FR2894166A1 - PROCESS FOR MODIFICATION OF SILICIC CRYSTALLINE MATERIALS - Google Patents

Abstract

The invention relates to a method for modifying silicic crystalline materials, comprising the steps of: i) contacting the crystalline silicic material with an aqueous solution of alkali metal silicate; andii) evaporating the water. It also relates to the composite obtainable by this process as well as the use of an alkali metal silicate solution for the inerting of crystalline silicic materials, especially asbestos.

Description

The present invention relates to a process for modifying materials

  crystalline silica, especially asbestos, and a composite material thus obtained. Silicone crystalline materials such as asbestos have been used extensively as flame retardant and insulating material, especially in the building. However, exposure to asbestos fibers and dust is dangerous for human health. The harmful properties of asbestos are related in particular to its fibrous structure and its crystalline character. Inhalation of asbestos particles can cause asbestosis, a disease of the lungs. This disease can be transformed, after a latency period of thirty years or more, into various cancers, in particular lung cancer and mesothelioma, a form of inoperable cancer of the chest and the abdominal wall. These risks led to the prohibition of the use of asbestos. It has also appeared necessary to proceed to the asbestos removal of public buildings and / or residential buildings. Now asbestos removal, carried out mechanically, is accompanied by a production of asbestos dusts important, requiring adequate protection of workers, but also, according to the applicable regulations, the inerting of asbestos products resulting from this operation, before being landfilled. Various methods for inerting asbestos are known. It is thus possible to heat treat asbestos to destroy its crystalline character. However, because of the refractory nature of asbestos, these heat treatments must be conducted at high temperatures, of the order of 1500 C. These processes therefore have a significant cost because of their energy consumption. Chemical inerting processes for asbestos have also been proposed.

  It is thus known from US Pat. No. 5,516,973 to attack asbestos with fluorides or hydrofluoric acid. These reagents dissolve the cationic components of asbestos, in particular magnesium oxides, as well as the silicic skeleton, forming hydrosoluble fluoro-silicates. However, the reagents used in this process require very expensive specific equipment. Finally, it has been proposed, in particular in the application WO 9203235, to coat the asbestos in a hydraulic binder. However, this method has the disadvantage of resulting in waste of considerable weight and volume which must then be buried. The object of the present invention is to propose a process for modifying silicic crystalline materials and in particular asbestos that does not have the drawbacks of the prior art, and in particular is inexpensive and results in waste that is not bulky. This object is achieved according to the invention by bringing the crystalline silica material into contact with a compound having a very strong physicochemical affinity for the structure of this material, which has the consequence of reducing the dangerous crystalline nature of these substances. materials. A stable amorphous / vitreous composite is then obtained which exhibits a notable mechanical strength, thus preventing the material from being crumbled. More specifically, the invention is directed, in a first aspect, to a process for modifying silicic crystalline materials, comprising the steps of: i) contacting the crystalline silicic material with an aqueous solution of alkali metal silicate; and ii) evaporate the water. By silicic crystalline material is meant a material having a substantially crystalline and not amorphous structure and composed at least in part of silicon oxide tetrahedra.

  Particularly targeted fibrous silicic crystalline materials include asbestos. Indeed, the method described is intended particularly for the treatment of asbestos fibers from asbestos removal operation, before the controlled landfill. Asbestos is composed of an agglomerate of extremely fine elementary fibrils with particular physical and chemical properties: resistance to high temperatures, incombustibility, high mechanical tensile strength, resistance to chemical and microorganisms, high electrical resistance , flexibility, easy to be spun and woven. Table 1: The different varieties and species of asbestos (From Kirk ûOthmer / Encyclopedia of Chemical Technology (Vol 3) 3rd Edition) Species N of Variety Chemical Composition Registry CAS Chrysotile * 12007-29-5 Serpentine 3MgO.2SiO2.2H2O Anthophyllite 17068-78-9 Amphibole 7MgO.8SiO2 • H2O Amosite * 12172-73-5 Amphibole 11 FeO.3MgO.16SiO2. 2H2O actinolite 13768-00-8 amphibole 2CaO.4MgO.FeO.8SiO2.H2O tremolite 14567-73-8 amphibole 2CaO.5MgO.8SiO2 • H2O Crocidolite * 12001-28-4 amphibole Na2O-Fe2O3.3FeO.8SiO2.H2O 15 * Asbestos species of commercial importance

  There are six varieties of asbestos, which can be grouped into two large families of mineral silicates according to their crystalline structure: amphiboles and serpentines. Chrysosite or white asbestos, class 20 serpentines, accounts for about 95% of global asbestos production. The risks represented by chrysolites, which some claim to be safe compared to other asbestos varieties, are controversial. Amphiboles comprise five minerals, two of which are used industrially: amosite or brown asbestos and crocidolite or blue asbestos.

  Apart from asbestos application, however, the process is also useful for inerting other hazardous silicic crystalline materials. Such materials are, for example, fine sands and all the divided forms of crystalline silica such as quartz, but especially cristobalite and tridymite which have fibrillated textures. In fact, these finely divided products, having a particle size of 0.5 to 100 microns, can, like asbestos, cause diseases such as silicosis when they are inhaled and therefore present in the same way a danger to human health. . The proposed process comprises as an essential step the contacting of the silicic crystalline material to be treated with an aqueous solution of alkali silicate. The solutions of alkali silicates have very particular properties which have earned these solutions the name of soluble glass D. These solutions actually contain silicic oligomers made ionic and mobile thanks to the alkali metal, for example the sodium present (Si bond -ONa breaking the silicic network) and also hydroxyls resulting from hydrolysis with water (Si-OH bonds). Dehydration at temperatures between 100 and 200 ° C results in a return to a substantially insoluble glassy solid state if dehydration is increased. Solutions of alkali silicates, especially sodium silicates, have a very strong physical and chemical affinity for surfaces and solid silicate interfaces, such as clays, natural or synthetic zeolites, precipitated silica or crystalline silica, kaolin. Silicate products, which are known as silicates, are among the products for which silicates in solution possess this very strong physicochemical affinity. This high affinity can be troublesome, for example in laundry formulations where it is desired to formulate together sodium silicate and synthetic ion exchange zeolites 4A. Indeed, it is observed that if one mixes and dries together, without precaution, silicate and a crystalline zeolite, one obtains an amalgam of the two products, in which the zeolite has lost its ion exchange properties, and this almost irreversibly . By contrast, in the context of the inerting of crystalline silicic materials such as asbestos, this property is very interesting and can be used to obtain, under mild conditions, a stable composite of crystalline material and alkali silicate. Preferably, the alkali silicate is a sodium or potassium silicate. The molar SiO 2 / Me 2 O ratio characterizing the alkali silicate (Me = alkali metal) is generally between 1 and 4, preferably between 2 and 3.5. In principle, it is preferred that the alkali silicate solution is concentrated. This limits the amount of water to be evaporated in step ii). By the term "concentrated solution" is meant a solution of a concentration of at least 20%, preferably 30 to 60%, and more particularly 30 to 50% alkali silicate solids. It may be advantageous for cation-rich materials, especially magnesium or iron, to add a cation sequestering / complexing agent to the silicate solution. This additive contributes to the destructuring of the crystal by trapping the cations inserted in the intervals of the crystalline structure. According to a preferred embodiment, the aqueous solution of alkali metal silicate 30 also comprises an additive complexing the ions, in particular magnesium or iron. Such additives can be chosen from amino phosphonated organic complexing agents, and more particularly from sodium ethylene diaminotetraphaphonate, sodium diethylenetriaminopentaphosphonate known under the trade names Briquest 423 and Briquest 543 (Rhodia). The amount of additive is not particularly limited and depends in particular on the amount of ions present in the material to be treated. Generally, the alkali silicate solution may contain 0 to 20%, preferably 0.1 to 10% by weight of amino phosphonate organic complex, itself in the form of an aqueous solution of concentration of between 25 and 50%. Step i) can be carried out by any means known in the art allowing complete wetting of the material with the solution. A simple way is to spray the liquid composition onto the crystalline material. Alternatively, the material to be treated may be contacted by immersion or soaking in the alkali silicate solution. The amount of inerting solution used is not critical, as long as it is sufficient to completely coat the material to be treated. As an indication, 25 to 150%, preferably 30 to 100% by weight of alkali silicate solution, expressed as dry product, relative to the weight of material to be treated can be added. The contact time of the material to be treated with the inerting solution is chosen so as to ensure good impregnation of the material to be treated. Generally, a few minutes are sufficient. Step ii) can be carried out by drying at a moderate temperature of between 20 and 110 ° C., possibly under the effect of ventilation. Preferably, it is carried out at ambient temperature, without any particular energy supply. The advantage of this process over existing systems is that: - the reagents are aqueous, weakly alkaline, not dangerous for the user and the environment; it can be used at room temperature; it is generally inexpensive; and it does not generate by-products to process. The process according to the invention can be applied to the inerting of crystalline silicic materials such as asbestos in various ways. Advantageously, it allows an asbestos inerting in place, and thus avoid the deflocating operation, generating dust. Alternatively, asbestos treated in situ according to the described method can be subsequently removed, but under better conditions, practically dust free. Finally, of course, the treatment can be applied to deflated asbestos in an appropriate processing unit prior to discharge. Surprisingly, the composite obtained after evaporation of water is water-resistant and has a mechanical strength substantially greater than the initial crystalline silica material. In particular, it has a high rigidity, is solid and free of dust or very fine microparticles. Thus, the invention therefore aims, in a third aspect, a composite of crystalline material and silicate obtainable by the method according to the preceding claims. Finally, according to a last aspect, the invention relates to the use of an alkali metal silicate solution for the inerting of crystalline silicic materials, such as in particular asbestos. The invention will be described in more detail in the nonlimiting examples below, and with regard to the single figure which shows: FIG. single: a photograph of an untreated asbestos sample (left) and an asbestos sample inert with sodium silicate according to Example 2 (right). EXAMPLE 1 A piece of 3.5 g of asbestos braid used as insulation was taken. It is noted at the time of cutting that a very visible amount of fine fibrous particles is produced. This test piece is immersed in a silicate solution having a molar ratio Rm SiO 2 / Na 2 O of 3.4, containing 35% by weight of dry matter. After 5 minutes of contact time, the piece of braid impregnated with silicate is deposited on a metal grid to be drained. After draining, the piece of wet silicate braid is weighed and a mass of 8.6 g is noted. Asbestos in braided form therefore absorbed 5.1 g of silicate solution. The specimen moistened with silicate is then left for 6 hours in the open air, at approximately 25 ° C., and then weighed again. There is a mass of about 5.7g. Asbestos therefore absorbed 2.2 g of partially dehydrated silicate. The rate of dry silicate / asbestos treatment is therefore 63%. The composite obtained is in a stiffened, solid form, free of all forms of dust or very fine microparticles. Microscopic examination confirms the disappearance of fibrillated forms of asbestos.

  EXAMPLE 2 The procedure is the same as in Example 1 except that the silicate-impregnated sample is dried for 2 hours at 120 ° C. The initial asbestos braiding mass is 2.6 g. The drained impregnated braid weighs 4.7g. Asbestos therefore absorbed 2.1 g of silicate solution. The braid after impregnation and drying weighs 3.5g. The rate of dry silicate / asbestos treatment is therefore 35%.

  It should be noted that the drying operation at 120 ° C. made it possible to dehydrate the silicate solution impregnated on asbestos almost completely. The material obtained has an amorphous amalgamated appearance, and is extremely rigid and resistant to breakage. As we see in the single figure, it shows no visible fibril.

  EXAMPLE 3 The procedure is the same as in Example 1 and 2, but replacing the silicate of Rm 3.4 with a silicate of Rm SiO2 / Na2O = 2.

  Two composite test pieces are thus obtained after drying for 6 hours at ambient temperature and drying for 2 hours at 120 ° C., respectively. The aspects of these two specimens are very close to those of Examples 1 and 2. In particular, they do not have apparent fibrils.

  EXAMPLE 4 Example 1 is repeated, but replacing the treatment solution with a mixture composed of 90% by weight of silicate solution Rm 3.4 at 35% solids and 10% of sodium dimethyl triaminopentaphosphonate. 40% solution (Briquest 543 from Rhodia). Drying is conducted at room temperature for 3 hours. The final treatment rate is 40%, expressed in silicate solution + dry Briquest relative to the mass of asbestos.

  The resulting composite is mechanically strong, rigid, and without apparent fibril. 15

Claims (10)

  A method of modifying silicic crystalline materials, comprising the steps of: i) contacting the crystalline silicic material with an aqueous solution of alkali metal silicate; and ii) evaporate the water. io   2. The process according to claim 1, wherein the crystalline silicic material is fibrous.   3. The process of claim 1 or 2, wherein the crystalline silicic material is asbestos.   4. The method of claim 1 or 2, wherein the crystalline silica material is fine sand.   5. Process according to one of claims 1 to 4, wherein the alkali metal silicate is sodium silicate.   6. Method according to one of claims 1 to 5, wherein step i) is carried out by spraying or dipping. 25   7. Method according to one of claims 1 to 6, wherein the aqueous solution of alkali metal silicate further comprises an additive complexing the ions.   8. Process according to one of claims 1 to 7, wherein step ii) is carried out at a temperature of 20 to 110 ° C.   9. Composite of crystalline material and silicate obtainable by the process according to the preceding claims.   10. Use of an alkali metal silicate solution for the inking of crystalline silicic materials.