Effect of Oxygen Content and Temperature On Stabilization of Arsenic in Copper Smelting System

Arsenic-bearing wastes from copper smelting system are usually disposed by trapping them in slag tailing. However, arsenic in slag tailing is not that stable, which can inltrate into the groundwater, threatening the environment and human health. The solidication/stabilization (S/S) of arsenic is the only way to deal with arsenic contamination. The ash smelting method shows relatively high S/S ability of arsenic, but the process and mechanism remain unclear. In this paper, we aim at revealing the effect of atmosphere on the S/S process of arsenic owing to the different content of oxygen in reaction shaft and sedimentation tank in copper smelting system. Calcium arsenate, SiO 2 , Fe 2 O 3 and iron powders were sintered at different temperature in air and argon to simulate the S/S reaction. The results show that the sintering product is Fe-Si oxide in air and fayalite in argon, and the fayalite possesses better capacity to solidify arsenic than that of Fe-Si oxide. The toxicity characteristic leaching procedure (TCLP) results reveal that the leached concentration of arsenic from fayalite fabricated at 1200 ℃ is only 2.916 mg L -1 , which satises the identication standard for hazardous substances in China. Furthermore, the theoretical calculation reveals that AsO 4 and SiO 4 tetrahedrons can be connected by O atoms when sintered in argon, and the Si-O-As covalent bond can evidently inhibit the release of As atom from fayalite. This work can not only clarify the S/S mechanism of arsenic in ash smelting process, but also provide theoretical guidances to dispose arsenic-bearing waste harmlessly.


Introduction
Owing to the excellent ductility, thermal conductivity and electrical conductivity, copper and its alloys have been widely utilized in various elds, such as wires, cables, vacuum devices and high strength bearings, etc.
The copper smelting industry has been rapidly developed in recent decades, and the annual output of copper in China has been more than 7 million tons (Yang et al. 2014). In general, the copper mineral in nature always exists as sul de associated with arsenic element. During smelting process, arsenic element can be released, which is harmful to the environment and human health. Statistically, the content of As in the copper mineral is as high as 0.2 wt.%, that is, the arsenic element brought into the environment through copper smelting system is up to 56000 tons every year in China. However, the capacity of arsenic containing products is rather limited, leading to the predicament that digesting all the arsenic-bearing waste is by no means a low-hanging fruit. Therefore, it is of great signi cance to seek an effective and safe method to dispose arsenic-bearing waste.
In order to reduce the toxicity of arsenic, solidi cation/stabilization (S/S) technology is always applied to dispose arsenic-bearing waste by restricting the mobility of As atoms. Several methods have been proposed to reduce arsenic contamination, such as cement solidi cation (Kundu and  Secondly, arsenic can react with cement and form insoluble calcium-arsenic compounds (Vandecasteele et al. 2002), such as Ca 3 (AsO 4 ) 2 and CaHAsO 3 ; Thirdly, arsenic can be absorbed onto the surface of calcium silicate hydrate (C-S-H) (Li et al. 2018). However, cement is not stable and arsenic-bearing wastes can be exposed again after damaging the cement layer. Importantly, calcium-arsenic compounds as well as absorbed arsenic can gradually dissolve into acid solution and release As element into the environment. Therefore, the solidi ed arsenic can be easily escaped and contaminate water source. As for the geopolymer solidi cation, three-dimensional network of inorganic polymer (Jaarsveld et al. 1997;Jaarsveld et al. 1999) can effectively solidify arsenic by chemical and physical absorption. Luo et al. (Luo et al. 2015) have found that the geopolymer material can transform amorphous As( ) into As( ) in the arsenic calcium slag inspired by alkaline condition. Then, Ca 2+ and Al 3+ hydrolyzed from geopolymer can react with As( ) and form insoluble salts with Al-As-O and Ca-As-O covalent bond, leading to the immobilization of As element.
However, the arsenic-bearing salts fabricated by geopolymer solidi cation can not guarantee long-term stability (Pantuzzo and Ciminelli 2010), especially in acid solution, and arsenic element can be released from the solidi ed products gradually (Sullivan et al. 2010). It is well accepted that SiO 4 tetrahedron is relatively stable, and AsO 4 tetrahedron can take place of SiO 4 tetrahedron during the vitri cation solidi cation process.
Zhao et al. (Zhao et al. 2017) have melted SiO 2 -Fe 2 O 3 -B 2 O 3 glass with sodium arsenate at 1200℃, and found that the formed Si-O-As and Fe-O-Si/As bonds could hinder the escape of As atoms from glass.
Furthermore, the glass phase shows better stability than cements and insoluble salts, indicating that the vitri cation solidi cation method is a potentially innocuous management to dispose arsenic-bearing wastes.
However, extra S/S process may bring out a severe nancial burden for smelting enterprises, especially for the high capacity of arsenic-bearing wastes. Actually, ash smelting process shows an evidently selfsolidi cation behavior (Lv and Zhang 2018;Zhang et al. 2019;Zheng and Lin 2015), exhibited by the high content of arsenosilicate glass in the smelting slag. It is a wise way to solidi ed arsenic-bearing wastes during the ash smelting process. Previous studies (Zhang et al. 2020) have shown that the arsenic exists as arsenosilicate, which is a stable arsenic-bearing phase obtained by the vitri cation of silicate at high temperature. During the ash smelting process, arsenic-bearing compounds and glass phases are apt to be vitri ed, and therefore arsenic atoms can be immobilized in the glass phase, achieving the innocuous treatment of As. However, owing to the short smelting time, the exact procedures and mechanisms of selfsolidi cation of As element during the ash smelting process remain unknown, especially for where the arsenic-bearing glass phase formed? The differences of temperature and oxygen content are vast between reaction tower and the sedimentation tank in the copper smelting system. Therefore, the effect of smelting parameters, such as temperature and content of oxygen, on the formation of arsenic-bearing glass phase is rather signi cant to guide the self-solidi cation of As, yet corresponding research is scarce.
In order to clarify the mechanism of arsenic S/S in copper smelting process, in this paper, we simulated the ash smelting process and studied the effect of temperature and oxygen content on the stabilization of As element. The microstructural evolution of simulated slag and the stabilized bonding of As elements was also studied in detail. This work not only offers an appropriate parameter for ash smelting process to solidify arsenic, but also provides an effective strategy for realizing the harmless treatment of arsenic-bearing wastes.

Flash smelting slag
The ash smelting copper slag in this work is provided by Tong Ling Nonferrous Metals, and the microstructure is shown in Fig. 1. It can be seen that the surface of slag is covered by aky Fe 3 O 4 , and the interior of slag is mainly composed of fayalite (Fe 2 SiO 4 ) and magnetite (Fe 3 O 4 ). The main chemical composition of the slag is measured by X-ray uorescence (XRF) and the result is shown in Table 1. The content of arsenic element in the slag is 0.25 wt.% while that of Fe and Si reach up to 34.31 wt.% and 16.24 wt.%, respectively.   Table. 1.

Selectively sequential extraction procedures
According to the different solubility of arsenic-bearing substances in various solutions, the selectively sequential extraction (   Step SCS Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Extracted components

Characterization
Phase constitutions of the slag were characterized by an X-ray diffraction RIGAKU D/Max 2550 PC) analyzer with a Cu Ka source (λ =1.54059 Å) at 40 kV at room temperature. The morphological characteristics of the smelting slag and the simulated slag were characterized by using a scanning electronic microscopy (SEM, Zeiss Merlin, operated at 15 kV) equipped with an energy dispersive X-ray analyzer (EDX). The analysis of chemical composition of slag was carried out by an X-ray uorescence spectrometer (ARL ADVANT'X, Thermo, USA). X-ray photoelectron spectroscopy (XPS) was performed by Thermo Fisher Scienti c ESCALAB 250Xi (Al Kα X-ray source, hν=1486.6 eV). The extraction agent can be prepared as follows: 5.7 ml acetic acid was added into deionized water, xed to the volume of 1 L, and guaranteed the pH value located in the range of 2.88 ± 0.05. Then, 2 g milled slag was added into 40 mL extraction agent and put into a inverted oscillation device. The solid-to-liquid ratio was 50 g L -1 , and the shaking time was 18 h at 25 ℃. Subsequently, the concentration of As element in the liquid was measured by EDX after separation of solid and liquid.

Theoretical calculation
In order to study the structure and the stability of simulated slag, the rst principle molecular dynamics In the MD simulations, the structure was heated to 1600 K (1327 ℃), each MD simulation in NPT ensemble lasted for 24ps with a time step of 1.0 fs, and the temperature was controlled by using the Nosé-Hoover method (Martyna et al. 1992). Owing to understand the atomic diffusion at sintering temperature, the mean square displacement of atoms in high temperature structure was calculated from the equation: where r i is the position of the atom, and brackets represent the average of all atoms in the group. After sintering at 1600 K, the structure was gradually cooled to 300 K, and then annealed at 300 K for at least 24ps.

Results And Discussion
3.1 Microstructures and phase determination of the simulated slag Fig. 4 shows the microstructures of the simulated slag sintered in air at different temperatures. The individual particles can be detected when sintered at 1000 ℃, which indicates that the particles have not been melted. When the sintering temperature rises up to 1100 ℃, the particles are partially melted and evident sintering necking can be seen. Besides, the size of particle increases obviously. When the temperature reaches up to 1200 ℃ and 1300 ℃, the microstructure shows the solidi cation structure of molten liquid, suggesting that severe reaction may occur during the sintering process. Besides, the locally globular protrusions can be clearly seen on the surface of the simulated slag even sintered at 1300 ℃. Fig. 5 shows the microstructure of simulated slag sintered in argon at different temperature. The slag shows two speci c morphologies: (I) When the sintering temperature is lower than 1100 ℃, the particles are relatively independent with lots of pores, which means that the reaction is insu cient. (II) The particles are melted and then form densi ed products with smooth and at morphology when sintered at 1200 and 1300 ℃. Compared with the simulated slag sintered in air, it can be inferred that the reaction is severer in argon, because of the disappearance of locally globular protrusions. When sintered in argon, fayalite phase can be formed even at 1000 ℃, as the XRD spectrum shown in Fig. 7. However, silica particles can not be reacted completely, and therefore inhibit the diffusion of atoms in the simulated slag. Thus, the simulated slag presents porous morphology when sintered below 1100 ℃. The Fe 3 O 4 and silica phases disappear when the sintering temperature reaches up to 1200 ℃, which can also be con rmed by the smooth and at morphology of the simulated slag. In addition, the diffraction peaks of arsenic-bearing phases can not be found because of the relatively low content of arsenic and its compound.

TCLP tests
TCLP tests have been utilized to evaluate the stabilization of As elements inside the simulated slag sintered at different temperature and atmosphere, as shown in Fig. 8. It can be seen that the leached arsenic concentration of the simulated slag sintered in air at 1000 ℃ and 1100 ℃ reaches up to 188.7 and 243.3 mg L -1 , respectively, much higher than the limitation of arsenic concentration (5.0 mg L -1 ) stipulated by the identi cation standard for hazardous substances (GB 5085. . The leaching concentration of As increases when the sintering temperature improves from 1000 ℃ to 1100 ℃ in air, which can be attributed to the improved mobility of arsenic atoms. When sintered at 1200 ℃ and 1300 ℃ in air, the leaching concentration of arsenic is as low as 2.916 and 0.339 mg L -1 , respectively. It can be inferred that As elements are stabilized in the simulated slag. As for the simulated slag sintered in argon, the leached arsenic concentration satis es the identi cation standard for hazardous substances except sintered at 1000 ℃.
According to the XRD spectrum, we can con rm that fayalite phase possesses better As stabilization ability than that of Fe-Si oxide phase.

XPS analysis of the simulated slag
In order to study the valence state of As and Si elements inside the simulated slag, XPS analysis has been applied. As shown in Fig. 9, the peak of Si2p shifts towards to the low binding energy with increasing temperature, indicating the increasing of the valence electron density of Si atoms (Dalby et al. 2007). The research (Zhao et al. 2016) certi es that the Si-O-Si bindings can be translated into Si-O-Fe and Si-O-As bindings when arsenic atoms are introduced into glass. As can be seen in Fig. 9(a) ) can be detected in As3d spectrum when sintered in air. The peak is shift towards to the low binding energy with the increase of temperature. It is due to the electron density of As is higher than Si in Si-O-As, causing the electron density around As atom increases and the binding energy of arsenic shift to lower energy. It can be deduced that the arsenic have participated in the glass structure in the form of Si-O-As. In argon, XPS spectra of As in Fig. 10 shows that the binding energy at 43.8 eV is regarded as As (III), while the binding energy at 44.9eV is presented as AsO 4 3-. The relative percentages of As (III) at these two temperatures are 29.3% and 20.7%, respectively. As (III) can be formed through the reduction of As ( ) by Fe. The As(V) is more stable and less toxic than As(III) (Wang et al. 2019; Xiu et al. 2016), and thus it is favorable to immobilize arsenic as As(V). With the increase of temperature, the percentage of As (III) decreases. Therefore, it is favorable for arsenic compound to replace SiO 4 and form As-O-Si covalent bond, resulting in the enhanced properties of arsenic solidi cation and stabilization of the simulated slag.

Theoretical calculation
In order to identify the promotional effect of diffusion rate by formed fayalite phase, the FPMD calculation is applied. Here, the (Ca 0.5 Fe 0.5 )H(As 0.5 Si 0.5 )O 4 structure is used to simulate the mixture of calcium hydrogen arsenate (CaHAsO 4 ) and fayalite. The diffusion rate and the mean square displacements of different atoms can be calculated according to the following equation: Where r i is the displacement of the atom and t represents the time. higher than that in CaHAsO 4 , which can be inferred that the fayalite phase can promote the movement of atoms and therefore enhance the diffusion rate. As discussed above, the simulated slag shows smooth and at morphology sintered in argon while that possesses locally globular protrusions sintered in air, which can be attributed to the formation of fayalite phase and thus accelerate the diffusion rate in argon. However, the promotional effect of diffusion rate by Fe 2 O 3 and Fe 3 O 4 phases are evidently lower than that of fayalite phase, and thus, the melting temperature of simulated slag is rather high when sintered in air. powders. The AsO 4 and SiO 4 tetrahedrons can be seen in the simulated structure, re ecting the occurrence of As-O-Si covalent bond. As mentioned above, As-O-Si covalent bond is relatively stable. Therefore, As atoms can not escape easily from the crystal, and therefore inhibit the leaching of As atoms.
In summary, we can con rm that the stabilizing ability of As elements is improved when the content of oxygen is relatively low, which can be ascribed as the following reasons: (I) The fayalite phase can be preserved at high temperature with low content of oxygen; (II) The fayalite phase inside the slag can reduce the melting temperature and enhance the diffusion rate; (III) As atom can take place of Si atom through diffusion process and form AsO 4 tetrahedron; (IV) The stability of Si-O-As covalent bond is rather high, which can inhibit the extraction of As from the slag.

Conclusion
1. The simulated slag forms Fe-Si oxide sintered in air, and the TCLP results can stipulate the identi cation standard for hazardous substances when the sintering temperature exceeds 1200 ℃.
2. In argon atmosphere, the sintered production is fayalite, which has excellent arsenic solidi cation ability when the sintering temperature is over 1100 ℃.
3. The fayalite phase can promote the movement of atoms and expedite the formation of Si-O-As bond.
4. AsO 4 can take place of SiO 4 tetrahedron with high stability, resulting in the high arsenic atom immobilization in fayalite.