Optimized Metal Recovery from Fly Ash from Municipal Solid Waste Incineration

Switzerland plays a pioneering role in sustainable waste management with a long tradition of waste incineration and the prohibition to landfill unburnt municipal solid waste since 2000. In recent years, the focus has been laid on further reduction of pollutants from incineration residues because the revised Swiss Waste Ordinance prescribes the recovery of metals from fly ash starting in 2021. Fly ash collected in the heat recovery section and the electrostatic precipitator contains high concentration of aluminosilicates, oxides, soluble salts, heavy metals and toxic organic compounds. Metals are either carried along with the flue gas as particles, forming enriched mineral aggregates or vaporized and condensed as complex chlorides or sulphates in fly ash. An efficient treatment of fly ash promises considerable ecological and economic benefits due to an improved quality for disposal and the recovery of the metals contained. At present acidic fly ash leaching (FLUWA) is the state‐of‐the‐art process, where up to 80% of Zn and minor amounts of Pb and Cu are recovered. This thesis contributes considerably to a better understanding of fly ash composition and leaching behaviour as a basis for improved metal separation. Detailed analyses of fly ash describing the chemical associations of metals were the basis for nearly 200 leaching experiments of various fly ashes. The achieved data set contains valuable information regarding the binding environment of metals in fly ash and the leaching behaviour covering a wide range of pH‐values, redox conditions, liquid to solid ratios, temperatures and leaching times. It could be shown that acidic fly ash leaching under oxidative conditions as well as a secondary leaching step using concentrated sodium chloride solution leads to an almost complete mobilization of Pb, Cu and Cd. Based on these findings at laboratory scale, an optimization of the acidic fly ash leaching on industrial‐scale was tested. It could be shown, that the trends of the chemical processes and metal recovery pointing in the right direction. Difficulties have been experienced in keeping and monitoring stable process conditions at the given system technology. The results of this thesis may also serve as a decision support for the upcoming implementation aid by the Federal Office for the Environment (FOEN), where the criteria for fly ash treatment and metal recovery efficiency have to be formulated

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

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

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

Dataset on ten-years monitoring of MSWI bottom ashes in six MSWI plants in the Canton of Zürich, Switzerland

The dataset presented in this article is the supplementary data for the research article “Ten-years monitoring of MSWI bottom ashes with focus on TOC development and leaching behaviour” by Glauser et al. (2020) [1]. From 2008 – 2018 bottom ashes have been monitored in six MSWI plants in the Canton of Zürich with regular sampling campaigns and analysis of important species defined in the Swiss Waste Legislation [2]. Both the size of the dataset and the long period of consistent and representative monitoring are unique for Switzerland. Relevant aspects of the monitoring data are discussed and interpreted in the above mentioned research article and complemented by simple emission forecast modelling. While only selected species were discussed in the research article, this data article covers all the monitoring data. The focus of the monitoring was laid on carbon-species with the analysis of total carbon (TC), total organic carbon (TOC), total inorganic carbon (TIC), degradable organic carbon (OC) and elemental carbon (EC). Total contents of nitrogen (N), sulphur (S), phosphorus (P), selected heavy metals (As, Cd, Cr, Cu, Ni, Pb, Sb and Zn) and loss on ignition (LOI) complete the solid chemical analysis. In addition, particulate ferrous (Fe) and non-ferrous (NF) metals and unburnt material were determined manually. Batch eluate tests according to Swiss Waste Legislation [3] were performed and analysed for dissolved organic carbon (DOC), ammonium (NH4+), nitrite (NO2-), fluoride (F-), sulphite (SO32-), sulphide (S2-), chromate Cr(IV) and the heavy metals Cu (aq) and Zn (aq) and Cr(IV). In addition, data on the biochemical oxygen demand (BOD) and the physical parameters pH and electrical conductivity complete the eluate analysis

Influence of sample matrix on the alkaline extraction of Cr(VI) in soils and industrial materials

An accurate and efficient determination of the highly toxic Cr(VI) in solid materials is important to determine the total Cr(VI) inventory of contaminated sites and the Cr(VI) release potential from such sites into the environment. Most commonly, total Cr(VI) is extracted from solid materials following a hot alkaline extraction procedure (US EPA method 3060A) where a complete release of water-extractable and sparingly soluble Cr(VI) phase is achieved. This work presents an evaluation of matrix effects that may occur during the hot alkaline extraction and in the determination of the total Cr(VI) inventory of variably composed contaminated soils and industrial materials (cement, fly ash) and is compared to water-extractable Cr(VI) results. Method validation including multiple extractions and matrix spiking along with chemical and mineralogical characterization showed satisfying results for total Cr(VI) contents for most of the tested materials. However, unreliable results were obtained by applying method 3060A to anoxic soils due to the degradation of organic material and/or reactions with Fe2+-bearing mineral phases. In addition, in certain samples discrepant spike recoveries have to be also attributed to sample heterogeneity. Separation of possible extracted Cr(III) by applying cation-exchange cartridges prior to solution analysis further shows that under the hot alkaline extraction conditions only Cr(VI) is present in solution in measurable amounts, whereas Cr(III) gets precipitated as amorphous Cr(OH)3(am). It is concluded that prior to routine application of method 3060A to a new material type, spiking tests are recommended for the identification of matrix effects. In addition, the mass of extracted solid material should to be well adjusted to the heterogeneity of the Cr(VI) distribution in the material in question

Chemical associations and mobilization of heavy metals in fly ash from municipal solid waste incineration

This study focusses on chemical and mineralogical characterization of fly ash and leached filter cake and on the determination of parameters influencing metal mobilization by leaching. Three different leaching processes of fly ash from municipal solid waste incineration (MSWI) plants in Switzerland comprise neutral, acidic and optimized acidic (+ oxidizing agent) fly ash leaching have been investigated. Fly ash is characterized by refractory particles (Al-foil, unburnt carbon, quartz, feldspar) and newly formed high-temperature phases (glass, gehlenite, wollastonite) surrounded by characteristic dust rims. Metals are carried along with the flue gas (Fe-oxides, brass) and are enriched in mineral aggregates (quartz, feldspar, wollastonite, glass) or vaporized and condensed as chlorides or sulphates. Parameters controlling the mobilization of neutral and acidic fly ash leaching are pH and redox conditions, liquid to solid ratio, extraction time and temperature. Almost no depletion for Zn, Pb, Cu and Cd is achieved by performing neutral leaching. Acidic fly ash leaching results in depletion factors of 40% for Zn, 53% for Cd, 8% for Pb and 6% for Cu. The extraction of Pb and Cu are mainly limited due to a cementation process and the formation of a PbCu0-alloy-phase and to a minor degree due to secondary precipitation (PbCl2). The addition of hydrogen peroxide during acidic fly ash leaching (optimized acidic leaching) prevents this reduction through oxidation of metallic components and thus significantly higher depletion factors for Pb (57%), Cu (30%) and Cd (92%) are achieved. The elevated metal depletion using acidic leaching in combination with hydrogen peroxide justifies the extra effort not only by reduced metal loads to the environment but also by reduced deposition costs

Ten-years monitoring of MSWI bottom ashes with focus on TOC development and leaching behaviour

In Switzerland MSWI bottom ash has to comply with the legal threshold value for TOC of <2 wt.-% in order to be landfilled. However, TOC contents of this magnitude lead to elevated DOC emissions and associated emissions of ammonium and Cu (aq). Since 2008 the Canton of Zürich therefore pursues a strategy to lower TOC contents in bottom ash by 2020 to 0.5 wt.-%. To observe the development of TOC and other constituents, bottom ash has been monitored from 2008–2018. Monitoring results indicate that TOC contents <0.5 wt.-% in bottom ash lead to DOC eluate concentrations <20 mg/l. DOC concentrations of this magnitude are close to Swiss legal criteria for discharge of landfill leachate into surface waters (10 mg/l). The emission results have been obtained by batch eluate tests according to Swiss Waste Legislation. Such laboratory tests only partially simulate real conditions occurring on landfills. To approximate landfill conditions, column tests with recent bottom ashes combined with tests on simple emission forecasting complete the study. The comparison of results from batch and column tests shows similar cumulative concentrations, indicating that batch tests are suitable to evaluate bottom ash quality. The tested modelling approach, based on constant conditions and exponential decrease in concentration, proved adequate to simulate column progressions. The modelled emission forecasts for DOC lies within 33% of column test results. Further, the model demonstrates the differences in flow regime between eluate tests and landfills and promotes better understanding of temporal aspects and the influence of landfill relevant parameters on pollutant mobilisation