Christensen, Thomas H�jlund (Solh�jpark 47, Farum, DK-3520, DK)
| 1. | A method for the treatment, in particular stabilization, of materials contai¬ ning environmentally noxious constituents, especially from the incineration of waste, in which the materials are treated with ferrous compounds and water, characterized in that the materials are washed and stabilized chemically using a solution containing ferrous ions (Fe++) which allows the formation of stabilizing ferric oxides (Fe+++) on the materials, the ferric oxide formation being carried out by means of an oxidation under controlled conditions with respect to the nature and the amount of oxidizing agent, temperature and pH, whereafter, if desired, the treated materials are subjected to a thermal aftertreatment. |
| 2. | A method as claimed in claim 1 , characterized in that the solution also allows the formation of stabilizing ferric oxides (Fe+++) in the washing solution. |
| 3. | A method as claimed in claim 1 or 2, in which by washing of the material a dissolution of easily soluble salts primarily takes place, characterized in that the same solution is used for washing as well as for stabilization of the material. |
| 4. | A method as claimed in claim 1 , 2 or 3, characterized in that the oxidation is carried out by means of air, oxygen, oxygen containing flue gas, hydrogen peroxide, ozone, chlorine, permanganate and/or dichromate. |
| 5. | A method as claimed in any of the preceding claims, characterized in that the oxidation is carried out under controlled temperature conditions in the interval from O to 1200°C. |
| 6. | A method as claimed in any of the preceding claims, characterized in that the oxidation is carried out under controlled pH conditions in the interval from 5 to 12, the pH value being adjusted as required. |
| 7. | A method as claimed in claim 6, characterized in that the oxidation is car¬ ried out under controlled pH conditions in the interval from 8 to 12. |
| 8. | A method as claimed in claims 1 to 3, characterized in that the product formed by precipitation in the washing solution is handled together with the product formed by stabilization of the material. |
| 9. | A method as claimed in any of the preceding claims, in which all or part of the materials are treated together or separately by washing and stabilization, charac¬ terized in that one or more of the said stabilized products are subsequently subjected to a thermal aftertreatment for obtaining a sintering or melting of the stabilized pro¬ ducts . |
| 10. | A method as claimed in claim 9, characterized in that the thermal after treatment is carried out in a temperature interval between 800°C and 1300°C 11 A method as claimed in claim 9, characterized in that the thermal after treatment is carried out in an incinerator for obtaining sintering together or melting to¬ gether with the slag formed in the incinerator. |
| 11. | A method as claimed in claim 11, characterized in that the stabilized pro¬ duct is passed to the inlet of a rotary kiln if the incinerator comprises a rotary kiln 13 A method as claimed in claim 11 , characterized in that the stabilized pro¬ duct is passed to the incinerator through a feeding system for grate screenings. |
| 12. | A method as claimed in claim 1 , 2 or 11 , characterized in that washing and stabilization is carried out as a part of the system for the returning of grate scree¬ nings 15 A method as claimed in any of the preceding claims, characterized in that the ferrous compound comprises ferrous sulfate and/or ferrous chloride. |
| 13. | method as claimed in claim 15, characterized in that the ferrous com¬ pound contains from 2 to 50 grams of Fe++ per litre 17 A method as claimed in claim 15 or 16, characterized in that in connecti on with chemical stabilization with ferrous compounds there is additionally carried out a stabilization with other chemicals which pπmaπly comprise silicon (Si), aluminium (Al) and/or phosphate (PO 3) 18 A method as claimed in any of the preceding claims, characterized in that it is carried out batchwise in a continuous system or in situ on already deposited or fixed material 19 A method as claimed in any of the preceding claims, characterized in that the stabilization of the material is subsequently combined with another coating of known type with cement, fly ash, lime or the like 20 A method as claimed in any of the preceding claims, characterized in that when a stabilization with ferric oxide is desired, materials of industrial dust, contamina¬ ted soil or other materials are treated alone or together with other materials 21 A plant for carrying out the method as claimed in claim 1 , characterized in that it comprises a silo (2) containing materials containing environmentally noxious constituents; a reactor (1) to which the said materials are fed; a tank (9) and a return water tank (7) and silos (3, 4) containing ferrous salt solution and, if desired, other chemicals, washingwater and the said chemicals being fed to the said reactor (1), the reactor contents being stirred; a sedimentation tank (5) to which the washingwater is fed; a centrifuge or filter press (6) in which the precipitate formed is dewatered; and means for aerating the water in the said tank (5) for decreasing the pH and oxidation of the water. |
| 14. | 22 A plant as claimed in claim 21 , characterized in that the reactor (1 ), the return tank (7) or a drying and oxidation station (8) is provided for returning thereto a precipitate formed in the tank (5). |
| 15. | 23 A plant as claimed in claim 21 or 22, characterized in that it comprises means for returning the treated material to an incinerator plant. |
| 16. | 24 A plant as claimed in claims 2123, characterized in that it comprises means for carrying out the oxidation in a separate drying plant or in the station (8). |
TECHNICAL FIELD
This invention relates to a method for the treatment, in particular stabilization of materials containing environmentally noxious constituents, especially from waste incineration plants for protection against the release of environmentally foreign or en- vironmentally noxious compounds during transport, use or disposal of the stabilized material. The method combines a washing-out of salts with the formation of coatings on the surfaces of the particles whereby the release of salts, metals etc. from the par¬ ticles is reduced, and the treated material can therefore more easily be reused or landfilled, and, if desired, a subsequent thermal treatment of the coated product for- med by the coating, e.g. in the incineration plant, in order to obtain in the plant a sinte¬ ring together with the slag.
The washing-out is carried out by means of a ferrous solution whereafter the material is oxidized so that the ferrous iron used with the material forms coatings of ferric oxides on the material and thereby immobilizes heavy metals in particular. The salt containing washing-water is also oxidized, whereby the metal ion content, if any, is reduced substantially by the sedimentation of ferric oxides. The method results in a stabilized solid product and a fairly pure salt solution. The method can be used in con¬ nection with all granular materials, which contain environmentally noxious compounds, such as slag, fly ash, flue gas cleaning products, sludge ash, industrial dust, contami- nated soil or combinations thereof, in particular however in connection with residuals from waste incineration plants.
BACKGROUND ART
By the incineration of waste, a reduction of the amount of waste is obtained, and it is made possible to use the released energy for power and/or heating purposes, but at the same time there is obtained a concentration of environmentally foreign or environmentally noxious constituents, such as heavy metals, in the ash and the
other solid particle fractions, and a flue gas which also carries hazardous compounds, both mineral and organic, such as halogen compounds, is also obtained
The flue gas from the incineration can be cleaned with respect to solid as well as gaseous contaminating constituents by means of various known methods whereby further particle fractions of flue gas cleaning products (FCP) can be produced
When ash fractions and other by-products from the incineration of waste are used or landfilled, a dissolution and leaching of the environmentally foreign or en¬ vironmentally noxious compounds can take place resulting in a contamination of the environment This effect can be removed or reduced to an acceptable level by extracting the environmentally noxious compounds for reuse or disposal of reduced volumes, or the compounds can be immobilized by coating the particles and/or binding the com¬ pounds chemically by means of additives or thermal treatment
It is known to return boiler ash, filter ash, flue gas cleaning products and grate screenings and to sinter or melt these materials together with the slag, e g by introdu¬ cing them in the rotary kiln inlet in a grate/rotary kiln plant
Likewise, much experience has been obtained by melting together various of these fractions in order to obtain stabilized materials
Processes are known for extracting, with e g acid containing water, a series of metals from the ash products, which are thereafter made into briquettes for being returned to the incineration plant so that the material can form part of the slag
It is also known to add cement, silicates, iron oxide or phosphate, whereby the materials are stabilized markedly with respect to the washing-out of heavy metals
Thus, it is possible to achieve that certain residual products or materials can be used as secondary building materials or be deposited
For more detailed descriptions of the said methods reference is made to the literature, e g the references cited in the following
However, the ash from waste incineration plants is typically subject to such variations that it has been difficult to provide a safe and economical method for secu- ring the reuse of the ash products
In the method known from the Danish Patent Application No 6379/89 there is, in principle, also used ferrous compounds in connection with stabilization In this
method aluminum silicates, lime, ferrous iron and water are mixed to a liquid mass and subsequently dried to a concrete-like solid waste product, but this method does not comprise a washing-out step, and ferrous iron is only one of several stabilizing chemi¬ cals which lead to the formation of a concrete-like product Likewise, it is known from Japanese Patent Application No J 62183896 that coal ash can be treated with iron containing compounds in order to improve the utility of the ash, but also this method does not comprise a combination with washing-out or subsequent oxidation and, if desired, thermal treatment
From e g EP Patent No 0 536 268 B1 it is known to introduce ashes and other flue gas cleaning products in the combustion zone of a waste incineration plant for thereby obtaining a melting together with the slag and thereby an immobilization of heavy metals etc in the slag, but one may hereby obtain an undesired release of noxi¬ ous constituents to the flue gas, e g Pb, Hg, As and Zn
It is the main object of the invention to provide such a safe and economical method for the stabilization of the residual products from waste incineration that they can be used without risk as filling materials or be landfilled without contamination of the environment
It is also an object of the invention to provide a plant for carrying out the said method
DISCLOSURE OF THE INVENTION
According to the invention there is provided a method as defined in the pre¬ amble of claim 1 by combining washing-out and stabilization, and by this combination there is obtained a substantial stabilization of the metal compounds in the material by the formation of iron oxide coatings on the particles as well as a substantial washing- -out of salts, avoiding that the salt-containing washing-water after the process contains substantial concentrations of especially metals By means of an optional subsequent thermal treatment in an incineration plant there can be obtained a further stabilization of the residual product by sintering/melting together with the other slag in the plant This is obtained by means of a method of the type defined above, the said method being characterized by the features set forth in the characterizing part of claim 1 Preferred and/or special embodiments of the method are defined in the dependent method claims
The invention also comprises a plant for carrying out the method of the pre¬ sent invention, the said plant being characterized by the features set forth in the cha¬ racterizing part of claim 21 Preferred and/or special embodiments of the plant are de¬ fined in the dependent plant claims The matenal to be stabilized is mixed in a reactor in a solution containing 0 2 to 5% of ferrous iron in an US ratio (liquid/solid) between 1 and 10, preferably 2-5, a pH regulation possibly being required for obtaining a pH in the mixture in the interval from 5 to 12, in particular from 8 to 11 7 The L/S ratio is adapted to inter alia the pro¬ perties of the residual material in question, including the amount of easily soluble salts which must and can be washed out, and the porosity, particle size distribution and ty¬ pes of the material The actual plant used can also influence the L/S ratio used
The concentration of ferrous iron (Fe ++ ) in the solution depends inter alia on the L/S ratio at which the plant is operated, and on the properties of the residual pro¬ duct and the requirement for coating The ferrous solution is preferably based on fer- rous sulphate and/or ferrous chloride If desired, the access of oxidizing agents to the mixture is restricted so that the oxidation of ferrous iron is limited and the ferrous ions have time for binding to the particle surfaces (10 minutes to 5 hours if a batch arran¬ gement is used) Washing-water and residual solid material are separated by traditio¬ nal methods such as sedimentation, centnfugation, filtering or combinations thereof The solid material is then oxidized and eventually heat treated under control¬ led conditions for the formation of iron oxide coatings consisting of goethite, hematite, magnetite, amorphous iron oxides or mixtures thereof Dependent on the composition of the starting material and critical requirements of the treated product there can be used various oxidizing agents (e g air, oxygen, oxygen containing flue gas, hydrogen peroxide, ozone, chlorine, permanganate, dichromate), various oxidation periods (up to 200 days) and various temperatures (0°C to 1300°C, preferably 5°C to 1300°C or 0°C to 1200°C) These factors will influence the types of iron oxides formed on the par¬ ticles Preferably air is used as the oxidizing agent The optional thermal treatment can be carried out in several steps, e g a treatment at 70°C to 90°C in order to obtain sta- ble ferric compounds, and a treatment at high temperature, e g 800°C to 1300°C, in furnace for obtaining melting or sintering with other products
The washing-water is then oxidized Dependent on other conditions further addition of ferrous solution and/or regulation of pH may be necessary, so that the
washing-water after finished oxidation and separation of particles/sludge does not contain substantial amounts of non-acceptable compounds and has a pH which does not limit its use and/or discharge The oxidation can be carried out with air, oxygen, oxygen containing flue gas, hydrogen peroxide, ozone, chlorine, permanganate or dichromate, preferably with air By the oxidation, ferric oxides are formed which will bind the metal compounds from the washing-water The separated particle/sludge fractions are optionally mixed with the stabilized residual product
In connection with drying and/or thermal treatment of the treated residual pro¬ ducts it can be expedient to perform a size reduction thereof to obtain a further stabili- zation
By reintroducing the problematic particle fractions in the combustion furnace the environmentally foreign compounds can be destructed, or they can be integrated in the slag and thereby be encapsulated the organic compounds such as PAH (polyaromatic hydrocarbons), dioxins and furanes are decomposed, and more heavy metals are bound in the silicate matrix of the ash, but a part of the metals such as Pb, Cd, Hg and Zn can partly evaporate again before they become bound and thereby give rise to an increased emission of trace substances
By means of the present invention it is possible to decrease this evaporation of metals in connection with the sintering/melting together with the bottom slag and further to minimize the washing-out of the compounds at the later use or landfilling
The stabilized residual products are fed to the grate (if the combustion furnace is a grate furnace) or to the rotary kiln inlet (if both a grate furnace and a rotary kiln are used) so that there is obtained a sintering together of these materials with the slag, while the volatile metals to a substantial degree remain chemically bound in the mate- rial after the thermal extra-treatment, so that the total slag can still easily be used as a secondary building material, e g as a foundation material for roadbuilding Hereby the¬ re is obtained a minimizing or removal of those amounts of residual products from waste incineration which are to be landfilled The return to the grate can, if desired, be carried out through the grate screenings return systems with which the combustion furnace can be provided Grate screenings collection systems are preferably transport systems in which the materials are transported partly under water They may, if desi¬ red, be provided with separation equipment which separates certain materials for reuse and e g separates magnetic and possible non-magnetic materials The stabili-
zed by-products are then preferably fed into the transport system after such a sorting has taken place
According to a special embodiment of the invention the washing-out can take place in situ thereby that the washing solution infiltrates the material, and this can be especially relevant in connection with landfilled materials and contaminated soil This embodiment provides less possibility of controlling the temperature but will, on the other hand, require less handling
BRIEF DESCRIPTION OF THE DRAWING AND OF A PREFERRED EMBODIMENT The present invention will be described in more detail in the following in con¬ nection with a preferred embodiment, reference being made to the drawing in which figure 1 is a schematic illustration of a plant according to the invention in which the method of the invention can be carried out
Description of a preferred embodiment based on a batchwise treatment of the residual product
The dry residual products which are collected in one or more silos 2 are dosed to reactor 1 to which there are also fed washing-water from a water tank 9 and a return water tank 7 as well as chemicals such as ferrous sulphate hydrate (FeSO 4 , π H 2 O) and optionally other chemicals from silos 3 and 4 with stirring The stirring is performed preferably for 10 to 20 minutes, whereafter the solid phase is sedimented, before the washing-water is passed to a sedimentation tank 5, and the precipitate is dewatered by being passed to e g a centrifuge or filter press 6 The water in the sedimentation tank 5 is treated by means of aeration to obtain a decrease in pH and aeration of the water Preferably purified flue gas is used for aeration, since the high carbon dioxide content can be used for the pH regulation which can also be carried out by the addition of acid A precipitate in the sedimentation tank 5 can be returned to the process in the reactor 1 , the return water tank 7 or a drying and/or aeration station 8 Following treatment and sedimentation of particles the water can typically be passed directly to a recipient which allows discharge of water with dissolved salts In the centrifuge or filter press 6 additional water is separated and returned to the process in the reactor 1 through the return water tank 7, while the treated product is passed through the drying and/or aeration station 8 where a further drying of the
product takes place If desired, the treated product is then passed on to the incinerati¬ on plant, and this is preferably done by feeding it to a grate screenings system 15
The process water used for the chemical treatment of the residual product in the reactor 1 is supplied from the water tank 9 and the return water tank 7 The water may be collected process water from other systems, e g from quenching and washing- -out of soluble salts such as chlorides from grate screenings 11 and 12, or from qu¬ enching and washing-out of slag
The chemically treated product must, if ferrous salts are used for stabilization, be subjected to an oxidation of the ferrous compounds to ferric oxides for obtaining the desired stabilization This oxidation can take place in a separate drying plant or in the station 8 during a thermal treatment in the incineration plant, or it can take place at both positions
The product is preferably, if it is fed to the incineration furnace through the grate screenings return system, supplied after an optional separation of magnetic ma- terial 13 and non-magnetic, coarse materials 14
The invention is described further in the following according to another embo¬ diment being carried out in the laboratory with subsequent measurement of the washing-out from a treated and an untreated flue gas cleaning product from waste incineration, respectively
Example
Flue gas cleaning product (FCP) from a waste incineration plant with a semi- dry flue gas cleaning process is mixed with an L/S-ratio (liquid solid) of 5 with a ferrous sulphate solution corresponding to 10 grams of Fe per 100 grams of RRP The mixing takes place under oxygen-free conditions and stirπng for about 10 minutes Thereafter, liquid and solid materials are separated by sedimentation and filtering
The solid material is left for 8 days in a drying chamber whereby ferrous iron bound to the solid material is oxidized and forms reddish brown coatings of iron oxides on the particles The material is then left for 2 weeks before being subjected to a leaching test in a column The washing-water is aerated by blowing in air for 24 hours whereby pH drops to about 7 5 and the concentration of lead to <0 01 mg per liter The amount of sludge (<1% of FCP) is mixed with the treated FCP
The treated FCP is filled in two columns in the laboratory and flushed with water in order to illustrate the washing-out from the material For comparison purposes two columns of the untreated FCP are made as well The results for the treated FCP are shown in Table 1 and 2, while the results for the untreated RRP appear from Table 3 and 4 The L/S ratio shows how much water (L) which has run through the column, in relation to the amount of FCP (S) in the column For lead which is one of the more cri¬ tical parameters in this context the washing-out has been reduced in the treated FCP with more than a factor 100
Table 1 and 2: Concentration of lead and other parameters in the effluent from two duplicated columns containing treated FCP from a waste incineration plant. The L/S ratio defines the amount of water which has run through the column in relation to the amount of material in the column.
Table 1
Table 2
Table 3 and 4: Concentration of lead and other parameters in the effluent from two duplicated columns containing untreated FCP from a waste incineration plant. The L/S ratio defines the amount of water which has run through the column in relation to the amount of material in the column.
Table 3
Table 4
References
1. J. Flyvbjerg & O Hjelmar, 1996 Restprodukter fra røggasrensningen ved affaldsforbrasnding Udrednmg af muhgheder for oparbejdning, genanvendel- se og deponenng Udarbejdet af Vandkvalitetsinstituttet for Miljøstyrelsens genanvendelsesrad, København
("Residual products from the flue gas cleaning by waste incineration Survey of possibilities for treatment, reuse and deposit Prepared by the Water Quali¬ ty Institute for the Environment Authority's Council for Reuse, Copenhagen")
2 J.J.J.M Goumans, H A. van der Sloot & Th G Aalbers Waste Materials in Construction, Proceedings of the International Conference on Environmental Implications of Construction with Waste Materials, Maastricht, The Nether¬ lands, 10-14 November 1991 , Elsevier, Amsterdam - London - New York - Tokyo 1991.
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