generally the fragile materials require low work to reach breaking.
The materials with low Young's module (with value lower than 104 MPa) have high deformations also when subjected to contained stresses, and generally, when the yield threshold is passed, they are able to absorb ever increasing deformations, also after reduced stress increases. Breaking occurs only after high deformations (also in the order of tens of millimeters). In these cases, the material is commonly identified as “plastic”, and since it is proportional to the area described by the curve stress-deformation up to breaking, the deformation work is sensibly higher than in case of fragile materials.
After this brief introduction, by analyzing the grinding devices known at the state of the art, it is to be noted that the same are distinguished by the modalities of energy transfer from the device to the mass, the dimensions of which are to be reduced.
In case of fragile materials, power is transferred to material as kinetic energy by means that compress the material abruptly by means of impacts. Said means are generally integral to a rotating element, and cause impacts on respective static (stator) organs.
Mills of the type known at the state-of-the-art function according to this principle, as for example crushing mills, reel mills and hammer mills.
Mills of the just described type are known with the most different geometrical shapes, yet all referable to the principle of providing a little stress locally between opposed grinding means.
According to another implementation of the same principle, the mills can comprise mobile organs, as rods or balls, which receive kinetic energy by the rotation of the grinding chamber case, or by the rotation of an inner rotor element which moves the mobile organs: it is the case of mills known as balls mills or rod mills. In each case, the functioning principle applied is that the impact on the material, given by two mill surfaces mobile and facing to each other, crushes the fragile material. Therefore, in the light of what just said about the materials features, it is clear that this technique cannot be applied to plastic materials, since the impacts obtainable by means of the described mills have a low shearing component and are not able to break the plastic material causing mainly their heating.
As it is known, after heating a plastic material a further flattening of the stress-deformation curve occurs. Therefore, the plastic material is never ground, but there occurs the formation of semi-melted aggregates which block the grinding machine. This is the reason why for plastic materials the mills known at the state of the art use a different grinding principle, and i.e. they provide locally a high shear stress by means of two tools, mobile to each other, characterized by the presence of sharp edge elements (blades) facing to each other at very little distances, substantially by sliding on each other. Stator and rotor function in these cases with relative high speeds and at reduced distances to each other (variable as a function of the desired particle size, and typically equal to about 1 mm in the common industrial usages). In this way, it is obtained the effect of concentrating mechanic action in limited areas and with very high local shear stress, so to obtain the reduction of material dimensions without an excessive heating. The mills known as blade mills or pin mills, which are used for dimension reduction of plastic material, rubbers, fibers, function according to this principle.
However, this technique cannot be applied to fragile materials, since the high shear stresses on fragile materials would cause a rapid deterioration by abrasion of the tools sharp edges. Moreover, yet modest contents of fragile materials in mixture with plastic materials cause important wear phenomena, due to the well known inclusion effect of fragile materials in the plastic matrix which multiplies the abrasive effect of the fragile portion
Therefore, the two categories of the just described grinding machines (respectively for fragile and plastic materials) have reciprocal incompatibility when it is desired their usage for materials of the other class of Young's modulus.
Anyway, there exist whole industrial fields, as the treatment of urban waste or the treatment of material yet stored in dumps, that have to handle the need to grind mixtures containing both fragile materials and plastic materials strongly different for composition (in the following also heterogeneous mixtures for brevity), for which at the state of the art there exist solutions only to provide coarse dimension reductions (up to average dimensions of some tens of millimeters), but there are no economic and reliable technical solutions able to obtain the grinding up to powders with average particle size of about 1 millimeter. The possibility to obtain from these materials powders of little dimensions would clear the way for many uses, recycle and treatment opportunities which at the moment are precluded because not apt to treat coarse ground materials.
The problem is so strong that there have been many attempts to grind heterogeneous mixtures with new concepts of grinding, some examples of which are described in the following.
A first solution known at the state of the art (THOR-ENEA) uses as grinding elements balls of hard material, kept in strong mechanic stirring by a plowshares rotor system. It exploits the principle of impact between grinding means, which produces the desired dimension reduction, but with improved efficacy of impacts yet definitely low. In fact, the system, according to the data available in literature, consumes some thousands of kWh (300-400 kWh) for each ton of ground material.
A second solution described in the Italian application MI2011A000320 describes a mill for grinding waste comprising at least two rotors, to each one of which a plurality of chains are connected, which sweep the grinding chamber, and in which the grinding chamber is obtained by the net sum of the grinding volumes, so that the whole grinding chamber is interested by the rotation of at least a chain. In the mill according to this document, in some embodiments, there are obstacles configured to avoid the waste accumulation in places of the grinding chamber not reached by the rotating chains. Briefly, this mill uses a plurality of chains, fixed at an end to at least two rotor organs which cause their unfolding under centrifugal force. The chains slide on a stator represented by a holed sheet. For the grinding, the device uses as principle the combination of the impact with the shear stress on the material induced by the sliding. The specific consumptions for ton of material treated are lower than THOR, but not significantly, above all for matrices with significative plastic materials content. Therefore, this solution cannot be applied with success to urban waste. Moreover, the device described is subject to rapid and clear wear of the grinding elements, and so the whole costs of device management are not acceptable in an industrial concept.
So, at the state of the art there remains unsolved the problem to provide a grinding process for heterogeneous mixtures of fragile and plastic materials which overcomes the limits linked to the processes known at the state of the art, as well as to provide a device able to implement said process efficiently.
More in particular, there remains unsolved the problem to provide a process allowing to grind heterogeneous mixtures comprising both fragile materials and plastic materials, with low specific energy consumptions, low wear of the tools and high productivity per hour with respect to the volume of the grinding chamber and the applied power, as well as a device implementing such process.
Yet, there remains unsolved the problem to provide a process and relative device which allow to obtain all the just described advantages and to grind heterogeneous mixtures up to average particle sizes in the dimension order lower than 1 mm. Yet, there remains unsolved the problem to provide a device which solves all the just described problems, and which has also modest wear and low whole management costs.
Therefore, aim of the present invention is to provide a grinding process which overcomes the limits linked to the processes known at the state of the art, as well as a device able to implement said process efficiently.
According to another aim, the present invention provides a process which allow to grind—also up to average particle sizes in the dimension order of 1 mm—heterogeneous mixtures comprising both fragile materials and plastic materials, with low specific energy consumptions, low wear of the tools and high productivity per hour with respect to the volume of the grinding chamber, as well as a device which implements such process.
Yet, according to another aim the present invention provides a device with all the just described advantages and which has also modest wear and contained whole management costs.
The present invention reaches the prefixed aims since it is a process for grinding heterogeneous matrices comprising fragile materials and plastic materials, comprising the steps of: (i) introducing in a mill or similar device, a heterogeneous matrix comprising plastic materials and fragile materials of variable dimension; (ii) applying, by suitable moving actuator means, kinetic energy to said heterogeneous matrix, by projecting the materials of the matrix at high speed against suitable fixed stop means provided on said mill; (iii) proceeding to grinding under the effect of impacts occurring between the material projected by the actuator means and the material accumulated on said stop means; (iv) discharging the portion of processed material reduced under a prefixed maximum diameter, the method being characterized in that said stop means are configured so that they can hold removably the material projected against the same in an outer region with respect to the volume interested by the movement of said actuator means. The invention provides also a device for grinding heterogeneous matrices comprising both fragile materials and plastic materials configured to carry out the process according to any one of the preceding claims, comprising: a holed drum (1) provided with holes (11) on its outer surface; a plurality of actuators (6) positioned inside said holed drum (1), fastened by means of respective flexible elements (61) to a central rotating shaft (7), characterized in that said holed drum (1) comprises also, on its inner surface, a plurality of stop means (12), configured to stop the material projected against the same by said actuator means, said stop means (12) being configured to cause the stop of material in grinding step in a peripheral area of said holed drum, not interested by rotation of said flexible elements (61) nor of said actuator means (6), said actuator means (6) being configured to be kept, during their rotation around said shaft (7) at a minimum distance from said stop means (12) higher than the diameter of said holes (11) provided on said drum (1).
These and other advantages will be clear from the detailed description of the invention, which is described in the following with reference to the appended
In
in
in
In
With reference to the appended
The device comprises also a charging hopper (3) configured to allow the introduction of material inside the drum (1), and a discharging mouth (4) configured to allow, preferably by gravity, the expulsion of material from the space comprised between the holed drum (1) and the case (2). It is just the case to specify that the hopper (3) and the discharging mouth (4) allow the continuous material supplying and discharging, also during the machine functioning. Preferably, the hopper (3) communicates with the central portion of the drum (1), so to reduce the possibility that the processed material is projected outside the machine by means of the charging hopper.
Inside the holed drum (1) there are provided a plurality of actuators means (6), fastened by means of flexible elements (61) to a central rotating shaft (7), moved by an engine (8) by means of suitable transmission means (9). As a way of not limiting example of the aims of the invention, said flexible elements (61) can be made up of metal cables or chains.
Preferably, said actuator means are fastened each by means of the respective flexible element (61) according to a regular arrangement of the rotating shaft (7). In a first embodiment, said actuator means are arranged aligned on more rows, the various rows being arranged angularly at equal distance (for example each one at 90°, as it is shown in
Thanks to their connection to the rotating shaft, when the machine is in function said actuator means (6) are radially arranged under the effect of the centrifugal force provided by the rotating shaft. Said actuator means, responsible for the kinetic energy transfer from the rotating shaft to the material mass to be ground, are configured so that, in a side view, their cumulated axial dimensions take up substantially the whole axial development of the machine.
In
This is visible in particular from section B-B and detail “C” of
These constructive measures allow the quantity of transmitted energy by the flexible elements (61) to the processed material to be modest. Since this kinetic energy is transmitted at modest speeds, not sufficient to obtain efficient impacts from the material (by efficient impacts meaning those ones able to cause the material grinding), reducing this energy allows to reduce also the energy waste, as well as to contain the processed material overheating.
The holed drum (1) comprises also, on the inner surface of its shell, a plurality of stop means (12), configured to stop against the same the material in grinding step (20).
The stop means (12) are specifically configured to cause the stop of the material in grinding step in a peripheral zone of the grinding chamber defined by the holed drum, not interested by the rotation of the flexible elements (61) nor of the actuator means (6)
An embodiment of said stop means is shown schematically in
In an embodiment shown in
It is suitable to specify that, by assuming that the drum (1) is cylindrically shaped, the zone not interested by the rotation of the actuator means (6) comprises the cylindrical crown, whose outer surface is defined by the drum (1) and whose thickness is given by the sum of the height of said stop means (12) and the minimum distance between said actuator means (6) and said stop means (12).
The actuator means (6) and the stop means (12) integral to the drum (1) are configured so that the actuator means (6), by rotating, are kept at a minimum distance from said stop means (12) well greater than the maximum particle size of the ground material desired, defined by the holes (11) of the drum (1).
From detail A of
According to a preferred embodiment said distance is between 1.3 and 5 times the maximum diameter defined by the holes (11). Therefore, as a way of example, it is to be specified that when the holes (11) are of dimensions equal to about 3 mm, said minimum distance between the actuator means (6) and the stop means (12) is between 4 and 15 mm.
According to another embodiment, when the holes (11) are of dimensions equal to 1 mm, said minimum distance between the actuator means (6) and the stop means (12) is between 1.5 and 5 mm, and the process allows to obtain ground material with maximum particle size equal to 1 mm and weighted average particle size between 200 and 600 pm.
In order to highlight the peculiarities of the device object of the description, it is to be specified that in all the mills known at the state of the art, grinding requires the use of two bodies in mutual movement opposed to each other at mutual distance lower than the particle size desired. This occurs or by means of a mechanism of blade type to induce shear stresses in plastic materials, or by means of a mechanism of impact type to produce impacts able to break the fragile materials.
In the mills known at the state of the art, the distance between the actuator means (6) and the stop means (12) is never kept at dimensions significantly greater than the maximum particle size desired, defined by the diameter of the discharging holes of the ground material. Indeed, by reasoning according to the logic known at the state of the art and yet described, in case of using holes of 3 mm to allow the ground material discharge, the expert in the field would implement mutual distances well lower than 3 mm between the actuator means (whether blades, hammers, balls or means of other type) and the respective stop means of the material (whether other blades, stator fixed parts or means of other type.
Moreover, in none of the grinding mills known at the state of the art there are provided specific measures to stop the ground material outside the area swept by the rotation means (i.e. flexible elements and actuators), thus subtracting the material in the peripheral zone from the direct action of the mechanic means in movement, and exposing it instead only to impacts with other particles of the mass in grinding step.
The distance between the actuator means (6) and the stop means (12) makes unnecessary sharp edges on the ones and on the other ones. Blade sharp edges are, as known, the first elements which wear in mills. The fact that the presence is not required reduces drastically the frequency and the difficulty of maintenance interventions, thus contributing together to all the other just described measures to contain the whole costs (energy and maintenance) for ton of obtained ground material.
In fact, as it is clear from detail A of
As a way of example, it can be said that the actuator means (6) have a connection radius (R) with length of the same dimension order as the free space between the actuator means (6) and the stop means (12).
Moreover, the possibility to realize both actuator means and stop means without sharp edges reduces the realization costs of the elements of the machine and increases the efficiency and duration of the treatments, as hardening, intended to increase the material hardness.
The just described constructive difference between the preferred embodiment and the mills known at the state of the art (or other differences described in the following) derives from the different grinding process implemented, object of the present invention as well.
The grinding process according to the invention comprises in fact the steps of:
(i) introducing in a mill or similar device a heterogeneous matrix comprising plastic materials and fragile materials of variable dimension;
(ii) applying, by suitable moving actuator means, kinetic energy to said heterogeneous matrix, by projecting the materials of the matrix at high speed against suitable fixed stop means configured to hold removably the material projected against the same in an outer region with respect to the volume interested by the movement of said actuator means;
(iii) proceeding to grinding under the effect of impacts occurring between the material projected by the actuator means and the material accumulated on said stop means;
(iv) discharging the portion of processed material reduced under a prefixed maximum diameter.
A preferred element of the just described process is that the speed of the actuator means used to provide kinetic energy to the material is between 20 and 60 m/s. Preferably, said speed is between 30 and 55 m/s, and more preferably between 40 and 50 m/s. According to tests carried out by the applicants, the use of the just described speed ranges for the actuator means allows to optimize the process productivity per hour and to reduce the energy consumption for tons of treated material. Preferably, moreover, the process is implemented in machines that allow the continuous supply of material to be processed and the continuous discharge of the ground material.
Preferably, moreover, the mobile actuator means and the fixed stop means never reach, during the process, mutual distances lower than the maximum particle size allowed for the ground material discharge.
As it can be observed, the process implemented does not comprise any shear, squeezing or similar operation of the material between two portions of the machine implementing the process, but only the obtainment of impacts between the particles of the matrix to be ground.
For this reason, in the just described preferred embodiment, the actuator means (6) are fastened to the rotating shaft (7) by means of flexible elements (61). In this way, if during the functioning a particle of dimensions higher than the distance between the actuator means and the stop means is compressed between the two of them, it will be observed a flexion of the flexible elements (61) which will move away the actuator means (6) from the stop means (12), not compelling the machine to squeeze nor to cut the material between two parts of the same machine.
In other terms, if the impact between the material mass dragged in rotation and the mass accumulated on the stop means (12) occurs overcoming the maximum stress provided by the machine project kinematic analysis, the flexibility of the elements (61) allows the connection to move away the actuator means (6) from the maximum circumference on which they are arranged under the centrifugal force effect, thus increasing the distance between the actuators (6) and the stop means (12).
It is to be specified that with the just described process, implemented by means of a machine of the described type, high values of productivity per hour have been recorded, between 2.0 and 3.5 ton/h for each m3 of grinding chamber volume, and a low energy consumption, between 40 and 80 kWh, for each ton of treated material.
It is suitable to specify that the energy consumption expressed in kWh/ton of treated material, is always valid. The volume productivity per hour, instead, is of common usage for commercial grinding machines, and for this reason it was used as indicator also for the machine of the invention.
The machine of the invention implements a principle of power transfer principally for useful impacts concentrated at the circular cylindrical periphery of the stator drum. Therefore, it would be logical to speak about the production of ground material “for surface unit”, and of capacity scale rules on the basis of the cylindrical surface of the stator drum. Anyway, the productivity per hour would be expressed according to an index not used, and practically little comprehensible, and therefore the distinctive productivity index above described remains according to the units commonly used.
The just described productivity and specific consumption values are referred to the grinding of various lots of undifferentiated urban waste, provided to the machine in form of coarse material obtained by a pre-treatment with clear span screens of 90 mm, up to obtaining a ground material with weighted average dimension of 1 mm, with the upper end of the distribution (defined by the diameter of the holes on the drum) equal to 3 mm. As it can be easily understood, the variability of the just described values derives from the variability of the features of the mixture incoming. In particular, while the percentage of the plastic components in the mixture to be ground increases, the specific consumptions increase.
Anyway, it is to be specified that the grinding process according to the invention does not have substantial efficacy losses while reducing the maximum dimensions admitted for the ground material, except an obvious increase of specific consumptions for ton of product. Naturally, it is possible to obtain ground materials of greater particle size by increasing further the specific productivity and reducing the energy consumption. For example, in this way ground materials with maximum particle size of 5 or 10 mm can be obtained, simply by varying the dimensions of the holes on the shell of the drum (1).
Substantial element of the process according to the invention is that the energy supplied by means of the rotating shaft is transmitted mostly to the actuator elements (6). Mostly means that the kinetic energy percentage provided to the actuator means (6) with respect to the whole kinetic energy supplied to the machine is between 50 and 90%. This means that the maximum portion of the energy provided to the machine is concentrated in the peripheral zone of the machine. Since in this zone, the speeds are maximum, it is energy which can produce impacts useful to grinding (i.e. impacts after which the material crushes).
In the just described process, the portion of the energy transmitted to the solid material to be processed at low speed is extremely limited, energy which produces inefficient impacts for grinding and generates heat.
Possibly, the process according to the invention can comprise the step of adding water to the material to be ground, in order to limit the increase of temperature caused by the treatment.
The need to add water or not depends on the type of material to be ground, which can comprise enough thereof yet.
The solid materials which can be treated by means of the process according to the invention comprise fragile materials characterized by a Young's modulus higher than 104 MPa and lower than 105 MPa, preferably in mixture with plastic and/or fibrous materials with Young's modulus lower than 104 MPa.
Aa a way of not limiting example, there can be treated mixtures containing one or more of the following materials: metals (as for example aluminum), organic materials (biomasses of various nature, wood), urban solid waste from undifferentiated collection, urban waste from differentiated collection (for example the plastic fraction, glass), rubbers.
It is the case to specify that in the industrial reality, many materials defined “rubbers” are compounds very rich of mineral charges, and components with Young's modulus higher than 104 MPa and lower than 105 MPa can exceed 30% of the composition.
Therefore, the term “mixed materials” is to be meant not only under the effect of features of product heterogeneity but also in the sense of base chemical composition.
Finally, the present invention provides a grinding process with low energy consumption for heterogeneous matrices, and the obtainment of fine particle sizes which is so able to make many techniques of today not convenient waste treatment competitive, since it allows:
| Number | Date | Country | Kind |
|---|---|---|---|
| 102019000011376 | Jul 2019 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/056340 | 7/6/2020 | WO |
