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IMPROVING THE REACTIVITY OF MUNICIPAL SOLID WASTE INCINERATED BOTTOM ASH THROUGH WET CARBONATION

The aim of the study is to improve the reactivity of municipal solid waste incinerated bottom ash through wet carbonation.

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Description

The interaction of CO2 with municipal solid waste incinerator (MSWI) bottom ash was studied in order to investigate the resulting changes in pH and bottom ash mineralogy and the impact that these changes have on the mobility of Cu and Mo. Carefully controlled carbonation experiments were performed on bottom ash suspensions and on filtered bottom ash leachates. Changes in leachate composition were interpreted with the geochemical model MINTEQA2, and neoformed minerals were investigated by means of chemical and spectroscopic analysis. The leaching of Cu and Mo during artificial carbonation is compared to the leachability of Cu and Mo from a sample of naturally carbonated bottom ash from the same incinerator. During carbonation in the laboratory, a precipitate was formed that consisted mainly of Al-rich amorphous material, calcite, and possibly gibbsite. Carbonation to pH ≈8.3 resulted in a reduction of more than 50% in Cu leaching, and a reduction of less than 3% in Mo leaching. The reduction in Cu leaching is attributed to sorption to the neoformed amorphous Al-minerals. During natural weathering/carbonation of bottom ash, additional sorption sites are formed which further reduce the leaching of Cu and Mo on a time scale of months to years.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

CHAPTER ONE

INTRODUCTION

1.1      BACKGROUND OF THE PROJECT

  • PROBLEM STATEMENT
  • AIM/OBJECTIVE OF THE PROJECT
  • SCOPE OF THE PROJECT
  • BENEFITS OF WET CARBONATION
  • DEFINITION OF TERMS

CHAPTER TWO

LITERATURE REVIEW

  • REVIEW OF THE STUDY
  • CHARACTERISTICS OF MUNICIPAL SOLID WASTE INCINERATED ASHES
  • BOTTOM ASH
  • WET CARBONATION
  • REVIEW OF THE RELATED STUDIES

CHAPTER THREE

3.0      MATERIALS AND METHODS

  • BOTTOM ASH DESCRIPTION
  • CARBONATION EXPERIMENTS WITH SUSPENDED MSWI BOTTOM ASH
  • CARBONATION EXPERIMENTS WITH FILTERED BOTTOM ASH LEACHATES
  • GEOCHEMICAL MODELLING

CHAPTER FOUR

RESULT AND DISCUSSION

  • THE EXTENT OF CO2 ABSORPTION AND RESULTING CHANGES IN BOTTOM ASH PH
  • GENERAL LEACHING TRENDS
  • MINERAL NEOFORMATION DURING CARBONATION
  • COPPER AND MOLYBDENUM
  • COMPARISON WITH NATURALLY WEATHERED BOTTOM ASH

CHAPTER FIVE

  • CONCLUSION AND RECOMMENDATION

REFERENCES

 

CHAPTER ONE

   1.0                                                      INTRODUCTION

1.1                                            BACKGROUND OF THE STUDY

Municipal solid waste incinerator (MSWI) bottom ash contains potentially toxic elements and is produced in very large quantities. In particular Cu and Mo have been shown to be leached in relatively large amounts from Dutch MSWI bottom ash (Born, 2024). Since MSWI bottom ash is formed at high temperatures and subsequently cooled fairly rapidly, the material is unstable under atmospheric conditions. Weathering will, therefore, change the mineralogical characteristics of the material. For a proper assessment of the long-term environmental impact of ash utilisation and disposal, it is necessary to understand these weathering processes and their effect on (trace) element leaching. Carbonation has been recognised to be an important weathering process affecting alkaline (waste) materials such as MSWI bottom ash (Comans et al., 2024), coal fly ash (e.g. Schramke, 2022), and oil shale solid wastes (e.g. Reddy et al., 2016; Essington, 2021). Carbonation involves the absorption of CO2 by an initially alkaline material; this causes the pH to decrease and calcite (CaCO3) to precipitate until the material is in equilibrium with (atmospheric) CO2. During the carbonation of MSWI bottom ash Al(OH)3(s) and amorphous aluminosilicates have also been found to precipitate (Zevenbergen et al., 2014). Aluminium minerals are likely to precipitate because the solubility of Al is strongly reduced when the pH decreases from >10 to 8–8.5 (Stumm et al., 2021). The CO2 required for carbonation may originate from the atmosphere or from the biodegradation of organic residues (Belevi et al., 2012). The effect of carbonation on trace-element mobility has been investigated recently for cement-solidified wastes (Lange et al., 2016), alkaline coal fly ash (Reddy et al., 2014), and oil shale solid wastes (Reddy et al., 2011). In these experiments a significant reduction in the leaching of Cu was observed and in some cases also in the leaching of Mo. Less is known about the impact of carbonation on the leaching of trace elements from MSWI bottom ash. Recent experiments, however, have shown that significantly less Cu and Mo are leached from naturally weathered MSWI bottom ash than from fresh ash (Meima et al., 2019). The lower leaching from weathered bottom ash has been attributed to the neutralisation of bottom ash pH and to the sorption of Cu and Mo to neoformed minerals (Meima et al., 2018).

1.2                                          STATEMENT OF THE PROBLEM

Recycling of waste material is a topic of growing importance in the field of waste management. Bottom ash, originating in the grate furnace of a municipal solid waste incinerator (MSWI), is an example of a material with a high recycling potential. It can replace the sand or gravel fraction in concrete bricks or can be used as such in foundations (IAWG, 2017). For bottom ash to be recycled, the material must comply with strict regulations, consisting of civil-technical (strength, grain size distribution etc) and environmental requirements. While the former requirements do not seem to be a problem for bottom ash, the latter are. Worldwide, leaching of heavy metals such as Cu, Pb and Zn is reported to exceed the local limit values substantially; the extent depending on the incineration process parameters and the local limit values. Leaching of metals from bottom ash and the reduction of its importance has been the subject of several investigations. Several techniques have been proposed ranging from physical ones such as sieving and (dia-) magnetic separation to chemical ones such as washing and carbonation (Sabbas et al., 2023). Carbonation seems to be one of the most promising techniques (Polle-tini et al., 2023) because it overcome limitation seen in other techniques such as time wasting and inaccuracy. The carbonation process is the reaction of CO2 from the air with hydroxides in the bot- tom ash, giving carbonates. This reaction is accompanied by a decrease of the material pH from 11–12 to 8–9. In several countries, bottom ash is left to mature for 6–12 weeks after quenching. After this natural maturation, leaching for several metals appears to be less important than from fresh bottom ash. It is believed that carbonation is an important process during this maturation (Marzi et al., 2018).

1.3                                    AIM AND OBJECTIVES OF THE STUDY

The aim of the study is to improve the reactivity of municipal solid waste incinerated bottom ash through wet carbonation. The objectives of the study are:

  1. To investigate the effect of carbonation on trace-element mobility.
  2. To investigates the reactivity of municipal solid waste incineration residues to aqueous carbonation, focusing on CO2 absorption rates.
  • To investigate the sorption of Cu and Mo to neoformed minerals.

1.4                                                   SCOPE OF THE STUDY

The scope of this work covers improving the reactivity of municipal solid waste incinerated bottom ash through wet carbonation. To improve the reactivity of municipal solid waste incinerated bottom ash (MSWI-BA) through wet carbonation, a slurry of MSWI-BA is exposed to CO2, leading to carbonate formation and potentially improving its suitability for recycling and reuse.

1.5                                       BENEFITS OF WET CARBONATION

  • Immobilization of PTEs:Carbonate formation can trap PTEs, reducing their leaching potential and making the material safer for reuse.
  • Improved Reactivity:The carbonation process can alter the mineralogical and chemical composition of the MSWI-BA, potentially enhancing its reactivity in certain applications, such as cement production.
  • CO2 Sequestration:Carbonation can contribute to CO2 sequestration, which is beneficial for mitigating climate change.
  • Waste Recycling:Carbonated MSWI-BA can be used as a replacement for natural aggregates in concrete or other construction materials, contributing to a circular economy.
  • Considerations:
    • Optimization of Process:The efficiency of the carbonation process depends on factors like CO2 source, reaction time, temperature, and pH. Optimization studies are often conducted to determine the most effective parameters.
    • Cost-Effectiveness:The cost of carbonation treatment needs to be considered in relation to the potential benefits of recycling the treated material.
    • Scale-up:Developing cost-effective and scalable carbonation technologies is essential for widespread implementation.

1.6                                                  DEFINITION OF TERMS

  • MSWI Bottom Ash:

Municipal solid waste incineration (MSWI) produces bottom ash, a residue containing various materials like metals, glass, and unburnt organic matter.

  • Reactivity and Challenges:

This bottom ash can have limited reactivity and may contain potentially toxic elements, posing challenges for its reuse.

  • Wet Carbonation Process:

Wet carbonation involves reacting the bottom ash with carbon dioxide (CO2) in the presence of water (or a liquid medium).

  • Mechanism:

The CO2 reacts with hydroxides and other compounds in the bottom ash, forming carbonates, which can improve the material’s properties.

Benefits of Wet Carbonation:

  • Improved Reactivity:

Carbonation can enhance the reactivity of the bottom ash, potentially making it suitable for use in construction materials or other applications.

  • Heavy Metal Stabilization:

Carbonation can lead to the stabilization of heavy metals within the bottom ash matrix, reducing the risk of leaching and environmental contamination.

CHAPTER FIVE

5.1                             CONCLUSIONS AND RECOMMENDATION

In this study, two important effects of carbonation have been identified. The most obvious effect is the neutralisation of the pH of the alkaline bottom ash, the second effect is the immobilisation of trace elements by sorption to neoformed minerals. Artificial carbonation experiments have shown that the leaching of Cu is reduced significantly by sorption to amorphous Al-minerals, but that the leaching of Mo remains largely unaffected. The low extent of Mo sorption is consistent with the low affinity of oxyanions for sorption to (hydr)oxide minerals at alkaline pH. However, during and after natural carbonation in the environment (on a time scale of months to years), the weathering of the bottom ash matrix creates additional sorption sites which further reduce the mobility of contaminants such as Cu and Mo. Identification of these sorption sites and mechanisms is mandatory to understand and predict the long- term behavior of waste materials such as bottom ash in the environment.

Carbonation technique and process parameters should be further optimized to reduce Cu leaching to below the limit value. In future research, carbonation will also be performed on other bottom ash fractions than the 0.1– 2 mm fraction.

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