Method for processing a solid material comprising asbestos fibres

Abstract

The invention relates to a method for processing a solid material comprising asbestos fibres, comprising the following steps: - grinding the solid material, - bringing the solid material into contact with an aqueous acid solution, resulting in at least some of the asbestos fibres being dissolved, - separating the liquid (5) and solid (4) fractions of the products resulting from the dissolving process. This method is characterised in that it further comprises the following steps: - diffusion dialysis of the liquid fraction (5) in order to produce a regenerated aqueous acid solution (9), - using the regenerated aqueous acid solution (9) in the step of bringing the ground solid material into contact with an aqueous acid solution. The invention also relates to a plant for carrying out the method according to the invention.

Description

[0001] DESCRIPTION

[0002] Title: Process for treating a solid material containing asbestos fibers

[0003] technical field

[0004] The present invention relates to the treatment of asbestos-containing waste. 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.

[0005] Previous technique

[0006] Asbestos is a group of fibrous minerals of the hydrated silicate type that, depending on the type of asbestos, may contain, in addition to silicon compounds, various elements such as magnesium, calcium, iron (II or III), aluminum, or sodium, in the form of oxides or hydroxides. Asbestos can be combined with binders, particularly in materials and products used in construction, as is the case, for example, in asbestos cement.

[0007] The ban on the use of asbestos, due to the health risks posed by this substance when fibers are released into the air, has led to the need to dispose of as waste asbestos products used in the past in building construction as well as in other manufacturing in industrial, railway, naval, and public works environments.

[0008] As an alternative to vitrification, which is very energy-intensive, and to the burial of asbestos-containing waste, which is environmentally unsatisfactory, it is possible to destroy asbestos fibers in mineral or organic acid solutions.

[0009] 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 can be used, alone or in mixtures with the 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.

[0010] 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.

[0011] Sulfuric acid then leads to the dissolution of magnesium ions contained in chrysotile, and the precipitation of calcium ions from the cement as insoluble calcium sulfate (plaster). The silica compounds from the asbestos and cement are found in the form of solid silica, as silicon dioxide is only slightly soluble in acid.

[0012] The solid materials thus produced are separated by filtration, possibly preceded by decantation, and can be used, for example in the manufacture of construction products.

[0013] The excess acid that did not react 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 concentration to treat another batch of asbestos-containing material. However, the acid bath gradually becomes enriched with dissolved materials (particularly magnesium sulfate), which slows the reaction rate. The acid solution must then be removed and replaced with a fresh one.

[0014] Among the dissolved substances, certain components such as iron and aluminum can be extracted by selective precipitation. This is achieved by neutralizing the acidity with a solution of sodium hydroxide or lime to reach 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 asbestos fibers have been destroyed and replacing it with fresh acid, results in a high consumption of both acid and neutralizing agent.

[0015] Documents WO8810234 and EP0348502, for example, propose such processes. They offer 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 can shorten the treatment time.

[0016] Description of the invention

[0017] The present invention aims to overcome these drawbacks by proposing a process for treating a solid material comprising asbestos fibers, comprising the following steps:

[0018] - grinding of solid material,

[0019] - contacting the crushed solid material with an aqueous acid solution, resulting in the dissolution of at least some of the asbestos fibers,

[0020] - Separation of the liquid and solid fractions of the products resulting from the dissolution. This process is unique in that it also includes the following steps:

[0021] - dialysis by diffusion of said liquid fraction, during which at least a portion of the H ions + and HsO + 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,

[0022] - use of said regenerated aqueous acid solution in the step of bringing the ground solid material into contact with an aqueous acid solution.

[0023] Thanks to these features, the aqueous acid consumption of the process according to the invention is particularly low, as the acid can be recycled and reused repeatedly, allowing a large quantity of material to be treated with a small amount of acid. This also enables a rapid treatment process, since the acid is not completely consumed during the dissolution reaction. This allows the reaction to be carried out with a significant excess of acid relative to the materials being 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.

[0024] 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.

[0025] The concentration of the acid in aqueous solution applied to the ground solid material can range from 2 to 6 N, effectively triggering the dissolution of asbestos fibers. The particle size of the ground solid material can range from 5 to 250 µm, providing a large surface area for contact with the acid, thus ensuring a rapid and efficient dissolution reaction.

[0026] During dissolution, the reagents can be maintained at a temperature between 40 and 100°C, which allows for rapid dissolution of the asbestos fibers in the acid.

[0027] A three-phase centrifuge can be used during the liquid and solid fraction separation stage, which is a robust and efficient embodiment of the invention.

[0028] 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.

[0029] During the diffusion dialysis stage, 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, thereby increasing the total surface area of ​​the membranes, and thus achieving faster and more efficient dialysis.

[0030] 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 can be carried out simultaneously, which makes it possible to obtain at least partially continuous solid material processing, and thus to process these materials more quickly.

[0031] 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 contacting the ground solid material with the aqueous acid solution, a solid / liquid separation module, and a dialysis module comprising at least one anion exchange membrane.

[0032] Thanks to these features, the aqueous acid consumption of the process according to the invention is particularly low, as the acid can be recycled and reused repeatedly, allowing a large quantity of material to be treated with a small amount of acid. This also enables a rapid treatment process, since the acid is not completely consumed during the dissolution reaction. This allows the reaction to be carried out with a significant excess of acid relative to the materials being 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.

[0033] Brief description of the drawings

[0034] 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:

[0035] [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,

[0036] [Fig 2] Fig. 2 is a schematic cross-sectional view of a dialysis module according to a preferred embodiment of the invention.

[0037] Description of the implementation methods

[0038] The process according to the invention, illustrated in Figure 1, makes it possible to treat a solid material containing asbestos fibers, such as asbestos cement, asbestos plaster, tile adhesives, flocking, insulating braids and fabrics, asbestos-containing cardboard ceiling panels, compressed asbestos pipe joints, insulating materials used as pipe insulation, etc. The asbestos mass content of this material varies. In some cases, the asbestos content may be very low (less than 1% in certain 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.

[0039] The asbestos dealt with in the context of the present invention is, for example, chrysotile asbestos (magnesium-rich hydrated silicate), which can be associated with cement in asbestos-cement products.

[0040] This material is, for example, construction waste, resulting from the demolition of a building or a road.

[0041] 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 a ball mill or a knife mill.

[0042] The ground solid material is then brought into contact with an aqueous solution of acid, in reactor 2, which causes the dissolution of at least some of the asbestos fibers.

[0043] Reactor 2 preferably includes stirring mechanisms to accelerate dissolution by increasing the exchange between the ground solid material and the aqueous acid solution. To increase the reaction kinetics of magnesium dissolution and asbestos fiber breakdown, and to make it compatible with industrial-scale process implementation, it is preferable to:

[0044] - Finely grind the asbestos-cement products to increase the contact surface between the acid solution and the asbestos fibers,

[0045] - Increase the temperature of the acid into which the ground product is introduced, without necessarily exceeding approximately 100°C for safety reasons.

[0046] - maintain a sufficient acid concentration throughout the reaction,

[0047] - use a sufficient quantity of acid relative to the quantity of solid materials ground, and

[0048] - limit the concentration of dissolved materials resulting from the dissolution of asbestos-containing materials.

[0049] Another factor to consider is the asbestos content of the solid material.

[0050] 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), scanning electron microscopy (SEM), and transmission electron microscopy with X-ray energy scatter analysis (TEA).

[0051] 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 of 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 building material. However, the present invention can be implemented with any Brønsted acid, or a mixture of Brønsted acids, such as, for example, HCl, HNO₃, H₃PO₄, or HF.

[0052] The concentration of the acid in aqueous solution, at the stage of contact with 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.

[0053] During the dissolution reaction, the reactants—that is, the aqueous acid solution and the ground solid material—can be subjected to a temperature that increases the kinetics of the dissolution reaction. This temperature is, for example, between 40 and 100°C. To achieve this, reactor 2 can incorporate heating and / or thermal insulation means for its internal volume.

[0054] After the dissolution reaction, the products of this reaction undergo solid / liquid separation in a solid / liquid separation module 3. Various methods known to those skilled in the art can be used. For example, a three-phase centrifuge can be used.

[0055] The solid fraction 4 resulting from the dissolution process, in the case of treating asbestos-cement solid material with H2SO4, consists mainly of gypsum and silica. These materials can be recovered for reuse in construction materials.

[0056] 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 includes 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 permeable to other cations, particularly polyvalent ones such as Mg ++ , That ++ Al +++ Fe ++ Fe +++ , Neither ++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. On the contrary, 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.

[0057] To achieve this, membrane 7 is preferably a thin membrane composed of a cross-linked organic polymer or copolymer.

[0058] 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 alternately supplied with water 8, and with a portion of the liquid fraction 5.

[0059] The regenerated acid solution, or diffusate 9, after possible concentration adjustment, can be reused in the asbestos fiber dissolution process. The invention thus reduces the amount of acid consumed in the asbestos treatment process, as the acid can be regenerated virtually indefinitely to treat large quantities of asbestos. It is also possible, for asbestos dissolution, to use larger quantities of acid proportional to the amount of solid material, which increases the reaction kinetics and allows for faster asbestos processing.

[0060] On the other side of the membranes 7, the crude acid solution exiting the dialysis process, called dialysate 10, depleted of acid, contains the dissolved ions resulting from the contact of the asbestos-containing materials with the acid solution. This solution can be 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 Figure 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 over existing processes, in which, without an acidity recovery step, it is necessary to neutralize the residual acidity of the liquid fraction from dissolution using basic reagents before the dissolved ions can be precipitated. The dialysis step is preferably preceded by pretreatment of the crude acid by microfiltration to prevent fouling of the intermembrane spaces through which the dialysate circulates. If there is a risk of dissolved organic substances in the crude acid, particularly from the wetting of asbestos-containing materials during their removal, an activated carbon filter can be installed upstream of the microfilter to prevent clogging of the ion exchange membranes.

[0061] 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 the 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 can 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.

[0062] In 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 to compensate for acid losses that could not be recovered during the dialysis step. This secondary supply can be achieved through a constant flow or a point-by-point supply, controlled, for example, by pH measurements.

[0063] 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 including an anion exchange membrane 7. Preferably, the plant also includes means for transferring materials between the successive elements, so as to allow the process to be implemented 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 materials 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.

[0064] 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 of solid material, - contacting the crushed solid material with an aqueous acid solution, resulting in the dissolution of at least some of the asbestos fibers, - separation of the liquid (5) and solid (4) fractions of the products resulting from the dissolution, characterized in that said process further comprises the following steps: - dialysis by diffusion of said liquid fraction (5), during which at least a part of the H ions + and H3O 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 bringing the ground solid material into contact 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. Processing method 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 N.

4. 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 pm.

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 separation module (3) solid / liquid, and a dialysis module (6) comprising at least one anion exchange membrane (7).

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