GARCÍA, Maria Fernanda (Via Petrarca 14, Follonica, I-58022, IT)
GARCÍA, Maria de Los Angeles (Via Magellano 5, Bagno di Gavorrano, I-58023, IT)
| CLAIMS 1. A method for treating a contaminated material, said contaminated material containing contaminating metals, in particular heavy metals, said method comprising the steps of: - prearranging a predetermined amount of said contaminated material; - mixing said amount of contaminated material with a predetermined quantity of a solution of water-soluble polyphosphates obtaining an intermediate material; - mixing said intermediate material with a predetermined quantity of calcium hydroxide obtaining a modified material that contains substantially water-insoluble compounds, such that said mixing steps cause said contaminating metals to be trapped as insoluble compounds within said modified material and a leaching and/or a dissemination into the environment of said contaminating metals is prevented. 2. A method according to claim 1 , wherein said soluble polyphosphates are selected from the group comprised of: - alkali metal polyphosphates, in particular sodium polyphosphates; - alkaline-earth metal polyphosphates, in particular monobasic alkaline-earth metal polyphosphates. 3. A method according to claim 1 , wherein said calcium hydroxide is put into contact with said intermediate material once a prefixed time has elapsed after said solution of water-soluble polyphosphates has come into contact and reacted with said contaminated material, wherein said prefixed time is at least 15 minutes, in particular at least 30 minutes. 4. A method according to claim 1 , wherein a pressing step of said modified material is provided such that said modified material turns from an original size to a reduced size obtaining a compressed material, said pressing step causing said contaminating metals to be further trapped within said compressed material and a leaching and/or a dissemination into the environment of said contaminating metals is further prevented. 5. A method according to claim 3, wherein said pressing step is carried out under a pressure that is higher than 700 Kilogram-force per square centimetre, in particular under a pressure higher than 1000 Kilogram- force per square centimetre, more in particular said pressure is higher than 2000 Kilogram-force per square centimetre. 6. A method according to claim 5, wherein said pressure is kept on a surface of said modified material during at least nine hours, in particular during at least twelve hours. 7. A method according to claim 3, wherein after said pressing step said reduced size is comprised between one half and one twentieth of said original size, in particular between one fourth and one sixth of said original size, more in particular about one fifth of said original size. 8. A method according to claim 3, wherein said pressing step is performed in a space selected from the group comprised of: - a plane surface, in particular a soil; - a concave surface, in particular a soil pit. 9. A method according to claim 3, wherein said pressing step comprises in turn the steps of: - prearranging a working surface defined by a frame, said frame having a prefixed height above a soil; - laying down a first layer of said modified material upon said working surface; - pressing said first layer until a prefixed size reduction of said first layer is achieved, a compressed first layer obtained from said first layer; laying down a second layer of said modified material upon said compressed first layer; - pressing said second layer until a prefixed size reduction of said second layer is achieved, - said first and said second layer having preferably a thickness set between 25 and 35 centimetres, as laid down. 10. A method according to claim 3, wherein said pressing step is performed in a mould, to obtain an article which has a prefixed shape, such that, in particular, bricks, tiles, blocks, filler materials are obtained. 11. A method according to claim 1 , wherein said predetermined amount of said contaminated material is obtained by selecting a finer contaminated material fraction which is formed by particles of said contaminated material that have a size that is smaller than 150 micron, in particular said finer contaminated material fraction formed by particles whose size is smaller than 75 micron. 12. A method according to claim 1 , wherein a moisture checking step is provided to check the moisture in said amount of contaminated material, said moisture checking step carried out prior to said step of mixing said contaminated material, and calculating the final humidity of the humidified material, and in case the calculated moisture content in the modified material exceeds 20%, carrying out a drying step of said contaminated material. 13. A method according to claim 1 , wherein said predetermined quantity of said polyphosphate solution is such that the weight ratio between said polyphosphate solution and said amount of contaminated material, said contaminated material considered in dry conditions, is set between 3% and 8%, in particular between of 5% and 6%. 14. A method according to claim 1 , wherein said calcium hydroxide is mixed as a solid to said intermediate material and said predetermined amount of calcium hydroxide is set between of 0,5% and 4% of said amount of contaminated material, said contaminated material considered in dry conditions, in particular said predetermined amount of calcium hydroxide is set between 1 ,5% and 2,5% of said amount of contaminated material. 15. A method for treating a contaminated material, said contaminated material containing a contaminating substance, in particular arsenic, said method comprising the steps of: - prearranging a predetermined amount of said contaminated material; mixing said intermediate material with a predetermined quantity of calcium hydroxide obtaining a modified material that contains substantially water-insoluble compounds, - pressing said modified material such that said modified material turns from an original size to a reduced size obtaining a compressed material, such that said contaminating substance is trapped within said compressed material and a leaching and/or a dissemination into the environment of said contaminating metals is prevented. |
METHOD FOR TRAPPING A CONTAMINATING SUBSTANCE IN A CONTAMINATED MATERIAL
DESCRIPTION Field of the invention
The present invention relates to a method for treating contaminated materials, such as earth or sludge, which contain contaminants heavy metals, or similar hazardous pollutants, by immobilizing such contaminants within a material such that the material can be safely exploited or disposed of. Background of the invention
An always growing need is felt of new methods for decontaminating polluted areas, in particular for decontaminating soils, wastewater sludge and, more in general, materials that are contaminated by leachable heavy metals or non-metallic substances like arsenic, selenium, antimony, which have similar polluting effects as heavy metals.
The well-known decontaminating methods can be roughly classified as:
- thermal treatments: they comprise melting a contaminated solid by heating it up to about 1600-2000 0 C, and then quickly cooling the melt to form an amorphous, glass-like material, in which the pollutants are trapped: such methods require a great deal of energy to reach and maintain the above mentioned high temperatures; besides, an important amount of silicon and aluminium is needed to assist vitrification; furthermore, mercury cannot be treated this way.
- biological treatments have also been proposed, which provide cultivating special plants on the contaminated material, from which they that can absorb specific contaminants, and then disposing the plants; such techniques are not useful, for instance, in case of particularly leachable, or high concentrated pollutants, in particular if these are deep dispersed in the soil.
- Chemical and physical-chemical treatments: the waste is mixed with such specific reagents as silicate and concrete materials, comprising aluminium and/or silicon compounds, or thermoplastic substances, or by vitrification, in order to form a matrix that retains the leachable polluting compounds. Among the latter type of treatments, the following are also known.
In particular, CN1418843 relates to a method to obtain a construction material from a mercury-contaminated, sand-containing petrochemical mud by intimately mixing with a catalysis solidifying agent which contains water, sodium polyphosphate, phosphoric acid, magnesium chloride, magnesium oxide and a polyester resin, along with other specific additives. The method provides furthermore injecting the resulting soft material into a mould, where a steam curing step is carried out at a temperature set between 100 and 180 0 C during 6 to 16 hours. The steam curing step is followed by a free or natural curing step, which takes place in a curing store yard, which requires 25 to 28 days. Such method is suited for treating a well defined class of contaminated materials, and involves a high temperature and energy consumption, and expensive equipment; besides, a long curing time is required to obtain a material that is suitable for further use as a construction material. US5037479 refers to a method of transforming leachable waste material that contains lead, cadmium and zinc into a non leachable form. The method provides mixing the waste with a buffering agent selected among magnesium oxide, magnesium hydroxide, reactive calcium carbonates and reactive magnesium carbonates, and with additional agents selected among triple superphosphate, ammonium and/or diammonium phosphate and/or boric acid and/or metallic iron. A disadvantage of this method is the high cost of the buffering agent and the additional agent.
US5202033 refers to a method of transforming leachable waste material that contains certain heavy metals into a non leachable form. Concerning chromium the method provides mixing the waste with ferrous sulphate. Concerning copper the method provides mixing the waste with an agent selected among sodium phosphate, sodium hydrogen phosphate, superphosphate, triple superphosphate, phosphoric acid, polyphosphoric acid, sodium carbonate, sodium bicarbonate, and calcium carbonate. Concerning arsenic it is treated mixing the waste with superphosphate, triple superphosphate, phosphoric acid, polyphosphoric acid, and adding a pH- control additive selected among magnesium oxide, magnesium hydroxide, calcium oxide and calcium hydroxide. Also in this case, a disadvantage is the high cost of the buffering agent and the additional agent. EP0335024 teaches how to immobilize lead and cadmium as a free flowing particulate mass in a free flowing dry particulate mass of a fly ash and bottom ash mixture, which result from the incineration of municipal waste. A step is provided of contacting the dry ash mixture with at least one water- soluble phosphate in a prefixed amount in the presence of a free lime source that may be lime, hydrated lime, flue gas scrubber products and combinations thereof. The method is limited to the case of immobilizing the two above- mentioned heavy metals in an ash that results from a municipal waste incineration; besides, it requires a large amount of reagents, in particular, of calcium hydroxide.
US2006/0229485 provides a method of reducing the solubility of combined heavy metal bearing material or waste within the limits set by various regulatory tests; the method comprises contacting it with at least one dry stabilizing agent selected among phosphates, cement kiln dust, lime kiln dust, Portland cement, silicates, quicklime, lime, phosphates, ferric sulphate, ferrous sulphate, ferric chloride and mineral complexing agent combinations, hexametaphosphate, polyphosphate, calcium orthophosphate, superphosphates, triple superphosphates, phosphate fertilizers, phosphate rock, bone phosphate, fishbone phosphates, hexametaphosphate polyphosphate, monocalcium phosphate, monoammonia phosphate, diammonium phosphate, dicalcium phosphate, tricalcium phosphate, trisodium phosphate, salts of phosphoric acid, and combinations thereof. This method simply reduces solubility but does not prevent a certain amount of heavy metals and dangerous substances to be leached when they are discharged in the environment.
Summary of the invention
It is therefore an object of the present invention to provide a safe-to-use method for treating a metal-contaminated, substantially solid material, such as sludge or earth that are polluted with heavy metals. It is another object of the present invention to provide such a method, which does not need a long time to be carried out. It is a further object of the present invention to provide such a method, which does not comprise high-temperature steps, and therefore does not require high energy inputs.
It is, furthermore, an object of the present invention to provide such a method that is suitable for treating a soil which does not contain appreciable amounts of silicon or aluminium, or other substances adapted to create an amorphous matrix.
It is still an object of the present invention to provide such a method, which is well suited for treating substantially solid polluted materials containing sulphates, chlorides, bromides, strong oxidant or strong acids.
It is another object of the present invention to provide such a method, which is adapted for treating soils that are contaminated with leachable pollutants.
These and other objects are achieved through a method for treating a contaminated material that contains contaminating metals, in particular heavy metals, the method comprising the steps of:
- prearranging a predetermined amount of the contaminated material;
- mixing the amount of the contaminated material with a predetermined quantity of a solution of water-soluble polyphosphates obtaining an intermediate material mixing the intermediate material with a predetermined quantity of calcium hydroxide obtaining a modified material that contains substantially water- insoluble compounds such said mixing steps cause the contaminating metals to be trapped as insoluble compounds within the modified material and a leaching and/or a dissemination into the environment of the contaminating metals is prevented.
In particular, the contaminated material that can be treated by the method according to the invention may be a soil, a sludge from an industrial or municipal wastewater plant, or it may be a demolition waste material. In order to trap the contaminating metals, an alternate route is therefore used to vitrification, which does not require the presence of aluminium and/or silicon compounds in the contaminated material to be treated. Between the contaminating metals and the reagents, i.e. polyphosphates and calcium hydroxide, ionic exchange reactions take place, by which water-insoluble, complex mixed salts are formed. Such reactions can be schematically written as follows:
[1] 2Na 2 HPO 4 + 2NaH 2 PO 4 + 3Me 2+ -* 2MeHPO 4 + Me(H 2 PO 4 ) 2 + 6Na +
[2] 2MeHPO 4 + Me(H 2 PO 4 ) 2 + 3Ca(OH) 2 ■ * Ca(MePO 4 ) 2 + Me(PO 4 Ca) 2 + 6H 2 O where a reference is made to the case of a solution of bibasic and monobasic sodium polyphosphates, i.e. a solution containing HPO 4 = and H 2 PO 4 - ions in a more or less hydrolyzed form, and Me is a generic contaminating metal, which is present, for instance, in the ionic form Me 2+ ; corresponding reactions can be easily written for contaminating metals in a different oxidation state. The mixing step is carried out by mixing the contaminated material with the polyphosphate solution and then adding and mixing calcium hydroxide, in order to prevent undesired side reactions between calcium hydroxide and polyphosphates, and calcium hydroxide can only react with the polluting metals. The presence of other compounds, such as anions or oxidant or reducing agents, does not interfere with the above mentioned ionic exchange reactions, as it is confirmed by laboratory leaching tests.
Advantageously, a step is provided of properly dosing calcium hydroxide, in order to prevent amphoteric metals, which are originally present as cations, from disproportionating and from being partially converted into soluble anions.
The soluble polyphosphates are advantageously alkali metal polyphosphates, in particular sodium polyphosphates. The latter are preferably obtained in a conventional way, by treating an aqueous solution of phosphoric acid with sodium hydroxide, in order to be produced in a very cheap way. In alternative, but not exclusively, the soluble polyphosphates can be alkaline-earth metal polyphosphates, in particular monobasic alkaline-earth metal polyphosphates.
The effectiveness of the treatment depends in fact upon polyphosphates water solubility: alkaline-earth metal polyphosphates are completely soluble, monobasic alkaline-earth metal polyphosphates are still enough soluble to be successfully used, whereas dibasic and tribasic polyphosphates are useless for the method according to the invention, due to their poor water solubility. No toxic effect associated with polyphosphates are known, in particular in the case of sodium polyphosphates, which are widely used as food additives: this reduces notably reagent handling related risks.
Advantageously, the calcium hydroxide is put into contact with the intermediate material once a prefixed time has elapsed after the solution of water-soluble polyphosphates has come into contact and reacted with the contaminated material, wherein the prefixed time is at least 15 minutes, preferably at least 30 minutes.
Advantageously, a pressing step of the modified material is provided such that the modified material turns from an original size to a reduced size obtaining a compressed material, the pressing step causing the contaminating metals to be further trapped within the compressed material and a leaching and/or a dissemination into the environment of the contaminating metals is further prevented. Advantageously, said pressing step is carried out under a pressure higher than 700 Kilogram-force per square centimetre. Preferably, said pressing step is carried out under a pressure higher than 1000 Kilogram-force per square centimetre. Most preferably, said pressing step is carried out under a pressure higher than 2000 Kilogram-force per square centimetre. The compression of the mixed material together with the reagent assures a more intimate contact between the pollutants and the reagent, which notably increases process efficiency in comparison with the well-known chemical treatments. In particular, the overall reagent amount required to carry out the method is further reduced. The advantages of this compression step can also be explained in terms of energy balance. The first law of thermodynamics, applied to the mixture of the contaminated material with the reagents, i.e. the water-soluble polyphosphates in aqueous solution and the calcium hydroxide is:
[3] ΔU = ΔH - L where:
- ΔU is the overall internal energy change due to the process involving the chemical reactions [1] and [2] and the pressing step;
ΔH is the overall enthalpy change due to the same process; - L is the work exchanged by the mixture during the same process, which is substantially equal to the work needed to carry out the pressing step. Since L = PxΔV, where P is the pressure at which the pressing step is carried out, and ΔV is the volume change due to the pressing step, equation [3] may be written in the form:
[4] ΔU = ΔH - PxΔV.
ΔV is a negative quantity; provided that pressure P is high enough, according to previously given values, compression energy PxΔV transferred to the material during the pressing step is a consistent part of overall internal energy change ΔU; in other words compression energy notably contributes to increasing overall internal energy change during completion of the reaction between the polluting metals and the reagents.
Furthermore, the treatment can be carried out at room temperature, which also reduces process energy costs. The pressing step can be carried out on a plane surface, for instance a soil.
Advantageously, said pressure is kept on a surface of said modified material during at least 9 hours; preferably said pressure is kept during at least 12 hours. For example, such pressing step can be carried out in a mould. Advantageously, the pressing step is such that the reduced size is comprised between one half and one twentieth of the original size, preferably the reduced size is comprised between one fourth and one sixth of the original size, most preferably the reduced size is about one fifth of the original size.
In alternative, but not exclusively, the pressing step can be performed on a concave surface, for example a soil pit and, in particular, a soil pit that has been dug during the step of prearrangement of the material to be treated.
In particular, said pressing step comprises the steps of: prearranging a working surface defined by a frame, said frame having a prefixed height above a soil; - laying down a first layer of said modified material upon said working surface;
- pressing said first layer until a prefixed size reduction of said first layer is achieved, a compressed first layer obtained from said first layer; - laying down a second layer of said modified material upon said compressed first layer; pressing said second layer until a prefixed size reduction of said second layer is achieved, - said first and said second layer having preferably a thickness set between 25 and 35 centimetres, as laid down before pressing.
Advantageously, the pressing step takes place in a mould, such that an article can be obtained, which has a prefixed shape; in particular bricks, tiles, blocks, or filler materials for asphalt or other various building material can be obtained. Besides being cheaper than most conventional depolluting procedures, the method easily allows therefore exploiting polluted material.
Preferably, a sample of the contaminated material to be treated is taken away for preliminary analysis of pollutants and moisture, in order to properly select and prepare a reagent formulation. Besides, to properly fix the reagent formulation, a lab simulation of the treatment can be usefully carried out prior to said mixing step, which comprises sampling and analysing liquid flowing down away from a treated sample, to establish how pollutants concentration changes as the treatment proceeds and therefore to assess suitability of a given formulation to treat the contaminated material.
Preferably, after the treatment with polyphosphates and calcium hydroxide, a control is effected on a sample taken from the treated material, the control step comprising; pressing the sample up to a prefixed pressure to be used in the subsequent pressing step; carrying out a leaching test, and analysing the liquid that flows down away from the sample.
Such a control is intended to establish whether the material is ready for pressing, or whether it needs a further reagent treatment. The prearranged amount of contaminated material is preferably obtained by selecting a finer contaminated material fraction which is formed by particles of said contaminated material that are smaller than 150 micron, preferably the particles of said contaminated material are smaller than 75 micron. In other words, only the finer fraction material is mixed with the polyphosphates solution, and with calcium hydroxide, because the pollutants are most likely to settle on loam and clay particles, i.e., on finer particles.
In particular, a moisture checking step is provided to check the moisture in the amount of contaminated material, the moisture checking step carried out prior to the step of mixing, the contaminated material, and calculating the final humidity of the humidified material, and in case the calculated moisture content in the modified material exceeds 20%, carrying out a drying step of the contaminated material.
Preferably, the predetermined quantity of the polyphosphate solution is such that the weight ratio between the polyphosphate solution and the contaminated material considered in dry conditions is set between 3% and 8%, preferably the quantity of the solution is set between 5% and 6%.
Preferably, calcium hydroxide is mixed as a solid to the intermediate material, and the predetermined amount of calcium hydroxide is set between 0,5% and 4% of the amount of contaminated material, the contaminated material considered in dry conditions, preferably the predetermined amount of calcium hydroxide is set between 1,5% and 2,5% of said amount of contaminated material. The use of calcium hydroxide as a solid is intended to prevent excessive residual moisture in the material after the reagent treatment, which would hinder and reduce the effectiveness of subsequent pressing step.
Preferably, the steps of mixing are performed in a device that comprises a tubular element and a screw that is rotatably arranged inside the tubular element, in particular the device is a screw mixer.
Some contaminating substances are known, which do not react with sodium polyphosphates, but are able to form water-insoluble salt by reacting with calcium hydroxide. Arsenic is one of these pollutants. If only such contaminating substances are present in the contaminated material, the above mentioned objects are reached also by a method that simply comprises the steps of: - prearranging a predetermined amount of contaminated material; mixing the intermediate material with a predetermined quantity of calcium hydroxide obtaining a modified material that contains substantially water- insoluble compounds, - pressing the modified material such that the modified material turns from an original size to a reduced size obtaining a compressed material, such that the contaminating substance is trapped within the compressed material and a leaching and/or a dissemination into the environment of the contaminating metals is prevented.
In this case, where polyphosphates may not be used, all the above defined steps of the method are applicable.
The above-mentioned objects are achieved through a method for making solid articles, such as bricks, tiles, blocks, filler materials, and the like, wherein such articles are manufactured as previously stated.
Description of preferred exemplary embodiments
The invention will be made clearer with the following description of its exemplary embodiments and of examples, exemplifying but not limitative.
Figure 1 shows a flow-sheet representing an exemplary embodiment 100 of the method according to the invention for treating earth or sludge, or other substantially solid material that is contaminated with metals, in particular, with heavy metals. The method provides a first step 10 of prearranging an amount of the contaminated material; in the case of earth, this step comprises excavation, down to a depth depending upon the thickness of the polluted layer and upon the degree of contamination; the depth may be established, for example, by analysing one or more core samples taken from the soil.
According to the nature of the polluted material, a step of preliminary separation can be carried out, for example a step of sieving 20 the prearranged material, in which a coarser material fraction is separated and put aside. Only a finer material fraction is retained for subsequent treatment, i.e., a fraction which is formed by particles whose size is less than 150 micron, on which pollutants like heavy metals are more likely to settle; the finer fraction comprises, in particular, loam and clay.
The method provides then a step 30 of collecting a sample, a step 31 of analysing the pollutants associated to this sample and a step 32 of evaluating the moisture amount in the sample, in order to properly select (step 35) a reagent formulation for treating the polluted material, wherein, in particular, a suitable concentration of polyphosphates in the aqueous reagent solution is calculated.
Concerning humidity content, it is normally not necessary to dry the material before putting it into contact with the reagents. The amount of water mixed with the material along with dissolved polyphosphates is preferably such that the residual moisture content in the treated material is not sensibly larger than 20%; this way, the subsequent pressing step is easier and more effective.
In order to properly select the reagent formulation, one or more lab simulation test 38 of the treatment may be usefully carried out prior to treating the material. In such tests, small amounts of polluted material are treated with a/respective trial reagent formulation/s. The effectiveness of the/each formulation is tested by collecting and analysing samples of liquid that flows away from the tested material.
A step follows of mixing 40 the prearranged and possibly sieved material, with a solution of polyphosphates, preferably alkali metal polyphosphates, in particular sodium polyphosphates.
An intermediate material is then obtained, for example according to reaction [1], in which hydrogen phosphates of the heavy metals or heavy pollutants are formed.
The intermediate material is preferably homogenised: to this purpose, mixing step 40 is carried out, for example, in a device comprising a tubular element and a screw mixer that rotatably arranged inside it. The duration of mixing step 40 is for example set about 30 minutes.
Afterwards, a step 50 of mixing the intermediate material with calcium hydroxide is provided, preferably with high purity calcium hydroxide. For example, the amounts of polluted material and of reagents are set within the following limits: polluted material (dry) 92,5 ÷ 95,0%
- sodium polyphosphates solution 4,5 ÷ 5,0% calcium hydroxide (> 99%) 0,5 ÷ 2,0% Calcium hydroxide operates for example according to reaction [2], by forming water-insoluble, complex mixed salts.
In case contaminating substances are present that do not react with sodium polyphosphates, but only react with calcium hydroxide, in any case a water-insoluble salt is formed. For example, arsenic can be present, as a polluting material, in an oxidation state of +2 or +5; calcium arsenite CaAsO 3 H, or calcium arsenate Ca 3 (AsO 4 ) 2 are then formed, according to respective sequences of reactions [5]-[6] or [7]-[8]:
[5] As 2 O 3 + 3 H 2 O ^ 2 H 3 AsO 3 [6] 2 AsO 3 H 3 + 2Ca(OH) 2 -* 4H 2 O + 2 CaAsO 3 H
[7] As 2 O 5 + 3 H 2 O -» 2 H 3 AsO 4
[8] 2 AsO 4 H 3 +3 Ca(OH) 2 -* 6 H 2 O + Ca 3 (AsO 4 ) 2 .
Both arsenic salts are water-insoluble. It is important that mixing step 50 is performed after the treatment with polyphosphates has been completed, to prevent calcium hydroxide from reacting directly with the polyphosphates.
Once the treatment with calcium hydroxide has come to completion, which normally happens within about 30 minutes, a control step 60 is carried out on a sample taken from the material being treated. This control step may comprise a control pressing step, which is carried out at the same pressure that is used in subsequent pressing step 70 of the whole amount of material, and one or more leaching tests, which consist in weighing and analysing a liquid that flows down away from the sample.
If the liquid which flows down away from the material still contains an amount C of a pollutant that exceeds the prefixed threshold limit C * , steps 40 and/or 50 must be repeated.
On the contrary, if the concentration of pollutants in this liquid is less than the prefixed threshold limit, the treated material is directed to pressing step 70. In particular, the pressing can be carried out after laying the treated material upon a soil, or after filling a soil pit with it; in this case a conventional soil or asphalt compactor can be advantageously used, which allows reducing the volume of the material, typically about five or six times. To this purpose, a pressure exceeding 700 Kilograms per square centimetre should be used, preferably a pressure ranging from 1500 to 3000 Kilograms per square centimetre, most preferably a pressure set about 2000 Kilograms per square centimetre.
The pressing step may also be performed within a working surface defined by a frame, which has a prefixed height above a soil; in particular, a first layer of the material which has been previously treated with the reagents is laid down upon said working surface, the frame assisting containing the material within the limits of the surface; then the first layer up to a prefixed size reduction of said first layer is compressed until a desired reduction of layer thickness is achieved; afterwards, subsequent layers are laid down upon previously compressed layers of treated material, and compressed similarly to previous layers. Each layer, as laid down before pressing, has preferably a thickness ranging from 25 to 35 centimetres, to contain which a frame height of about 50-60 centimetres is preferable.
In alternative, pressing step 70 can be carried out within forms or molds, which allows obtaining filler materials for use in asphalt or in other various building material, or allows obtaining such articles as bricks, tiles or blocks.
To obtain a particularly enhanced process result, the pressure should be exerted or repeatedly applied on said modified material for at least 9 hours, preferably during at least 12 hours. Below, three examples are given of respective contaminated sludge samples, prepared in laboratory. For each example the operating conditions of the treatment, the amount of reagents used, the concentration of the pollutants before and after the treatment are shown.
In all the following examples, the final concentration of each pollutant, given in column (III) of each table, is far lower than the maximum admissible values in land intended for garden and communal areas, or for residential and private use, according to many national regulation. For example, such values according to Italian regulation are given in the column (IV) of each table.
Example 1 A sludge sample that contained 94,3 g dry solid and contaminated with metals as in columns (I), (II) of table 1 , was treated with 5,2 g aqueous solution of sodium polyphosphates and then with 2,0 g calcium hydroxide, obtaining modified material with a sensible pollutants concentration reduction.
Example 2 The modified material of Example 1 was pressed up to 350 KN/cm2 at room temperature, obtaining a size reduction to about 1/5 the initial volume. TABLE 1
(I) (II) (HI) (IV)
Arsenic 90 20
Cadmium 5,35 Below limits of 2
Cobalt 65,3 column (IV) 20
Total chromium 172,5 150
Chromium Vl <1 2
Mercury 0,65 1
Nichel 6381 120
Lead 18529 100
Copper 580 120
Selenium 1 ,45 3
Vanadium 31 ,5 90
Zinc 23585 150
A leaching test was carried out before the treatment and 12 hours after pressing, according to EN 1245-2:2004 (see below Table 5). Initial and final concentration are given in Table 2 respectively, in column (II) and (III) of the same table, and are expressed in mg/Kg.
TABLE 2
(I) (II) (HI)O (IV)
Arsenic 90 0,7 20
Cadmium 5,35 0,4 2
Cobalt 65,3 1 ,09 20
Total chromium 172,5 0,56 150
Chromium Vl <1 not detectable 2
Mercury 0,65 not detectable 1
Nichel 6381 4 120
Lead 18529 5,15 100
Copper 580 3,7 120
Selenium 1 ,45 not detectable 3
Vanadium 31 ,5 1 ,12 90
Zinc 23585 14,17 150 Example 3
A sludge sample that contained 93,0 g dry material and contaminated with metals as in columns (I), (II) of table 2, was treated with 5,0 g aqueous solution of sodium polyphosphates and with 1 ,5 g calcium hydroxide, obtaining a sensible pollutants concentration reduction; initial and final concentration are given, respectively, in column (II) and (III) of the same table, and are expressed in mg/Kg. Then the compression step and the leaching step have been carried out as in Example 2.
TABLE 3
(I) (II) (IN)O (IV)
Cadmium 5,35 0,39 2
Cobalt 65,3 1 ,04 20
Total chromium 172,5 0,82 150
Nichel 6381 4,93 120
Lead 18529 6 100
Copper 580 4,78 120
Vanadium 31 ,5 0,88 90
Zinc 23585 10,97 150
Example 4
A sludge sample that contained 93 g dry material and contaminated with metals as in columns (I), (II) of table 3, was treated with 5,0 g aqueous solution of sodium polyphosphates and with 2,0 g calcium hydroxide, obtaining a sensible pollutants concentration reduction, initial and final concentration are given, respectively, in column (II) and (III) of the same table, and are expressed in mg/Kg.
Then the compression step and the leaching step have been carried out as in Example 2. TABLE 4
(I) (II) (lll)o (IV)
Cadmium 5,35 0,39 2
Cobalt 65,3 1 ,05 20
Total chromium 172,5 0,85 150
Nichel 6381 6,34 120
Lead 18529 7,25 100
Copper 580 4,47 120
Vanadium 31 ,5 0,82 90
Zinc 23585 9,79 150
TABLE 5: List of the procedures in use for carryir concerning the metals of column (V).
(V)
Arsenic, Mercury, Selenium ICP-OES (hydride method)
Cadmium, Cobalt, chromium total, Nichel, Lead, copper, Vanadium, zinc UNI EN ISO 11885
Chromium Vl UV/ Spectrometry
The foregoing description of a specific embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such an embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.