WO1996019304A1

WO1996019304A1 - Stabilisation of ashes

Info

Publication number: WO1996019304A1
Application number: PCT/SE1995/001367
Authority: WO
Inventors: Sven Ragnarsson
Original assignee: ABB Fläkt AB
Priority date: 1994-12-22
Filing date: 1995-11-17
Publication date: 1996-06-27
Also published as: SE9404453D0; JPH10510759A; AU4319696A; SE510942C2; SE9404453L

Abstract

A method and a device for the stabilisation of ash, preferably fly ash, are disclosed. In the method, the ash is first extracted with an acid aqueous solution, whereupon it is preferably washed with an aqueous solution before being alkalised with an aqueous solution of a water-soluble alkali, such as soda, and treated with an aqueous solution of a water-soluble sulphide, such as sodium sulphide, in order to convert leachable material in the ash to insoluble sulphide. The thus-stabilised ash is then washed, preferably with water. After each treatment stage, the ash is preferably separated from the liquid. The device consists of a band filter (15) having an inlet end for ash that is to be stabilised, as well as an outlet end for ash that has been stabilised. As seen from the inlet end to the outlet end, the band filter comprises a station (25) for acid extraction of the ash, a washing station (30), a station (36) for alkalisation of the ash, a station (42) for sulphide-treatment of the ash, and a washing station (48).

Description

Stabilisation of ashes

The present invention relates to a method and a device for the stabilisation of ash, preferably fly ash.

The incineration of combustible solids, such as refuse, generates gaseous combustion products as well as solid combustion products (ash). The ash may form coarse particles, so-called bottom ash, or melt during the inci¬ neration, thus forming melted ash. The bottom ash and the melted ash are accumulated at the bottom of combustion chamber in the form of slag. The ash may further consist of fly ash, i.e. combustion residues which are so minute and lightweight that they accompany the flue gases up into the chimney, where they are normally intercepted by a filter. The slag is dominated by compounds that do not evaporate at the incineration temperature, such as iron oxide and silicon dioxide, while the fly ash contains a large number of metal compounds that have evaporated in the incineration but have been sublimated in the cooling of the flue gas. The metal compounds in the fly ash are, inter alia, present in the form of chlorides and sul- phates. Among the different metals forming part of the metal compounds in the fly ash, mention may be made of alkali metals, such as potassium and sodium, and heavy metals, such as mercury, cadmium, zinc, tin, lead, chro¬ mium and nickel. It goes without saying that the composi- tion of the fly ash varies according to the incinerated material, but the above composition is an enumeration of substances that may occur in the incineration of refuse, such as garbage containing a great many different pro¬ ducts, such as paper, plastic, food scraps and sulphurous material, and requiring, when incinerated, extensive cleaning to avoid pollution.

Whereas the slag has a fairly low concentration of heavy metals, the fly ash has a high concentration there¬ of, such that, say, the concentration of mercury may be several hundred times higher in the fly ash than in the slag (for instance 0.5 ppb in the slag and 150 ppb in the fly ash). Moreover, the fly ash has a much higher chlo¬ ride concentration than the slag ( for instance respec- tively 7% and 0.1 %).

Considering that slag has a low solubility while much of the fly ash is soluble, this means that the fly ash is much more dangerous to the environment than the slag and that the fly ash involves an environmental hazard and thus cannot be deposited just like that.

In an effort to reduce the environmental hazards as¬ sociated with fly ash, various solutions have been sug¬ gested. According to one such solution, the fly ash is mixed with slag, whereupon cement, lime or phosphates are added to bind the heavy metals and produce a mass which does not absorb water. However, this solution involves additional costs for the binder added, as well as addi¬ tional costs for the larger volume of material that has to be deposited. Furthermore, it is far from certain that the fly ash stabilised by being thus embedded meets the stipulated requirements as regards, for instance, maximum contents of leachable heavy metals, the leaching test being, at least in some countries, carried out on crushed material. Another suggestion for a solution to the problem of how to reduce the environmental hazards associated with the fly ash is to leach the ash, before deposition, with an acid solution in order to extract the heavy-metal impurities found therein. SE 457,773, for instance, thus teaches acid leaching of fly ash with a view to removing soluble heavy metals subsequently precipitated in the form of sulphides. The sulphide precipitation takes place in an acid solution and is therefore unsatisfactory. There does not occur any chemical stabilisation of the remainder of the fly ash after the acid leaching, but it is only indicated that the remainder of the ash can be recycled to the incineration plant or be fixed by bitumi- nisation. Another instance is DE 39 18 292, which also teaches acid extraction of fly ash, the extracted heavy metals being precipitated in the form of sulphides along with the fly ash. The thus-precipitated heavy-metal sul- phides and the fly ash are then separated from the acid solution, which subsequently is neutralised. Also here does the sulphide precipitation take place in an acid solution, which results in unsatisfactory precipitation. In the above acid leaching of fly ash, it is prima- rily chlorides and sulphates that are leached out, and approximately 70-80% by weight of the heavy metals found in the fly ash are dissolved. However, approximately 20-30% by weight of the heavy metals remain in the fly ash, and there is a risk of these heavy metals being leached out when the ash is deposited. In addition, the prior-art acid sulphide precipitation of the leached-out heavy metals is unsatisfactory, i.e. the leached-out heavy metals are not optimally precipitated and stabilis¬ ed as stable sulphides but remain to a certain extent in soluble form.

This invention aims at obviating the above drawbacks and achieving a stabilisation of ash, preferably fly ash, such that leachable components of the ash are so fixed that the ash can be deposited without constituting an environmental hazard.

According to the invention, this aim is achieved by a method for stabilising ash, preferably fly ash, in which the ash is extracted with an acid aqueous solution, said method being characterised in that the remaining ash is then treated in at least two stages comprising an al¬ kalisation stage as well as a stage in which a sulphide- ion source is added.

In accordance with the invention, the ash is thus first extracted with an acid aqueous solution, as in the prior art. As a result, easily-soluble compounds and salts are extracted, and the main part of the heavy-metal impurities are dissolved. As mentioned in the foregoing, there does, however, remain a not insignificant amount of heavy-metal impurities in the leaching residue of the ash, these remaining heavy-metal impurities being poten¬ tially leachable and thus constituting an environmental hazard. In order to obviate this environmental hazard, the heavy-metal impurities in the remaining ash are sta¬ bilised by an addition of a sulphide-ion source and a resulting conversion to stable sulphide compounds. As also mentioned in the foregoing, sulphide precipitation in an acid environment yields unsatisfactory results, and the sulphide precipitation is therefore, in the inven¬ tion, preceded by an alkalisation of the remaining ash. This alkalisation results in maximum and stable sulphide precipitation. Preferably, the alkalisation takes place to a pH of approximately 7 or above, preferably a pH of approxi¬ mately 7-11, and most preferred a pH of approximately 10. Furthermore, the alkalisation preferably involves an aqueous solution of a water-soluble alkali, such as soda or sodium hydroxide. Soda is especially preferred, having a buffer effect on the fly ash.

Moreover, it is preferred that the sulphide-ion source added consists of an aqueous solution of a water- soluble sulphide. Examples of preferred water-soluble sulphides are inorganic and organic sulphides, such as sodium sulphide, dithiophosphoric acid derivatives, thio- carboxylic acid derivatives or mercaptotriazine deriva¬ tives.

Preferably, the extraction of the ash with the acid aqueous solution is carried out at a pH of approximately 1-2, the extraction being preferably carried out with washing liquid from a scrubber for the wet cleaning of a gas containing hydrogen chloride. The hydrogen-chloride- containing gas may, for instance, consist of flue gas resulting from the incineration upon which also the ash was generated, in particular refuse incineration generat¬ ing a flue gas containing hydrogen chloride as well as sulphur dioxide. Such a flue gas is wet-cleaned in at least two stages for the removal of respectively hydrogen chloride and sulphur dioxide, and it is the acid washing liquid from the scrubber for the removal of hydrogen chloride that is used in this invention for the ash extraction. Using this washing liquid in the present invention is especially advantageous, since there is then no need of any separate, additional washing liquid, which reduces the evaporation need in the subsequent treatment of the washing liquid. For further details on the design of the wet cleaning of a hydrogen-chloride-containing gas, reference is made to laid-open Swedish Patent Appli¬ cation 9300169-1 (corresponding to DE 44 01 441.4).

According to the invention, it is especially prefer- red that the remaining ash is separated from the liquid after each stage. Consequently, the remaining ash is separated from the acid aqueous solution after the extraction involving this solution, and the remaining ash is likewise separated from the alkaline solution after the alkalisation as well as from the aqueous solution with the sulphide-ion source after the addition thereof. Such separation is also preferred in the event of further treatment stages, to be described in more detail below. The method according to the invention is rendered even more effective if the ash is, prior to alkalisation, washed with water, and such washing is therefore espe¬ cially preferred. As a result of the washing operation, the acid components are essentially washed away from the remaining ash, and the alkalisation of the ash is faci- litated, resulting in a reduction of the amount of alkali required.

Similarly, it is preferred that the ash is washed with water after the addition of the sulphide-ion source. As a result of the washing operation, any excess of sul- phide ions is washed away.

In order to put the washing liquid to optimum use, it is furthermore preferred that liquid from the washing operation following the sulphide treatment is transferred to the alkalisation stage. Moreover, it is preferred that liquid from the alkalisation stage is transferred to the washing stage following the acid extraction. For optimum utilisation of the washing liquid, it is finally prefer¬ red that liquid be transferred from the washing stage following the acid extraction to the hydrogen-chloride scrubber.

The invention further provides a device for the sta- bilisation of ash, preferably fly ash, said installation being characterised in that it comprises a band filter having an inlet end for ash that is to be stabilised and an outlet end for ash that has been stabilised, said band filter further comprising, as seen from the inlet end to the outlet end, a station for acid extraction of the ash, a station for alkalisation of the ash, and a station for sulphide treatment of the ash. Preferably, the band fil¬ ter further comprises washing stations arranged before the alkalisation station and after the sulphide-treatment station.

Preferably, each station of the inventive device has a feed means arranged above the band filter to feed liquid, as well as a discharge means arranged below the band filter to draw off liquid. Conveniently, each sta- tion disposed after the station for acid extraction of the ash comprises a circulation circuit having a liquid supply, a means for circulating liquid in the circuit, a feed means arranged above the band filter to feed liquid, as well as a discharge means arranged below the band fil- ter to draw off liquid.

In an especially preferred embodiment, the device comprises means for transferring liquid from the washing station disposed after the sulphide-treatment station to the alkalisation station, as well as means for transfer- ring liquid from the alkalisation station to the washing station disposed before the alkalisation station. Following the above general description, the inven¬ tion with its distinctive features will now be described in more detail with regard to the stabilisation of fly ash and with reference to the accompanying drawings, in which

Fig. 1 is a schematic flow chart illustrating a pre¬ ferred mode of implementation of the method according to the invention, and

Fig. 2 is a schematic view of a preferred embodiment of the device according to the invention.

In Fig. 1, reference numeral 1 designates fly ash resulting from incineration, especially refuse incine¬ ration, and containing leachable impurities in the form of heavy-metal impurities and so forth. First, the fly ash 1 is treated in an extraction stage 2 with an acid aqueous solution, preferably obtained from a hydrogen- chloride scrubber 3 in a flue-gas-cleaning plant (not shown). As indicated by the arrow 4, the spent acid aqueous solution from the extraction is drawn off to a plant (not shown) for the treatment of wastewater. In this plant, the impurities, such as heavy metals, dis¬ solved in the acid aqueous solution are precipitated and recovered. It is especially preferred that the treatment plant is the plant normally connected to the hydrogen- chloride scrubber mentioned above. For more detailed information on such plants, reference is made to the description of laid-open SE Patent Application 9300169-1 mentioned in the foregoing.

After the extraction with the acid aqueous solution, the remaining fly ash is separated from the acid aqueous solution and conducted to a washing stage 5, where it is washed with an aqueous solution. This aqueous solution may consist of fresh water, but preferably consists of an aqueous solution taken from any of the subsequent stages. After the washing operation, the fly ash is separated from the washing liquid, and the spent washing liquid is preferably conducted to the hydrogen-chloride scrubber 3, so as to there be used for cleaning flue gas of hydrogen chloride. After being separated from the washing liquid, the fly ash is conducted to an alkalisation stage 6 in which use is made of an alkaline aqueous solution. Pre- ferably, the alkalisation treatment is carried out with the aid of an aqueous soda solution, since this also has a buffer effect. The purpose of the alkalisation treat¬ ment is to impart to the remaining fly ash an alkaline pH of approximately 7 or above, preferably a pH of approxi- mately 7-11, and especially a pH of approximately 10, thereby to optimise the effectiveness of the subsequent sulphide precipitation. After the alkalisation treatment, the remaining fly ash is separated from the alkaline aqueous solution and, as indicated by the arrow 7, the spent alkaline aqueous solution is drawn off to be used in the preceding washing stage.

The remaining fly ash is then treated in a stage 8 with a sulphide-ion source, such as an aqueous solution of sodium sulphide, thereby to convert any soluble sub- stances remaining in the fly ash to insoluble sulphides. The supply of the sulphide-ion source is indicated by the arrow 9. Since the treatment with the sulphide-ion source is carried out at an alkaline pH, preferably a pH of approximately 10, it is highly effective. Having been treated with the sulphide solution, the fly ash is sepa¬ rated therefrom and is washed in a subsequent stage 10 with water. Preferably, use is made of fresh water sup¬ plied as indicated by the arrow 11. After the washing operation, the fly ash is separated from the washing water, and the spent washing water is preferably con¬ ducted to the preceding alkalisation stage 6, as indi¬ cated by the arrow 12, where it forms an alkaline aqueous solution together with supplied alkali 13, such as soda. After the remaining fly ash has been separated from the washing water, the remainder is made up of stabilised fly ash 14, which does not contain any leachable compo¬ nents and thus may safely be deposited at a waste depo- sit. Since the thus-stabilised fly ash does not contain any leachable components, the deposition will be fairly inexpensive. For instance, it may be mentioned that fly ash treated in accordance with the inventive method meets the requirements of German Deposition Class I which, among other things, stipulates a water-soluble proportion below 3% by weight, a sulphate content below 500 mg/1, a chloride content below 500 mg/1, a nickel content below 0.2 mg/1, a mercury content below 0.005 mg/1, a copper content below 1 mg/1, a cadmium content below 0.05 mg/1, and a lead content below 0.2 mg/1.

Moreover, Fig. 2 illustrates a preferred device for implementing the inventive method described above. This device, which is schematically illustrated in Fig. 2, consists of a band filter 15 having an inlet end 16 for the fly ash that is to be stabilised, as well as an out¬ let end 17 for the fly ash that has been stabilised. The band filter 15 includes an endless filter band 18, for instance a plastic wire, which is tensioned round two rollers 19 and 20, of which one or both may be driven. Fly ash 1 is supplied either directly to the band filter or, as illustrated in Fig. 2, to a container 21, where it is mixed with an acid aqueous solution 22. From the container 21, the fly ash and the acid aqueous solu- tion are fed with the aid of a pump 23 through a conduit 24 to a station 25 for acid extraction of the fly ash. This acid extraction, which really begins in the con¬ tainer 21, is completed by the acid aqueous solution being drawn off through the filter band 18 and drained off via an outlet conduit 26 to a collecting container

27. From this container 27, the spent acid aqueous solu¬ tion is, by means of a pump 28, fed to a plant for waste¬ water treatment, as indicated by the arrow 29. As men¬ tioned in the foregoing with reference to Fig. 1, it is preferred that this plant is the one normally connected to the hydrogen-chloride scrubber in connection with flue-gas cleaning. Having been separated from the acid aqueous solu¬ tion, the fly ash 1 is further conveyed by the band fil¬ ter to a washing station 30, where it is washed with an aqueous solution from a conduit 31 so as to be rid of any remainders of the acid aqueous solution. Washing liquid is drawn off through the filter band 18 and is removed via a conduit 32 to a container 33. From this container

33, the washing liquid can be recycled by means of a pump

34. Part of the washing liquid may be drawn off via a branch conduit 35 in order to be used as washing liquid in the hydrogen-chloride scrubber.

Having been separated from the washing liquid, the fly ash is alkalised in the next station 36 by being treated with an alkaline aqueous solution supplied via a conduit 37. As mentioned in the foregoing, the alkaline aqueous solution preferably is a soda solution, the alka¬ lisation treatment taking place to a pH of approximately 10. The alkaline aqueous solution is drawn off via a con¬ duit 38 to a container 39, whence it can be recycled by means of the pump 40. A certain amount of the alkaline aqueous solution can be drawn off via a branch conduit 41 to the container 33 in the preceding washing stage.

Having been alkalised and rid of the alkaline aque¬ ous solution, the fly ash is treated with a view to fix- ing any remaining impurities as sulphides in the station 42 by the supply of a sulphide-ion-containing aqueous solution, for instance a sodium sulphide solution, via a conduit 43. The spent sulphide solution is drawn off via a conduit 44 to a container 45. The required replenish- ment with sodium sulphide may be carried out via a con¬ duit 46, and the sulphide-containing aqueous solution is supplied from the container 45 to the conduit 43 by means of a pump 47.

After the sulphide treatment and the drawing-off of the sulphide-containing aqueous solution have been com¬ pleted, the stabilised fly ash is washed in a station 48 with water supplied via a conduit 49. After the washing operation, the water is drawn off via a conduit 50 to a container 51, which is replenished with fresh water via a conduit 52. By means of a pump 53, the water can be recycled to the washing station 48. Part of the water is drawn off via a branch conduit 54 to the container 39 for the alkaline aqueous solution and there forms, together with alkali supplied via the conduit 55, a fresh alkaline aqueous solution.

Having been dewatered on the band filter 15, the stabilised and washed fly ash is discharged at the outlet end of the band filter and collected in a container 56 with a view to subsequent deposition.

As described in the foregoing, the method according to the invention is implemented continuously, but it is to be understood that the inventive method may also be implemented batchwise and that other devices than the illustrated one described above may be used. Thus, each of the above stages may, for instance, be performed in a separate tank with associated decantation of liquid. It is further conceivable to combine the band filter and the tanks in one plant.

Although the invention has been described in the foregoing with reference to certain preferred embodiments thereof, it will be appreciated that the invention is not restricted thereto and that several modifications are conceivable within the scope of the appended claims.

Claims

1. A method for stabilising ash, preferably fly ash, in which the ash is extracted with an acid aqueous solu¬ tion, c h a r a c t e r i s e d in that the remaining ash is then treated in at least two stages comprising an al¬ kalisation stage as well as a stage in which a sulphide- ion source is added.

2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the alkalisation is carried out to a pH of approximately 7 or above, preferably a pH of approximately 7-11, and especially a pH of approximately 10.

3. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the remaining ash is separated from the liquid after each stage.

4. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the extraction with the acid aqueous solution is carried out at a pH of approximately 1-2.

5. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the extraction of ash with the acid aqueous solution involves a washing liquid obtained from a scrubber for the wet cleaning of a hydrogen-chloride-containing gas.

6. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the ash is washed with water prior to the alkalisation.

7. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the ash is washed with water after the addition of the sulphide-ion source.

8. A method as claimed in claim 7, c h a r a c - t e r i s e d in that liquid from the washing with water is transferred to the alkalisation stage.

9. A method as claimed in claim 6, c h a r a c ¬ t e r i s e d in that liquid from the alkalisation stage is transferred to the washing with water preceding the alkalisation stage.

10. A method as claimed in claims 5 and 6, c h a r a c t e r i s e d in that liquid is transferred from the washing with water to the scrubber.

11. A device for the stabilisation of ash, prefer¬ ably fly ash, c h a r a c t e r i s e d in that it com- prises a band filter (15) having an inlet end (16) for ash (1) that is to be stabilised and an outlet end (17) for ash that has been stabilised, said band filter fur¬ ther comprising, as seen from the inlet end to the outlet end, a station (25) for acid extraction of the ash, a station (36) for alkalisation of the ash, and a station (42) for sulphide treatment of the ash.

12. A device as claimed in claim 11, c h a r a c ¬ t e r i s e d in that the band filter (15) comprises washing stations (30, 48) disposed before the alkalisa- tion station and after the sulphide-treatment station.

13. A device as claimed in claim 11 or 12, c h a r a c t e r i s e d in that each station (25, 30,

36, 42, 48) comprises a feed means (24, 31, 37, 43, 49) arranged above the band filter (15) to feed liquid, as well as a discharge means (26, 32, 38, 44, 45) arranged below the band filter to draw off liquid.

14. A device as claimed in any one of claims 11-13, c h a r a c t e r i s e d in that each station (30, 36, 42, 48) disposed after the station (25) for acid extrac- tion of the ash comprises a circulation circuit having a liquid supply (33, 39, 45, 51), a means (34, 40, 47, 53) for circulating liquid in the circuit, a feed means (31,

37, 43, 49) arranged above the band filter (15) to feed liquid, as well as a discharge means (32, 38, 44, 45) arranged below the band filter to draw off liquid.

15. A device as claimed in any one of claims 11-14, c h a r a c t e r i s e d in that it comprises means ( 54) for transferring liquid from the washing station (48) disposed after the sulphide-treatment station (42) to the alkalisation station (36).

16. A device as claimed in any one of claims 11-15, c h a r a c t e r i s e d in that it comprises means (41) for transferring liquid from the alkalisation station (36) to the washing station (30) disposed before the alkalisation station (36).