EP0635282B1 - Method for treatment of residual material of incinerators - Google Patents

Description

TECHNICAL AREA

The invention relates to a method for treating residues in waste incineration plants, in which, in addition to slag, hydrogen chloride is also obtained from the flue gas scrubbing.

TECHNOLOGICAL BACKGROUND AND PRIOR ART

When municipal waste is incinerated, various residual material fractions occur in the process: slag (250 kg / ton of waste), filter dust (20 - 30 kg / ton of waste), hydrogen chloride (7 kg / ton of waste) from the wet scrubber of the flue gas cleaning system and sulfur dioxide (2 kg / Ton of garbage). It is not possible to use these residues without previous work-up.

Filter dust is classified as hazardous waste due to the high content of heavy metals and organic materials. A recycling, e.g. in sidewalk construction, but is possible by glazing the dust. Gypsum can be produced from sulfur dioxide as a material by skillful process control. The processing of the hydrogen chloride is problematic since extremely complex processes with multi-stage rectification are necessary to produce usable hydrochloric acid of technical quality. Finally, the slag is contaminated with heavy metals and is therefore often disposed of in an orderly manner.

Various methods for treating waste in waste incineration plants are known from EP-A-0 538 598 and DE-A-40 35 042

SUMMARY OF THE INVENTION

The invention has for its object to provide a method for treating waste in waste incineration plants, which leads to a practically heavy metal-free end product with good hydraulic properties.

This object is achieved according to the invention in a process of the type mentioned at the outset by washing the slag with the crude hydrochloric acid from the wet scrubber of the flue gas cleaning system after prior mechanical comminution, and reducing the chloride content in the slag treated in this way by washing with water. The heavy metal chlorides in both wash waters are precipitated by adding a precipitant, preferably soda.

The invention is based on the following considerations, which have been confirmed by practical tests:

It is not possible to reduce the heavy metal content in slag from waste incineration plants by washing with water alone. Slag has an alkaline reaction due to its calcium oxide (CaO) content. $${\text{CaO + H}}_{\text{2nd}}{\text{O → Approx}}^{\text{2+}}{\text{+ 2 OH}}^{\text{-}}$$

   [Me: zweiwertiges Schwermetall]For this reason, insoluble heavy metal compounds are formed when eluting with water $${\text{Me}}^{\text{2+}}{\text{+ 2 OH}}^{\text{-}}{\text{→ Me (OH)}}_{\text{2nd}}$$

[Me: divalent heavy metal]

A sufficiently high content of alkaline substances is desirable because heavy metal ions are immobilized in this way. In addition, CaO gives the slag good building material properties. For the alkalinity are in the relevant Regulations set minimum values. For example, the "Swiss Technical Ordinance on Waste" (TVA) stipulates a minimum value of 1 mol / kg, which is converted to CaO 2.8%.

In an acidic washing process, predominantly soluble compounds are formed, which can be separated from the slag by a solid-liquid separation. According to the invention, hydrochloric acid is now suitable for such a washing process, since this acid is obtained in any case in the flue gas cleaning of the system.

Heavy metals that are present as oxides are converted into a soluble form. $${\text{MeO + 2 HCl → MeCl}}_{\text{2nd}}{\text{+ H}}_{\text{2nd}}\text{O}$$

Alkaline material, such as CaO, is partially neutralized by acid washing. $${\text{CaO + 2 HCl → CaCl}}_{\text{2nd}}{\text{+ H}}_{\text{2nd}}\text{O}$$

As laboratory studies have shown, the total alkalinity can only be destroyed if the material is ground very small. In this case, the proportion of CaO that is neutralized in the washing process would then have to be added again to stabilize the residue. So if the starting product is only crushed comparatively roughly, e.g. on grain sizes between 5 and 30 mm in a hammer mill, there is no need to stabilize the residue. This means that acidic washing is also possible with a smaller amount of hydrochloric acid than that theoretically calculated from the total alkalinity.

A major advantage of the invention is that in a single process, two critical residues, heavy metal-containing slag and crude are economically available Hydrochloric acid, can be eliminated or converted into reusable material. The end products of this treatment are slag in building material quality and usable table salt.

An exemplary embodiment of the invention and the advantages which can be achieved thereby are explained in more detail below with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, an embodiment of the invention is shown schematically in the form of a flow chart.

WAYS OF CARRYING OUT THE INVENTION

According to the figure, slag is discharged from the incinerator 1 of a waste incineration plant by means of tappet deslagger 2 through a water bath from the furnace 1. In a first process step, the so-called coarse or oversize is separated in a mechanical separation device 3. Coarse or oversize are not or incompletely burned components of the waste with diameters of approximately 250 mm or larger. The iron fraction is then separated in a magnetic separator 4, which can be done, for example, by overband magnets. The grain fraction between about 30 and 250 mm leaving the magnetic separator is mechanically comminuted in a mill 5, for example a hammer mill, to a grain size of 32 mm and less. The grain fraction prepared in this way then passes into a first scrubber, the acid scrubber 6, where it is washed with crude hydrochloric acid. An acid scrubber suitable for this purpose includes, for example, a stainless steel tank with an integrated agitator. The crude hydrochloric acid comes from the wet scrubber 7 of the flue gas cleaning system which is already present in waste incineration plants 8. This washing process predominantly forms soluble compounds which can be separated from the slag by a known solid-liquid separation.

The residue obtained in this way has a not inconsiderable residue of chlorides, which is reduced by washing with water in a second washer 9, the water washer. The separation of slag and washing water loaded with chlorides takes place, as in the first washing process, by means of a solid-liquid separation which is known per se.

The slag separated in the previous process step can now optionally in a lime stage 10 with lime, e.g. in the form of milk of lime. This measure serves to stabilize the slag. Traces of heavy metal remaining in the slag are immobilized in this way and are no longer leached out.

$${\text{CaCl}}_{\text{2}}{\text{+ Na}}_{\text{2}}{\text{(CO}}_{\text{3}}{\text{) → CaCO}}_{\text{3}}\text{+ 2 NaCl}$$

$${\text{MeCl}}_{\text{2}}{\text{+ Na}}_{\text{2}}{\text{(CO}}_{\text{3}}{\text{) → MeCO}}_{\text{3}}\text{+ 2 NaCl}$$

   [Me: zweiwertiges Schwermetall]The washing water from the acid scrubber 6 and the water scrubber 9 - they contain, as essential heavy metal impurities Chloride (MeCl ₂₎ and calcium chloride (CaCl ₂₎ - are fed to a waste water treatment plant 11 and further treated there. Soda (Na ₂ (CO ₃ )) is preferably used as the precipitant, with which neutralization and simultaneous precipitation is possible: $${\text{2 HCl + Na}}_{\text{2nd}}{\text{(CO}}_{\text{3rd}}{\text{) → 2 NaCl + H}}_{\text{2nd}}{\text{O + CO}}_{\text{2nd}}$$

$${\text{CaCl}}_{\text{2nd}}{\text{+ Well}}_{\text{2nd}}{\text{(CO}}_{\text{3rd}}{\text{) → CaCO}}_{\text{3rd}}\text{+ 2 NaCl}$$

$${\text{MeCl}}_{\text{2nd}}{\text{+ Well}}_{\text{2nd}}{\text{(CO}}_{\text{3rd}}{\text{) → MeCO}}_{\text{3rd}}\text{+ 2 NaCl}$$

[Me: divalent heavy metal]

The heavy metal concentration can be reduced very deeply by adding sulfidic reagents, for example trimercaptotriazine (trade name TMT15).

Recyclable table salt, e.g. NaCl for chlor-alkali electrolysis.

In a typical waste incineration plant, 250 to 300 kg of slag are produced per ton of waste. Measurements have shown that the alkalinity - calculated as CaO - is between 10 and 13%. These are converted to the amount of substance between 800 and 1000 mol of alkaline equivalents per ton of waste. In the flue gas cleaning system, approx. 7 kg of HCl are produced, corresponding to around 200 mol of acid.

In-depth laboratory tests have shown that even with these substance quantity ratios, the pH during washing (in the acid level) is acidic. Heavy metals that sit on the surfaces of the slag grains are effectively removed by the washing process.

Label list

1

Incinerator

2nd

Pestle purifier

3rd

Separator

4th

Magnetic separator

5

Hammer mill

6

Acid scrubber

7

Wet scrubber from 8

8th

Flue gas cleaning system

9

Water washer

10th

Lime scale

11

Wastewater treatment plant

Claims (4)

1. Process for residue treatment in refuse incineration plants in which, in addition to slag, hydrogen chloride arises from the flue gas scrubbing, characterized in that the slag, after prior mechanical comminution, is washed with the crude hydrochloric acid from the wet scrubber of the flue gas purification plant, and the chloride content in the slag thus treated is reduced by washing with water.
2. Process according to Claim 1, characterized in that after the separation of slag and wash water, the metal chlorides are precipitated in both wash waters by addition of soda.
3. Process according to Claim 2, characterized in that, for the further reduction of the heavy metal concentration in the wash water, sulphide reagents, preferably trimercaptotriazine, are added to the wash water.
4. Process according to one of Claims 1 to 3, characterized in that, after the separation of slag and wash water, the slag is treated with lime, preferably in the form of milk of lime, for the purpose of immobilizing the traces of heavy metals remaining in the slag.

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