ECOLOGICAL PROCESS FOR PURIFICATION AND REACTIVATION OF CARBON BLACK OBTAINED FROM THE PYROLYSIS OF USED TIRES

Abstract

The present invention relates to a purification process for carbon black obtained from the pyrolysis of used tires by solvent extraction, characterized by low or zero toxicity and environmental impact, preferably, but not exclusively, derived from raw materials from renewable and non-fossil sources, i.e., by inert atmosphere heat treatment or by inert atmosphere heat treatment followed by solvent extraction, in order to remove the pyrolyzed rubber residues deposited on the surface of the carbon black obtained from pyrolysis and the polycyclic aromatic hydrocarbons contained therein.The present invention also relates to a process for extracting zinc from carbon black obtained from pyrolysis, previously purified by solvent extraction or by heat treatment or by heat treatment followed by solvent extraction, using carboxylic acids of natural origin, such as citric acid and tartaric acid obtained from completely renewable sources.

Description

ECOLOGICAL PROCESS FOR PURIFICATION AND REACTIVATION OF CARBON BLACK OBTAINED FROM THE PYROLYSIS OF USED TIRES The present invention relates to an ecological process for the purification and reactivation of carbon black obtained from the pyrolysis of used tires. In particular, the present invention relates to a purification process for carbon black obtained from the pyrolysis of used tires by solvent extraction characterized by low or zero toxicity and environmental impact, preferably, but not exclusively, derived from raw materials from renewable and non-fossil sources, i.e., by inert atmosphere heat treatment or by inert atmosphere heat treatment followed by solvent extraction, in order to remove the pyrolyzed rubber residues deposited on the surface of the carbon black obtained from pyrolysis and the polycyclic aromatic hydrocarbons contained therein. The present invention also relates to a process for extracting zinc from carbon black obtained from pyrolysis, previously purified by solvent extraction or by heat treatment or by heat treatment followed by solvent extraction, using carboxylic acids of natural origin, such as citric acid and tartaric acid obtained from completely renewable sources. It is known that, among the various proposals for the recovery of used tires, pyrolysis appears to be one of the best solutions, as it allows for the implementation of the principles of the circular economy, that is, recovering and recirculating the recovered materials without emitting CO2 into the atmosphere [see JD Martínez et al., Sustainable Energy Reviews, vol. 23, pp. 179-213, (2013)]. In fact, the pyrolysis of used tires could allow for the reuse of certain pyrolysate fractions in new rubber compounds [see F. Cataldo, (2006). Progress in Rubber, Plastics and Recycling Technology, vol. 22, pp. 147-164 and 243-252, (2006)], but above all it allows the recovery and reuse of pyrolytic carbon black (hereafter abbreviated CBp) in the preparation of new rubber compounds, both for technical articles and for new tires [see F. Cataldo, Macromolecular Materials & Engineering, vol. 290, pp.463-467, (2005)]. Industrial production of CBp is already underway in several EU countries, as well as in non-EU countries, with limited use of CBp as a low-strength filler in the rubber industry. In fact, currently produced CBp has a number of drawbacks that somewhat limit its potential diffusion. First, the surface of currently produced CBp is covered and inactivated by approximately 10-22 wt% of carbonized rubber, produced by the incomplete pyrolysis of the tire rubber from which it is derived, hereinafter referred to in this description as residue or bituminous fraction, with the aggravating circumstance that this carbonized rubber fraction also contains polycyclic aromatic hydrocarbons, hereinafter referred to in this description as PAHs, some of which are known to be potentially carcinogenic [see F.Cataldo, Fullerenes Nanotubes Carbon Nanostructures, vol. 28, pp. 368-376, (2020)]. It should also be noted that the total PAH content in rubber compounds is limited by strict regulations, such as Regulation (EC) No. 1907 / 2006. Carbonized rubber, a bituminous residue from pyrolysis, deposited on the surface of CBp, reduces its surface area (a fundamental parameter for the reinforcing effect of carbon black in general and CBp in particular) and covers the active sites on the CBp surface. If these sites were free, they would be the basis for the rubber-black interaction, leading to a greater reinforcing effect if the CBp were reused as a filler in the manufacture of new rubber compounds [see F. Cataldo, Carbon, vol. 40, pp. 157-162, (2002); F. Cataldo, Polymer International, vol. 50, pp. 828-834, (2001)].Furthermore, the presence of carbonized rubber on the surface of CBp somewhat limits its processability, preventing crushing after production. In other words, the carbonized rubber behaves like a bituminous resin that dries out any mill designed to crush CBp, a problem whose solution has been proposed in patents NL2015888B1 and WO2013 / 095145A1 through controlled heat treatment in the presence of air to volatilize and burn the bituminous fraction. However, such treatment is not without risks of fire and explosion, which have indeed occurred at an industrial plant scale. Another drawback of conventional carbon black particulates (CBp) is the high content of a mineral fraction, obviously not present in carbon blacks produced by combustion of heavy oil and / or coal fractions in the absence of oxygen, using a process known as kiln combustion. This mineral fraction, sometimes referred to as ash in the technical literature, represents approximately 13.5% by weight of CBp and consists essentially of zinc sulfide mixed with silica and silicates. These silicates are added to tire compounds along with carbon black as fillers and accumulate in the CBp after pyrolysis. The problem with this mineral fraction in CBp is that it is tightly trapped within and on the surface of the CBp, limiting its reinforcing effect and interfering with the sulfur-based vulcanization system if it is reused in new rubber compounds. A carbonized material is, by definition, insoluble or sparingly soluble in organic solvents. Surprisingly, a number of organic solvents have been found—and are therefore the subject of the present invention—that are capable of extracting the carbonized rubber fraction present in CBp. This leaves CBp devoid of this fraction, which previously limited its processability during grinding, as it lacked the carbonized fraction that occupied the active sites and, to some extent, limited its surface-measured reinforcing action. Furthermore, extraction with the solvents that are the subject of the present invention also effectively removes all types of PAHs present in CBp, rendering CBp completely purified and ready for use as a filler in new rubber compounds, without the limitations imposed on its use in place of primary CBp. The solution according to the present invention fits within this context, proposing to provide a purification process for carbon black obtained from the pyrolysis of used tires by solvent extraction, characterized by low or zero toxicity and environmental impact, preferably, but not exclusively, derived from raw materials from renewable and non-fossil sources, or by thermal treatment in an inert atmosphere. QJ QQbn / 77n7 / q / YILI followed by solvent extraction, in order to remove the residues of pyrolyzed rubber deposited on the surface of the carbon black obtained from pyrolysis and the polycyclic aromatic hydrocarbons contained therein. Furthermore, the present invention proposes to provide a process for the extraction of zinc from carbon black obtained from pyrolysis previously purified by solvent extraction or by thermal treatment or by thermal treatment followed by solvent extraction, making use of carboxylic acids of natural origin, such as citric acid and tartaric acid obtained from completely renewable sources. The objective of the present invention is, therefore, to provide an environmentally friendly process for the purification and reactivation of carbon black obtained from the pyrolysis of spent tires that allows overcoming the limitations of processes according to the state of the art and obtaining the technical results described above. Another objective of the invention is that such an ecological process of purification and reactivation of carbon black obtained from the pyrolysis of spent tires can be carried out at substantially contained costs. No less important, the objective of the invention is to propose an ecological process for the purification and reactivation of carbon black obtained from the pyrolysis of used tires that is simple, safe and reliable. Therefore, the specific objective of the present invention is a process for purifying carbon black obtained from the pyrolysis of spent tires by extraction with a medium polarity solvent, or by inert atmosphere heat treatment or by inert atmosphere heat treatment followed by extraction with a medium polarity solvent, where preferably such medium polarity extraction solvent has a polarity of between 36.5 < ET(30) < 41.6 kcal / mol on the Reichardt scale and even more preferably selected from: methyltetrahydrofuran (Me-THF), tetrahydrofuran (THF), ethyl acetate (AcOEt), dimethyl carbonate (DMC), methyl acetate (AcOMe), dichloromethane (CH2Cl2), ethyl formate (HCO2Et), 3:1 vol / vol azeotropic compound of ethyl acetate and ethanol (AcOEt / EtOH) and wherein the inert atmosphere heat treatment is preferably carried out at a temperature between 500 °C and 750 °C and more preferably at a temperature between 550 °C and 600 °C, as well as under a stream of an inert gas preferably selected from: N2, Ar, CO2, superheated steam or a combination of the same and more preferably under a stream of CO2, superheated steam or a combination of the same. Furthermore, according to the present invention, the purification process of carbon black obtained from the pyrolysis of spent tires may further comprise a step of selectively extracting zinc from the carbon black previously purified by solvent extraction and / or inert atmosphere heat treatment, wherein the selective zinc extraction step comprises the use of an aqueous solution comprising a carboxylic acid selected from: citric acid and tartaric acid, preferably in combination with hydrofluoric acid. EXAMPLES 1-20 The present invention will be described below by way of non-limiting illustration with special reference to some illustrative examples. In the examples, solvent extractions were performed in a Soxhlet-type solid-liquid extractor [see L. Gattermann, Die Praxis der Organischen Chemikers, de Gruyter, (1957)], with a 500 mL extraction chamber. For each extraction batch, approximately 262 g of CBp were placed in a special cellulose thimble, which was then inserted into the Soxhlet extraction chamber. For each example reported in Table 1, the extraction was carried out over a standard time span of 8 hours, occasionally using the solvents reported in Examples 1–20 of Table 1 below. At the end of the extraction, the cellulose thimble containing the extracted CBp was removed from the Soxhlet extractor and allowed to dry under a fume hood and then in a dryer until the weight was constant.Subsequently, the dried black was removed from the cellulose thimble, gently pulverized in a mortar or using a special powder mill if necessary, and then weighed. The weighing determined the amount of purified CBp remaining in the thimble after extraction. The weight percent of CBp recovered after extraction was subtracted from the weight of the purified CBp, thus eliminating the weight percent of the bituminous fraction. Using the method in ASTM D655919, the surface area of ​​CBp was also determined after solvent extraction. Primary CBp, as such, has a surface area of ​​41 m² / g. Only solvents effective in extracting the bituminous fraction are capable of significantly increasing the surface area of ​​CBp, which does not occur with the other ineffective solvents reported for comparison.Examples 1 to 5 shown in Table 1 are comparative and show how some solvents, in this case a series of aliphatic and cycloaliphatic (or naphthenic) hydrocarbons such as pentane, hexane, decalin, solvent naphtha, turpentine essence, and limonene, are ineffective at removing the residual bituminous fraction from CBp pyrolysis, and treatment with these often has no apparent effects on the CBp surface area. Table 1. Extraction of CBp using solvent. Examples 1-20 Example No. Solvent Solvent polarity according to Reichardt ET(30) (in kcal / mol) % of CBp recovered after exhaustive extraction Bituminous fraction of CBp extracted from the solvent (% of initial CBp) Surface area of ​​CBp (m2 / g) ASTM D6559 Comments on extraction efficiency of the bituminous fraction of CBp CBp as such na ('j na (**) na D 41 1 Pentane 30 99 1 41 Comparative example, solvent NOT effective 2 Hexane 31 98 2 41 Comparative example, NOT effective solvent 3 Decalin 31.2 98 2 41 Comparative example, NOT effective solvent 4 Naphtha solvent (artificial turpentine) 32 98 2 41 Comparative example, NOT effective solvent 4 Turpentine essence (turpentine) 32.2 98 2 41 Comparative example, NOT effective solvent 5 Limonene 32.3 98 2 41 Comparative example, NOT effective solvent 6 Xylenes (mixture of isomers) 33.1 91 9 58 Comparative example, NOT effective solvent 7 Toluene 33.9 91 9 61 Comparative example, NOT effective solvent 8 Benzene 34.3 90 10 64 Comparative example, NOT effective solvent 9 Trichloroethylene 35.9 90 10 66 Comparative example, NOT effective solvent Effective 10 2-methyltetrahydrofuran 0 36.5 88 18 75 Effective solvent 11 Tetrahydro rupture no 37.4 86 20 76 Effective solvent Q 7 QQbn / 77n7 / q / YILI 12 Ethyl acetate 38.1 85 18 74 Effective solvent 13 Dimethyl carbonate 38.2 87 16 74 Effective solvent 14 Methyl acetate 38.9 85 18 74 Effective solvent 15 Dichloromethane 40.7 79 21 77 Effective solvent 16 Ethyl formate 40.9 85 17 74 Effective solvent 17 Ethyl acetate / ethanol (3:1) 41.6 86 14 74 Effective solvent 18 Acetone 42.2 89 11 69 Comparative example, NOT effective solvent 19 Ethanol 51.9 92 8 67 Comparative example, NOT effective solvent 20 Water 63.1 100 0 41 Comparative example, NOT effective solvent (*) Reichardt C. Solvents and Solvent Effects in Organic Chemistry 3rd Ed., Table 7.3, WileyVCH (2003). (**) na = not applicable Therefore, it was possible to determine that naphtha, the solvent mentioned by J. Piskorz et al. in the article [Energy and Fuels vol. 13, pp. 544-551 (1999)], appears to be completely ineffective as a solvent for the bituminous fraction of CBp, along with all the other aliphatic and cycloaliphatic solvents tested. On the other hand, certain aromatic solvents, such as xylenes in a mixture of isomers, toluene, and benzene (examples 6-8 in Table 1), appear to have better extraction effects than acids. QJ QQbn / 77n7 / q / YILI aliphatic and naphthenic, but with results not entirely satisfactory, with respect to the series of solvents, the objective of the present invention, reported in Examples 10-17 of Table 1. Therefore, Examples 6-8 of Table 1 are also reported as comparative examples and show that benzene, toluene, and xylene, mentioned in the US patent [GW Denison, US PAT. 2004 / 047779 A1] as extraction and purification agents of carbon black, are not the best solvents suitable for this operation, being far surpassed in efficiency by the solvents identified in the present invention and object of the present invention, reported in Examples 10-17 of Table 1. In fact, it has been surprisingly found, and therefore forms the object of the present invention, that a number of solvents with medium polarity, measured by the Reichardt method and scale and designated ET(30) [see C.Reichardt, Solvents and Solvent Effects in Organic Chemistry. WHey-VCH edition of (2003) and the updated edition of (2011)] are highly effective in the extraction of the bituminous fraction of CBp, far surpassing in efficiency both the aliphatic, cycloaliphatic, terpenic and aromatic hydrocarbons mentioned above (examples 1-8 in Table 1), as well as certain chlorinated compounds such as trichloroethylene, example 9 in Table 1, mentioned by Piskorz et al. [Op.cit.] as a possible solvent for CBp extraction, even surpassing in efficiency certain highly polar solvents such as acetone and ethanol (examples 18 and 19 in Table 1), while acetone is already recommended by Piskorz et al. [Op.cit.] as a suitable solvent for the purification of CBp. In fact, the extraction efficiency, as well as the unique performance of the solvents, the objective of the present invention (examples 10-17 in Table 1), is measured by the amount of bituminous fraction extracted from CBp in the standard Soxhlet extraction time, combined with the increase in surface area measured in the purified CBp, which is unmatched and not reflected in any of the solvents reported for comparison. Extraction with the solvents, the objective of the present invention (examples 10-17 in Table 1), results in a significant increase in the surface area of ​​CBp, which in examples 10-14 and 16-17 reaches 74-76 m² / g and culminates in example 15 with 77 m² / g, compared to 41 m² / g for primary CBp. In particular, Table 1 clearly shows that the extraction efficiency of the studied solvents follows the trend of the solvent polarity scale proposed and measured by Reichardt, ET(30) [see C. Reichardt, Op.cit.The maximum extraction efficiency can be identified for solvents with ET(30) values ​​of 36.5 < ET(30) < 41.6 kcal / moL. Therefore, the preferred solvents for CBp extraction, which are the objective of the present invention, are those of Examples 10-17, specifically methyltetrahydrofuran (Me-THF), tetrahydrofuran (THF), ethyl acetate (AcOEt), dimethyl carbonate (DMC), methyl acetate (AcOMe), dichloromethane (CH2Cl2), ethyl formate (HCO2Et), and the 3:1 vol / vol azeotropic compound of ethyl acetate and ethanol (AcOEt / EtOH). It is important to emphasize that all the solvents, which are the objective of the present invention, have environmentally friendly characteristics. In fact, MeTHF and THF are obtained from completely renewable natural sources, i.e., for example, corn cobs, rice husks, sugarcane bagasse and other similar agricultural residues, and MeTHF has an interesting toxicological and environmental profile [see V. Pace et al. ChemSusChem, Vol. 5, pp.1369-1379, (2012)]. Ethyl acetate, methyl acetate, and ethyl formate can also be obtained from renewable sources, respectively acetic fermentation, alcoholic fermentation, and acetone-butyl fermentation [see M. Giua, Chlmica Industriale, vol. 8, pp. 283-386, (1975)], and are readily biodegradable and therefore have almost no environmental impact. DMC has very low toxicity and a rapid biodegradability profile with a very favorable environmental impact profile [see F. Mizia et al. Chlmica & Industria, vol. 83, pp. 47-54, (2001)]. In Example 15 of Table 1, it can be seen that CH2CI2 is among the most efficient purification solvents for CBp and is therefore included among the solvents that are the subject of the present invention. CH2CI2 can also be obtained, at least in part, from renewable sources, such as, for example, by chlorination of biomethane.Although there are concerns regarding its toxicity, it should be noted that these concerns could be overcome if CH2Cl2 were used in a properly designed closed-loop system. In fact, CH2Cl2 has undeniable advantages due to its low cost, high extraction efficiency, low boiling point (approximately 40 °C), low enthalpy of evaporation at 80.5 kcal / kg (important in the solvent recycling step), and, above all, the almost unique characteristic among the solvents tested in Examples IOI 7 of Table 1 of being non-flammable. From an environmental impact perspective, dichloromethane decomposes rapidly if dispersed in the environment and has no effect on stratospheric ozone [see C.M. Trudinger, et al., Journal of Geophysical Research: Atmospheres, vol. 109, issue D22, (2004)].Table 1 also shows that the removal of pyrolyzed rubber residues from the CBp surface by solvent extraction leads to an increase in the CBp surface area, as measured according to ASTM D6559-19. A larger surface area corresponds to an increased reinforcing effect of the carbon black, as the active sites on the CBp surface responsible for the black-rubber interaction become available. Therefore, removing the extractable fraction of CBp not only eliminates PAHs (contained in the bituminous fraction) but also enhances the reinforcing properties of CBp if it is reused as a filler in new rubber compounds. EXAMPLES 21-28 In another embodiment of the present invention, the CBp is heat-treated in a tubular furnace under a selected gas, which in any case is of a different chemical nature than either air or oxygen. The heat treatment was carried out between 500 °C and 750 °C, and preferably between 550 °C and 600 °C. Within this temperature range, the bituminous fraction of the CBp is distilled and carried away from the CBp by the flow of the selected gas. After a suitable treatment time, as shown in the examples reported in Table 2, the CBp is completely purified and freed from the bituminous fraction. In preferred embodiments thereof, a quartz tube with an internal diameter of 2 cm and a length of 30 cm, with two male cones at its ends, is inserted into a Carbolite Type MTF 10 / 25 / 130 tubular furnace. The quartz tube is charged with the mass of primary CBp to be treated, carefully arranged within the usable section of the tube covered by the furnace, as reported in Table 2, Examples 21-28. The ends of the quartz tube are closed with two valves, each connected to a female cone. A porous septum made of sintered glass with G1 porosity is inserted between the female cone and the valve to prevent the CBp powder from dispersing outside the reactor. Examples 21-28 in Table 2 were carried out by heating the oven at a gradient of 20 °C / min to the chosen temperature (shown in Table 2) and this temperature was maintained for the time indicated in Table 2, before turning off the oven and waiting for it to cool to room temperature. Table 2 - Removal of the bituminous fraction of CBp by heat treatment - Examples 21-28 Example no. Process Gas Mass of primary CBp in the tube furnace (g) Temperature (°C) x Treatment Time (min) % of CBp recovered after heat treatment Bituminous fraction of CBp removed by heat (% of initial CBp) Surface area of ​​CBp (m2 / g) ASTM D6559-19 Comments on the extraction efficiency of the bituminous fraction of CBp CBp as such na (*) na (j na (j na (*) 41 21 Nitrogen 2 550 °C x 15 min 80 20 68 Effective treatment 22 Nitrogen 5 550 °C x 45 min 82 18 67 Effective treatment 23 Argon 10 600 °C x 30 min 82 18 70 Effective treatment 24 Carbon dioxide (CO2) 10 600 °C x 60 min 78 20 75 Effective treatment 25 Carbon dioxide (CO2) 10 600 °C x 120 min 76 20 82 Effective treatment 26 Carbon dioxide (CO2) 10 600 °C x 180 min 74 20 86 Effective treatment 27 Water vapor 10 600 °C x 120 min 74 20 85 Effective treatment 28 Water vapor 10 600 °C x 180 min 71 20 89 Effective treatment (*) na = not applicable Throughout the heating gradient, during the holding of the process temperature, and throughout the cooling stage, the CBp was rinsed inside the reaction tube with the chosen gas (flow rates between 0.1 and 0.5 L / min): nitrogen for Examples 21–22, argon for Example 23, carbon dioxide (CO2) for Examples 24–26, and finally with superheated steam at approximately 600 °C for Examples 27 and 28. The superheated steam was produced by drawing it from a special generating boiler and passing it through a copper coil heated externally by a Bunsen burner with a large flame until it reached red-hot [see Gattermann, Op. Cit., Aut similia]. Examples 21–23 in Table 2 show that inert gases such as nitrogen and argon are able to entrain the bituminous fraction coating the CBp when the latter is heated to 550–600 °C. The removal of the bituminous fraction by inert gas heat treatment leads to an increase in the surface area of ​​the purified CBp, as already observed in solvent extraction and summarized in Table 1. In other words, the heat treatment of CBp under N₂ or Ar has the same effects as solvent extraction. On the other hand, as illustrated in Table 2 of Examples 24–28, the treatment of CBp under a stream of carbon dioxide (CO₂), under a stream of superheated steam, or a combination thereof, leads not only to the complete removal of the bituminous fraction but also to a striking increase in the surface area of ​​the resulting CBp. It is known from the literature that this activation phenomenon of the carbonaceous substrate occurs according to the following reactions: CO2 + C 2CO (1) as well as H2O + C CO + H2(2) HsO + C → CO2 + H2(3) but requires much higher temperatures, i.e., between 850 °C and 1,000 °C, but allows for achieving common surface areas of activated carbons [see López, FA, et al. Journal of the Air & Waste Management Association, vol. 63, pp. 534-544 (2013)]. On the other hand, it has been surprisingly found, and therefore forms the objective of the present invention, that it is possible to activate CBp at only 600 °C in both a CO2 atmosphere and a superheated steam atmosphere. Most likely, the impurities present in CBp in the form of zinc compounds, silica, and traces of metallic iron and iron oxides allow for catalytic activation at low temperatures.On the other hand, the surface area increases of CBp in Examples 24-28 compared to Examples 21-23 are quite modest, but of absolute importance if the ultimate purpose of using the CBp produced in this patent is its reuse in new tire rubber compounds, where surface areas of approximately 100 m² / g are effective. Therefore, the heat treatment of CBp, as described in this invention, also allows the surface area of ​​the purified CBp to be modulated at will, keeping it relatively low through treatments under N₂ and Ar for applications in tire compounds where semi-reinforcing blacks are required, and instead yielding actual reinforcing carbon blacks of up to approximately 90-100 m² / g with longer heat treatments if CO₂ and / or superheated steam are used.In particular, the activation of CBp at 600 °C under CO2 and / or superheated steam is surprising, and the fact that the resulting carbon blacks are intended for use in tire compounds and not as activated carbon with very high surface areas, designed for other uses [see López et al. Op. cit.]. Confirmation of the realization of reactions (1), (2) and (3) in the corresponding examples 24-28 of Table 2 is derived from the combustion mass observed in the CBp treated at 600 °C under CO2, which amounts to 2% in example 24, 4% in example 25 and 6% in example 26, while at 600 °C under superheated steam the combustion mass ranges from 6 to 9%, respectively, in examples 27 and 28. The combustion mass is the mass of C that has been gasified by the chemical reaction of CO2 or steam with the surface of CBp as expected from reactions (1), (2) and (3). Q ! CQfríWZZnZ / ? / YILI From the foregoing, it follows that, in their preferred embodiments, the present invention describes two purification processes for primary CBp that are quite equivalent to each other: a solvent extraction process or a heat treatment process under inert or reactive gases. However, the heat treatment allows for modification of the surface area of ​​the resulting modified CBp, which is not possible with the solvent process.However, it should be mentioned that, for certain niche applications, such as, by way of example but not limitation, carbon blacks for rubbers and plastics that must come into contact with food and / or for cosmetic use, a sequence of processes can be proposed in which CBp is first thermally purified and subsequently solvent extracted in order to remove even the last traces of PAHs, in order to offer the market for these niche applications strictly clean CBp that is absolutely in line with the strictest regulations regarding residual PAH content. The chemical analysis of a common CBp [see F. Cataldo, Fullerenes Nanotubes and Carbon Nanostructures, Op. Cit] showed that zinc represents 5.5% of CBp, while the total ash which includes, in addition to zinc, also silica and silicates, represents 13.5% of CBp and finally the sulfur, which is mainly in the form of zinc sulfide, represents 3.2% of CBp. As a further part of the present invention, it has surprisingly been discovered that it is possible to quantitatively extract zinc from previously purified CBp by solvent extraction as illustrated in Examples 10-17 reported in Table 1 or by heat treatment according to Examples 21-28 in Table 2, using carboxylic acids of natural origin, in this case citric acid and tartaric acid obtained from completely renewable sources and, therefore, with an absolutely favorable toxicological and environmental profile [see M. Giua, Chimica Industriale, Op. Cit.].In fact, it has been surprisingly discovered, and is an additional objective of the present invention, that aqueous solutions of citric acid or aqueous solutions of tartaric acid are effective for extracting the zinc contained in CBp previously purified by solvent extraction according to Examples 10-17 reported in Table 1 or by heat treatment according to Examples 21-28 in Table 2. Furthermore, since the zinc in CBp is mainly in the form of zinc sulfide (ZnS), the removal of zinc also automatically involves the removal of mineral sulfur, and aqueous solutions of citric and tartaric acids are able to decompose ZnS and bring Zn2+ to solution, which other common carboxylic acids such as acetic acid cannot accomplish. EXAMPLES 29-35 The solvent-extracted CBp, as described in either Examples 10-17, or prepared by heat treatment as in Examples 21-28, was weighed out in the amount of 250 g and placed in a flat-bottomed, three-necked flask fitted with a mechanical stirrer passing through the central neck of the flask. A bubble condenser was mounted in another neck of the flask, with a valve inserted at its top for gas outlet. The gas was bubbled into a large Drechsell bottle [see Gattermann, Op. Cit.] containing 600 mL of a 10% NaOH solution. A tube was inserted into the third neck for the introduction of a flow of compressed air (generated by a small laboratory compressor). Next, 1,000 mL of an aqueous acid solution, the nature and concentration of which are specified in Examples 29 to 37 of Table 3, were added to the flask. Both the stirrer and the compressor were switched on.The gas that developed from the CBp consisted essentially of hydrogen sulfide (H₂S), due to the decomposition of ZnS present in the CBp by the acidic solution. The developed H₂S was carried by the compressor airflow to the Drechsell bottle and absorbed by aqueous NaOH. The treatment was continued for 3 hours, heating the reaction slurry using an external oil bath maintained at 80–120 °C. At the end of the treatment, the CBp slurry was pump-filtered through still-hot Rapid A filter paper. The CBp on the filter was washed thoroughly with demineralized water. The filtrate was analyzed for the quantitative determination of dissolved zinc using the Hach LCK 360 spectrophotometric method. After washing to neutral, the CBp was dried in an oven at 80 °C for 12 hours with very slow stirring. Table 3 - Removal of zinc from previously solvent-extracted CBp - Examples 29-37 Example No. Type CBp Type of Acid Acid Molarity (mol / L) % Ash in CBp % Zinc in CBp % Zinc Extracted from CBp Zinc Extracted in g / kg from CBp na CBp as such None na 13.5 5.5 0 0 na CBp extracted from examples 1017 0 21-28 None na 15.7 6.5 0 0 29 CBp extracted from examples 1017 or 21-28 Hydrochloric acid. 2.0 10.4 2.9 55 36 30 CBp extracted from examples 1017 0 21-28 Hydrochloric acid. 4.0 7.6 1.1 83 54 31 CBp extracted from examples 1017 or 21-28 Hydrochloric acid. 6.0 6.3 0.2 97 63 32 CBp extracted from examples 1017 0 21-28 Hydrochloric acid / Hydrofluoric acid 6.0 / 3.0 0.0 0.1 98 64 33 CBp extracted from examples 1017 or 21-28 Acetic acid 2.5-6.0 13.1 4.7 27 18 34 CBp extracted from examples 10- Citric acid 0.5-2.0 7.2 0.8 88 57 17 0 21-28 35 CBp extracted from examples 1017o21-28 Citric acid / hydrofluoric acid 0.5-2.0 / 3.0 0.5 0.2 97 63 36 CBp extracted from examples 1017o21-28 Tartaric acid 0.5-2.0 6.4 0.3 96 62 37 CBp extracted from examples 1017o21-28 Tartaric acid / hydrofluoric acid 0.5-2.0 / 3.0 0.4 0.1 98 64 The ash content of the purified CBp was determined by thermogravimetric analysis in the airflow. Table 3 shows all the analytical data for Examples 29-37, together with the analytical data for CBp as such and those extracted with solvent according to Examples 10-17 reported in Table 1 or by heat treatment according to Examples 21-28 in Table 2. From Examples 34 and 36 in Table 3, it can be deduced that both citric acid and especially tartaric acid are equivalent to 6 M hydrochloric acid in the almost complete extraction efficiency of Zn present in CBp, with unprecedented advantages in terms of absence of toxicity, zero environmental impact, and derivation from completely renewable sources for citric and tartaric acids. In fact, zinc has fair commercial value and it is considered advantageous to extract it from CBp for the purpose of overall economic efficiency in the CBp purification process. Furthermore, the removal of Zn from CBp also entails the simultaneous and almost complete removal of sulfur, the latter being largely in the form of ZnS. If a completely ashless and absolutely clean CBp is to be produced, Example 32 illustrates that the combination of hydrochloric / hydrofluoric acids is effective in the complete removal of the mineral fraction from the CBp. In fact, Examples 35 and 37 show that the combination of citric acid and hydrofluoric acid, as well as the combination of tartaric acid and hydrofluoric acid, are equally effective in removing the mineral fraction from the CBp. The present invention has been described in its preferred embodiments. It is understood that those skilled in the art may make changes, for example, in the selection of solvents or carboxylic acids, without departing from the relative scope of patent protection.

Claims

1. The purification process of carbon black obtained from the pyrolysis of spent tires by extraction with a solvent with a polarity between 36.5 < ET(30) < 41.6 kcal / mol on the Reichardt scale.

2. The purification process of carbon black obtained from the pyrolysis of spent tires according to claim 1, wherein the extraction solvent is selected from: methyltetrahydrofuran (Me-THF), tetrahydrofuran (THF), ethyl acetate (AcOEt), dimethyl carbonate (DMC), methyl acetate (AcOMe), dichloromethane (CFI2Cl2), ethyl formate (HCO2Et), 3:1 vol / vol azeotropic compound of ethyl acetate and ethanol (AcOEt / EtOH).

3. The purification process of carbon black obtained from the pyrolysis of spent tires according to claim 1 or 2, wherein a heat treatment is carried out in an inert atmosphere prior to solvent extraction.

4. The purification process of carbon black obtained from the pyrolysis of spent tires according to any of the preceding claims, further comprising a selective zinc extraction step from the previously purified carbon black by solvent extraction or solvent extraction and inert atmosphere heat treatment, wherein the selective zinc extraction step comprises the use of an aqueous solution comprising a carboxylic acid selected from: citric acid and tartaric acid.

5. The purification process of carbon black obtained from the pyrolysis of spent tires according to claim 4, wherein the aqueous solution comprises hydrofluoric acid, in combination with the carboxylic acid selected from: citric acid and tartaric acid.