Effect of Cl Removal in MSWI Bottom Ash via Carbonation with CO₂ and Decomposition Kinetics of Friedel’s Salt

Abstract

In this study, the effect of Cl removal in bottom ash via a carbonation treatment with CO₂ was investigated by comparing it with a water washing treatment. First, this was also focused on examining the existence of Cl contained in the bottom ash. The overall (soluble and insoluble) Cl content was close to that of bottom ash with fine particle. Next, the washing with water was confirmed and it was not effective in decreasing the Cl content because of the existence of insoluble Cl. Whereas, the removal effect of Cl via carbonation with CO₂ was very high compared to the washing treatment because of the decomposition of Friedel’s salt (main insoluble Cl).

In addition, the kinetics data pertaining to the decomposed Friedel’s salt as the carbonation process proceeds was confirmed. The theoretical was well fitted to the kinetics data. The variation of the rate is constant upon decomposition with the reaction temperature followed the Arrhenius equation (19.676 kJ/mol of activation energy) and the orders with respect to water-to-solution and particle size were also obtained. The decomposition rate of Friedel’s salt based on diffusion through the product layer of shrinking core model could be expressed by the equation.

Fig. 10 Effect of particle size on the decomposition kinetics of Friedel’s salt (a) and effect of particle size on the decomposition rate constants (b) (30% CO₂ concentration, 10 mL/g water-to-solid, and 20°C temperature). d₀ indicates the mean diameter of bottom ash; 75 µm, mean of under 0.15 mm; 225 µm, mean of 0.15–0.3 mm; and 450 µm, mean of 0.3–0.6 mm.

1. Introduction

Although the incineration method, which can reduce the volume of the waste by 85–90%, is significant for treating municipal solid waste, the ash remained after incineration has been generated in an amount of nearly 400 thousand tons, in Korea. Approximately 90% of the ash is bottom ash with the remaining 10% fly ash. In the case of fly ash, it is unstable environmentally because of a high concentration of heavy metals. Whereas municipal solid waste incineration (MSWI) bottom ash, which can be described as heterogeneous particles consisting of ferrous and non-ferrous metals, synthetic and natural ceramics, glass, minerals, etc., can be a potential substitute resource, especially as an aggregate material in the construction industry.¹⁾ However, the bottom ash is subject to rigorous requirements with the limit values of pH, Cl, hazardous elements, amongst others, designated by the law before recycling.

Among these limit values, the existence of a high concentration of Cl may become the priority of strict requirement because of the living habits of Koreans, who like to eat salty food. In fact, the food with high Cl content is well collected after being discarded and reused as the recycled-product contributing to the agriculture, such as fertilizers and feeds. Nonetheless, about 20–30% of a food waste is treated in incinerator intentionally because of the energy- and the cost-effectiveness regarding the disposal of food waste. After incineration, the remained main Cl in bottom ash is composed of soluble salts such as NaCl and KCl, which can easily be dissolved by water. For the purpose of dechlorination, water washing is considered as a cheap but effective method. However, it is difficult to reduce the Cl content to the desired condition for recycling only by means of washing due to the existence of insoluble Cl.

If the bottom ash is not subject to rigorous requirements with the limit values of Cl, it can cause many environmental problems. For example, when the bottom ash is recycled as the application with construction fill, sub-base material in road construction, raw material of cement, etc., the remaining insoluble Cl has a negative effect on the properties of construction materials; potential Cl destroys the passivation of metal and results in many material and equipment corrosion. According to Kikuchi,²⁾ the bottom ash containing insoluble Cl is not applicable to ordinary Portland cement, because it can be dissolved after a certain period of time due to pH change and carbonation with CO₂. In addition, an insoluble Cl can lead to surrounding environmental problems; the Cl released to the environment has a negative influence on the growth of plants and aquatic ecosystems. According to Gryndler et al.,³⁾ increased concentrations of soil chloride can increase the ability of the soil to degrade and produce chlorinated organic compounds. Moreover, the formation and behavior of toxic heavy metals are mainly affected by the presence of Cl.^4,5)

For this problem-solving with dechlorination effect of insoluble Cl, a number of related studies are currently underway. According to Ito et al.,⁶⁾ the acid leaching process has been employed by using sulfuric or other acid solutions for removal of insoluble Cl. However, it has some demerits. Even if insoluble Cl is ionized by acid solutions, unwanted elements are dissolved along with the target Cl and additional method is thus needed to treat the remaining waste-acid-solution. In another method, the thermal treatments such as sintering, roasting and calcinations are often used prior to the washing step because the insoluble Cl can be decomposed, some chlorides are vaporized, and the dechlorination effect is enhanced.^7,8) However, a demerit of thermal treatment is their high energy requirements and potential environmental contamination (generation of dust, gas, and etc.). Instead, to find the effective method leading to the dechlorination, it is good to understand Friedel’s salt (Ca₂Al(OH)₆Cl·2H₂O) because Friedel’s salt was identified in the bottom ash as a major insoluble Cl. It also has important phenomenon with the decomposition by CO₂ gas.^9,10) For this reason, the accelerated carbonation using a gas with a higher CO₂ percentage can be perceived as one of the important treatment process for the removal of insoluble Cl.^11,12)

Therefore, in this study, the effect of Cl removal in bottom ash via accelerated carbonation with CO₂ was investigated by comparison with washing using water. Carbonation was carried out by using batch-type reactor kept at desired temperature and 30% CO₂ was injected into it. For confirming the behavior of dechlorination after the desired carbonation time, the quantitative analysis of Friedel’s salt, which existed as a major insoluble Cl in bottom ash, was performed using the peak intensity of XRD pattern. In addition, the decomposition kinetics of Friedel’s salt during carbonation reaction was analyzed according to the shrinking core model. The equation was formulated with the rate constant and decomposition fraction vs. carbonation-time and fitted to the data of decomposition kinetics. Diffusion through the product layer was found suitable to explain the decomposition kinetics, considered as the function of reaction temperature, water-to-solid ratio, and particle size.

2. Experimental Methods

2.1 Material

Bottom ash sample was taken from a MSWI incineration facility, which has the quenching process with a water-cooling system (cooling down the incinerated bottom ash with high-temperature), located in a metropolitan area in Korea.

The bottom ash was sampled twice while on a conveyor belt after being quenched using the water-cooling system; the quantity sampled each time was 500 kg and a total of 1000 kg was obtained. After sampling, the coarse particles with the size larger than 20 mm were sieved out because most of these coarse particles are metals, glass, and ceramics and are non-representative in this study (more contaminants including Cl are likely to be present in the fine particle). Next, the bottom ash was dried at 100°C for 24 hrs. After drying, a magnetic separation was used to remove iron scrap. Then a size classification was carried out according to a sieving method with under 0.15, 0.15–0.3, 0.3–0.6, 0.6–1.18, 1.18–2.36, 2.36–4.75, and over 4.75 mm.

2.2 Characterization

The initial amount of Cl in each fresh (untreated) bottom ash sieved as required particle size fraction (CL₀; mg/kg) was obtained after dissolving it in acid solution. The concentration of Cl dissolved after acid digestion was measured using an Ion Chromatography (ICS-3000, Dionex).

To determine the initial amount of soluble Cl (CL_SC; mg/kg), 100 g of the bottom ash sieved as required particle size fraction was mixed with 1000 mL of distilled water. The mixture was shaken on a vibration table at 300 rpm and 20°C for 120 min (according to the conditions obtained from the result of Cl removal percentage after desired washing time, mentioned in section 2.3). Then, the leachate was separated using a filter paper with a 0.5 um pore-size. The concentration of Cl of the leachate was measured using an Ion Chromatograph.

For the quantitative analysis of the initial amount of insoluble Cl that exists as a Friedel’s salt (CL_ISC–F; mg/kg) in bottom ash, the author used the measurement of the peak intensity in the XRD pattern.¹³⁾ First, Friedel’s salt was prepared by using pure reagents with 3CaO·Al₂O₃ and CaCl₂.¹⁴⁾ A suspension of 0.02 mol 3CaO·Al₂O₃ and 0.02 mol CaCl₂ in 300 ml of distilled water was stirred by a magnetic bar at 300 rpm in a 1000 mL batch-type reactor kept at 50°C. Then, the synthesized sample was dried. The contents of Cl, Ca, and Al were measured using an Ion Chromatograph and an inductively coupled plasma atomic emission spectrometry (OPTIMA 5300DV, Perkin Elmer), respectively. It was found that the sample contained 7.21 mass% Al, 7.81 mass% Cl, and 24.84 mass% Ca, indicating that molar ratio is Al:Cl:Ca = 1.2:1:2.8. Al and Ca in the sample were higher than that of the theoretical pure Friedel’s salt, which is Al:Cl:Ca = 1:1:2. If all Cl exists as the Friedel’s salt, the synthesized Friedel’s salt was 61.7 mass% as follows:   

\begin{align} &\text{Calculated assay (mass%)} \\ &\quad = 7.81\ (\text{mass%})\times [280.59\ (\text{Friedel's salt molar mass;} \\ &\qquad \text{ g/mol})/35.5\ (\text{chloride molar mass; g/mol})] \end{align} (1)

Here, four standard products with different concentrations of Friedel’s salt (2.1 mass%, 10.4 mass%, 30.2 mass%, and 61.7 mass%) were prepared by mixing the synthesized sample (61.7 mass% of Friedel’s salt) and the reagent of SiO₂ (99.9% chemical grade; Sigma Aldrich, Ltd.). Then, 10 mass% MgO was added to each standard product for using the ratio of strongest XRD intensity between 2θ = 11.2° (Friedel’s salt) and 2θ = 43.0° (MgO) and these products were measured using an X-ray diffractometer (Cu K_α radiation, 45 kV, 100 mA, PW3040/00, Philips). Figure 1, showing the standard line obtained by measuring the ratio of peak intensity (Friedel’s salt/MgO), helped to bring the initial amount of Friedel’s salt. For example, if the XRD peak of the mixture of MgO and desired bottom ash shows the ratio of Friedel’s salt/MgO of 4.66, 17800 mg/kg of CL_ISC–F can be calculated by using the graph in Fig. 1.

Fig. 1

Standard line for quantitative analysis of Friedel’s salt by XRD peak intensity. The abbreviation represents: (CL_ISC–F) initial amount of insoluble Cl that exists as a Friedel’s salt.

The difference between CL₀ and (CL_SC + CL_ISC–F) determined the initial amount of insoluble Cl that exists as other chlorides except Friedel’s salt (CL_ISC–EF; mg/kg), i.e. sodalite (Na₈Si₆Al₆O₂₄Cl₂), mentioned in section 3.1.

In addition, the samples were taken from each sieved bottom ash and were measured using an X-ray diffractometer, to confirm the mineralogical phases.

2.3 Washing with water

To confirm the effect of Cl removal as a function of washing time, the washing experiments with the bottom ash sample being less than 0.15 mm were performed in a 2000 mL batch-type reactor kept at 20°C of temperature. The water-to-solid ratio with mixture of 1000 mL of distilled water and 100 g of bottom ash was 10 mL/g. The reactor was shaken on a vibration table at 300 rpm during various washing times ranging from 0 to 120 min. Then, the leachate was separated using a filter paper and the concentration of Cl was measured using an Ion Chromatography. The experimental data taken at different washing times were made to fit the remaining amount of Cl in bottom ash after the desired washing time (CL_AW; mg/kg). Using CL_AW, the removal percentage of Cl after desired washing time (R_CL–AW) can be calculated from the following equation:   

\begin{equation} R_{\text{CL--AW}} = (1 - \textit{CL}_{\text{AW}}/\textit{CL}_{0})\times 100 \end{equation} (2)

To confirm the particle size distributions of the bottom ash before and after washing, the measurements were performed using a particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd.).

In addition, the samples were taken from before and after washed-bottom ash and were measured using an X-ray diffractometer, to confirm the mineralogical phases.

2.4 Carbonation with CO₂

The bottom ash sample taken for carbonation with CO₂ had a particle size of less than 0.6 mm owing to the high rate of Friedel’s salt content and specific surface area suggesting further evidence of the removal effect of Cl and the decomposition kinetics in this study.

All the experiments were performed by putting desired amount (67, 100, and 200 g) of bottom ash with different particle sizes (under 0.15, 0.15–0.3, and 0.3–0.6 mm) into 2000 ml distilled water (10, 20, and 30 mL/g water-to-solid ratios) in a 3000 ml batch-type reactor kept at desired temperature (20, 30, 40, and 50°C) and CO₂ (30%). The mixture was stirred by a magnetic bar at 300 rpm during carbonation times ranging from 0 to 120 min. Then, the leachate was separated and pH and R_CL–AC (removal percentage of Cl in bottom ash after desired carbonation time) were measured. The concentration of Cl of the leachate was measured using an Ion Chromatography. The R_CL–AC can be calculated from the following equation:   

\begin{equation} R_{\text{CL--AC}} = (1-\textit{CL}_{\text{AC}}/\textit{CL}_{0})\times 100 \end{equation} (3)

Here, CL_AC represents the remaining amount of Cl in bottom ash after desired carbonation time (mg/kg).

The samples were measured using an X-ray diffractometer, to confirm the changes of the mineralogical phases of the carbonated bottom ash.

3. Results and Discussions

3.1 Characterization of bottom ash containing Cl

Table 1 shows CL₀, CL_SC, CL_ISC–F, and CL_ISC–EF for each particle size fraction of the bottom ash; the sum of CL_SC, CL_ISC–F, and CL_ISC–EF is CL₀ in each particle size. It is noticed that the CL₀, CL_SC, CL_ISC–F, and CL_ISC–EF are correlated with the particle size. They increased with decreasing particles sizes and the fraction with a particle size under 0.15 mm has the highest value. According to Chen et al.,¹⁵⁾ a negative correlation, showing 0.886 of the correlation coefficient (R²), exists between Cl content and the particle size, indicating that Cl accumulates easily on fine particles. As shown in Fig. 2, it presents that the overall Cl content (sum of CL₀s) is close to that of bottom ash with fine particles. The fraction with the size smaller than 0.15 mm only accounts for 7.7% of the total weight of bottom ash, but accumulates more than 34.7% of the overall Cl content. Almost half of overall Cl content is contained in the fraction of the size smaller than 0.3 mm. In addition, CL_SC and CL_ISC–F also increased with a decrease in the particle size. In case of CL_SC, almost half of the amount is contained in the size smaller than 0.6 mm, whereas more than half of the overall CL_ISC–F (about 55.4%) is contained in the size smaller than 0.15 mm. However, the values of CL_ISC–EF were relatively low in all particle sizes.

Table 1 CL₀, CL_SC, CL_ISC–F, and CL_ISC–EF for each particle size fraction of untreated bottom ash. The abbreviations represent: (CL₀ = CL_SC + CL_ISC–F + CL_ISC–EF) initial amount of Cl; (CL_SC) initial amount of soluble Cl; (CL_ISC–F) initial amount of insoluble Cl that exists as a Friedel’s salt; (CL_ISC–EF) initial amount of insoluble Cl that exists as other chlorides except Friedel’s salt.

Fig. 2

Weight percentages of CL_SC, CL_ISC–F, and CL_ISC–EF per overall Cl content (sum of CL₀s) and bottom ash in each particle size per total bottom ash. The abbreviations represent: (CL₀ = CL_SC + CL_ISC–F + CL_ISC–EF) initial amount of Cl; (CL_SC) initial amount of soluble Cl; (CL_ISC–F) initial amount of insoluble Cl that exists as a Friedel’s salt; (CL_ISC–EF) initial amount of insoluble Cl that exists as other chlorides except Friedel’s salt.

Next, the mineralogical phase of bottom ash was confirmed by the XRD patterns in Fig. 3. As shown in patterns, the peaks of Friedel’s salt, ettringite, portlandite, etc. existed and the intensities of them increased with a decrease in the particle size. Particularly, the XRD pattern revealed that the intensity of Friedel’s salt was the largest. In addition, it can be seen that the small peak of sodalite (Na₈Si₆Al₆O₂₄Cl₂), which is one of other insoluble Cl except Friedel’s salt, was detected. The facts about insoluble Cl support the aforementioned results in Table 1. To explain the reason for the existence of Friedel’s salt, it is necessary to understand the water quenching process. Water quenching is the process with water cooling system for cooling down the bottom ash with high-temperature after incineration and it has been determined to have a high impact on bottom ash characteristics. Inkaew et al.¹⁶⁾ examined that water quenching affected the change of bottom ash’s morphology by reducing temperature, the alteration of the chemical composition, and the enhancement of the quench products formation (i.e., portlandite, ettringite, and Friedel’s salt). The chemical composition of the bottom ash shown in many researcher’s data indicated that the quenching process plays a crucial role to provide the hydrate precipitation of Cl such as Friedel’s salt on the bottom ash.^8,17,18) In addition, the existence of Ca is related to controlling the formation of portlandite (Ca(OH)₂) during the quenching process, as well as calcite (CaCO₃) formed by the reaction between Ca(OH)₂ and CO₂ in the air. Ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O) can also be controlled by the quenching process.

Fig. 3

XRD patterns of untreated bottom ash as a function of the particle size.

3.2 Washing treatment

The waste washing has its disadvantages in terms of two respects. First, the insoluble Cl is not expected to be decomposed leading to the decrease of the Cl content. According to the washing method in section 2.3, the results in Fig. 4, showing the behavior of R_CL–AW as a function of washing time, indicate that the washing process is effective for the removal of the only soluble Cl in a short time; the only half of CL₀, indicating 48.3% of R_CL–AW, was removed and the solution reached almost its maximum at less than 20 min because of the insoluble Cl content. Second, the unexpected ettringite (mentioned as quench product) is formed from the water washing. According to the literatures,^19,20) the pH range of stability for ettringite is between 10.5 and 13. According to the following equation, the formation is favored over mono-sulfate at below 50°C.   

\begin{align} &\text{6Ca$^{2+}$} + \text{2Al$^{3+}$} + \text{3SO$_{4}^{2-}$} + \text{38H$_{2}$O} \\ &\quad \to \text{12H$^{+}$} + \text{Ca$_{6}$Al$_{2}$(SO$_{4}$)$_{3}$(OH)$_{12}{\cdot}$26H$_{2}$O} \end{align} (4)

Fig. 4

R_CL–AW of washed bottom ash and R_CL–AC, and pH of carbonated bottom ash as a function of the washing or carbonation time (washing or carbonation conditions: 30% CO₂ concentration (carbonation only), 10 mL/g water-to-solid ratio, 20°C reaction temperature and particle size <0.15 mm). The abbreviations represent: (R_CL–AW) removal percentage of Cl after desired washing time; (R_CL–AC) removal percentage of Cl after desired carbonation time.

In case of a washing process, as the washing time proceeds, the washing water becomes high alkaline condition with a pH in excess of 12 at the temperature of 25°C, and this condition leads to the formation of ettringite during the washing process. Indeed, the XRD diffractions of untreated and washed bottom ash show the formation of ettringite, as shown in Fig. 5. The peaks of KCl and NaCl disappeared after washing, whereas that of ettringite increased. In addition, the data for the change of particle size distributions before and after washing, as shown in Fig. 6, may provide further evidence related with the dissolution of soluble Cl particles and the formation of ettringite; some of them were agglomerated with the bottom ash, as shown in scanning electron microscopy image of the surface of bottom ash after washing (in Fig. 6).

Fig. 5

XRD patterns of untreated bottom ash and bottom ash treated by washing with distilled water (washing conditions: 120 min washing time, 10 mL/g water-to-solid ratios, 20°C reaction temperature, and particle size <0.15 mm).

Fig. 6

Particle size distribution of untreated and washed bottom ash (washing conditions: 120 min washing time, 10 mL/g water-to-solid ratios, 20°C reaction temperature, and particle size <0.15 mm).

Ettringite has an important phenomenon. This structure consists of columns of {Ca₆[Al(OH)₆]₂·24H₂O}⁶⁺ with the inter-column space (channels) occupied by $\text{SO}_{4}^{2 - }$ molecules, and it can be substitutable for heavy metal’s oxyanions, such as $\text{CrO}_{4}^{2 - }$ and $\text{AsO}_{4}^{2 - }$.^21,22) However, the important phenomenon^23,24) is that heavy metal ions substituted into ettringite can be easily released from the particle surface to the outside because the ettringite is easily decomposed by carbonation with CO₂. In addition, the occurrence of volume change due to the carbonation with CO₂ in the bottom ash can be also expected. Therefore, when recycled as the application with construction fill, sub-base material in road construction, etc. their phenomenon may not lead to suitable substitute for natural resource.

3.3 Accelerated carbonation treatment

For confirmation of the carbonation effect, the behaviors of R_CL–AC and pH observed at different carbonation times were investigated by comparing with R_CL–AW in Fig. 4. As the result, the pH fell into under pH 9.5 after 20 min of carbonation. It indicates that the neutralization of pH after carbonation is controlled mainly by the decomposition of quench products (alkaline compounds with high value of pH). Next, the removal effect of Cl is very high compared to washing treatment. The removal percentage of Cl, which was only 50.8% (R_CL–AW) using washing, increased to 95.4% (R_CL–AC) at 120 min of carbonation time. However, it couldn’t reach 100% because of the existence of other insoluble chlorides except Friedel’s salt, such as sodalite, which is not easily decomposed with CO₂. According to the literatures,^23,24) the quench products are the main alkaline inorganic materials in the reaction with CO₂. These carbonation reactions of Friedel’s salt (eq. (5)), portlandite (eq. (6)), and ettringite (eq. (7)) are described below:   

\begin{align} &\text{2[Ca$_{2}$Al(OH)$_{6}$Cl${\cdot}$2H$_{2}$O]} + \text{3CO$_{2}$}\\ &\quad \to \text{3CaCO$_{3}$} + \text{Al$_{2}$O$_{3}{\cdot}$xH$_{2}$O} + \text{CaCl$_{2}$} + \text{($10-\mathrm{x}$)H$_{2}$O} \end{align} (5)

  

\begin{equation} \text{Ca(OH)$_{2}$} + \text{CO$_{2}$}\to \text{CaCO$_{3}$} + \text{H$_{2}$O} \end{equation} (6)

  

\begin{align} & \text{Ca$_{6}$Al$_{2}$(SO$_{4}$)$_{3}$(OH)$_{12}{\cdot}$26H$_{2}$O} + \text{3CO$_{2}$}\\ &\quad \to \text{3CaCO$_{3}$} + \text{3[CaSO$_{4}{\cdot}$2H$_{2}$O]} + \text{Al$_{2}$O$_{3}{\cdot}$xH$_{2}$O} \\ &\qquad +\text{($26-\mathrm{x}$)H$_{2}$O} \end{align} (7)

Figure 7, showing XRD patterns of the bottom ash at different carbonation times, provides an evidence of the above chemical reactions. The intensities of Friedel’s salt, portlandite, and ettringite in untreated bottom ash decreased with the carbonation reaction and no quench product peak was observed after 120 min of carbonation while that of calcite and gypsum increased. However, even though the peak of Al₂O₃·xH₂O is expected through the formation of a new Al-material from eqs. (5) and (7), it was not detected because Al would precipitate mainly as amorphous Al-material (gel-like).²⁵⁾

Fig. 7

XRD patterns of bottom ash carbonated at different reaction times (carbonation condition: 30% CO₂ concentration, 20°C reaction temperature, 10 mL/g water-to-solid ratio and a particle size <0.5 mm).

3.4 Decomposition kinetics of Friedel’s salt

Before studying the decomposition kinetics of Friedel’s salt, we have to know the carbonation mechanism with CO₂ dissolution into water (step 1) and the reaction between CO₂ dissolved into liquid and bottom ash (step 2).²⁴⁾ In the case of Step 1, CO₂ dissolution into liquid (eqs. (8)–(10)) is dependent on injection type, CO₂ pressure and concentration, temperature, etc. Carbon dioxide enters the water through equilibrium with the atmosphere (eq. (8)) and it can react with the water to form carbonic acid (eq. (9)). Dissolved CO₂ in the form of H₂CO₃ may lose protons to form bicarbonate, HCO₃³⁻, and carbonate, CO₃³⁻ (eq. (10)).   

\begin{equation} \text{CO$_{\text{2(g)}}$}\leftrightarrow \text{CO$_{\text{2(aq)}}$} \end{equation} (8)

  

\begin{equation} \text{CO$_{\text{2(aq)}}$} + \text{H$_{2}$O}\leftrightarrow \text{H$_{2}$CO$_{3}$} \end{equation} (9)

  

\begin{equation} \text{H$_{2}$CO$_{3}$}\leftrightarrow \text{HCO$_{3}^{-}$} + \text{H$^{+}$}\leftrightarrow \text{CO$_{3}^{2-}$} + \text{2H$^{+}$} \end{equation} (10)

Since these consecutive chemical reactions in Step 1 are much faster than those of Step 2, the carbonation reaction between the bottom ash and CO₂ in liquid (Step 2) becomes a rate-determining step. In addition, in Step 2, the carbonation reaction may be generally controlled by the solid-liquid heterogeneous system.^23,24) Thus, it can be expressed as the shrinking core model with three steps such as surface chemical reaction, diffusion through the product layer, and fluid film diffusion control. Because no fluid film covers the unreacted bottom ash particle as the reaction proceeds, there could be only surface chemical reaction and diffusion through the product layer, as follows.   

\begin{equation} \text{Surface chemical reaction:}\ [1-(1-X_{\text{t}})^{1/3}] = k_{\text{R}}t \end{equation} (11)

  

\begin{align} &\text{Diffusion through the product layer:}\ \\ &\quad [1-(2/3)X_{\text{t}} - (1 - X_{\text{t}})^{2/3}] = k_{\text{D}}t \end{align} (12)

Assuming that this carbonation process is simply described by the decomposition of Friedel’s salt (eq. (5)), k_R and k_D are the reaction rate constants (h⁻¹) of Friedel’s salt decomposition for chemical reaction and diffusion control, respectively. The calculation was then performed as eq. (13); X_t is the decomposed fraction vs. carbonation time t.   

\begin{equation} X_{\text{t}} = X_{\text{CO2}}/\textit{CL}_{\text{ISC--F}} \end{equation} (13)

In this equation, X_CO2 is the amount of dissolved Cl, which existed as a Friedel’s salt initially, after desired carbonation time (mg/kg). The values of X_CO2 were measured by using the measurement of the peak intensity in the XRD pattern and the standard line for quantitative analysis of Friedel’s salt, as mentioned in section 2.2.

To consider the temperature effect on the decomposition kinetics of Friedel’s salt, the carbonation of the bottom ash with less than 0.15 mm particle size was performed using 30% CO₂ concentration at different temperatures ranging from 20 to 50°C. The water-to-solid ratio was 10 mL/g. Figure 8(a) indicates that the decomposition rate decreased with a decrease in the temperature and the reaction under all conditions was reached the maximum value of X_t after 120 min. Figure 8(b) and Fig. 8(c) present the experimental data plotted on 1 − (1 − X_t)^1/3 and 1 − (2/3)X_t − (1 − X_t)^2/3, respectively. The results show that the correlation coefficients (R²) of diffusion through of the product layer are closer to 1 than those of surface chemical reaction. In addition, if considered as the three functions of temperature, waste-to-solid ratio, and particle size, the decomposition rate constant of Friedel’s salt can be expressed as follows:   

\begin{equation} k_{\text{D}} = k_{0}'e^{-\text{Ea}/RT}(W/S)^{m}d_{0}{}^{n} \end{equation} (14)

Fig. 8

Effect of the reaction temperature on the decomposition kinetics of Friedel’s salt (a), plots of 1 − (1 − X_t)^1/3 versus reaction time for different reaction temperatures (b), plots of 1 − (2/3)X_t − (1 − X_t)^2/3 (c) and effect of reaction temperature on the decomposition rate constants (d) (30% CO₂ concentration, 10 mL/g water-to-solid, and particle size <0.15 mm).

Where Ea is the activation energy (kJ/mol); T, the reaction temperature (K); R, the ideal gas constant, 8.314 × 10⁻³ (kJ/mol); W/S, water-to-solid ratio (mL/g); d₀, particle size (µm); m and n, constants; and $k'_{0}$ is pre-exponential factor. Rearranging eq. (14), three equations are obtained as follows:   

\begin{align} k_{1} &= k_{1}'e^{-\text{Ea}/RT}\ \\ &\quad (\text{between rate constant and activation energy}) \end{align} (15)

  

\begin{align} k_{2} = k_{2}'(W/S)^{m}\ &(\text{between rate constant and }\\ &\ \text{water-to-solid ratio constant}) \end{align} (16)

  

\begin{align} k_{3} = k_{3}'d_{0}{}^{n}\ &(\text{between rate constant and particle }\\ &\ \text{size constant}) \end{align} (17)

Where k_1–3 (here $k_{\text{D}} = k_{1}*k_{2}*k_{3}$) and $k'_{1\text{--}3}$ (here $k'_{0} = k'_{1}*k'_{2}*k'_{3}$) are the rate constants and the pre-exponential factors, respectively. In eqs. (15)–(17), the ln k_1–3 values calculated from these k_1–3 values were plotted against 1/T, ln W/S, and ln d₀.

When eq. (15) is applied, the rate constants obtained from the linear relationship between ln k and 1/T in Fig. 8(c) were used to determine the Arrhenius plot and activation energy was 19.676 kJ/mol, as shown in Fig. 8(d). For examining the effect of water-to-solid ratio, the rate constants for various W/S values (10, 20, and 30 mL/g) were determined by using the linear regressions obtained from the data in Fig. 9(a). A linear plot between ln k and ln W/S in Fig. 9(b) indicates that m of the water-to-solid ratio constant was calculated to be −0.065. This value shows no effect of water-to-solid ratio on the decomposition rate. In addition, the data in Fig. 10(a) determined the effect of particle sizes (mean diameter d₀; 75 (under 0.15 mm), 225 (0.15–0.3 mm), and 450 µm (0.3–0.6 mm)) on the decomposition rate of Friedel’s salt and the decrease in particle size increased the decomposition rate. The linear plot between ln k and ln d₀ in Fig. 10(b) indicates that n of the particle size constant was calculated to be −0.234.

Fig. 9

Effect of water-to-solid ratio on the decomposition kinetics of Friedel’s salt (a) and on the decomposition rate constants (b) (30% CO₂ concentration, 20°C temperature, and particle size <0.15 mm).

Fig. 10

Effect of particle size on the decomposition kinetics of Friedel’s salt (a) and effect of particle size on the decomposition rate constants (b) (30% CO₂ concentration, 10 mL/g water-to-solid, and 20°C temperature). d₀ indicates the mean diameter of bottom ash; 75 µm, mean of under 0.15 mm; 225 µm, mean of 0.15–0.3 mm; and 450 µm, mean of 0.3–0.6 mm.

The calculated activation energy and the constants of water-to-solid ratio and particle size conform to the shrinking core model for diffusion through the product layer. Thus the decomposition of Friedel’s salt during the carbonation reaction with CO₂ can be clearly presented as follows:   

\begin{align} &[1 - (2/3)X_{\text{t}} - (1 - X_{\text{t}})^{2/3}] \\ &\quad = k_{\text{D}}t = k_{0}'(W/S)^{-0.065}d_{0}{}^{-0.234}\exp (-19.676/\text{RT})\,t \end{align} (18)

Plotting of 1 − (2/3)X_t − (1 − X_t)^2/3 against (W/S)^−0.065d₀^−0.234 exp(−19.676/RT)t gives a $k'_{0}$ amount of 47.5.

4. Conclusion

In this study, Cl removal effect, related with the decomposition of Friedel’s salt, on the accelerated carbonation of MSWI bottom ash was investigated by comparing with a water washing. In addition, the decomposition kinetics of Friedel’s salt was also examined. The obtained results were as follows.

1. (1)    In Korea, the bottom ash generated after incineration of municipal solid waste has a high Cl content. It consists of the soluble and insoluble chloride, especially Friedel’s salt, which is formed during quenching process with water in incineration facility, was identified as a major insoluble Cl. The overall Cl content was close to that of bottom ash with fine particle.
2. (2)    Water washing, considered a cheap and simple method for the purpose of dechlorination, was carried out and it was not effective in reducing the Cl content because of the existence of insoluble Cl; the only half of Cl was removed. In addition, the result indicated that the washing does not provide stable condition because the ettringite formed during washing has a high reactivity with CO₂ to affect the volume change.
3. (3)    Removal effect of Cl was obtained from the decomposition of Friedel’s salt according to the reaction with CO₂ and was very high compared to the washing. The removal percentage of Cl, which was only half of Cl using washing, increased to more than 95% after carbonation.
4. (4)    Theoretical model was well fitted to the kinetics data pertaining to the decomposed Friedel’s salt. Considering as the function of temperature (Ea = 19.676 kJ/mol), water-to-solid ratio (W/S), and particle size (d₀), the decomposition rate based on diffusion through the product layer of shrinking core model can be expressed by the following equation (here X_t is the decomposed fraction vs. carbonation time t.):   
   \begin{align*} &[1 - (2/3)X_{\text{t}} - (1 - X_{\text{t}})^{2/3}]\\ &\ = kt = 47.5(W/S)^{-0.065}d_{0}{}^{-0.234}\exp (-19.676/\text{RT})\,t. \end{align*}

Acknowledgement

This work was supported by a grant from the National Institute of Environmental Research (NIER), funded by the Ministry of Environment (MOE) of the Republic of Korea (NIER-2014-01-01-015).

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