AND RELATIVE PROCESS

DESCRIPTION

The present disclosure refers generally to the sector of treating of contaminated soils, and in particular to a vitrifying and melting mixture used for the treating of a lead-contaminated soil, and a relative process.

A way of proceeding for treating a lead-polluted soil is that to provide a step wherein the soil is subjected to melting in presence of silica (vitrifying agent) and fluxing agents. In practice, this is a vitrification, i.e. a step wherein the soil and the fluxing agents are melted at high temperatures (at least to silica melting temperature) to obtain a vitrification product.

The vitrification of a soil containing lead, wherein the lead derives, for example, from battery remains or paints, is an already known process.

The vitrification of such soil, contaminated by lead of battery or paints, occurs by way of a fine adjustment (with a precise control of concentrations) of a plurality of fluxing agents such as barium carbonate, calcium carbonate, dolomite, fluorite, sodium carbonate, lithium and potassium oxide.

In relation to known processes, however, it was found out a drawback due to the need of lengthy trials and preliminary tests, to try and find out which of the several fluxing agents and their relative combination gives the best results on the basis of the chemical and physical properties of the soil to treat, and therefore an operating difficulty due to the need of carrying out in advance a large number of trials, in order to identify which of the many available fluxing agents, or combination of fluxing agents, is suitable for the soil to be treated, and also to determine the relative concentrations of fluxing agents.

A technical problem at the basis of the present disclosure is that of providing a vitrifying and melting mixture and a process for the treating of a lead-contaminated soil which enable to overcome the drawbacks relating to the known art.

The above-mentioned technical problem is solved by a vitrifying and melting mixture according to claim 1 , by a mix according to claim 8 to claim 10, by a process for the treating of a lead-contaminated soil according to claim 11 , as well as by a vitrification product according to claim 13.

Secondary features of the subject of the present disclosure are defined in the corresponding dependent claims thereof.

By means of the subject-matter of the present disclosure it is possible to achieve some significant advantages.

An advantage of the mixture according to the present disclosure lies in that a high versatility of application of the vitrifying and melting mixture is allowed, because - as it was verified - just small adjustments in the relative quantity percentages of the first, second, third aliquot of the mixture, as defined in claim 1 , in relation to the composition of the natural soil of the site where the contamination occurred, and to the quantity and quality of the contamination itself in the soil, are needed.

This versatility and simplicity of use is related to the use of a limited number of aliquots that are added to the soil for the formation of the mix, and to the type of materials used. In fact, these aliquots include, with different amounts from each other, quantities of silica, and other components, such as sodium, potassium, alumina and magnesium.

In particular, the porphyry, which contains silica in a prevailing quantity, and other minerals in relatively small percentages, acts as a vitrifying agent as much as a fluxing agent. For example, the porphyry contains silica between 60% and 80% by weight, alumina between 10% and 20% by weight, sodium between 1 % and 5% by weight and potassium between 1 % and 10% by weight of said first aliquot.

As to the ashes, since the ashes include silica in reduced or insignificant quantity, and mostly minerals, such ashes act as fluxing agents. For example, the ashes include silica between 10% and 20% by weight, alumina between 5 and 10% by weight, sodium between 1 % and 10% by weight, potassium between 1 % by weight and 10% by weight, calcium between 30% and 45% by weight, and magnesium between 1 % by weight and 7% by weight of said second aliquot.

As to the glass, it is noted that this glass, being a end product, has already been produced previously, i.e. it has been previously melted, and thus includes per se other components that complete the amalgam/mix and the final product to be obtained, wherein these additional components determine a decrease of the melting temperature.

Within the scope of the present disclosure, the term "glass" means a vitrification product which has been already subjected to melting, and that therefore it is subjected to melting again in the mix according to the present disclosure. For example, the glass includes silica between 60% and 80% by weight, sodium between 10% and 20% by weight, calcium between 5% and 15% by weight and magnesium between 1% and 5% by weight of said third aliquot.

Since a first aliquot, like the porphyry, that acts both as vitrifying agent and as fluxing agent, a second aliquot like the ashes that act mostly as fluxing agent, and a third aliquot that acts mostly as vitrifying agent, are used at once, it was confirmed that it is sufficient to modify the relative percentages of the above-mentioned first, second and third aliquots, on the basis of the type of soil to treat, to modify, even slightly, the quantities of vitrifying or fluxing agent needed.

In other words, a modification, "with a rough change", of the relative percentages of the above-mentioned first, second and third aliquot, determines a modification, in a precise and accurate way, thus obtaining "a fine change", of the total percentages of vitrifying agent silica and of other fluxing components (for example, sodium, potassium, alumina and magnesium), as a function of the chemical and physical properties of the soil to be treated. For example, a 1% increase of the first aliquot, determines an increase of about 0.5% in silica and of 0.01% in sodium (so two orders of magnitude lower). Rather, if it is needed a fine change more in fluxing agent than in vitrifying agent, it is sufficient to modify the percentage of ashes.

This opportunity to make a fine adjustment of the composition of the mix is very important, since, as known, a vitrification process is very sensitive, and always requires a fine adjustment of all the components, both in terms of vitrifying agent and fluxing agent (silica, sodium, potassium, alumina and magnesium) wherein a change in vitrifying agent often is correlated to a change in fluxing agent.

It was further verified that a mixture that of the present disclosure permits to rapidly adjust, in a simple way, the quantity of vitrifying and fluxing agent.

Moreover, the process according to the present disclosure provides the chance, by way of the mixing of polluted soil with fluxing and vitrifying agents (glass, powders, and ashes), to obtain a stable vitrification product, with minimum leaching properties and consistent with legal standards.

In other words, the soil, after being subjected to the treating according to the present disclosure, shows extremely low water leaching properties, thus enabling its further use in the environment. This treating permits to avoid a transfer to the landfill, with a significant saving of resources.

It follows that the vitrification product obtained by the process according the present disclosure can be reused. This opportunity permits to minimize the resort to the landfill transfer, that is typically very expensive for this kind of wastes, and involves a very demanding environmental management of the same. Moreover, landfills for wastes classified as "hazardous" are less common and difficult to access. In addition, it is pointed out that an hazardous waste placed in landfill does not change nature or improves, but remains dangerous forever.

Therefore, the vitrification product obtained by means of the process according to the present disclosure, has reduced leaching, confirmed through the performing of release tests set in the (Italian) Ministerial Decree of 5 February 1998 and performed according to Appendix A of UNI 10802 standard, according to the method set by the UNI EN 12457-2 standard. The obtained results show that, in spite of the lead content inside the glass, the treating permits to maintain the release level to the environment within legal limits. In the present disclosure, the release tests must be intended as the methods carried out according to UNI 12457 standard.

It is thus possible to manage the decontamination of such locations with an extremely reduced impact on the environment.

The process according to the present disclosure is applied in several decontamination activities of metal or metallorganic-forms polluted sites, such as the tetraethyl-lead.

Further advantages are the following.

A volatile-substances polluted soil, such as tetraethyl-lead, that otherwise could not be used for any purpose, is valorized by the vitrification treating, obtaining a vitrification product with stable properties and that can be reused for different purposes, without any risk for the environment and for people.

In one embodiment, the first aliquot of fragmented porphyry rock is residual slime from porphyry cutting by means of rotary blade or diamond wire, i.e. it is the remaining of an industrial cutting process of porphyry, so it is a finely ground rock, belonging to the feldspar family.

The residual slime from porphyry cutting is an already prepared material, and with no value per se, in fact designed to the landfill. Thanks to that embodiment the slime is, in that circumstance, ennobled. Therefore, the process according to the present disclosure, according to this last embodiment, includes a collection step of residual slime from porphyry cutting.

Within the scope of the present disclosure, the fragmentation dimensions of the rock are not binding since the mix (slime included) is subjected to the melting step.

In one embodiment, analyzing the average composition of the residual slime from porphyry cutting, it is noted that it includes substantially 73% by weight of silica, substantially 14% by weight of alumina, 3% by weight of sodium and 5% by weight of potassium.

In one embodiment, the above-mentioned ashes are powders coming from a solid urban waste incinerator, i.e. ashes held by incinerator's filters. Also in this embodiment, it is to be pointed out that, within the scope of the present process, the ashes are ennobled, in fact they would alternatively be designed exclusively to an hazardous waste landfill. Therefore, the process according the present disclosure, according to this last embodiment, includes a collection step of ashes coming from a waste incinerator.

In one embodiment, based on an analysis of an average composition of ashes held by filters of a solid urban waste incinerator, it is noted that these ashes include 16% by weight of silica, 7% by weight of alumina, 5% by weight of sodium, 4% by weight of potassium, 38% by weight of calcium and 3% by weight of magnesium. The remaining percentages represent a so called ignition loss (including water part and C0 ₂ ).

As to the glass, in one embodiment, the glass is collected from glass bottles crushing, even more preferably from a scrap from glass sorting, coming from glass bottles crushing. In fact, it is noted that the bottles and other glass containers are usually collected separately and crushed; the resulting gravel is selected and the good part routed to glassworks for the production of new glass bottles; the inferior part, consisting of granules and glass powder that are impossible to separate from the dirt with a minimum economic return, is used as glass in the process according to the present disclosure.

In other words, also this material is ennobled, because otherwise it would be designed to a landfill. Therefore, the process according to the present disclosure, according to this last embodiment, includes a collection step of glass coming from a waste separate collection. In practice, according to an aspect of the present disclosure, the proviso of one of these embodiments, or a combination of them, allows to use, as first, second and third aliquot of mix, materials that are wastes, readily available, and low cost, or materials only designed to the landfill and with high costs for disposal.

It follows that, according to an aspect of the present disclosure, the process can be used not only for the treating of a contaminated soil, but also of other wastes, because the ashes collected from incinerator, the slime from porphyry cutting, and the glass from bottles, that go in the melting mix, could be scrap materials, that in the melting and subsequent cooling step are definitely stabilized and made inert.

in one embodiment of the present disclosure, based on the analysis of the average composition of glass from bottles, it is noted that this glass includes 71 % by weight of silica, 13% by weight of sodium, 10% by weight of calcium and 2% by weight of magnesium.

In one embodiment of the present disclosure, the lead is included in the soil with a percentage between 0.5% and 3% by weight.

To permit a sufficient lead absorbing in the mix treated during the melting and cooling step, the mix includes between 30% and 50% by weight of soil, between 10% and 30% by weight of porphyry, between approximately 10% and 30% by weight of ashes and between 10% and 30% by weight of glass.

Also the melting temperature depends on the soil, on the percentages of the components that build up the mix, and on the furnace used for the melting. Generally, the melting temperature ranges between 1600 °C and 1500°C.

In one embodiment, the mix includes 40% of contaminated soil (having an average content as is of total lead of 13525 mg/kg SS, that substantially corresponds to 1 ,3525% of lead content upon the whole of the soil, of which tetraethyl-lead equal to 1210 mg/kg SS, 20% of ashes, 20% of porphyry and 20% of glass. The amount of 13525 mg/kg SS shown means that each kg of dry substance of the sample contains 13525 mg of lead. SS stands for dry substance (Sostanza Secca), according to the law request included in the (Italian) Legislative Decree n. 152/2006.

According to one aspect of the process according to the present disclosure the process includes:

- an excavation step of the contaminated soil,

- a first transportation step of the contaminated soil; - an accumulation step of the contaminated soil transported during said first step; wherein all of the three excavation and transportation and accumulation steps are performed in a closed place, for example a closed shed (movable if needed, or fixed), the closed place being provided with an air suction system with filters suitable for suppression of contaminants in the form of vapour and of particles or powders.

The filters for the air suction and for the disposal of contaminants permit a protection of the areas surrounding the excavation and the accumulation zones.

According to another aspect of the process according to the present disclosure, the process includes a second transportation step of the contaminated soil from the accumulation zone to a zone where vitrification occurs, whereas the accumulation and vitrification steps are not performed in the same place. In the first transportation step and during the second transportation step suitable motor vehicles are used, in order to not disperse vapours, smells, powders or perculation liquids.

In one embodiment, in the zone designed for the vitrification it is carried out a preparation step of the contaminated material for the vitrification. In particular, the contaminated material is sieved to separate an eventual coarse portion (stones and gravel up to a size of about 1 millimeter) from a fine portion (slime and clay).

The coarser part could be washed and reused, according to the law, as road foundation or similar, while the fine contaminated part is subject to the vitrification process.

According to an aspect of the present disclosure, the ashes, the porphyry and the glass are added only to the fine portion obtained after the sieving.

The addition of ashes, of porphyry and of glass to the fine portion of the contaminated soil occurs in single tanks.

The use of distinct tanks permits to properly measure the percentages of the composition of the mixture to vitrify, and to ensure a homogeneous mixing.

Each mix of each tank is tested with a sampling to control the result after a melting. If the result corresponds to a default value, the entire content of the tank is subjected to melting. Each mix is melted at a temperature of about 1500°C, in a conventional vitrification furnace. Such a high furnace temperature forces the organic lead to be reduced and rendered immobile during the melting rather than escape free in the chimney.

The mix is preferably melted by oxy-combustion, i.e. using methane as fuel and pure oxygen as comburent, in a stoichiometric ratio of 1 :2 respectively, which optimizes combustion, instead of using methane and atmospheric air. In fact, the methane burns in an optimal way with the double of pure oxygen, whereas it burns with almost five parts of atmospheric air, since oxygen, which revives the flame, is present in the atmosphere at 21 %; therefore, the volume of fuel decreases by almost four times and the exit and the speed of the fumes significantly slows down, better keeping in the combustion chamber vapors and gases to be degraded.

The mix transforms in liquid glass and casts in a crucible, where it suddenly cools with fresh water and crushes, forming a vitreous frit (irregularly shaped glass pieces), that is suitable for various operations of subsequent reuse.

Further advantages, characteristic features and modes of use of the mixture subject of the present disclosure will become clear from the following embodiments, provided solely by way of a non-limiting example.

It is clear, however, that each embodiment above and later on shown may have one or more of the advantages listed above; in any case it is not required that each embodiment should have simultaneously all the advantages listed.

Example 1.

It has been extracted a sample of about 20 kg of organic and inorganic lead-polluted soil from a real site.

The process has been carried out as follows.

It is carried out an excavation of contaminated soil (20 kg of soil). The soil has an average content of total lead of 13525 mg/kg SS, that corresponds to 1 ,3525% of lead content upon the whole of the soil, where the amount of tetraethyl-lead is equal to 1210 mg/kg SS. Subsequently a preparation of the mix for the vitrification takes place, according to the following methods.

An aliquot of contaminated soil (about 20 kg) is sieved to separate an eventual coarse portion (stones and gravel up to a size of about 1 millimeter) from a fine portion (slime and clay); the coarser part could be washed and reused, according to the law, as road foundation or similar, while the fine contaminated part is subject to the vitrification process. Referring to the starting 20 Kg, the coarse part, assumable in a case of 50% , i.e. 10 kg, is reused, while the fine part becomes part of the mix (the remaining 10 kg).

The sieving should be done with water, to reduce the powders containing dangerous substances, not to spread odors, and to maintain the working temperature as lowest as possible, so as not to volatilize the organic lead. Thus, it should be avoided, even it is more convenient, the drying process. The sieving is carried out with sieves immersed in water, with method very similar to the sieving of the sand of rivers.

To the fine part, so obtained after the sieving, are added, in suitable tanks, 5 kg of porphyry (first aliquot), 5 kg of ashes (second aliquot) and 5 Kg of residual scrap glass (third aliquot). It is obtained a first lot of mix of overall 25 kg.

Further lots of mix, equal to that previously described, are prepared in the same way. In this regard it is convenient that each lot of the mix is obtained always in the same way and it should be tested by a special chemical laboratory; the laboratory staff should, in fact, cook, before the use of the melting and every mixing, a small amount of mixture in a small test furnace to be sure, in advance, that the mix has the same non-leaching properties required by the current regulations. In the case of any discrepancy, it will be possible to make the appropriate changes to the mixture, prior to melting it, adding quantity of ashes, or of slime or of glass up to adjust its properties.

The whole is cooked at high temperature, in the order of 1500°.

Example 2.

The process is carried out as follows.

It is carried out an excavation, collecting organic and inorganic lead-contaminated soil from a real site. The soil has an average content of total lead of 13525 mg/kg SS, that corresponds to 1 ,3525% of lead content upon the whole of the soil, of which tetraethyl- lead is equal to 1210 mg/kg SS.

Then it is carried out a first transportation of the excavated soil and an accumulation of the soil in a closed shed provided with an air suction system with suitable filters for the suppression of contaminants in the form of vapour, of particles or powders.

Also the excavation has been carried out inside a mobile shed provided with suitable filters for the air suction for the removal of contaminants and the protection of the areas surrounding the excavation.

Subsequently it is carried out a second transportation of the excavated soil, from the storage shed to the treatment plant.

Both the first transportation and the second transportation occur with devices suitable for retaining vapors, smells, powders or perculation liquids. With suitable devices it is meant, for example, motor vehicles with a seal proof container or anyway a tank that does not disperse liquids.

Subsequently, a preparation of the vitrification mix occurs, according to the following methods.

A large quantity of contaminated soil is sieved to separate an eventual coarse portion (stones and gravel up to a size of about 1 millimeter) from a fine portion (slime and clay); the coarser part could be washed and reused, according to the law, as road foundation or similar, while the fine contaminated part is subject to the vitrification process. The sieving should be done with water, to suppress the powders containing dangerous substances, not to spread odors, and to maintain the working temperature as lowest as possible, so as not to volatilize the organic lead. It should be avoided, even if it is more convenient, the drying process. The sieving is carried out with sieves immersed in water, with method very similar to the sieving of the sand of rivers.

The fine part, obtained after the sieving, is added with 20% of porphyry (first aliquot), 20% of ashes (second aliquot) and 20% of residual scrap glass (third aliquot), in large suitable tanks. It is thus obtained a first large lot of mix.

In particular, the ashes include 16% of silica by weight, 7% by weight of alumina, 5% by weight of sodium, 4% by weight of potassium, 38% by weight of calcium and 3% by weight of magnesium. The remaining percentages represent a so called ignition loss (including water part and C0 ₂ ).

The porphyry is the residual slime from porphyry cutting and includes substantially 73% by weight of silica, substantially 14% by weight of alumina, 3% by weight of sodium and 5% by weight of potassium.

The glass includes 71% of silica by weight, 13% of sodium by weight, 10% of calcium by weight and 2% of magnesium by weight.

Further lots of mix, equal to that previously described, are prepared in the same way. In this regard it is convenient that each lot of the mix is obtained always in the same way and it should be tested by a special chemical laboratory; the laboratory staff should, in fact, cook, before the use of the melting and every mixing, a small amount of mixture in a small test furnace to guarantee that the mix has the same non-leaching properties required by the current regulations. In the case of any discrepancy, it will be possible to make the appropriate changes to the mixture, prior to melting it, adding quantity of ashes, or of slime or of glass up to adjust the properties.

Depending on the final location of the vitrified material, it is possible to design the composition so that the produced glass could be used, besides as stabilized, even for the production of concretes or for a use in a cement factory inside the mix for the production of cement.

According to an aspect of the present disclosure, the melting step is carried out in a glass furnace at a temperature of about 1500°C. The high temperature of the furnace melts the inserted mixture, whose siliceous parts melts with the lead and the other components, producing an amalgam, wherein the organic part is completely burned and the inorganic part is stably rendered immobile inside the glass.

In one embodiment, the melting is obtained through a so called oxy-combustion system, wherein the combustion is carried out in presence of methane: oxigen in a stoichiometric ratio of 1 :2 respectively, which optimizes combustion.

The mix transforms in liquid glass and casts in a crucible, where it suddenly cools with fresh water and crushes, forming a vitreous frit (irregularly shaped glass pieces), that is suitable for various operations of subsequent reuse.

The fumes deriving from the melting pass through a standard filter for glass production plants to be suitably scrubbed in order to comply with the more stringent regulations on emissions.

The air intake flow rate is proportional to the size of the filter and to the amount of fumes to be purified, and therefore to the size of the furnace. A filter suitable for use is a bag filter, such that hot and humid fumes pass through the fabric depositing the powders on the outside part of it. Periodically and automatically the bags are shaked, with a strong jet of compressed air opposite to the air intake, such that the powder falls off and settle in the bottom of the filter. A Venturi tube is inserted before the filter, to disperse in the air intake a small quantity of calcium hydrate, to eliminate fluorides and chlorine eventually present in the fumes, if present.

The residual powders, that avoided the melting, settle in the lower part of the filter and are re-inserted in the subsequent mixture.

The process stabilizes and reduces the contaminating organic and inorganic compounds included in the soil, because they are transformed, together with the finer part of the soil itself, through the high temperature, in a glass, i.e. a stable material, insoluble and irreversible. That material, besides being more easily managed (it has no volatility or leaching problems) can also be reused, with the necessary permissions, for various purposes.

From the point of view of control tests, it is pointed out that the reuse of glass obtained from the application of the above-mentioned disclosure is subject to the execution of release tests and composition analysis. Depending on the final use, it is stressed that these tests should be compared with different limits, depending on the intended use of the vitrification product. In practice, the release test is done after the melting, as a test of the quality of the vitrification product.

The original soil, subjected to release tests, shows lead-release ranges of about 500-1300 μ9/Ι. The soil, after the vitrification show release values that vary between 3 and 25 g/l. The legal limit for the reuse in a simplified procedure of such a waste is equal to 50 μg/l. It is noted that the waste has significantly improved its leaching properties.

In particular, the release test to evaluate the leaching was carried out according to the technical specifications described in UNI 12457 standard.

In particular, the test consists of the fragmentation of the product to be tested, to obtain a material of a granulometry lower than 4 mm. The so obtained material is inserted into a container with deionized water, in a 1 :10 proportion (a part of solid to test with 10 parts of water). The so obtained mixture is kept in agitation for 24 hours. At the end of this period, the material is separated from the water by filtration, and the water is analyzed to measure the content of the elements of interest, which will be exclusively due to the leaching of the material tested.

The subject of the present disclosure has been hereto described with reference to some examples. It is understood that other embodiments might exist, all comprised in the protective scope of the claims hereinafter.