Page 94 - EER-1-1

Explora: Environment
and Resource Accelerated carbonation for heavy metal inhibition in incineration bottom ash

(Ca Al(OH) Cl·2H O), and ettringite 3.2. Carbonation of IBA with aqueous (NH ) CO and
6
2
2
3
4 2
(Ca Al (SO ) (OH) ·26H O) serve as the foundation effect on IBA mineralogy
42
2
2
6
12
4 3
for the carbonation of IBA, resulting in the formation of In this study, ammonium carbonate, (NH ) CO , was
4 2
3
insoluble CaCO . IBA with a particle size below 2.0 mm employed as the carbonating agent. The rationale behind
3
boasts the highest Ca content and it inherently possesses lies in our intent to integrate the well-established carbon
the highest carbonation capacity due to smaller particle capture process using aqueous ammonia and carbonation
size and thus larger surface area (Table 2). In the case of of IBA. Specifically, IBA samples with a particle size of
Na, K, and Fe, it seems that larger particle sizes correspond <0.6 mm were selected for carbonation, given that this
to higher concentrations, while Al and Mg show similar fraction displayed the highest leaching rate of heavy
content in different sizes of IBA. Regarding the heavy metals (discussed in section 3.3) and a more homogeneous
metals of interest, IBA with a particle size of 1.12 − 2.0 mm composition attributed to finer particles. Besides, it
exhibits a higher content of Pb and Ni, while for other has been shown by Arickx et al. that the accelerated
30
heavy metals, IBA with particle size <0.6 mm shows higher carbonation is most efficient on finer fractions and has
content. It was reported that IBA with particle size <4 mm very little impact on coarser fractions as they contain more
contains significantly higher level of heavy metals. The inert components.
43
content of S and Cl varies not much for IBA samples with
different particle sizes. For the accelerated carbonation with enhanced
concentration of CO , a three-step mechanism was
2
proposed: (i) Leaching of mineral ions, primarily Ca ,
2+
Table 1. Elemental composition in IBA with different from solid to liquid; (ii) dissolution of CO leading to the
2
particle size formation of carbonate ions; and (iii) carbonation reactions
Oxide <0.6 mm 0.6−1.12 mm 1.12−2 mm 2−2.8 mm resulting in the formation of calcite particles and layers,
CaO 47.56 46.67 43.90 37.74 which significantly contributes to the inhibition of heavy
metals. An increase in temperature has been identified
SiO 2 15.37 14.44 18.05 25.48 to enhance calcium leaching, but high temperature is
Fe O 6.57 7.33 7.59 9.45
2 3 unfavorable to the dissolution of CO . In the mineral
2
Al O 6.66 5.90 6.02 6.01 sequestration process of CO , the formation of carbonate
2 3 2
MgO 2.48 2.68 2.40 2.87 ions in the liquid phase is influenced by variables such as
TiO 2 1.33 1.44 1.54 1.07 CO concentration, temperature, moisture level, pressure,
2
Na O 1.02 1.05 1.22 1.52 and time, while the leaching of calcium ions is markedly
influenced by factors such as total calcium content, particle
2
K O 0.85 0.91 1.06 1.16
2 sizes, and microstructures of the raw mineral materials. 35,44
Zn 2969 1556 - - Instead, when ammonium carbonate replaces CO as
2
Cu 2019 1283 - - carbonating agent, the carbonation reaction is much
2+
Pb 294 364 - - more straightforward: the dissolved Ca from the mineral
Cr 406 196 - - reacts with dissociated carbonate ions to form CaCO . The
3
Ni 403 771 - - concentration of ammonium carbonation may become
Cd 4 3 a critical parameter that influences the carbonation
efficiency for a certain mineral.
S 5.83 5.24 4.84 3.85
Cl 2.86 2.67 2.60 2.12 To investigate the impact of (NH ) CO concentration
4 2
3
on the carbonation efficiency of IBA, (NH ) CO solutions
Note: Values in boldface are derived from ICP-MS and expressed in 4 2 3
mg/kg, while others are derived from XRF and expressed in wt%. with varying concentrations were prepared. Maintaining
Abbreviations: IBA: Incineration bottom ash; ICP-MS: Inductively a fixed volume while adjusting concentration allowed for
coupled plasma-mass spectrometer; XRF: X-ray fluorescence. the alteration of the molar ratio of (NH ) CO to alkaline
4 2
3
minerals in IBA. The quantification of CO absorption was
2
Table 2. Brunauer–Emmett–Teller results of IBA with estimated by calculating the weight loss between 550°C
different particle size and 800°C of IBA before and after carbonation using TGA,
which is attributed to the decomposition of CaCO . 36,45
IBA particle <0.6 mm 0.6−1.12 mm 1.12−2 mm 2−2.8 mm 3
size Mineralogical transformations after carbonation
Surface area 14.34 10.98 8.51 9.75 were evidenced by XRD analysis on IBA-AC10 (IBA
2
(m /g) <0.6 mm, 50 mL of 10 wt% AC, 1 h), and the results are
Abbreviation: IBA: Incineration bottom ash. shown in Figure 1A. Crystalline phases of hydrocalumite
Volume 1 Issue 1 (2024) 4 doi: 10.36922/eer.4609