ITFI20010201A1 - METHOD AND PLANT FOR THE PRODUCTION OF PELLETS - Google Patents

Description

DESCRIPTION

The present invention relates to a method and a plant for the production of pellets.

It is known that the pelletization on an industrial scale of materials of various kinds involves the use of machines, so-called "pelletizers", which are equipped with dies, cylindrical or of other shape, through which the materials to be pelletized are forced to pass.

Pelletizing machines are described in documents EP 846554, WO 93/22132, FR 1371346 and GB 1 153862, IT BO / 93 / A / 1 16, IT BO / 93 / A / 1 15.

The plants that use the pelletizing machines known today make a production of pellets with inconsistent characteristics, that is variable over time, both with reference to the degree of compaction and to the liquid content or degree of humidity. Furthermore, current plants have relatively high energy consumption and, therefore, the resulting cost is excessive compared to the economic value of the pellets, especially when these are obtained from the recycling of waste material.

The main object of the present invention is to propose an operating method and a plant which allow to optimize energy consumption in relation to production and to produce pelletized material with substantially constant characteristics.

This result has been achieved, in accordance with the invention, by adopting an operating method and a plant having the characteristics indicated in the independent claims. Other features of the present invention are the subject of the dependent claims.

Thanks to the present invention, it is possible to standardize and optimize production, obtaining pellets with constant structural and morphological characteristics, and to reduce energy consumption. Furthermore, a plant according to the invention is relatively simple to manufacture, occupies relatively modest spaces and allows the treatment of pelletizable materials of various kinds, sizes and original humidity, such as dehydrated biological sludge from purification plants, biological fertilizers and chemicals, pulp and pulp resulting from industrial processes, feed, combustible fractions of municipal solid waste, compost, paper and cardboard, twigs, textile waste, wood chips, wood sawdust, plastic materials and pelletizable chemicals.

These and further advantages and characteristics of the present invention will be better understood by every person skilled in the art from the following description and with the help of the attached drawings, given as a practical example of the invention but not to be considered in a limiting sense, in which:

Fig. 1 represents a diagram of a possible embodiment of a plant according to the invention; Fig. 2 represents a simplified block diagram of the process parameter control system for the plant of Fig. 1;

Figs. 3, 4 and 5 show three diagrams relating to possible further 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 dies;

Fig. 7 is a schematic side view of a device for measuring parameters of the pellets leaving the plant. Reduced to its essential structure and with particular reference to Fig. 1 of the attached drawings, a plant according to the invention comprises:

a first hopper (11), into which the material to be pelletized is introduced, the outlet base of said hopper being provided with a metal mesh sieve with pre-established mesh width;

an aeraulic separator (12) downstream of the loading hopper (1 1), which is connected to the latter by means of a duct that extends between the inlet section of the separator itself (12) and the outlet section of an auger (1 10), the latter being placed below the hopper (1 1) to receive the material that comes out of it;

a refiner mill (13), placed downstream of said separator a first aspirator (9), whose inlet is connected to the outlet of said refiner (13) by means of a duct (90), the outlet of the aspirator (9) unloading in a silo (6) for collecting the material intended for pelletization;

a motorized screw extractor (7) placed at the base of the silo (6), with which the extractor is in communication;

a motorized belt or screw conveyor (8) placed between said extractor (7) and a second hopper (1) through which a pelletizing machine (2) is fed, between said second hopper (1) and the machine (2 ) a screw feeder (100) being arranged, the outlet section of which communicates with the inlet of the machine (2);

means for pneumatic extraction of the pellets from said machine (2), with a second aspirator (30) which conveys the pellets into a separator cyclone (3) and also sucks up the dust from the latter to send them to the aforementioned silo (6):

a conveyor belt (4) at the outlet section of said cyclone (3);

a sieve (5) downstream of said belt (4);

a station for stuffing the pellets downstream of said sieve (5).

Using a pelletizer of the type with one or more cylindrical dies, a part of the material that is introduced is drawn, that is, transformed into pellets, while the remaining part.

as is known to those skilled in the art, it escapes the action of the dies and, therefore, is not pelletized.

In the diagram of Fig. 1, the arrow (I) indicates the incoming material to be pelletized, the arrow (U) indicates the outgoing pelletized material, the arrows (S) indicate both the material sent to the silo (6) and coming from the duct (90) and the air extracted from the cyclone (3) by means of the aspirator (30), the arrows (P) indicate the pellets coming out of the dies (20) of the machine (2), the arrows (R) indicate the material not drawn and put back into circulation and the arrow (G) indicates the material rejected in a container (14) which will be described later. The material coming out of the silo (6) and destined for the machine (2) is indicated by the arrows (Q).

The sieve of the hopper (11) only allows the passage, and therefore the treatment, of material of size not exceeding a predetermined limit. The auger (1 10) feeds the separator (12) with the material that passes through the meshes of the sieve (1 1). The separator (12) separates heavier objects from said material, such as metal or ceramic bodies, stones, etc., in a manner known per se. to send them to the container (14) intended for the collection of discards. The refiner (13) crushes and dimensionally homogenizes the material leaving the separator (12). The aspirator (9) sucks the treated material from the refiner (13) and sends it into the silo (6), together with the material (R) that has not correctly passed the pelletizing process in the machine (2) and is introduced into the duct (90 ).

The silo (6) is equipped with a battery of filters (60) to discharge the appropriately dedusted air into the environment. The screw extractor (7) unloads the material from the silo (6) in a variable quantity according to the rotation speed of its axis. The conveyor (8) transfers the material from the extractor (7) to the hopper (1). From the latter, through the dosing screw (100), the material (Q) is fed into the pelletizer (2). The latter, as previously mentioned, can be of the type with one or more cylindrical toothed dies (20) and, according to methods known to those skilled in the art, compresses the materials that reach it, forcing them to pass through the holes of the same. dies. As previously stated, machines of this type are described in documents EP 846554, WO 93/22132, FR 1371346 and GB 1 153862, to which documents can be referred to for further details. The second aspirator (30) is connected to the cavities (21) of the dies of the machine (2), in which cavities the pellets are collected, by means of corresponding connection ducts (whose path is identified by the arrows P in the diagram of Fig. 1 ), and sucks up the pellets and introduces them into the separator cyclone (3). This has an inlet and an outlet for the pellets, the outlet being at its lower base and facing the conveyor belt (4). The powder that accompanies the pellets in this phase of the work cycle reaches the sieve (5) together with the pellets. The sieve (5), arranged downstream of the conveyor belt (4), is of the vibrating mesh type with meshes of predetermined width. The pellets retained by the net are bagged in a bagging station not described because of a known type. The fragments of pellets, residual dust and pellets smaller than the predetermined size pass through the meshes of the sieve (5) and are sucked up by the first aspirator (9), which is pneumatically connected to the lower base of the sieve (5) , and thus reintroduced into circulation, or conveyed in the aforementioned conduit (90).

All the noisiest equipment can be placed in a soundproof booth (10).

Advantageously, in accordance with the invention, it is possible to adjust the flow rate of the air stream produced by the aspirator (9) according to the degree of humidity of the material introduced into the pelletizing machine (2). More particularly, it is envisaged to increase said flow rate if the material to be pelletized has a degree of humidity higher than a predetermined limit, and vice versa. The humidity of the material can be detected by means of probes (IG), of a known type, placed in the silo (6) and / or in the hopper (1).

The possibility of adding water or other liquids to the material to be pelletized is also advantageously provided if its humidity degree is less than a predetermined value.

In practice, with the regulation of said air flow rate and with the possible addition of water or other liquids, it is possible to give the material to be pelletized the degree of humidity considered optimal.

For example, if a probe (IG) placed in the silo (6) detects that the material present therein has a degree of humidity lower than 16 ÷ 18%, then water is introduced into the silo (6), into the feeder (8 ) or in the machine (2) until, with a subsequent detection, a degree of humidity corresponding to the programmed value (in the example, a value between 16% and 18%) is detected.

It can also be advantageously provided to detect the power absorbed by the motor [or motors) of the machine (2) and the temperature of the dies to regulate the quantity of material being treated on the basis of the values of these two parameters. More particularly, regardless of the nature of the material being processed, if when fully operational the values assumed by these parameters exceed corresponding predetermined values, then the rotation speed of the screw (7) and / or of the screw (100) enslaved to the hopper (1) is reduced ).

For example, if a current draw greater than 100A is detected by the motor unit of the machine (2), then a decrease in the number of revolutions of the motor (MC) is commanded to introduce a smaller amount of material into the machine (2) .

It can also be advantageously provided to detect the degree of humidity and the apparent specific weight of the outgoing pellets (defining the apparent specific weight as the ratio between the weight of the pellets contained in a known volume and the value of this volume). The degree of humidity of the pellets at an outlet can be detected by means of hygroscopic probes (IGU) placed in a container of known size (i.e. of known volume) positioned on load cells and intended to receive the pellets leaving the plant for allow to detect the apparent specific weight as previously mentioned. For example, this detection can be made on samples of pellets taken out of the plant, similarly to what is shown in Fig. 7. In Fig. 7 the container intended to receive the pellets from a conveyor belt is represented by (70) (75) which can be provided at the outlet (U) of the system. The container (70) can have an openable bottom (73) for unloading the pellets and can be equipped with load cells (72) and one or more hygroscopic probes (71) suitable for providing relative signals respectively to a unit for the calculation of the apparent weight "pα" (PA) and a moisture calculation unit "u" (IGU). In the remainder of this description, pa, pr, u, ur will indicate, respectively, the apparent specific weight of the output pellets (pα), a corresponding predetermined reference value (pr), the humidity degree of the output pellets (u ) and a corresponding predetermined reference value

(Ur).

The quantity of material introduced into the machine (2) is linked to the current absorbed by the motor (or motors) of the same machine, since, with the same material consistency, the treatment of larger quantities of material corresponds to a higher absorbed current and vice versa . In relation to this, a maximum quantity of material can be defined as that quantity of material for which the current absorbed by the machine (2) is maximum, i.e. corresponding to a value of power developed by its motor group beyond which structural failures can occur. the dies (20) are interesting.

For this purpose, a control can be provided on the quantity of material fed into the machine (2) at full speed, so that if the current absorbed by the motor unit of the machine (2) equals the maximum preset value, then action is taken on the motor of the machine (2). feeder (100) to reduce its speed and thus reduce the amount of material fed into the machine (2).

When fully operational, the quantity of material introduced into the machine (2) is expected to be such that the power absorbed by the motor unit of the same machine is equal, within the tolerance limits allowed by the measuring equipment used, to the corresponding rated power. In practice, the system is self-regulating, leading for example to an increase in the quantity of material introduced until the absorbed power is less than the nominal one (this indicates the lack of resistance offered by the material to the action of the dies) or a decrease in this quantity. or an increase in the quantity of water if the absorbed power is greater than the nominal one.

Having said this, if, when u> ur, despite the complete closure of the water supply ducts and the maximum allowed increase in the air flow rate in the aspirator suction (9), the humidity of the outgoing pellets is even greater than limit value Ur, then it is possible to command the introduction of material into the machine (2) at the maximum flow rate allowed by the feeder (100) for a predetermined time (for example, 5 minutes), even if during this time the power absorbed by the group machine motor (2) should be greater than the rated one. If during this period of time the current absorbed by the motor unit of the machine (2) equals the aforementioned maximum safety value and is still u> ur, then the system is shut down and checks are carried out. If, on the other hand, during this time there is u = ur, then the normal power supply of the machine is restored (2).

Continuing to use the abbreviations indicated above, if at the check output pα <pr and u = ur, then: the amount of water introduced into the system decreases to the limit, excluding the water supply; the quantity of material fed into the machine (2) is reduced to allow for a prolongation of its residence time in the holes of the dies; the flow rate of the recirculating air flow sucked by the machine (2) is increased. Said adjustments are carried out in the order described and at the end of each of them the value of pα is measured. If, however, it is pα <pr, then the plant is stopped and checks are carried out.

If it is verified that pα> pr and u = ur, then: water is introduced; the material in recirculation is reduced; the input material is increased and an absorption of 10% greater than the nominal one is allowed for a predetermined time. These latter operations are also carried out in the order described.

If pα = pr and u> ur, then: the recirculation is increased; the quantity of water introduced decreases. These operations are also performed in the order described.

If pα = pr and u <ur, then: the quantity of water introduced is increased; the quantity of material introduced is increased; material recirculation is reduced. These operations are also performed in the order described.

By way of example, experimental tests have shown that in the production of wood pellets with medium hardness (for example spruce) using a pelletizing machine with dies with nominal values of diameter equal to 600 mm and height (thickness) equal to 150 mm , with holes with a nominal diameter of 10 mm, with a 75 kW motor, there is a power absorption of about 50 kW per 1000 Kg / h of material fed into the machine (2), which corresponds to about 100 A of absorption from part of the relative motor unit, to obtain a pellet considered optimal with pα = 680 ÷ 720 Kg / m <3> and u = 6 ÷ 9%. In Fig. 6, where two dies (20) are schematically represented, the diameter of the die (20) has been marked with (d), the thickness of the die (20) with (h) and the diameter of the hole (21) with (b).

It is therefore possible to produce pellets characterized by a predetermined degree of humidity and a predetermined apparent specific weight, within tolerance limits linked to the selected measuring equipment.

In the example of Fig. 2 a simplified diagram of the system that supervises the above operating phases is provided. An ammeter (or a wattmeter) (AW) is provided with a digital output, per se known, connected to the power supply of the machine (2) and to a programmable microprocessor (UE). Also connected to the microprocessor (UE) are the sensor (T) intended to detect the temperature of the die of the machine (2), the sensor (IG) to detect the degree of humidity of the material to be treated (for example, of the material in the silo 6 ), the sensor (IGU) to detect the degree of humidity (u) of the pellets at the outlet, and the unit (PA) to calculate the apparent weight (pa). At the outlet, the microprocessor (UE) is connected to the motor (MA) of the aspirator (30), to that (MT) of the dies (20), to that (MC) of the auger (100) and to that (MMC) of the auger (7). Any known microprocessor system suitable for the purpose can be used. In the same diagram of Fig. 2 a keyboard (TA) is also provided for entering and / or selecting the reference values of the process parameters defined above.

The plant relating to the example of Fig. 3 provides, immediately downstream of the hopper (1 1) for loading the material to be treated, a deferizer (80) to separate the metallic materials from the non-metallic ones and downstream of which a metal detector (81). In correspondence with the unit formed by the hopper (1 1), the iron separator (80) and the metal detector (81) there is a conveyor belt (82) which receives the material to be treated from the hopper (1 1) and transports it according to the direction indicated by the arrow (W). Above the end section of the belt (82) is the mouth of the manifold (90) enslaved by the aspirator (9). When the metal detector (81) detects the presence of metal products, the suction through the duct (90) is deactivated so that the metal products (PM) are sent by the belt (82) to a station (83) for collecting them. . In accordance with this example of embodiment of the plant, two refiners (13) are provided, both interlocked 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 means of another aspirator (84), into a cyclonization chamber (85). The aspirator (84) receives the material dispensed from the silo (6) through two dispensing valves of the star type or rotary cells (600). From the chamber (85) the lighter fraction of the material is reintroduced into circulation (R), while the heavier one is fed into the pelletizing machine (2) with the aid of two motorized vertical screw feeders (86). The pellets (P) coming out of the machine (2) are pneumatically conveyed into an aeraulic separator (87) which on one side is enslaved by a corresponding aspirator (88) which conveys the dust towards the filters (60) and, on the opposite side, it unloads the pellets in a final deduster (89) through a rotary valve or rotary valve (870). The dust intercepted by the filters (60) and by the dust collector (89) are sucked into the collector (90) and then reintroduced into the circulation, as happens for the pellets of smaller size than the predetermined limit and for the material escaped from the action of the dies in the machine (2) . Also for the plant schematized in Fig. 3 what has been said regarding the control of the process parameters (operating temperatures of the dies, degree of humidity of the material being treated and of the outgoing pellets, etc.) is valid.

The plant relating to the example of Fig. 4 provides for the use of a pusher (1 10) associated with the loading hopper (1 1), which discharges into an underlying mill - slow crusher (1 1 1). The crushed material is sent pneumatically into the silo (6) thanks to the vacuum cleaner (900). The rotary valve or rotary valve (901) associated with the silo (6) unloads the material onto the conveyor belt (82), which can be operated to move the material according to the directions indicated by the arrows (K) and (L). The metal detector (81) is associated with the conveyor belt as in the previous example. If the presence of metal products (PM) is detected, the belt is driven to direct the latter towards the corresponding collection point (83), away from the system (arrow K). On the opposite side (arrow L), is the manifold (90) enslaved to the aspirator (9) to pneumatically convey the material to the crusher (13) and from here to a silo - lung (61). Below the silo - lung (61) there is a horizontal auger (62) which takes the material from it (through the rotary valve 610) to supply it to a vertical auger doser (63).

The latter feeds the machine (2). Water can be introduced into the horizontal auger (62) to regulate the degree of humidity of the material intended for the machine (2) as described previously. Furthermore, oil can be introduced into the machine (2) if necessary to favor the formation of pellets in it. Similarly to what is foreseen in the diagram of Fig. 3, the pellets coming out of the machine (2) are pneumatically fed into an aeraulic separator (87) enslaved by an aspirator (88) and which discharges the pellets of size not lower than the predetermined limit in a final dust collector (89).

The plant relating to the example of Fig. 5 provides for the use of a crusher (1 1 1) connected at the outlet to an aspirator (900), which is connected to a separator cyclone (990).

The separator cyclone (990) has an upper outlet for powders (992) connected to a filtering battery (60) and a lower outlet on which a rotary valve (991) is acting which connects to an iron separator (80).

Downstream of the iron separator (80) there is a gravimetric separator (993) equipped with two outlets, one (999) for unloading heavy materials (HC) and one connected to a silo (6), by means of a fan (998).

The silo (6) also has an upper outlet connected to the filtering battery (60) and, at the bottom, it has an outlet equipped with a rotary valve (600) which discharges the material onto a conveyor belt (82) equipped with a metal detector (81) .

If the presence of metal products (PM) is detected, these are discarded at the metal detector (81), while the material suitable for the pelletizing process is sent to the relative machine (2) by means of the belt (994), which in Fig 5 is shown broken into two portions to connect the two levels of the drawing. Downstream of the pelletizer (2) there is a main duct (995), intended for the transfer of the pellets produced and an aeraulic separator (87), and a secondary duct (996), intended to bring the waste upstream, i.e. in correspondence with the duct marked with (997) in Fig. 5, arranged upstream of the fan (998) which is in turn provided upstream of the silo

(6).

The pellets arriving at the aeraulic separator (87) are subjected to the action of the aspirator (88), which is also connected to the filtering battery (60) and, before exiting (U) from the processing cycle, they pass through a final dust collector (89).

For simplicity of the drawing, in Fig. 5 the filtering battery (60) is not graphically connected to the various ducts leading to it. Furthermore, the components with the same numbering as the corresponding components of the previous figures have the same operation.

If necessary, a part of the pellets intended for bagging can also be reintroduced into the silo (6) in order to introduce into the holes of the dies material offering greater resistance, that is, adequate for the formation of pellets.

In practice, the details of execution can in any case vary in an equivalent manner in the shape, dimensions, arrangement of the elements, nature of the materials used, without however departing from the scope of the idea of the solution adopted and therefore remaining within the limits of the protection granted by this patent for invention

industrial.

Claims (10)

CLAIMS 1) Plant for the production of pellets, comprising means for feeding the material to be pelletized, at least one pelletizing machine, means for pneumatic extraction of the pellets coming out of said machine, characterized by the fact that it includes means for putting back into circulation, that is to return to the pelletizing machine at least a part of the products coming out of it. 2) Plant according to claim 1 characterized by the fact that it comprises means for reintroducing powders and pellets of smaller size than a predetermined value into circulation. 3) Plant according to claim 1 characterized by the fact that said means for reintroducing at least a part of the products leaving the pelletizing machine into circulation comprise a filter (5) with a mesh sieve with predetermined size meshes. 4) Plant according to claim 1 characterized by the fact that it comprises a sensor (T) for detecting the operating temperature of said pelletizing machine and programmable means and microprocessor (UE) which receive the data coming from the sensor (T) to regulate the operating speed of said feeding means on the basis of predetermined temperature parameters. 5) Plant according to claim 1 characterized by the fact that it comprises a sensor (AW) for detecting the power and / or current absorbed by the motor of the said pelletizing machine and programmable means with microprocessor (UÈ) which receive the data coming from the said sensor (AW ) to adjust the operating speed of said power supply means on the basis of predetermined power and / or current parameters. 6) Plant according to claim 1 characterized by the touch that comprises one or more probes (IG) to detect the degree of humidity of the material introduced into the pelletizing machine, which probe is connected to programmable means with microprocessor (UE) which receive data from said probe (IG) to regulate the degree of humidity of said material in relation to a predetermined reference value. 7) Plant according to claim 1 characterized by the fact that it comprises one or more probes (IGU) for detecting the degree of humidity of the outgoing pellets. 8) Plant according to claim 1, characterized by the fact that it comprises means (PA) for detecting the apparent specific weight of the outgoing pellets. 9) Plant according to one or more of the preceding claims characterized by the fact that at least one of said pelletizing machines is of the type with one or more dies. 10) Method for the production of pellets, comprising feeding the material to be pelletized and pelletizing with one or more pelletizing machines, characterized by the fact that it involves reintroducing at least a part of the products coming out of said one or more pelletizing machines . 1 1) Method according to claim 10 characterized by the fact that it involves the control of the humidity degree of the material to be pelletized and / or the control of the humidity degree of the output pellets and / or the control of the apparent specific weight of the output pellets and / or the control of the power or current absorbed by said one or more pelletizing machines and / or of the operating temperature of said one or more pelletizing machines. 12) Method according to claim 10 characterized by the fact that it involves reintroducing powders and pellets of smaller size than a predetermined value into circulation.