WO2012176160A1

WO2012176160A1 - Method and apparatus for the treatment of bottom ashes

Info

Publication number: WO2012176160A1
Application number: PCT/IB2012/053163
Authority: WO
Inventors: Paolo Bevilacqua; Paolo AVANZI; Paolo BELVIOLANDI; Francesco POVIANI; Laura POVIANI; Fulvio RONCARI
Original assignee: ENVIROTECH Srl; Aprica SpA
Current assignee: ENVIROTECH Srl; Aprica SpA
Priority date: 2011-06-23
Filing date: 2012-06-22
Publication date: 2012-12-27
Anticipated expiration: 2013-12-23
Also published as: EP2723499A1; ITMI20111141A1

Abstract

The present invention relates to a method for the treatment of bottom ashes deriving from combustion processes, said method comprising a step of storing and aging bottom ashes and a step of grinding stored and aged bottom ashes, as well as a subsequent water washing step of said stored and aged bottom ashes. The water washing step is divided into two distinct washing stages, in every one of which water washing is combined with at least one screening step and one step of sand slime removal and hydrocyclone treatment and between which a further grinding step of the bottom ashes hold by screening in the first washing stage is carried out. The water used to wash bottom ashes is made to continuously recirculate in each washing stage after a step of clariflocculation with chemical treatment and fresh water is supplied in the second washing stage only. Thanks to these features, it is possible to reduce the content in polluting substances contained in the bottom ashes within the limits provided by the present standards concerning their reuse as inert materials, by using a much smaller amount of fresh water than that used in known treatment apparatuses for bottom ashes based on water washing. The invention also relates to an apparatus configured so as to carry out the treatment method.

Description

METHOD AND APPARATUS FOR THE TREATMENT OF BOTTOM ASHES

The present invention relates to the treatment of bottom ashes deriving from combustion apparatuses and in particular to a treatment method and an apparatus that provide for washing bottom ashes with water.

Combustion is used as a treatment process for a wide range of products, substances and materials, such as municipal solid waste. During combustion solid residues of various types are produced, the major fraction of which is made up of "bottom ashes". It is known that bottom ashes can be advantageously re-used as inert building materials, provided that they are subjected to treatments suitable to eliminate the main polluting substances they contain, in particular heavy metals such as Cu, Cr, Pb, Zn, Sb and Mo, and salts such as chlorides and sulfates. Bottom ashes in fact have good technical properties that allow their use in civil construction works such as road surfaces and noise barriers. In addition to this, bottom ashes can be used as inert materials in the production of cement, concrete and aggregates, as well as waste covering materials at landfills.

In general, the treatment of bottom ashes comprises their aging by way of carbonation with C0₂, which causes the precipitation of certain polluting substances contained in them in the form of insoluble carbonates, in particular heavy metals. Aging is generally combined with grinding and screening of bottom ashes, as well as with the recovery, by separation, of metallic materials contained in them, as is typically the case of municipal solid waste.

The use of bottom ashes is regulated by national laws and standards, such as e.g. the Italian Ministerial Decree 186/2006 and its subsequent amended versions, which establish limits on the leaching of pollutants from bottom ashes when in contact with water.

The environmental quality of bottom ashes in terms of leaching of pollutants is evaluated through the so-called "leaching test" described in the UNI-EN 12457-2 standard, which allows to measure their attitude to the leaching of organic and inorganic pollutants when in contact with water.

The use of the inert materials deriving from the treatment of bottom ashes is also regulated by technical standards in the field, such as for example:

- UNI EN 12620: Aggregates for concrete;

- UNI EN 13043: Aggregates for bituminous mixtures;

- UNI EN 13139: Aggregates for mortar;

- UNI EN 13242: Aggregates for civil engineering works.

The limits currently established by the Italian standards for the use of bottom ashes as inert building materials are relatively strict with respect to the European context, which results in a rather limited use thereof, that is about 20% of the total amount produced every year.

In order to improve the quality of bottom ashes in view of their disposal and/or reuse according to the law, water washing processes are more and more employed.

Water washing is used primarily to remove components highly soluble in water, such as, for example, salts, heavy metals and unburaed residues of organic substances, thus considerably limiting the leaching of these pollutants during the use of bottom ashes as inert building materials or as landfill coating materials, which could cause significant damages to structures and people as well as to the surrounding environment.

In order to obtain bottom ashes meeting the existing standards in terms of leaching of polluting substances, washing must be carried out by using rather high weight ratios between fresh water and bottom ashes, which are about 3:1, so that once washing is finished waste water discharged into sewage systems or surface waters has a content of pollutants, chlorides in particular, within the limits established by the standards. Thus, for example, in order to reuse a ton of bottom ashes, about 3 m³ of fresh water are needed.

This results in a very high water consumption, which strongly challenges the opportunity to recover large amounts of bottom ashes from both an economic and an environmental point of view. The large amounts of water used in washing treatments in fact are rich in pollutants and require further purification treatments subjected to severe environmental constraints, so the costs associated with the use of water are added to those related to its subsequent purification treatments and disposal to sewage systems or surface waters.

There is therefore the need to provide a method and an apparatus for the treatment of bottom ashes by water washing allowing to overcome the above-mentioned drawbacks, and in particular to reduce the consumption of fresh water, which is an object of the present invention.

It is also an object of the present invention to provide a method and a apparatus for the treatment and recovery of bottom ashes by water washing, which allow to recover a percentage of inert materials from treated bottom ashes larger than the percentage that is currently recoverable by known methods and treatment apparatuses.

An idea of solution underlying the present invention is to divide the water washing treatment of bottom ashes in two distinct washing stages in every one of which washing is combined with at least one screening step and at least one step of slime removal and hydrocyclone treatment. In both water washing stages the water used to wash the bottom ashes is continuously recirculated after a step of clariflocculation with chemical treatment, and fresh water for rinsing bottom ashes is used only in the second washing stage.

Moreover, according to the present invention the bottom ashes held by screening in the first washing stage are subjected to a grinding step before the second washing stage.

Thanks to these characteristics it is possible to effectively reduce the content of polluting substances in the bottom ashes by using a total of an amount of fresh water much lower than that used in a traditional washing apparatus, resulting in a benefit for the environment.

An advantage offered by the invention is that the grinding step between the two washing stages provides the bottom ashes with a grain size corresponding to the requirements of the leaching tests according to the present standards in the field, whereby it is possible to check the compliance of the treated bottom ashes to the requirements of the standards directly at the outlet of the apparatus, without subjecting them to further grinding steps that might release further polluting substances.

Another advantage offered by the invention is that between the two washing stages a step of separation and recovery of metallic ferrous and non ferrous materials is carried out. The provision of a separation step of metallic materials between the two washing stages allows to recover almost the total amount of metallic materials present in the bottom ashes, because they have a suitable size for the separation thanks to the grinding step that precedes the first washing stage and to the screening step carried out in the first washing stage.

Moreover, thanks to the first washing stage the metal materials that are separated and recovered are substantially free of the main pollutants that characterize bottom ashes, which considerably improves their quality in view of their subsequent use.

The separation of metallic materials may be advantageously combined with the screening of bottom ashes, thus allowing to optimize the recovery process depending on the grain size of the bottom ashes coming out from the first washing stage.

Still another advantage offered by the invention is that the water subjected to clariflocculation and treatment that is made to recirculate in the second washing stage may be fed back to the first washing stage in order to compensate for possible apparatus losses.

Further advantages and features of the method and apparatus for the treatment of bottom ashes according to the present invention will become clear to those skilled in the art from the following detailed and non-limiting description of embodiments thereof with reference to the accompanying drawings in which:

- Figure 1 is a block diagram schematically showing the main sections of the apparatus according to the invention;

- Figure 2 is a block diagram schematically showing the first washing stage of the apparatus according to the invention;

- Figure 3 is a block diagram schematically showing the second washing stage of the apparatus according to the invention;

- Figure 4 is a block diagram schematically showing the separation section of the metallic materials of the apparatus according to the invention.

Referring to Figure 1 , the treatment method according to the invention comprises an initial step of storing and aging of bottom ashes in a section S 1. The ashes deriving from combustion apparatuses, for example, from an incineration apparatus of municipal solid waste, are stored in piles and exposed for a predetermined period of time to atmospheric agents. The contact with carbon dioxide and moisture promotes a series of chemical reactions of carbonation which cause the precipitation of some contaminants as insoluble carbonates, especially heavy metals, thus reducing the environmental risks associated with the future disposal and/or reuse of the bottom ashes.

Moreover, aging reduces the risk of incrustation and deposits in machinery intended to carry out washing of the bottom ashes, as it causes the precipitation of elements such as calcium in the form of carbonates.

The aging carried out by simple exposure to atmospheric agents generally requires times ranging between 90 and 180 days. Alternatively, accelerated aging processes may be used, wherein the bottom ashes are wetted by irrigation and treated with flows of carbon dioxide.

Aged bottom ashes are typically handled by way of buckets moved by overhead cranes and fed to a loading hopper of the treatment apparatus. Alternatively, the bottom ashes may be collected and handled by wheel loaders.

The bottom ashes so loaded are fed to a section S2 wherein they are subjected to a grinding step in a shredder/crusher e.g. provided with counter-rotating shafts, by which they are dimensionally reduced to a grain size suitable for washing, for example lower than 200 mm. The ground ashes are collected on a conveyor belt and sent to the washing.

During the grinding step possible inert- fractions of large size present among the bottom ashes are greatly reduced dimensionally, resulting in the benefit of protecting the machines that carry out washing and of releasing possible metallic inclusions present in them.

The grinding step also results in a uniform distribution of the ashes on the conveyor belt arranged downstream of the shredder/crusher, which allows to measure and accurately adjust the flow of ashes fed to the washing stage, for example by a weighing system comprising load cells applied to the conveyor belt.

Downstream of the grinding section S2 the bottom ashes are subjected to washing with water.

According to the present invention, the washing of bottom ashes is divided into two distinct stages, respectively indicated as S3 and S4, in every one of which washing is combined with at least one screening stage and one step of slime removal and hydrocyclone treatment. Moreover, between the two washing stages bottom ashes are subjected to a further grinding step.

Referring now to Figure 2, in the first washing stage S3, the ground ashes travelling on the conveyor belt are loaded into a washer 31 and dragged in its interior by means of a water flow. The weight ratio between water and bottom ashes in the first washing stage S3 is very high, for example corresponding to about 3 to 5: 1, which allows to dissolve almost the total amount of salts. Inside the washer 31 the heavier materials tend to proceed more slowly, whereas a lighter fraction, mainly consisting of unburned organic substances and mineral formations that have, for example, a lower density than water, floats and is carried away by the water flow, and can thus be separated from the bottom ashes.

The water is preferably made to circulate in countercurrent with respect to the feeding direction of the bottom ashes, which allows to separate the lighter fraction more easily.

The washer 31 is preferably of the type comprising a rotary drum, wherein the rotary drum is provided ith a plurality of vanes that allow to establish washing mechanisms that are based not only on the passage of the contaminants into solution, but also on a series of mechanical interactions among the particles of the ashes, such as friction, abrasion, impacts, and the like.

The lighter fraction separated by the washer 31 is fed to a vibrating sieve 32.

The vibrating sieve 32 is configured to hold unburned organic substances, which are collected on a conveyor belt (not shown) and transported to a suitable storage container.

The mineral formations and the water that instead pass through the meshes of the vibrating sieve 32 are fed e.g. by gravity to a group 33 of slime removal and hydrocyclone treatment, which allows their recovery in the form of fine sands, for example having a grain size lower than or equal to 2 mm. The group 33 of slime removal and hydrocyclone treatment may comprise an attrition cell 33a and spiral gravimetric separators 33b, which allow to further wash the mixture of water and sand separated by the hydrocyclone. In particular, the attrition cell 33a subjects the mixture of water and sand to a strong agitation so as to promote impacts and friction among the particles, which prolong the mechanical separating action of the pollutants. The mixture of sand and water that comes out from the attrition cell 33a flows e.g. by gravity in the spiral gravimetric separators 33b, which allow the separation and removal of the residues of organic fractions and light mineral fractions that contain most of the pollutants from the recoverable inert materials thanks to the combined effect of centrifugal force and gravity.

The residues of organic fractions and light mineral fractions coming out from the spiral gravimetric separators that have a grain size lower than 2 mm are dried on a vibrating dryer 34 and subsequently stored in order to be disposed. The remaining inert fraction having a grain size lower than or equal to 2 mm is instead sent to a vibrating dryer 36 together with the inert materials having a grain size lower than or equal to 4 mm coming from the screening and subsequently stored to be recovered. The water recovered by the vibrating dryer 34 is fed back to the group 33 of slime removal and hydrocyclone treatment, thus allowing to recover further organic and mineral fractions.

Downstream of the washer 31 a screening group 35 is also arranged, which comprises a plurality of sieves the meshes of which have a progressively smaller size suitable to treat the heavier materials that pass through the washer 31 and that are separated from the unburned organic materials and the lighter mineral fractions.

In the embodiment illustrated in Figure 2 there are schematically shown three sieves that are arranged in series and are configured to hold fractions having a grain size greater than 40 mm, 10 mm and 4 mm, respectively. More generally, the screening group 35 is preferably configured to hold fractions of the washed ashes having a grain size greater than 4 mm.

The sieves of the screening group 35 may advantageously be integrated in the washer 31.

The inert materials collected downstream of the screening group 35 and those coming from the group 33 of slime removal and hydrocyclone treatment are sent to the vibrating dryer 36 and then stored in a special container. Also in this case the water recovered from vibrating dryer 36 is fed back to the group 33 of slime removal and hydrocyclone treatment.

All the water used for washing the bottom ashes in the first washing stage S3 is made to recirculate on a continuous base, i.e. once started the apparatus the first washing stage S3 does not require any supply of fresh water during its operation. The recirculation path of the water in the first washing stage S3 is indicated in figure 2 by way of dashed lines.

In order to recirculate the muddy water, i.e. the water containing a fine mineral fraction, it is necessary to provide a suitable step of clariflocculation with chemical treatment. To this aim, the apparatus according to the invention comprises a clariflocculation group 37 and a tank 38 for chemical treatment to which the water coming out from the group 33 of slime removal and hydrocyclone treatment and from the vibrating dryers 34 and 36 is sent. In the clariflocculation group 37 a process of clariflocculation of the water is carried out, which also allows to recover dehydrated sludge. The dehydrated sludge, which has a moisture content of about 40%, contains most of the salts present in solution in the water used to wash the bottom ashes and may be recovered for the production of cement in cement factories.

Moreover, before being fed back to the washer 31 the water subjected to the clariflocculation process is collected in a storage tank 38 wherein a chemical treatment is carried out by dosing additives for adjusting pH and for precipitating polluting substances, in particular heavy metals, as well as by dosing anti-foaming and anti- fouling agents.

The presence of unburned organic materials in the bottom ashes fed to the washer of the first washing stage may cause unpleasant odors due to the turbulent motions to which bottom ashes are subjected during washing. For this reason dosing of an oxidizing solution, such as sodium hypochlorite, in the water that feeds the washer 31 may advantageously be carried out, for example at the outlet of the recirculation pump. In this way an adequate contact time between the solution of sodium hypochlorite and the volatile organic substances within the washer 31 is ensured, thus favoring their oxidation and suppressing unpleasant odors as they arise.

Considering for example the treatment of bottom ashes deriving from the combustion of municipal solid waste, the first washing stage S3 allows to treat the 100% of the bottom ashes and to separate about the 8% in the form of dehydrated sludge, about the 22 % in the form of inert materials with a grain size lower than 4 mm and about the 20% in the form of metallic materials, whose separation mode will be discussed later. The remaining 50% of the bottom ashes is sent to the second washing stage S4.

The inert materials recovered in the first washing stage S3 have a concentration of pollutants, especially chlorides and organic compounds (evaluated through the COD, i.e. Chemical Oxygen Demand, parameter), slightly higher than the limits set by the leaching test, but lower than the limits established by the UNI- EN field standards for the reuse as inert construction materials.

Still referring to Figure 1, according to the present invention the bottom ashes held by the sieves of the screening group 35 of the first washing stage S3 are subjected to a further grinding step in order to provide them with a grain size within the limits set by the leaching test required by the standards, in particular a grain size lower than or equal to 4 mm. This further grinding step is carried out in a section S5 of the apparatus. The bottom ashes further dimensionally reduced are then sent to the second washing stage S4.

The grinding of the bottom ashes carried out prior to the second washing stage S4 allows to separate more polluting elements and salts that can thus be effectively eliminated by the second washing.

Referring now to Figure 3, similarly to the first washing stage S3 also the second washing stage S4 comprises a washer 41, such as a rotary drum washer as in the first washing stage S3, downstream of which there are arranged a sieve 42, which allows to hold possible residues having a grain size greater than 4 mm, and a group 43 of slime removal and hydrocyclone treatment that allows to separate the lighter mineral fraction coming out from the washer 41.

The bottom ashes having a grain size greater than 4 mm held by screening may advantageously be fed back upstream of the first washing stage S3, thus allowing a new washing and a new grinding thereof.

The inert materials with a grain size lower than or equal to 4 mm recovered through the group 43 of slime removal and hydrocyclone treatment are sent to a vibrating dryer 44 and then stored in a special container.

Similarly to the first washing stage S3, in the second washing stage S4, the water used in the washer 41 is preferably made to circulate in countercurrent with respect to the feeding direction of the bottom ashes, thus allowing to remove the pollutants more effectively.

In the second washing stage S4 the weight ratio between water and bottom ashes is lower than that of the first stage S3 and equal to about 2:1, which is possible because, as explained above, the bottom ashes fed to the second washing stage S4 are about the 50% of those loaded in the apparatus.

A part of the water used in the second washing stage S4 is fresh water, which is exclusively fed to the second washing stage S4 of the apparatus with a weight ratio of about 1:1 with respect to the bottom ashes treated therein. Still with reference to the example of the processing of bottom ashes deriving from the combustion of municipal solid waste, given that the bottom ashes fed to the second washing stage are about the 50% of the total amount of bottom ashes, the weight ratio between fresh water and bottom ashes is therefore about 0.5:1, i.e. nearly one-sixth of the weight ratio that is typical of a washing apparatus of a traditional type, which ratio characterizes the apparatus according to the invention.

As in Figure 2, also in Figure 3 the water circulation path is schematically indicated by dashed lines. The supply path of fresh water is instead shown by ways dashed and dotted lines.

The source (not shown) of fresh water that feeds the washer 41 of the second washing stage S4 may also be connected to the sieve 42 and possibly also to the vibrating dryer 44, thus allowing to carry out a further washing or rinsing of the inert materials before their collection.

Similarly to the first washing stage S3, also the second washing stage S4 comprises a clariflocculation group 45 similar to the clariflocculation group 37 of the first washing stage S3. Also in this case dehydrated sludge are recovered, which contain residues of salts and polluting substances present in the ashes subjected to the second washing stage.

Downstream of the clariflocculation group 45 there is a group 46 for the depuration of overflow waters before their discharge into public sewage systems or surface waters, so that they meet the requirements of the present standards. The overflow waters are subjected to a chemical-physical treatment with flocculation, addition of additives for pH adjustment and precipitation of heavy metals and subsequent clariflocculation. The overflow water can also be subjected to a biological treatment preferably by way of activated sludge. Finally, there is provided a step of sand filtration with quartzite filters, which serves to further ensure compliance of overflow water with the limits set by the standards as to suspended solids.

Still with reference to the example of the treatment of bottom ashes deriving from the combustion of municipal solid waste, the second washing stage S4 allows to treat the remaining 50% of bottom ashes and to recover approximately the 45% in the form of inert materials with a grain size lower than 4 mm and about the 5% in the form of dehydrated sludge.

The inert materials recovered in the second washing stage S4 have pollutant concentrations below the values set by the UNI-EN standards for reuse as inert building materials and within the limits established by the leaching test at national level.

According to a further aspect of the invention, a section S6 adapted to allow separation of metallic materials which may be present in the bottom ashes may be arranged between the first and the second washing stages S3 and S4. This is typically the case of bottom ashes deriving from a combustion apparatus of municipal solid waste.

According to the present invention the section S6 is also arranged upstream of the further grinding section S5,

The separation of metallic materials carried out between the first and the second washing stages allows to recover metallic materials that have been washed and are thus substantially free from the pollutants present in the bottom ashes.

Moreover, separation is carried out on bottom ashes already subjected to grinding and therefore having a smaller size with respect to the bottom ashes present at the inlet of the apparatus, for example a size lower than 200 mm that is more suitable for the separation.

Referring to Figure 4, the section S6 for the separation of metallic materials comprises at least one treatment line connected to the first washing stage S3 downstream of the screening group 35. The treatment line is provided with at least one magnetic separator 61 and/or at least one eddy current separator 62, for example of the type comprising a conveyor belt, arranged in series, and a possible further magnetic separator 63, for example of the rotary drum type, arranged therebetween and adapted to collect weakly magnetic materials.

Since the screening group 35 of the first washing stage S3 comprises a number of sieves that hold fractions having a progressively smaller grain size, it is possible and advantageous to carry out the separation of metallic materials on parallel treatment lines that are respectively connected downstream of each sieve of the screening group 35 of the first washing stage S3.

In the embodiment illustrated in Figure 4 there are schematically shown three parallel separation lines operating on bottom ashes having a grain size greater than or equal to 40 mm, 10 mm and 4 mm, respectively.

A grinding step of non-metallic materials having a grain size greater than 40 mm may also be advantageously provided, which is carried out by means of a crusher 64 arranged downstream of the magnetic separator 61 of the first separation line of metallic materials, thus allowing their washing and recirculation in the washer of the first washing stage.

This way of separating metallic materials from bottom ashes having a grain size greater than 40 mm, and of making inert materials having a grain size greater than 40 mm and then reduced to a size lower than or equal to 40 mm to recirculate, facilitates the washing of inert materials and allows to feed to the subsequent sections materials with a grain size comprised between 40 mm and 10 mm and between 10 mm and 4 mm in order to separate metallic ferrous and non ferrous materials on a number of lines in parallel.

This mode of separating metallic materials in parallel is particularly advantageous, because it is well known that the magnetic and eddy current separators are all the more effective the more uniform is the size of the materials to be separated.

The inert non-metallic materials collected downstream of the section S6 of separation of metallic materials are fed to the grinding section S5 and then to the second washing stage S4.

It is clear that the embodiments of the method and the apparatus herein described and illustrated are just examples susceptible to numerous variants. For example, the water made to recirculate, which is clarified and accumulated in the second washing stage S4, may be fed to the first washing stage in order to compensate for possible water losses of the apparatus. Furthermore, it is possible to provide for one or more by-pass lines allowing to collect materials without subjecting them to all the treatment steps of the apparatus. For example, in the section S6 for the separation of metallic materials it is possible to arrange a by-pass line that allows to recover a certain amount of inert materials with a grain size greater than 4 mm before the grinding step carried out in the section S5 that precedes the second washing stage S4. Furthermore, possible metal fractions resulting from the grinding of the bottom ashes in section S5 between the first and the second washing stages S3, S4 may be fed back to the section S6 for the separation of metallic materials and separated therein by way of a suitable magnetic separator 61.

Claims

1. A method for the treatment of bottom ashes deriving from combustion processes, said method comprising a step of storing and aging bottom ashes and a step of grinding stored and aged bottom ashes, as well as a subsequent water washing stage of said stored and aged bottom ashes, characterized in that said water washing stage is divided into two distinct washing stages, in each one of which water washing is combined with at least one screening step and one step of sand slime removal and hydrocyclone treatment and among which a further grinding step of the bottom ashes held by screening in the first washing stage is carried out, in that the water used to wash bottom ashes is continuously recirculated in each washing stage after a step of clariflocculation with chemical treatment and in that fresh water is also supplied in the second washing stage only.

2. A treatment method according to claim 1, wherein in the first washing stage the weight ratio between water and bottom ashes is 3 to 5:1.

3. A treatment method according to claim 1 or 2, wherein in the second washing stage the weight ratio between water and bottom ashes is 2:1, and wherein the weight ratio between the fresh water supplied and the bottom ashes is 1 : 1.

4. A treatment method according to any of claims 1 to 3, wherein in each washing stage water is supplied in countercurrent with respect to the movement direction of the bottom ashes.

5. A treatment method according to any of claims 1 to 4, wherein in the first and second washing stage a light fraction of the bottom ashes that can be dragged from the water is subjected to sand slime removal and to hydrocyclone treatment.

6. A treatment method according to claim 5, wherein in the first washing stage said light fraction of the bottom ashes is separated from the unburned organic materials by screening.

7. A treatment method according to any of claims 1 to 6, wherein inert materials recovered downstream of the screening, sand slime removal and hydrocyclone treatments in the first washing stage are respectively subjected to treatments of vibration drying and subsequently collected.

8. A treatment method according to any of claims 1 to 7, wherein the screening steps associated with each washing stage are so configured to separate and hold washed bottom ashes having a size larger than 4 mm.

9. A treatment method according to any of claims 1 to 8, wherein in said further grinding step the ground bottom ashes have a grain size less than or equal to 4 mm.

10. A treatment method according to any of claims 1 to 9, further comprising a step of separation of metallic materials contained in the bottom ashes, said separation step being carried out between the first and second washing stages on bottom ashes separated by screening in the first washing stage and before their further grinding step.

11. A treatment method according to claim 10, wherein in said separation step non-metallic materials having a grain size larger than 40 mm are ground and then fed back to the first washing stage.

12. An apparatus for the treatment of bottom ashes deriving from combustion processes, said apparatus comprising a storage and aging section (SI) wherein bottom ashes are stored and aged and a grinding section (S2) wherein stored and aged bottom ashes are ground, as well as a subsequent section wherein bottom ashes are subjected to water washing, characterized in that said water washing section is divided into two separated washing stages (S3, S4) respectively comprising a washer (31, 41), a screening group (35, 42) and sand slime removing and hydrocyclone treating group (33, 33a, 33b, 43) arranged downstream of the washer (31, 41), as well as a clariflocculation group (37, 45) arranged downstream of the sand slime removing and hydrocyclone treating group (33, 33 a, 33b, 43) and suitable to allow the recirculation of washing water, wherein between said washing stages (S3, S4) a further grinding section (S5) is arranged, and wherein only the second washing stage (S4) is connected to a fresh water supply.

13. An apparatus according to claim 12, wherein said fresh water supply is connected to the washer (41) and to the screening group (42) of the second washing stage (S4).

14. An apparatus according to claim 12 or 13, wherein the washers (31, 41) of each washing stage (S3, S4) are rotary drum washers.

15. An apparatus according to any of claims 12 to 14, wherein the screening groups (35, 42) of each washing stage (S3, S4) respectively comprise one or more sieves configured to separate and hold bottom ashes having a grain size larger than 4 mm.

16. An apparatus according to claim 15, wherein the screening group (35) of the first washing stage (S3) comprises three sieves arranged in series, said sieves being adapted to separate and hold bottom ash having a grain size larger than 40 mm, 10 mm and 4 mm, respectively.

17. An apparatus according to any of claims 12 to 16, wherein the first and second washing stages (S3, S4) also comprise vibrating dryers (34, 36, 44) arranged downstream of the sand slime removing, hydrocyclone treating and screening groups (33, 43, 35, 42).

18. An apparatus according to any of claims 12 to 17, wherein the first washing stage (S3) also comprises a vibrating sieve (32) adapted to receive a light fraction of bottom ashes separated by the washer (31).

19. An apparatus according to claim 18, wherein said vibrating sieve (32) is arranged between the washer (31) and the sand slime removing and hydrocyclone treating group (33).

20. An apparatus according to any of claims 12 to 19, further comprising a separation section (S6) suitable to separate metallic materials contained in the bottom ashes, said separation section (S6) being arranged between the first and second washing stages (S3, S4) and upstream of the further grinding section (S5).

21. An apparatus according to claim 20, wherein said separation section (S6) comprises at least one separation line provided with at least one magnetic separator and/or at least one eddy current separator arranged in series, said separation line being connected to the first washing stage (S3) downstream of its screening group (35).

22. An apparatus according to claim 21„ wherein the separation section (S6) comprises a plurality of separation lines operating in parallel on bottom ashes having a different grain size, and wherein each separation line is connected downstream of a sieve of the screening group (35) of the first washing stage (S3).

23. An apparatus according to any of claims 20 to 22, wherein the separation section (S6) further comprises a grinder (64) adapted to grind non-metallic materials having a grain size larger than 40 mm.

24. An apparatus according to any of claims 12 to 23, wherein the second washing stage (S4) further comprises a depuration group (46) for depurating overflow waters in view of their discharge into public sewer or surface water, said depuration group (46) being arranged downstream of the clariflocculation group (45) for the clariflocculation of recirculated water.