Article pubs.acs.org/est
Decomposition of Potent Greenhouse Gas Sulfur Hexafluoride (SF6) by Kirschsteinite-dominant Stainless Steel Slag Jia Zhang,† Ji Zhi Zhou,† Zhi Ping Xu,‡ Yajun Li,† Tiehua Cao,† Jun Zhao,† Xiuxiu Ruan,† Qiang Liu,*,† and Guangren Qian*,† †
School of Environmental and Chemical Engineering, Shanghai University, No. 333 Nanchen Rd., Shanghai 200444, P. R. China ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia
‡
S Supporting Information *
ABSTRACT: In this investigation, kirschsteinite-dominant stainless steel slag (SSS) has been found to decompose sulfur hexafluoride (SF6) with the activity higher than pure metal oxides, such as Fe2O3 and CaO. SSS is mainly made up of CaO·FeO· SiO2(CFS)/MgO·FeO·MnO(RO) phase conglomeration. The SF6 decomposition reaction with SSS at 500−700 °C generated solid MF2/MF3 and gaseous SiF4, SO2/SO3 as well as HF. When 10 wt % of SSS was replaced by Fe2O3 or CaO, the SF6 decomposition amount decreased from 21.0 to 15.2 or 15.0 mg/g at 600 °C. The advantage of SSS over Fe2O3 or CaO in the SF6 decomposition is related to its own special microstructure and composition. The dispersion of each oxide component in SSS reduces the sintering of freshly formed MF2/MF3, which is severe in the case of pure metal oxides and inhibits the continuous reaction of inner components. Moreover, SiO2 in SSS reacts with SF6 and evolves as gaseous SiF4, which leaves SSS with voids and consequently exposes inner oxides for further reactions. In addition, we have found that oxygen significantly inhibited the SF6 decomposition with SSS while H2O did not, which could be explained in terms of reaction pathways. This research thus demonstrates that waste material SSS could be potentially an effective removal reagent of greenhouse gas SF6.
1. INTRODUCTION Sulfur hexafluoride (SF6) is one of the six greenhouse gases in Kyoto Protocol.1 It has a greenhouse effect of 23 900 times as CO2, and is also an extremely stable gas with an atmospheric lifetime of 3200 years.1,2 Nowadays, global SF6 concentration has grown from less than 1 ppt in 1975 to more than 7 ppt.3 Therefore, increasing attentions have been paid to SF6 control and removal from the discharge sites. In the past decade, SF6 is generally removed by adsorption, separation and decomposition methods.4−13 In general, inorganic materials, such as zeolites and carbon nanotubes, are used to adsorb SF6, but at a limited removal amounts.4−7 By contrast, a separation procedure could remove 100% of SF6 and reuse it, but need an additional equipment, especially high pressure setup.8 On the other hand, 100% of SF6 decomposition has been achieved using plasma/electrical discharge/spark, or catalysts, such as polyisoprene and metal phosphate,14−17 but some corrosive and toxic byproducts would be formed in these methods, such as SO2F2 and SF4.14−17 Furthermore, additional equipment, such as microwave radiation, UV radiation or evaporator, has been required to assist the decomposition. Therefore, it is necessary to search for a cost-effective way in removing SF6. In our previous work, electroplating sludge has proven to be an effective SF6 decomposition reagent.18 With this reagent, © 2013 American Chemical Society
complete decomposition of SF6 was achieved, together with a few byproduct gases that can be mostly absorbed by an alkaline solution. In that research, we had concluded that metal and silicon oxides were the effective components in decomposing SF6.18 Similarly, we believe that stainless steel slag (SSS), which is rich in metal and silicon oxides, could also be used as an effective SF6 decomposition reagent. Stainless steel slag is a kind of electric arc furnace (EAF) slag, with high contents of CaO, SiO2, Fe2O3 and FeO. From the viewpoint of mineralogy, stainless steel slag is a kirschsteinite-based steel slag, and its dominant components are CFS (CaO·FeO·SiO2) mineral and RO (MgO·FeO·MnO) solid solution phases.19,20 Thus it is our belief that the stainless steel slag would be a potential SF6 decomposition reagent. On the other hand, there is a huge amount of stainless steel slag produced each year.21 In 2011, global production of stainless steel was up to 35 million tons, resulting in about 7 million tons of stainless steel slag. At the moment, stack and landfilling are still the major way to dispose these slags, with Received: Revised: Accepted: Published: 599
September 3, 2013 November 13, 2013 December 11, 2013 December 11, 2013 dx.doi.org/10.1021/es403884e | Environ. Sci. Technol. 2014, 48, 599−606
Environmental Science & Technology
Article
or 700 °C, and the Si-ti curve was further plotted in SI Figure S2C. 2.4. Characterization. SF6 was monitored by a gas chromatography (GC 9800) with a 2 m × 3 mm stainless steel column filled with GDX-104 (80 meshes), coupled with a thermal conductivity detector (TCD). The column, injection and detector temperature was 50, 100, and 150 °C, respectively. Helium was used as the carrier gas at a flow-rate of 25 mL/min. The time needed for one GC run was about 3 min. XRD patterns of the slag before and after the catalytic reaction were recorded on a Rigaku D/max RBX X-ray diffractometer with Cu−Kα radiation (λ = 0.154 nm) at a scanning rate of 4°/min in the 2θ range of 5−90°. Surface areas were determined by the BET method using Micromeritics ASAP 2020. FTIR spectra were obtained on a Nicolet 380 spectrometer by using KBr method. The CO2-TPDs of slag before and after reaction were also recorded. The morphology and composition of the samples were examined in a HITACHI SU-1510 SEM equipped with EDS. The concentrations of F−, SO32‑ and SO42‑ in the gas stream-absorbed solution were determined by an ion chromatography (ICS 1100), which was equipped with suppressed conductivity detection system. Separation was achieved with IonPacCS11 column using 25 mmol/L NaOH at 1.2 mL/min as the eluent. Then the detected concentration (mg/L) was changed into the total mass amount (mg).
only fewer reports that investigated the metal recovery, such as nickel and chromium, from these wastes.22 As a result, the slag disposal does not only occupy a large area of lands, but also wastes the potential useful materials as potential hazardous materials to the environment. Thus utilization of stainless steel slag to decompose SF6 can recycle this solid waste into a useful material. Therefore, the objectives of this research were to (1) examine the feasibility of stainless steel slag in greenhouse gas SF6 decomposition, including effects of the reaction temperature and reagent particle size; (2) investigate the influence of water and oxygen on SF6 removal process as the SF6 gas stream often contains water and oxygen; (3) compare the activity of SSS with that of typical metal oxides in terms of the decomposition amount.
2. EXPERIMENTAL SECTION 2.1. Slag Reagent. The stainless steel slag used in this research was supplied by Baosteel Co., Ltd. Before being used in SF6 decomposition, the slag was first dried at 120 °C for 24 h. Then the dried slag was milled to get the particle portion through 32, 50, or 100 meshes, and stored in a desiccator for further use. Moreover, samples
