Process for treating asbestos

Abstract

The present invention relates to a process for treating a material containing asbestos, which makes it possible to transform the asbestos into amorphous silica fibres or into a fibrosity-free mineral material. The process comprises a step of pretreating asbestos-based material in which the material is ground, followed by a hydro-thermal denaturation step, which provides for an addition of an acid and a heating to obtain the amorphous silica, or followed by a thermal denaturation step to obtain a mineral material. Optionally, the amorphous silica fibres may be further treated with a post-treatment which may be of chemical, physical and / or hydrothermal type. The products deriving from the process of the invention can be valorised as secondary raw materials without any residual hazard.

Description

[0001] “Process for treating asbestos”

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a process for treating a material containing asbestos, which allows the asbestos to be transformed into amorphous fibrous silica or into a mineral fibrosity-free material. The products deriving from the process of the invention can be valorised as secondary raw materials without any residual hazard.

[0004] STATE OF THE ART

[0005] Asbestos is the trade name given to some natural minerals with a fibrous structure, belonging to the class of the silicates. In modern times, some of these minerals have been widely used for their excellent technological properties: they have good resistance to heat and fire, to the action of chemical and biological agents, abrasion and wear, they have a high mechanical strength, good flexibility, they easily bond with construction materials and have good sound-absorbing and heat-insulating properties. For all these properties and the low cost, asbestos has been widely used in artifacts and industrial applications, constructions, in means of transport and in the home. In particular, the raw fibre was processed to obtain various products adaptable to multiple uses. In these products the asbestos fibres can be both free and strongly or weakly bound. In case the fibres are weakly bonded, we are talking about brittle materials, i.e. which can be crumbled with manual action alone due to poor internal cohesion. In case, on the other hand, the fibres are strongly bonded, we are talking about compact materials, which can only be crumbled into powder through the aid of mechanical machinery. Brittle matrix materials are certainly the most hazardous, as the fibres can be dispersed in the air with extreme ease and can therefore be inhaled. Compact matrix asbestos, on the other hand, by its nature, does not tend to release fibres and a hazardous situation exists only if it is abraded, deteriorated or sawn.

[0006] There are a very large number of types of asbestos-containing materials (ACM) that have extremely varied and differentiated characteristics and uses. The U.S. Federal Register lists over 3,000 finished objects containing asbestos. ACMs can be classified into three categories: (a) surface materials: these include ACMs sprayed or distributed by spatulation on surfaces (load-bearing elements, walls, ceilings) for sound-absorbing, heatinsulating and decorative purposes;

[0007] (b) thermal insulation materials: these include ACMs used to prevent the formation of condensation in pipes, ducts, boilers, tanks and in various components of water cooling systems, but also in heating, ventilation and air conditioning systems; c) miscellaneous materials: all other ACMs, such as false ceilings, sheaths, fabrics, etc., are grouped into this category.

[0008] The most widespread use of asbestos concerns the construction sector, in particular in the form of asbestos composite with cement, the so-called asbestos-cement. In addition, in order to avoid or limit damage to structures in the event of fire, asbestos has been widely used as a coating through spraying and spatulation techniques on beams or floors. The heat-resistant mixture was composed of varying percentages of asbestos and other materials (vermiculite, sand or cellulose fibres) and binding materials (gypsum and / or calcium carbonate): the result was a continuous and soft- touch layer with a colour varying from dark grey to white. Asbestos minerals were used as additives in cementitious conglomerates to improve the mechanical characteristics thereof. They were usually: Portland cement, water, inerts, chrysolite fibres, crocidolite and / or amosite, asbestos in amounts varying up to 50% by weight of the composite.

[0009] Today asbestos is universally recognized to be one of the materials having the highest hazard to human health among those present in living and working environments, which hazard results in very serious diseases mainly affecting the respiratory system. Although an aetiological link had been hypothesized since the beginning of the last century between the inhalation of airborne asbestos fibres and the onset of specific diseases, only in the 1990s was legislation introduced in the various countries consistent with the hazard of the material.

[0010] The assessment of the damage that asbestos caused to workers has forced the governments of all nations to ban the use of asbestos and to provide precise protocols for the replacement and disposal of asbestos already used.

[0011] Currently, the disposal of the materials containing asbestos is performed by landfilling. However, the accumulation of asbestos-containing waste (ACW) in landfills does not solve the problem, but rather passes it on to future generations: the need is therefore strongly felt for new processes for the inertisation of asbestoscontaining materials allowing the transformation of hazardous asbestos fibres into inert materials that can potentially be used as non-hazardous secondary raw materials and therefore can be employed for other uses.

[0012] The processes allowing the inertisation of the ACWs currently available are processes for conditioning the asbestos-containing material in matrices of different nature that avoid the dispersion of asbestos fibres in the environment, or they are “transformation” processes intervening directly on the fibrous structure of the mineral itself, transforming it into other phases that are not hazardous to human health. The main ACW transformation processes are based on acidic chemical treatments, heat treatments, mechanical-chemical treatments and biochemical and microbiological methods.

[0013] With regard to the acid treatments, several methodologies have been developed involving the use of acids, both organic and mineral, for the transformation of ACWs, obtaining secondary materials that are recyclable and often reusable in the ceramics industry. In particular, the effects of mineral acids, such as the hydrofluoric, hydrochloric and sulphuric acids, and of organic acids, such as formic and oxalic acid, have been studied.

[0014] With regard to the heat treatments, it is known that asbestos materials are unstable at high temperatures. Chrysotile, for example, has a tendency to start losing hydroxyl groups at 500-600 °C and to be transformed into a different inert mineral phase, forsterite, which recrystallizes at 820 °C. The application of this principle allows to obtain inert materials from ACW, as such or ground, treated in furnaces at a temperature of 800-950 °C. In addition, if firing precedes material compaction, the consequent disorientation of the crystals allows the final product to be used as electrical insulator or refractory material. This process is called ceram ification. It is also possible to operate a vitrification of the ACWs through various processes that are based on melting asbestos-containing waste, conducted with the addition of different additives in a wide temperature range (1300-1800 °C), followed by rapid cooling with the production of inert material with a glassy amorphous structure. However, this solving method requires a lot of energy in order to bring the melting furnaces to extremely high and constant temperatures. In glass-ceramification, on the other hand, the waste is melted at temperatures comprised between 1300 and 1400 °C together with particular additives, such as blast furnace slag or industrial sludge, forming a mixture with a high metal content. The slag that is obtained is crystallized at a controlled temperature: in this way, products with very high mechanical resistance are obtained and are particularly suitable as coating and protection surfaces in the construction, mechanical and chemical industry.

[0015] Another technique consists of the so-called lithification, which is based on melting ACWs, resulting from the de-insulation of railway carriages, at a temperature of 1300-1400 °C. Slow cooling results in the crystallisation of pyroxenes, olivine and iron oxides. The final result of the treatment is the production of inert materials, which can be recovered for various applications.

[0016] With regard to the biological treatments, the microbiological action of mosses and lichens on various rock substrates containing asbestos fibres has been studied both in vivo and in vitro: lichen and fungal hyphae are able to penetrate and secrete chemical compounds (oxalic acid is one of the primary metabolites), some of which can alter the mineralogical structure of the asbestos fibres (see for example the article by S.E. Favero-Longo, M. Girlanda, R. Honegger, B. Fubini, R. Piervittori; Mycological Research, Vol. 111 , Issue 4, pages 473-481 (2007)).

[0017] Microbiological methodologies have also been developed for the transformation of asbestos through the use of bacteria, in particular Lactobacillus casei and Lactobacillus plantarum (see for example the article by I. A. Stanik, K. Cedzyhska, S. Zakowska; Fresenius Environmental Bulletin, Vol. 15, Issue 7, pages 640-643 (2006)).

[0018] Unfortunately, the methods for transforming asbestos-containing materials (ACMs) known to date have non-negligible disadvantages. In particular, the acid treatment leads to the accumulation of a large amount of waste products, also to be disposed of. In addition, it should be borne in mind that, for the treatment of millions of tons of ACWs (the approximate estimate only for the Italian territory exceeds 30 million tons, while in Europe it is around 250 million tons still to date to be disposed of), it would be necessary to use huge quantities of reagents, which would entail non- negligible environmental impacts and very high costs. With regard to the heat treatment, the disadvantage is due to the enormous energy demand in order to bring the furnaces to very high and constant temperatures.

[0019] Landfilling for the disposal of the ACWs also has serious disadvantages: in fact, it is necessary to transport the ACWs over long distances, with consequent environmental risks and logistical costs, as the landfills in which to bury the ACWs are increasingly distant. In addition, the creation of landfills increases soil consumption, i.e. the loss of originally agricultural, natural or semi-natural surface in favour of artificial land covering.

[0020] In addition, there are very expensive possibilities for the treatment of the ACWs, such as plasma torch, used in France: such use is however extremely limited and expensive.

[0021] Therefore, there are no asbestos elimination treatments on the market that are not excessively expensive and lead to final products that can be used as secondary raw materials. The Applicant has therefore posed the problem of studying and developing a process for treating asbestos-containing materials that overcomes the inconveniences deriving from the use of large amounts of reagents, the accumulation of a large amount of waste products following acid treatment and the use of an excessive amount of energy and in which it is possible to use waste compounds coming from other processes and reuse the products obtained from the process itself.

[0022] DEFINITIONS

[0023] Unless otherwise defined, all the terms of the art, notations and other scientific terms used herein are intended to have the meanings commonly understood by those skilled in the art to which this description pertains. In some cases, terms with commonly understood meanings are defined herein for the sake of clarity and / or for ease of reference; the inclusion of such definitions in the present description should thus not be interpreted as representing a substantial difference from what is generally understood in the art.

[0024] The terms “comprising”, “having”, “including” and “containing” are to be understood as open-ended terms (i.e. the meaning of “comprising, but not limited to”) and are to be considered as a support also for terms like “consist essentially of”, “consisting essentially of”, “consist of” or “consisting of”. For all the ranges specified in the text, in the figures and in the claims of the present patent application, it is understood that the endpoints of these ranges are included. The terms “obtainable”, “obtained”, “obtainable directly from”, and “obtained directly from” are considered equivalent.

[0025] SUMMARY OF THE INVENTION

[0026] The process of the invention comprises an initial pre-treatment by at least one grinding, either dry or in aqueous phase, preferably dry, of a material comprising asbestos fibres and a matrix, to obtain a powder or an aqueous suspension, comprising ground asbestos fibres and a ground matrix.

[0027] The grinding is followed by a separation of the asbestos fibres from the matrix, performed by electrostatic separation when the grinding is performed dry, thus obtaining dry asbestos fibres, or by froth-flotation when the grinding is performed in the aqueous phase, thus obtaining a suspension of asbestos fibres.

[0028] If the grinding is carried out dry, the subsequent separation will allow to obtain dry ground asbestos fibres separated from the matrix; while if the grinding is carried out in the aqueous phase, the subsequent separation leads to a suspension of asbestos fibres and to a waste sludge containing the ground matrix.

[0029] The obtained suspension of asbestos fibres can be subjected to filtration and drying, thereby obtaining dry asbestos fibres.

[0030] After separation, the process of the invention comprises subjecting the suspension of asbestos fibres or the dry asbestos fibres after suspension in water, obtained in the separation step, to hydrothermal denaturation performed by the addition of an acidic aqueous solution and heating, thus obtaining an amorphous fibrous siliceous material; or it comprises subjecting the dry asbestos fibres obtained in the separation step and possible filtration and drying, to thermal denaturation, obtaining a mineral material.

[0031] Thermal denaturation is carried out on ground asbestos fibres. When grinding is performed dry, denaturation can be carried out directly on the ground asbestos fibres, separated from the matrix.

[0032] Instead, in the case of grinding in aqueous phase, the asbestos fibres to be denatured must be filtered from the aqueous suspension of asbestos fibres obtained after separation of the matrix and must be dried before carrying out thermal denaturation.

[0033] According to a preferred aspect, the process of the invention may further comprise a post-treatment of the amorphous silica fibres to eliminate any potential hazards and allow a reuse thereof as a secondary raw material without specific restrictions or provisions. The post-treatment of amorphous silica fibres can be of chemical, physical and / or hydrothermal type.

[0034] Object of the invention is also the mineral material obtainable by the process of the invention, as well as object of the invention is the use thereof as a secondary raw material. According to a preferred aspect, the material obtainable by the process of the invention is forsterite.

[0035] BRIEF DESCRIPTION OF THE FIGURES

[0036] Figure 1 shows the dimensional analysis of the crushed material in step a) of Example 1 , showing an average particle size comprised between 2 pm (SMD) and 9 pm (VMD).

[0037] DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention relates to a method for treating a material comprising asbestos fibres and a matrix, comprising the steps of: a) pre-treating the material comprising asbestos fibres and a matrix by at least one grinding, either dry or in aqueous phase, obtaining a powder or an aqueous suspension, comprising ground asbestos fibres and a ground matrix; b) separating the asbestos fibres from the matrix by electrostatic separation when the separation of step a) is performed dry, thus obtaining dry asbestos fibres, or by froth-flotation when the grinding of step a) is performed in the aqueous phase, thus obtaining a suspension of asbestos fibres; b’) optionally subjecting the suspension of asbestos fibres obtained in step b) to filtration and drying obtaining dry asbestos fibres; c) subjecting the suspension of asbestos fibres or the dry asbestos fibres after suspension in water, obtained in step b), to hydrothermal denaturation performed by the addition of an acidic aqueous solution and heating, thus obtaining an amorphous fibrous siliceous material, or c’) subjecting the dry asbestos fibres obtained in step b) and b’) to thermal denaturation, obtaining a mineral material.

[0039] According to a preferred aspect, the at least one grinding of the asbestos fibres and a matrix of step a) is carried out dry.

[0040] According to another preferred aspect, the mineral material obtained at the end of step c’) is forsterite.

[0041] In one embodiment, the method of the invention provides for a further step d) of post-treating the amorphous fibrous siliceous material, allowing to obtain a fibrosity- free siliceous material. The post-treatment can be carried out by a chemical, physical and / or hydrothermal treatment and leads to an amorphous siliceous material substantially free of fibres.

[0042] The starting material comprising asbestos fibres and a matrix is selected between a material comprising asbestos fibres dispersed in a brittle matrix (i.e. in a matrix different from the cementitious matrix) or a material comprising asbestos fibres dispersed in a cementitious matrix.

[0043] In particular, the material containing asbestos fibres may be:

[0044] • a surface covering panel having sound-absorbing, heat-insulating, flameretardant and / or decorative properties comprising asbestos in admixture with an inorganic matrix, for example a matrix based on silicates, gypsum and / or calcium carbonate. The asbestos panel is generally used to obtain flame barriers in ducts for electrical systems and for the insulation of railway carriages, ships, buses;

[0045] • a material obtained by mixing asbestos fibres with cement (known as asbestos-cement), in which the asbestos fibres are present in quantities of up to 50% by weight, generally comprised between 8 and 15% by weight, with respect to the total weight of the material. Asbestos-cement has generally been used for manufacturing artifacts for construction (e.g. tiles, partition walls, pipes, roofing elements, tanks, flat or corrugated slabs, etc.).

[0046] Asbestos fibres are preferably a material in the form of ropes, tapes or sheaths obtained by weaving asbestos fibres, generally in admixture with other natural or synthetic fibres, used for the wrapping of pipes to be thermally insulated or of electrical cables close to sources of intense heat, or for the production of flame- retardant fabrics or with properties of resistance to the corrosive action of acids and bases.

[0047] According to a preferred aspect, the asbestos-containing starting material is asbestos-cement.

[0048] According to a further preferred aspect, the process of the invention is used to treat asbestos-cement artifacts, such as for example eternit, comprising an amount of asbestos of 8-15% by mass with respect to the total mass, while the remaining part of 85-92% by mass with respect to the total mass, is generally composed of cement, gypsum, or other inert matrix.

[0049] Both the material containing asbestos fibres and a matrix and the asbestos fibres are treated with the method of the invention at the end of their life, i.e. following their removal and subsequent disposal. Therefore, the method of the invention can be considered as a method for neutralizing and / or recycling asbestos-based materials that are to be progressively disposed of. The amorphous fibrous silica obtained at the end of step c), the non-fibrous one obtained at the end of step d) and the fibrosity-free mineral material obtained at the end of step c’) are secondary raw materials that can be used for various uses.

[0050] According to a preferred aspect, the mineral material obtained with the process of the invention is forsterite.

[0051] The method of the invention therefore makes available an effective neutralization system for hazardous asbestos-based materials having low costs as it has been optimized to require a reduced consumption of energy and chemical reagents.

[0052] In the pre-treatment step a), the material comprising asbestos fibres and a matrix is subjected to at least one grinding, either dry or in aqueous phase, obtaining a powder or an aqueous suspension, comprising ground asbestos fibres and a ground matrix, preferably with at least two grindings.

[0053] Grinding is carried out through known technologies, such as for example hammer mills, ball mills, grinders, crushers and the like. Preferably, grinding is carried out with a ball mill which allows better control over the degree of grinding.

[0054] When grinding is carried out dry the material comprising asbestos fibres and a matrix can be ground after a first coarse crushing or directly, given their good friability.

[0055] When grinding is carried out in aqueous phase, the material comprising asbestos fibres and a matrix is mixed with water, preferably in a ratio comprised between 1 :6 and 1 :10 and then at least one grinding, preferably at least two grindings, is carried out.

[0056] Grinding in aqueous phase has the multiple advantages of simplifying the handling of the material (which can be handled by pumping), ensuring better efficiency and homogeneity of grinding and minimising the risks associated with the dispersion of asbestos powders in the environment, while dry grinding allows the subsequent filtration and drying steps (step b’) necessary to be able to proceed when choosing the thermal denaturation of asbestos fibres to be avoided.

[0057] The at least one grinding is carried out up to a homogeneous particle size, obtaining particles of particle size less than 5 mm, preferably less than 4 mm, more preferably comprised between 5 pm and 4 mm, even more preferably between 5 and 100 pm. In a preferred embodiment, a first grinding is carried out to obtain a particle size of less than 5 mm, preferably comprised between 1 and 4 mm and subsequently a second grinding to obtain a particle size of less than 100 pm, preferably less than 50 pm, more preferably comprised between 5 and 50 pm.

[0058] The result of the at least one grinding, when carried out in aqueous phase, is an aqueous suspension of ground asbestos fibres comprising a ground matrix, otherwise when the grinding is carried out dry a powder comprising ground asbestos fibres and ground matrix is obtained, preferably with the particle size indicated above.

[0059] When the grinding is carried out in aqueous phase, an aqueous suspension comprising the ground asbestos fibres, suspended and dispersed in a ground matrix, is obtained. The ground matrix imparts a basic pH to the suspension, typically comprised between 11 -12, in case the matrix is of cementitious type.

[0060] In this embodiment, the aqueous suspension obtained from grinding is treated in step b) to separate the asbestos fibres from the matrix, exploiting the fibrous morphology of the asbestos fibres that is maintained even after grinding and that gives it different chemical-physical characteristics compared to the matrix. The chemical-physical characteristics that can be exploited in this sense are, for example, apparent density, surface potential (zeta potential), surface area, polarizability, etc.

[0061] The separation of step b) can be conducted by gravimetric, physical or chemical- physical separation techniques or combinations thereof, such as centrifugation, gravimetric concentration (such as for example Knelson or Mozley machinery), vibrating table (such as for example Wilfey machinery), flotation, preferably frothflotation, electrostatic separation, and / or electrocoagulation. These techniques exploit the differences in density, surface chemistry, or polarizability existing among the asbestos fibres and the ground matrix particles.

[0062] When the grinding of step a) is carried out dry, the separation of the asbestos fibres from the matrix in step b) is carried out by electrostatic separation, thereby obtaining dry asbestos fibres. Electrostatic separation can be carried out with tools such as triboelectric roller separators or plate precipitators.

[0063] In the case of dry grinding, a powder of asbestos fibres separated from the powder of the matrix, which is an inert solid, is obtained from the separation.

[0064] When the grinding of step a) is carried out in the aqueous phase, the material to be separated is already in aqueous suspension, therefore, for the separation, the frothflotation technique is used.

[0065] Froth-flotation is a separation technique in which two mineral fractions are separated on the basis of their different interfacial properties (such as density, wettability, contact angle, etc.). This technique gives rise to floating foams enriched in one of the two components with respect to the other, which tends to settle on the bottom of the aqueous suspension. Since the separation efficiency of this process is additive, after a sufficient number of froth-flotation steps a complete separation of the two mineral fractions can be obtained.

[0066] Froth-flotation should not be confused with floc-flotation, a technique used in the field of water purification to remove contaminants from water by flotation, after they have been chemically flocculated.

[0067] An aqueous suspension of asbestos fibres and a waste sludge, preferably basic in the case where the matrix is cementitious, are obtained from the separation. The latter is subjected to further recycling steps illustrated below.

[0068] When the flotation technique is used in step b), at least one additive is added in advance to the aqueous suspension of ground asbestos fibres, optionally comprising a ground matrix. Preferably, the aqueous suspension is allowed to condition before performing air insufflation. The at least one additive may be selected from: a collector, such as for example a xanthate, a sulfosuccinate, a thiophosphate, a thiocarbamate, a thiourea, a thiocarbanilide, a mineral or natural oil; a foaming agent, such as for example a carboxylic acid or a sulphate derivative thereof (for example, ether sulfonate, or benzenesulfonate), an alkylammonium salt or a polyethoxylated alcohol; an activator such as for example copper sulphate; a suppressant, such as for example a polyelectrolyte, a phytic acid derivative, a cyanide or a dichromate; a modifier, such as for example a starch, a dextrin, a polyalcohol or a phosphate.

[0069] After a conditioning time of the aqueous suspension preferably comprised between 1 and 24 hours, the suspension is transferred to the flotation tower, where a jet of air is blown at the base of the mixture through nozzles forming a foam enriched in fibres, while the sludge accumulates at the base. The fibre-rich foam is collected at the surface of the upper part of the flotation tower.

[0070] The sludge can be subjected to separation, preferably by flotation, several times in order to guarantee a residue of fibres therein considered non-hazardous, preferably not detectable by instrumental analytical techniques.

[0071] According to a preferred aspect, the residue of asbestos fibres present in the sludge is less than 0.1 % by weight with respect to the total weight of the sludge, preferably less than 0.01 % by weight with respect to the total weight of the sludge and even more preferably, the sludge is substantially free of asbestos fibres, i.e. the asbestos fibres are present in the sludge in an amount lower than the determination limit of common measuring instruments.

[0072] The waste sludge now free of asbestos fibres is subjected to a separation carried out by filtration, for example using a filter press, centrifugation and / or sedimentation. An inert solid and an effluent are obtained from the separation.

[0073] When the starting material is asbestos-cement, the inert solid, coming both from the separation after dry grinding, and from the separation after wet grinding, is composed almost exclusively of calcium silicates and aluminates (the remaining part being other materials making up the cement in a minority fraction, i.e. carbonates, hydroxides, oxides or other inerts) and can be used as a secondary raw material for different industrial applications: for example it can be recycled as a construction inert filler or as a secondary raw material in cement factories for the production of new concrete. The use of this material for the production of concrete advantageously results in a reduced CO2 emission compared to the limestone materials commonly used. In addition, the inert solid may be supplemented with other substances and be used as an agricultural soil improver.

[0074] The effluent obtained from the separation performed after wet grinding is alkaline (with pH 10-12), when the starting material is asbestos-cement.

[0075] In this case, the effluent can be considered one of the materials to be recovered from the process with different potential uses: this can be for example reused in the process of the invention to neutralize the acidic waste produced at the end of step c) of hydrothermal denaturation, described in detail below, obtaining to precipitate and recover the metals in solution and allowing to recover the water to be reused in the process; alternatively, the alkaline effluent for example can be destined, in whole or in part, to the capture of CO2 and the formation of carbonates. That is, fumes containing high CO2 contents generated by any industrial process or rather by the plant's own feed system, can be washed with alkaline effluent through a scrubber or directly bubbled through the alkaline effluent, reducing CO2 and generating carbonates.

[0076] For example, when the starting material is asbestos-cement, this alkaline solution has a capacity to sequester CO2 calculated in the order of 300 kg of carbon dioxide per tonne of asbestos-cement treated by the method; at the same time a neutralization and purification of the alkaline process waters are obtained without the use of any external additives.

[0077] When the starting material contains a brittle matrix, the effluent is an aqueous suspension of matrix powder having neutral pH. For example, it is a neutral aqueous suspension of gypsum powder, if the starting material is a brittle matrix of gypsum. The aqueous suspension containing asbestos fibres obtained from separation step b) when grinding was carried out in aqueous phase, is subjected to a step b1) after separation by froth-flotation. The aqueous suspension is in the form of a foam enriched with asbestos fibres and is washed and filtered in order to separate the asbestos fibres from the water containing the flotation additives. The water containing the additives is recovered and recycled by re-introducing it into the flotation process. The filtered asbestos fibres are resuspended again in water, preferably in a weight ratio of 1 :10 fibres / water, before proceeding to the next step c) of hydrothermal denaturation, or they are dried (step b’)) before proceeding to the next step c’) of thermal denaturation.

[0078] Step b’) provides that the suspension of asbestos fibres of point b) is subjected to filtration and drying, obtaining dry asbestos fibres, before proceeding with thermal denaturation.

[0079] Any known filtration technique can be used for filtration, such as, for example, vacuum filtration, filter press, centrifuge, etc.

[0080] Likewise, any known technique may be used for drying.

[0081] According to a preferred aspect, the fibres entering the denaturation treatment should have a moisture residue of less than 1 %.

[0082] Step c) of hydrothermal denaturation treatment is performed on the suspension of asbestos fibres or on the dry asbestos fibres after suspension in water, obtained in step b).

[0083] Hydrothermal denaturation provides for the addition of an acidic aqueous solution to the aqueous suspension and heating, obtaining an amorphous fibrous siliceous material at the end of the treatment.

[0084] The acidic aqueous solution is obtained, for example, starting from mineral acids such as phosphoric acid and sulphuric acid. Said mineral acids may be used alone or in admixture with each other.

[0085] Alternatively or in addition to the mineral acids, use can be made of aqueous acidic waste or effluents coming from various industrial sectors, for example the steel industry, galvanic treatments, the agri-food industry, and the energy technology sector. The waste must preferably be characterized by a high acid reserve (i.e. it must have an amount of acid that can freely react within the mixture, which may be different from the nominal acid concentration) and may or may not comprise other contaminants that can be defined as hazardous contaminants (such as heavy metals, toxic substances, fluorides, etc.). In case said contaminants are present, they can be separated downstream of the process of the invention and reused in various industrial applications.

[0086] If in the industrial waste the acid component is sufficiently low and not sufficient to ensure a sufficient supply of acid reagent throughout the process, a mineral acid, for example sulphuric or phosphoric acid or a mixture thereof, is further added. In particular, the mixture may comprise one part of sulphuric acid and two parts of phosphoric acid (by volume), which entails a better workability of the solution and avoids the uncontrolled formation of insoluble precipitates.

[0087] In the embodiment in which the acidic waste is used as an acidic additive, it can be used in an amount of from 10% to 100% of the final liquid volume, depending on the characteristics of the waste. For example, in the case of effluent from batteries, an amount thereof equal to 30% of the final volume is used. It is therefore necessary to add an amount of acid in the hydrothermal denaturation step so as to reach the necessary acid equivalents per amounts of asbestos fibres.

[0088] The aqueous mixture of asbestos fibres and acids is subjected to heating, preferably with a single heat treatment step, at a temperature comprised between 170 °C and 250 °C, preferably between 180 and 220 °C. The reaction time is comprised between 2 and 10 hours, preferably for 3-7 hours, preferably in a sealed reactor.

[0089] The applied heat generates an equilibrium pressure comprised between 9 and 12 bar, preferably between 9.5 and 10.5 bar, which is preferably maintained for the duration of the treatment.

[0090] The application of acidic and thermal conditions makes it possible to convert essentially all of the asbestos fibres into amorphous fibrous siliceous material, that is, into a material in which the asbestos is below the detection limit of the most common analytical techniques.

[0091] An object of the invention is therefore also the fibrous siliceous material obtainable with the process of the invention and the use thereof as a secondary raw material. Step c’) of thermal denaturation treatment is performed on the dry asbestos fibres obtained in steps b) and b’), and leads to obtaining a mineral material.

[0092] The dry asbestos fibres are placed in a furnace or other heating apparatus, such as for example resistance furnaces, flame furnaces, burners, magnetron (microwave) etc., and a temperature higher than 900 °C, preferably between 900 °C and 1 ,100 °C, is applied.

[0093] According to a preferred aspect, the temperature above 900 °C is maintained for a period of time comprised between 0.3 and 5 hours, preferably between 0.5 and 1 hour.

[0094] Dehydroxylation of the asbestos structure is thus obtained: the terminal OH groups are freed from the structure in the form of H2O leaving magnesium silicates. The thermal dehydroxylation reaction begins to take place at temperatures of 450- 550 °C, but in this step it only concerns the outermost hydroxyl groups of the asbestos tubular structure. To obtain the removal of the innermost hydroxyls, it is necessary to supply a greater thermal energy to the system and, in order to reach complete dehydroxylation, i.e. complete destruction of the asbestos structure, it is necessary to reach temperatures comprised between 900 and 1 ,100 °C.

[0095] It is important to reduce the ambient moisture content at this step, as moisture slows down the dehydroxylation process.

[0096] After surpassing 800-820 °C, at the same time as the loss of the last most strongly bound hydroxyl groups, new mineral phases are formed: The free magnesium atoms and the silicate groups, which were present as amorphous material, begin to crystallize, mainly in the form of forsterite. Forsterite is a magnesium silicate belonging to the olivine group, and is obtained as the main product of the thermal degradation of asbestos.

[0097] Once the reaction is completed, the fibrous morphology has also completely disappeared, since the new mineral phase is obtained by recrystallization process, and not simply phase transition. Forsterite is a mineral that can demonstrate good market value given the different applications thereof: in the most common cases it is used as a hard stone, i.e. as an abrasive material, in the production of special ceramics, and in the production of jewellery (in the case of sufficiently large and transparent stones).

[0098] The process of the invention allows to overcome the various disadvantages related to the usual technique for thermal denaturation of asbestos.

[0099] In fact, the various thermal denaturation techniques (different variants have been proposed, such as for example the use of different gas mixtures in the furnace or the use of microwaves for heating, etc.), in addition to being extremely energy- intensive, act on asbestos, which is often only a minority fraction in the composition of the artifacts. This results in an unequivocal economic unsustainability of the process, however refined it may be. The most direct consequence is that in order to transform the asbestos fibres contained in an artifact, it is necessary to bring its entire mass to inertisation temperatures. If we consider the most common form of artifacts made of asbestos, they are represented by eternit, consisting of 5-15% asbestos fibres (content by weight) and the remaining 85-95% cement. Having to bring all this amount of cement to denaturation temperatures also introduces further negative effects in addition to energy consumption: - during heating at high temperature, the cement also undergoes endothermic phase transitions, thus leading to a further increase in the thermal demand of the process and longer times to reach thermal equilibrium, compared to simple mass heating;

[0100] - the carbonate fraction contained in the cement (up to 20% by weight) undergoes decarboxylation leading to a further CO2 emission;

[0101] - the concomitant presence of other elements including an abundance of calcium ions competes with and hinders the crystallization of forsterite, which is consequently produced in smaller quantities, and of lower purity. The forsterite produced in the presence of the cementitious matrix is also completely mixed with the other minerals obtained from the degradation of the cement, such as for example calcium and aluminium oxides and silicates, thus limiting the potential use thereof as a secondary raw material.

[0102] Advantageously, in the process of the invention, thermal denaturation is carried out only on asbestos fibres, previously separated from the matrix, in particular from the cementitious matrix, and this allows only about 5-15% of the initial mass to be brought to a high temperature. With the process of the invention, it is thus obtained directly forsterite not mixed with other materials and free of impurities and contaminants and, consequently, of better quality. In addition, the initial grinding steps bring the asbestos to a reduced and homogeneous particle size, thus obtaining a high exchange surface area; this allows a faster and more regular dehydroxylation reaction to be carried out until complete conversion, thus eliminating the risk that particularly large fibres or aggregates remain only partially denatured, which condition would even increase their carcinogenicity according to what has reported by recent studies.

[0103] The process of the invention also allows the degradation of asbestos even in case amphiboles other than chrysotile asbestos are present. In fact, in a small percentage of cases, particular forms of asbestos have also been used in the production of eternit, such as for example amosite or crocidolite (they represent mostly compositional variations given by the presence of impurities of other elements such as for example sodium or iron), which can create problems in the denaturation processes, as they are particularly inert.

[0104] Steps a) and b) allow to maintain the energy costs of the hydrothermal denaturation and thermal denaturation treatment at industrially acceptable values as the matrix, normally alkaline, of the materials comprising asbestos fibres is separated before the hydrothermal or thermal denaturation step, thereby lowering respectively the amount of acid or the energy demand necessary to obtain the denaturation effect. The amount of water necessary for the fluidization of the system and the execution of the hydrothermal reaction is also contained, with the consequence that the energy demand necessary for the heat treatment is also reduced.

[0105] The reactor used for hydrothermal denaturation is designed so as to ensure and withstand the creation of an equilibrium pressure inside it caused by water vapour. It can be made of special steel or other corrosion-resistant material, it can have different geometries and provide various heating systems: electric by resistance, through microwave, by induction, flame, jacketed with diathermic oil or molten salt circuit, etc.

[0106] For the purposes of energy efficiency, which contributes significantly to the sustainability of the process, there is a heat recovery system, for example a liquidliquid exchanger allowing to recover thermal energy from the liquid exiting the reactor after a thermal cycle and use it to preheat a new volume of liquid entering the reactor, reducing the energy requirement of the heating step.

[0107] In addition, it can be provided to use a cogeneration system to feed the plant: the concomitant (on-site) production of electrical and thermal energy can provide for feeding the process, both in terms of thermal requirement and for the activation of all electronic systems. The introduction of a cogenerator makes it possible to exploit a further synergy of efficiency and impact reduction: a part of the alkaline effluent can be destined to intercept and abate the CO2 contained in the fumes produced by the electro-thermal generator with the bubbling / capture system mentioned above.

[0108] From the hydrothermal step of step c) a hot, dense, muddy suspension of amorphous fibrous siliceous material is obtained at the outlet which, after cooling, is separated by filtration, centrifugation, sedimentation or filter pressing (step c1)) into its solid fraction and its liquid fraction.

[0109] The liquid fraction is composed of an acidic aqueous solution rich in ions, i.e. the positive magnesium ions freed by the denaturation reaction and by the counter-ions provided by the consumed acids. This aqueous fraction can be mixed in appropriate proportions with the previously obtained alkaline effluent. The proportions are those necessary to produce a quantitative neutralization of the acidic and basic species present in the two solutions. Neutralization also involves the concomitant precipitation of salts (mainly phosphates and sulphates) of magnesium, poorly soluble at pH close to 7.

[0110] The recovered magnesium salts are preferably valued as a secondary raw material; for example, they are useful in the steel field for the manufacture of light alloys or as nutrients in the formulation of agricultural fertilizers.

[0111] After separation of the magnesium salts, the neutralized water is recovered and reintroduced into the initial steps of the process, thus containing the water consumption of the method and contributing to the sustainability and circularity thereof.

[0112] The solid fraction is composed of amorphous silica fibres (amorphous fibrous siliceous material), which are a subject-matter of the invention, as well as a subject- matter of the invention is their use as a secondary raw material.

[0113] Amorphous silica fibres are the result of the denaturation of asbestos fibres, of which they partially maintain the morphology, but not the composition. The fibres are composed of hydrated amorphous silica with a purity higher than 95%; they mostly maintain fibrous morphology, demonstrating mesoporosity and nanoporosity that give them surface area values greater than 200 m2 / g, up to even 400 m2 / g. These characteristics result in a market value for these silicas, especially by virtue of their high purity, which is certainly desired in different sectors, such as the production of glasses for technological applications. These chemical-morphological characteristics generate at the same time a potential residual hazard with respect to their use. In reality, to date there is no legislation unequivocally stating the hazard thereof, also due to the fact that it is a material not very widespread, but their average size (5-50 microns in length, width less than 6 microns, with a length / width ratio greater than 6) places them in what is called the breathable fraction of pulviscular fibres. Their composition also makes them potentially similar to the subgroup of vitreous artificial fibres (AVFs) recognised as hazardous and carcinogenic (i.e. those with an alkali / alkaline-earth metal oxide content of less than 18%). Therefore, the method of the invention provides, in one embodiment, to perform a post-treatment (step d)) of the amorphous silica fibres to eliminate any potential hazards thereof and allow the reuse thereof as a secondary raw material without restrictions or specific provisions. The post-treatment of amorphous silica fibres can be of chemical, physical and / or hydrothermal type.

[0114] The chemical post-treatment can be an alkaline treatment with strong bases: the silica fibres are contacted with an aqueous solution with a high concentration (equal to or greater than 8M) of alkali metal hydroxides, preferably sodium hydroxide or potassium hydroxide. A volume of such solution sufficient to completely wet the mass (at least 3 times the weight of the fibres) is kept in contact with the fibres for at least 2-4 hours under ambient conditions. The extreme alkalinity leads to a deformation and partial dissolution of the silicas, which will eventually result in morphologies that can be no longer assimilated to the fibres.

[0115] Another example of chemical treatment to densify silica fibres consists in immobilizing the fibres by adding at least one binder, preferably a reactive silica- based compound capable of “cementing” the fibres together. Concentrated aqueous solutions of sodium or potassium or lithium silicates, rather than aqueous dispersions of non-capped silica nanoparticles, are usable for this purpose.

[0116] By mixing the silica fibres with the appropriate amounts of the aforesaid solutions (i.e. the minimum amount useful for completely wetting the fibrous material), after a fast drying time, the vitrification of the binder and the resulting solidification of the glassy mass with the effect of blocking the silica fibres and preventing the dispersion thereof occur.

[0117] If it is desired to maintain a high surface area or it is wished not to affect the compositional purity of the silicas obtained after hydrothermal denaturation, it is possible to resort to a post-treatment by means of physical treatments, such as a heat treatment optionally combined with pressing. The melting temperature of pure silica is comprised between 1670 °C and 1900 °C; bringing the material above these temperatures would certainly guarantee the loss of fibrosity, but the process would be complex and energy-intensive. In addition, it is known that above 1390 °C recrystallization phenomena can occur leading to the formation of Cristobalite (crystalline phase of silica known to exhibit carcinogenicity if breathed). It was also found that in the temperature range from 900 °C to 1400 °C the silica shows oscillations in its intrinsic density value, presenting a minimum around 950 °C.

[0118] Therefore, a fast heating in the range between 940 °C and 960 °C, preferably at 950 °C, combined with a mechanical hot pressing action allowing to obtain a compaction of the partially softened fibres (even if below the nominal Tg of the glass at 1260 °C) can be used in the post-treatment. A compacted material, but not fully densified, with characteristics dependent on the action time of the press is thus obtained. 30% to 70-80% of the initial surface area value can be lost. The procedure results in a cross-linking of the fibres, which immobilises them and prevents their dispersion and volatilisation, allowing safe handling.

[0119] Alternatively, the amorphous fibrous silica can be densified under hydrothermal conditions, with lower energy expenditure. The hydrothermal conditions allow similar results to be obtained by working in aqueous phase within a static reactor at high pressures and applying temperatures of 300-400 °C together with a pressure comprised between 50 and 200 Mpa, preferably for times of 60-120 minutes.

[0120] The object of the invention is also the fibrosity-free siliceous material obtainable with the method of the invention when it comprises step d) and the use thereof as a secondary raw material.

[0121] Finally, the object of the invention is also the mineral material obtainable by the process of the invention when step c’) of thermal denaturation is performed, as well as the use thereof as a secondary raw material.

[0122] According to a preferred aspect, the mineral material obtainable at the end of step c’) is forsterite.

[0123] In particular, forsterite can be used as an abrasive material, in the production of special ceramics, and in the production of jewellery (in the case of sufficiently large and transparent stones).

[0124] EXAMPLES

[0125] Example 1

[0126] Step a) 10 kg of corrugated sheets in eternit fibre cement (material containing asbestos fibres, contained in asbestos fibres = 8.6% w / w by laboratory analysis) previously coarsely crushed (about 4 mm) were mixed with 60 litres of water and ground in a rotating Alumina ball mill for about 10 minutes, obtaining a dense mixture composed of the suspended fibres and the ground matrix. The dimensional analysis of the material demonstrated a mean particle size comprised between 2 pm (SMD) and 9 pm (VMD) (Figure 1 ).

[0127] Step b) 2 litres of the previously prepared mixture were transferred to a bench flotation cell with a total capacity of 5 litres. Additional 2 litres of water and 0.8 g of Amyl Xanthate were then added; the mixture was mixed rapidly, then held for 12 hours still under conditioning. The flotation process was then activated using air insufflation with a 73 mm impeller and maintaining rotation speed at 200 rpm; after 15 minutes the foam produced was collected and gradually separated from the sludge and filtered. Laboratory analyses performed on the resulting cementitious sludge demonstrated a reduced Asbestos fibre content from 8.6% to 4.7% w / w after the first step. The collected cementitious sludge was repeatedly subjected to the flotation process under the same conditions, bringing the fibre content to 3.2% after the second cycle and 1 .2% after the third cycle.

[0128] The flotation process was repeated on different batches of the initially prepared mixture, and the fractions of foam enriched in fibres collected in the various steps were brought together until obtaining about 10 g of material.

[0129] Step c) the material enriched in asbestos fibres was then supplemented with 70 ml of water, 30 ml of acid effluent deriving from spent lead-acid batteries and 6.7 ml of 95% concentrated sulphuric acid. The mixture thus composed (pH lower than 2, verified with litmus paper) was transferred into a laboratory stainless steel reactor equipped with a central heating system. Once the reactor containing the liquid was sealed, the hydrothermal denaturation treatment was carried out, by bringing it to 180 °C and maintaining the temperature for 4 hours. Once the reactor was cooled, the treated material was filtered, dried and analysed to determine the asbestos content thereof, whose result was non-detectable (< 0.01 %, instrumental limit of detection).

[0130] Step d) Although no material with a composition that can be assimilated to asbestos was identified with SEM analysis in the sample subjected to the hydrothermal treatment, particles are observed that maintain fibrous / tubular morphology that EDX analysis revealed to be composed of Silica (SiC>2) as indicated in table 1 below:

[0131] Table 1

[0132] The sample was then treated with a solution of 8M Potassium Hydroxide (2 ml of solution for one gram of material) initially resulting in a deformation of the fibrous / tubular structures, and within 30 minutes in a complete dissolution of the silica fibres.

Claims

CLAIMS1. Method for treating a material comprising asbestos fibres and a matrix, comprising the steps of: a) pre-treating the material comprising asbestos fibres and a matrix by at least one grinding, either dry or in aqueous phase, obtaining a powder or an aqueous suspension, comprising ground asbestos fibres and a ground matrix; b) separating the asbestos fibres from the matrix by electrostatic separation when the grinding of step a) is performed dry, thus obtaining dry asbestos fibres, or by froth-flotation when the grinding of step a) is performed in the aqueous phase, thus obtaining a suspension of asbestos fibres; b’) optionally subjecting the suspension of asbestos fibres obtained in step b) to filtration and drying obtaining dry asbestos fibres; c) subjecting the suspension of asbestos fibres or the dry asbestos fibres after suspension in water, obtained in step b), to hydrothermal denaturation performed by the addition of an acidic aqueous solution and heating, thus obtaining an amorphous fibrous siliceous material, or c’) subjecting the dry asbestos fibres obtained in step b) and b’) to thermal denaturation, obtaining a mineral material.

2. Method according to claim 1 , wherein the at least one grinding of the asbestos fibres and a matrix of step a) is carried out dry.

3. Method according to any one of the preceding claims, wherein the grinding of the material comprising asbestos fibres and a matrix in step a) is carried out at least twice.

4. Method according to any one of the preceding claims, wherein the grinding of step a) is carried out until particles with a particle size of less than 5 mm, preferably less than 4 mm, more preferably comprised between 5 pm and 4 mm, even more preferably between 5 and 100 pm are obtained.

5. Method according to any one of the preceding claims, wherein when in step b)the separation by flotation is performed, in addition to the suspension of asbestos fibres a waste sludge containing the ground matrix is obtained, which is subjected to separation to obtain an effluent and an inert solid comprising the ground matrix.

6. Method according to claim 1 , wherein when the separation is carried out by flotation, at least one additive, preferably selected from a collector, a foamer, an activator, a suppressant and a modifier, is added in advance to the aqueous suspension of ground asbestos fibres and matrix.

7. Method according to claim 6, wherein after the froth-flotation a step b1 ) of separating the asbestos fibres from the at least one additive is performed.

8. Method according to any one of the preceding claims, wherein, when the hydrothermal treatment is performed, the acidic aqueous solution added in step c) is obtained from mineral acids, preferably sulphuric and / or phosphoric acid, from acidic waste, acidic aqueous effluents or mixtures thereof.

9. Method according to claim 1 , wherein the heating performed in step c) in case of hydrothermal denaturation is carried out at a temperature comprised between 170 °C and 250 °C, preferably between 180 °C and 220 °C.

10. Method according to claim 1 , wherein the thermal denaturation of step c’) is carried out at a temperature comprised between 900 °C and 1 ,100 °C.11 . Mineral material obtainable according to any one of claims 1 to 8 and 10 when step c’) of thermal denaturation is performed.

12. Method according to claim 1 , wherein after the hydrothermal denaturation a step c1 ) of separating a solid fraction composed of an amorphous fibrous siliceous material and a liquid fraction composed of an acidic aqueous solution rich in ions is performed.

13. Method according to claim 1 , comprising, when step c) of hydrothermaldenaturation is performed, a further step d) of post-treating the fibrous siliceous material to obtain a fibrosity-free siliceous material, by chemical, physical and / or hydrothermal treatment.

14. Method according to claim 13, wherein the chemical type post-treatment is an alkaline treatment carried out by contacting the fibrous siliceous material with an aqueous solution of alkali metal hydroxides having a concentration higher than 8M.

15. Method according to claim 13, wherein the chemical type post-treatment consists in immobilising the fibres of the fibrous siliceous material by addition of at least one binder.

16. Method according to claim 13, wherein the physical type post-treatment is a heat treatment carried out in the range between 940 °C and 960 °C, preferably at 950 °C, optionally with pressing.

17. Method according to claim 13, wherein the hydrothermal type post-treatment is carried out within a static reactor at high pressures and applying temperatures of 300-400°C together with a pressure comprised between 50 and 200 Mpa.

18. Amorphous fibrous siliceous material obtainable by the process according to any one of claims 1 to 9, when step c) of hydrothermal denaturation is performed.

19. Fibrosity-free siliceous material obtainable with the process according to any one of claims 13 to 17.

20. Use of the mineral material of claim 11 , of the amorphous fibrous siliceous material of claim 18 or of the fibrosity-free siliceous material of claim 19, or of the liquid fraction composed of an acidic aqueous solution rich in ions of claim 12 or of the inert solid comprising the ground matrix of claim 5, as secondary raw material.21 . Use of the effluent of claim 5 to abate the CO2 content from gaseous emissions, when the starting material is asbestos-cement.

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