Thermodynamic model of flue gas cooling path and implications on heavy metal recovery from MSWI fly ash

Solid residuals of municipal solid waste incineration (MSWI), i.e. bottom ash and fly ash contain significant quantities of heavy metals. Nearly 80’000 tons of MSWI fly ash is produced annually in Switzerland. From the combustion of waste wood, another 6’000t of waste wood fly ash arise annually, which show heavy metal concentrations in the range of MSWI fly ash. In Switzerland, the current practice is to stabilize and dispose of the residues from waste incineration on landfills or underground storage. For MSWI fly ash, heavy metal recovery through acid leaching will be mandatory from 2026 onwards. For waste wood fly ash, heavy metals will have to be recovered from 2024 onwards and it is a matter of discussion, whether a co-treatment with MSWI fly ash could be expedient. The heavy metals are recovered from MSWI fly ash through acid fly ash leaching (FLUWA process). The MSWI fly ash here represents a mix of the different ash fractions (boiler- and electrostatic precipitator ashes) that arise along the flue gas cooling path. With regard to the obligation of heavy metal recovery from MSWI fly ash, it has not yet been conclusively defined whether the heavy metal recovery from boiler ash is expedient from a technical and ecological point of view, as little data exists on its geochemical characteristics and heavy metal binding forms. The recovery of metals from the waste makes a valuable contribution to a sustainable closure of material cycles, as metals from primary raw material mining can partly be returned to the material cycle and the environmental pollution though land-filling is minimized. This thesis focusses on inventory of volatile heavy metals (e.g. Zn, Pb, Cu, Cd) in MSWI fly ash. Detailed characterization in combination with leaching experiments allow new insights on their chemical and mineralogical composition, heavy metal binding forms and leachability. Thermodynamic modeling was used as complemen-tary approach to investigate ash-forming processes and transport mechanisms occurring during cooling of the flue gas. The influence of flue gas composition on phase assemblages and the formation conditions of PCDD/-F thereby represent further investigated aspects. It has been shown that the MSWI boiler ashes are significantly different materials than the electrostatic precipitator ash and that their share on the bulk fly ash is approximately 25 - 30%. The boiler ash consists mainly of acid-buffering and inert refractory minerals, as well as amorphous glass phase and shows considerably lower heavy metal concentrations than the electrostatic precipitator ash. The thermodynamic simulations predict, that Zn and Cu in the boiler ash fractions are preferably stable as silicates (Zn2SiO4) and oxides (e.g. ZnO, Fe2ZnO4, CuO), phases that have also been analytically verified. The elec-trostatic precipitator ash is dominated by a chloride and sulfate matrix which shows a low acid-buffering capacity and shows much higher concentrations in the heavy metals of interest (Zn, Cd, Cu, Pb), predominantly in the form of easily soluble salts. These obser-vations suggest that the electrostatic precipitator ash shows a higher potential for heavy metal recovery. The thermodynamic simulations confirm that the S/Cl ratio in the flue gas primarily controls the predicted equilibrium phase assemblages, while variations in O2 concentration affects the composition of the ash to a minor extent. The thermodynamic modelling further indicate that the formation of gaseous PCDD/-F should take place at extremely reducing conditions which are not expected for typical bulk flue gas conditions. It is suggested that necessary reducing conditions may establish, e.g. in close vicinity to the combustion of plastic or other solid particles. The results of this thesis give new in- sights into the ash-forming processes during the cooling of the flue gas and contribute to a better understanding of speciation of heavy metals in the ash, as well as of the leachability of individual fractions of MSWI fly ash and waste wood fly ash. The data may serve as a basis for life cycle assessment and to concretize the enforcement aid for heavy metal recovery. In particular, further development of thermodynamic modelling approach opens opportunity for theoretical investigation of the combustion condition in the incineration plant, optimization of the combustion process and potential minimization of toxic organic pollutants

Waste Wood Fly Ash Treatment in Switzerland: Effects of Co-Processing with Fly Ash from Municipal Solid Waste on Cr(VI) Reduction and Heavy Metal Recovery

In Switzerland, waste wood fly ash (WWFA) must be treated before deposition on landfills due to its high pollutant load (Cr(VI) and heavy metals). Acid fly ash leaching, the process used for heavy metal recovery from municipal solid waste incineration fly ash (MSWIFA), represents a possible treatment for heavy metal depletion and Cr(VI) reduction in WWFA. The co-processing of WWFA with MSWIFA during acid fly ash leaching was investigated in laboratory- and industrial- scale experiments with different setups. Of interest were the effects on heavy metal recovery efficiency, the successful outcome of Cr(VI) reduction and consumption of neutralizing chemicals (HCl, H2O2). Detailed chemical and mineralogical characterization of two WWFA types and MSWIFA showed that MSWIFA has higher concentrations in potentially harmful elements than WWFA. However, both WWFA types showed high concentrations in Pb and Cr(VI), and therefore need treatment prior to deposition. Depending on the waste wood proportion and quality, WWFA showed chemical and mineralogical differences that affect leaching behavior. In all experimental setups, successful Cr(VI) reduction was achieved. However, WWFA showed higher consumption of HCl and H2O2, the latter resulting in a particularly negative effect on the recovery of Pb and Cu. Thus, co-processing of smaller WWFA portions could be expedient in order to diminish the negative effects of Pb and Cu recovery

Characterization of MSWI fly ashes along the flue gas cooling path and implications on heavy metal recovery through acid leaching

This study reports on detailed chemical and mineralogical characterization of the different municipal solid waste incineration ashes forming along the flue gas path of plants with separate dust removal and neutralization. In pursuit of optimizing heavy metal recovery through acid leaching, the metal extractability from empty pass ashes (EA), boiler ashes (BOA) and the electrostatic precipitator ashes (ESPA) was evaluated and compared. The focus was laid on matrix phases affecting leachability (e.g. alkalinity, oxidation–reduction potential), as well as on distribution and concentration of recoverable heavy metals and their binding forms. The data showed, that EA and BOA are geochemically similar and are essentially composed of two different materials: the heavy metal poor airborne ash particles and the Zn- and Pb-rich sulfate deposits that condensate on heat exchanger surfaces. Variation in relative amount and chemical composition of the deposits is responsible for fluctuations in bulk composition of EA and BOA. The ESPA shows different chemical and mineralogical characteristics than EA and BOA. The ESPA is enriched in the volatile heavy metals Zn, Pb, Cu, Cd and Sn, which are mainly incorporated in chlorides and sulfates. The high content of salt-bound and thus easily soluble heavy metals together with the lower alkalinity and lower oxidation–reduction potential indicates, that ESPA has a better leachability compared to EA and BOA. The EA and BOA, on the other hand, do not show any significant differences in leachability. The data may contribute to a basis for re-evaluating disposal routes of ash fractions with poor extraction properties

Inventory of MSWI Fly Ash in Switzerland: Heavy Metal Recovery Potential and Their Properties for Acid Leaching

From the year 2021 on, heavy metals from Swiss municipal solid waste incineration (MSWI) fly ash (FA) must be recovered before landfilling. This is predominantly performed by acid leaching. As a basis for the development of defined recovery rates and for the implementation of the recovery process, the authorities and plant operators need information on the geochemical properties of FA. This study provides extended chemical and mineralogical characterization of all FA produced in 29 MSWI plants in Switzerland. Acid neutralizing capacity (ANC) and metallic aluminum (Al0) were additionally analyzed to estimate the effort for acid leaching. Results show that all FA samples are composed of similar constituents, but their content varies due to differences in waste input and incineration conditions. Based on their geochemical properties, the ashes could be divided into four types describing the leachability: very good (6 FA), good (10 FA), moderate (5 FA), and poor leaching potential (8 FA). Due to the large differences it is suggested that the required recovery rates are adjusted to the leaching potential. The quantity of heavy metals recoverable by acid leaching was estimated to be 2420 t/y Zn, 530 t/y Pb, 66 t/y Cu and 22 t/y Cd

Thermodynamic model of flue gas cooling path and implications on heavy metal recovery from MSWI fly ash

Solid residuals of municipal solid waste incineration (MSWI), i.e. bottom ash and fly ash contain significant quantities of heavy metals. Nearly 80’000 tons of MSWI fly ash is produced annually in Switzerland. From the combustion of waste wood, another 6’000t of waste wood fly ash arise annually, which show heavy metal concentrations in the range of MSWI fly ash. In Switzerland, the current practice is to stabilize and dispose of the residues from waste incineration on landfills or underground storage. For MSWI fly ash, heavy metal recovery through acid leaching will be mandatory from 2026 onwards. For waste wood fly ash, heavy metals will have to be recovered from 2024 onwards and it is a matter of discussion, whether a co-treatment with MSWI fly ash could be expedient. The heavy metals are recovered from MSWI fly ash through acid fly ash leaching (FLUWA process). The MSWI fly ash here represents a mix of the different ash fractions (boiler- and electrostatic precipitator ashes) that arise along the flue gas cooling path. With regard to the obligation of heavy metal recovery from MSWI fly ash, it has not yet been conclusively defined whether the heavy metal recovery from boiler ash is expedient from a technical and ecological point of view, as little data exists on its geochemical characteristics and heavy metal binding forms. The recovery of metals from the waste makes a valuable contribution to a sustainable closure of material cycles, as metals from primary raw material mining can partly be returned to the material cycle and the environmental pollution though land- filling is minimized. This thesis focusses on inventory of volatile heavy metals (e.g. Zn, Pb, Cu, Cd) in MSWI fly ash. Detailed characterization in combination with leaching experiments allow new insights on their chemical and mineralogical composition, heavy metal binding forms and leachability. Thermodynamic modeling was used as complemen- tary approach to investigate ash-forming processes and transport mechanisms occurring during cooling of the flue gas. The influence of flue gas composition on phase assemblages and the formation conditions of PCDD/-F thereby represent further investigated aspects. It has been shown that the MSWI boiler ashes are significantly different materials than the electrostatic precipitator ash and that their share on the bulk fly ash is approximately 25 - 30%. The boiler ash consists mainly of acid-buffering and inert refractory minerals, as well as amorphous glass phase and shows considerably lower heavy metal concentrations than the electrostatic precipitator ash. The thermodynamic simulations predict, that Zn and Cu in the boiler ash fractions are preferably stable as silicates (Zn2SiO4) and oxides (e.g. ZnO, Fe2ZnO4, CuO), phases that have also been analytically verified. The elec- trostatic precipitator ash is dominated by a chloride and sulfate matrix which shows a low acid-buffering capacity and shows much higher concentrations in the heavy metals of interest (Zn, Cd, Cu, Pb), predominantly in the form of easily soluble salts. These obser- vations suggest that the electrostatic precipitator ash shows a higher potential for heavy metal recovery. The thermodynamic simulations confirm that the S/Cl ratio in the flue gas primarily controls the predicted equilibrium phase assemblages, while variations in O2 concentration affects the composition of the ash to a minor extent. The thermodynamic modelling further indicate that the formation of gaseous PCDD/-F should take place at extremely reducing conditions which are not expected for typical bulk flue gas conditions. It is suggested that necessary reducing conditions may establish, e.g. in close vicinity to the combustion of plastic or other solid particles. The results of this thesis give new in- sights into the ash-forming processes during the cooling of the flue gas and contribute to a better understanding of speciation of heavy metals in the ash, as well as of the leachability of individual fractions of MSWI fly ash and waste wood fly ash. The data may serve as a basis for life cycle assessment and to concretize the enforcement aid for heavy metal recovery. In particular, further development of thermodynamic modelling approach opens opportunity for theoretical investigation of the combustion condition in the incineration plant, optimization of the combustion process and potential minimization of toxic organic pollutants

Optimization of Metal Recovery from MSWI Fly Ash by Acid Leaching: Findings from Laboratory- and Industrial-Scale Experiments

A major part of Swiss fly ashes (FA) from municipal solid waste incineration (MSWI) are treated with the acid fly ash leaching process (FLUWA) in order to recover heavy metals prior to deposition. The FLUWA process uses scrub water from wet flue gas cleaning to leach heavy metals from FA. The leaching efficiency is strongly dependent on the leaching conditions (e.g., pH, Eh, L/S-ratio). This case study presents the optimization of the FLUWA process at the MSWI plant Linth, Switzerland, through determination of ideal process parameters for optimal metal recovery. By means of laboratory- and industrial-scale experiments, the process was adjusted towards a more efficient leaching of Zn, Pb, Cu, and Cd. This included the use of an oxidizing agent (hydrogen peroxide). Laboratory experiments proved to be a powerful tool for simulating process optimizations at industrial scale. An ideal leaching pH of 3.8 was determined and it was observed that the process stability is significantly influenced by the L/S-ratio applied to the leaching process. In the course of the study, the recovery could be improved to 67% Zn, 66% Pb, 30% Cu, and 91% Cd. It can be concluded that for optimal metal recovery the process has to be individually adjusted to the composition of the processed FA and scrub water of each specific FLUWA process

Urban Mining

Gespräch mit Dr. Mirjam Wolffers und Dr. Gisela Weibel, Fachstelle Sekundärrohstoffe, Institut für Geologie, Universität Bern, im Rahmen der Veranstaltungsreihe «Fokus Forschung», aufgenommen am 27.04.2023. Moderation: Eri Amsler, Universitätsbibliothek Bern. Primäre Rohstoffe zu beschaffen ist Energie- und CO2-intensiv, gleichzeitig landet in der Schweiz noch immer ein grosser Teil der Abfallmenge auf Deponien. Gisela Weibel und Mirjam Wolffers untersuchen Materialien aus verschiedenen Stoffflüssen auf ihr Wiederverwertungspotenzial, um besiedelte Gebiete als sekundäre Rohstofflagerstätten – sogenannte Urban Mines – nutzbar zu machen. Im Gespräch erklären die Forscherinnen wie sie mineralische Abfälle vom Tunnelbau oder Metalle aus Rückständen von Verbrennungsanlagen untersuchen und erzählen über die Zusammenarbeit mit der Industrie und das gemeinsame Bestreben für einen nachhaltigeren Materialkreislauf

Mein abc ist sekundär

Ausstellung und Gespräch über das Potential und die Herausforderungen der Verwertung von mineralischen Abfällen im Sinne der Kreislaufwirtschaf

Heavy Metal Recoverability Of MSWI Fly Ashes Along The Flue Gas Cooling Path

Each year, around 75’000t of fly ash from municipal solid waste incineration (MSWI fly ash) are produced and deposited on landfills in Switzerland. MSWI fly ash consists of boiler- and electrostatic precipitator ash and contains significant concentrations of heavy metals (e.g., Zn, Cu, Cd, Pb, Sb). From the year 2026 onwards, it will be mandatory to treat Swiss MSWI fly ashes with acid leaching in order to recover the heavy metals prior to deposition. However, it has not yet been legally determined whether the boiler ash is to be treated as fly ash. It could also be treated and deposited together with the bottom ash. Currently, the data available for boiler ashes are very limited, and their metal recovery potential has not been fully explored. Therefore, detailed chemical and mineralogical characterization was performed on the different MSWI ashes that form along the flue gas path (empty pass-ash (EA), boiler ashes (BOA) and electrostatic precipitator ash (ESPA)). Using a broad combination of methods (XRF, XRD, SEM), the ashes from six Swiss MSWI plants were characterized with respect to the chemical and mineralogical composition of major- and minor phases. Important parameters to estimate the suitability of a treatment are the contents of recoverable heavy metals and the extractability of the ashes. The focus was therefore laid on matrix phases affecting leachability (e.g. alkalinity, oxidation-reduction potential), as well as on the distribution and concentration of recoverable heavy metals and their binding forms. In order to estimate the need for metal recovery before landfilling, the contents of non-mobilizable pollutants such as Sb was also recorded along the flue gas path. EA and BOA showed comparable bulk chemical and mineralogical composition and are composed of two significantly different materials: the airborne ash particles (quenched melt droplets and refractory particles) and deposits formed on heat exchanger surfaces. It is mainly the deposits that contribute to the elevated heavy metal concentration, explained by the well-developed, large (Na,K)-PbSO4 crystals and the Zn-bearing matrix sulfates. The variation in the amount and chemical composition of the deposits controls the fluctuations in the bulk composition of EA and BOA. The ESPA shows different chemical and mineralogical characteristics than EA and BOA. The ESPA is enriched in the more volatile metals Zn, Pb, Cu, Cd; which are mainly present as chlorides and sulfates. The high content of salt-bound and thus easily soluble heavy metals together with the lower alkalinity and oxidation-reduction potential indicates, that ESPA has a better leachability compared to EA and BOA. These observations suggest that individual treatment of ESPA has higher potential for heavy metal recovery. Comparing the EA and BOA, however, no significant differences could be found in the parameters affecting extractability. The obtained results provide important insights into the formation of the different ash fractions and its geochemical characteristics. The data may serve as basis for re-evaluating disposal routes of ash fractions with poor extraction properties