FR2900076A1 - Decontaminating polluted soil, specifically to remove hydrocarbons, by mixing with carbon granules to absorb pollutant, treating with water and separating soil and granules in decanter - Google Patents

Abstract

Decontamination of polluted soil (T) involves (a) mixing the soil particles with carbon granules (C) in a reactor (20), with residence time sufficient to transfer pollutants to the granules by solid-solid contact and adsorption, (b) treating with water to initiate separation of decontaminated soil and polluted carbon and (c) completing separation in a decanter (30), where floating polluted granules and decontaminated soil are recovered from the surface and base respectively.

Description

The present invention relates to the decontamination of polluted soil, and

  more particularly the decontamination of lands polluted by hydrocarbons (gasoline, fuel oil, diesel, kerosene ...).

  BACKGROUND OF THE INVENTION There are already numerous techniques for the de-contamination of polluted soils, and mention may be made of so-called "off-site" treatments with a burial or incineration at an approved center outside the site, or inerting, or physicochemical washing in a fixed installation. Such processing techniques are useful for achieving complete elimination of pollutants of all kinds, but their cost is generally prohibitive. Mention may also be made of so-called "in situ" treatments, for example skimming with selective pumping of the floating products above the sheet, or forced ex-traction of the vapors, or in situ biodegradation with accelerated consumption. of the pollutant by bacteria in the unsaturated zone. There are also leaching, inerting, stabilization or thermal desorption treatments with hot water. In this regard, reference may be made to US-A-5,741,761, US-A-5,664,911 and US-A-5,076,727 for in-situ decontamination treatments, and US-A-5,482,402. and EP-A-0 597 154 for essentially thermal in-situ treatments. There are also treatments that are essentially chemical or electrochemical treatments. For example, it is possible to refer to the documents WO-A-2004/032624, WO-A-95/21034, WO-A-93/24249, US-A-5,008,019, US-A-4,761,221, US Pat. A-4 167 973, and FR-A-2 691 949. The technological background of the invention can also be completed by mentioning thermal decontamination techniques using a vibrated bed, with the disadvantage of a very difficult treatment. hard polluted soils 2 which are practically incinerated and therefore unusable. Finally, biological treatments using bacteria, which treatments are very long (more than a year for example) and therefore unusable industrially, and phytoremediation treatments using plants in which can accumulate can be mentioned. certain organic compounds via root absorption, in addition to the process of metabolization by microorganisms of the rhizosphere. The invention is particularly interested in pollution by hydrocarbons, as it comes for example on parking sites, service stations or hydrocarbon storage. Oil spills through oil leaks or fuel pumps cause pollutant impregnation of the soil by accumulating more or less. For example, public authorities are increasingly trying to identify the various polluted sites and the various types of pollution associated with them on databases. These data are interesting in the context of a change in the land use plan, for example when a decision is made to transform a parking area into a residential area.

  However, the aforementioned prior techniques are unsuitable, either because of their cost, or because of the duration of treatment, or because of the nuisance generated by the necessary facilities. OBJECT OF THE INVENTION The object of the invention is to design a method for decontaminating polluted soils, in particular soil polluted by hydrocarbons, which is easy to implement and requires only small and easily movable installations, It may be possible to intervene on service station sites in the urban periphery or even urban housing sites. Another object of the invention is to design a decontamination process which makes it possible to obtain rapid decontamination at a level of decontamination sufficient to envisage an urban-type use. The invention finally aims to design a decontamination process that allows rehabilitation of the polluted site, with reuse of de-contaminated soil comparable to the use of healthy soils. GENERAL DEFINITION OF THE INVENTION The above technical problem is: solved in accordance with the invention by a process for decontaminating polluted soil, in particular soil polluted by hydrocarbons comprising the following successive stages. a) in a reactor are mixed particles of soil polluted with coal aggregates, with a predetermined residence time in said reactor which is sufficient for the solid-solid contacting thus carried out adsorbs the pollution of the earth to coal; b) the ground-carbon mixture is treated with water to initiate the separation of the contaminated earth particles and the polluted coal granules; c) the decant is allowed to settle in a decanter to achieve the complete separation of the decontaminated earth particles and allow the recovery of polluted char-good floating on the surface, the decontaminated earth being recovered at the bottom of the decanter. Thus, the decontamination process according to the invention is based on the adsorption of hydrocarbons on the coal particles, by solid-solid type transfer. This phenomenon makes it possible to separate the organic materials from the earthy matrix, which are often mainly composed of inorganic materials. However, this will not always be the case, for example for applications such as the treatment of forest soils or peatlands. The agglomerates formed (coal plus hydrocarbons) are then separated from the earth matrix by treatment with water and decantation. It should be noted that a solid-solid mixture is subsequently used which is separated from agglomerates by flotation without the use of surfactants or other facilitating agents. separation and agglomeration. It is only the difference in density between water and coal that allows the physical separation of these two phases. Preferably, the mixing step a) is preceded by a preliminary step of coarse particle size of the polluted earth particles, in order to retain only particles not exceeding a given particle size, for example 5 mm. In fact, with particles of excessive granulometry, the decanta- lization process would be less effective because the pollution is in principle rather concentrated on the fine particles. According to a first embodiment, the water treatment carried out in step b) is carried out directly in the reactor in which the mixture of polluted earth particles and coal aggregates is produced. In a variant, the water treatment carried out in step b) is carried out in a separate dispersion tank arranged downstream of the reactor in which the mixture of the polluted earth particles and the coal granulates is made, said mixture being - coal being transferred into said dispersion tank once the predetermined residence time of said mixture in said reactor. Preferably, the water and the earth-coal mixture are stirred in the dispersion tank.

  Advantageously, the coal granules, when in a polluted but unsaturated state, are recovered from the settling tank for recycling into the mixing reactor, preferably after a dewatering phase. When the coal aggregates present in the settling tank are saturated, said aggregates can be subjected to a regeneration treatment by washing with a solvent in order to be depolluted and recycled in the mixing reactor.

  It may also be provided that the decantation carried out in step c) is carried out with the addition of air for aeroflotation of the polluted coal aggregates and / or addition of surfactants. It may also be advantageous to provide that the decontaminated earth particles resting at the bottom of the settling tank are subjected to a final stage of treatment with water in a dispersion tank arranged downstream of the settling tank, and separation by decantation. in a separation tank arranged downstream of said dis- persion tank for recovering the last polluted coal granulates floating on the surface, the decontaminated earth being recovered at the bottom of said separation tank. The coal used during the implementation of the process according to the invention may be charcoal or activated carbon if it is desired to treat soil polluted by heavy metals or organic compounds. Finally, in practice, the quantity of coal admitted into the mixing reactor will be determined as a function of the mass content of pollutants of the soil to be decontaminated. Other features and advantages of the invention will appear more clearly in the light of the following description and the accompanying drawings, illustrating a particular embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Reference is made to the figures of the accompanying drawings, in which: FIG. 1 schematically illustrates an installation for implementing the method according to the invention, FIG. 2 is a view illustrating a variant of FIG. Previous installation in which a dispersant tank is used; FIG. 3 illustrates an installation of the type of that of FIG. 2 which is completed by a final water treatment and separation by settling installation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 schematically illustrates a treatment plant 100 for implementing a polluted soil decontamination method according to the invention. T-lands are brought, after a preliminary preliminary maceration, into a hopper 1, and conveyed by a line 2 to a coarse size sorting means 10. This sorting means 10 makes it possible to remove polluted earth particles whose granulometry exceeds a predetermined threshold, for example 5 mm. A line 3 then makes it possible to discard the particles of excessive particle size, while the particles of polluted earth of particle size smaller than the predetermined threshold exit by a line 4. Such a preliminary sorting is not mandatory at all, but it is nonetheless interesting to obtain a process that is both fast and efficient.

  The installation comprises a mixing reactor 20, which is here represented diagrammatically with a central shaft 21 carrying stirring members 22 integral in rotation with said shaft 21. A line 5 makes it possible to bring coal aggregates C into the interior of the reactor 20.

  The term "aggregates" should be understood in its broadest acceptance, and include both coal powder and granular particles, optionally compressed in the form of balls or bulk. In practice, we will use aggregates whose average size is of the order of a millimeter. As much as possible, avoid the presence of fine particles of coal that can cause lung infections for personnel on the site. For the decontamination of soil polluted by hydrocarbons, preference will be given to charcoal, which has the advantage of being an inexpensive material. The invention is however not limited to this particular material, and it will be possible to consider using active charcoal, in order to treat soil polluted by heavy metals or particular organic compounds. According to an essential characteristic of the process of the invention, polluted earth particles T are mixed in the reactor 20 with coal granules C, with a predetermined residence time in the reactor which is sufficient for the solid contacting - solid thus carried by adsorption pollution of the earth coal. The physical adsorption phenolene allows the soil to be polluted by contact between the pollutant molecules (hydrocarbons) and the adsorption medium (here coal). It is recalled that several physical processes take place during adsorption, namely a diffusion of the pollutant molecules towards the coal particles by mixing polluted soil - coal, an external diffusion of the constituent through the boundary layer located in the vicinity of the adsorbent medium, internal diffusion of the constituent in the porous grain structure, desorption of diluent or other molecules of the adsorbent, and finally adsorption of the constituent on the adsorbent. In practice, it will be provided that the quantity of coal C admitted into the mixing reactor 20 is determined according to the pollutant mass content of the soil to be decontaminated T concerned. This mass content is previously determined by a physico-chemical analysis of the soil concerned. As an indication, with a soil initially polluted at 2% by mass of total soil, ie 20 000 ppm, it will be possible to use a quantity of coal in mass which is of the order of six times that of the initial pollutant. Of course, the greater the amount of coal used, the higher the decontamination rate increases, but the cost: also increases, which requires to find a satisfactory compromise. With such data, sufficient mixing is achieved so that an essential part of the pollution is transferred to the coal after the lapse of a residence time of the order of twenty to thirty minutes. After this first mixing step, a water treatment of the earth-carbon mixture T, C is carried out to initiate the separation of the contaminated soil particles and aggregates from the polluted coals. In the context of the installation 100 illustrated in FIG. 1, the treatment water is admitted directly into the reactor 20 via a feed line 6 equipped with a valve 26. As an indication, the reactor 20 will be admitted into the reactor 20 a quantity of water corresponding to approximately 75% of water by weight relative to 25% of earth by mass. With these data, the treatment will be carried out for a time of the order of twenty minutes. In practice, it will be used for this water treatment of water in the liquid phase, but it may alternatively be envisaged to use water in the vapor phase. At the end of the water treatment process, the earth-carbon mixture thus treated is discharged from the reactor 20 via a line 9 equipped with a valve 25, to arrive in a settler 30. In this settler 30 separates: almost complete decontaminated earth particles, which allows the recovery of polluted coal C which floats on the surface forming a mass of thick scum, the decontaminated earth T resting at the bottom of the decanter 30 and can be recovered by an outlet 35 of the decanter. It may be possible to provide decantation with the addition of air for aeroflotation of polluted coal aggregates and / or addition of surfactants. This is illustrated by the presence of a line 32 allowing the addition of air or surfactants directly into the water of the decanter 30. More generally, the recovery of the adsorbed contaminant may be carried out other than by flotation, by any other physico-chemical process such as centrifugation, pressing or filtration. In FIG. 1, two interesting possibilities for optimizing the process are shown in dotted lines.

  Firstly, a line 8 associated with the recycling of water was provided, making it possible to transfer part of the water 31 contained in the decanter 30 to the water inlet line 6 in the reactor 20. More interesting is the possible recycling of coal in the state of flotation on the surface in the decanter 30. Indeed, a line 7 has been provided for recycling the coal granules C into the reactor 20, preferably after a dewatering phase. This makes it possible to reduce the consumption of the coal ad-put in the reactor by the normal feed line 5. This is indeed possible when the coal aggregates C are in a polluted state but not yet saturated. In this case, the aggregates can be easily recovered from the decanter by simple skimming, to be recycled to the mixing reactor 20, preferably after a dewatering phase. When the C 10 coal aggregates present in the decanter 30 are saturated, it will of course be possible to subject said aggregates to an incineration process, but it is more advantageous to be able to recover this waste by subjecting the aggregates to washing with a solvent in order to These aggregates are depolluted, and their recycling is also carried out in the mixing reactor 20. As an indication, the time necessary to obtain a satisfactory separation in the decanter 30 is of the order of twenty minutes. This means that the method of the invention allows a decontamination treatment of polluted soil taking place over a time of the order of one hour, with twenty minutes for the solid-solid mixture, twenty minutes for the treatment with water , and twenty minutes for separation by settling. Such a performance is extremely interesting compared to the very long processes of the state of the art mentioned above. FIG. 2 illustrates a variant of the previous installation, the installation concerned 101 being different from the installation 100 previously described and illustrated in FIG. 1 by the fact that the treatment with water of the earth mixture - Charcoal T, C, is no longer carried out in reactor 20, but in a separate dispersion vessel. There is thus provided a dispersion tank 40 which is arranged downstream of the reactor 20, being supplied with a mixture of earth and carbon via the line 9 via the valve 25. The dispersion tank 40 is supplied with water by a line 41 equipped with A valve 26. Preferably, the assembly 42 contained in the dispersion tank 40, which consists of water and the earth-carbon mixture T, C, is continuously stirred in the dispersion vessel. 40. There is illustrated here a stirring propeller 43 mounted at the end of a vertical rotating shaft 44. The triphasic water-earth-carbon mixture in the presence 11 is then kept very homogeneous, which increases the shear separation and also avoids a clogging of the system of emptying the tank by sedimentation. After the time provided for the water treatment step of the earth-coal mixture T, C, for example about twenty minutes, the whole is sent to the decanter 30 via an output line 45 equipped with a valve 27. The other equipment and associated implementation process correspond to the same with those described with reference to Figure 1, and will not be described again. If a concrete mixer, or a router of the type conventionally mounted on the construction trucks, is used as the reactor, it will be preferable to use the reactor solely for mixing the polluted earth particles with the coal aggregates, and then unload the reactor in a dispersion tank, according to the installation 101 il-lustrée in Figure 2. The installation which is then put in place on the site is limited to the dispersion tank 40 and decanter 30, which allows to consider a small installation and easily movable installation. It may optionally complete the installation of Figure 1 or Figure 2 by a complementary finishing facility if it is desired to obtain an even higher level of decontamination. FIG. 3 thus illustrates an installation which comprises the installation 101 of FIG. 2, and which is completed by a finishing treatment installation 200. It goes without saying that the finishing installation 200 could equally well equip the 100 of Figure 1. The earth T which has been recovered at the bottom of the decanter 30 at the end of the first process cycle, via the outlet 35 of said decanter, is now transferred to a dispersion vessel 60 similar to the dispersion tank 40 above. This transfer is for example carried out by a column 50 enclosing a transfer screw 51 of the Archimedean screw type. At the outlet 52, the already largely cleared land falls into the dispersion tank 60 which contains treatment water admitted via an associated line 61 equipped with a valve 29. The assembly 62 contained inside the The dispersion tank 60 is preferably subjected to permanent stirring, by means of a stirring propeller 63 mounted at the end of a rotating green shaft 64. This is followed by a final stage of treatment with water in the vessel. dispersion 60 for a predetermined time, pa ::. example of the order of twenty minutes, and the assembly is then discharged through the line 65 equipped with a valve 28 to arrive in a separation tank 70 similar to the decanter 30 above. There is then a separation process by settling in the separation tank 70 which is arranged downstream of the dispersion tank 60, to recover by skimming the polluted coal floating on the surface, the further decontaminated land being recovered at the bottom separation tank 70. For this purpose, separation tank 70 has an outlet 75 making it possible to recover the soil which is then decontaminated at a higher level. As previously, the water 71 of the separation tank 70 may give rise to recycling via a line 78. Similarly, the last aggregates of polluted coals can be recovered by a line 77, to be treated and optionally recycled. Finally, there may be provided an air injection line 72 for aeroflotation polluted aggregates, or surfactants, similar to the line 32 above.

  It has thus been possible to achieve a method of decontamination which is both economical and effective, while allowing implementation by means of a lightweight and easily transportable installation. Such a method is therefore particularly suitable for site rehabilitations or urban site work. The invention is not limited to the embodiment which has just been described, but on the contrary covers any variant using, with equivalent means, the essential characteristics mentioned above.

Claims (12)

  1. A method for decontaminating polluted soil, in particular soil polluted by hydrocarbons, characterized in that it comprises the following successive steps. a) polluted soil particles (T) are mixed in a reactor (20) with coal aggregates (C), with a predetermined residence time in said reactor which is sufficient for the solid-solid contacting thus carried out by adsorption the pollution of the earth with coal; b) treating the earth - coal mixture (T, C) with water to initiate the separation of the decontaminated earth particles and the polluted coal granules; c) it is allowed to settle in a decanter (30) to achieve complete separation of the decontaminated earth particles and allow the recovery of polluted coal floating on the surface, the decontaminated earth being recovered at the bottom of the decanter.   2. Method according to claim 1, characterized in that the mixing step a) is preceded by a preliminary step of sorting granulometric: Lque coarse (10) polluted earth particles (T), in order to retain only particles not exceeding a given particle size, for example 5 mm.   3. Method according to claim 1 or claim 2, characterized in that the water treatment carried out in step b) is carried out directly in the reactor (20) in which the mixture of the particles of polluted earth and coal aggregates.   4. Method according to claim 1 or claim 2, characterized in that the water treatment carried out in step b) is carried out in a separate dispersion tank (40) arranged downstream of the reactor ( 20) in which the mixture of the polluted earth particles and the coal granules is carried out, said earth-carbon mixture (T, C) being transferred into said dispersion tank once the predetermined residence time of said mixture in said reactor has elapsed.   5. Method according to claim 4, characterized in that the water and the earth-coal mixture (T, C) are stirred in the dispersion tank (40).   6. Method according to one of claims 1 to 5, characterized in that the coal aggregates (C), when they are in a polluted but unsaturated state, are recovered from the decanter (30) to be recycled in the mixing reactant (20), preferably after a dewatering phase.   7. Method according to claim 6, characterized in that, when the coal aggregates (C) present in the decanter (30) are saturated, said aggregates are washed with a solvent to be cleaned and recycled in the mixing reactor (20).   8. Method according to one of claims 1 to 7, characterized in that e decantation performed in step c) is carried out with addition of air for an aeroflotation of polluted coal aggregates and / or addition of surfactants .   9. Method according to one of claims 1 to 8, characterized in that the decontaminated earth particles resting at the bottom of the settling tank are subjected to a final step of treatment with water in a dispersion tank (60) arranged downstream decanter (30), and decantation separation in a separation tank (70) arranged downstream of said dispersion vessel to recover the last aggregates of polluted coal which float on the surface, the decontaminated earth being: recovered at the bottom of said separation vessel.   10. Method according to one of claims 1 to 9, 16 characterized in that the coal (C) used in the implementation of said process is charcoal.   11. Method according to one of claims 1 to 9, characterized in that the coal (C) used in the implementation of said process is activated carbon, in order to treat soil polluted by heavy metals or organic compounds .   12. Method according to one of claims 1 to 11, characterized in that the amount of coal (C) fed into the mixing reactor is determined as a function of the mass content of pollutants of the earth (T) to decontaminate concerned.