WO2008093368A1

WO2008093368A1 - Process for disposal of asbestos and plant for carrying out such a process

Info

Publication number: WO2008093368A1
Application number: PCT/IT2007/000066
Authority: WO
Inventors: Nicola Criscuolo
Original assignee: Irsai S.R.L.
Priority date: 2007-01-31
Filing date: 2007-01-31
Publication date: 2008-08-07

Abstract

Process for disposal of asbestos or materials containing asbestos suitable for decomposing the asbestos in non- hazardous materials, comprising the following steps: - decomposing the asbestos or the materials containing asbestos with fluosilicic acid obtaining fluosilicates; - decomposing the fluosilicates with at least one acid or a mixture of acids having such strength as to decompose the fluosilicates into silicon tetrafluoride and hydrogen fluoride; - regenerating the fluosilicic acid by reacting the silicon tetrafluoride and the hydrogen fluoride with each other; and - disposing the compounds obtained at the end of the two decomposition steps.

Description

DESCRIPTION

PROCESS FOR DISPOSAL OF ASBESTOS AND PLANT FOR CARRYING OUT SUCH A PROCESS

[0001] The object of the present invention is a process for disposal of asbestos or material containing asbestos, and plant for making such process . BACKGROUND OF THE INVENTION

[0002] As known, different varieties of asbestos exist in nature, which differ according to the crystalline structure and the chemical composition.

[0003] In general, the different varieties of asbestos exhibit a parallel aggregation structure with very fine elongated crystals, easily splittable into textile fibres, provided with excellent characteristics of fire resistance, endurance, thermal insulation and abrasion resistance.

[0004] For these characteristics, in the past asbestos was widely used in different fields, in particular in building, as thermal insulator or as basic component for materials intended for withstanding fire.

[0005] Asbestos has already been recognised as highly carcinogenic material for a few decades . Its dangerousness is increased by the tendency of the material to break up and thus release microfibres which can be easily inhaled. [0006] Over the last decades, urged by this problem, several sites asbestos contaminated sites have been decontaminated, in particular residential and commercial buildings, but also industrial structures. [0007] Before being removed, materials containing asbestos must be suitably treated, for example coated with suitable resins, to prevent fibre dispersion and after that, they must be packaged in sealed containers to be carried without risks. The materials thus treated keep their dangerousness characteristics unchanged and must therefore be disposed in special dumps. As a consequence, this implies high disposal costs and a considerable impact on the environment . [0008] As known, the carcinogenicity of asbestos is essentially related to its crystalline structure of the microfibre type.

[0009] To solve the problems related to the disposal of asbestos to the root, several disposal processes have therefore been proposed in the past capable of destroying the microfibre structure thereof.

[0010] A first type of disposal processes envisages the thermal decomposition of asbestos fibres. Even though this solution is technically effective, however it is uneconomical due to the large plant investments it requires and to the considerable operating costs (mainly energy) .

[001I]A second type of processes envisages the destruction of asbestos by these chemical reagents, in particular acid substances, capable of attacking and destroying the crystalline structure of asbestos. In some cases, the break-up action of the acids is also associated to a thermal action.

[0012] An example of this second type of disposal processes is described in European patent EP372084 Bl. The process envisages the use of a treatment agent consisting of a mixture of one or more mineral acids (for example sulphuric acid, hydrochloric acid or hydrofluoric acid) and one or more inorganic polymers, containing Si-O, Ti-O and Al-O links, in the form of aqueous solution or dispersion. The treatment agent is sprayed on the material containing asbestos and then left to react even for a few days.

[0013] This process, devised to be carried out on site and to prevent problems related to the removal of materials containing asbestos, however, exhibits the disadvantage of requiring very long treatment times, even a few days. [0014] Another disadvantage is related to the fact that the process requires the use of a treatment agent that must be suitably prepared and is therefore quite expensive. [0015] Another example of disposal process by chemical agents is described in American patent US 6005158. The process envisages the use of a treatment agent consisting of an inorganic acid, for example hexafluorophosphoric acid, or of a mixture of hydrofluoric acid and phosphoric acid, or a mixture of hexafluorophosphoric acid and phosphoric acid. The process provides for the material containing asbestos to be disposed to be directly sprayed with the above treatment agent, or for the material to be immersed in a tank containing a solution of the above treatment agent. The disadvantage of this process is that it basically requires a treatment lasting a few days. [0016] Finally, a disadvantage in general common to all disposal processes by chemical agents of the known type is that of requiring a considerable consumption of the reagents used for the decomposition of asbestos to the disadvantage of the inexpensiveness of the process itself. SUMMARY OF THE INVENTION

[0017] The object of the present invention is to provide a process for disposal of asbestos or materials containing asbestos which should allow complete decomposition of the asbestos treated within a few hours while allowing limiting the consumption of reagents used for the decomposition of asbestos. [0018] A further object of the present invention is to provide a process for disposal of asbestos which should generate end residues deliverable without any further treatment to dumps for non-toxic waste or waste reusable in the industrial field.

[0019] A further object of the present invention is to provide a process for disposal of asbestos which should envisage the use of low cost consumable reagents.

[0020] A further purpose of the present invention is to provide a plant for carrying out the above disposal process . [0021] Such objects are achieved by a process for disposing of asbestos or materials containing asbestos, as described in the following claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Further features and advantages of the process according to the invention will appear more clearly from the following description of some preferred embodiments thereof, made by way of a non-limiting example with reference to the annexed figures, wherein:

[0023] figure 1 shows a block diagram of the process steps according to a preferred embodiment of the invention;

[0024] figures 2a and 2b show two simplified diagrams of the plant according to two different embodiments of the invention; [0025] figures 3 to 8 show the main reactions involved in the process according to the invention as the variety of asbestos treated varies; and

[0026] figures 9 and 10 show the main reactions involved in the process according to the invention in relation to the decomposition of a cement matrix. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0027] The process for disposal of asbestos or materials containing asbestos according to the present invention is suitable for decomposing asbestos minerals into materials not hazardous for the environment.

[0028] In general, the expression "materials not hazardous for the environment" herein refers to substantially inert materials, that can be delivered to standard dumps, for example for inert materials resulting from building. More in particular, such expression refers to substances resulting from asbestos minerals, but morphologically modified so as to not exhibit the carcinogenicity features typical of the latter. [0029] The term "asbestos" herein in general refers to any morphological variety of asbestos, that is, serpentine asbestos and amphibole asbestos.

[0030] In particular, the general term of "asbestos" therefore refers in particular to the following varieties: chrysotile (or white asbestos) of general formula Mg₃(Si₂O₅)(OH)₄; actinolite of general formula Ca₂(Mg, Fe²⁺)_S(Si₈O₂₂)(OH)₂; amosite (or dark asbestos) of general formula (Mg, Fe²⁺J₇(Si₈O₂₂)(OH)₂; anthophyllite of general formula Mg₇(Si₂O₅) (OH)₂; crocidolite (or blue asbestos) of general formula Na₂(Mg, Fe²⁺J₇(Si₈O₂₂) (OH)₂; and tremolite of general formula Ca₂Mg₅Si₈O₂₂ (OH) ₂.

[0031] The expression "materials containing asbestos" herein in general refers to materials obtained by mixing asbestos fibres and fillers/binders consisting of substantially inert materials. A typical example consists of cement-asbestos, wherein asbestos fibres are buried in a cement matrix. These materials may incorporate a single variety of asbestos, as well as one or more varieties. In general, the contents of asbestos may range from values of 2-3% by weight to 60-70% by weight of the material as a whole .

[0032] Assuming a cement matrix made with Portland cement, the above materials containing asbestos exhibit: tricalcium silicate 3CaCSiO₂ (50%) ; dicalcium silicate 2CaCSiO₂ (25%) ; tricalcium aluminate 3CaO.Al₂O₃ (12%) ; and tetracalcium aluminoferrite (4CaO.Al₂O₃. Fe₂O₃ (8%) . The above percentages refer to the cement matrix only. [0033] Hereinafter, for simplicity of description, the term "asbestos" shall be used to refer to both pure asbestos minerals and to materials containing asbestos, unless otherwise specified for requirements of description. [0034] According to the invention, the process for disposal of asbestos, comprises the following operating steps: [0035] - decomposing asbestos or the materials containing asbestos with fluosilicic acid obtaining fluosilicates; [0036] - decomposing said fluosilicates with at least one acid or mixture of acids having such force as to decompose said fluosilicates into silicon tetrafluoride and hydrogen fluoride; [0037] - regenerating the fluosilicic acid reacting the silicon tetrafluoride and the hydrogen fluoride with each other; and

[0038] - disposing compounds obtained at the end of the above two decomposition steps . [0039] Advantageously, the process according to the invention comprises a step of milling or grinding the asbestos to be carried out before the step of decomposition with fluosilicic acid.

[0040] Advantageously, the process according to the invention comprises a step of unpacking the asbestos during which the protective enclosure used for safely carrying the materials from the decontamination sites to the disposal plant, is removed. This step, which may be manual or robotised, is envisaged upstream of the entire process, that is, before the grinding step. [0041] "Protective enclosure" is herein meant to refer in general to containers, rigid or capable of adjusting to the shape of the contents, used for containing the asbestos pr materials contaminated by asbestos. Therefore, reference is not made to any protective layers of polymeric products sprayed on the contaminated items in order to prevent the dispersion of asbestos particles during the site decontamination.

[0042] Advantageously, the process according to the invention comprises a step of liquid-solid separation of the reaction products of the above steps of decomposition. [0043] Advantageously, it is also possible to envisage a step of neutralisation of the acidity present in the reaction products inside the first reactor, to be carried out before the above step of liquid-solid separation. [0044] Advantageously, the step of disposing of the compounds generated by the steps of decomposition is carried out after the above step of separation, with end disposal methods that preferably differ in the liquid phase and in the solid phase. [0045] More in detail, the solid phase that is substantially inert, may be delivered to dumps for non- hazardous waste or be reused in the production of building materials . The liquid phase that mainly consists of magnesium salts in solution, may be advantageously reused as reagent in the chemical industry. [0046] Advantageously, the unpacking and grinding operations, the decomposition and regeneration reactions take place in a confined vacuum environment, in order to prevent the dispersion of particles of asbestos or other harmful substances. The above operations may take place in the same confined environment or in different environments .

* * *

[0047] A preferred embodiment of the invention shall now be described in detail, with reference to Figures 1 and 2a annexed herein.

[0048] In accordance with this preferred embodiment, the disposal process is carried out in a discontinuous manner, at least with reference to the two steps of decomposition. The steps of unpacking and grinding may in fact be optionally carried out continuously envisaging an accumulation of the material to be then treated in the next steps. Preferably, in order to prevent the need of having sealed storage containers available, also the unpacking and grinding steps are carried out discontinuously so as to process only the amounts intended to be treated in the next process steps. [0049] The items contaminated by asbestos or integrally made of asbestos are subject to the unpacking step. As already said, this operating step takes place in a confined vacuum environment, and may be carried out manually or preferably, in a robotised manner.

[0050] The items are then subject to the grinding step.

Advantageously, grinding may be carried out in multiple stages in order to cut the blocks or pieces of items to the desired size. To this end it is possible to use any equipment of known type suitable for the purpose, such as a roller grind.

[0051] If the item to be treated is a mix of asbestos fibres and fillers/binders, for example of the cement type (as in the case of asbestos cement) , the grinding step is continued up to obtaining particles with a mean grain size not exceeding 1 cm.

[0052] As will appear more clearly in the following description, in order to reduce the overall duration of the disposal process, grinding is preferably continued up to obtaining particles with a mean grain size comprised within the range from 0.5 mm to 1 mm.

[0053] If the item to be treated consists of asbestos only and can therefore be broken up into fibres, the grinding step is continued up to obtaining fibres with a mean length not exceeding 20 mm.

[0054] At this point the asbestos decomposition step, which is carried out in a first batch reactor, can begin. [0055] The process envisages a first step of charging the reactor, during which the amount of asbestos to be treated and a predetermined amount of fluosilicic or hexafluorosilicic acid (H₂SiF₆) are introduced in the reactor . [0056] In this first loading step the fluosilicic acid is introduced in the first reactor as aqueous solution with a concentration by weight comprised between 10% and 40%. [0057] Preferably, the acid has a concentration equal to 30%, this being the concentration at which the fluosilicic acid is more easily found on the market.

[0058] Operatively, the fluosilicic acid is introduced in the first reactor with an excess comprised between 5% and 15% as compared to the stoichiometric amount for the decomposition of the silicates present in the charged material to be disposed of . Preferably, the excess is substantially equal to 10%.

[0059] The amount of fluosilicic acid to be introduced in the first reactor therefore depends on the overall amount of silicates, which varies according to the amount and the variety of asbestos (chrysotile, amosite, anthophyllite, etc.) to be decomposed inside the reactor, but also to the presence of a cement matrix (for example consisting of Portland cement) . [0060] In general, the decomposition reaction of asbestos by the attack of the fluosilicic acid envisages the decomposition of the silicates with the production of fluosilicates, silica and water. This reaction varies according to the variety of asbestos involved, as can be seen in Figures 3 to 8. [0061] If materials containing asbestos with cement base are subject to the disposal process, the attack reaction carried out by the fluosilicic acid involves also the different types of silicates present in the cement with further generation of fluosilicates, as can be seen in Figure 9.

[0062] During this decomposition step, the contents of the first reactor are subject to continuous mixing. Preferably, the mixing of the reactor contents takes place by a double diaphragm pump, so as to prevent contact of the inside parts of the pump with the acid substances in the first reactor.

[0063] Asbestos with its structure of silicates (and optionally the silicates of the cement matrix) is left to react with the fluosilicic acid for a first reaction period tl having a duration comprised in the range between 15 minutes and 35 minutes.

[0064] Advantageously, the duration of this first period of reaction is defined according to the mean grain size of the particles or to the mean length of the fibres loaded in the reactor. The smaller the dimensions of the loaded particles, the shorter the length of such period tl. [0065] Table 1 hereinafter shows the preferred ranges of time of this first period of reaction tl according to the mean grain size D of the particles loaded in the first reactor.

Table 1

[0066] Advantageously, in this first period of reaction tl heat is not supplied from the exterior and the temperature that generates inside the reactor is related to the normal occurrence of the decomposition reaction. [0067] Advantageously, during the asbestos decomposition step an air current is blown into the first reactor. The air blow-in becomes necessary if the asbestos to be decomposed if of the crocidolite type. In fact in that case, as will be described in detail hereinafter, also molecular oxygen 02 operates in the decomposition reaction.

[0068] Preferably, the asbestos decomposition by attack with fluosilicic acid is carried out with pH values not exceeding 5. [0069] At the end of this first period of reaction tl, the fluosilicate decomposition step begins. Preferably, this step is carried out inside the first reactor, in conjunction with the asbestos decomposition step. [0070] The process envisages a second step of loading the reactor, during which an acid or a mixture of acids having such strength as to decompose the fluosilicates into silicon tetrafluoride and hydrogen fluoride, are introduced in the reactor. [0071] Preferably sulphuric acid (H₂SO₄) is used as acid, but also other acids may advantageously be used, having such strength as to decompose the fluosilicates into silicon tetrafluoride and hydrogen fluoride, for example phosphoric acid (H₂PO₄) . [0072] In this second loading step the sulphuric acid is introduced in the first reactor as aqueous solution, with a concentration by weight comprised between 30% and 98%, and preferably equal to 98%.

[0073] Operatively, the sulphuric acid is introduced in the first reactor with an excess comprised between 5% and 15% as compared to the stoichiometric amount for the decomposition of the fluosilicates intended to generate subsequent to the attack of the fluosilicic acid on the loaded material to be disposed of. Preferably, the excess is substantially equal to 10%. If the material to be disposed of contains a cement matrix, the aluminates present in the latter should be also taken into account in the balance.

[0074] As with the fluosilicic acid, the amount of sulphuric acid to be introduced in the first reactor therefore depends on the overall amount of fluosilicates that will generate in the reactor and which varies according to the amount and the variety of asbestos (chrysotile, amosite, anthophyllite, etc.) to be decomposed inside the reactor, but also to the presence of a cement matrix (for example consisting of Portland cement) that contains silicates that can be attacked by the fluosilicic acid and aluminates that can be attacked by the sulphuric acid. [0075] In general, the decomposition reaction of the fluosilicates by sulphuric acid attack leads to the formation of gaseous silicon tetrafluoride (SiF₄) and hydrogen fluoride (HF) , as well as to the formation of magnesium sulphate (soluble) . According to the variety of decomposed asbestos, molecular hydrogen (H₂) may also form (in the case of actinolite, amosite and crocidolite) , as well as sodium sulphate, ferrous sulphate and/or hydrated calcium sulphate. Any by-products resulting from impurities, always present in the original material, are not mentioned herein. [0076] The decomposition reaction of fluosilicates varies according to the variety of asbestos involved, as described in Figures 3 to 8.

[0077] If the materials to be disdposed of exhibit a cement base, the attack with fluosilicic acid also leads to the decomposition of the aluminates (tricalcium aluminate and/or tetracalcium aluminioferrite) with the production of ferric sulphate Fe₂ (SO₄) ₃ and/or aluminium trisulphate

Al₂ (SO₄) ₃, as can be seen in Figure 10.

[0078] Advantageously, the above second loading step envisags that after the loading of the sulphuric acid in the first reactor, an amount of dilution water is introduced.

[0079] In this way it is possible to rise the temperature inside the reactor, up to values approximately comprised within the range from 50 to 60⁰C, using the heat that develops from the dilution of sulphuric acid with water.

The rise in temperature favours the decomposition reaction of asbestos and also the decomposition reaction of fluosilicates . [0080] Preferably, the dilution water is introduced in said first reactor in a quantity by weight comprised between

10% and 20% of the quantity of sulphuric acid loaded.

[0081] The decomposition reactions of both fluosilicates

(and aluminates) by the sulphuric acid, and of the residual silicates (asbestos) by the fluosilicic acid, are left to run for a second period of reaction t2 having a duration comprised in the range between 30 min and 70 min.

[0082] Advantageously, the duration of this second period of reaction is defined according to the mean grain size of the particles or to the mean length of the fibres initially loaded in the reactor. The smaller the dimensions of the charged particles, the shorter the length of such period t2. [0083] Table 1 hereinbefore shows the preferred ranges of time of this second period of reaction t2 according to the mean grain size D of the particles loaded in the first reactor. [0084] Advantageously, the introduction of sulphuric acid tends to decrease the pH values favouring the completion of the decomposition of asbestos still present . [0085] Preferably, during this second reaction period t2 the contents of the first reactor are subject to continuous mixing. [0086] Advantageously, during this second reaction period t2 the gaseous reaction products (SiF₄, HF and in some cases also H₂) are continuously discharged from the first reactor, so as to favour the decomposition reaction of the fluosilicates . [0087] Preferably, the discharge of gaseous products is carried out by blowing in a current of air under pressure. [0088] Substantially, at the same time as the decomposition step of fluosilicates, that is, during the second reaction period t2, the process according to the invention envisages the performance of fluosilicic acid regeneration step.

[0089] Advantageously, the regeneration step is carried out in a second reactor. [0090] Operatively the acid regeneration takes place by recombination in gaseous phase of the silicon tetrafluoride with hydrogen fluoride in the presence of water with acid generation in aqueous solution, according to the following general reaction: SiF₄(g)+2HF(g)^t→H₂F₆Si(l) [0091] Preferably, the second reactor consists of a maze- like water spray chamber, inside which the gaseous flow from the first reactor is conducted.

[0092] In order to favour the combining reaction between SiF₄ and HF inside the second reactor, an overpressure comprised preferably between 1.2 and 2 bar is maintained. [0093] Operatively, the aqueous solution of H₂SiF₆ that generates inside the second reactor is collected at the bottom and is at least partly recirculated to the spray nozzles, optionally integrated with a pure water flow. The flow rate of nebulised water is substantially calculated on the basis of the amount of asbestos treated in the first reactor, that is, on the basis of the amount of SiF₄ and HF to be treated. Preferably, the regeneration step continues for a time sufficient to bring the density of the fluosilicic acid solution to a value of about 1.3 gr/cm³.

[0094] Advantageously, the regeneration step allows recovering between 90% and 95% of the fluosilicic acid loaded in the reactor. [0095] At the end of the regeneration step, the amount of fluosilicic acid regenerated may be reused in a second disposal cycle, optionally integrated with fresh acid in order to balance the consumption occurred in the previous cycle . [0096] Preferably, in order to prevent hydrogen fluoride leaks outside the plant, the above second reactor is located in a confined vacuum environment .

[0097] At the end of the above second reaction period t2 , the asbestos decomposition reaction yield is close to 100%, as results from analyses of samples of reaction residues by X-ray diffractometry.

[0098] The yield of the fluosilicate decomposition reaction on the other hand is comprised between 90% and 95%. [0099] Preferably, before proceeding with the step of disposing of the reaction products, it is possible to proceed with the step of neutralisation of the acidity- present in the first reactor so as to allow the delivery of the residues to the dump. The acidity is essentially given by the residual sulphuric acid. [00100] Operatively, it is possible to proceed for example by introducing a suitable amount of lime, calculated on the basis of pH measurements. [00101] Advantageously, at this point, the step of disposing of the reaction products inside the first reactor is carried out .

[00102] As already mentioned hereinbefore, the reaction product is always magnesium sulphate, present in aqueous solution. According to the variety of asbestos subject to decomposition, also sodium sulphate, ferrous sulphate and/or hydrated calcium sulphate may be normally found in solid phase, in addition to the silica directly resulting from the asbestos decomposition. Any by-products resulting from impurities that may be present in the original material, are not mentioned herein. [00103] If the asbestos is associated to a cement matrix, at the end of the decomposition steps in solid phase there may also be calcium sulphate, ferric sulphate Fe₂(SO₄J₃ and/or aluminium trisulphate Al₂(SO₄J₃. [00104] Advantageously, the process envisages the step of liquid-solid separation of the contents of the first reactor, at least in order to separate the silica in suspension from the magnesium sulphate solution. [00105] To this end, it is possible to use any suitable filter of the known type, such as a tape filter or an Oliver filter.

[00106] As already mentioned before, preferably, the end disposal methods differ for the liquid phase and the solid phase.

[00107] More in detail, the solid phase that is substantially inert, may be delivered to dumps for non- hazardous waste or be reused in the production of building materials. The liquid phase that essentially consists of magnesium salts in solution, may be advantageously reused as reagent in the chemical industry. * * *

[00108] In accordance with a second embodiment of the invention, illustrated in Figure 2b, the above decomposition steps are carried out using two or more first reactors connected to each other in parallel . In this way it is possible to better exploit the potential of the unpacking, grinding and separation stations of the plant itself, thus approaching a continuous operation.

* * *

[00109] The reactions involved in the process according to the invention based on the variety of asbestos to be disposed of, shall now be described in further detail. [00110] The asbestos decomposition reaction is indicated with letter (a) , the fluosilicate decomposition reaction with letter (b) , whereas the fluosilicic acid regeneration reaction with letter (c) .

[00111] If the asbestos is present as chrysotile variety, the reactions involved in the disposal process are shown in Figure 3. [00112] If the asbestos is present as amosite variety, the reactions involved in the disposal process are shown in Figure 4. In particular it is noted that, besides magnesium sulphate, ferrous sulphate is produced, which is in the solid phase obtained by separation of the end contents of the first reactor. [00113] If the asbestos is present as actinolite variety, the reactions involved in the disposal process are shown in Figure 5. In particular it is noted that, besides magnesium sulphate, ferrous sulphate and hydrated calcium sulphate are produced, which are in the solid phase obtained by separation of the end contents of the first reactor.

[00114] If the asbestos is present as crocidolite variety, the reactions involved in the disposal process are shown in Figure 6. In particular it is noted that, besides magnesium sulphate, sodium sulphate and ferrous sulphate are produced, which are in the solid phase obtained by separation of the end contents of the first reactor.

[00115] If the asbestos is present as anthophyllite variety, the reactions involved in the disposal process are shown in Figure 7.

[00116] If the asbestos is present as tremolite variety, the reactions involved in the disposal process are shown in Figure 8. In particular it is noted that, besides magnesium sulphate, hydrated calcium sulphate is produced, which is in the solid phase obtained by separation of the end contents of the first reactor.

* * *

[00117] Below are some examples of application of the process according to the preferred embodiment of the process described above.

Example Ia

[00118] In a confined vacuum environment, 10 kg of white asbestos (chrysotile) where subject to grinding. The asbestos was subject to grinding with a roller grind up to obtaining fibres with a length not exceeding 20 mm.

[00119] The material thus treated was then loaded in a batch reactor, wherein an aqueous solution of fluosilicic acid (H₂SiF₅) had been previously introduced, with a concentration equal to 30% by weight, for a quantity of acid equal to about 17.2 kg, calculated with a 10% more than the stoichiometric quantity.

[00120] The material was left to react with the aqueous solution of fluosilicic acid for about 20 minutes. The reaction contents were continuously mixed by a double diaphragm pump connected to the reactor.

[00121] 98% sulphuric acid for an overall acid quantity of about 11.8 kg (10% more than the stoichiometric quantity), and then water for a quantity of 1.8 kg, were then introduced in the reactor in a sequence. Mixing sulphuric acid and water rose the temperature inside the reactor to a value comprised between 5O⁰C and 6O⁰C. This process step continued for about 45 min. The reaction contents were continuously mixed by the above double diaphragm pump .

[00122] The gaseous reaction products (silicon tetrafluoride and hydrogen fluoride) were continuously discharged from the reactor through a current of air under pressure. The gaseous mixture in output from the reactor was then sent in a maze-like water spray chamber, wherein the silicon tetrafluoride and the hydrogen fluoride reacted to form fluosilicic acid again. The pressure inside the reactor was kept at a value of about 1.5 bar, recirculating a flow rate equal to about 4000 litres/hour. The regeneration continued up to obtaining a density of the 30% solution equal to 1.3 gr/cm³. About 15.4 kg of fluosilicic acid were recovered at the end of the process .

[00123] After about 45 minutes from the addition of sulphuric acid and water, the reactor was emptied. The contents essentially consisted of an aqueous solution of magnesium sulphate and a solid phase consisting of silica. The X-ray diffractometry analysis of samples of the contents excluded the presence of chrysotile. Example Ib

[00124] In confined vacuum environment, 10 kg of a cement mix consisting of white asbestos (chrysotile) by about 40% by weight and cement matrix (Portland cement) by the remaining 60%, were subject to grinding. The grinding was carried out in multiple stages up to bringing the material to a mean grain size of about 1 cm. [00125] The process continued as in example Ia. The quantity of fluosilicic acid (H₂SiF₆) loaded in the reactor was equal to about 15.9 kg (of which about 6.9 kg for asbestos and 9 kg for the cement matrix) . The first reaction period tl lasted about 30 minutes. The quantity of sulphuric acid loaded was about 13.9 kg (of which about 4.7 kg for the fluosilicates resulting from the asbestos decomposition and 9.2 kg for the fluosilicates and the aluminates of the cement matrix) , whereas the quantity of dilution water equal to about 2 kg. The second reaction period t2 lasted about 60 minutes. About 14.3 kg of fluosilicic acid (~ 90% recovery) were recovered at the end of the process. [00126] The end contents of the reactor essentially consisted of an aqueous solution of magnesium sulphate and a solid phase consisting of silica, calcium sulphate, aluminium trisulphate and ferric sulphate. The X-ray diffractometry analysis of samples of the contents excluded the presence of chrysotile.

Example Ic

[00127] The process was as in example Ib, but continuing the grinding up to obtaining particles having a mean grain size of about 2 mm. The first reaction time tl was reduced to 20 minutes, whereas the second reaction period t2 to 45 minutes, substantially obtaining the same results.

Example 2a [00128] In confined vacuum environment, 10 kg of cement mix consisting of actinolite by about 32% by weight and cement matrix by the remaining 68%, were subject to grinding. The grinding was carried out in multiple stages up to bringing the material to a mean grain size of about 1 cm. [00129] The ground material was loaded in a batch reactor (first reactor) , wherein an aqueous solution of fluosilicic acid (H₂SiF₆) had been previously introduced, with a concentration equal to 30% by weight, for a quantity of acid equal to about 13.5 kg (3.3 kg for asbestos and 10.2 kg for the cement matrix), with a 10% more than the stoichiometric quantity. The first reaction time tl was equal to about 35 minutes. After that, 98% sulphuric acid was introduced in the reactor for an overall acid quantity equal to 14.25 kg (3.85 kg for asbestos and 10.4 kg for the cement matrix), with a 10% more than the stoichiometric quantity. After that, water was introduced in the reactor for a quantity of 2.1 kg. The second reaction time t2 was equal to about 70 minutes. The gases produced in the first reactor were continuously sent in a spray chamber (second reactor) inside which a pressure of about 1.5 bar was kept, recirculating a flow rate equal to about 4000 litres/hour. The regeneration continued up to obtaining a density of the solution equal to 1.3 gr/cm³. About 12.2 kg of fluosilicic acid were recovered at the end of the process. At the end of the second reaction period t2, lime was introduced in the first reactor to neutralise the residual acidity up to a value of pH=7 (controlled by a pH sensor) . The contents were filtered by an Oliver filter producing an aqueous phase containing magnesium sulphate and a solid phase containing silica, hydrated calcium sulphate, ferrous sulphate, calcium sulphate, aluminium trisulphate and ferric sulphate. The X-ray diffractometry analysis of samples of the solid phase and of the aqueous phase excluded the presence of actinolite.

Example 2b

[00130] The process was as in example 2a, but continuing the grinding up to obtaining particles having a mean grain size of about 0.8 mm. The first reaction time tl was reduced to 18 minutes, whereas the second reaction period t2 to about 35 minutes, substantially obtaining the same results.

Example 3 [00131] In confined vacuum environment, 10 kg of cement mix consisting of dark asbestos (amosite) by about 28% by weight and cement matrix by the remaining 72%, were subject to grinding. The grinding was carried out in multiple stages up to bringing the material to a mean grain size of about 5 mm. [00132] The material thus ground was then loaded in a batch reactor, wherein an aqueous solution of fluosilicic acid (H₂SiF₆) had been previously introduced, with a concentration equal to 30% by weight, for a quantity of acid equal to about 13.6 kg (2.8 kg for asbestos and 10.8 kg for the cement matrix) , with about 10% more than the stoichiometric quantity. The first reaction time tl was equal to about 25 minutes. The temperature of the reactor was substantially ambient. The reaction contents were continuously mixed by a double diaphragm pump. [00133] After that, 98% sulphuric acid was introduced in the reactor for an overall acid quantity equal to 14.6 kg (3.6 kg for the fluosilicates resulting from the asbestos and 11 kg for the fluosilicates and the aluminates of the cement matrix) , with a 10% more than the stoichiometric quantity. After that, water was introduced in the reactor for a quantity of 2.2 kg. This process step continued for about 50 min. The reaction contents were continuously mixed by the above double diaphragm pump . [00134] The gaseous reaction products (silicon tetrafluoride, hydrogen fluoride and molecular hydrogen) were continuously discharged from the reactor through an air current. The gaseous mixture in output from the reactor was then absorbed in water in a maze-like spray chamber, wherein the silicon tetrafluoride and the hydrogen fluoride reacted with each other to form fluosilicic acid again. The pressure inside the reactor was kept at a value of about 1.5 bar, recirculating a flow rate equal to about 4000 litres/hour. The regeneration continued up to obtaining a density of the solution equal to 1.3 gr/cm3. About 12.2 kg of fluosilicic acid were recovered at the end of the process. [00135] After about 50 minutes from the addition of sulphuric acid and water, lime was introduced in the reactor to neutralise the residual acidity up to a value of pH=7 (controlled by a pH sensor) .

[00136] The reactor was then emptied. The contents were filtered by an Oliver filter producing an aqueous phase containing magnesium sulphate and a solid phase containing silica, ferrous sulphate, calcium sulphate, aluminium trisulphate and ferric sulphate. The X-ray diffractometry analysis of samples of the solid phase and of the aqueous phase excluded the presence of amosite.

Example 4 [00137] In confined vacuum environment, 10 kg of cement mix consisting of anthophyllite by about 25% by weight and cement matrix by the remaining 75%, were subject to grinding. The grinding was carried out in multiple stages up to bringing the material to a mean grain size of about 0.5 mm.

[00138] The material thus ground was then loaded in a batch reactor, wherein an aqueous solution of fluosilicic acid (H₂SiF₆) had been previously introduced, with a concentration equal to 30% by weight, for a quantity of acid equal to about 14.9 kg (3.6 kg for asbestos and 11.3 kg for the cement matrix) , with a 10% more than the stoichiometric quantity.

[00139] The material was left to react with the aqueous solution of fluosilicic acid for about 15 minutes. The temperature of the reactor was substantially ambient. The reaction contents were continuously mixed by a double diaphragm pump.

[00140] After that, 98% sulphuric acid was introduced in the reactor for an overall acid quantity equal to 13.9 kg (2.4 kg for the fluosilicates from the asbestos and 11.5 kg for the fluosilicates and the aluminates of the cement matrix) . After that, water was introduced in the reactor for a quantity of 2 kg. [00141] The mixing between sulphuric acid and water rose the temperature inside the reactor to a value comprised between 50⁰C and 60⁰C, favouring the completion of the reaction between the anthophyllite (and cement matrix) silicates and the fluosilicic acid and the reaction between the fluosilicates and the aluminates with sulphuric acid. This process step continued for about 30 min. The reaction contents were continuously mixed by the above double diaphragm pump. [00142] The gaseous reaction products (silicon tetrafluoride and hydrogen fluoride) were continuously discharged from the reactor through an air current. The gaseous mixture in output from the reactor was then absorbed in water in a maze-like spray chamber, wherein the silicon tetrafluoride and the hydrogen fluoride reacted to form fluosilicic acid again. The pressure inside the rector was kept at a value of about 1.5 bar, recirculating a flow rate equal to about 4000 litres/hour. The regeneration continued up to obtaining a density of the solution equal to 1.3 gr/cm³. [00143] 13.4 kg of fluosilicic acid were recovered at the end of the process .

[00144] After about 30 minutes from the addition of sulphuric acid and water, lime was introduced in the reactor to neutralise the residual acidity up to a value of pH=7 (controlled by a pH sensor) . [00145] The reactor was then emptied. The contents consisted of an aqueous solution of magnesium sulphate and a solid phase essentially consisting of silica, calcium sulphate, aluminium trisulphate and ferric sulphate. The X-ray diffractometry analysis of samples of the contents excluded the presence of anthophyllite.

Example 5

[00146] In confined vacuum environment, 10 kg of cement mix consisting of blue asbestos (crocidolite) by about 35% by weight and cement matrix by the remaining 65%, were subject to grinding. The grinding was carried out in multiple stages up to bringing the material to a mean grain size of about 3 mm.

[00147] The material thus ground was then loaded in a batch reactor, wherein an aqueous solution of fluosilicic acid (H₂SiFe) had been previously introduced, with a concentration equal to 30% by weight, for a quantity of acid equal to about 13.5 kg (3.7 kg for asbestos and 9.8 kg for the cement matrix) . [00148] The material was left to react with the aqueous solution of fluosilicic acid for about 22 minutes in the presence of oxygen supplied by continuous blow in of an air current in the reactor. The reaction contents were continuously mixed by a double diaphragm pump. [00149] After that, 98% sulphuric acid was introduced in the reactor for an overall acid quantity equal to 14.6 kg (4.7 kg for the fluosilicates from the asbestos and 9.9 kg for the fluosilicates and the aluminates from the cement matrix) . After that, water was introduced in the reactor for a quantity of 2.2 kg. [00150] The mixing between sulphuric acid and water rose the temperature inside the reactor to a value comprised between 50⁰C and 60⁰C, favouring the completion of the reaction between the crocidolite and cement matrix silicates and the fluosilicic acid and the reaction between the sulphuric acid and the fluosilicates (sodium, iron, and magnesium and calcium) and aluminates. This process step continued for about 45 min. The reaction contents were continuously mixed by the above double diaphragm pump. [00151] The gaseous reaction products (silicon tetrafluoride, hydrogen fluoride and molecular hydrogen) were continuously discharged from the reactor through the above air current. The gaseous mixture in output from the reactor was then absorbed in water in a maze-like spray chamber, wherein the silicon tetrafluoride and the hydrogen fluoride reacted with each other to form fluosilicic acid again. The pressure inside the rector was kept at a value of about 1.5 bar, recirculating a flow rate equal to about 4000 litres/hour. The regeneration continued up to obtaining a density of the solution equal to 1.3 gr/cm³. About 12.2 kg of fluosilicic acid were recovered at the end of the process. [00152] After about 45 minutes from the addition of sulphuric acid and water, lime was introduced in the reactor to neutralise the residual acidity up to a value of pH=7 (controlled by a pH sensor) .

[00153] The reactor was emptied. The contents were filtered by an Oliver filter producing an aqueous phase containing magnesium sulphate and a solid phase containing silica, sodium sulphate, ferrous sulphate, calcium sulphate, aluminium trisulphate and ferric sulphate. The X-ray diffractometry analysis of samples of the solid phase and of the aqueous phase excluded the presence of crocidolite. Example 6

[00154] In confined vacuum environment, 10 kg of cement mix consisting of tremolite by about 38% by weight and cement matrix by the remaining 62%, were subject to grinding. The grinding was carried out in multiple stages up to bringing the material to a mean grain size of about 8 mm.

[00155] The material thus ground was then loaded in a batch reactor, wherein an aqueous solution of fluosilicic acid (H₂SiF₆) had been previously introduced, with a concentration equal to 30% by weight, for a quantity of acid equal to about 14.6 kg (5.3 kg for asbestos and 9.3 kg for the cement matrix) .

[00156] The material was left to react with the aqueous solution of fluosilicic acid for about 30 minutes. The temperature of the reactor was substantially ambient. The reaction contents were continuously mixed by a double diaphragm pump .

[00157] After that, 98% sulphuric acid was introduced in the reactor for an overall acid quantity equal to 13.1 kg (3.6 kg for the fluosilicates from the asbestos and 9.5 kg for the fluosilicates and the aluminates from the cement matrix) .

[00158] After that, water was introduced in the reactor for a quantity of 2 kg. [00159] The mixing between sulphuric acid and water rose the temperature inside the reactor to a value comprised between 50⁰C and 60⁰C, favouring the completion of the reaction between the tremolite and cement matrix silicates and the fluosilicic acid and the reaction between the sulphuric acid and the fluosilicates (calcium and magnesium) and aluminates. This process step continued for about 60 min. The reaction contents were continuously mixed by the above double diaphragm pump. [00160] The gaseous reaction products (silicon tetrafluoride and hydrogen fluoride) were continuously discharged from the reactor through an air current. The gaseous mixture in output from the reactor was then absorbed in water in a maze-like spray chamber, wherein the silicon tetrafluoride and the hydrogen fluoride reacted with each other to form fluosilicic acid again. The pressure inside the rector was kept at a value of about 1.5 bar, recirculating a flow rate equal to about 4000 litres/hour. The regeneration continued up to obtaining a density of the solution equal to 1.3 gr/cm³. About 13.15 kg of fluosilicic acid were recovered at the end of the process.

[00161] After about 60 minutes from the addition of sulphuric acid and water, lime was introduced in the reactor to neutralise the residual acidity up to a value of pH=7 (controlled by a pH sensor) .

[00162] The reactor was then emptied. The contents were filtered by an Oliver filter producing an aqueous phase containing magnesium sulphate and a solid phase containing silica, hydrated calcium sulphate, calcium sulphate, aluminium trisulphate and ferric sulphate. The X-ray diffractometry analysis of samples of the solid phase and of the aqueous phase excluded the presence of tremolite.

* * * [00163] In accordance with an alternative embodiment of the invention, the fluosilicic acid to be used in the asbestos decomposition step is obtained by reacting the silicon tetrafluoride and the hydrogen fluoride generated by the decomposition with acids of a fluosilicic acid salt, according to the reactions shown hereinafter, where MF₆Si indicates a metal fluosilicate: MF₆Si+H₂SO₄ ~Jh£→MSO₄ +SiF₄(g)+2HF(g) SiF₄(g)+2HF(g)-Z2→H₂F₆Si(l) [00164] In this case the first reactor is initially used as reaction environment for the metal fluosilicate decomposition.

* * *

[00165] In accordance with another alternative embodiment of the invention, the fluosilicic acid to be used in the asbestos decomposition step is obtained by- reacting the silicon tetrafluoride and the hydrogen fluoride generated by the decomposition with fluosilicate acids resulting from the decomposition of asbestos with hydrofluoric acid, according to the reactions shown hereinafter:

42HF+Ca₂Mg₅Si₈O₂₂(OH)₂ -»2CaF₆Si+5MgF₆Si+SiO₂ +22H₂O

2CaF₆Si+5MgF₆Si+7H₂SO₄ ^⁰ > 2CaSO₄2H₂O(s)+5MgSO₄(I)+JSiF₄(g)+UHF(g)

ISiF₄(g)+UHF(g)-^£→IH₂F₆Si(I)

[00166] The example shows the case where asbestos is of the tremolite variety, but it may be extended to the other varieties of asbestos as well .

[00167] In this case, the process envisages a first cycle wherein the asbestos decomposition step is carried out using hydrofluoric acid instead of fluosilicic acid. * * *

[00168] Plant 1 for disposal of asbestos or materials containing asbestos according to the invention comprises at least a first reactor 10 inside which the decomposition of asbestos or materials containing asbestos is intended to take place by the attack with fluosilicic acid. Also the decomposition of the fluosilicates into silicon tetrafluoride and hydrogen fluoride is intended to take place in this first reactor 10 by the attack with an acid or a mixture of acids. [00169] The plant further comprises at least a second reactor 20 inside which the gaseous reaction products resulting from the first reactor 10 are intended to be conveyed. In particular, silicon tetrafluoride and hydrogen fluoride are intended to react inside such second reactor for forming fluosilicic acid.

[00170] Advantageously, as can be seen in Figures 2a and 2b the plant is provided with an asbestos unpacking station 30. Preferably, the unpacking is carried out with a robotised system, but it may be carried out manually. [00171] Preferably, the above unpacking station 30 is located in a confined vacuum environment, to prevent the dispersion of asbestos microfibres to the external environment . [00172] Advantageously, plant 1 comprises a grinding station 40 of the asbestos or materials containing asbestos that must be disposed of. Station 40 comprises a multiple stage grinding equipment suitable for breaking up the asbestos or the materials containing asbestos into particles having a mean grain size not exceeding 1 cm. For example, equipment that may be used is a roller grind. [00173] Advantageously, the grinding station is located in a confined vacuum environment.

[00174] Plant 1 further comprises liquid-solid separation means 50 suitable for separating the contents of the first reactor in liquid phase L and in a solid phase S, so as to carry out a separate disposal or reuse the residues .

[00175] Tape filters or Oliver filters may for example be used as separation means. [00176] In accordance with a preferred embodiment of the invention, illustrated in Figure 2a, the plant is provided with a first reactor 10, of batch type. [00177] As mentioned hereinbefore, preferably reactor 10 is provided with at least one double diaphragm pump 11 for mixing the contents thereof.

[00178] More in detail, the first reactor is connected to the grinding station through a first duct 101, inside which it is possible to seat for example a screw conveyor. In this way it is possible to reduce the dispersion of asbestos particles in the reactor charging step.

[00179] The first reactor is hydraulically connected by a second duct 102 to a tank (not shown) for storing fresh fluosilicic acid. This second duct 102 is connected - in an intermediate portion - to the second reactor 20, for allowing the recirculation of the fluosilicic acid regenerated from the previous treatment cycle. [00180] The first reactor is hydraulically connected through a third duct 104 to another tank (not shown) for the storage of acids, for example sulphuric acid or phosphoric acid, and through a fourth duct 104 to the water mains for water supply. Moreover, a fifth duct 105 is provided for loading a basic product (such as lime) in the reactor. [00181] The reactor is further connected to a fan or a compressor through a sixth duct 106 for allowing the blow in of an air current under pressure therein. [00182] The first reactor 10 is connected at the top with the second reactor 20 through a seventh duct 107 for allowing the discharge of the gaseous reaction products and their conveyance into the second reactor 20, and at the bottom, through an eighth duct 108, to the liquid - solid separation means 50.

[00183] Advantageously, the first reactor 10 is provided with pH detecting means so as to calculate the amount of basic product (for example lime) to be introduced in the reactor for neutralising the end products inside the first reactor before treating them in the separation means 50. [00184] Advantageously, as can be seen in Figure 2a, the second reactor 20 consists of a maze-like water spray chamber, inside which the gaseous flow from the first reactor 10 is conducted.

[00185] In order to favour the combining reaction between SiF₄ and HF inside the second reactor, an overpressure comprised preferably between 1.2 and 2 bar is maintained.

[00186] Operatively, the aqueous solution of H₂SiF₆ that generates inside the second reactor is collected at the bottom and is at least partly recirculated to the spray nozzles (located on top of the chamber) , optionally integrated with a pure water flow. The flow rate of sprayed water is substantially calculated on the basis of the amount of asbestos treated in the first reactor, that is, on the basis of the amount of SiF₄ and HF to be treated.

[00187] Preferably, both the first reactor 10 and the second reactor 20 are located in confined vacuum environments . [00188] In accordance with this preferred embodiment, the disposal plant is carried out in a discontinuous manner, at least with reference to the activity of the first reactor 10.

[00189] The unpacking and grinding stations may operate continuously, separately from the first reactor, but in this case envisaging an accumulation of the ground material .

[00190] Preferably, in order to prevent the need of having sealed storage containers available, also the unpacking and grinding stations are carried out discontinuously so as to process only the amounts intended to be treated in the first reactor 10. [00191] In accordance with an alternative embodiment of the invention, illustrated in Figure 2b, plant 1 is provided with at least 2 first reactors 10' and 10", connected in parallel to each other.

[00192] Compared to the plant diagram illustrated in Figure 2a, the different ducts 101 - 108 are suitably branched to serve both reactors . [00193] Operatively, the two reactors 10' and 10" operate in a coordinated manner so as to reduce idle operating times in upstream and downstream stations . [00194] It is possible to envisage plants provided with a larger number of reactors 10, so as to approach a continuous operation, further reducing the idle processing times and exploiting the potential of the separation means and of the unpacking and grinding stations.

[00195] As can be understood from the above examples, the process for disposing of asbestos according to the invention allows having complete decomposition of the asbestos treated within a few hours while limiting the consumption of reagents used for the decomposition of asbestos. The step of regeneration of the fluosilicic acid in fact allows recovering between 90% and 95% of the fluosilicic acid used, moving the consumption to reagents like sulphuric acid or phosphoric acid that have a significantly lower market price.

[00196] The process for disposing of asbestos according to the invention allows obtaining end residues deliverable without any further treatment to dumps for non-toxic (inert) waste or waste reusable in the industrial field.

[00197] This in the first place ensures high environmental sustainability of the process as no hazardous products to be disposed of are output . Secondly, it ensures economic sustainability of the process thanks to the possibility of reselling the treatment residues as industrially reusable materials. [00198] The invention thus conceived thus achieves the intended purposes.

[00199] Of course, in the practical embodiment thereof, it may take shapes and configurations differing from that illustrated above without departing from the present scope of protection. [00200] Moreover, all the parts may be replaced by technically equivalent ones and the sizes, shapes and materials used may be whatever according to the requirements .

*** * ***

Claims

1. Process for disposal of asbestos or materials containing asbestos suitable for decomposing the asbestos in non-hazardous materials, comprising the following steps: decomposing asbestos or the materials containing asbestos with fluosilicic acid obtaining fluosilicates;

- decomposing said fluosilicates with at least one acid or mixture of acids having such force as to decompose said fluosilicates into silicon tetrafluoride and hydrogen fluoride;

- regenerating the fluosilicic acid reacting the silicon tetrafluoride and the hydrogen fluoride with each other; and - disposing compounds obtained at the end of the above two decomposition steps.

2. Process according to claim I₇ wherein the asbestos or the materials containing asbestos are subject to a grinding step before being subject to said decomposition step with fluosilicic acid.

3. Process according to claim 2, wherein said grinding step is continued up to obtaining particles with a mean grain size not exceeding 1 cm.

4. Process according to claim 2 or 3 , wherein said grinding step is continued up to obtaining particles with a mean grain size comprised in the range from 1 mm to between 0.5 mm.

5. Process according to any one of the previous claims , wherein in said grinding step the asbestos is ground up to obtaining fibres with a length not exceeding 20 mm.

6. Process according to any one of claims 2 to 5 , wherein said grinding step is carried out in confined vacuum environment .

7. Process according to any one of the previous claims, wherein said asbestos decomposition step is carried out in at least a first reactor.

8. Process according to any one of the previous claims, wherein said fluosilicate decomposition step is carried out in said first reactor.

9. Process according to claim 7 or 8 , wherein said first reactor is a batch reactor.

10. Process according to any one of the previous claims, comprising a first step of loading said first reactor wherein asbestos and/or materials containing asbestos and fluosilicic acid are introduced.

11. Process according to the previous claim, wherein said asbestos decomposition step is carried out for a first reaction period (tl) after said first loading step.

12. Process according to any one of the previous claims, comprising a second loading step of said first reactor, wherein said acid or said mixture of acids is introduced for the decomposition of fluosilicates, said second loading step being carried out at the end of said first reaction period (tl) .

13. Process according to any one of the previous claims, wherein said fluosilicate decomposition step is carried out for a second reaction period (t2) subsequent to said first reaction period (tl) .

14. Process according to the previous claim, wherein said asbestos decomposition step also continues during said second reaction period (t2) at the same time as said fluosilicate decomposition step.

15. Process according to any one of the previous claims, wherein said first reaction period (tl) has a duration comprised between 15 min and 35 min.

16. Process according to any one of the previous claims, wherein said first reaction period (tl) has a duration comprised between 15 min and 20 min if the grain size of the particles is comprised in the range between 0.5 mm and 1 mm.

17. Process according to any one of the previous claims, wherein said first reaction period (tl) has a duration comprised between 20 min and 25 min if the grain size of the particles is comprised in the range between 1 mm and 5 mm.

18. Process according to any one of the previous claims, wherein said first reaction period (tl) has a duration comprised between 25 min and 35 min if the grain size of the particles is comprised in the range between 5 mm and 1 cm.

19. Process according to any one of the previous claims, wherein the asbestos decomposition is carried out at pH values not exceeding 5.

20. Process according to any one of the previous claims, wherein said second reaction period (t2) has a duration comprised in the range between 30 min and 70 min.

21. Process according to any one of the previous claims, wherein said second reaction period (t2) has a duration comprised between 30 min and 40 min if the initial grain size of the particles is comprised in the range between 0.5 mm and 1 mm.

22. Process according to any one of the previous claims, wherein said second reaction period (t2) has a duration comprised between 40 min and 50 min if the initial grain size of the particles is comprised in the range between 1 mm and 5 mm.

23. Process according to any one of the previous claims, wherein said second reaction period (t2) has a duration comprised between 50 min and 70 min if the initial grain size of the particles is comprised in the range between 5 mm and 1 cm.

24. Process according to any one of the previous claims, wherein the yield of the asbestos decomposition reaction is substantially equal to 100%.

25. Process according to any one of the previous claims, wherein the yield of the fluosilicate decomposition reaction is comprised between 90% and 95%.

26. Process according to any one of the previous claims, wherein during said asbestos decomposition step an air current is blown into said first reactor.

27. Process according to any one of the previous claims, wherein during said fluosilicate decomposition step an air current is blown into said first reactor.

28. Process according to any one of the previous claims, wherein at least during said fluosilicate decomposition step the gaseous reaction products are discharged from said first reactor.

29. Process according to the previous claim, wherein the discharge of gaseous products is carried out by blowing in a current of air under pressure.

30. Process according to any one of the previous claims, wherein in said first loading step the fluosilicic acid is introduced in said first reactor as aqueous solution with a concentration by weight comprised between 10% and 40%.

31. Process according to any one of the previous claims, wherein in said first loading step the fluosilicic acid is introduced in said first reactor as aqueous solution with a concentration by weight equal to about 30%.

32. Process according to any one of the previous claims, wherein the fluosilicic acid is introduced in said first reactor with an excess comprised between 5% and 15% as compared to the stoichiometric amount for the decomposition of the silicates present in the material loaded in said first reactor.

33. Process according to any one of the previous claims, wherein the fluosilicic acid is introduced in said first reactor with an excess substantially of 10% as compared to the stoichiometric amount for the decomposition of the silicates present in the material loaded in said first reactor .

34. Process according to any one of the previous claims, wherein in said fluosilicate decomposition step sulphuric acid is used as acid.

35. Process according to the previous claim, wherein in said second loading step the sulphuric acid is loaded in said first reactor as aqueous solution with a concentration by weight comprised between 30% and 98%.

36. Process according to any one of the previous claims, wherein the sulphuric acid is introduced in said first reactor with an excess comprised between 5% and 15% as compared to the stoichiometric amount for the decomposition of the fluosilicates present in the material loaded in said first reactor.

37. Process according to any one of the previous claims, wherein the sulphuric acid is introduced in said first reactor with an excess substantially of 10% as compared to the stoichiometric amount for the decomposition of the fluosilicates present in the material loaded in said first reactor.

38. Process according to the two previous claims, wherein said second loading step envisages in a sequence the introduction of the sulphuric acid in said first reactor and then the introduction of dilution water.

39. Process according to the previous claim, wherein the dilution water is introduced in said first reactor in a quantity by weight comprised between 10% and 20% of the quantity of sulphuric acid loaded.

40. Process according to the previous claim, wherein the heat developed by the mixing of sulphuric acid with water leads to an increase of the temperature inside the first reactor.

41. Process according to any one of the previous claims, wherein in said fluosilicate decomposition step phosphoric acid is used as acid.

42. Process according to any one of the previous claim, wherein at least in a first initial cycle of said process, the fluosilicic acid to be used in said asbestos decomposition step is obtained by reacting the silicon tetrafluoride and the hydrogen fluoride generated by the decomposition with acids of a fluosilicic acid salt.

43. Process according to any one of claim 1 to 39, wherein at least in a first initial cycle of said process, the fluosilicic acid to be used in said asbestos decomposition step is obtained by reacting the silicon tetrafluoride and the hydrogen fluoride generated by the decomposition with fluosilicate acids resulting from the asbestos decomposition with hydrofluoric acid.

44. Process according to any one of the previous claims, wherein said first reactor is located in a confined vacuum environment .

45. Process according to any one of the previous claims, wherein said regeneration step is carried out in a second reactor .

46. Process according to any one of the previous claims, wherein said second reactor is located in a confined vacuum environment.

47. Process according to any one of the previous claims, wherein said second reactor consists of a maze-like water spray chamber.

48. Process according to any one of the previous claims, wherein said second reactor receives the gaseous products of the decomposition of fluosilicates discharged from said first reactor.

49. Process according to any one of the previous claims, wherein inside said second reactor an overpressure comprised between 1.2 bar and 2 bar is maintained for favouring the combination between silicon tetrafluoride and hydrogen fluoride .

50. Process according to any one of the previous claims, wherein with said regeneration step, between 90% and 95% of the hexafluosilicic acid loaded in said first reactor is recovered.

51. Process according to any one of the previous claims, comprising a step of neutralising the acidity present into said first reactor to be carried out after said second reaction period (t2) by the addition of a predetermined amount of a basic substance.

52. Process according to any one of the previous claims, comprising a step of liquid-solid separation of the reaction products generated in said decomposition steps.

53. Process according to the previous claim, wherein said separation step is carried out after said neutralisation step.

54. Process according to any one of the previous claims, wherein said step of disposing of the compounds generated from the decomposition steps is carried out after said separation step .

55. Process according to any one of the previous claims, wherein the solid phase obtained from said liquid-solid separation is substantially inert and may be delivered to dump for non-hazardous waste and/or be reused in the production of building materials.

56. Process according to any one of the previous claims, wherein the liquid phase obtained from said liquid - solid separation is a magnesium phosphate or sulphate solution and may be reused as reagent in the chemical industry.

57. Process according to any one of the previous claims, comprising a step of unpacking the asbestos or the materials containing asbestos.

58. Process according to the previous claim, wherein said unpacking step is automated.

59. Process according to any one of the previous claims, wherein said unpacking step is carried out in a confined vacuum environment.

60. Plant for disposing of asbestos or materials containing asbestos, comprising:

- at least a first reactor inside which the decomposition of the asbestos or of said materials containing asbestos into fluosilicates is intended to take place by attack with fluosilicic acid, as well as the decomposition of said fluosilicates into silicon tetrafluoride and hydrogen fluoride by attack with an acid or a mixture of acids ; and at least a second reactor inside which the gaseous reaction products resulting from said first reactor are intended to be conveyed and the silicon tetrafluoride and hydrogen fluoride are intended to react with each other for forming fluosilicic acid.

61. Plant according to claim 60, wherein said first reactor is a batch reactor.

62. Plant according to any one of the previous claims, wherein said first reactor is provided with at least one double diaphragm pump suitable for mixing the contents of said first reactor.

63. Plant according to any one of claims 60 to 62, wherein said first reactor is located in a confined vacuum environment.

64. Plant according to any one of the previous claims , wherein said first reactor is connected in parallel to one or more similar reactors inside which said decomposition steps are intended to take place in a coordinated manner.

65. Plant according to any one of claims 60 to 64, wherein said second reactor consists of a maze-like water spray chamber.

66. Plant according to any one of the previous claims, wherein said second reactor is located in a confined vacuum environment.

67. Plant according to any one of the previous claims, wherein said second reactor is connected to said one or more first reactors connected to each other in parallel .

68. Plant according to any one of the previous claims, comprising a multiple stage grinding system for grinding said asbestos or said materials containing asbestos.

69. Plant according to any one of the previous claims, comprising a multiple stage grinding system suitable for breaking up said asbestos or said materials containing asbestos into particles having a mean grain size not exceeding 1 cm.

70. Plant according to any one of the previous claims, wherein said grinding system is located in a confined vacuum environment .

71. Plant according to any one of the previous claims, comprising liquid-solid separation means suitable for separating the contents of said first reactor in a liquid phase and a solid phase.

72. Plant according to any one of the previous claims, comprising a system for unpacking loads of asbestos or materials containing asbestos intended to be disposed of in said plant.

73. Plant according to any one of the previous claims, wherein said unpacking system is located in a confined vacuum environment.