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Switzerland has a long tradition of waste incineration and today combustible municipal waste that cannot be recycled has to be thermally treated in one of the 31 municipal solid waste incineration (MSWI) plants. Advantages of incineration are reduction of mass (75%) and volume (90%) as well as the inertisation of some metals and destruction of organic compounds. However, ca. 600 000 tons of bottom ash and 60 000 tons of fly ash annually still remain and have to be deposited due to their elevated concentrations of toxic substances. Today, one third of the Swiss MSWI plants are performing an acidic leaching process (FLUWA) to separate heavy metals from the fly ash. The depleted ashes show a reduced impact on the environment after deposition and the recovered metals can even be reused as valuable metals. The incorporation in mineral phases or glasses reduces the mobility of the metals. Predictions about the extractability of an element and thus the optimisation of an extraction process are only possible if the type of chemical bonding is known. The presented results focus on a detailed chemical and mineralogical characterisation of fly ash and the residual filter cake after acidic leaching. Furthermore, the FLUWA-process was performed in a laboratory scale to calculate the depletion factors of elements of interest such as Cd, Cu, Pb or Zn. In the course of these investigations hydrogen peroxide (H2O2) was additionally added to the suspension in order to increase the extraction rate.

The collected fresh fly ash (FA) is mixed with the residues of the wet flue gas treatment (neutral and acidic scrub water) in the FLUWA-process and extracted in a multistage cascade. The scrub water is used to extract heavy metals such as Zn, Pb, Cd and Cu from the FA. The suspension is then processed by vacuum belt filtration into a compact and metal depleted filter cake (FC) and a metalliferous filtrate.

FLUWA-process

Out

look

Filter cake (FC)Fresh fly ash (FA)

SEM/EDS study of fresh fly ash SEM/EDS study of acidic leached filter cake

Fresh fly ash

FLUWA-process FLUWA-process + H2O2-addition

Metal depletion in filter cakes after FLUWA-processThe FLUWA-process was transformed to laboratory scale to adjust and vary parameters such as concentrations, extraction time or temperature. 100g FA is washed with 400 ml scrub water and 8 ml of 30% hydrogen peroxide (H2O2) for 60 minutes at 60°C. After 30 minutes the pH is fixed at 4.4 and after 60 minutes the hot suspension is filtered.

The addition of hydrogen peroxide during the FLUWA-process forces the oxidation of metallic phases andtherefore cementation is supressed. During the cementation relative noble ions are reduced to zero valence and bond to surface of less noble metals. A common cementation occurs between Cu-ions and metallic iron. The added H2O2 immediately oxidises the metallic iron to Fe(III) and avoids this reaction.

The most important differences with the use of H2O2 are:

- Iron is completely transformed into Fe(III) and precipitates as Fe(III) oxide hydroxide. The amount removed from the FA is therefore <10%.

- Copper shows a depletion of 37%. Without H2O2 Cu is cemented completely and no Cu is extracted.

- Zinc is present in high concentrations (8 wt.%) and >60% are extracted from the FA. The fact that Zn does not cement nobler ions such as Fe, Cd, Pb or Cu during extraction indicates that Zn is relatively easily oxidised under the present extraction conditions.

- Lead is removed much more efficient with the addition of H2O2. 21% are extracted by the FLUWA-process and 44% after the peroxide addition.

- Cadmium is washed out nearly complete during the FLUWA-process.

- Antimony shows no depletion after the FLUWA-process.

Morphology: Three general particle morphologies are identified by SEM/EDS which demonstrate the complex and diverse bonding conditions of th metals.

1. Fine-grained condensed phase with increased concentration of volatile elements such as O, Zn, Na, S, Cl, K.2. Larger metal-bearing particles which are encapsulated by the condensate.

3. Refractory minerals such as SiO2.

Matrix:

Hollow glassy cenospheres, quatz, alumosilicate-endmembers (e.g. orthoglase, albite, anorthite), anhydrite and complex sulphate phases such as omongwaite (Na2Ca5(SO4)6•3H2O or gorgeyite (K2Ca5(SO4)•H2O).

Examples of metal associations:

Palmierite (K,Na)2Pb(SO4)2 Potassium zinc chloride (K2ZnCl4) Brass (Cu2Zn3 )

Morphology: Acidic leaching causes distinct changes in particle composition.

- New phase formation and phase dissolution (salts) through the interaction between fly ash and scrub water during FLUWA-process.

- After fly ash leaching the fine-grained condensed phase is disappeared

almost completely.

Matrix: - Increase of relative content of insoluble minerals (quartz, alumosilicates) and the amorphous part from ca. 60 wt.% to >70 wt.%. - New formation of gypsum (CaSO4•2H2O).

Examples of new formed metal associations:

Cotunnite (PbCl2) Pb-Cu-bearing phase Hemimorphite (Zn4Si2O7(OH)2•2H2O)

It has been shown that acidic leaching clearly alters ash composition and constituents of metal-bearing phases and the matrix. For a detailed insight of the ongoing redox-processes and new phase formations and dissolutions additional lab-experiments are ongoing, e.g. the influence of the pH rise and velocity from around 1 at the beginning to 2.5 after 30 minutes in respect to the acid buffering capacity of the ashes.To determine the bonding forms of the metals, various extraction experiments of separates are required. SEM image analyses are ongoing for better quantify metals phases using the tool Image SXM.

Salts, sulphates and newly formed melilithes are the dominant minerals in the FA.

Salts and newly formed melilithes were removed and sulphateswere preferentially hydrated.

elctrostaticprecipitator (ESP)

FLUWAconveyor belt

vacuum belt filter

extractorH2O2

ion exchangersscrub water (acidic+neutral)

fresh fly ash silo

combined flue gas scrubberboiler

landfill

leached filter cake

Mineralogy (XRD) Mineralogy (XRD)

Bulk chemistry (WD-XRF) Bulk chemistry (WD-XRF)

Initial concentration of relevant elementsprior to FLUWA-process (TD-ICP-OES)

LOI SiO2 CaO Al2O3 MgO wt.% 13.2 7.1 16.2 3.2 1.0 Na2O K2O Fe2O3 TiO2 MnO wt.% 10.5 6.9 1.5 1.0 0.06 SO3 P2O5 Cl V Cr mg/kg 54000 4660 127800 83 487 Co Ni Cu Zn Br mg/kg 25 68 2410 69700 4760 Rb Sr Y Zr Cd mg/kg 180 340 8 150 633 Sn Sb Ba Pb Bi mg/kg 2500 4270 1820 11400 187

LOI SiO2 CaO Al2O3 MgO wt.% 9.8 12.1 25.0 6.0 0.8 Na2O K2O Fe2O3 TiO2 MnO wt.% 0.6 0.8 3.0 1.3 0.07 SO3 P2O5 Cl V Cr mg/kg 130100 6630 7710 100 822 Co Ni Cu Zn Br mg/kg 39 106 2310 24800 199 Rb Sr Y Zr Cd mg/kg 39 457 9 287 41 Sn Sb Ba Pb Bi mg/kg 3130 5080 3020 7000 246

67%

46%

0% 1%

21%

9%

1%

61%

20%

79%

37%

37%

0%

44%

6%

24%

63%

0%0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Cd Mn Cu Sb Pb Fe Al Zn Ca

% re

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om F

A

filtrate

0.9% Calcite1.3% Quartz

11.7% Anhydrite

13.7 % Gypsum

Amorphouspart 71%

12.9% Melilithe

1.5% Quartz1.5% Calcite

6.4% Halite

8.7% K2ZnCl4

7.3% Anhydrite

Amorphous part

58.9%

78005 mg/kg Zn

34639 mg/kg Al

15458 mg/kg Fe

1190

3 mg/k

g Pb

3518

mg/k

g Sb

2623

mg/

kg C

u

764

mg/

kg M

n

529

mg/

kg C

d

170528 mg/kgCa

Extraction of Heavy Metals from MSWI Fly AshGisela Weibel1, Ivo Budde1, Urs Eggenberger1, Stefan Schlumberger2

1) Institut für Geologie, Baltzerstrasse 1+3, 3012 Bern, Switzerland2) KEBAG Kehrichtbeseitigungs-AG, Emmenspitz, 4528 Zuchwil, Switzerland