Arsenic removal from copper slag matrix by high temperature sulfide-reduction-volatilization

Highlights

• Pyrite is proposed to volatilize glassy arsenic by sulfide-reduction-volatilization reaction.

• The optimum process parameters for pyrite volatilizing glassy arsenic have been studied.

• The mechanism of arsenic volatilization by pyrite has been determined.

Abstract

Arsenic contamination has been a major problem in copper slag utilization. Arsenic is easily incorporated into the silicate-based matrix, making the arsenic difficult to volatilize. In this study, pyrite was selected to depolymerize the matrix structure and volatilize the glassy arsenic by sulfide-reduction-volatilization reaction. The optimum technological parameters and mechanism of glassy arsenic volatilization by pyrite were further studied. The optimum operating parameters for glassy arsenic volatilization by pyrite were determined to be a temperature of 1200 °C, a holding time of 60 min, a heating rate of 5 °C/min, a basicity of 0.3, and a pyrite addition content of 15%. The arsenic volatilization ratio reached 80.9% under these experimental conditions. Besides, the mechanism of glassy arsenic volatilization was elucidated by XRD, XPS, FTIR, and SEM analyses. These results indicate that, with the increase in temperature, the pyrite decomposes to generate a variety of sulfur-based reducing substances (FeS, FeS_1-x, S₂(g)). Through “oxygen capture reaction”, these sulfur-based reducing substances depolymerize the bridging oxygen structure from the glass former ([AsO₄], [FeO₄], and [SiO₄]) by the conversion of (Q² +Q³)→(Q⁰ +Q¹) and result in the precipitation of glass former ([AsO₄], [FeO₄] and [SiO₄]) combining with the nearby cation. In this process, the glassy arsenic is released by the glass network and participates in reductive volatilization reaction with sulfur-based reducing substances, converting the glassy arsenic with high thermal stability to volatile arsenic oxide and arsenic sulfide. These findings provide a theoretical support for the in situ volatilization of arsenic in copper smelting and centralized control of arsenic contamination.

Introduction

The pollution caused by arsenic, lead, cadmium, and other impurities affects the whole process in copper metallurgy. With the sharp reduction in the supply of high-grade and low-impurity copper concentrates, a large quantity of copper concentrates with high arsenic content has been used as the raw materials. Thus, a large amount of arsenic-containing pollutants and byproducts are inevitably produced. Especially for arsenic-containing copper slag, according to previous studies, 2.2–3 tons of slag is generated for one ton of copper production (Siddique et al., 2020). Hence, a large amount of arsenic-containing slag is inevitably produced in copper production industry. Recently, some attempts have been made to use copper smelting process to dispose arsenic-containing materials, including flue dust (Montenegro et al., 2013), smelting dust (Montenegro et al., 2008), and arsenic-bearing gypsum sludge (Li et al., 2018), to recover valuable metal resources and stabilize arsenic in the waste. Thus, it will increase arsenic concentration in the copper slag and hinder the resource utilization of copper slag. Therefore, it is essential to reduce the arsenic content in copper slag.

The arsenic occurrence in copper slag is directly linked to the arsenic separation effect. Copper slag is a typical multiphase complex, mainly including fayalite, magnetite, and silicate glass matrix. Owing to isomorphism mechanism, silicate-based matrix is the key phase to host arsenic in the copper slag (Zhang et al., 2021, Zhang et al., 2020). Arsenic can be embedded in the matrix due to a similar atomic radius (the atomic radii of As⁵⁺ and Si⁴⁺ are 0.046 nm and 0.042 nm (Onishi and Sandell, 1955), respectively). Moreover, As⁵⁺ can be incorporated as a glass former ion with four-fold oxygen coordination to replace [Si⁴⁺O₄] tetrahedra and to further build a glass network (Paje et al., 2003). The formation of glassy arsenic can enhance the arsenic dissolution capacity and thermal stability in copper slag (Park and Heo, 2002), which inhibits the arsenic volatilization. In fact, the high volatilization of arsenic from copper smelting process contributes to the arsenic collection as particulate matter, which can be used as the raw materials to generate arsenic metal etc. The dissolution of arsenic into copper slag lowers the efficiency of arsenic flue-gas collection device. Worse still, the increased concentration of arsenic improves the environmental risk for the depth utilization of copper slag. Thus, the depolymerization of arsenic-containing matrix and inducing arsenic volatilization from copper slag are important for the control of arsenic pollution in copper metallurgical.

The glassy arsenic phase transformation in copper slag is the key point for arsenic volatilization (Yao et al., 2020). A previous study (Cui et al., 2014) indicates that [AsO₄] has the highest thermal stability, and it is difficult to volatilize. However, low-valence arsenic can easily volatilize. Therefore, the transformation of glassy arsenic to low-valence arsenic can make arsenic volatilize from the silicate-based matrix in copper slag. Yang et al. (2019) used charcoal powder to reduce waste calcium arsenate, converting the arsenic from As⁵⁺ to As⁰, to obtain elemental arsenic. The result shows that 99.0% of arsenic could be extracted by volatilization. Jiang et al. (2010) used anthracite coal to reduce nMeO·As2O5 to As4O6, and up to 88.54% of total arsenic can be volatilized from the high-arsenic iron concentrate. Although a high arsenic volatilization rate can be obtained by carbon reduction reaction, the iron compounds in copper slag will compete with arsenic for carbon reducing agent, leading to high amount of carbon consumption. Besides, the addition of carbon to volatilize arsenic may affect the structure of copper slag, which is not convenient for subsequent copper flotation recovery. The sulfide reduction method is a promising alternative to volatilize glassy arsenic from copper slag. Liu et al. (2017) found that a higher sulfur content in coal has a positive effect on arsenic volatilization. Dosmukhamedov et al. (2018) used sulfiding agents to reduce the amount of nonferrous metals in converter slag. The authors found that arsenic accumulates in flue gas by volatilization from the slag in the presence of sulfiding agent. These studies prove that sulfide reduction method can achieve arsenic volatilization. However, few detailed studies are reported on the glassy arsenic volatilization by reduction volatilization in copper slag. The glassy arsenic sulfide-reduction-volatilization behavior and mechanism in copper slag need to be studied further.

Owing to the high thermal stability of pyrite and its decomposition products that do not deteriorate the copper slag, pyrite was selected as the sulfiding agent to volatilize glassy arsenic in this study. The process parameters of glassy arsenic volatilization, including the volatilization temperature, holding time, heating rate, glass alkalinity, and dosage of pyrite, were determined. Furthermore, the phase transformation and volatilization mechanism of glassy arsenic were studied by XRD, XPS, FTIR, and SEM analyses. These findings provide a theoretical support for controlling arsenic pollution in copper metallurgy.