Asbestos waste destruction and valorization method

Abstract

A method for destructing and valorizing an asbestos waste including the steps of: determining the asbestos mineralogical group contained in the waste, performing a treatment on the waste which includes of: an acid treatment when the waste comprises only a chrysotile, a base treatment when the waste comprises only an amphibole, the acid treatment followed by the base treatment when the asbestos waste includes a mixture of a chrysotile and an amphibole, and valorizing at least one of the products obtained on completion of the performing of the treatment. An embodiment also concerns a treatment of a chrysotile waste through an acid treatment followed by a thermal treatment.

Claims

1. A method for destructing and valorizing an asbestos waste, comprising: a) determining asbestos mineralogical group(s) contained in said waste, said group comprising a chrysotile and an amphibole, b) performing at least one treatment on said asbestos waste, said treatment being: an acid treatment when the asbestos waste comprises only a chrysotile, said acid treatment comprising in the immersion of the asbestos waste in a strong acid solution, at a temperature of at most 100° C., so as to obtain an acid solution and a solid comprising a mesoporous silica, a base treatment when the asbestos waste comprises only an amphibole, said base treatment consisting in the immersion of the asbestos waste in a solution of a strong base in a hermetically sealed medium so as to obtain a base solution containing dissolved silica, said acid treatment followed by said base treatment when the asbestos waste comprises a mixture of a chrysotile and an amphibole, so as to obtain after said acid treatment an acid solution and a solid mixture containing a mesoporous silica and the unaltered amphibole, said solid mixture being separated from the acid solution to be subjected to said base treatment so as to obtain a base solution containing dissolved silica, and c) valorizing at least one of the products obtained on completion of step b) of the treatment.

2. The destruction and valorization method according to claim 1, wherein the asbestos waste is an asbestos cement waste or a gypsum-based asbestos flocking.

3. The destruction and valorization method according to claim 1, wherein the acid treatment when the asbestos waste comprises only a chrysotile is followed by a thermal treatment comprising a heating of the mesoporous silica at a temperature of at least 600° C.

4. The destruction and valorization method according to claim 1, wherein when an acid treatment has been performed, step c) comprises using the mesoporous silica for the entrapment or the filtering of molecules and/or as a silicon precursor for the synthesis of a zeolite.

5. The destruction and valorization method according to claim 4, wherein when the acid solution is a nitric acid solution, the mesoporous silica is used to synthesize a nitrate-cancrinite type zeolite of formula Na8[Al6Si6O24](NO3)2-4H2O.

6. The destruction and valorization method according to claim 1, wherein when an acid treatment has been performed, step c) comprises selectively extracting or isolating ions present in the acid solution obtained on completion of the acid treatment.

7. The destruction and valorization method according to claim 1, wherein when a base treatment has been performed, step c) comprises in using the base solution obtained on completion of the base treatment for the production of a hydrated calcium silicate type material and/or for the synthesis of a zeolite.

8. The destruction and valorization method according to claim 1, wherein the acid solution contains at least one strong monoacid selected from nitric acid and hydrochloric acid.

9. The destruction and valorization method according to claim 1, wherein the strong base of the base solution is selected from soda and potash.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood using the following detailed description, with reference to the appended drawing representing, as a non-limiting example, the experimental results obtained from three asbestos wastes and a chrysotile waste:

(2) FIG. 1 represents the X-ray diffractograms of the three asbestos wastes;

(3) FIG. 2 represents the X-ray diffractograms of the solids collected on completion of the acid treatment of the three asbestos wastes;

(4) FIG. 3 represents the X-ray diffractogram of the nitrate-cancrinite obtained from one of the asbestos wastes;

(5) FIG. 4 represents the X-ray diffractograms of a chrysotile waste before and after the acid treatment;

(6) FIG. 5 represents the thermogravimetric analysis of the hydrated mesoporous silica obtained after the acid treatment;

(7) FIG. 6 represents the X-ray diffractograms of the mesoporous silica after thermal treatment at different temperatures.

DETAILED DESCRIPTION

(8) I—Experiments on Asbestos Cement Wastes:

(9) In a 1.sup.st series of experiments, the destruction and valorization method according to the invention that has been described hereinabove has been implemented on the following three asbestos cement wastes: the 1.sup.st asbestos cement waste was a roof tile; the 2.sup.nd asbestos cement waste was a hose gasket; the 3.sup.rd asbestos waste was a flocking sample comprising a mixture of gypsum, asbestos and fibers of alkaline earth silicates.

(10) First of all, in order to determine the mineralogical group(s) contained therein, these three wastes have been analyzed by X-ray diffractometry with a diffractometer commercialized by the company BRUCKER under the commercial name «D2 PHASER» using the radiation of the Kα spectral line of copper (λ=1.54 Å) after filtering by a nickel filter. The measurement step was 0.014° in 2θ.

(11) FIG. 1 represents the diffractogram of the 1.sup.st, 2.sup.nd and 3.sup.rd wastes.

(12) The upper diffractogram is that of the 1.sup.st waste, the intermediate diffractogram is that of the 2.sup.nd waste and the lower difractogram is that of the 3.sup.rd waste.

(13) In these three diffractograms, the characteristic diffraction peaks of the chrysotile, crocidolite, calcium carbonate (CaCO.sub.3), the cement matrix (ettringite of formula Ca.sub.6Al.sub.2(SO.sub.4).sub.3(OH).sub.12,26H.sub.2O) and of gypsum are respectively signaled by the annotations: «Chry», «Cro», «CC», «E» and «G».

(14) As regards the presence of asbestos, the diffractogram of the 1.sup.st waste has only the characteristic peaks of the chrysotile (in particular at 12.05° and at 24.30° in 2θ), the diffractogram of the 2.sup.nd waste has the characteristic peaks of the chrysotile and crocidolite (in particular at 28.76°) and the diffractogram of the 3.sup.rd waste has only the peaks of the chrysotile.

(15) The three wastes have also been analyzed by scanning electron microscopy using an electronic microscope of the type FEI Quanta 200 FEG equipped with a vacuum secondary electrons detector. The acceleration voltage was 15 kV. This analysis has been coupled with an energy dispersive analysis (EDS) for the chemical identification of the different phases.

(16) These analyses have allowed determining that: the 1.sup.st waste (roof tile) contained only the chrysotile; the 2.sup.nd waste (hose gasket) contained a mixture of the chrysotile and an amphibole: crocidolite; the 3.sup.rd waste contained the chrysotile and fibers of amorphous, non-crystallized alkaline earth silicates.

(17) Because the three wastes contained a chrysotile, they have been subjected to an acid treatment which consisted in immersing them in a 1 L reactor which contained a nitric acid solution at a concentration of 4 mol/L and in maintaining the reactive medium thus obtained under stirring at a temperature of 80° C. for a 7 day time period.

(18) The treatment in the acid medium has allowed reducing by 90% the mass of the three asbestos cement wastes.

(19) A chemical analysis by X-ray fluorescence spectrometry of the solid and of the acid solution collected after the acid treatment has allowed establishing the material balance which is detailed in Table 1 hereinbelow:

(20) TABLE-US-00001 TABLE 1 Material balance: evolution of the chemical compositions before and after treatment in the acid medium Samples % Mg % Al % Si % Ca % Fe 1.sup.st waste 15 8 34 37 5 Acid solution 2 22 0 65 12 Remaining solid 0 0 >98 0 0 2.sup.nd waste 10 6 25 52 8 Acid solution 1 18 0 68 13 Remaining solid 1 0 93 0 6 3.sup.rd waste 4 7 15 69 5 Acid solution 7 10 0 79 4 Remaining solid 0 0 >99 0 0

(21) The chemical analysis by X-ray fluorescence spectrometry has been carried out using a spectrophotometer commercialized by the company PANalytical under the commercial name «Epsilon 3.sup.X» and which was equipped with a silver tube (under 30 kV and 3 mA) and different filters (Ag, Al and Ti).

(22) With regards to the detailed results in Table 1, it is observed that the acid solutions on completion of the acid treatment contain calcium, iron, magnesium and aluminum. The cement matrix has been dissolved. From the waste before the treatment, silicon is the only element that has not been dissolved. It is present in the remaining solid on completion of the acid treatment.

(23) In addition, the chemical analysis of the 1.sup.st waste (roof tile) after the acid treatment shows that the cement matrix has been dissolved and that the chrysotile has been transformed.

(24) As regards the 2.sup.nd waste (hose gasket), the chemical composition of the solution after the acid treatment is similar to that obtained after this treatment on the 1.sup.st waste, but in the solid, iron and magnesium remain indicating that the amphibole (crocidolite) has not been dissolved by the acid treatment.

(25) As regards the 3.sup.rd waste (flocking), the chemical composition of the remaining solid shows that the gypsum matrix, as well as the fibers of amorphous alkaline earth silicates have been dissolved and that the chrysotile has been transformed into pure silica.

(26) The solids of the three wastes collected on completion of the acid treatment have been subjected to an X-ray diffractometry analysis.

(27) FIG. 2 represents the diffractograms of the solids collected on completion of the acid treatment of the 1.sup.st 2.sup.nd waste and 3.sup.rd waste.

(28) The upper diffractogram is that of the solid of the 1.sup.st waste, the intermediate diffractogram is that of the solid of the 2.sup.nd waste and the lower diffractogram is that of the solid of the 3.sup.rd waste.

(29) In the 2.sup.nd diffractogram, the characteristic diffraction peaks of crocidolite are signaled by the annotation: «Cro».

(30) With regards to the diffractograms represented in FIG. 2, there is observed: the absence of the characteristic peak of the chrysotile in the three diffractograms of the solids derived from the three wastes; the presence of the characteristic peaks of crocidolite in the diffractogram of the solid of the 2.sup.nd waste.

(31) This shows that the acid treatment eliminates the chrysotile but not the amphibole in an asbestos waste.

(32) The solution resulting from the acid treatment mainly contains the Fe.sup.3+, Mg.sup.2+, Al.sup.3+ and Ca.sup.2+ ions. These ions can be selectively precipitated by addition of soda (NaOH) or potash (KOH) according to successive precipitation/filtering sequences at the following pH values: 1.2±0.3; 3±0.3; 10.4±0.3 and 12.4±0.3 to respectively collect the Fe.sup.3+, Al.sup.3+, Mg.sup.2+ and Ca.sup.2+ ions in the form of hydroxides Fe(OH).sub.3, Al(OH).sub.3, Mg(OH).sub.2 and Ca(OH).sub.2. After having collected the aforementioned ions, the final solution contains either sodium nitrate (NaNO.sub.3) in the case of a neutralization with soda (this nitrate may be used for the synthesis of nitrate-cancrinite), or potassium nitrate (KNO.sub.3) in the case of a neutralization with potash (this nitrate may be used in agriculture).

(33) As explained hereinabove, the structure of the chrysotile has been destructed by dissolution of the brucite layer, Mg(OH).sub.2. The dissolution of the brucite layer is due to the structure of the chrysotile which makes this layer accessible to an acid solvent. However, the acid treatment does not destroy the amphibole because of its structure: the octahedron soluble layer MO.sub.6 is confined between two layers of SiO.sub.2 which are insoluble in an acid solution.

(34) A base treatment consisting in the immersion of the solid of the 2.sup.nd waste (collected on completion of the acid treatment) in a NaOH solution at a concentration of 10 mol/L and heated to a temperature of 180° C. in a 50 mL autoclave has been performed for a 5 day time period.

(35) Furthermore, this same base treatment has been applied to samples of pure crocidolite and amosite.

(36) All the solids subjected to this base treatment have been completely dissolved and base solutions have been obtained which have been analyzed by X-ray fluorescence spectrometry.

(37) Table 2 details the chemical analyses by X-ray fluorescence spectrometry of the residual solutions after the base treatment.

(38) TABLE-US-00002 TABLE 2 X-ray fluorescence spectrometry analyses of the solutions after the base treatment Samples Na Mg Si Fe Crocidolite solution 82% 1% 15% 1% Amosite solution 83% 0% 16% 1% 2.sup.nd waste solution 81% 0% 18% 1%

(39) With regards to Table 2, it is observed that the composition of the solutions for dissolving the pure amphiboles (crocidolite and amosite) is similar to the composition of the solution obtained after the dissolution of the solid of the 2.sup.nd waste thus showing the total destruction thereof. Silicon is present in the base solutions obtained on completion of the base treatment. These results prove that the base treatment completely destroys the asbestos of the amphiboles group.

(40) Moreover, the mesoporous silica obtained on completion of the acid treatment applied on the 1.sup.st waste (that is to say the tile) has been collected by filtering and used for the synthesis of a nitrate-cancrinite type zeolite.

(41) This zeolite has been synthesized in the following manner:

(42) The following precursors have been provided: silicon precursor: 3.09 g of the mesoporous silica obtained on completion of the acid treatment on the 1.sup.st waste; aluminum precursor: 1.64 g of Al.sub.2O.sub.3.Na.sub.2O; sodium nitrate precursor: 29.01 g of NaNO.sub.3; 4.36 g of NaOH.

(43) In a 1.sup.st beaker filled with 35 mL of distilled water, NaOH, the mesoporous silica and then after a few minutes NaNO.sub.3, have been added in the that order.

(44) In a 2.sup.nd beaker, Al.sub.2O.sub.3.Na.sub.2O has been dissolved in 5 mL of distilled water.

(45) The two mixtures have been added in a 3.sup.rd vessel. An instantaneous gelation occurs. The vessel has been closed and stirred very strongly.

(46) Afterwards, this vessel is placed in the oven at a temperature of 90° C. for one day. Then filtering with a cellulose-based filter-paper and washing with distilled water have been performed. The product thus obtained, which was a nitrate-cancrinite type zeolite, has been dried.

(47) Indeed, its X-ray diffractogram is represented in FIG. 3, as well as that of the structural model of the nitrate-cancrinite (that is to say the «theoretical» or also so-called «calculated» diffractogram). It is observed that the two diffractograms almost superimpose one another. This is confirmed by the fact that we have an X.sup.2 factor equal to 5.3 which is an excellent value in crystallography. Thus, FIG. 3 shows the excellent purity of the product thus synthesized with a very good compliance with the structural model.

(48) II—Experiments for Treating a Chrysotile Waste:

(49) In a 2.sup.nd series of experiments, a pure chrysotile waste has been treated by firstly subjecting it to an acid treatment by immersion in a nitric acid solution with a concentration of 4 mol/L for 7 days at 80° C.

(50) FIG. 4 represents the X-ray diffractograms: of the chrysotile waste before the acid treatment (upper diffractogram); of the chrysotile waste after the acid treatment (lower diffractogram).

(51) In the upper diffractogram, there are observed the characteristic diffraction peaks of the chrysotile which are absent in the lower diffractogram which features a large characteristic diffusion peak of an amorphous compound.

(52) Thus, on completion of the acid treatment on the chrysotile, a mesoporous silica is obtained which is an amorphous (that is to say non-crystallized) silica. The crystalline structure of the chrysotile has disappeared on completion of this treatment.

(53) A porosity analysis has been performed on the silica thus obtained: specific surface area: 455 m.sup.2/g; pore volume: 0.37 cm.sup.3/g; pore diameter: 3.2 nm.

(54) Afterwards, the mesoporous silica has been separated from the acid solution by filtering and subjected to a thermal treatment.

(55) The thermal treatment has consisting in submitting the mesoporous silica to the following temperatures: 200° C., 600° C., 700° C. and 800° C.

(56) A thermogravimetric analysis has been performed with an equipment of the type Netzsch STA449F from 25° C. to 1400° C. with a heating rate of 5° C./min under an argon stream.

(57) FIG. 5 represents the record of the thermogravimetric analysis. There is observed an abrupt loss of mass up to 100° C. which corresponds to the elimination of «free» water. The loss of mass between 100° C. and 600° C. corresponds to the loss of water «bonded» to the siliceous framework of the nanotubes. This loss of water is accompanied by a progressive rearrangement of the SiO.sub.4 tetrahedrons therebetween. This rearrangement causes a crystallization of the walls of the nanotubes as of 700° C. and a loss of the porosity of the walls by their condensation.

(58) FIG. 6 represents the X-ray diffractograms of the mesoporous silica after the thermal treatment at: 600° C. (lower diffractogram); 700° C. (intermediate diffractogram); 800° C. (upper diffractogram).

(59) It is observed that at 600° C., the silica is still amorphous (there is no diffraction peak). As of 700° C., crystallization peaks of a cristobalite-type crystallized phase of the silica start appearing.

(60) Table 3 below details the specific surface area, the size of the pores and the pore volume of the mesoporous silica before the thermal treatment, then after the thermal treatment at 200° C., 600° C. and 800° C. Each of the thermal treatments has been performed in a furnace commercialized by the company CARBOLITE GERO under the commercial name «CWF», and that for 10 hours.

(61) TABLE-US-00003 TABLE 3 Specific surface area, size of the pores and pore volume of the silica before and after the thermal treatment Specific Size of the surface area pores Pore volume (m.sup.2/g) (nm) (cm.sup.3/g) After the acid treatment 455 3.23 0.37 After the thermal treatment 418 3.5 0.37 at 200° C. After the thermal treatment 208 5.2 0.27 at 600° C. After the thermal treatment 174 5.3 0.21 at 800° C.

(62) With regards to Table 3, it is observed that the volume of the pores and the specific surface area decrease while the diameter of the pores increases with the treatment temperature.

(63) These results show that the walls of the silica nanotubes are rebuilt by condensing as explained hereinabove. Indeed, during the acid treatment, the brucite layers are eliminated; which leaves large spaces between the silica sheets which constitute the wall of the tubes. The tubes have a 3.2 nm diameter but with barely structured walls and therefore a considerable specific surface area (surface developed by non-contiguous sheets) and a large pore volume. When the temperature is increased, these sheets reconnect together with the formation of SiO.sub.4 tetrahedrons; which results in a decrease in the specific surface area, the total volume also decreases and the center of the tube (diameter of the tube) increases.