DESCRIPTION

Technical field

The present invention relates to the field of plants for treatment of polluted soils.

Scope of the Invention

The technique for treating contaminated soils by heating the ground up to vaporization of volatile and semi-volatile or ganic compounds has long been known: the removal of contaminated ground fractions, as deep as it is considered appropriate, and the subsequent heating thereof in appropriate kilns, up to tem peratures sufficient to vaporize the volatile and semi-volatile organic compounds are involved.

The systems in use generally provide that the kiln operates at high temperatures by means of flame, combustion fumes or py rolysis, by overheating the contaminated soil. Such treatments are usually performed in dedicated plants, normally hundreds of kilometres away from the polluted site: this necessarily implies that the soil to be reclaimed is temporarily housed in pre arranged areas, for highly variable periods of time before being treated in the kiln.

These procedures clearly cause environmental damage, both due to the fact that losses of harmful substances are very like ly to occur in transport and storage, and because of the obvious economic and logistical disadvantages related to the need of providing safe transports of the soil to be treated to the plant areas, and the subsequent transports aimed at carrying over the soil to the previously contaminated area.

The complexity of organizing such operations causes delays in reclamation operations, with the risk that contamination would undesirably spread; moreover, such a procedure leaves room for wrong - albeit unintended - actions during transport and storage .

Furthermore, the prior art systems involve a considerable ther mal energy consumption, in order to obtain the proper reclama- tion of the soil from pollutants, without any form of recovery being provided, thereby increasing the pollutants released into the air, to which the gases that are produced during convention al soil treatment operations associate.

The object of the present invention is therefore to provide a mobile plant for soil decontamination, i.e., such that it can be performed each time near the contaminated area, thus reducing logistical problems and - at least part of - economic ones, and that it is able to reduce energy costs and limit the risks of pollution and contamination of originally clean areas, in case of a proper soil reclamation from pollutants.

Said object is achieved by a plant for the reclamation of contaminated soils, characterized in that it consists of a pre treatment and feeding compartment, a thermal desorption compart ment and a thermal cooling and recovery compartment, separate from each other and interconnected, each of said compartments being arranged on a deck provided with handling means .

A description of a preferred embodiment of the plant ac cording to the invention is reported below, with reference to the appended figures, set forth herein only by way of illustra tion, wherein:

Fig. 1 is a top front perspective view of a mobile plant according to the invention, wherein

Fig. 2 is the rear perspective view;

Fig. 3 is a top perspective view of the desorption compart ment according to the invention;

Fig. 4 is a top perspective view of the connecting member between the desorption compartment and the thermal cooling and recovery compartment according to the invention;

Fig. 5 is a top perspective view of the thermal cooling and recovery compartment according to the invention;

Fig. 6 is a top perspective view of the pre-treatment and feeding compartment according to the invention;

Fig. 7 is a perspective view of the release part of the re claimed material of the plant according to the invention; Fig. 8 is the schematic view of the energy sources of the plant according to the invention.

In the description set forth below, similar elements of different parts are identified with the same reference number for the sake of convenience, in order to make the reading easi er .

As is well understood from Fig. 1, the mobile plant con sists of a series of parts that make the structure autonomous and fully operational, which identify the processing steps.

Notably, there are provided:

- a pre-treatment and feeding compartment 1;

- a thermal desorption compartment 2; and

- a cooling and heat recovery compartment 3.

Notably, the pre-treatment and feeding compartment 1 con sists of a material storage area, material collection means, means for screening, mixing and feeding to the remainder of the plant .

The storage area consists of a space designed for the tem porary conservation of the soil to be reclaimed, which is locat ed, for the sake of convenience, in the proximity of a hopper 4 for the collection of material and the transmission to the fol lowing treatment steps.

At the hopper 4 there is also provided a module for screen ing the substance injected and for separating large bodies, so that only the substantially more homogeneous material is for warded to the following steps, and a more accurate processing is allowed. In the case of coarse or large-sized materials, it is also possible to provide a crushing mill to reduce the size of the material to be treated. The screening and crushing modules, which are known per se in the art, have not been shown in the figures, in order to make the whole plant easier to understand.

The entire pre-treatment and feeding compartment 1 is mounted on a deck 5 provided with wheels 6 and telescopic secur ing feet 7 to allow rapid displacement, both for the on-site settlement and for the transfer to other sites. Downstream of the first compartment 1, the second thermal desorption compartment 2 is provided, connected to the first through a conveyor belt 8 for conveying the reclamation materi al. Said conveyor belt 8 has been arranged as tilted with re spect to the road surface, and is provided with transport plane vibrating means, so that the soil to be treated enters the de sorption area with the proper moisture. In this way, the combi nation of these two properties, the tilting of the belt body and the vibration of the floor, cause the residual water present in the soil to drip: a mechanical drying step has thus been imple mented, being useful for the desorber and the processing proce dure, as it ensures less moisture in the following steps.

The desorption compartment 2, where the material flows through the conveyor belt 8, consists mainly of a casing 9 de signed to thermally insulate a rotating desorption drum, or de sorber. The contaminated and screened soil is introduced into the drum, whose internal surface is provided with blades ar ranged according to a spiral geometry: this expedient ensures - in use - the right path followed by the soil inside it. The de sorption compartment also comprises heat generators, typically air vein burners, intended for maintaining the temperature of the drum at about 900 °C.

Just as the pre-treatment and feeding compartment, also the desorption compartment is mounted on a deck 5 provided with wheels 6 and telescopic securing feet 7 to allow rapid displace ment, both for the on-site settlement and for the transfer to other sites.

The desorbed soil undergoes the final screening at the tow er 12, in order to recover sands and powders separately for sub sequent purposes, maintaining a 300 pm riddle. In order to fa cilitate the proper management of aggregates, at this step there is also provided a humidifier 10, which reduces the risk of pow ders and makes the soil easy to manage and transport.

The steam spreading inside the desorber is rich in contami nants, and must be subject to further treatment, in order to further reduce its polluting load. To this end, the steam is carried through a duct 10 to an after-burning chamber 11, where in the contaminating substance which has not yet been spread in side the desorber is oxidized. To that end, the environment in side the after-burning chamber 11 is kept at a temperature of about 750°C. The oxidation of the oxidized contaminant releases energy in the form of thermal energy, which is conveyed inside the duct 10, so that it can be brought to a micromesh bag filter 13 for collecting the powders, and then transferred to the chim ney 14.

Inside the duct 10 a tube bundle - not shown, having a con ventional structure - is provided for heating countercurrently the ambient, and therefore clean, air introduced into the duct 10 through a fan 15, in order to head towards the inside of the desorber 9.

Just as the other two compartments described, also the thermal cooling and energy recovery compartment is mounted on a deck 5 provided with wheels 6 and telescopic securing feet 7 to allow rapid displacement, both for the on-site settlement and for the transfer to other sites. Furthermore, in order to ensure compliance with road regulations, there is provided the use of a telescopic chimney, whose height varies also according to the type and the temperature of emission at the end of the treat ment, as it will be better understood from the description of the operation that follows.

In the plant, in addition to the fan mentioned, other fans are provided for controlling and managing the fumes, to allow the proper operation and movement of gases, and to minimize the deposition of particulates along the ducts.

Moreover, preferably, the drum within the desorber is placed with a slight depression to prevent the steams from com ing out of the drum itself.

The entire plant is managed by a control unit 16 and is powered by a power station 17, which in this embodiment are pro vided upstream of the hopper 1, and mounted for transport on the deck 5 of the pre-treatment and feeding compartment 1.

The operation of the plant described herein looks relative- ly simple: after the decortication of the soil, the operators, after taking care of decorticating the soil, insert it into the hopper 1, where the first screening phase takes place. The screen is adjusted, according to the mesh number and size, as a function of the specific chemical and physical conditions of the soil .

If the screened product is considered suitable, it is for warded for recovery; otherwise, the material is transferred to a crushing module, to make the excessively large material liable to be treated, and subjected again to screening.

Once the desired size of the material to be reclaimed has been obtained, it is carried through a conveyor belt into the desorber. In the transfer from the hopper to the desorber, the soil is mechanically dehumidified to allow a more rapid and ef ficient spreading of the polluting substances.

Once the soil has entered the desorber, the rotating drum - by means of the blades arranged along its internal surface - lifts it and causes it to fall by gravity, while the overheated air coming from the heat generators associated with the desorber and enriched with the hot air carried by the heat exchanger in vests the soil to be reclaimed countercurrently, thus allowing the first separation of various contaminating materials from the soil .

At the end of the treatment, the steam rich in contaminants is brought to the after-burner through the duct for the follow ing treatment, while the solid substance is sent to a screen, and then distributed for the following specific activities for the type of product obtained.

The gaseous part is brought into the post-combustor for the following oxidation: within the after-burner - due to the oxida tion of the pollutants, which are thus rendered harmless for the emission in the environment - a surplus of heat to be reused in the plant is generated. This surplus of heat, which can not even be emitted into the environment without causing harmful ef fects on the ecosystem, in particular is conveyed into a tube bundle to achieve heat exchange, thanks to the countercurrent transit of ambient air directed to the desorber. In the ex change, the steams coming out of the after-burner reduce their temperature and are cleaned by a particulate filter, which could still have pollutants and which would certainly reduce the draught efficiency of the chimney, before reaching the open air - through the chimney itself - at a temperature low enough to be not harmful, but high enough to prevent the occurrence of con densation along the chimney itself.

The energy balance of the system therefore provides that the temperature inside the desorber is about 900 °C, whereas in side the after-burner the temperature reaches 750°C. The ambient air which is overheated by the heat exchanger fed by the fumes generated during the thermal treatment of the soil, at the end of its the path, that is when entering into the desorber, reach es the temperature of about 450°C. Therefore, the temperature of the steam that is brought to the bag filter is about up to 294 °C. At the end of the treatment, the gas that goes up the chimney has a temperature that is around 200°C.

As it is better understood by analysing Fig. 8, the de scribed plant provides a series of control elements for the right operating temperature. Notably, in the described preferred embodiment, in the drum there are provided burners 18, prefera bly fed with methane gas or LPG, with modulating operation, with fixed air, fed at a pressure of 300 mbar; the power voltage to the panel is 400V-50Hz 3 phases.

The control of the power in the burner of the kiln per formed by the thermoregulator 19 on the basis of the discharge temperature of the solid material detected by the infra-red probe 19a, so that the variations in the supplied gas correspond to variations in temperature with respect to the adjusted value.

To obtain the right temperature inside the after-burner, a thermocouple 20 detects the temperature of the fumes at the out let and sends it to the thermoregulator 20a: if the temperature exceeds a settable alert value (e.g., 160°C), the flame of a burner 21 provided at the after-burner 8 is switched off. At the same time, a probe 22 detects the temperature in the after- burner by means of the thermocouple 22a, and it is substantially ensured that the temperature inside the after-burner is kept constant .

In order to reduce consumption of energyj [Francescol ] , the 5 rotating drum is powered by a thermal group consisting of a spe cial burner with a refractory head, working with fixed air, pre heated by the heat exchanger 10.

The following table 1 sets forth the technical data of a plant according to the invention, in order to obtain a proper dimen- 10 sioning.

Table 1

Main features of the thermal treatment plant

¹ (depending on the moisture and concentration of the contami- 15 nant in it)

Economic assessment in operation of the plant

In order to properly evaluate the operating cost of the plant, let us assume a soil with a moisture of 13%. Based on 0 this data, table 2 sets forth the expected costs per tonne of reclaimed waste, which are the following:

Table 2

C

Since about 50% of the material transferred as a result of being removed from the contaminated soil consists of gravel and pebbles, and therefore is not subject to treatment, the economic

5 and energy balance must be halved. The cost per tonne of treat ment therefore becomes around 2 €/t.

The calculation was made considering that the transferred mate rial contains 15% of moisture to be eliminated, and therefore a weight of about 150 kg can be expected for each tonne of materi

10 al subject to treatment.

Since the energy needed to evaporate the water is 600 kcal/1, about 90,000 kcal/t should be used only to evaporate the water in a tonne of material.

Considering the total energy power of the desorber, the

15 theoretical power of the plant reaches about 50 t/hr.

Based on the data set forth above, i.e., with the maximum capac ity with 15% moisture equal to 50 t/hr, with an advantageous flow rate of 30 t/hr (hourly working capacity when in full oper ation), the desorber proceeds to 60% of its capacity.

20 From the foregoing, it is understood that the plant has been designed by evaluating the opportunity to fully transform the contaminating substance into thermal energy during the treatment within the after-burner. This allows to reduce in an evident and extremely satisfactory way the energy requirement

25 coming from the outside, that is to minimize the contribution of the burners within the drum for the desorption step, since the contaminant has been substantially recycled into thermal energy.

As it can be understood, therefore, the plant thus produced allows a direct relationship between the contamination of the

30 soil to be reclaimed and the amount of power that must be sup- plied from the outside for the treatment: the greater is the contamination, the less is the external contribution of energy. This makes the whole plant highly environmentally friendly.

This plant is particularly profitable when it is necessary to 5 reclaim the soil from hydrocarbons: in this case, the energy production is also given by pyrolysis, which takes place direct ly in the drum, especially if the contamination is very high. Such reaction helps to raise the temperature inside the rotating drum, causing a decrease in the consumption of methane gas. The 10 greater is the contamination, the moisture being equal, the low er the consumption will be.

To get an idea of the substances that have been spread at the end of a conventional soil treatment according to a plant of the present invention, a practical example is set forth in table 15 3.

20

Table 3

Features of emission and plant efficiency

From the above description, and from the practical numerical ex ample that has been set forth, the advantages obtained by the plant of the present invention can be summarized as follows:

5 - Same general advantages of the known art plants, since it is able of not forming dioxins and furans, the contaminated soil being still able to preserve most of its organic and chemical properties, etc.;

- a very adaptable system from a structural point of view,

10 with the possibility of providing additional modules to meet the most diverse treatment needs and energy sources;

- very limited treatment costs, both due to the reduction of power supplied to the system for the treatment, and to the minor logistical problems related to ex situ treatments

15 - limited exploitation of energy resources.

Furthermore, the mobile thermal treatment plant according to the invention ensures a reduction of the organic pollutants (from C4 to C40) up to 99% compared to the concentrations of the input pollutants. The use of thermal oxidation systems also allows to

20 eliminate the organic compounds in the exhaust gases, with effi ciencies in the range of 99 to 99.9% and with a process speed of 32-54 t/hr.

It is therefore perfectly clear that the intended objects have thus been achieved, by realizing a plant with reduced ener

25 gy and environmental impact and limited logistical impact .

Compared to the thermo-destruction treatments, moreover, the op erating conditions are able to proceed just with the volatiliza tion of pollutants, without oxidation or direct destruction, avoiding the risk of forming dioxins or furans.

30 It is understood that the above description relates to a specific preferred embodiment, not limiting the technical scope as defined by the appended claims.