EP1306199A2

EP1306199A2 - Method and plant for producing pellets

Info

Publication number: EP1306199A2
Application number: EP02425640A
Authority: EP
Inventors: Fulvio Soldaini
Original assignee: Ecotre System Srl
Current assignee: AVS SLOVAKIA S.R.O.
Priority date: 2001-10-26
Filing date: 2002-10-24
Publication date: 2003-05-02
Also published as: CN1413826A; EP1306199A3; ITFI20010201A1

Abstract

The invention refers to a plant and a method for the production of pellets; the plant comprises means for feeding material to be pelletized, at least one pelletizing machine, means for the pneumatic extraction of pellets on output from said machine and means for recirculating, that is, feeding back to the pelletizing machine, at least part of the products on output from the same machine.

Description

The present invention refers to a method and plant for the production of pellets.
It is known that in the pelletization on an industrial-scale of various type of materials, use is made of so-called "pelletizing" machines being provided with cylindrical or of other shape drawplates which the materials to be pelletized are made to go through.
Pelletizing machines are described in EP 846554, WO 93/22132, FR 1371346 and GB 1153862, IT BO/93/A/116, IT BO/93/A/115.
The plants using pelletizing machines currently known give outputs of pellets having unsteady characteristics, that is, such characteristics that vary in the course of time, in compaction degree, liquids content and humidity level.
Moreover, existing plants have relatively high energy consumptions and, accordingly, their cost is too large with respect to the market value of the pellets, especially when these are obtained by recycling waste material.
The main object of the present invention is to provide an operational method and plant which allow the optimization of energy consumptions in relation to the production of a pelletized material having substantially steady characteristics.
This result has been achieved, according to the invention, by adopting an operational method and plant having the characteristics indicated in the independent claims. Further characteristics of the present invention being set forth in the dependent claims.
The present invention makes it possible to provide a uniform and optimized production of pellets having constant structural and morphological characteristics while reducing the energy consumptions. Moreover, a plant according to the invention is relatively easy to make, requires relatively small space and allows the treatment of pelletizable materials of various nature, size and original humidity, such as biological dehydrated muds from depuration plants, waste pastes and pulps from industrial processes, fodders, combustible fractions of urban solid waste, compost, paper and paperboard, brushwood, textile waste, wooden shaving, sawdust, plastics materials and chemical products able to be pelletized.
These and other advantages and characteristics of the invention will be best understood from a reading of the following description in conjunction with the attached drawings given as a practical exemplification of the invention, but not to be considered in a limitative sense, wherein:

Fig. 1 is an outline of a possible embodiment of a plant according to the invention;
Fig. 2 is a block diagram of the system for controlling the process parameters for the plant of Fig. 1;
Figs. 3, 4 and 5 show schematic representations of possible embodiments of a plant according to the invention;
Fig. 6 is a schematic representation of the interaction between the material to be pelletized and the drawplates;
Fig. 7 is a schematic side view of an apparatus for evaluating the parameters of pellets on output from the plant.

Reduced to its basic structure, and reference being made to Fig. 1 of the attached drawings, a plant according to the invention comprises:

a first hopper (11) wherein the material to be pelletized is introduced, the lower outlet of said hopper being provided with a wire sieve having meshes of preset width;
an aeration separator (12), downstream of the loading hopper (11), which is connected to the latter via a conduit developing between the inlet section of the same separator (12) and the outlet section of a feed screw (110), the latter being located below the hopper (11) to receive the exiting materials;
a refining mill or refiner (13), located downstream of said separator (12);
a first aspirator (9), whose inlet is connected with the outlet of said refiner (13) via a conduit (90), the outlet of the aspirator (9) leading to a silo (6) for collecting the material to be pelletized;
a motor-driven screw extractor (7) located at the bottom of the silo (6) and in communication with the latter;
a motor-driven belt or screw conveyor (8), located between said extractor (7) and a second hopper (1), for supplying a pelletizing machine (2), a screw meter (100) being disposed between said second hopper (1) and the machine (2), the outlet section of said meter being in communication with the inlet of the machine (2);
means for the pneumatic extraction of pellets from said machine (2), with a second aspirator (30) which conveys the pellets to a cyclone separator (3) and provides as well for aspirating dust from the latter and deliver the same dust to said silo (6);
a belt conveyor (4) in correspondence of the outlet section of said cyclone (3);
a sieve (5) downstream of said belt (4);
a station for bag-filling the pellets downstream of said sieve (5).

By using a pelletizing machine of a type having one or more cylindrical drawplates, a portion of the material fed thereinto result drawn, that is, formed into pellets, while the remaining part, as well known to those skilled in the art, escapes from the action of the drawplates and, therefore, results non pelletized.
In the layout of Fig. 1, arrow (I) indicates the input material to be pelletized, arrow (U) indicates the output pelletized material, arrows (S) indicate the material fed to the silo (6) from the conduit (90) as well as the air extracted from the cyclone (3) by the aspirator (30), arrows (P) indicate the pellets on output from the drawplates (20) of machine (2), arrows (R) indicate the material not drawn and made to recirculate, and arrow (G) indicates the material thrown back into a vessel (14) to be described later on, the material on output from the silo (6) to the machine (2) being indicated by arrows (Q).
The sieve of hopper (11) allows merely the passage and thus the treatment of material having a size non exceeding a preset limit. The feed screw (110) supplies the separator (12) with material going through the meshes of the sieve (1).
The separator (12) provides, with procedures known per se, for separating from said material heavier objects such as metal bodies, ceramics, stones, etc., and send them to a waste-collecting vessel (14). The refiner (13) provides for crushing and dimensionally homogenizing the material on output from the separator (12). The aspirator (9) sucks the treated material from the refiner (13) and sends it to the silo (6), together with the material (R) that hasn't been properly pelletized within the machine (2) and introduced into the conduit (90).
The silo (6) is provided with a set of filters (60) for discharging air, suitably freed from the dust, out in the environment. The screw extractor (7) provides for unloading the material from the silo (6) in an amount varying as a function of the rotary speed of its axis. The conveyor (8) transfers the material from the extractor (7) to the hopper (1). From the latter, via the metering screw (100), the material (Q) is introduced into the pelletizing machine (2).
The latter, as above mentioned, may be of a type having one or more cylindrical drawplates (20) and provides, according to procedures known to those skilled in the art, for compressing the materials arriving therein, by forcing them to go through the holes of the same drawplates. As stated above, machines of this type are described in EP 846554, WO 93/22132, FR 1371346 and GB 1153862, to which documents reference can be made for further details. The second aspirator (30) is connected to the cavities (21) of the drawplates of machine (2) - the pellets being collected within said cavities via corresponding connecting conduits (the route of which is indicated by arrows P in the layout of Fig. 1) - and provides for aspirating the pellets and putting them into the cyclone separator (3). The latter exhibits an inlet and an outlet for the pellets, the outlet being in correspondence of the separator's lower base and facing the belt conveyor (4). The dust going along the pellets in this step of the work cycle arrives at the sieve (5) together with the pellets. The sieve (5), located downstream of the belt conveyor (4), is of vibration type and with meshes of preset width. The pellets caught by the wire net are bag-filled at a well known bag-filling station (not described). The pellet fragments, residual dust and pellets which are of a size smaller than the one preset, pass through the meshes of the sieve (5) and are aspirated by the first aspirator (9), which is pneumatically connected with the lower base of the sieve (5), and are thus put again in circulation, that is, conveyed into the said conduit (90).
All the too-noisy apparatuses can be disposed within a soundproof cabin (10).
Advantageously, according to the invention, provision is made for regulating the flowrate of the air flow generated by the aspirator (9) as a function of the humidity of the material fed into the pelletizing machine (2). More particularly, provision is made for increasing said flowrate when the material to be pelletized exhibits a humidity of a level higher than a preset limit, and vice versa. The humidity of the material can be detected by means of probes (IG), of known type, located within the silo (6) and/or the hopper (1).
Also advantageously, there is provided for possibly adding water or other liquids to the material to be pelletized when the humidity level is lower than a preset limit.
In practice, by regulating said flowrate of air, and by adding water or other liquids, as necessary, the material to be pelletized can be given its optimal humidity level.
For example, if a probe (IG) within the silo (6) senses that the material present therein has a humidity lower than 16÷18%, water is put into the silo (6), into the feeder (8) or into the machine (2), until one of subsequent detections reveals a humidity level corresponding to the programmed value (in this example, a value in the range of 16-18%).
Also advantageously, provision may be made for detecting the power absorbed by the motor(s) of the machine (2) and the temperature of drawplates, in order to adjust the quantity of material under treatment according to the values of these two parameters. More particularly, if under normal operating conditions, and regardless of the nature of the material in the process of formation, the data of said parameters exceed corresponding preset values, the rotational speed of the screw (7) and/or screw (100) associated to hopper (1) is reduced.
For example, should an absorption of current by the motor unit of machine (2) be detected higher than 100 A, then a reduction of motor (MC)'s rpm would be operated in order to supply the machine (2) with a smaller quantity of material.
Also advantageously, provision may be made for detecting the humidity and bulk density of output pellets (the bulk density being defined as the ratio between the weight of pellets contained in a known volume and the value of such volume).
The humidity degree of the output pellets can be evaluated by means of hygroscopic probes (IGU) disposed in a container of known dimensions (that is, of known volume) positioned upon load cells intended for receiving pellets on output from the plant to measure the bulk density, as above mentioned. For example, said measurement can be carried out on pellet samples on output from the plant, in the same way as illustrated in Fig. 7. In Fig. 7 numeral (70) designates the container for receiving the pellets from a belt conveyor (75) possibly provided in correspondence of the outlet (U) of the plant. The container (70) may be provided with a bottom (73), able to be open for the discharge of pellets, with load cells (72) and with one or more igroscopic probes (71) suitable for sending signals respectively to a unit (IGU) for computing the bulk density "p_a" (PA) and a unit for computing the humidity "u" (IGU). In the present description, p_a, p_r, u and u_r refers, respectively, to the bulk density of output pellets (p_a), a corresponding reference value (p_r), the humidity of output pellets (u) and a corresponding reference value (u_r).
The amount of material introduced into the machine (2) is related to the current absorbed by the motor(s) of the same machine, since, the material's consistency being equal, the treatment of a greater quantity of material corresponds to a higher absorbed power, and vice versa. With this respect, a maximum amount of material can be defined as the quantity of material in consequence of which the power absorbed by the machine (2) is the highest, that is, a quantity corresponding to a value of power developed by the motor unit beyond which structural collapses in the drawplates (20) are likely to occur.
Accordingly, provision is made for checking the quantity of material put in the machine (2) under normal operating conditions, so that, if the current absorbed by the motor unit of machine (2) is equal to the preset maximum value, the speed of the motor of feeder (100) is reduced thereby reducing the amount of material introduced into the machine (2).
Under steady operating conditions, provision is made so that the quantity of material fed into the machine (2) be such that the power absorbed by the motor unit of the same machine be equal, within the tolerances allowed by the accuracy of the measuring devices, to the corresponding rated power. In practice, the system is self-controlled, thereby providing, for example, either an increasing quantity of material, until the power absorbed is less than the rating (this being an index of poor resistance of the material to the drawplates' action), or a reduction of such quantity, or an increase of water quantity if the absorbed power is higher than the rated one.
That being stated, if, when u>u_r, in spite of the complete closing of water-admitting conduits and maximum allowed increase of flowrate of air sucked by the aspirator (9), the humidity of output pellets results still higher of the limit value u_r, then it is possible to operate the introduction of material in the machine (2) at the highest flowrate allowed by the feeder (100) for a predetermined time (for example, 5 minutes) even if during this time the power absorbed by the motor unit of machine (2) results higher than the rated power. Should the power absorbed by the motor unit of machine (2) be equal, during this time, to said maximum safety value, and again u>u_r, then the plant would be stopped to be checked. If, instead, u=u_r during this time, the normal supply for machine (2) is re-established.
By still using the abbreviations previously indicated, if p_a<p_r and u=u_r upon the output, the following measures are taken: reduction of the quantity of water admitted in the plant, possibly to the point of excluding the water admission; reduction of the quantity of material fed into the machine (2) in order to extend the time of permanence thereof within the holes of the drawplates; increase of the flowrate of recirculation of air aspirated by the machine (2). The said adjustments are carried out in the described order and upon completion of each adjustment the value of p_a is measured. If, however, it results p_a<p_r, then the plant is stopped and checked.
If there is occurs that p_a>p_r and u=u_r, then: water is let in; the quantity of recirculated material is decreased; the quantity of material fed-in is increased and, for a preset time, an absorption of 10% higher than rated one is allowed. Again, these operations are performed in the described order. If p_a=p_r and u>u_r, then: the recirculation is increased; the quantity of admitted water is decreased. Again, these operations are performed in the described order.
If p_a=p_r and u<u_r, then: the quantity of admitted water is increased; the quantity of material fed-in is increased; the recirculation of material is reduced. Again, these operations are performed in the described order.
By way of example, experimental tests have shown that in the production of wooden pellets with medium hardness (a spruce hardness, for example), by using a pelletizing machine with drawplates having nominal diameter of 600 mm and height (thickness) of 150 mm, with holes having nominal diameter of 10 mm, and a 75-kW motor, the absorption of power necessary for obtaining a pellet - regarded as optimal - with p_a = 680÷720 kg/m³ and u = 6÷9%, is about 50 kW per 1000 kg/hour of material admitted in the machine (2), which corresponds to about 100 A of absorption by the relevant motor unit. In Fig. 6, wherein two drawplates (20) are schematically shown, the diameter of the drawplate (20) has been designated by (d), the thickness of drawplate (20) by (h) and the diameter of hole (21) by (b).
It results thus possible to produce pellets characterized by a humidity and bulk density of preset values, within the tolerances allowed by the accuracy of the measuring devices being used.
In the example of Fig. 2 a diagram is given of the system which carries out the said operating steps. A known per se amperometer (or a wattmeter) (AW) with digital output is provided for connection with the supplier of machine (2) and with a programmable microprocessor (UE). Also connected to the microprocessor (UE) is the sensor (T) intended for detecting the temperature of the drawplate of machine (2), the sensor (IG) for detecting the humidity of the material to be treated (for example, the material in the silo 6), the sensor (IGU) for detecting the humidity (u) of the output pellets, and the unit (PA) for computing the bulk weight (p_a). The output of microprocessor (UE) is connected to the motor (MA) of aspirator (30), to motor (MT) of drawplates (20), to motor (MC) of feed screw (100) and to the motor (MMC) of feed screw (7). Any suitable, known, microprocessor-based system can be used for the purpose. In the same diagram of Fig. 2 a keyboard (TA) is for inputting and/or selecting reference values of the process parameters above defined.
The plant relative to Fig. 3 includes, immediately downstream, the hopper (11) for the loading the material to be treated, a deferrization unit (80) for separating the metal materials from non-metal ones and, downstream thereof, a metal detector (81). Provided in correspondence of the complex formed by the hopper (11), deferrization unit (80) and metal detector (81), is a belt conveyor (82) which receives the material to be treated from the hopper (11) and carries the same material along in the direction of the arrow (W). Disposed above the terminal section of the belt (82) is the mouth of the manifold of a conduit (90) associated with the aspirator (9). When the metal detector (81) detects the presence of metal products, the aspiration through the conduit (90) is deactivated so that the metal products (PM) are conveyed by the belt (82) to a station (83) for collecting the same.
According to this exemplary embodiment of the plant, two refiners (13) are provided, both of them being associated, on one side, with the manifold (90) and, on the opposite side, with the aspirator (9). The material (Q) coming out of the silo (6) is pneumatically conveyed, by a further aspirator (84), in a cyclone chamber (85). The aspirator (84) receives the material delivered by the silo (6) through two delivery valves of stellar type or rotocells (600). From the chamber (85), the lighter fraction of the material is put again in circulation (R), whereas the heavier one is introduced in the pelletizing machine (2) with the aid of two vertical, motor-driven screw feeders (86). The pellets (P) going out from the machine (2) are pneumatically conveyed to an aeration separator (87) which, on one side, is associated with a corresponding aspirator (88) providing for conveying the dust toward the filters (60) and, on the opposite side, unloads the pellets into a final dust remover (89) via a rotocell or stellar valve (870). The dust intercepted by the filters (60) and dust remover (89) are aspirated into the manifold (90) and then recirculated, as it occurs for pellets of a size smaller than a preset limit and for the material escaped from the action of the drawplates of machine (2).
What has been stated in regard to the control of the process parameters (operating temperatures of drawplates, humidity of material under treatment and humidity of output pellets, etc.) applies also to the plant schematically represented in Fig. 3.
The plant related to Fig. 4 includes the use of a pusher (110) associated with the loading hopper (11), which discharges into an underlying slow crusher mill (111). The material thus crushed is pneumatically sent into the silo (6) by the action of aspirator (900). The rotocell or stellar valve (901) associated with the silo (6) unloads the material onto the belt conveyor (82), which can be operated for moving the material in the directions of arrows (K) and (L).
Associated with the belt conveyor is the metal detector (81), likewise in the previous example. If the presence of metal products (PM) is detected, the belt is actuated for directing the latter to the corresponding collection point (83) by moving them away from the system (arrow K). Located on the opposite side (arrow L) is the manifold (90) associated with the aspirator (9) to pneumatically convey the material to the crusher (13) and, from here, to a storage unit silo (61).
Provided below the silo (61) is a horizontal feed screw (62) which takes material therefrom (via the rotocell 610) to supply it to a vertical screw meter (63). The latter feeds the machine (2) with said material. Water (from the container WO) can possibly admitted into the horizontal feed screw (62) to adjust the humidity of the material fed to the machine (2), as previously described. Moreover, oil (from container OL) can be introduced into the machine (2), as necessary, to ease the formation of pellets in the same machine. Likewise in the layout of Fig. 3, the pellets coming out of the machine (2) are pneumatically put into an aeration separator (87) associated with an aspirator (88), the same aspirator unloading pellets, of a size no less than the preset limit, into a final dust remover (89).
The plant relative to the example of Fig. 5 provides for the use of a crusher-mill (111) whose outlet is connected to an aspirator (900), which is in turn connected with a cyclone separator (990).
The cyclone separator (990) has an upper outlet for the dust (992) connected with a filtering bank (60) and a lower outlet on which a rotocell (991) is made to act for the connection thereof to a deferrization unit (80).
Provided downstream of the deferizzation unit (80) is a gravimetric separator (993) provided with two outlets, one (999) for unloading heavy materials (HC) and one connected with a silo (6) via a fan (998).
The silo (6) also has an upper outlet connected to the filtering bank (60) and an outlet below provided with a rotocell (600) which discharges the material onto a belt conveyor (82) provided with metal detector (81).
If there is detected the presence of metal products (PM), these are disposed of in correspondence of the metal detector (81), while the material suitable for the pelletizing process is fed to the relevant machine (2) by the belt (994) which, in Fig. 5, is represented cut into two portions for the sake of clarity. Located downstream of the pelletizing machine (2) is a main conduit (995) for the transfer of produced pellets, an aeration separator (87), and a secondary conduit (996) which is for carrying the waste material back to the upstream section, that is, in correspondence of the conduit (997) shown in Fig. 5, located upstream of fan (998), which is in turn disposed upstream of silo (6).
The pellets arriving at the aeration separator (87) are subjected to the action of aspirator (88), also connected to the filtering bank (60) and, before leaving the work cycle, go through a final dust remover (89).
For the sake of clarity, in the drawing of Fig. 5 the filtering bank (60) is depicted as non-connected with the various conduits leading thereto. Besides, the components which are given the same numerals as the corresponding components of preceding figures are similarly operated.
As necessary, provision may also be made for re-introducing into the silo (6) a portion of the pellets intended for bag-filling, in order to put further material into the holes of the drawplates to provide higher resistance, that is, a resistance suitable for the formation of pellets.

Claims

Plant for the production of pellets, comprising means for feeding material to be pelletized, at least one pelletizing machine, means for the pneumatic extraction of pellets on output from said machine, characterized in that it comprises means for recirculating, that is, feeding back to the pelletizing machine, at least part of the products on output from the same machine.
Plant according to claim 1, characterized in that it comprises means for recirculating dust and pellets which are of a size smaller than a preset value.
Plant according to claim 1, characterized in that the said means for recirculating at least a portion of the products on output from the pelletizing machine comprise a filter (5) with a net sieve having mesches of predetermined width.
Plant according to claim 1, characterized in that it comprises a sensor (T) for detecting the operating temperature of said pelletizing machine and programmable microprocessor-based means (UE) which receive data from the sensor (T) to adjust the operating speed of said feeding means on the basis of preset temperature parameters.
Plant according to claim 1, characterized in that it comprises a sensor (AW) for detecting the power and/or the current absorbed by the motor of said pelletizing machine, and programmable microprocessor-based means (UE) which receive data from the sensor (AW) to adjust the operating speed of said feeding means on the basis of preset power and/or current parameters.
Plant according to claim 1, characterized in that it comprises one or more probes (IG) for detecting the humidity of the material fed into the pelletizing machine, which probe is connected with programmable microprocessor-based means (UE) which receive data from said probe (IG) to adjust the humidity level of said material in relation to a predetermined reference value.
Plant according to claim 1, characterized in that it comprises one or more probes (IGU) for detecting the humidity level of the output pellets.
Plant according to claim 1, characterized in that it comprises means (PA) for detecting the bulk density of the output pellets.
Plant according to one or more preceding claims, characterized in that at least one of said pelletizing machines is of a type with one or more drawplates.
Method of producing pellets, comprising the supply of material to be pelletized and the pelletization by one or more pelletizing machines, characterized in that it comprises recirculating at least a portion of the products on output from said one or more pelletizing machines.
Method according to claim 10, characterized in that it comprises checking the humidity level of the material to be pelletized and/or of the humidity level of output pellets and/or of the bulk density of output pellets and/or of power or current absorbed by one or more pelletizing machines and/or of the operating temperature of said one or more pelletizing machines.
Method according to claim 10, characterized in that it comprises recirculating dust and pellets which are of a size smaller than a preset value.