Encapsulation behaviors of metals in slags containing various amorphous volume fractions.

ABSTRACT

In this study, a melting process with addition of Si[O.sub.2] was applied to treat incinerator fly ash. To describe the encapsulation behaviors of metals quantitatively, the amorphous volume fraction (AVF) of slags was initially determined. Vitrification appeared to reduce the mobility of Cr, Cu, Mn, and Ni instead of significantly immobilizing Cd, Pb, and Zn. It was verified that Si[O.sub.2] enhanced the formation of an amorphous glassy structure. With the increase of Si[O.sub.2], the crystalline phases would gradually diminish and transform into a higher silica-connected species. During the formation of slag matrix, Al, Ca, and Mg could modify the glass network, and consequently the encapsulation behaviors of these species would noticeably affect the chemical stability of slags. Significant immobilization of crust metals could be achieved only when a more compact and interconnected amorphous glass network was formed. Hence, it indicated that a higher AVF silica-based slag had a better potential to resist acid attack. In conclusion, for environmental protection, it is important to investigate the correlation between the encapsulation behaviors of metals and the crystalline characteristics of slag structure.

INTRODUCTION

Incineration is a popular technology used for the treatment of solid wastes in industrialized countries. However, it may cause secondary pollutions because of the generation of flue gas and ashes, which contain large amount of heavy metals and organic toxics. (1-3) Direct landfilling of ashes is no longer regarded as an appropriate policy because of rapid growth of municipal waste and deficiency of landfill sites. (4) With the benefit of transforming waste into reusable resources, vitrification is believed to be a promising technology to solve the problems of ash treatment. (5) It can lead to a destruction of dioxins (>99%), (6) recover heavy metals by phase separation or vaporization, (7,8) and significantly reduce the mobility of hazardous metals. (9)

The previous studies stated that the immobilization of metals in slags was because of the encapsulation of amorphous glassy matrix. (10,11) In a vitrification process, CaO and Si[O.sub.2] acted as stabilizing and vitrifying agents, respectively. (12) The mass ratio of CaO/Si[O.sub.2] (basicity) significantly affected the crystalline characteristics of slag structures. (13) In addition, higher basicity during the melting process would lead to a more crystalline structure with better mechanical and thermal property of slag. (14) On the contrary, the decrease of basicity by adding Si[O.sub.2] or cullet is helpful to form an amorphous glassy structure, and such a structure is proved to be more favorable to reduce the leaching of heavy metals. (15,16) Vitrification can incorporate toxic elements into glassy matrix, (17) and consequently the characteristics of structure may affect the physical property and the chemical stability of slags. (9)

The purpose of this study was to provide further information about the encapsulation behaviors of amorphous glassy structure during vitrification. It was initially attempted to analyze the amorphous volume fraction (AVF) of slags quantitatively and to describe the influence of Si[O.sub.2] on the formation of the crystalline phases in slags quantitatively. Therefore, the relation between the amorphous phase and the encapsulation behaviors of metals could be definitely presented.

EXPERIMENTAL WORK

Sampling and Melting Process

Fly ash used for this experiment was sampled from the fabric filters in a semidry scrubbing process at a municipal solid waste incinerator site in Southern Taiwan. High purity of Si[O.sub.2] powder was used as an additive to mix with fly ash to adjust the basicity with Si[O.sub.2]/fly ash mixing ratios (S/A) of 0, 0.1, 0.2, and 0.3, respectively. Specimens with weights of 25 g were held in a graphite crucible and vitrified in a furnace. The mixture were heated to 1450 [degrees]C at 6 [degrees]C/min, held isothermally for 4 hr, and then cooled down in room temperature. The fly ash and slags, respectively, denoted as A-0, S-0, S-1, S-2, and S-3, were all pulverized to a size that passed through a mesh 100 sieve, precisely weighed, and then extracted by a sequential procedure to determine the phase distribution of metals.

Analytical Methods and Apparatus

Similar sequential procedure used to estimate the mobility of solid wastes has already been applied in the previous studies. (10,18) In this paper, three representative phases, including easily reducible phase (metal fixed at amorphous oxides), moderately reducible phase (metal bound to crystalline oxides), and residual phase (metal resistant fixed in crystal lattice) were determined to describe the encapsulation behavior of samples. The fractions of easily reducible phase, moderately reducible phase, and residual phase indicated that the metals might be leached out in the period of the decomposition of slag structure (no decomposition, initial decomposition, and thorough decomposition). (9) The summation of the three phases revealed the composition of specimens.

Samples, each in 2-g aliquots, were separately mixed with 35 mL of 0.1 M hydroxylammonium chloride and shaken overhead for 15 hr. They were next centrifuged for 15 min at 2000 rpm, filtrated with a 0.8-[micro]m mixed cellulose ester filter, and then analyzed for the easily reducible phase. The residual samples were conducted with 35 mL of 0.1 M ammonium oxalate/oxalic acid for 15-hr shaking for the moderately reducible phase. Similar centrifuging, filtrating, and analyzing procedures were conducted for the residual samples. As for the residual phase, the sample was digested with a mixed acid (1 mL of HF + 5 mL of HN[O.sub.3] + 10 mL of HCl[O.sub.4]) hermetically in a Teflon vessel at 210 [degrees]C for 5 hr. The mixture was then diluted to 25 mL and filtrated. This extraction scheme partitioned the samples into three fractions with the increasing availability of the metals. The concentration of metal species in extracts, including Al, Ca, Cd, Cr, Cu, Fe, Mg, Mn, Ni, Pb, and Zn, was analyzed by an inductively coupled plasma-atomic emission spectrometry (JY-38 Plus ICP-atomic emissions spectroscopy).

Qualitative Analysis of Microstructure

Microstructure of slag surface was qualitatively examined by scanning electron microscopy-energy dispersive spectroscopy (Jeol JXA-840 SEM-EDS). Specimens were adhered on a coin and coated with Au using an ion sputter coater. The X-ray diffraction (XRD) analysis, which was performed to identify the crystalline phases, was carried out by a powder diffractometer (Geigerflex 3063) with Ni-filtered Cu[K.sub.[alpha]] radiation on powders, at particle size below 20 [micro]m, at 4[degrees]/min, in the 2[theta] = 10-80[degrees] range. In this paper, the volume fraction of crystalline phase in slags was determined by quantitative XRD analysis with an internal standard method. (19) High-purity silica powder was used to mix with specimens with an Si/sample mass ratio of 0.1 to serve as an internal standard. The quantitative XRD analysis was performed to measure the approximate amount of the crystalline phase, and details of this procedure are given in a previous report. (20) The amount of crystalline phase can be determined according to the area of their specific peaks. Then the AVF of slags can be calculated by the following equation:

AVF = 1 - [i=n.summation over (i=1)] [CP.sub.i] (1)

where [CP.sub.i] is the volume fraction of the ith specific crystalline phase.

RESULTS AND DISCUSSION

Composition of Fly Ash and Slags

The composition of fly ash and slags (S-0 as a representative) was presented in Table 1. The metal species in samples were further divided into two categories of crust and anthropogenic elements, which were presented in oxide and elemental form, respectively. The major crust elements in fly ash and slags were CaO, Si[O.sub.2], and [Al.sub.2][O.sub.3], the common constituents in fly ash. (21) An elevation of crust elements in slags could be observed, and it could be explained that an approximately 50% weight loss of fly ash after a heating treatment caused these species to be concentrated. However, some anthropogenic metals, including Cd, Pb, and Zn, showed a drastic difference between fly ash and slags. The decrease of the metal content in slag was caused by the evaporation of metals during the melting process. (8)

[FIGURE 1 OMITTED]

Morphology of Fly Ash and Slags

Figure 1 displayed the morphology of the fly ash and slags. In A-0, many agglomerates with an uneven surface structure and a uniform size distribution were observed. After the melting treatment, S-0 showed a highly dense but porous structure, revealing that these fly ash particles could be merely bound via high temperature. With an addition of Si[O.sub.2], S-1 became more rough edged, and its glassy surface was smoother. The glass-like characteristic was more conspicuous with the increase of the S/A ratio. According to the examination of the slags' surface, it could be obviously observed that the addition of Si[O.sub.2] led to a smoother and more amorphous structure.

Crystalline Characteristics of Slag Structure