Copper Stabilization in Glass-Ceramics: Effect of CaO and SiO 2 on CuAl x Fe 2- x O 4 Formation

Abstract

Industrial activities discharge heavy metal-laden wastes; and copper is one of these metals that can be stabilized after ceramic sintering by Al- or Fe-containing precursors. Besides traditional precursors, Al- and Fe-containing solid wastes (e.g., sewage sludge incineration ash, coal fly ash) were found equally capable of copper stabilization when they were adopted in the reaction matrices. However, the effect of CaO and SiO₂, the predominant components in most of the inorganic solid wastes, on metal transformation and stabilization has not been addressed. Therefore, a CaO-SiO₂-CuO-Al₂O₃-Fe₂O₃ reaction matrix was proposed in this study, focusing on the effect of CaO and SiO₂ on copper transformation and immobilization. Results showed that a single-phase CuAl_xFe_2-xO₄ spinel solution was developed in the system without CaO and SiO₂, whereas the addition of CaO led to the generation of two separate spinel phases: CuAl₂O₄ and CuFe₂O₄. Compared with CaO, the existence of SiO₂ showed less impact on the phase transformation. The effect of sintering temperature and additive dosage was also illustrated in this study and the results provided useful guidance for the optimization of reaction systems to achieve beneficial utilization of solid wastes for heavy metal stabilization.

Introduction

Heavy metal pollution is always an intractable environmental problem, and people have been increasingly aware of the seriousness of global population explosion accompanied by rapid industrialization without proper effluent disposal (Kuo, 2009; Yabe et al, 2010). Copper is usually recognized as one of the common heavy metals, and is always discharged by industrial activities, including mining, electroplating, metallurgy, iron and steel industries, etc. (Mahmood et al, 2012). It would cause severe health problems once the level of copper in human body exceeds specific value (1.3 mg/L), such as, stomach and intestine diseases, neurotoxicity, jaundice, and liver intoxication (Cheng et al, 2010; Hanafiah and Ngah, 2009). Treatments like adsorption or chemical precipitation are usually used for heavy metal removal from aqueous environment, but these methods result in quite a large amount of secondary metal-laden sludge, which requires further treatment (Cheng et al, 2010).

Many technologies have been reported to prevent the release of heavy metals from the industrial sludge (Chen et al, 2020; Çoruh and Ergun, 2006; Dong et al, 2022; Malviya and Chaudhary, 2006; Shen et al, 2018; Ucaroglu and Talinli, 2012), among which ceramic sintering has been proved to effectively stabilize the hazardous metals long term in the ceramic products (Shih et al, 2006; Tang et al, 2016; Tang et al, 2014b; Tang et al, 2011a; Tang et al, 2011b; Tang et al, 2010). Traditional ceramic precursors, such as Al₂O₃, kaolinite, and Fe₂O₃, were found to stabilize the hazardous copper successfully in a spinel crystal structure, whereas Al- or Fe-containing solid wastes were also reported to positively stabilize copper when the ash was used to replace the traditional ceramic precursors (Su et al, 2019). Those solid wastes can be waterworks sludge from drinking water treatment processes (Tang et al, 2014a), incineration ash from the sludge of municipal wastewater treatment processes (Ma et al, 2020; Ma et al, 2019), or even fly ash after coal combustion in a power plant (Terzano et al, 2005; Wu et al, 2018).

The usage of solid wastes to replace traditional ceramic precursors does not only relieve the environmental burden caused by waste accumulation but also beneficially reutilizes the valuable sources discarded previously. The ceramic sintering technique has been widely recognized as a promising “waste-to-resource” strategy to achieve the sustainable waste management.

Although the solid wastes can be potentially accepted as raw materials for ceramic industries, researchers have pointed out that the complicated matrix has made it difficult to conduct a mechanism study on metal transformation. However, it can still be noticed that the major components show great similarity for common solid wastes such as waterworks sludge (Tang et al, 2014a), incineration residues from both municipal solid waste (Chen et al, 2016; Min et al, 2017) and sewage sludge (Lynn et al, 2018), etc. Elements such as Al, Fe, Si, and Ca have been reported to be predominantly contained in the aforementioned solid wastes. The total content of Al, Fe, Si, and Ca in the incinerated waterworks sludge collected in Hong Kong occupies about 93% from elemental analysis results normalized in the oxide forms (Tang et al, 2011a). About 48% of Al, Fe, Si, and Ca has also been reported in Taiwan when the municipal solid waste incineration ash was used as glass-ceramic material (Vu et al, 2012).

The Al and Fe oxides have proven to stabilize Cu by generating spinel phases of CuAl₂O₄ and CuFe₂O₄, whereas the presence of two other major components (CaO and SiO₂) in the solid wastes can also impact the reaction between Cu and Al/Fe and lead to various phase constitution in the final products (Li et al, 2017; Shen et al, 2018). Therefore, the effect of CaO and SiO₂ on the phase transformation in a CaO-SiO₂-CuO-Al₂O₃-Fe₂O₃ system is crucial to understand the immobilization mechanisms and to further improve the stabilization efficiency. However, to our knowledge, no systematic investigations on the effect of CaO and SiO₂ have been carried out yet in a CaO-SiO₂-CuO-Al₂O₃-Fe₂O₃ system. The goal of this study is to bridge the knowledge gap with respect to the stabilization mechanisms when various amounts of CaO/SiO₂ exist in the chosen ceramic reaction systems.

In the current study, the reaction system between heavy metal-laden sludge and various solid wastes acting as stabilization precursors can be simplified as the reaction among CuO, Al₂O₃, Fe₂O₃, CaO, and SiO₂, to avoid the influence of other factors and therefore concentrate on the study of CaO and SiO₂ effect. The investigation was carried out by fixing the molar ratio of CuO:Fe₂O₃:Al₂O₃ to be 2:1:1 and then adding various dosages of CaO and SiO₂ (separately or simultaneously) to simulate different precursor materials. In the previous study, it was reported that the optimal sintering temperature for formation of spinel phase was around 1,000°C (Tang et al, 2011a). Therefore, the reaction system was sintered at 900°C, 950°C, 1,000°C, and 1,050°C in our study to evaluate the temperature influence on the phase transformation mechanisms during sintering. X-ray diffraction (XRD) was applied as the main characterization method in this experiment to study the phase composition after thermal treatment.

The results will assist in the comprehensive study of the effect of CaO and SiO₂ on the original reaction system, respectively, and jointly. Moreover, the current study will provide new perspectives with respect to the heavy metal immobilization using solid wastes as precursors during the ceramic processes.

Materials and Methods

To investigate the effect of CaO and SiO₂ on the Cu immobilization process, respectively, and jointly, various reaction systems were adopted in this study for comparison. The detailed information is summarized in Table 1. Among these reaction systems, the molar ratio of Cu:Al:Fe was always kept at 1:1:1 for a total dry weight of 60 g. The above ratio was chosen to ensure complete reaction of all oxides to generate spinel structure. Then additives, such as CaO, SiO₂, and CaO+SiO₂ with different weight fractions (1.0%, 5.0%, and 10%) were blended into the CuO+Al₂O₃+Fe₂O₃ systems, respectively. For ease of reference, each reaction system was cited in the following text using abbreviations, as indicated in Table 1. For example, CuO-Al₂O₃-Fe₂O₃ system with 10 wt% SiO₂ and 10 wt% CaO as additives is denoted as BS-10(Ca+Si). The CuO, Al₂O₃, Fe₂O₃, SiO₂, and CaO powders were purchased from Sigma-Aldrich.

Table 1.

The Compositions and Denotation of the Reaction Systems

Additives	Denoted as	Weight of components (g)
Additives	Denoted as	CuO	Al₂O₃	Fe₂O₃	SiO₂	CaO
No additives	Base system (BS)	4.52	3.00	4.56	0	0
1.0 wt% SiO₂	BS-1Si	4.52	3.00	4.56	0.12	0
5.0 wt% SiO₂	BS-5Si	4.52	3.00	4.56	0.60	0
10 wt% SiO₂	BS-10Si	4.52	3.00	4.56	1.20	0
1.0 wt% CaO	BS-1Ca	4.52	3.00	4.56	0	0.12
5.0 wt% CaO	BS-5Ca	4.52	3.00	4.56	0	0.60
10 wt% CaO	BS-10Ca	4.52	3.00	4.56	0	1.20
1.0 wt% SiO₂+1.0 wt% CaO	BS-1(Ca+Si)	4.52	3.00	4.56	0.12	0.12
5.0 wt% SiO₂+5.0 wt% CaO	BS-5(Ca+Si)	4.52	3.00	4.56	0.60	0.60
10 wt% SiO₂+10 wt% CaO	BS-10(Ca+Si)	4.52	3.00	4.56	1.20	1.20

The reaction series were homogenized through ball milling the mixed powders in water slurry for 18 h, and were then dried and homogenized again by mortar grinding to offset the influence of stratification of various components with different densities after drying. The mixture was pressed into 20 mm pellets at 12 MPa with the dwelling time of 1 min. The as-prepared pellets were then fired at 950°C, 1,000°C, and 1,050°C for 3 h at a heating rate of 10°C/min.

The sintered pellet sample was broken into pieces, one of which was cold mounted into the resin and then polished on a grinder polisher (Ecomet™ 250; Buehler) using sandpapers to a smooth surface. The microstructure was characterized using a scanning electron microscopy (SEM; Zeiss, Merlin) equipped with energy-dispersive spectroscope (EDS). The rest of the pellet sample was ground into powder for the subsequent XRD test (Rigaku Smartlab diffractometer). Phase composition and transformation were investigated by powder XRD with Cu Kα radiation (45 kV, 200 mA), operated in step scanning mode with 2θ range of 10°–90°, step size of 0.02°, and a scan speed of 0.3 s/step. Phase identification was executed by matching powder XRD patterns with those retrieved from the standard powder diffraction database of the International Center for Diffraction Data (ICDD PDF-2, Release 2008). HSC Chemistry version 7.1 software was introduced for thermodynamic calculations.

Results and Discussion

The influence of CaO and SiO₂ on the Cu-Al-Fe-O reaction system

As shown in Fig. 1, controlling the molar ratio of Cu:Al:Fe at 2:1:1 and firing the mixture of CuO+Al₂O₃+Fe₂O₃ (BS) at 950°C led to a spinel solid solution structure in the final product, which indicated a sufficient reaction between the compounds in the system. Considering the molar ratio of Cu:Al:Fe was 2:1:1, the spinel solid solution most likely has a formula of CuAlFeO₄. Single-phase cubic spinel ferrite CuAlFeO₄ has been reported previously by prefiring CuO, Al₂O₃, and Fe₂O₃ at 950°C and further homogenized at 1,000°C (Ata-Allah, 2004). The XRD patterns of the final product from BS after sintering at 950°C in our study coincided with the patterns of CuAlFeO₄ reported in the literature (Ata-Allah, 2004). The corresponding reaction equation in the current study is given as:

FIG. 1.

The XRD patterns and identified phases of different reaction systems: base systems without additives, with 10 wt% CaO, with 10 wt% SiO₂, and with 10 wt% (CaO+SiO₂), after sintering at 950°C. XRD, X-ray diffraction.

2 C u O + A l_{2} O_{3} + F e_{2} O_{3} \to 2 C u A l F e O_{4}

(1)

Comparing the XRD patterns of 10 wt% CaO-added reaction system (BS-10Ca) to that of BS after sintering, it illustrated that the existence of CaO had a considerable effect on the final formula of the spinel solid solution. In Fig. 1, the XRD pattern of BS-10Ca suggested that the raw materials developed into two separate spinel phases—CuAl₂O₄ and CuFe₂O₄, rather than one spinel solid solution structure after firing. The existence of CaO dramatically inhibited the substitution process between Al atoms and Fe atoms in the spinel structure, and therefore prevented the generation of CuAl_xFe_2-xO₄ spinel solid solution (0 < x < 1). Due to the lack of substitution between Al atoms and Fe atoms, the product finally developed into alumina-spinel and ferro-spinel, separately. Besides, a tiny peak of CuO was observed at the position around 36° in the XRD patterns of sintered BS-10Ca, which indicated excess CuO in the reaction system, resulting from the consumption of Al₂O₃ from reacting with CaO to form CaAl₄O₇ phase and leading to insufficient Al₂O₃ to react with CuO. The presence of CaAl₄O₇ phase in the final product was verified by the XRD result in Fig. 1.

The possible reaction equation can be given as below:

When x mol of CaO participated in the reaction shown in Equation (2), there would be 2x mol of CuO remaining unreacted due to the fixed molar ratio of Cu:Al:Fe = 1:1:1 in all reaction systems. From Equation (2), it is demonstrated that the yield of CuFe₂O₄ was superior to the yield of CuAl₂O₄ due to the consumption of Al₂O₃ in forming CaAl₄O₇. This phenomenon was further confirmed by the XRD patterns (Fig. 1, BS-10Ca), in which the peak intensity of CuFe₂O₄ was dramatically stronger compared with CuAl₂O₄.

To evaluate the reaction preference in the systems with and without CaO addition, thermodynamic calculations were performed, and the calculated Gibbs free energy of the reaction as a function of temperature is present in Fig. 2. In the CaO-free system (BS), the high configurational entropy in CuAl_xFe_2-xO₄, which is related to the number of ways of arranging all the atoms in the system, is inclined to stabilize solid solution formation (Larosa et al, 2019). Since the configurational entropy of CuAl_xFe_2-xO₄ is much higher compared with CuFe₂O₄ and CuAl₂O₄, it leads to a lower reaction Gibbs free energy when generating a single spinel phase, and therefore, only CuAl_xFe_2-xO₄ spinel phase existed in the final product in BS.

FIG. 2.

Gibbs free energies plotted as a function of temperature for the two reaction systems with and without CaO addition.

However, the addition of CaO into the system led to a significant decrease in Gibbs free energy of formation of both CuFe₂O₄ and CuAl₂O₄ phases, which became even lower than the Gibbs free energy of formation of the single CuAl_xFe_2-xO₄ phase. Thus, two separate spinel phases were favored in BS-10Ca during sintering. CaO tends to react with Al₂O₃ to generate stable CaAl₄O₇ phase (Mao et al, 2016) and abated the potential of Al₂O₃ to react with other compounds in the system.

Besides, the generation of CaAl₄O₇ phase would act as obstacles at the grain boundaries to hinder further penetration and the substitution process between Al atoms and Fe atoms (Wang et al, 2015). As a result, the system with CaO addition finally developed two separate spinel phases, CuAl₂O₄ and CuFe₂O₄.

In the reaction system, which applied 10 wt% SiO₂ as additive (BS-10Si), two spinel solid solutions were also generated, as shown in Fig. 1. Two series of characteristic peaks were very close to each other with partial overlap, but still distinguishable. Steep and intense peaks of SiO₂ overwhelmed other characteristic peaks, including the peaks of spinel solid solution. Unlike the reaction in BS-10Ca, there is no detection of CuO phase in BS-10Si after sintering, indicating nearly complete reaction of CuO during sintering. It was found that the peak intensity of Al-rich spinel phase and Fe-rich spinel phase was comparable in BS-10Si, which was different from the condition in BS-10Ca, where the peak intensity of CuFe₂O₄ was markedly stronger compared with CuAl₂O₄. Li et al (2017) found that though Cu could be stabilized by forming CuAl₂O₄ and CuFe₂O₄ during sintering, it could not be incorporated into silicate phases, because the reaction of CuO with SiO₂ was not triggered at temperatures below 1,100°C. Other than SiO₂, no new Si-containing phase was observed from the XRD patterns, suggesting a lack of reaction between SiO₂ and other raw materials in the system.

However, SiO₂ was not completely isolated from other components in the reaction system, since the addition of SiO₂ led to the forming of two spinel phases, in contrast to one single CuAlFe₂O₄ spinel phase in BS. The melting temperature of SiO₂ is higher than the sintering temperatures used in this study (Ringdalen and Tangstad, 2016) and the solid SiO₂ mainly acted as obstruction to the substitution process between Al atoms and Fe atoms, which increased the energy barrier for reactions and thus slowed down the reaction rate to achieve one uniform spinel solid solution phase. Therefore, with the addition of 10 wt% SiO₂, the reaction system finally generated two separate spinel solid solution phases.

Compared with the XRD patterns of BS-10Si, the characteristic peak of SiO₂ at the position around 27° was significantly weaker in BS-10(Ca+Si). On the contrary, the peak intensities of spinel phases were enhanced in BS-10(Ca+Si) when comparing to those in BS-10Si. Apart from the detection of a tiny peak of SiO₂, the XRD patterns of BS-10(Ca+Si) were extremely similar to BS-10Ca, which suggested that the extra 10 wt% SiO₂ did not play a significant role on the reactions and final phases during sintering. Therefore, the influence of CaO on the Cu-Al-Fe-O reaction system surpassed that of SiO₂ and played a much more decisive role.

The influence of sintering temperature on different reaction systems

To investigate the influence of sintering temperature on different reaction systems, Fig. 3 collates the XRD results of four studied systems after sintering at 950°C, 1,000°C, and 1,050°C for 3 h. From Fig. 3a corresponding to the reaction system without additives (BS), it was found that elevating sintering temperature from 950°C to 1,000°C did not affect the phase constitution in the final product, but led to a higher degree of crystallinity in the product, which was confirmed through stronger intensity and narrower width of the peaks (Ma et al, 2020). Nevertheless, the formation of the spinel solid solution does not seem to be affected by the sintering temperature monotonously. The degree of crystallinity in the spinel solid solution dropped slightly with less sharp peaks when the sintering temperature was further increased from 1,000°C to 1,050°C. Wei et al (2002) also reported the decreased degree of crystallinity with increasing sintering temperature.

FIG. 3.

The XRD patterns of the reaction systems: (a) base system, (b) base system with 10 wt% CaO, (c) base system with 10 wt% SiO₂, and (d) base system with 10 wt% (CaO+SiO₂) after sintering at 950°C, 1,000°C, and 1,050°C for 3 h.

When the sintering temperature was further increased from 1,000°C to 1,050°C, on the one hand, part of spinel grains would meet and bridge with each other to generate larger grains, which is known as normal growth process; on the other hand, a few large-angle grain boundaries arose with high mobility, therefore leading to a quite fast growth in grain size and clusters (Huang and Liu, 2020). The cluster distribution was incapable of evolving fast enough and resulted in the slightly lower crystallinity in the spinel structure (Kelton and Greer, 2010).

Figure 3b illustrates the temperature influence on the reaction system BS-10Ca. The degree of crystallinity in Fig. 3b kept growing with increased sintering temperature and achieved its maximum value at 1,050°C. It seems that the addition of 10 wt% CaO slowed down the reaction rate in the system, therefore, unlike the case in Fig. 3a, the spinel phase in BS-10Ca cannot obtain its optimal crystallinity after sintering at 1,000°C. However, this phenomenon did not necessarily mean monotonous influence of the sintering temperature on the spinel solid solution formation.

As mentioned in The Influence of CaO and SiO2 on the Cu-Al-Fe-O Reaction System section, the existence of 10 wt% CaO prevented the formation of one type of spinel solid solution and promoted the development of two independent spinel structures—CuAl₂O₄ and CuFe₂O₄ at 950°C. Such tendency of generating two separate spinel phases did not change after increasing the sintering temperature to 1,000°C and 1,050°C. In addition, the difference of peak intensity between the CuAl₂O₄ and CuFe₂O₄ phases continued to increase at higher sintering temperature due to the continuous consumption of Al₂O₃ in the reaction with CaO to generate CaAl₄O₇.

This was consistent with further decreasing the Gibbs free energy when the temperature was increased to 1,000°C and 1,050°C, as illustrated in Fig. 2. The peak of CuAl₂O₄ almost disappeared at the sintering temperature of 1,050°C and the majority of the final product was CuFe₂O₄. Therefore, the Fe-rich materials will be a prior option as the immobilization precursor for copper in the comparison to Al-rich materials when the existence of CaO cannot be averted.

The XRD results of the reaction system BS-10Si are exhibited in Fig. 3c. As described in The Influence of CaO and SiO2 on the Cu-Al-Fe-O Reaction System section, the XRD pattern illustrates two types of spinel solid solution after sintering at 950°C for 3 h. While the peak position of spinel phase stayed consistent as the sintering temperature increased in Fig. 3b, the peaks of two spinel solid solutions in Fig. 3c moved toward each other and eventually converged into one type of spinel solid solution at sintering temperature above 1,000°C.

Accordingly, the presented XRD pattern of BS-10Si at 950°C could be explained by insufficient reaction in the given thermal reaction condition. The provided energy was insufficient for complete homogenization of the Cu, Al, and Fe atoms to form CuAlFeO₄ spinel solid solution since SiO₂ acted as obstacles for interdiffusion of Fe and Al (Li et al, 2016), therefore, the reaction in the fired mixture ceased in the state of coexisting of two spinel solid solutions during cooling down. When the sintering temperature was raised over 1,000°C, the energy and time were abundant for sufficient reaction and the final product only contained one type of spinel solid solution phase.

It is worth mentioning that the intensity of the characteristic peak of SiO₂ at around 27° diminished remarkably at a higher temperature. In contrast, the crystallinity of spinel solid solution improved prominently at a higher temperature and arrives its maximum at 1,050°C.

The XRD patterns of the reaction system BS-10(Ca+Si) are shown in Fig. 3d, which is similar to the XRD patterns of BS-10Ca. This is because, compared with SiO₂, the presence of CaO had an overwhelming influence on the final-phase composition during sintering. At 950°C, the peak intensity of CuFe₂O₄ phase was higher compared with the CuAl₂O₄ phase due to the consumption of Al₂O₃ by CaO to generate CaAl₄O₇, and the difference between the peak intensity of CuAl₂O₄ and CuFe₂O₄ was further enhanced at 1,000°C. When the sintering temperature was increased to 1,050°C, the characteristic peaks of CuAl₂O₄ almost disappeared, which was the same as the situation in Fig. 3b. The peak intensity of SiO₂ declined significantly at elevated temperatures. While the characteristic peak of SiO₂ at round 27° was still recognizable at 1,000°C, it eventually faded away at 1,050°C. The evolution of SiO₂ peak intensity suggested that the addition of SiO₂ did play a role in the phase transformation during sintering.

Although SiO₂ did not react with other compounds in the investigated system, it influenced the phase transformation, as well as the composition of final product, by hindering the substitution process between Al atoms and Fe atoms during the formation of spinel phase. Therefore, the barrier energy of reaction to generate one uniform spinel structure increased and eventually resulted in two separate spinel phases, CuFe₂O₄ and CuAl₂O₄. In BS-10(Ca+Si) system, the crystallinity of CuFe₂O₄ was suppressed most conspicuously at 1,050°C, which was revealed by a broadened peak width compared with the XRD patterns in Fig. 3b at 1,050°C. In other words, the CuFe₂O₄ phase could obtain better crystallinity without the presence of SiO₂.

Figure 4 shows SEM images of different reaction systems after sintering at 1,000°C for 3 h, illustrating the microstructure evolution with different dosages of CaO and SiO₂ addition. The corresponding EDS spectra were also collated in Fig. 4 to show the composition of the phases in the final product. For BS in Fig. 4a, the microstructure was relatively dense with distributed holes. The corresponding EDS showed simultaneous existence of Cu, Al, and Fe, suggesting the generation of one spinel structure after sintering. With 10 wt% CaO added to the system, the product became less compacted with serried microvoids (Fig. 4b). It was likely that the CaO addition could intensify the reactions in the system and thereby accelerated the growth of grain boundaries and left more voids behind (Li et al, 2016). The corresponding EDS result showed that the tested area was abundant in Ca and Al elements, which was in accordance with the generation of CaAl₄O₇ in BS-10Ca after sintering. From Fig. 4c, we can see that the microstructure of BS-10Si was similar to that of additives-free system, with compact structure and scattered holes.

FIG. 4.

SEM images of different reaction systems after sintering at 950°C: (a) base system, (b) base system with 10 wt% CaO, (c) base system with 10 wt% SiO₂, and (d) base system with 10 wt% (CaO+SiO₂), with corresponding EDS spectrum. EDS, energy-dispersive spectroscope; SEM, scanning electron microscopy.

The corresponding EDS spectrum from a spot also confirmed the coexistence of Cu, Al, and Fe, implying the formation of CuAl_xFe_2-xO₄ phase, which was consistent with the phase identification in Fig. 3c. Figure 4d illustrated the microstructure of BS-10(Ca+Si), which showed alike morphology as BS-10Ca. With a comparison of the results in Fig. 4b and d, it was feasible to conclude that, compared with SiO₂, CaO had a more significant effect on the phase transformation and morphology of the final product.

The influence of additive dosage on different reaction systems

Various amounts of additives were blended with CuO, Al₂O₃, and Fe₂O₃ to investigate the influence of additive dosage, and Fig. 5 displayed the XRD patterns of the investigated systems sintered at 950°C. Figure 5a shows the XRD patterns of a series of CaO-added reaction systems, indicating the superiority of Al element to Fe element in spinel structure formation in BS-1Ca reaction system with slightly higher CuAl₂O₄ peak intensity, compared with that of CuFe₂O₄ (∼36°). In the BS-1Ca system, it was interesting to find that CaO reacted with Fe element to form Ca₃Fe₁₅O₂₅, which was confirmed through the characteristic peak at around 35°. The generation of Ca₃Fe₁₅O₂₅ resulted in the Fe element deficiency in the spinel structure, and led to stronger peak intensity of CuAl₂O₄ in comparison with CuFe₂O₄. Due to the consumption of Fe₂O₃ by CaO, Fe element became deficient to stabilize Cu, and therefore led to residual CuO in the final product with tiny CuO peaks detected in the XRD patterns (Supplementary Fig. S1). Besides, a tiny peak was found around 26° and the corresponding phase was identified as CaAl₄O₇.

FIG. 5.

The XRD patterns of different reaction systems that have been applied (a) CaO, (b) SiO₂, and (c) CaO+SiO₂ as additives with varying dosage after sintering at 950°C for 3 h.

When CaO addition was increased to 5.0 wt% (BS-5Ca system), the Ca₃Fe₁₅O₂₅ phase disappeared while the intensity of CaAl₄O₇ phase slightly increased, ascertained by the peak at ∼26°.

The increased content of CaO may result in a significant change in the boundaries of phase diagram and lead to the transition of Ca₃Fe₁₅O₂₅ to CaAl₄O₇ in the final products (Macgregor, 1970). In BS-5Ca system, two sets of spinel peaks perfectly matching CuFe₂O₄ and CuAl₂O₄, respectively, were detected, but with CuFe₂O₄ peak intensity (∼36°) surpassing that of CuAl₂O₄. In the case of BS-5Ca, it was more difficult for Al element to participate in the formation of spinel structure than Fe element due to the consumption of Al₂O₃ by CaO to form CaAl₄O₇. Besides, the increased content of CaO led to rising consumption of Al₂O₃, and therefore more CuO was left unreacted from the evidence of obvious CuO peak in the XRD patterns. The case for BS-10Ca was similar to the BS-5Ca system, with the peak intensity of CuAl₂O₄ overwhelmed by CuFe₂O₄ due to the further consumption of Al element to generate CaAl₄O₇ phase. In summary, the peak intensity of CuAl₂O₄ was slightly stronger compared with CuFe₂O₄ when the addition of CaO was set to be 1.0 wt%.

Then the peak intensity of CuFe₂O₄ surpassed that of CuAl₂O₄ in the BS-5Ca system, and the difference between their peak intensities continued to enlarge with the CaO addition increasing to 10 wt%, accompanied by the development of CaAl₄O₇ and CuO peaks in the XRD patterns.

The XRD patterns of the various amounts of SiO₂-added systems are shown in Fig. 5b. The addition of 1.0 wt% SiO₂ (BS-1Si) into the reaction system led to the formation of two spinel phases: CuFe₂O₄ and CuAl₂O₄, instead of one CuAlFeO₄ spinel solid solution as in BS. However, two types of spinel structures in BS-1Si system showed poor crystallinity and the majority of the characteristic peaks was overlapping with each other. It appeared that the addition of SiO₂ mainly acted as a kinetic hindrance to the formation of one homogeneous CuAlFeO₄ spinel phase and the crystallization tendency, which was directly related to the initial components in the mixtures (Xuan et al, 2018).

It was also interesting to note that, although the molar ratio of Fe₂O₃ to Al₂O₃ was 1:1, the peak intensity of CuFe₂O₄ spinel was stronger compared with CuAl₂O₄ spinel. It was reported that SiO₂ addition can significantly suppress the reaction of Al₂O₃ and the crystallization process by posing a kinetic barrier for the transformation (McHale et al, 1997). In the BS-1Si system, the obstruction effect from SiO₂ was severer on the reaction of Al₂O₃, compared with Fe₂O₃, and therefore resulted in a higher peak intensity of ferro-spinel.

The characteristic peak of SiO₂ at around 27° cannot be detected until the SiO₂ addition was raised up to 5.0 wt%. The XRD patterns of BS-5Si and BS-10Si systems were similar to that of BS-1Si with two types of spinel phase coexisting in the final products. However, two sets of spinel peaks were further separated when the dosage of SiO₂ was set to be 10 wt%, indicating that higher dosage of SiO₂ was favorable for forming two distinct spinel structures. Besides, the crystallinity degree of both CuFe₂O₄ and CuAl₂O₄ in BS-10Si system was further declined with comparable peak intensities for two spinel phases, due to stronger obstruction to Al₂O₃ and Fe₂O₃ reactions from increased content of SiO₂. The excessive addition of SiO₂ will also result in relatively high intensity of SiO₂ peak and made the peaks of spinel phases obscure.

Figure 5c shows the XRD patterns of reaction systems with both CaO and SiO₂ additives at different levels. In accordance with expectation, the XRD pattern in Fig. 5c is similar to that in Fig. 5a, which again demonstrated that the influence of CaO on the reaction system prevailed over SiO₂. In Fig. 5b, the peak of SiO₂ is invisible in the XRD patterns of BS-1(Si+Ca) system, as observed in the BS-1Si system. With the increased addition of CaO and SiO₂, the CaAl₄O₇ phase took the place of Ca₃Fe₁₅O₂₅ phase and led to CuFe₂O₄ as the predominant phase in the final product. The peak of CuO was clearly detected at all levels of CaO and SiO₂ addition, as shown in Fig. 5c.

Conclusions

The current research investigated the effect of CaO and SiO₂ on phase transformation and metal incorporation during sintering in CaO-SiO₂-CuO-Al₂O₃-Fe₂O₃ systems. The thermodynamic calculations were performed to interpret the incorporation mechanisms. In the additive-free system, only one CuAl_xFe_2-xO₄ spinel phase was generated in the reaction systems at all tested temperatures (950–1,050°C). However, when CaO was added to the system, two separate spinels (alumina-spinel and ferro-spinel) were developed, due to the significantly decreased Gibbs free energy of reaction caused by the addition of CaO and the restrain of the substitution process between Al and Fe atoms.

In contrast, the role of SiO₂ addition was less conspicuous, with no newly generated Si-containing phases detected in the final product. Although the added SiO₂ did not take part in the reaction and phase incorporation, it still hindered the substitution process between atoms and led to the formation of two spinel phases at 950°C. When the sintering temperature was increased to 1,000 and 1,050°C, the energy was sufficient to overcome the obstruction from SiO₂ and the reaction system with SiO₂ addition eventually developed a single CuAl_xFe_2-xO₄ spinel phase.

CaO tended to react with Fe₂O₃ to generate Ca₃Fe₁₅O₂₅ when CaO addition was 1.0 wt%, but CaAl₄O₇ phase took the place of Ca₃Fe₁₅O₂₅ phase when CaO addition was increased to 5.0 and 10 wt%. The findings in our study provide basic understanding for the effect of CaO/SiO₂ on the incorporation and stabilization mechanisms of Cu and can shed light on following studies about heavy metal stabilization in the CaO/SiO₂-laden solid wastes. A waste-to-resource method would be expected in the future by utilizing the thermally stabilized solid waste for industry products.

Footnotes

Acknowledgment

The authors are sincerely grateful for the assistance of SUSTech Core Research Facilities.

Authors' Contributions

F.M. and Y.T. provided the conceptualization, supervised the research, and were also in charge of funding acquisition. Z.L. and Y.L. performed the experiments and collected the XRD data. J.Z. and Y.X. focused on data curation and formal analysis. F.M. and Y.X. wrote the original draft. All authors reviewed and edited the final article.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work is supported financially by the Natural Science Foundation of Guangdong Province in China under Grant 2019A1515011836; and the National Natural Science Foundation of China (NSFC) under Grant 41977329 and 21707063.

Supplementary Material

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Supplementary Material

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Copper Stabilization in Glass-Ceramics: Effect of CaO and SiO 2 on CuAl x Fe 2- x O 4 Formation

Abstract

Introduction

Materials and Methods

Results and Discussion

The influence of CaO and SiO2 on the Cu-Al-Fe-O reaction system

The influence of sintering temperature on different reaction systems

The influence of additive dosage on different reaction systems

Conclusions

Footnotes

Acknowledgment

Authors' Contributions

Author Disclosure Statement

Funding Information

Supplementary Material

References

Supplementary Material

The influence of CaO and SiO₂ on the Cu-Al-Fe-O reaction system