DESCRIPTION

The present invention relates to a waste treatment method, especially for waste materials containing heavy metals.

Waste materials are typically released during energy production, e.g. pit coal or lignite fired power plants, incineration of garbage, as by-products of industrial processes, such as blast furnace, steel and phosphorous slag, and in alteration procedures of landscapes, for example digging and dredging.

A traditionally common disposal strategy of these waste materials is their incorporation into cement, concrete and further building materials.

However, this approach proves unsatisfactory due to the inadequate leaching properties of the final products.

An alternative disposal strategy is disclosed in U.S. patent 5, 626, 552 (to Nomura et al . ) , wherein waste incineration products are mixed with a water glass solution and subsequently cured under heating for obtaining a substantially solid product.

However, also the process illustrated in this document has several drawbacks.

In particular, the solidification of the solid product explained therein requires a heating step of the mixture to a predetermined temperature, ranging between 40°C to 100°C. It will be promptly understood that, due to huge amounts of waste materials at issue (e.g. the annual quantity of the mere incineration fly ashes to be disposed of in the U.S. is of about 40 million ton), the finding of economic digestion processes is a primary concern worldwide.

It is a purpose of the present invention to solve the drawback of the prior-art methods and, in particular, the disadvantages mentioned herein before.

This purpose is achieved through a waste treatment method according to the present invention, that is especially suitable for waste materials containing heavy metals.

This method comprises first steps of providing at least a fly ash and of providing at least one fragmented, preferably powdered, waste material, wherein the fly ash and the fragmented waste material overall contain at least a heavy metal, or a plurality thereof.

In the following description, with the wording "overall contain" it is meant that the system formed of the fly ash and the fragmented waste material contains at least a heavy metal; consequently, this metal is contained either only in the fly ash, or only in the fragmented waste material, or in both of them.

As a non-limiting example, the possible heavy metals contained in the waste material are lead, arsenic, barium, bismuth, cadmium, chromium, copper, iron, nickel, manganese, selenium, silver and/or zinc.

Preferably, the fly ash and the fragmented waste material are mixed in a ratio between 1:10 to 10:1.

Even more preferably, the mass of fly ash is comprised between 5 and 50% wt of the total waste material.

According to a further advantageous embodiment, the mass of fly ash is comprised between 10 and 20% wt .

Preferably, the fly ash according to the present invention originates from pit coal and lignite fired power plants and/or from an incineration process.

According to a preferred embodiment of the invention, the step of providing the fragmented waste material comprises a step of providing one or more of the compounds selected from the group comprising flue-gas treatment (FGT) , coal ash (CA) , flue-gas desulphurization (FGD) residues and/or further ashes from industrial and/or farming processes.

A FGD residue is a product typically generated in processes for reducing SO ₂ emissions from the exhaust gas system of an incinerator. The physical nature of this material varies from a wet sludge to a dry powdered material depending on the process originating the material. According to an embodiment of the invention, the method further comprises a step of drying said waste material .

In fact, an at least partial drying step proves to be advantageous in the cases wherein the waste material is muddy or in the form of a sludge, i.e. in the circumstances wherein the heavy metal, and optionally the halides explained herein after, is unacceptably diluted in the waste material.

Furthermore, according to another alternative, the step of providing at least one fragmented waste material comprises a step of fragmenting, e.g. by milling, powdering, cutting and similar, a big-sized solid waste material .

This is the case wherein the starting waste material is in a physical form that would prevent obtaining an acceptable homogenization between the different reagents depicted herein below.

According to another variant, the step of providing the fragmented waste material comprises a step of providing one or more between contaminated sludges, slurries, soils, wastewaters, sewage water, mine water, spent carbon and/or sands, spent catalysts, exhausted adsorbents from gas and liquid purification processes, further ashes from industrial and/or farming processes. The method further comprises a step of providing an aqueous solution comprising colloidal silica.

Preferably, the amount of the aqueous solution is between 5 and 40% wt of the waste material to be treated.

In other words, the above amount of aqueous solution is evaluated with respect of the overall mass of fly ashes and fragmented waste material.

According to advantageous embodiments, the amount of the aqueous solution is between 5 and 10% wt, between 10 and 15% wt, between 15 and 20% wt, between 20 and 25% wt, between 25 and 30% wt, between 30 and 35% wt, or between 35 and 40% wt of the waste material to be treated.

In any case, the suitable amount of aqueous solution should be determined on a case-by-case basis, in order to obtain a mixture with suitable mechanical properties for the further method steps of the invention.

In other words, the mixture between the aqueous solution and the waste material should be enough liquid so as to make it enough workable, e.g. in the case the method foresees a step of shaping, but not too liquid so as to make the subsequent solidifying step of the mixture difficult or even impossible.

Preferably, the amount of colloidal silica within the above aqueous solution is between 15 and 45% wt .

According to preferred embodiments, the amount of colloidal silica within the aqueous solution is between 15 and 20% wt, between 20 and 25% wt, between 25 and 30% wt, between 30 and 35% wt, or between 35 and 40% wt, or between 40 and 45% wt .

Even more preferably, the amount of colloidal silica within the aqueous solution is between 25 and 35% wt .

In a further step of the invention, the fly ash, the fragmented waste material and the aqueous solution are mixed, obtaining a substantially homogenous colloidal mixture .

As a non-limiting example, the mixing step is performed with a continuous stirred-tank reactor (CSTR) for the time necessary for obtaining the homogenous colloidal mixture.

Depending form the mixing speed of the reactor and from the amount of reagents involved, this time ranges between 30 minutes and some hours.

According to a preferred embodiment, the mixing step and the fragmenting step are performed at least partially simultaneously, e.g. at the same time in the same reactor.

In a subsequent step of the method, the colloidal mixture is solidified in an inert semi-finished product, wherein the heavy metal is entangled in a substantially insoluble metal-silica compound.

In other words, after the colloidal mixture has been solidified, the heavy metal remains trapped inside the semi-finished product. This way, the heavy metal is prevented from solving and from propagating in the environment during its use.

Furthermore, the solidifying step comprises a step of resting the colloidal mixture at a temperature below 40°C.

For this reason, innovatively, the method according to the present invention allows an economical disposal of waste materials containing heavy metals.

In fact, due to the presence of fly ashes in the colloidal mixture, solidification happens substantially at environmental temperatures comprised between 10 and 35 °C, variable from the relevant location and season.

Preferably, the resting step is performed substantially at room temperature, i.e. between 20 and 25°C, optionally at atmospheric pressure.

According to an advantageous embodiment, the resting step is performed for a resting time of at least 24 hours .

According to further alternatives, the resting time is comprised between 24 and 120 hours, preferably between 24 and 36 hours, between 36 and 48 hours, between 48 and 60 hours, between 60 and 72 hours, advantageously between 72 and 84 hours, between 84 and 96 hours, between 96 and 108 hours, or between 108 and 120 hours. According to a preferred embodiment, the fly ash and the fragmented waste material overall further comprise at least a water-soluble halide.

As a non-limiting example, the possible halides contained in the waste material are chloride, fluoride, bromide and/or iodide of various metals, e.g. lead and/or sodium.

It is important to point out that these, halide compounds, besides provoking obvious corrosion problems to the equipment, drastically limit possible (re-) uses of the semi-finished product because the latter is a still hazardous material.

Due to these environmental and safety grounds, incorporation of the semi-finished product in construction materials like cement or concrete is out of the question.

According to this embodiment, the method according to the invention further comprises a step of removing the water-soluble halide.

In other words, depending whether the step of removing is performed before or after solidification of the semi-finished product, during this method step the dangerous and corrosive water soluble halides are advantageously removed from the colloidal mixture and/or form the semi-finished product. This way, the semi-finished product obtained according to the present invention has an increased variety of possible (re-) uses, in relation to the solid products traditionally obtained.

Preferably, the step of removing comprises a step of washing the colloidal mixture and/or the semi-finished product .

A preferred embodiment foresees a washing step with water, because of its advantageous availability and cost.

According to a further embodiment, the step of removing is followed by a step of recovering the soluble halide from the washing liquid, i.e. from water in a preferred variant.

According to preferred alternatives of the present method, the step of recovering comprises a step of mechanical (ultra- ) filtration, centrifugation, decantation, membrane separation, evaporation, extraction, distillation, electrolysis and/or crystallization.

Advantageously, this step of removing further comprises a step of removing soluble and/or leachable compounds, e.g. sulfates, nitrates and/or carbonates, optionally comprising a step of recovering also these compounds, e.g. by washing and optionally recovering them as explained herein before. Preferably, the solidifying step comprises step of gelation of the colloidal mixture by water removal, pH variation, addition of at least one salt and/or organic solvent, and/or subjecting said mixture to ultrasounds.

According to a further embodiment, the method comprises a step of shaping the semi-finished product, e.g. by (co) extrusion, injection, molding and similar, wherein this product is formed in a desired shape.

This desired shape may be decided in relation to the final use of the semi-finished product.

As a non limiting example the semi-finished product may be used as a construction material, such as a tile, a brick or another a structural element, as a (powdered) filler, e.g. for polymer materials or different matrices in order to improve their mechanical properties, as an extender or a general additive to rubber, engineering polymers, asphalt, concrete, cement, as well as rubber, polymer-, asphalt-, concrete- and cement-based construction products, and similar applications.

Preferably, the shaping step is performed at least partially simultaneously with the solidifying step.

For example, the colloidal mixture may be cast in a mould having the desired shape, or may be forced through an extrusion dye in a semi-solid state of said mixture.

Preferably, the method further comprises a step of incorporating the semi-finished product in a final product .

As a non limiting example, the final product comprises rubbers, engineering polymers, asphalt, concrete, cement, as well as rubber, polymer-, asphalt-, concrete- and cement-based construction products.

The present invention furthermore relates to the use of the semi-finished product, obtained according to the method explained herein before, as a construction material, as a paving or roofing substrate, as a filler for any these uses, and/or as a stabilization material e.g. for geology purposes.

The object of the present application will now be explained on the basis of two non limiting examples. EXAMPLE 1

Manufacturing of the semi-finished product.

The following fragmented and dried waste materials are provided: flue-gas treatment (FGT) , coal ash (CA) and flue-gas desulphurization (FGD) residues. The powders are mixed in the percentage of FGT 65% wt, CA 15% wt and FGD 20% wt.

An aqueous solution of colloidal silica Ludox SM 30 and deionized water is added to the powders mixture in the percentage respectively of 25% wt and 50% wt to obtain an homogenous mixture by continuously stirring in a CSTR reactor for 1 hour. The obtained colloidal mixture rests in air for hours

EXAMPLE 2

Removal of the water-soluble halides.

Afterwards, the inert semi-finished product obtained as before is ground to powder and then leached in water for 12 hours to remove the soluble salts.

From a TXRF analysis conducted on the leaching water, it can be seen that a high amount of chlorine is present. It is important to point out that the water used for leaching was initially completely free from chlorine, so it derives that this element was released from the semi-finished product during the method according to the present invention.

An XRD analysis of the inertized material shows that soluble salts are mainly made from sodium chloride which is no more present in the semi-finished product.

Innovatively, the method of the present invention is suitable for an economical disposal of waste materials containing heavy metals.

In fact, the method of the present invention, besides allowing more convenient and environmentally sustainable process conditions, is performed with low- cost and high-availability chemicals, that contribute to maintaining an overall inexpensive process. Furthermore, the solid semi-finished product therewith obtained is environmentally safe because of drastically improved leaching properties. For this reason the traditionally widespread landfilling disposal strategy may be discarded as the semi-finished product according to the present application may be re-used as a secondary material.

Advantageously, the method of the present invention is highly versatile, so that it can be used with a variety of different waste materials, irrespective of their origin.

Advantageously, the method of the present invention does not require a preliminary removal of halides, sulfates, nitrates, carbonates, and/or of further salts, that could e.g. hinder further method steps or limit the possible uses of the semi-finished or of the final product, as these components do not hinder the correct execution of the main steps.

Advantageously, the method of the present invention is suitable for being performed with low-technology appliances, so that any waste-generating industrial plant may produce the semi-finished product in-situ with a r

little economical effort and a high environmental advantage .

Advantageously, due to the reduced leachability of the semi-finished product, the variety of possible uses of the semi-finished product according to the present invention is drastically increased.

Even if not previously specified, a person skilled in the art could envisage, by referring to the customary skill of the sector, to replace or substitute some of the aspects referred above with other technically equivalent elements.

Also these replacements and substitutions remain within the scope defined by the following claims.

Furthermore, each alternative illustrated in relation to a particular embodiment can be performed independently from the other described embodiments.