Accelerated Carbonation of Municipal Solid Waste Incineration Fly Ash Using CO 2 as an Acidic Agent for Clinker Production

Material

The fly ash was obtained from an MSWI facility located in Mudu of Suzhou, China. The plant began to operate in 2006 with a capacity of 1000 tonnes per day and produces electric power (12,000 kW). The plant uses a grate furnace (SEGHERS) and is equipped with an air pollution control system that consists of a semi-dry flue gas cleaning tower, an active carbon absorption reactor, and a bag filter. The flue gas and dioxin emission standard complies with Eu I and Eu II respectively.

The major elements of the sample were examined by X-ray Fluorescence (XRF-1700; Shimadzu Corporation), and the heavy metals were determined by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, PRODIGY Type; Thermo Electron Co., Ltd) after an HF/HClO₄/HNO₃ digestion (Li et al., 2004; Liu et al., 2005). The corresponding data of the sample are shown in Table 1.

Methods

Carbonation experiments were performed at room temperature by mixing dried ash (20 g) and water at a certain liquid-to-solid (L/S) ratio (w/w) (5, 10, and 20, respectively) in the reactor. The MSWI fly ash was first dried at 105°C for 24 h to reach a constant weight to prevent the small amount of water content in the fly ash from having an effect on the carbonation process. The chamber was sealed to prevent the air from affecting the results, and three small holes were made in the sealing device aimed at the introduction of CO₂, the discharge of unused CO₂, and for the pH analyzing meter, respectively. A CO₂ stream with a purity of 99.99% was then bubbled into the mixer at a flow rate of 40 mL min⁻¹, controlled by a mass flow controller (D07-7B), and samples were stirred using a magnetic stirrer at 120 rpm to increase the inter-reactions. The slurry was filtered through 0.45 μm membrane filters, the pH values were determined immediately, and the water effluent was then acidified with 10% nitric acid to pH<2 for the analysis of the heavy metals.

pH change with CO₂ bubbling

Figure 1 shows the pH change of the mixture of fly ash and water during the carbonation process. First, the pH decreased very slowly. Many ions, including HCO₃⁻, CO₃^2–, OH⁻, and Ca²⁺, co-exist in the mixture, and OH⁻has the most important effect on the pH of the solution. Second, the pH decreased very quickly because of the depletion of Ca(OH)₂, which was completely precipitated into CaCO₃. Finally, the pH again decreased slowly and was controlled by the dissolution of CO₂ and the generation of Ca(HCO₃)₂.

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FIG. 1.

pH change with CO₂ bubbling for three different particle sizes (L/S=150; CO₂ flow rate=10 mL min⁻¹; magnetic stirrer speed=120 rpm). L/S, liquid to solid.

Composition change

Table 2 compares the compositions of the MSWI fly ash before and after carbonation. The carbonated fly ash (CFA) is very similar in composition with the cement raw material used in Beijing cement plants. The Cl content in the un-carbonated fly ash (UFA) was 14.54%. In the CFA with pH of 10 and 7, the Cl content was reduced to 0.57% and 0.27%, respectively, and the reduced Cl entered into the water effluent as a soluble salt. This level is comparable to the Cl content in the raw material of cement (0.005%) after the substitution of raw material at a relatively low percentage.

Leaching of chloride and sulfate

The leaching of chloride and sulfate during the accelerated carbonation process was determined using ionic chromatography (Dionex, Co., Ltd). The leaching of Cl⁻ is independent of the pH, relatively rapid, and not controlled by solubility limitations. The L/S has a significant effect on the release of Cl. At an L/S of 20, the released Cl accounts for 85%–90% of the total Cl in the fly ash (Appendix Fig. A4).

The leaching of sulfate also seemed to have no relationship to the pH but was greatly affected by the L/S. As the L/S increased, the leaching of sulfate also increased and accounted for 20%–25% of the total sulfate at an L/S of 20. The PHREEQC calculations show that the solutions were slightly undersaturated with anhydrite but were saturated with gypsum (Fig. 2). The saturation index (SI) is estimated to be −0.20 for anhydrite for the experiments with an L/S ratio of 10:1 L/S, and the calcium sulfate entered into the solution quickly but precipitated as gypsum. The solutions were supersaturated with regard to calcite and aragonite but were significantly undersaturated (SI=−8) with regard to portlandite.

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FIG. 2.

Common logarithm of the sulfate activity versus the calcium activity for all experiments. The solid line for anhydrite is also shown.

Retention of amphoteric heavy metals

Figure 3 presents the leached amounts of heavy metals in the water effluent from carbonations at different pHs. The fly ash reaches an initial pH of 12.3 when it contacts water for a very short time. The leached amount of the heavy metals at pH 12.3 is high up to 333.98, 17.35, and 19.62 g kg⁻¹ for Pb, Cu, and Zn, respectively. After carbonation, the leached amount greatly decreases with the change of pH for most of the heavy metals, especially for Pb. The leached amount of Pb is lowered from 333.98 to 3.33 g kg⁻¹ and 0.058 g kg⁻¹ when the pH of the mixture decreases to 10 and 7, respectively. The carbonation process is also effective for Cu and Zn. When the pH of the mixture decreases from 12.3 to 9, the leached amount of Cu and Zn is reduced from 17.35 to 0.11 g kg⁻¹ and from 19.62 to 0.47 g kg⁻¹, respectively, in the solution (Appendix Table A1). Additionally, after the carbonation process, the concentration of Cd was lower than 0.01 mg L⁻¹.

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FIG. 3.

Amounts of heavy metals leached after the carbonation process (g kg⁻¹): (a) Pb, (b) Zn, (c) Cu.

Mineralogy of CFA

Figure 4 shows the major mineral compounds in the fly ash before and after carbonation identified by XRD (D/max-2500 diffractometer; Rigaku Co.) at 50 KeV and 200 mA from 10° to 100° at a scanning rate of 6° 2θ min⁻¹. The soluble salts halite and sylvite were not identified in the CFA. Calcium hydroxychloride (CaClOH) was identified as a major chloride compound in the UFA, which was the intermediate phase of the reaction of CaO with HCl (Partanen et al., 2005). In the diffractogram of the CFA with pH 10, a very small amount of Ca(OH)₂ was shown at about 18° and 34°, but the peaks were not marked in the diffractogram, as they were too weak. When the pH of the fly ash decreased, the CaClOH peaks in the diffractogram were substituted by CaCO₃ peaks. The possible reactions may be due to the reaction of CaClOH with water first [Eq. (1)], then the carbonation of the hydrated products of CaClOH [Eq. (2)]. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} {\rm 2 CaClOH} \hbox{(s)} + \chi {\rm H}_{2} \hbox{O} \rightarrow {\rm CaCl}_{2}\cdot \chi {\rm H}_{2} \hbox{O} (l) + \hbox{Ca (OH)}_{2}\hbox{(s)} \tag{1} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \hbox{Ca (OH)}_{2} + {\rm CO}_{2} \rightarrow {\rm CaCO}_{3} + {\rm H}_{2} \hbox{O}\tag{2} \end{align*} \end{document}

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FIG. 4.

XRD diffractograms of the un-carbonated and carbonated fly ash produced at L/S of 10: (a) un-carbonated fly ash; (b) carbonated fly ash with pH 10; (c) carbonated fly ash with pH 7. Arrows indicate that the crystal phase did not change in the original and carbonated fly ash.

Thermal behavior of CFA

Figure 5 shows the thermogravimetric (TG) curve and differential thermal analysis (DTA) curve obtained for the CFA and the UFA. The ash samples were heated from 40°C to 1350°C at a heating rate of 30°C min⁻¹ in an atmosphere of CO₂ with a flow rate of 80 mL min⁻¹. The sample increases in weight at ∼340°C because of the generation of CaCO₃. When the temperature is increased further, the generated and pre-existing in the original fly ash begins to decompose. The weight loss between 600°C and 920°C for UFA was probably related to the decomposition of CaCO₃ and a consequent weight loss of about 5%. A weight loss of 15% occurred between 1020°C and 1250°C, which was associated with decomposition of sulfate and the evaporation of NaCl and KCl (Bethanis et al., 1998; Mangialardi, 2003).

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FIG. 5.

Thermogravimetric/differential thermal analysis (TG/DTA) curves of the un-carbonated and carbonated fly ash.

In contrast, the TG/DTG analyses of the CFA revealed the presence of a weight gain and three weight losses. The UFA does not show a significant weight loss after 907°C, because almost all of the NaCl and KCl and part of the anhydrite have dissolved into the water during the carbonation process. The DTA curves indicate the presence of a third endothermic effect at 1290°C, which corresponds to the melting of the sample.

Clinker properties

Figure 6a shows the main composition of the ordinary clinker and the CFA-added clinker. Figure 6b shows the compressive strengths for the ordinary mortar and the CFA-added mortar after (1, 7, and 28) days. Figure 6c shows the initial and final setting times for the ordinary mortar and the CFA-added mortar. The results show that incorporating the CFA into the clinker has no considerable effect on the main composition, the compressive strength, and the initial and final setting times.

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FIG. 6.

Properties of the clinker with and without the addition of carbonated fly ash. (a) The main composition; the C₃A (3CaO·Al₂O₃) and C₄AF (4CaO·Al₂O₃·Fe₂O₃) content are shown on the secondary y-axis. (b) The compressive strength. (c) The setting times.