Patent Publication Number: US-2023134279-A1

Title: Method and plant for processing reconstituted tobacco

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a plant for the production of reconstituted tobacco, both of the conventional type and of the non-conventional type, the latter also referred to as HNB (Heat Not Burn). 
     BACKGROUND 
     In general, reconstituted tobacco is obtained with the use of tobacco by-products and processing scraps (ribs, small pieces of leaves, powder, etc.) which, properly ground up to be practically reduced to powder and mixed with water, glycerin binders and other liquid additives, allow to obtain an extremely fluid mixture (slurry) having a liquid content of about 70% by weight, which is then poured in the form of a veil on a steel strip and with this transferred to a drying. Here the evaporation of the liquid fraction of the mixture takes place, so that the solid residue forms a sort of continuous strip of tobacco having approximately the same width as the steel strip. Subsequently, the dried mixture strip is separated from the steel strip and is cut into pieces of various sizes according to the request. These pieces are then transformed into thin filaments which, suitably mixed, are fed to a traditional cigarette packing machine. 
     Depending on the raw materials used and, in particular, depending on whether shredded tobacco by-products up to a particle size between 20 μm and 220 μm are used, or whether ground tobacco leaves with dimensions between 5 and 10 mm are used, the reconstituted tobacco is distinguished into conventional or unconventional. 
     WO 2016/050469, WO 2016/050470, WO 2016/050471, WO 2016/050472 describe known techniques for the production of reconstituted tobacco, which however require large plants and involve high energy consumption to carry the slurry, which when it is product is rather fluid, to the consistency of a sheet of tobacco. It is sufficient to point out that a drying oven can reach up to 100 m in length. 
     Another drawback of the known techniques for the production of reconstituted tobacco with the use of by-products consists in the fact that the formation of the sheet starting from the slurry film is rather irregular, since the starting products are not homogeneous and their distribution on the steel is not uniform; it follows that the reconstituted tobacco sheet does not allow it to be reeled or cut regularly. 
     WO2019/157576 describes a method for the preparation of strips of reconstituted vegetable material in which, between a pre-lamination phase and the final lamination phase, a remixing of the pre-laminated sheet is provided inside a mixer in order to obtain a homogeneous mass from then submit to the final lamination. 
     WO2020/058814 discloses a method for the preparation of reconstituted tobacco in which the solid components of shredded tobacco are mixed with water, at least one binding agent and at least one material to form an aerosol until a slurry with a liquid content of about 30-50%, preferably about 35-40%. 
     WO2016/067226 describes a method a process for preparing reconstituted tobacco comprising: a first drying unit, a grinding unit, a mixing unit for the solid components (i.e. tobacco powder with solid powder of natural binders), a mixing unit for the liquid components (i.e. liquid/nanogel with propylene glycol and glycerin), a unit for mixing the solid mixed components with the liquid mixed components, from one to three lamination units to obtain a 0.15 film-0.3 mm, and a dryer to reduce the moisture content of the film. 
     The object of the invention is to eliminate these drawbacks and to produce both conventional and non-conventional reconstituted tobacco, with much smaller plants. 
     Another object of the invention is to produce reconstituted tobacco with limited energy consumption. 
     Another object of the invention is to produce reconstituted tobacco using apparatuses that are partly already available on the market, even if never used in this specific technical sector. 
     Another object of the invention is to produce reconstituted tobacco in an alternative way to traditional methods. 
     Another object of the invention is to produce reconstituted tobacco with characteristics suitable for satisfying different market demands. 
     Another object of the invention is to produce reconstituted tobacco by operating at low temperatures and therefore preserving all the aromas of the tobacco. 
     SUMMARY 
     According to the invention, all these objects and others that will result from the following description are jointly or separately achieved with a method of producing reconstituted tobacco and a plant as set forth in the appended claims. 
     In particular, the method according to the invention to produce reconstituted tobacco is characterized by the fact that it includes the carrying out, in sequence, of the following steps: 
     the solid components of tobacco are ground up to bring them to a particle size of about 20-220 μm, preferably about 80-180 μm, 
     the ground product thus obtained is mixed with water, powdered cellulose, at least one binding agent and at least one material to form an aerosol until a slurry is obtained with a liquid content of about 30-50%, preferably about 35-40%, 
     said mixture is subjected to a first lamination to obtain a continuous strip with a thickness of about 1-20 mm, preferably about 1-10 mm, 
     said strip, already subjected to said first lamination, is subjected to a series of further lamination steps, until a strip having a significantly constant thickness of about 90-280 μm, preferably of about 140-200 μm, is obtained, 
     said strip is dried until its liquid content is about 8-15%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is further clarified hereinafter in some of its preferred embodiments reported for purely illustrative and non-limiting purposes with reference to the attached drawings, in which: 
         FIG.  1    shows a general schematic view of a plant for the production of reconstituted tobacco according to the invention, 
         FIG.  2    shows its feed section if the plant is intended for the production of conventional reconstituted tobacco, 
         FIG.  3    shows its feed section if the plant is intended for production of reconstituted tobacco of unconventional type (HNB), 
         FIG.  4    shows its rib feeding section, 
         FIG.  5    shows a partial scheme of the plant with two distinct lines of pre-treatment of leaves and tobacco ribs, 
         FIG.  6    shows its section for grinding, mixing and storage, 
         FIG.  7    shows a schematic view of it refiner with cylinders, 
         FIG.  8    shows in plan its stratification section in a different embodiment, 
         FIG.  9    schematically shows a hot air dryer thereof, and 
         FIG.  10    schematically shows its hot air dryer in a different embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As can be seen from the figures, the plant for producing reconstituted tobacco according to the invention comprises several sections designed to operate on the incoming raw materials until they are transformed into a continuous strip of reconstituted tobacco to be sent to the subsequent cigarette packaging operations. 
     In particular, the plant according to the invention for the production of reconstituted tobacco comprises: 
     a unit for shredding the solid components of tobacco, to bring them to a particle size of about 20-220 μm, preferably about 80-180 μm; it 
     a mill  20  and/or a grinder  24  and/or a pin mill  54 ; 
     a mixer  80 , which is fed with metered quantities of shredded material, with water, with at least one binding agent and with at least one material to form an aerosol, and which is configured to form a mixture with a liquid content of about 30-50%, preferably about 35-40%, 
     a first rolling unit  100  to obtain from said mixture a continuous strip having a thickness of about 1-20 mm, preferably about 1-10 mm, 
     a rolling line  116  located downstream of said first rolling unit  100  to bring said continuous strip to a thickness of about 90-280 μm, preferably of about 140-200 μm, 
     a dryer  122 , located downstream of said rolling line  116 , to bring the liquid content of said strip, rolled and exiting said rolling line  116 , to about 8-15%. 
     Advantageously, the plant also comprises a mixture forming unit  92  for forming a plurality of portions  97  of said mixture; suitably, therefore, the first rolling unit  100  is configured to form from the portions  97  of said mixture a continuous strip with a thickness of about 1-20 mm, preferably about 1-10 mm. 
     Preferably, the plant according to the invention comprises: 
     a pre-treatment section of the starting solid products (tobacco leaves, ribs, leaf fragments, powder, etc.) for their preparation for subsequent grinding treatments, 
     a grinding and storage section awaiting subsequent mixing with suitable treatment liquids; suitably, said grinding section comprises said grinding unit, 
     a mixing section of solid and liquid materials to obtain a homogeneous mixture with a rather dense consistency, 
     a section for converting the mixture, and in particular of a plurality of portions of said mixture, into a continuous belt, 
     a rolling line of the continuous strip for its reduction to the desired final thickness, 
     a drying section of the laminated web. 
     Conveniently, the preparation and pretreatment section of the starting solid products is different depending on whether the plant is intended to produce reconstituted tobacco of a conventional type ( FIG.  2   ) or of an unconventional type ( FIG.  3   ). Moreover, advantageously, a section for the preparation and pre-treatment of the tobacco ribs can also be provided ( FIG.  4   ), to be used for the production of both conventional and unconventional reconstituted tobacco. 
     Advantageously, if the preparation and pre-treatment section is intended to feed a plant for the production of reconstituted tobacco of a conventional type ( FIG.  2   ), it comprises a tilter  2  of the cartons containing the tobacco by-products, aimed at overturning their content on a feeder  4  of a vibrating conveyor  6 , which separates any heavy bodies from the product to be treated. The heavy bodies are collected in a suitable container  8 , while the product to be treated is transferred, through a pneumatic conveying line  10 , a cyclone  12 , a conveyor belt  14 , equipped with a metal detector  16  for the removal of any metal bodies, and a pneumatic conveying line  18 , to the mill  20 , preferably of the cryogenic type. 
     Advantageously, if the preparation and pretreatment section is provided for the preparation of reconstituted tobacco of an unconventional type ( FIG.  3   ), it comprises a feeding station with a bench  22  for unpacking the bales of tobacco leaves from cartons of about 200 kg, which generally contain them, and the transfer of these to a grinder  24 . 
     Conveniently, the output of this grinder  24  is connected, through a pneumatic conveying line  26 , to a cyclone  28 , in which the transport air is separated from the solid product, which is transferred to a vibrating sieve  30  for the separation of the fine parts from the remaining parts of the product. The outlet of the fine parts is directly connected to the cryogenic mill  20 , while the outlet of the remaining parts of the product feeds a traditional twine levering machine  32 , which eliminates any twine previously not removed from the bales of tobacco leaves. 
     The outlet of the twine lever machine  32  feeds a classification chamber  34  for the separation of any heavy foreign bodies from the ground tobacco leaves, which through a pneumatic conveying line  36 , a cyclone  38 , a conveyor belt  40 , equipped with metal detector  42  for the removal of any metal bodies, a weighing system  43  (master scale), and a pneumatic conveying line  44 , are transferred to storage and mixing silos  45 , from which they can then be transported through another line of pneumatic transport  44 ′, to the cryogenic mill  20 . These silos  45  are sized so as to contain the quantities of product necessary to form the batch according to the particular recipe to be prepared. 
     Advantageously, if the preparation and pre-treatment section is provided for the preparation of the tobacco ribs to be used for the production of both conventional and unconventional reconstituted tobacco ( FIG.  4   ), it comprises a tilter  46  of cartons containing the tobacco ribs, a rib feeder  48  to a vibrating conveyor  50 , for the separation from these of any heavy bodies, and a pneumatic conveying line  52  for their transfer to a pin mill  54 , where they are ground. 
     The pin mill  54  has its outlet in turn connected, by means of a pneumatic conveying line  56  equipped with cyclone filters  58 , to one or more storage silos  60 . 
     The output of the silo or silos  60  is at its connected, by means of a screw conveyor  62 , to a weighing system  64  (slave scale), which doses the ground ribs in the percentage required by the particular recipe to be prepared, before sending them through the same pneumatic transport line  44 ′, to the storage and mixing silos  45 . 
     As mentioned, the plant according to the invention also includes the mill  20  ( FIG.  5   ), which grinds the various products received until it reaches an average particle size of about 20-220 μm, preferably about 80-180 μm. 
     There are various types of mills that can be used, although it is more advantageous to use a cryogenic pin mill, which allows the product to be kept at low process temperatures and therefore to retain the aromas of the tobacco. 
     The pin mill is traditional in itself and comprises within a closed structure a fixed disc and a rotating disc or two counter-rotating discs, provided with facing and partially interpenetrating pins. Being a traditional apparatus in itself, it has been globally indicated with  20  in  FIGS.  5  and  6    but is not shown in its internal construction characteristics or in its operating modes. 
     Preferably, the pin mill  20  is designed to carry out cryogenic grinding, that is, grinding in the presence of liquid nitrogen. 
     As stated, in a reconstituted tobacco production plant a cryogenic pin mill  20  is somewhat more advantageous than a traditional mill, essentially due to the different ways in which the products to be ground are treated. In fact, grinding at room temperature can lead to poor quality products while grinding in the presence of liquid nitrogen allows the physical properties and the chemical and organoleptic characteristics of the products to be preserved. 
     The amount of liquid nitrogen used in cryogenic grinding processes is a key part to consider when investigating the pros and cons of the process, and can vary depending on the materials being processed. Liquid nitrogen at a temperature of −175° C. is injected onto the product inside the chamber of a screw conveyor  68  which feeds the mill  20  and its residence time in contact with the nitrogen is approximately 2 to 5 sec., which is also the transit time of the product inside the screw that feeds the pin mill. The temperature of the product that leaves the mill  20  is advantageously lower than 10° C., so that the nitrogen vapors, which are released almost instantaneously upon contact with the tobacco to be cooled, go through the entire feeding system of the mill in countercurrent the desired pre-cooling effect. The flow of liquid nitrogen in the pre-cooling system and in the mill is controlled by thermocouples, which make the cryomind process fully automatic. 
     In summary, the positive factors of cryogenic grinding are: 
     higher yields, 
     better quality of the final product without breaking or tearing of the molecular structure, 
     decrease in energy required, 
     better quality of the final product, 
     less waste due to overheating and oxidation, 
     more homogeneous and finer final product, 
     less amount of material to be reprocessed in the grinding system. 
     Conveniently, the outlet of the cryogenic pin mill  20  is connected to a fluid bed sieve  70 , which has the function of separating the ground product, which comes out of the mill itself and generally has an average particle size of about 20-220 μm, preferably of about 80-180 μm, from larger sizes, inevitably present. 
     Conveniently, the fluidized bed sieve  70  therefore has the function of classifying the product and reintroducing the one with fractions greater than 220 μm into the mill  20 , after having separated them from those between 20 μm and 220 μm, which through a pneumatic transport  72  are sent to one or more mixing and storage silos  74 . 
     Advantageously, the outlet of the mixing and storage silos  74  feeds, through a pneumatic transport line  76 , a cyclone filter  78 , which has the function of breaking down the dusty air and more specifically to separate the dust, which is then recovered and reintroduced into the cycle, from the air, which can then be expelled. 
     Conveniently, the outlet of the cyclone filter  78  feeds, through a continuous dosing system, preferably with a screw, the mixer  80 , which can be of various types, for example of the horizontal tilting type or vertical spiral type. 
     The mixer  80  is fed with metered quantities of ground tobacco, water, at least one binding agent and at least one material to form an aerosol, and is configured to obtain a mixture with a liquid content of about 30-50%, preferably of about 35-40%. 
     In particular, the values of liquid or humidity, indicated in the present description, are intended to be determined according to the measuring system on a wet basis. In particular, the humidity values are defined as the percentage of water contained in the total mass of the corresponding product and, in other words, as the percentage ratio between the quantity of water and the total mass of the assembly. Conveniently, these values are obtained using the traditional methods provided in the literature to measure the amount of water in a product, such as those presented in “Tobacco Moisture, Water and Oven Volatiles-A status report of common moisture methods used within the tobacco industry “By Nils Rose ET AL. in “Analytical and bioanalytical chemistry” (1 Jul. 2014, pages 1-16). 
     Preferably, at least one duct for inlet water, a material for the formation of aerosols (for example glycerin) and at least one binding agent (binder) is connected to the mixer  80 . Conveniently, one or more inlet ducts can be provided for other additives required by the particular recipe to be prepared. 
     More particularly, the plant comprises one or more storage tanks  82  for aerosol formation material and one or more premixers  84 , into which said material for aerosol formation can be introduced and, preferably, a plurality of additives dosed in the right proportions to form the liquid to be introduced into the mixer  80 . 
     In order to increase the resistance of the sheet of finished product and at the same time to increase the density of the product itself, the invention envisages introducing it into the mixer  80 , together with the other components of the mixture, including powdered cellulose. Conveniently, the cellulose powder used is made up of an organic fiber obtained from natural cellulose, and not a compound derived from cellulose. The powdered cellulose thus added preferably has a particle size comprised between 50 and 100 μm and is preferably in a percentage comprised between 2% and 10% by weight with respect to the ground tobacco. 
     The added powdered cellulose is predominantly or exclusively of natural origin. In particular, the powdered cellulose thus added is not synthetic and is not obtained by chemical treatments. 
     Conveniently, the powdered cellulose thus added does not have a binding function but the function of lowering the specific weight of the finished product and of reducing the solid components of tobacco, thus reducing the costs of the finished product, since cellulose has a much lower cost than tobacco. Furthermore, the addition of cellulose makes the finished product less brittle by increasing the tensile strength, and therefore it is more easily workable, and this is particularly useful in the case in which a pleated sheet of finished product is to be obtained. 
     This powdered cellulose, which before its use is contained in bags or big-bags, can be directly introduced into the mixer  80  ( FIG.  1   ) and in this case, after being poured into a traditional hopper, it is fed to a cyclone filter  78 ′, which through a continuous dosing system, preferably of the screw type, introduces it in a metered quantity into the mixer  80 . 
     Alternatively, the powdered cellulose can be introduced, again through a cyclone filter  78 ′ and a continuous dosing system, in the pneumatic conveying line  44 ′ which feeds the mixing and storage silos, from which it is then transferred to the mill  20  ( FIG.  5   ) through the pneumatic conveying line  44 ′, together with the other components of the mixture, present in the line itself. From the mill  20  the contents of the mixing and storage silos  45  are then transferred, through the conveyor line  76 , to the weighing device placed at the inlet of the mixer  80 . 
     Examples of preferred materials for aerosol formation (and in particular for the formation of a visible aerosol) include polyhydric alcohols (e.g., glycerin, propylene glycol, triethylene glycol and tetraethylene glycol), aliphatic esters of mono-, di- or polycarboxylic acids (e.g. methyl stearate, dimethyl dodecanedium and dimethyl-tetradecandioate), as well as their mixtures. Suitably, glycerin, propylene glycol, triethylene glycol and tetraethylene glycol can be mixed together to form an aerosol-forming material. The aerosol forming material can also be provided as a portion of the binding agent (e.g., when the binding agent is propylene glycol alginate). Advantageously, suitable combinations of materials for aerosol formation can also be provided. 
     Preferably, said at least one binder includes at least one of hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylcellulose, microcrystalline cellulose, methylcellulose, carboxymethylcellulose (CMC), corn starch, potato starch, guar gum, locust bean gum, pectin and alginates (e.g., ammonium alginate and sodium alginate). 
     Preferably, the binder and the added powdered cellulose are defined by materials different from each other. Conveniently, the powdered cellulose mainly serves to form a three-dimensional framework with a high thickening effect, a pseudoplastic behavior and a good capacity to retain liquids, while the binding agent serves exclusively or mainly to bind together the various components to be mixed. 
     Advantageously, the outlet of the premixers  84  is connected to an inlet of a hydrator  86 , having other inlets connected to a water supply line  88  and to a compressed air supply line  90 . 
     Preferably, the outlet of the mixer  80  feeds the mixture forming unit  92  to obtain a plurality of portions  97 , preferably shaped like loaves/loaves and separated from each other. Conveniently, the forming unit  92  comprises a pair of forming cylinders  96 , affected by grooves preferably parallel to the axis of the cylinders themselves and intended to pick up the mixture at the inlet and to supply the portions  97  at the outlet. Advantageously, the unit  92  is also configured for roughing the mixture and for this purpose, preferably, comprises a hopper  94  provided with an internal lump breaker and said pair of forming cylinders  96  on the bottom. 
     Advantageously, at the outlet of the forming unit  92  a conveyor belt  98  is provided for the transfer of the portions  97  to the first rolling unit  100 . 
     Preferably, the first rolling unit  100  comprises a lobe feeder  102  to homogenize the mixture formed by the portions  97 . 
     Advantageously, along the transfer path from the forming unit  92  to the lobe feeder  102 , a further metal detector  104  can be provided, having the function of removing any metal parts, which could still be present in the mixture and could damage subsequent processing units. These metal parts are conveyed along a distinct path to the inlet of the lobe feeder  102  and are collected within a suitable container  106 . 
     The lobe feeder  102  comprises a series of lobed feed rollers, between which the portions are made to pass  97  (which come out of the forming rolls  96  of the forming unit  92 ) so as to be mixed together and homogenized before being pushed between a pair of rolling rolls  108 , which are configured to form a continuous strip of thickness about 1-20 mm, preferably about 1-10 mm. 
     Conveniently, therefore, the lobe feeder  102  causes homogenization of the product which leaves the forming unit  92  and which could have lumps. Advantageously, moreover, the lobe feeder  102  also advances the product to push it into the inlet between the pair of laminating cylinders  108 . 
     Conveniently, therefore, the first laminating unit  100  comprises a homogenization module, preferably defined by the feeder lobe  102 , which is positioned immediately upstream with respect to a pre-lamination module which is defined by at least one pair of lamination cylinders  108 , which are configured to form a continuous strip with a thickness of approximately 1-20 mm, preferably about 1-10 mm. Advantageously, the strip thus obtained has a greater elasticity. 
     Conveniently, in a version not shown of the plant, the rolling line  116  can be provided directly downstream of the lobe feeder  102 . In particular, in this case, the rolling line receives the single-layer continuous strip having a thickness of about 1-20 mm, preferably of about 1-10 mm, which comes out of the first lamination unit  100  provided in the lobe feeder  102  Advantageously, downstream of the first lamination unit  100  and upstream of the lamination line  116 , a layering unit  110  can be provided. Preferably, it is configured to arrange on several layers the single-layer continuous strip, having a thickness of about 1-10 mm, which comes out of the first rolling unit  100 , so as to transform it in a multilayer belt with a thickness of about 2-20 mm, which is then sent to the inlet of the rolling line  116 . 
     Preferably, said stratification unit  110  consists of an upstream conveyor belt  112 , the which has the function of depositing the product belt on an underlying downstream conveyor belt  114 , preferably belonging to the rolling line  116 , arranging it so that it is layered on said downstream conveyor belt  114 , for example by multiple folding on itself. Preferably, the upstream conveyor belt  112  is elevated with respect to the downstream conveyor belt  114  and is equipped with a continuous forward motion with respect to its support structure, and at the same time with an alternating motion with its support structure, parallel to the its longitudinal axis. 
     Conveniently, the layering unit  110  feeds the subsequent and underlying rolling line  116 , and depending on the type of plant the upstream conveyor belt  112  of the layering unit  110  can be arranged parallel or perpendicular to the rolling line  116 . In in particular, if the downstream conveyor belts  114  of the rolling line  116  have a width substantially equal to the width of the product belt that leaves the stratification unit  110 , the upstream conveyor belt  112  is arranged parallel to the downstream conveyor belts  114  provided in the rolling line  116  ( FIG.  1   ), while if the downstream conveyor belts  114  of the rolling line  116  are wider than the product belt exiting the layering unit  110 , it is preferable that the upstream conveyor belt  112  is arranged orthogonally to the downstream conveyor belts  114  provided in the rolling line  116  ( FIG.  8   ), so that with its movements can distribute the product strip over the entire useful width of the rolling line  116 . 
     Conveniently, in both cases, the alternating movement of the support structure of the upstream conveyor belt  112  of the layering unit  110  causes a stratification of the product belt, which leaves the first rolling unit  100  on the underlying first downstream conveyor belt  114  of the rolling line  116  and the formation of a stratified belt having a width substantially equal to the useful width of the rolling line itself. 
     The rolling line  116  is formed by several rolling stations, each comprising a pair of cylinders  118 , which delimit an increasingly narrow passage between them to progressively reduce the thickness of the strip of product being processed. In particular, the rolling line  116  is configured to progressively bring the continuous strip to a thickness of 90-280 μm, preferably of about 140-200 μm. 
     Preferably, between each rolling station and the next there is a conveyor belt  114  having a length preferably of about 1.5-2 m, which has the function of allowing the product to rest before it is subjected to the subsequent rolling step. 
     Advantageously, the rolling line  116  is then completed with one or more calibration stations, each formed by a pair of calibrating lamination rolls  120 . 
     It is advantageously provided that the  118  and possibly also the calibrating rolls  120  can be heated, so as to being able to start the drying phase already during lamination. Conveniently, downstream of the rolling line  116  there is a dryer  122 , preferably with air recirculation ( FIG.  7   ), to bring the liquid content of said rolled strip to about 8-15%. Advantageously, the dryer  122  can be divided into two units  124 ,  126 , placed in series with each other. More particularly, the upstream unit  124  is provided for carrying out the first drying phase and is equipped inside with a steel belt or net conveyor belt for transporting the product leaving the rolling line  16 ; the downstream unit  126  is provided for carrying out the second drying phase and the subsequent cooling phase and is internally equipped with a network conveyor belt. 
     Furthermore, the dryer  122  is advantageously provided at the inlet and outlet with sensors  128 , preferably with infrared rays, which monitor the product along its entire length. 
     The operation of the plant just described for the production of conventional reconstituted tobacco ( FIG.  2   ) is as follows. 
     Preferably, the tobacco waste containers are placed on the tipper  2 , which overturns the products onto the feeder  4 , which transfers them to the vibrating conveyor  6 . Here the separation of any heavy bodies from the tobacco by-products takes place: the former are collected in the container  8  while the latter are transferred by an air flow along the pneumatic conveying line up to the cyclone  12 , which separates the air from the solid products and lets these fall on the conveyor  14 , for their transfer, through the pneumatic line  18 , to the cryogenic mill  20 . 
     Preferably, for the production of reconstituted tobacco of an unconventional type instead ( FIG.  3   ), the cartons containing the tobacco leaves are placed in the undressing bench  22 , where the individual bales of tobacco leaves are removed from the cartons and sent to the grinder  24 , which reduces the leaves themselves to a substantially uniform size between 5 and 10 mm. 
     Conveniently, the ground product is then transferred along the pneumatic conveying line  26  to the cyclone  28 , which separates it from the air and makes it fall onto the vibrating sieve  30 . 
     Here the separation of the finer parts takes place, which are sent directly to the cryogenic mill  20 , from the remaining parts which, after passing through the twine levering machine  32 , reach the classification chamber  34 . In this, any heavy bodies are separated from the ground leaves, which after being subjected to the control of the metal detector  42  are sent to the cryogenic mill  20 . 
     Conveniently, if the recipe requires it, also shredded tobacco ribs can be transferred to the same cryogenic mill  20 , which can be used for the production of both conventional and unconventional reconstituted tobacco. 
     In this case ( FIG.  4   ) the containers with the ribs are placed on the overturning device  46 , which feeds the ribs themselves to the vibrating conveyor  50  for the removal of any heavy bodies. The ribs are then transferred through the pneumatic line  52  to the pin mill  54 , which shreds them to reduce them to a size between 5 and 8 mm. 
     From here the shredded ribs, separated in the cyclones  58  from the transport air, are transferred to the storage silos  60 , from which the different types of ribs, coming from different qualities of tobacco, can be picked up and transferred through the screw conveyor  62  to the rib dispenser  64 , which doses them according to the particular recipe to be prepared. 
     The ground and dosed ribs in the correct quantities are transferred through the pneumatic line  66  to the cryogenic mill  20 . 
     Advantageously, regardless of the type of reconstituted tobacco to be produced, and therefore of the type of solid parts of tobacco introduced into the shredding unit, by this last comes a ground product with an average particle size of about 20-220 μm, preferably about 80-180 μm. Preferably, the ground product, which comes out of the fluid bed sieve fed by the cryogenic mill  20 , has an average particle size of about 20-220 μm, preferably of about 80-180 μm. 
     Advantageously, the product thus ground is sent to the mixing and storage silos  60 , from which the products can then be withdrawn according to the needs and transferred to the mixer  80 . 
     In addition to the ground tobacco and the cellulose and in general all the solid products coming from the mixing and storage silos  45 , water, at least one binding agent and at least one material to form an aerosol are also introduced. Advantageously, compressed air and other additives can also be introduced, including in particular ground cloves. 
     Conveniently, the whole is then mixed together to form a slurry having a percentage of liquids (humidity) of about 30-50%, preferably about 35-40%, by weight on a wet basis, i.e., a rather dense consistency. 
     Preferably, the mixture thus obtained is transferred to the forming unit  92 , from which a plurality of portions  97 , preferably shaped like loaves, come out. 
     These portions of mixture  97 , which come out of the forming unit  92  are suitably transferred to the first rolling unit  100  which is configured to homogenize the mixture and to provide at the outlet a continuous strip with a thickness of about 1-20 mm, preferably of approximately 1-10 mm. This continuous strip, which comes out of the first lamination unit  100 , is transferred directly to the lamination line  116  or—by means of the stratification unit  110 —is folded on itself to be thus deposited in the form stratified on the inlet belt  114  of the rolling line  116 . 
     Conveniently, as mentioned, the stratification is obtained by letting the continuous belt fall on the conveyor belt  112 , which is made to advance with respect to its support structure, which is moved by alternating motion, so as to arrange the product belt on several layers on said inlet conveyor belt  114 . Depending on the plant and the direction of the alternating movement, the support structure of the conveyor belt  112  immediately downstream of the layering unit  110 , the product belt is arranged on several layers parallel to the longitudinal direction of the rolling line  116  or orthogonally thereto. 
     Conveniently, at each passage from one station to another of the rolling line  116 , the product strip undergoes a thickness reduction, until it reaches the desired thickness in correspondence with the output calibrating cylinders  120 , which has a significantly constant value of about 90-280 μm, preferably about 140-200 μm. Advantageously, moreover, at the exit from the rolling line  116  the strip has a liquid content lower than 20% or even 15%, if the laminating cylinders  118  are heated and the removal of the water has already begun during the rolling process. 
     The web of product leaving the rolling line  116  is then subjected to drying in the dryer  122 , where its liquid content is brought to about 8-15%. 
     Preferably, the dryer  122  is air-recirculated, and compared to the dryers traditionally used in reconstituted tobacco production plants it is somewhat more advantageous both in terms of manufacturing complexity and in terms of overall dimensions and in terms of energy consumption. This is because traditional plants treat a very fluid and not very stable product (slurry), unlike the product treated by the plant according to the invention, which is much denser and much more stable. Consequently, while the plants that treat slurry require traditional irradiation and conduction dryers, the plant according to the invention can advantageously use a recirculating air dryer  122  with a network conveyor belt or a combined system of steel conveyor belts for the first drying phase and net conveyor belts for the second drying phase and for the cooling phase. In this way, with the same performance, reduced dimensions are obtained (about 45 m compared to over 100 m of a traditional dryer) and lower energy consumption due to the lower quantity of water to be removed (using about 1000 kg/hour of steam/hour compared to over 5000 kg/hour of steam from a traditional dryer). 
     Conveniently, at the exit of the dryer  122  the product is ready to be wound in reels or to be cut into threads of predetermined dimensions, to be used for the packaging of cigarettes. 
     Conveniently, in the method according to the invention, the homogenization of the mixture is carried out mainly or exclusively by means of the lobe feeder  102  and is in any case carried out always and only upstream, preferably immediately upstream of the pre-rolling module, which transforms the homogenized mixture in a continuous strip with a thickness of about 1-20 mm, preferably about 1-10 mm, to be sent in the final rolling line  116 . 
     In particular, once the homogenized mixture has been transformed in a continuous strip with a thickness of about 1-20 mm, preferably about 1-10 mm, the latter is no longer remixed, but is sent in the form of a strip to the rolling line  116  to be thus brought to the desired thickness, which has a significantly constant value of about 90-280 μm, preferably of about 140-200 μm. Advantageously, the fact that the rolling line  116  receives a strip at the inlet, and not a deformed mixture, allows to guarantee a constant inlet flow, increasing the accuracy of the thickness of the sheet in the subsequent rolling steps and also allows to reduce the number of laminating cylinders  118  of the same line, thus reducing the cost and the overall dimensions of the plant. 
     Conveniently, if the plant is envisaged for the production of reconstituted tobacco of an unconventional type, in addition to using a different preparation and treatment section already described, it uses, as an alternative to the molding unit  92  or in addition and to upstream of this, a cylinder refiner  130  which has the task of bringing the solid components of the mixture to a grain size not exceeding 20 μm. 
     The refiner ( FIG.  7   ) comprises inside a closed container a plurality of cylinders  132  arranged in sequence in close proximity to each other, so as to delimit corresponding grinding slots. The lower cylinder  132 ′ is mounted with the axis outside the plane containing the axis of all the other cylinders  132  and functions as a feeder of the mixture which is taken from the bottom of the container and made to rise upwards so as to pass between the lower cylinder and the one immediately above and then to follow among all the others. The various pairs of cylinders  132 , between which the mixture passes, rotate at different speeds, in the sense that the upper cylinder rotates at a higher speed than the lower cylinder, with which it cooperates, so as to subject the mixture to stretching during the passage between the cylinders  132  of each pair and to thus reduce the size of the particles of the mixture itself. In fact, one of the fundamental parameters for the success of the refining process is precisely the different speed of the different cylinders  132 , on which the passage of the whole mass of mixture that has passed through the grinding slot depends. 
     The pressure between the cylinders is hydraulically controlled. 
     All the cylinders  132  are cooled with cold water which circulates inside each cylinder and in this way contrasts the heat, which develops from the mixture due to the friction due both to the movement of the cylinders and to the rubbing with the product. In this way the temperature of the product mass is reduced until it reaches 25° C. 
     Thanks to the refiner  130  just described, the friction action, which is exerted on the mixture by the cylinders  132  of the latter, develops a considerable binding action of the cellulose fibers contained in the tobacco and in particular in the ribs of the latter, and this involves the double advantage of developing the aromatic components of the product and eliminating the need to introduce more fiber into the mixture to obtain the required binding effect. 
     The operation of the plant in this different embodiment provides that the shredded leaves and the shredded ribs coming from the preparation and pre-treatment stations are fed to the cryogenic pin mill  20  in proportionally dosed quantities according to the recipe to be obtained, and are from this leads to a particle size of about 20-220 μm, preferably about 80-180 μm. 
     The product is then transferred in the manner already described into the mixer  80 , in which a product mixture is formed as described above. 
     The mixture thus obtained is then fed to the cylinder refiner  130 , which has the task of bringing the solid components of the mixture to a grain size not exceeding 20 μm. In this way, the friction action exerted on the mixture by the cylinders  132  of the refiner  130  develops a considerable binding action of the cellulose fibers contained in the tobacco and in particular in the ribs thereof, and this entails the double advantage of developing, from a on the other hand, the aromatic components of the product and, on the other hand, to eliminate the need to introduce more fiber into the mixture to obtain the required binding effect. 
     In  FIG.  1    schematically indicates the position of the refiner  130  between the mixer  80  and the forming unit  92 , but the invention also provides that the refiner  130  can be an alternative to the forming unit  92 , and in this case the mixture which it leaves the refiner  130  and is transferred directly to the first rolling unit  100 , for the continuation of the processing cycle according to the methods already described.