Process for treating a solid material containing asbestos fibers

Abstract

The invention relates to a process for treating a solid material containing asbestos fibers, comprising the following steps: - grinding the solid material, - contacting the solid material with an aqueous acid solution, resulting in the dissolution of at least a portion of the asbestos fibers, - separating the liquid (5) and solid (4) fractions of the products resulting from the dissolution. This process is particular in that it further comprises the following steps: - dialysis by diffusion of said liquid fraction (5), in order to produce a regenerated aqueous acid solution (9), - use of said regenerated aqueous acid solution (9) in the step of contacting the ground solid material with an aqueous acid solution. The invention also relates to an installation for carrying out the process according to the invention. Figure for the abstract: Fig 1

Description

Title of the invention: Process for treating a solid material comprising asbestos fibers technical field

[0001] The present invention relates to the treatment of waste containing asbestos. More particularly, it relates to a process for dissolving asbestos fibers by contacting them with an aqueous acid solution. This process allows for rapid dissolution and consumes little energy and smaller quantities of aqueous acid solution than known prior art processes. Prior art

[0002] Asbestos is a group of fibrous minerals of the hydrated silicate type that may contain, depending on the type of asbestos, in addition to silicon compounds, various elements such as magnesium, calcium, iron(II) or iron(III), aluminum, or sodium, in the form of oxides or hydroxides. Asbestos may be combined, particularly in materials and products used in construction, with binders, as is the case, for example, in asbestos cement.

[0003] The prohibition of the use of asbestos, due to the health risks presented by this substance when fibers are found in the air, has led to the need to dispose of as waste the asbestos products used in the past in building construction as well as in other manufacturing in industrial, railway, naval, and public works environments.

[0004] As an alternative to vitrification, which is very energy-intensive, and to the burial of waste containing asbestos, which is environmentally unsatisfactory, it is possible to destroy asbestos fibers in solutions of mineral or organic acids.

[0005] For this purpose, concentrated sulfuric acid (in a solution of the order of 2 N to 6 N), hydrochloric acid, or even phosphoric or hydrofluoric acid, alone or in mixture with each other and possible addition of an organic acid allowing the complexation of iron ions, which facilitates the dissolution of types of asbestos containing it such as crocidolite or amosite.

[0006] For example, the destruction of chrysotile asbestos (magnesium-rich hydrated silicate), where appropriate associated with cement in asbestos-cement products, by sulfuric acid at a temperature between 30°C and 100°C, has been demonstrated.

[0007] The sulfuric acid then leads to the dissolution of the magnesium ions contained in the chrysotile, and a precipitation of the calcium ions from the cement in the form of Insoluble calcium sulfate (plaster). Silica compounds from asbestos and cement are found in the form of solid silica, as silicon dioxide is poorly soluble in acid.

[0008] The solid materials thus produced are separated by filtration, possibly preceded by decantation, and can be recovered, for example in the manufacture of construction products.

[0009] The excess acid that has not reacted remains in the liquid phase (filtrate and settling supernatant) and contains dissolved materials from the asbestos and cement. This acid solution can be regenerated by adding concentrated acid to obtain a sufficient acid concentration to attack another batch of asbestos-containing product. However, the acid bath gradually becomes enriched with dissolved materials (particularly magnesium sulfate), which slows down the reaction kinetics. The acid solution must then be removed and replaced with a fresh solution.

[0010] Among the dissolved substances, it is possible to extract certain components such as iron and aluminum by selective precipitation, neutralizing the acidity with a sodium hydroxide or lime solution to achieve a pH of approximately 5 to 7. Magnesium can then be extracted by precipitation as magnesium hydroxide by raising the pH to a value corresponding to the formation of this sparingly soluble compound. This pH adjustment, achieved by neutralizing the residual acidity after the destruction of the asbestos fibers and replacing it with fresh acid, results in a high consumption of both acid and neutralizing agent.

[0011] Documents WO8810234 and EP0348502, for example, propose such processes. They have the advantage of lower energy consumption than a vitrification process and can be implemented with conventional equipment used in the chemical industry. However, these processes require a significant quantity of reagents, particularly acids and neutralizers, which makes them expensive and / or time-consuming; using a larger quantity of reagents shortens the treatment time. Description of the invention

[0012] The present invention aims to overcome these drawbacks by proposing a process for treating a solid material comprising asbestos fibers, comprising the following steps: - grinding of solid material, - contacting the ground solid material with an aqueous solution of acid, resulting in the dissolution of at least part of the asbestos fibers, - separation of the liquid and solid fractions of the products resulting from the dissolution.

[0013] This process is particular in that it also comprises the following steps: - dialysis by diffusion of said liquid fraction, during which at least a portion of the H+ and H3O+ ions present in said liquid fraction pass through at least one anion exchange membrane, said membrane being impermeable to salts, in order to produce a regenerated aqueous acid solution, - use of said regenerated aqueous acid solution in the step of bringing the ground solid material into contact with an aqueous acid solution.

[0014] Thanks to these provisions, the consumption of aqueous acid solution in the process according to the invention is particularly low, as the acid can be recycled and reused several times, making it possible to treat a large quantity of material with a small amount of acid. This also allows for a rapid treatment process, since the acid is not completely consumed during the dissolution reaction, this reaction can be carried out with a significant excess of acid relative to the materials to be treated, without resulting in excessive acid consumption. Furthermore, since the acidity is practically removed from the process effluents, it is not necessary, as with existing processes, to treat these effluents, particularly by using large quantities of basic reagents.

[0015] The aqueous acid solution brought into contact with the ground solid material can be an aqueous solution of H2SO4, which has the advantage of not generating toxic fumes, and of being readily available as a by-product of various industrial processes.

[0016] The concentration of the acid in aqueous solution in contact with the ground solid material can be between 2 and 6 N, which effectively produces the dissolution reaction of the asbestos fibers.

[0017] The particle size of the ground solid material can be between 5 and 250 qm, which allows for a large contact surface with the acid, and thus ensures that the dissolution reaction is rapid and efficient.

[0018] During dissolution, the reactants can be maintained at a temperature between 40 and 100°C, which allows for rapid dissolution of the asbestos fibers in the acid.

[0019] A three-phase centrifuge can be used during the liquid and solid fraction separation step, which is a robust and efficient embodiment of the invention.

[0020] During the diffusion dialysis step, said liquid fraction can be brought to one side of said at least one membrane, and water can be brought to the other side, in the opposite direction, which is a simple and efficient embodiment of the invention.

[0021] During the diffusion dialysis step, a plurality of membranes can be used, the membranes being arranged parallel to each other, and the spaces between consecutive membranes being alternately supplied with water and a portion of said liquid fraction, which increases the total surface area of ​​the membranes, and thus results in faster and more efficient dialysis.

[0022] A stream of ground solid material can be brought into contact with a stream of regenerated aqueous acid solution, the steps of bringing the ground solid material into contact with an aqueous acid solution and dialysis by diffusion being able to be carried out simultaneously, which makes it possible to obtain a process at least partially continuous for the treatment of solid materials, and thus to treat these materials more quickly.

[0023] The present invention also relates to an installation for processing a solid material comprising asbestos fibers for the implementation of a process according to the invention, comprising a grinding module, a reactor for bringing the ground solid material into contact with the aqueous acid solution, a solid / liquid separation module, and a dialysis module comprising at least one anion exchange membrane.

[0024] Thanks to these provisions, the consumption of aqueous acid solution in the process according to the invention is particularly low, as the acid can be recycled and reused several times, making it possible to treat a large quantity of material with a small amount of acid. This also allows for a rapid treatment process, since the acid is not completely consumed during the dissolution reaction, this reaction can be carried out with a significant excess of acid relative to the materials to be treated, without resulting in excessive acid consumption. Furthermore, since the acidity is practically removed from the process effluents, it is not necessary, as with existing processes, to treat these effluents, particularly by using large quantities of basic reagents. Brief description of the drawings

[0025] The present invention and its advantages will become more apparent from the following description of several embodiments given by way of non-limiting examples, with reference to the accompanying drawings, in which:

[0026] [Fig-1] [Fig. 1] is a schematic view of a preferred embodiment of the process according to the invention, and of the installation for implementing said process,

[0027] [Fig.2] [Fig.2] is a schematic cross-sectional view of a dialysis module according to a preferred embodiment of the invention. Description of the implementation methods

[0028] The process according to the invention, illustrated in [Fig. 1], makes it possible to treat a solid material containing asbestos fibers, such as, for example, asbestos cement, asbestos plaster, tile adhesives, flocking, braids and insulating asbestos fabrics, asbestos-containing cardboard-based suspended ceiling panels, Compressed asbestos pipe joints, insulating materials used as pipe insulation, etc. The asbestos content by mass of this material varies. In some cases, the asbestos content may be very minimal (less than 1% in some adhesives or sealants), in asbestos cement the content is approximately 7 to 14%; and in other products the content can reach 90% or even more.

[0029] The asbestos treated within the framework of the present invention is, for example, chrysotile asbestos (hydrated magnesium silicate), which can be associated with cement in asbestos-cement products.

[0030] This material is, for example, construction waste, resulting from the demolition of a building or a road.

[0031] The first step of the process according to the invention is the grinding of the solid material in a grinding module 1. After grinding, the solid material can have a particle size of less than 500 µm, for example between 5 and 250 µm. To obtain such a particle size, different solutions can be used, such as, for example, a ball mill or a knife mill.

[0032] The ground solid material is then brought into contact with an aqueous solution of acid, in a reactor 2, which causes the dissolution of at least part of the asbestos fibers.

[0033] Reactor 2 preferably includes stirring means, allowing to accelerate dissolution by increasing the exchanges between the ground solid material and the aqueous acid solution.

[0034] To increase the reaction kinetics of magnesium dissolution and asbestos fiber breakdown, and to make it compatible with industrialization of the process, it is preferable to: - Finely grind the asbestos-cement products to increase the contact surface between the acid solution and the asbestos fibers, - Increase the temperature of the acid into which the ground product is introduced, without necessarily exceeding approximately 100°C for safety reasons. - maintain a sufficient acid concentration throughout the reaction, - use a sufficient quantity of acid relative to the quantity of solid materials ground, and - limit the concentration of dissolved materials resulting from the dissolution of asbestos-containing materials.

[0035] Another factor to be taken into account is the asbestos content of the solid material.

[0036] These various parameters can be adjusted so that all or almost all of the asbestos fibers are dissolved, and preferably so that no asbestos content is detectable by officially recognized analytical methods, for example polarized light optical microscopy (PLOM), electron microscopy scanning electron microscopy (SEM), and transmission electron microscopy with X-ray energy dispersion analysis (TEA).

[0037] The aqueous acid solution is preferably an aqueous solution of H2SO4. This type of acid has the advantage of being non-volatile, and therefore not generating toxic gases, and of being easy to source, for example as a by-product of an industrial process. Another advantage of H2SO4 is that the dissolution reaction, in the case where the solid material is a cement-asbestos mixture, will generate the precipitation of calcium ions as gypsum. This gypsum can then be extracted by separating the liquid and solid phases, and is easily recyclable as a construction material.

[0038] The present invention can however be implemented with any Brønsted acid, or a mixture of Brønsted acids, such as, for example, HCl, HNO3, H3PO4, or HF.

[0039] The concentration of the acid in aqueous solution, during the step of contacting the ground solid material, defined as equivalent concentration, is for example between 2 and 6 N. For example, with regard to H2SO4, a concentration of 1 mol / 1 corresponds to 2 N.

[0040] During the dissolution reaction, the reactants, i.e., the aqueous acid solution and the ground solid material, can be subjected to a temperature, thereby increasing the kinetics of the dissolution reaction. This temperature is, for example, between 40 and 100°C. To achieve this, reactor 2 can include means for heating and / or thermally insulating its internal volume.

[0041] After the dissolution reaction, the products of this reaction undergo solid / liquid separation in a solid / liquid separation module 3. Various means known to those skilled in the art can be used. For example, a three-phase centrifuge can be used.

[0042] The solid fraction 4 resulting from the dissolution, in the case of the treatment of solid asbestos-cement material treated with H2SO4, consists mainly of gypsum and silica. These materials can be recovered for reuse in construction materials.

[0043] The liquid fraction 5 resulting from the dissolution is then treated so that the acid can be recovered and reused in the dissolution reaction. To this end, the process according to the invention comprises a diffusion dialysis step (also called acid dialysis) in a dialysis module 6. The liquid fraction 5, which can be considered as a spent acid solution to be regenerated, flows in contact with at least one anion exchange membrane 7. On the other side of these membranes 7, a flow of clean water 8, preferably demineralized water, flows, preferably counter-currently. The membranes 7 are permeable to anions and hydrogen ions, but very slightly to Other cations, particularly polyvalent ones such as Mg++, Ca++, Al+++, Fe++, Fe+++, and Ni++, are present. The difference in acid concentration across membranes 7 causes the acid to diffuse through them, while cations such as magnesium, calcium, aluminum, iron(II), iron(III), and nickel ions are largely retained by membranes 7 in the spent acid. The counter-current flow of water and spent acid, and a process of osmosis, results in an acid concentration in the solution recovered downstream of the membranes (diffusate 9), on the side of the membrane through which the water entered, close to the acid concentration of the incoming liquid fraction. Membrane 7 contains ionized chemical groups whose positive charge is balanced by anions present in the solution with which the membrane is in contact.Due to the difference in anion concentration on either side of membrane 7, these diffuse through membrane 7 from the spent acid solution into the diffusate, accompanied by hydrogen ions whose small size and low charge also allow them to cross membrane 7. Conversely, divalent and trivalent metal ions are mostly kept away from the membrane by fixed positive charges, which allows the recovery in the diffusate of an acid free of these metal ions.

[0044] To achieve this, the membrane 7 is preferably a thin membrane composed of a cross-linked organic polymer or copolymer.

[0045] The dialysis module 6 may comprise a single membrane 7, or a plurality of membranes 7. The membrane 7 may be flat, and be arranged, for example, in a parallelepiped-shaped dialysis chamber, or the membrane 7 may be arranged in a spiral shape, and be arranged in a cylindrical dialysis chamber. In a preferred embodiment of the invention, illustrated in [Fig. 2], the dialysis module comprises a plurality of membranes 7, parallel to one another. The spaces between two consecutive membranes 7 are then supplied alternately with water 8, and with a portion of the liquid fraction 5.

[0046] The regenerated acid solution, or diffusate 9, after possible adjustment of the concentration, can be reused in the asbestos fiber dissolution process. The invention thus makes it possible to reduce the amount of acid consumed by the asbestos treatment process, as the acid can be regenerated virtually indefinitely to treat large quantities of asbestos. It is also possible, for the dissolution of asbestos, to use larger quantities of acid proportionally to the amount of solid material, which increases the reaction kinetics and allows for faster treatment of the asbestos.

[0047] On the other side of the membranes 7, the crude acid solution exiting the dialysis process, called dialysate 10, depleted in acid, contains the dissolved ions resulting from the contact of the asbestos-containing materials with the acid solution. This solution can The solution is neutralized by the addition of a basic reagent 11 to selectively precipitate the dissolved ions (iron, aluminum, then magnesium, etc.), for example, in a stirred tank 12. A filter 13 can then be used to recover the targeted material in solid form. In the example illustrated in [Fig. 1], two successive precipitation stages are used to precipitate different types of ions by progressively increasing the pH. The present invention makes it possible to carry out this step without consuming large quantities of basic reagent (such as sodium hydroxide, potassium hydroxide, or lime). This is a significant advantage compared to existing processes, in which, without an acidity recovery step, it is necessary to neutralize the residual acidity of the liquid fraction resulting from the dissolution using basic reagents before the dissolved ions can be precipitated.

[0048] The dialysis step is preferably preceded by pretreatment of the crude acid by microfiltration, in order to avoid fouling the intermembrane spaces through which the dialysate circulates. In the event of a risk of the presence of dissolved organic substances in the crude acid, particularly from the humidification of asbestos-containing materials with a wetting agent during their removal, an activated carbon filter can be installed upstream of the microfilter to prevent the risk of clogging the ion exchange membranes.

[0049] In a preferred embodiment of the process according to the invention, it is carried out at least partially continuously. In this case, a stream of ground solid material is brought into contact with a stream of regenerated aqueous acid solution, the steps of contacting the ground solid material with an aqueous acid solution and dialysis by diffusion being carried out simultaneously. If the process is carried out continuously from the grinding stage, a stream of solid material may be present upstream and downstream of the grinding module 1. It is also possible for the grinding to be carried out in batches, and for a stream of ground solid material to be generated downstream of the grinding module 1.

[0050] In the case of a continuous process, the main supply of aqueous acid solution is made at the start of the process. The acid from this main supply may be sufficient to treat a certain quantity of solid material, being successively regenerated by dialysis. In some embodiments, a secondary supply of aqueous acid solution can be provided throughout the process, the role of which is to compensate for the losses of acid that could not be recovered during the dialysis step. The secondary supply can be provided by a constant flow, or by a point-by-point supply, controlled, for example, by pH measurements.

[0051] The process according to the invention can be implemented in a treatment plant comprising a grinding module 1, a reactor 2 for contacting the ground solid material with the aqueous acid solution, a solid / liquid separation module 3, and a dialysis module 6 comprising an exchange membrane 7 of anions. Preferably, the installation also includes transfer means between successive elements, so as to be able to implement the process in at least a partially continuous manner. The installation may thus include a first transfer means 14 suitable for transferring a continuous flow of ground material to reactor 2, and / or a second transfer means 15 suitable for transferring a flow of products from the dissolution reaction from reactor 2 to the separation module 3, and / or a third transfer means 16 suitable for transferring a flow of liquid phase 5 from the separation module 3 to the dialysis module 6 and / or a fourth transfer means 17 suitable for transferring a flow of diffusate 9 from the dialysis module 6 to reactor 3.

[0052] The present invention is not limited to the embodiments described but extends to any modification and variant obvious to a person skilled in the art, within the limits of the appended claims. Furthermore, the technical features of the various embodiments and variants mentioned above may be combined, in whole or in part.

Claims

Demands

1. A process for treating a solid material comprising asbestos fibers, comprising the following steps: - grinding the solid material, - contacting the ground solid material with an aqueous acid solution, resulting in the dissolution of at least a portion of the asbestos fibers, - separating the liquid (5) and solid (4) fractions of the products resulting from the dissolution, characterized in that said process further comprises the following steps: - diffusion dialysis of said liquid fraction (5), during which at least a portion of the H+ and H3O+ ions present in said liquid fraction pass through at least one anion exchange membrane (7), said membrane (7) being impermeable to cations, with the exception of hydrogen cations, in order to produce a regenerated aqueous acid solution (9), - use of said regenerated aqueous acid solution (9) in the step of contacting the ground solid material with an aqueous acid solution.

2. Processing method according to claim 1, characterized in that the aqueous acid solution brought into contact with the ground solid material is an aqueous solution of H2SO4.

3. A treatment process according to any one of claims 1 to 2, characterized in that the concentration of the acid in aqueous solution in contact with the ground solid material is between 2 and 6 NT

4. IN. Processing method according to any one of claims 1 to 3, characterized in that the particle size of the ground solid material is between 5 and 250 qm.

5. Processing method according to any one of claims 1 to 4, wherein during dissolution, the reactants are maintained at a temperature between 40 and 100°C.

6. Processing method according to any one of claims 1 to 5, wherein a three-phase centrifuge (3) is used during the step of separating the liquid and solid fractions.

7. A treatment method according to any one of claims 1 to 6, wherein during the diffusion dialysis step, said liquid fraction (9) is brought to one side of said at least one membrane (7), and water (8) is brought to the other side, in the opposite direction.

8. A treatment method according to claim 7, wherein during the diffusion dialysis step, a plurality of membranes (7) are used, the membranes (7) being arranged parallel to one another, and the spaces between consecutive membranes being alternately supplied with water (8) and with a portion of said liquid fraction (9).

9. A treatment method according to any one of claims 1 to 8, wherein a stream of ground solid material is brought into contact with a stream of regenerated aqueous acid solution, the steps of bringing the ground solid material into contact with an aqueous acid solution and dialysis by diffusion being carried out simultaneously.

10. Installation for processing a solid material comprising asbestos fibers for carrying out a process according to any one of claims 1 to 9, comprising a grinding module (1), a reactor (2) for contacting the ground solid material with the aqueous acid solution, a solid-liquid separation module (3), and a dialysis module (6) comprising at least one anion exchange membrane (7).

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