FR2759000A1 - Decontaminating granular material (soil) from low density pollutants - Google Patents

Abstract

Device for cleaning granular materials contaminated with pollutants of a density lower than 1 and which are mainly hydrophobic, containing a vessel with a flat base (15), means of injecting air and means of injecting water (5) where the means of injecting air are orifices (17) positioned on the base of the vessel so the material loaded into the vessel and saturated with water is mixed uniformly by the injected air. Process of cleaning granular materials contaminated with pollutants of a density lower than 1 by placing a quantity of the materials in a vessel containing means of injecting air and water, as described above; the material is saturated with water so a layer of water sits on the material; the air is injected to mix the granular material and the layer of water containing the pollutants is pumped out or extracted.

Description

 The present invention relates to a device and a method for depolluting soil contaminated by pollutants whose density is less than or substantially equal to 1 and of mainly hydrophobic type, for example hydrocarbons, vegetable oils, cutting oils, or else anthracene oils. The implementation of the present invention has the particular advantage of being able to be easily carried out on site since the device is easily transportable and of relatively low cost.

 The principle of the treatment is based on the release of the polluting product by rupture of the menisci formed between the grains constituting the soil, thanks to an optimized injection of air in a determined volume of polluted material and saturated with water. The rupture, by the action of air, of the contacts between the grains causes the polluting product to rise with a density substantially lower than that of the water in which the soil grains bathe. In addition, the mixing of the soil caused by the injection of air into a medium saturated with water, promotes aeroflottation of fine particles loaded with polluting product.

Thus, the entrainment of the fines on the surface contributes to the effectiveness of the depollution, while cleaning the larger particles of the fines. In a variant, the injection of water into the polluted material is done with means comparable to those used for the injection of air.

 Thus, the present invention relates to a device for depolluting granular materials contaminated by pollutants with a density substantially less than 1 and of the mainly hydrophobic type. The device comprises a tank with a substantially flat bottom, air injection means and water injection means. The air injection means have orifices distributed over the bottom of the tank so that the materials, once loaded into the tank and saturated with water, are mixed substantially uniformly by the air injected.

 The water injection means may include orifices distributed over the bottom so that the materials, loaded into the tank and saturated with water, are mixed substantially uniformly by the injection of air and by the injection of 'water.

 An impermeable band can be placed around the perimeter of said tank.

 The air injection means and the water injection means can be constituted by tubes having perforations distributed along the tubes so that the flow rate of the fluid is substantially equivalent at the outlet of the orifices similarly nature.

 The air injection tubes can be parallel and regularly spaced on the bottom of the tank and the water injection tubes can be interposed between the air injection tubes.

The invention also relates to a method for depolluting granular materials contaminated by pollutants with a density substantially less than 1 and of the mainly hydrophobic type. In the process, the following steps are carried out:
- A quantity of said materials is placed in a tank comprising air injection means and water injection means, said air injection means comprising orifices distributed on the bottom of the tank
- the quantity of materials is saturated with water by the water injection means so that a sheet of water overcomes the materials stored in the tank,
air is injected by said air injection means so as to mix all the granular materials with air,
- the blade of water loaded with pollutants is removed by pumping and / or dumping.

 A preliminary step of mixing the materials can be carried out before loading the brewing tank.

 A water injection step can be carried out by said water injection means which have orifices distributed over the bottom of the tank.

 An air and water injection step can be carried out simultaneously.

 An impermeable band can be placed in the tank around the perimeter of the tank.

 The fluid from the water slide can be collected in a settling tank.

The present invention will be better understood and its advantages will appear more clearly on reading the example below, which is in no way limiting and illustrated by the appended figures, among which:
- Figure 1 shows an exploded perspective of an embodiment of the
device according to the invention, and
- Figure 2 shows a detail of the air injection means or
of water.

 FIG. 1 and FIG. 2 describe a sealed treatment tank 15 provided with a drain valve 11 and an overflow valve 10. Upstream of the drain valve 11 is a filter (millimeter mesh cloth) 12 intended to retain the material during the drying of the sands at the end of treatment.

The tank 15 is supplied by a network of drains supplied with compressed air and each provided with a valve 1 and a network of drains constituted by tubes supplied with water, and each provided with a valve 6. There are also two valves leak 7 and 8 allowing a gradual increase in pressure of the air and water supply. The drains 5 are placed on the bottom of the tank 15, substantially horizontally. Ports 17 are arranged along the air tubes, the section of the ports and their number are determined so that, depending on the available air flow rate and the surface of the tank bottom, the outlet flow rate of the air at the level of all the orifices is substantially equal.

Thus, the most regular and homogeneous air mixing is obtained, which favors the rupture of the greatest number of menisci of pollutants. The arrangement and shape of the water orifices 18 on the water injection tubes are carried out according to the same principle, but adapted to the water.

 A plate 4 is arranged along the periphery (width of about 30 cm) of the treatment tank so as to prevent the privileged rise of air at the edge of the tank (edge effect). This plate 4, which plays the role of an air baffle can be metallic or plastic, and directly attached to the edges of the tank.

 The air inlet is at 13 and the water inlet at 14. When the tank 15 is in place on the site, the material to be treated 3 is deposited on the bottom of the tank and saturated in water by filling the tank, preferably by water drains. The injection of air begins when saturation is complete, that is to say when a layer of water 2 appears above the material. Preferably, the air and water injections continue concomitantly until the water layer 2 has a sufficient height taking into account the quantity of pollutants and fines which float as and when of brewing.

Preferably, the water injection is then stopped.

 The process is improved when the quantity of material to be treated is homogenized beforehand in kneading means, for example of the concrete mixer type or equivalent. Another tank can also be fitted with mixing means of the propeller or Archimedes screw type.

 During the treatment, the water slide 2 of the treatment tank is rejected or poured into another settling tank (not shown). Pumps or spouts can be used to adjust the level of the water slide during air mixing with joint water injection.

 A treatment test is carried out using the device as shown in FIGS. 1 and 2, on a material whose initial content of pollutant of the paraffinic oil type is 5.1 g / kg (5100 ppm). Paraffinic oil is characterized by a viscosity of 18 cst at 11 "C, a surface tension of 28.5 mN / m, an interfacial tension of 21 mN / M and a density at 20" C of 0.87 g / cm3. The polluted material is composed of old Seine alluvial deposits with a bimodal type particle size: 20% of the elements are less than 0.2 mm in size and 80% of the grains are between 0.2 mm and 2 mm. Of course, the device according to the invention is suitable for soils whose particle size distribution is different from the present example. Preferably, the soils to be treated are classified as sands or gravels.

 The depollution process is carried out as follows.

 The polluted material is placed in the treatment tank (7 m3 of earth for an area of 10 m2). First, water is injected through the network of water drains to saturate the material. Then, air and water are injected through the two networks of drains located at the base of the tank. The drains are arranged horizontally so as not to initiate the injection of water or air into certain portions of the drains. The spacing between air drains (same for water drains) is 50 cm. Reducing the spacing between drains (25 cm for example) can be envisaged and should improve the effectiveness of pollution control. In the case of the present test, each drain has a length of 4 meters and has slots of 0.15 mm 2, at the rate of 11 slots per meter.

We used a 35x42 mm PVC strainer (width of the strainer slots: 0.3 mm) from which a large part of the slots were closed. The purpose of this particular arrangement of the orifices is to correctly distribute the air or water injection rate over the length of the tubes. In the present example, the air is injected by a fan at the rate of 350 m3 / h at a pressure of 0.24 bars. Water is injected at the base of the tank (saturation is generally obtained for 1 m3 of water per m3 of polluted soil).

 The action of air sets in motion the material saturated with water. To improve the movement, it is advantageous to jointly inject air and water, especially in a preliminary phase. The hydrocarbon and the fines loaded with hydrocarbon rise to the surface in a layer of water located above the material. This blade of water is then pumped to a settling tank. The supernatant hydrocarbon is then recovered there (the quantity is small: of the order of a liter taking into account the original content of 5.1 g / kg, relatively low). The volume of fine clayey materials (whose oil content is then around 30 g / kg) is 1.5 m3. The water contained by the treated material is evacuated to the sewer without special treatment (the hydrocarbon content of the order of 3 mg / l) using a drain valve located at the bottom of the treatment tank. When the supernatant hydrocarbon is recovered from the settling tank, the water layer (above the fines) is also evacuated to the sewer (for a correct sedimentation of the fines, it is advisable to wait approximately two days).

 The air and water injection durations were 5h and 2h30 respectively.

 The residual hydrocarbon content in the material, after treatment, is 1.8 g / kg (1800 ppm). For the material tested in the present case and for the type of pollutant considered, the yield of said process (the reduction in the pollutant content) is - r '-? - of 60r.

 In addition, the sand has been cleaned of fine particles.

 This method and this pollution control device can be applied to various types of polluting products insofar as they are lighter than water and of the mainly hydrophobic type.

Claims (11)

 1) Device for depolluting granular materials contaminated by pollutants with a density substantially less than 1 and of mainly hydrophobic type, characterized in that it comprises a tank with a substantially flat bottom (15), air injection means and water injection means (5), in that said air injection means comprise orifices (17) distributed over the bottom of the tank so that the materials once loaded into the tank and saturated with water, are stirred substantially uniformly by the injected air.  2) Device according to claim 1, wherein the water injection means (5) have orifices (18) distributed over the bottom so that the materials loaded in the tank and saturated with water, are substantially stirred uniformly by air injection and water injection.  3) Device according to one of claims 1 or 2, wherein there is an impermeable strip (4) on the perimeter of said tank.  4) Device according to one of claims 1 to 3, wherein the air injection means and the water injection means are constituted by tubes (5) having perforations (17; 18) distributed the along the tubes so that the fluid flow rate is substantially equivalent to the outlet of all the orifices of the same kind.  5) Device according to claim 4, wherein the air injection tubes are parallel and regularly spaced on the bottom of the tank and wherein the water injection tubes are interposed between the injection tubes air.  6) Method for depolluting granular materials contaminated by pollutants with a density substantially less than 1 and of mainly hydrophobic type, characterized in that the following steps are carried out:  - a quantity of said materials is placed in a tank (15) comprising air injection means and water injection means (5), said air injection means comprising orifices (17) distributed on the bottom of the tank,  - the quantity of materials is saturated with water by the water injection means so that a sheet of water overcomes the materials stored in the tank,  air is injected by said air injection means so as to mix all the granular materials with air,  - the blade of water loaded with polluting product is removed by pumping and / or spilling.  7) The method of claim 6, wherein a preliminary step of kneading said materials is carried out before filling the brewing tank.  8) Method according to one of claims 6 or 7, wherein one performs a step of injecting water by said water injection means which have orifices (18) distributed over the bottom of the tank.  9) Method according to one of claims 6 or 8, wherein a step of injecting air and water is carried out simultaneously.  10) Method according to one of claims 6 to 9, wherein there is in the tank an impermeable strip (4) on the perimeter of the tank.  11) Method according to one of claims 6 to 10, wherein the fluid from the water slide is collected in a settling tank.