DE3918292A1 - Treating flue ash in combustion gases from incineration plant - by treating with acid soln. to dissolve heavy metals and reprecipitating as sulphide cpd(s) on ash residue - Google Patents

Abstract

Heavy metal-contg flue ash which is present in the combustion gases of incineration plants is treated by washing with acidic soln to dissolve the heavy metals from the ash. Prior to mechanically sepg the ash from the washing soln sulphur cpds which cause the precipitation of heavy metal-sulphur cpds are added. After precipitation the ash together with the heavy metal-sulphur cpd precipitation is sepd from the washing soln. USE/ADVANTAGE - treating flue gas from waste and refuse incineration plants. The mixt of ash and heavy metal precipitate can be safely dumped in a land-fill due to the high degree of inactivity afforded to the heavy metal in the present state. Alternatively the material can be used in road and underground construction.

Description

The invention relates to a method for the treatment of fly ash containing heavy metals from the flue gas of Incineration plants, especially waste or Waste incineration plants in which the fly ash with a acid wash solution is treated, the washed Fly ash mechanically separated from the washing liquid and heavy metals released from the fly ash by adding at least one sulfur compound as Heavy metal sulfur compound (sulfide compound) precipitated will.

Such a method is known from DE-OS 33 20 466, where the fly ash with the condensate is one of the Fly ash from the separation plant separating the flue gas downstream condensation / washing system treated after treatment with the acidic washing liquid the fly ash of a solid-liquid separator fed and after disconnecting the incinerator is fed to be incorporated into the slag will.

The solution coming from the solid-liquid separation becomes a sulfide precipitation to form the  Subjected to heavy metal sulfur compounds.

The flake-like heavy metal Sulfur compounds are from the precipitation solution separated and the precipitation solution becomes one Subjected to neutralization so that then wastewater containing neutral salt is fed to a receiving water can be.

The heavy metal connections represent a possible Recycling available. In the case of recovery from Mercury, lead, cadmium from the resulting sulfides should even eliminate the residue completely Heavy metal pollution may be possible. If a suitable one Floodwater is not available, the fly ash not washed, but only that with the condensate from the Flue gas excreted mercury from a sulfide precipitation subjected and together with the unwashed Fly ash fixed by bituminization. It can of it it is assumed that recycling, i. H. a Heavy metal extraction from the heavy metal concentrate according to DE-OS 33 20 466, not economically worthwhile, so that the heavy metal concentrate itself as hazardous waste must be disposed of. The fly ash disposal problem is by returning the fly ash after washing not solved in the incinerator either, because the one there generated slag must also be disposed of.

It is the object of the present invention Procedure to indicate with the fly ash with her Heavy metal content can be processed according to landfill.

This object is achieved in that the addition of the leading the heavy metal sulfur compounds Sulfur compounds before separating the Washing liquid is made from the fly ash and after the Precipitation of the heavy metal sulfur compounds  Fly ash and the heavy metal sulfur compounds be separated from the washing liquid together.

In addition to the heavy metals, the laundry also removes the easily soluble substances, primarily chlorides, from the fly ash. Suitable sulfur compounds are preferably hydrogen sulfide (H ₂ S) and sodium sulfide (Na ₂ S), but also organic sulfur compounds (such as dithiophosphoric acid, thiocarboxylic acid or mercaptotriazine derivatives).

Of particular advantage with the invention Procedure is that the failing insoluble Heavy metal sulfur compounds largely on the Surface of the fly ash particles are deposited. This facilitates the mechanical separation of the fly ash and the heavy metal sulfur compounds by z. B. Filtration, sedimentation or centrifugation.

The pH of the washing solution is preferably lower than that of acid rain (pH = 4-5) to the Immobilization of pH-related Heavy metal hydroxides and other acid-soluble Heavy metal connections in and on the fly ash save and use chlorides and other soluble To safely remove components from the fly ash can. The pH may be higher during the precipitation. However, it must be so low that hydroxide formation is prevented, preferably ≦ 6.

To as far as possible precipitate the solved To reach heavy metals, it is useful if first the main part of dissolved heavy metals with a Sulfur compound is precipitated and then the remaining heavy metals in solution in one Precipitation can be precipitated.  

In order to precipitate the main part, inexpensive inorganic sulfur compounds, such as H ₂ S and Na ₂ S, are preferably used to precipitate the sulfide, while in the second stage, more expensive organosulfides are used, which are only used in smaller quantities to reprecipitate problematic substances, such as mercury and / or Cd -Connections are required. To further improve sulfide precipitation, in particular fine sulfide precipitation, it is also possible to add surface-active substances, such as polyvinylpyrrolidone or polyvinyl alcohol. Precipitation can also be achieved without a sulfur compound.

On the one hand, with the main and the fine precipitation possible that the precipitated heavy metals and the Fly ash only after the fine precipitation of the Wash liquid to be separated, and to be another after the failure of the main part of the solved Heavy metals this and the fly ash from the Wash liquid separated, and only then the separated one Washing liquid is subjected to fine precipitation; the after the fine precipitation separated solids become Treatment of the initial fly ash with the acid Wash solution returned.

An excess of inorganic sulfide or hydrogen sulfide ions forming substances, such as. B. H ₂ S or Na ₂ S, is oxidized after the precipitation of the heavy metals by adding an oxidizing agent to the washing liquid before or after their separation from the solids. Hydrogen peroxide (H ₂ O ₂ ) and ozone (O ₃ ) are more preferably suitable for this. This prevents odor nuisance in the environment from the treated fly ash or the discharged waste water. However, it would also be possible to selectively strip the superfluous sulfur compounds by adding steam and air.

After the mechanical separation of fly ash and the Main part of the heavy metal sulfur compounds on the one hand, and precipitation solution on the other, it is expedient to use the separated precipitation solution Neutralize alkalis, so that a neutral salt Wastewater is available. This contains in essential alkali or alkaline earth compounds, especially chlorides and can in general without further Treatment in the sewage network or the receiving water be delivered.

If fine precipitation is used, the neutralization can the fine precipitation and separation of the precipitation products or done together with the precipitation.

It may be for better washing out of Neutral salts make sense, the solid mass from fly ash and heavy metal sulfur compounds after the mechanical separation still after washing undergo, which in turn is advantageous if that Post-wash water to treat fresh fly ash is used, d. H. if appropriate after acidification as Wash solution is used.

A sulfuric acid is preferably used as the washing solution or a hydrochloric acid wash solution as this Acids in sufficient quantities and relatively cheap to buy To be available.

If the flue gas from the incinerator after Separate the fly ash with a washing liquid is dealt with, as is the case in the conduct of the process in accordance with DE-OS 33 20 466 is the case, it is in fact from economic advantage if as a washing solution for the Fly ash that accumulates during flue gas washing hydrochloric acid waste water can be used. The pH of this Waste water is generally in the pH range 1-2 and  is so low that no separate acid is added must become. The advantage here is that in the wastewater the heavy metals contained in the flue gas scrubber together with the heavy metals released from the fly ash immobilized and separated together with the fly ash will. This eliminates the need for otherwise additional wastewater treatment.

Although the fly ash treated according to the invention because of their in the practically relevant pH range pH-independent heavy metal immobilization without disposed of further treatment in a normal landfill as all other easily soluble salts, chlorides, in particular, can be washed out May be appropriate to solidify the landfill, by relatively small additions of hydraulically setting Additives, such as B. lime or cement added will.

The hardening of filter dust with cement is in itself known.

The procedural steps: treatment of the fly ash with a acidic washing solution, sulfide or hydrogen sulfide precipitation Heavy metals using inorganic and / or organic Sulfur compounds, especially sulfides, possibly Oxidation of unused chemicals, mechanical separation of fly ash and washing solution and a possibly Neutralization before or after mechanical separation can be used either for individual fly ash batches successively in a container or in continuous Continuous operation in succession in succession carry out switched containers.

Two procedures should now be based on the attached Block diagrams are explained in more detail. Show it:  

Fig. 1 is a process in the main part and fine precipitation before a solid-liquid separation, and

Fig. 2 is a process procedure in which the fine precipitation at which the fine precipitation and neutralization of the washing liquid are carried out in one step takes place after separation of the washing liquid from the treatment residue and precipitation of the main part.

In the process procedure shown in FIG. 1 waste 1 is supplied to a combustion plant 2 and from the incinerator flue gases exiting by means of a dust separator 3, deposited an electrostatic filter, fly ash, 4 in a preferred manner. The flue gases leaving the dust separator 3 are subjected to an exhaust gas scrubber 5 , in which HCl and Hg are essentially removed from the flue gas and are produced in an acidic scrubbing solution 6 . The flue gas is then fed to a desulfurization system 7 and, if appropriate, to a downstream denitrification system.

The fly ash and the acidic washing solution are introduced into an extraction container 8 , in which the easily soluble compounds and the heavy metals are released from the fly ash.

Thereafter, the content of the extraction container 8 is transferred to a first precipitation container 9 a , in which an inorganic sulfide or hydrogen sulfide ion-forming substance is introduced, such as. B. Na ₂ S or H ₂ S, or mixtures of such substances (supply line 10 ). It is also possible to add acid 11 via feed line in order to set the required pH.

After sulfide precipitation 9 a , the solid-liquid mixture is transferred to a fine sulfide precipitation container 9 b , to which an organosulfide 12 and optionally surface-active substances 13 are fed.

After fine sulfide precipitation 9 b , the excess of chemicals for sulfide precipitation in oxidation state 14 can be oxidized by adding an oxidizing agent 15 . In addition to the oxidation stage 14 , a dashed line 16 for selective stripping with steam and air is shown in the figure.

After the oxidation or stripping treatment that may be required is carried out, a solid-liquid separation 17 takes place . The liquid of the separation stage 17 is neutralized by adding alkalis 18 in a neutralization stage 19 , so that waste water containing neutral salt can be drawn off from the neutralization stage 19 .

The solids content of the solid-liquid separation stage 17 is subjected to a post-wash 20 with pure water 21 and the post-wash water 22 is transferred into the extraction container 8 . If necessary, fresh acid 23 is added to adjust the required pH.

After the after-washing 20 , the solid consisting of fly ash and attached heavy metal-sulfur compounds 24 can be brought to a landfill. Because of the high degree of immobilization, other uses could also be conceivable, such as. B. Road construction and other civil engineering tasks.

Examples

Fly ash from an electrostatic precipitator Household waste incineration plant with a Cd content of  600 mg / kg and a Zn content of 45,000 mg / kg each for 30 minutes at 60 ° C using different Extraction media in relation to fly ash too Extraction medium from 1 to about 2 treated. The results are listed in the table attached to the application.

When treating the fly ash with water in one pH of 6.3 becomes half of the Cd, but only 5.6% of Zn washed out.

When using hydrochloric acid as a wash solution is used a pH of 3 a comparable amount of Cd extracted and it becomes around half of the Zn content drained. After washing the treated fly ash however, the post-wash solution still contains water impermissibly high Cd and Zn concentrations.

When treating the fly ash with a sulfuric acid Washing solution, the degree of extraction for Cd increases sharply, however, the post-wash also shows very high Cd- and Zn concentrations.

With an alkaline treatment of the fly ash with Lime milk or lime water at a pH of 11.1 only very small amounts of Cd and Zn are extracted, so that there is no aftertreatment of the extract can be. The post-wash with water also performs very low Cd and Zn concentrations. However, the alkaline treated fly ash not with water, but with hydrochloric acid (comparable to acid rain) in one Washed pH of 3, so the Concentrations of Cd and especially Zn on impermissible high values, i.e. H. the pH-related immobilization the heavy metals are largely removed.

The last three rows of the table concern Procedures according to the invention. During treatment  the fly ash with a hydrochloric acid solution at pH = 3 and a precipitation of the heavy metals with hydrogen sulfide After precipitation, pH = 3 is only an extraction degree of 0.12% for Cd and less than 0.01% for Zn. Wash with hydrochloric acid at pH 3 leads to permissible heavy metal contents in the Post-wash solution, so that obviously a leaching Fly ash deposited according to the invention also at acid rain is not to be feared. The penultimate Example shows that treatment with a hydrochloric acid washing solution at pH = 3 and a precipitation with Sodium sulfide at pH = 5.5 also very small Extraction levels can be achieved and also with a Wash with hydrochloric acid or with water The pollutant content of the post-wash solutions is very low.

Treatment with a sulfuric acid wash solution and precipitation with sodium sulfide at a pH of 5.5 also shows low extraction levels and tolerable heavy metal contents in the post-wash solution.

In describing the process procedure shown in FIG. 2, the process sequence changes after the precipitation in the first precipitation tank 9 a.

The washing liquid and the solids contained in it are fed directly to a solid-liquid separation stage 17 ', in which the leached fly ash is separated off together with the heavy metal sulfides. As in the process according to FIG. 1, a wide variety of filters such as band filters, disc filters, drum filters or the like are suitable as separation stage 17 .

The solid is subjected to the post-wash 20 as in the procedure according to FIG. 1.

The liquid of the separation stage 17 is fed to a simultaneous fine precipitation and neutralization stage 25 . The liquid is by adding an alkali, e.g. B. a calcium hydroxide suspension in the pH value, so that further heavy metal sulfides - ie the excess of S ^- from the precipitation 9 a - and partly also hydroxides fail. To reduce the free residual sulfide ion concentration, an iron salt solution, e.g. B. FeCl ₃ and / or H ₂ O _{2 can be} added.

The solids are then separated off in a further solid-liquid separation stage 26 , preferably by sedimentation in a thickener, lamella thickener or the like, and the sediment 27 is returned to the extraction container 8 . As a result, acid-soluble heavy metal compounds are dissolved again. This procedure ensures that heavy metals can only be removed from the system via the precipitation 9 a , the solid-liquid separation stage 17 '.

The liquid of the solid-liquid separation stage 26 is fed to an oxidation stage 28 , which contains an oxidizing agent 29 , such as. B. air, pure oxygen, H ₂ O _{2 is} supplied.

In this way it is ensured, as in the case of the procedure according to FIG. 1, that only waste water laden with neutral salt is drawn off from the procedure.

In the process procedure shown in FIG. 2 for fine precipitation can, of course, used alone or also a organosulfide be used in addition.

Both procedures show that for the sulfidic precipitation of the main part of the dissolved heavy metals a heavy metal fine precipitation should preferably be added in order to be able to reach the required waste water limit values in all cases. Here, the process according to FIG. 2 is particularly advantageous, in which the heavy metal fine precipitation is combined with the neutralization of the washing liquid in one process step.

If the fly ash comes from a furnace reduced by an SNCR process with a reducing agent such as NH ₃ , urea or the like with respect to the NO _x content of the exhaust gases, 4 ammonium compounds can adhere to the fly ash and dissolve during the washing treatment 8 , or it can be entered in the wash water during the washing 5 of the flue gas ammonium compounds. The dissolved ammonium compounds remain in the wastewater from stages 19 and 28 and must be removed from it depending on the requirements of the discharger. The process according to the invention thus offers the additional advantage that no ammonium compounds remain in the waste product 24 when using the SNCR process.

Claims (15)

1.Process for the treatment of fly ash containing heavy metals from the flue gas of incineration plants, in particular waste or waste incineration plants, in which the fly ash is treated with an acidic washing solution, the washed fly ash is mechanically separated from the washing liquid and heavy metals released from the fly ash by addition at least a sulfur compound is precipitated as a heavy metal sulfur compound (sulfide compound), characterized in that the sulfur compounds leading to the heavy metal sulfur compounds are added before the washing liquid is separated from the fly ash and the fly ash and the heavy metal after the precipitation of the heavy metal sulfur compounds - Sulfur compound to be separated from the washing liquid together. 2. The method according to claim 1, characterized characterized in that the pH of the Washing solution is lower than that of acid rain, the pH-related immobilization of Heavy metal hydroxides and other acid-soluble Heavy metal connections in and on the fly ash pick up and this with chlorides and others soluble components safely from the fly ash to be able to detach.   3. The method according to claim 1 or 2, characterized characterized that the main part dissolved heavy metals with a sulfur compound is precipitated and then the rest in solution heavy metals in a fine precipitation be canceled. 4. The method according to one or more of claims 1-3, characterized in that the precipitated heavy metals and the fly ash only after the fine precipitation from the fly ash. 5. The method according to one or more of claims 1-3, characterized in that after the failure of the main part of the solved Heavy metals this and the fly ash from the Wash liquid separated and only then separated washing liquid subjected to fine precipitation and that those separated after the fine precipitation Solids for treating the starting fly ash with the acid washing solution can be recycled. 6. The method according to one or more of claims 1-5, characterized in that a Excess sulfur compounds, especially inorganic sulfide or hydrogen sulfide ions forming substances, after the precipitation of the Heavy metals by adding an oxidizing agent to the Washing liquid before or after its separation from the Solids is oxidized. 7. The method according to one or more of claims 1-5, characterized in that a Excess sulfur compounds, especially inorganic sulfide or hydrogen sulfide ions forming substances, by adding steam and air is selectively stripped.   8. The method according to one or more of claims 1- 7, characterized in that the solid mass consisting of fly ash and Heavy metal sulfur compounds after the mechanical separation still subjected to a wash becomes. 9. The method according to claim 8, characterized characterized that the post-wash water is used to treat fresh fly ash. 10. The method according to one or more of claims 1- 9, characterized in that as a washing solution a sulfuric acid or a hydrochloric acid Wash solution is used. 11. The method according to one or more of the claims 1-10, characterized, that when treating the flue gas Incinerator after separating the fly ash with a washing liquid as a washing solution for the Fly ash that accumulates during flue gas washing hydrochloric acid waste water can be used. 12. The method according to one or more of the claims 1-11, characterized, that for the precipitation of the main part of the solved Heavy metals at least one sulfur compound selected from the group: inorganic sulfide and Hydrogen sulfide ion-forming substances Organosulfides or mixtures thereof are used is, preferably hydrogen sulfide and / or Sodium sulfide.   13. The method according to one or more of claims 1-12, characterized in that after the mechanical separation of fly ash and the Main part of the heavy metal sulfur compounds on the one hand and the washing solution on the other separated washing liquid by means of alkalis is neutralized. 14. The method according to claim 13, characterized characterized that neutralization after the fine precipitation. 15. The method according to claim 13, characterized characterized that neutralization is done together with the fine precipitation.