Patent Publication Number: US-2021171400-A1

Title: Porcelain stoneware products

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a technology for the production of porcelain stoneware products, wherein the treatment process allows the re-use of recycled materials deriving from the treatment of urban waste. 
     In particular, the present invention relates to the production of products for the construction of driveways. 
     BACKGROUND 
     It is known that quarries are capable of producing natural materials such as basalts, granites and marbles are almost exhausted and however they are increasingly difficult to cultivate both for problems of quantity, and for problems linked to quantity, and for problems linked to environmental issues. 
     Similarly, the production of very particularly products, such as for example the “sanpietrini” made of leucite, is increasingly facing more difficulties linked to finding raw material and to environmental issues connected to the impact of quarries. 
     The “sanpietrino” is a small block of rock cut in truncated-pyramid shape, typically obtained from working volcanic rocks coming from the quarries located at the foot of Colli Albani (but even in the volcanic area of Viterbo). The name is due to the fact that at first they were used to pave St. Peter&#39;s Square in Rome. During time, their use has widely spread, especially in the old towns. 
     By extension the same definition is used for similar small blocks too, even having shapes not exactly equal to the classical sanpietrino, and/or made of other material such as for example porphyry. 
     For these reasons, today the need for a solution is much felt which could safeguard tradition and then allow to continue using products such as for example sanpietrini, at the same time by avoiding depletion of quarries, with beneficial effects both in economic terms and in terms of environmental impact. 
     SUMMARY OF THE INVENTION 
     The technical problem faced and solved by the present invention is then to provide a product alternative to sanpietrino and other similar product which however keeps the structural features thereof, of shape and mechanical resistance, and which then could usefully replace sanpietrino without losing tradition and effectiveness. 
     This is obtained through an element for driveways as defined by claim  1 . 
     Additional features of the present invention are defined in the corresponding depending claims. 
     The idea underlying the present invention is to detect a material which, as alternative to rocks of various nature, could allow to obtain a product for the construction of driveways having at least the same, if not better, features in terms of mechanical resistance, appearance, and so on. 
     Such alternative material has been found in the porcelain stoneware. 
     As it is known, stoneware is a ceramic material having hard, compact, sound, waterproof paste, obtained by cooking until incipient vitrification of the mixture; it is often covered by a waterproof glaze. The nature of starting mixture varies depending upon the type and use. Usually a distinction is made among: ordinary (or common) stoneware, fine stoneware, porcelain stoneware, chemical stoneware. 
     The ordinary stoneware is obtained by mixing refractory plastic clays, generally containing iron, sometimes mixed with silica, limestone, feldspar, baked clay, and so on. Recipients are manufactured therewith to preserve food substances, pipes for liquids, floor or coating tiles. 
     The porcelain stoneware or fireclay, is manufactured with impure kaolin, vitrified in the mass like porcelain, it is used for hygienic-sanitary apparatuses wherein the aesthetical needs are limited and good resistance properties are required. 
     However, nowadays, no technique for producing porcelain stoneware products is known, having features with shape and structure such as those of the sanpietrino (or similar products). 
     The current technology for the treatment of ceramic mixtures for producing porcelain stoneware, allows to obtain products having thickness of 10-20 mm. 
     In fact, implementing a porcelain stoneware product having larger thickness than this, would compromise the quality of the product itself, to the point of making it clearly unusable for the prefixed purposes. In fact, a little resistant, too friable product, prone to break, and so on, would be obtained. 
     Another limit is constituted by the fact that producing high-thickness porcelain stoneware would require anomalous quantities of raw materials. It is to be reminded, for example, that 1 mq of stoneware having thickness of 10 mm weights about 20 kg, whereas if one produced with the same techniques a material with thickness of 60 mm its weight would be about 140 kg/mq. 
     Therefore, it results evident that the more the thickness increases, the more the unitary cost of the finished material increases, reaching probably not justifiable production costs. 
     With the present invention one wanted to give an answer even to these problems, by developing a process for producing porcelain stoneware products which indeed allows to obtain the above-outlined results, without the disadvantages which the current techniques would involve. 
     At the basis of this process solution there is the idea of re-using, in the preparation of the ceramic mixture, recovery materials, or better waste materials of other companies which usually are intended to be carried to special dumps indeed with often high transfer costs. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention, by overcoming the problems of known art, involves several and evident advantages which, together with the features and use modes of the present invention, will result evident by the following detailed description of preferred embodiments thereof, shown by way of example and not for limitative purposes and therefor the figures of the enclosed drawings will be referred to, wherein: 
         FIGS. 1A to 10  show examples of sanpietrini according to the present invention; 
         FIGS. 2A to 2C  show a flowchart of a possible implementation of a process according to the present invention; 
         FIGS. 3A to 3E  illustrate by way of example a possible plant for demetallizing ashes according to the present invention; and 
         FIG. 4  is a graph of a possible firing curve according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Some embodiments of the present invention will be described hereinafter, by making reference to the above-mentioned figures. 
     By way of preamble, the ceramic mixture therewith traditionally the porcelain stoneware is produced is usually constituted by the following components:
         one or more alkaline feldspars;   usually they are sodium or potassium feldspars which can be found in Italy (Calabria or Sardinia) or in Turkey or in Pyrenees. Usually the feldspar enters the mixture with amounts not lower than 40-45%, and such raw material has the purpose of giving fusibility to the mixture itself, so that the melting temperature remains at values lower than 1200° C. The dosing of these ingredients, as well as their grinding level, allows to optimize the firing curve of the product and to give a structure without porosity thereto.   one or more plastic clays;   they are usually clays with high percentage of alumina (20-25%) with low amounts of iron oxide (light firing), or higher iron amounts (red-brown firing). They enter the mixture around 35%. They have the purpose of keeping together the not plastic ingredients so as to provide malleability and workability to the mixture before cooking. The most precious clays can be found in Germany and Ukraine and in lower percentage in Spain.   Inerts or fillers;   The inerts can be quartz or feldspathic sands, pre-cooked kaolins, chamotte. Usually they enter the ceramic mixture with % around 15-20% and usually the come from local quarries and then represent even the less expensive portion of the mixture.       

     With the objective which was indicated above, many solutions were tried to integrate recovery materials in the mixtures for producing porcelain stoneware, with the purpose of detecting the one which would meet at best all so far highlighted requirements. 
     To this purpose, crystals of automobiles, phials of medicines, glasses of bottles (all treated suitably), or ashes of power plants, that is the ashes which constitute the solid residue, which the thermoelectric power plants produce during the coal combustion phase, were then tested. 
     However, none of these materials allowed to obtain the wished results. 
     Then, the inventors thought to use the ashes produced by the incinerators of urban waste. 
     In fact, in the modern incinerators, these, after having been crushed and minced, are treated in specific self-combustion ovens by obtaining as main product electric energy and as by-product (waste product) combustion ashes up to now destined to special dumps. 
     With the present invention one has arrived to introduce into the mixture for producing porcelain stoneware such ashes coming from waste-to-energy plants (with partial replacement of other raw materials) without the technical features of the product being altered and consequently the use of this raw material (ashes) involves the evident advantage of lowering sensibly the cost of the mixture and solve then the problem of the cost of the mixture for producing high thicknesses. 
     By firstly referring to  FIGS. 1A to 10  they show some examples of porcelain stoneware products according to the present invention, in particular sanpietrini. 
     For example,  FIG. 1  shows a sanpietrino  1  made of porcelain stoneware usable as element of a driveway. 
     The sanpietrino  1  has a tread surface  2  and a base surface  3 , and it is characterized in that it has an average thickness Sm, measured between the tread surface  2  and the base surface  3 , which on the average is greater than or equal to 4.5 cm. 
     An indication of the average thickness is provided, considering that the tread and base surfaces can be not perfectly regular (even intentionally to give a particular aspect to the sanpietrino). 
     As already explained, advantageously, an element for the construction of driveways according to the present invention, made of porcelain stoneware, could include in its composition a percentage Pc other than zero of recycled material coming from the treatment of waste. 
     In particular, the recycled material could advantageously come from processes for treating urban waste and substantially comprise the residual ashes from the combustion of such waste. 
     Preferably, the percentage Pc of recycled material, in particular ashes, is between 1% and 40% of the volume of the mixture used for the production of porcelain stoneware. 
     According to a preferred embodiment, such percentage Pc is between 10% and 30%, still more preferably between 15% and 25%. 
     From a geometrical point of view, according to a possible embodiment, an element according to the present invention has a tread surface  2  the sizes thereof are about 10×10 cm. 
     However, it is to be meant that even elements with different shape, such as for example special pieces for pavements, angles, and so on, can be implemented with the same chemical/physical feature. 
     For example, elements could be provided, the tread surface thereof ( 2 ) has the sizes of about 20×20 cm or 20×40 cm or 40×60 cm. 
     Still, according to an additional embodiment, an element can be provided, the tread surface thereof  2  has sizes of approximately 4.5×4.5 cm. 
     Generally, all elements produced according to the present invention, have a base surface  3 , the extension thereof, with respect to that of the tread surface  2 , can be in a variable ratio between 0.1 and 1.3, proportion corresponding to sizes which indicatively can vary between 1×1 cm to 13×13 cm. 
     According to the preferred embodiments, an element of the invention has a substantially truncated-pyramid shape, the tread surface  2  of which represents a large base thereof. Then, excepting some special pieces, in this case the base surface  3  will always have a lower extension than that of the tread surface  2 . 
     However, it will have to be meant that the base surface  3  could also have an extension equal to zero, or almost zero, this involving a substantially pyramidal shape. 
     At last, as already reminded, the shapes attributed to such elements, have not to be considered necessarily regular in the strictly mathematical sense. Actually, deviations from the regular geometrical shapes can be also wished and inserted intentionally to provide the pieces a more ‘artisan’ and less ‘standard aspect. 
     As already stated, the average thickness Sm of such elements is greater than or equal to 4.5 cm. However, preferred embodiments thereof provide an average thickness Sm greater than or equal to 6 cm. 
     By referring to the flow chart of  FIGS. 2A to 2C , this illustrates the main phases of a process according to the present invention. 
     In particular, the ashes coming from the incinerators before being used have to remain in the storage for at least 20-30 days, with the purpose of lowering the PH thereof through carbonation reactions. 
     At this point, the ashes can be in case treated to be reduced in sizes, deferrized and-metallized. 
     Although there are apparatuses for treatments of this type on the ashes, for the implementation of this phase, the inventors also devised and developed a process and an innovative apparatus, innovative on itself, providing effectively more performing results than known art and which, generally, could also be used in other fields, outside the present invention. 
     Consequently, a plant was devised for implementing this deferrization process.  FIGS. 3A to 3E  relate to the de-metallization plant according to the present invention. With reference to such figures, the plant elements are identified by numbers shown in the following legend: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 21 
                 Mill opening power unit 
               
               
                 20 
                 Conveyor belt mm 650 × 12,000 
               
               
                 19 
                 Conveyor belt mm 650 × 8,500 
               
               
                 18 
                 Conveyor belt mm 500 × 5500 
               
               
                 17 
                 Separator EMC 1500 
               
               
                 16 
                 Vibrating table 1400 × 750 
               
               
                 15 
                 Magnetic drum TM-N30.140 
               
               
                 14 
                 Magnetic drum TM-N30.140 
               
               
                 13 
                 Vibrating table 1400 × 1500 
               
               
                 12 
                 Conveyor belt mm 650 × 9000 
               
               
                 11 
                 Conveyor belt mm 650 × 17,300 
               
               
                 10 
                 Tertiary mill BSI type PFC 105 
               
               
                 9 
                 Mill vibrating feeder 
               
               
                 8 
                 Conveyor belt mm 600 × 9000 
               
               
                 7 
                 Vibrating screen 2 Planes mm 1200 × 3000 
               
               
                 6 
                 Bandaboard belt 650 × 8170 
               
               
                 5 
                 Deferrizing belt 
               
               
                 4 
                 Conveyor belt mm 800 × 4500 
               
               
                 3 
                 Extracting belt 800 × 6260 
               
               
                 2 
                 Grid control hydraulic power unit 
               
               
                 1 
                 Loading hopper with light grid 80 mm 
               
               
                   
               
            
           
         
       
     
     According to such process, the ashes are subjected to two deferrization phases. 
     A primary deferrization phase provides that the material is extracted from an inlet hopper and re-launched on a belt  5  wherein a powerful deferrizer having permanent magnets separating the coarser ferrous portions is mounted. 
     By way of example, for this phase a belt magnetic separator  5  can be used having the following features:
         Plate with permanent magnets made of Neodymium (sizes 900×700 h 180 mm);   Drums with diameter 300 mm with keyed and interchangeable through shaft with diameter 50 mm;   Interaxis drums 1,500 mm;   Rubber mat class 400, three cloths 4+2 with strips h=30 mm pitch 400;   Installed power 2.2 Kw The so roughly deferrized material is transported, through a belt system, to a vibrating screen  7  which separates the fine fraction 0+8 mm from the coarse fraction 8+80 mm.       

     By way of example, the vibrating screen  7  can preferably have the following features:
         Screening planes: two   Sizes of the screening planes 1,200×3,000 mm.   Screening surface of each plane: 3.60 mq   Power motorization: 5.5 kw       

     At this point, the coarse fraction is transferred to an impact mill  10  wherein it is subjected to a grinding phase and it is subjected to a granulometric reduction (with a reduction ratio of about ⅙-⅛). The more one pushes on crushing, the more effective the separation level is if the objective is not that of re-using the inert as MPS (Secondary Raw Material), a granulometric reduction as high as possible is preferred. 
     By way of example, the impact mill can be a hammer mill  10  with the following features:
         Casing made of properly stiffened sheet   Inner armour plate made of chromium alloy   Hammers made of reversible chromium alloy   Side plates made of steel   Complete motorization (motor, belts, pulley and protection carter)   Coated discharge caisson   Supply vent   Vibrating feeder complete with suspensions   Hydraulic plant for the machine opening   Complete with sustaining structure   Side ramps complete with hand rails   Access ladder       

     Downwards of the mill  10  the crushed material is joined again with the previously separated fine fraction to be subsequently treated in a secondary deferrization phase. 
     In this secondary deferrization phase, preferably through a vibrating feeder  13 , the material is fed on a pair of high-efficiency magnetic drums  14 ,  15  which perform further separation of the ferrous fraction. 
     By way of example, the vibrating feeder  13  can have the following features:
         Plate sizes 1,400×1,500 mm.   Two electromechanical vibrators with adjustable masses   Suspensions with elastic springs   Vibrating casing coated with anti-wear material   Installed power 2×1.5 kw
 
Still by way of example, the magnetic drums  14 ,  15  can have the following features:
   Diameter 300 mm.   Drum length 1.400 mm.   Constructed with permanent magnets made of Neodymium having high magnetic power   Rotating structure made of Stainless steel AISI 304 with cross strips on the surface for iron detachment   Rotating support and fixed support for fastening   Motor and reducer       

     Then, downwards of the magnetic drums the material is supplied, preferably by means of a vibrating feeder  16  (similar to the first vibrating feeder  13 ), to a separator EMC  17  which separates, through a system with induced currents, the aluminium portions existing in the material. 
     By way of example, the induction separator  17  can have the following features:
         Magnetic rotor: diameter 350 mm.—useful width 1.500 mm.—dynamic balancing level G 0.4 (maximum precision level)—keyed and replaceable through shaft—special magnets made of Neodymium, locked mechanically and sealed from the external environment, for a maximum safety and to prevent iron powder from entering and the consequent damage thereof—supplementary protection of the magnetic rotor implemented with cylinder made of stainless steel—outer cylinder made of fiberglass for belt rolling—CONCENTRIC technology—   Belt traction roller: steel with biconical turning for the belt self-centring—keyed and replaceable through shaft   Conveyor belt: PVC material with two antistatic cloths with “Bandebord” for the containment of the materials to be treated   Motorization: for traction of magnetic rotor—for traction of conveyor belt   Switchboard: equipped with two inverters for adjusting the speed of the belt and of the magnetic rotor—emergency system which in case of black-out allows to remove the material present on the belt.       

     The ashes coming from the pre-treatment phase, preferably, according to the present invention have a composition similar to the one indicated in the following Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Ideal composition of the ashes 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Na2O 
                 MgO 
                 Al2O3 
                 SiO2 
                 P2O5 
                 SO3 
                 Cl2O 
                 K2O 
                 CaO 
                 TiO2 
                 Fe2O3 
                 CuO 
                 ZnO 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 % 
                 2.84 
                 2.75 
                 17.08 
                 36.08 
                 1.23 
                 0.85 
                 1.12 
                 1.06 
                 30.21 
                 1.74 
                 4.03 
                 0.45 
                 0.56 
               
               
                   
               
            
           
         
       
     
     Preferably, the percentage Pc of ashes to be used according to the invention, is between 1% and 40% of the volume of the mixture used for the production of porcelain stoneware. 
     According to a preferred embodiment, such percentage Pc is between 10% and 30%, still more preferably between 15% and 25%. 
     Although it is to be meant that the other raw materials (apart from the ashes) which can be used to implement the porcelain stoneware according to the present invention could be evidently selected among those normally and typically used for such purpose, according to a preferred embodiment, raw materials having specific physical/chemical features can be used. 
     These materials were selected, also among those which can be found on the market, according to the following Table 2: 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Raw materials 
               
            
           
           
               
               
               
            
               
                   
                   
                 Percentage in 
               
               
                   
                 Raw material 
                 the mixture 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 1 
                 Kaolin BG (Industrial Minerals): 
                 12-15% 
               
               
                 2 
                 MF/TV (Svimisa) 
                 12-15% 
               
               
                 3 
                 Sigillo Clay (Saxa Gres) 
                 25-45% 
               
               
                 4 
                 Sasifo Sand (Sibelco) 
                 10-16% 
               
               
                 5 
                 Sigillo (Plasticizer clay) 
                  6-12% 
               
               
                   
               
            
           
         
       
     
     The producer of the commercial product is shown in brackets, however it is clear that raw materials with similar features could be used. The features of the raw materials shown in Table 1 are illustrated hereinafter. 
     Kaolin BG 
     
       
         
           
               
             
               
                   
               
             
            
               
                 XRF Chemical analysis: 
               
            
           
           
               
               
               
            
               
                   
                   
                 % 
               
               
                   
                   
               
               
                   
                 SiO 2   
                 79.5 
               
               
                   
                 Al 2 O 3   
                 14.0 
               
               
                   
                 Fe 2 O 3   
                 1.40 
               
               
                   
                 TiO 2   
                 0.15 
               
               
                   
                 CaO 
                 0.05 
               
               
                   
                 MgO 
                 0.10 
               
               
                   
                 K 2 O 
                 0.25 
               
               
                   
                 Na 2 O 
                 0.10 
               
               
                   
                 P.F. 
                 4.60 
               
               
                   
                 S 
                 0.08 
               
               
                   
                   
               
            
           
           
               
            
               
                 Average grainsize distribution: 
               
            
           
           
               
               
               
            
               
                   
                 Class 
                 Weight 
               
               
                   
                 mm 
                 % 
               
               
                   
                   
               
               
                   
                 +20.0 
                 6 
               
               
                   
                 20.0-10.0 
                 18 
               
               
                   
                 10.0-1.0  
                 69 
               
               
                   
                 1.0-0.1 
                 3 
               
               
                   
                  −0.1 
                 4 
               
               
                   
                 Moisture 
                 9% 
               
               
                   
                   
               
            
           
           
               
            
               
                 Average mineralogical analysis: 
               
            
           
           
               
               
               
            
               
                   
                   
                 % 
               
               
                   
                   
               
               
                   
                 Quartz 
                 63 
               
               
                   
                 Kaolin 
                 32 
               
               
                   
                 
                   
                 
                 3 
               
               
                   
                 Other 
                 2 
               
               
                   
                   
               
               
                   
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
     MF/TV 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
             
            
               
                 CHEMICAL 
                   
                   
                   
                   
                   
               
               
                 ANALYSIS 
                 [%] 
                   
                 FIRED PROPERTIES 
               
               
                   
               
               
                 SiO 2   
                 78.5  
                 
                   
                 
                 Firing shrinkage 
                 [%] 
                 10.5-11.5 
               
               
                 Al 2 O 3   
                 
                   
                 
                 
                   
                 
                 Absoprtion H 2 O 
                 [%] 
                 &lt;0.1 
               
               
                 Fe 2 O 3   
                 
                   
                 
                 
                   
                 
                     Colorimetric 
                   
                 
                   
                 
               
               
                 CaO 
                 0.3 
                   
                 coordinates 
               
               
                 MgO 
                 0.4 
                   
                     Cooking cycle 
                 
                   
                 
                 
                   
                 
               
               
                 Na 2 O 
                 2.2-2.5 
                   
                 Temperature 
                 [° C.] 
                 1230 
               
               
                 K 2 O 
                 4.2-4.5 
                   
                     Oven thermal work 
                 [° C.] 
                 1130 
               
               
                 TiO 2   
                 
                   
                 
               
               
                 MnO 2   
                 Tr 
               
               
                 
                   
                 
                 
                   
                 
               
               
                 Carbon 
                 absent 
               
               
                 Sulphur 
                 absent 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 MINERALOGY 
                   
                   
                   
                   
               
               
                 ANALYSIS 
                 [%] 
                 OTHER PROPERTIES 
               
               
                   
               
               
                 QUARTZ 
                 40 
                 Supplying humidity (max) 
                 [%] 
                 10 
               
               
                 ORTHOCLASE 
                 35 
                 Granulometry 
                 [mm] 
                 &lt;3.6 
               
               
                 ALBITE 
                 12 
                 Resistance to bending (drying) 
                 
                   
                 
                 20.0 
               
               
                 CLAY 
                 15 
                 Test piece forming pressure 
                 
                   
                 
                 350 
               
               
                   
                   
                 Post pressing expansion 
                 [%] 
                 0.3 
               
               
                   
               
               
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
     Sigillo Clay 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Oxides 
                 % 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 SiO 2   
                 73.60 
               
               
                   
                 B 2 O 3   
               
               
                   
                 Al 2 O 3   
                 13.15 
               
               
                   
                 Fe 2 O 3   
                 3.30 
               
               
                   
                 PbO 
               
               
                   
                 CaO 
                 2.70 
               
               
                   
                 BaO 
               
               
                   
                 MgO 
                 1.25 
               
               
                   
                 ZnO 
               
               
                   
                 K 2 O 
                 2.70 
               
               
                   
                 Na 2 O 
                 2.30 
               
               
                   
                 Li 2 O 
               
               
                   
                 MnO 2   
                 0.10 
               
               
                   
                 TiO 2   
                 0.75 
               
               
                   
                 P 2 O 5   
                 Tr 
               
               
                   
                 Calc. Loss 
                 2.25 
               
               
                   
                 Total 
                 99.85 
               
               
                   
                   
               
            
           
         
       
     
     Sasifo Sand 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 SiO 2  [%] 
                 Al 2 O 3  [%] 
                 Fe 2 O 3  [%] 
                 K 2 O [%] 
                 Na 2 O [%] 
                 TiO 2  [%] 
                 CaO [%] 
                 MgO [%] 
               
               
                   
               
             
            
               
                 89.55 
                 5.88 
                 0.365 
                 2.130 
                 1.709 
                 0.085 
                 0.190 
                 0.082 
               
               
                   
               
            
           
         
       
     
     All raw materials belonging to the formulation are dosed in the prefixed portion and carried, for example through a sequence of weighing caissons working continuously, to a grinding compartment. Herein they are finely ground, with granulometry preferably lower than 120 microns. 
     To this purpose, for example, the “hard” raw materials (for example ashes, Sasifo sand, MF/TV) enter continuously the mill containing ground bodies such as pebbles and/or high-density alumina spheres, from the bottom thereof they come out finely ground, with granulometry preferably lower than 150 microns, preferably lower than 120 μm. 
     The other “soft” raw materials (for example clays) which do not require energy action, are finely dispersed, for example in a bowl mill. 
     At this point the two channels of raw materials join again, by forming one single mixture of components, still according to a controlled dosage. 
     Such mixture is then homogenized, thereafter through a wetting action during the rotation phase they are “granulated” that is brought to a humidity rate between 4% and 8%, preferably around 6%, and made basically spherical or granular to provide a correct evenness, this for example by wetting in bentonite dispersed in water. 
     At this point the mixture is ready to proceed to the subsequent steps. 
     The mixture is stored in silos, wherein it can rest and mature. The accumulation in silos further allows to implement a “lung”, that is a reserve of material always available for the subsequent process stages. 
     After such storage, the process provides a phase for forming the product which can follow two different ways: pressing and extrusion. 
     The pressing, for example performed with presses SACMI PH 690, consists in carrying the mixture powders within a mould made of hardened steel wherein they are subjected to a pressing with a load comprised between 100 Kg/cmq and 400 Kg/cmq, preferably between 130 Kg/cmq and 250 Kg/cmq, providing the shape to the piece. 
     For example, for a typical sanpietrino, each piece will have sizes of about 11×11 cm for its tread surface, 8×8 cm for its base surface, and a thickness of about 7 cm. Upon preparing the piece, the shrinkage the product is subjected to during cooking is taken into consideration and therefore that the end product will have sizes of about 10×10 cm for the tread surface, circa 7×7 cm for the base surface and about 6 cm for the average thickness. 
     Some types of product provide geometrical features and complex shapes, which do not make them suitable to a treatment by pressing. 
     For example, still by making reference to the sanpietrino, some specific pieces (pavement stone, tiles in 20×20, 20×40, 40×60 format, small cubes 4.5×4.5×4.5) could be required for the construction of particular areas of the paving. 
     For these pieces, it is preferable to add to the mixture powders taken from the storage silos, previously turbo-dissolved specific clays, which make the mixture more elastic so as to be able to drawing it, that is forcing it through a screw die or extruder wherein the suitable shape and size is provided to the product. 
     For example, plasticizing clays, such as the same SIGILLO, or even other clays (for example English ball Clay) can be added by 5-12%. 
     The main extrusion parameters, according to the present invention, are a depression between 65 and 71 mm Hg, and a pressure between 18 and 21 kg/cmq. 
     In fact, in order to extrude, plasticity or malleability of the mixture (20% of humidity) has to be increased, thereafter a suitable depression is created to remove air and the mixture is forced, by using a screw, through a slit therewith the material assumes the wished shape. 
     Once come out of the press and/or the extruder, the pieces are positioned on wagons manufactured with materials suitable to withstand the oven&#39;s high temperatures. 
     Within the present invention, it was seen that with ovens having single-layered rollers, traditionally used for the treatment of porcelain stoneware, it would not be possible to cook the pieces sufficiently. 
     Therefore, according to the present invention, the cooking process takes place preferably in tunnel-like ovens, of the type used for sanitary articles and products for building (bricks and brick tiles). 
     Then the pieces are conditioned with humidity lower than 1%, preferably lower than 0.7%, still more preferably around 0.5%, thereafter they enter a tunnel-like oven wherein within 6/12 hours they are brought to about 1150° C. and subsequently brought back to low temperature, the whole according to a predefined firing curve. 
     Generally, a firing curve according to the present invention, is so as to allow to heat the pieces, from an inlet temperature Ti at the oven inlet between 50° C. and 100° C., until a maximum temperature Tm between 1,100° C. and 1,300° C. in a heating time Th between 6 and 10 hours. 
     Therefore, a cooking time Tc at this maximum pressure Tm between 40 min and 90 minutes is provided. 
     Subsequently, the pieces are subjected to a controlled cooling to as to bring them back to an outlet temperature Tu from the oven comprised between 100° C. and 200° C. in a cooling time TI comprised between 6 hours and 8 hours. 
     In all phases of course a punctual control of temperatures is provided, so that the resulting firing curve has no tracts with particularly steep course, and which keeps its course substantially monotonous (increasing, constant, decreasing) in the three main phases (heating, cooking, cooling). 
       FIG. 4  shows a preferred example of firing curve according to the present invention, wherein, as it can be seen from the graph, the parameters are the following: 
     Heating Phase 
     Ti: ≈70° C.; 
     Tm: ≈1,200° C.; 
     Tr: ≈9 h; 
     Cooking Phase: 
     Tc: ˜1 h; 
     Cooling Phase; 
     Tu: ˜120° C.; 
     Ti: ˜7 h. 
     In  FIG. 4 , the curves of temperature HT are shown at each one of the thermocouples present in the oven. It is evidently to be meant that these curves HT substantially and ideally should be all coincident therebetween. 
     The curve at the bottom LT is the reading of temperatures below the oven plane, wherein there are materials sensible to high temperatures and then it is required that in these areas the temperature does not increase too much to avoid damages. 
     At the outlet of the oven a selection phase can be provided to remove in case defective pieces. 
     At last, the pieces can be packaged before being sent to the warehouse of finished products. 
     The present invention has been so far described with reference to preferred embodiments thereof. It is to be meant that each one of the technical solutions implemented in the embodiments described herein by way of example, could advantageously be combined differently therebetween, to create other embodiments, belonging to the same inventive core and however all within the protective scope of the herebelow reported claims.