Pyrite Passivation by Triethylenetetramine: An Electrochemical Study

Hindawi Publishing Corporation Journal of Analytical Methods in Chemistry Volume 2013, Article ID 387124, 8 pages http://dx.doi.org/10.1155/2013/387124 Research Article Pyrite Passivation by Triethylenetetramine: An Electrochemical Study Yun Liu,1, 2 Zhi Dang,2, 3 Yin Xu,1 and Tianyuan Xu1 1 Department of Environmental Science and Engineering, Xiangtan University, Xiangtan 411105, China The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, China 3 Higher Education Mega Center, School of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China 2 Correspondence should be addressed to Yun Liu; Received 24 November 2012; Accepted 29 December 2012 Academic Editor: Fei Qi Copyright © 2013 Yun Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The potential of triethylenetetramine (TETA) to inhibit the oxidation of pyrite in H2 SO4 solution had been investigated by using the open-circuit potential (OCP), cyclic voltammetry (CV), potentiodynamic polarization, and electrochemical impedance (EIS), respectively. Experimental results indicate that TETA is an efficient coating agent in preventing the oxidation of pyrite and that the inhibition efficiency is more pronounced with the increase of TETA. The data from potentiodynamic polarization show that the inhibition efficiency (𝜂%) increases from 42.08% to 80.98% with the concentration of TETA increasing from 1% to 5%. These results are consistent with the measurement of EIS (43.09% to 82.55%). The information obtained from potentiodynamic polarization also displays that the TETA is a kind of mixed type inhibitor. 1. Introduction Pyrite, FeS2 , is one of the most common sulfide minerals. It is frequently present in tailings, waste rock dumps, many valuable mineral raw materials, and coal [1]. It is easy to be oxidized under natural weathering conditions. The oxidation of pyrite results in sulfuric acid and toxic trace metals formation in acid mine drainage (AMD), which is one of the most serious environmental problems facing the mining industry [2]. That is why many studies have been carried out on the mechanism of pyrite’s oxidation during the last six decades by investigators from different areas, such as metallurgy and environment science [3–9]. Now researchers have found that the oxygen and ferric iron play a very important role for the pyrite’s oxidation [10, 11] and that some acidophilic microorganisms, for example, Acidithiobacillus ferrooxidans [12] and Leptospirillum ferrooxidans [13], can accelerate the oxidation of pyrite greatly. According to the knowledge of people for the mechanism of pyrite decomposition, if it is no contact between pyrite and oxidants (e.g., O2 and Fe3+ ), the rate of pyrite oxidation could be suppressed. For years, several techniques have been developed to reduce the oxidation of sulfide minerals, including bactericides [14], neutralization [15, 16], and cover treatment [17–19]. However, most of these technologies are costly, short-term solutions, and difficult to apply. In parallel, many researchers have used certain chemical reagents that can create effective oxygen barriers to protect the surface of iron sulfide from oxidation. For example, both iron phosphate precipitates and silica precipitates have been shown to suppress pyrite oxidation efficiently. However, these treatments require initial surface oxidation with hydrogen peroxide, which is difficult to handle in a real application [20, 21]. Similarly, although some passivating agents such as acetyl acetone, humic acids, ammonium lignosulfonates, oxalic acid, and sodium silicate also have the capability to inhibit pyrite from oxidation, these treatments also need peroxidation, and the coating with oxalic acid requires a temperature control at 65∘ C [22]. In addition, Elsetinow et al. [23] have concluded that the formation of a passivating layer on the pyrite surface after exposure to the lipid could suppress pyrite oxidation by either interrupting the advection of aqueous 2 oxidants or the electron transfer between oxidants and the pyrite. Using the property of formation of strong insoluble chelating complex with Fe3+ , Lan et al. [24] have investigated the possibility of using 8-hydroxyquinoline as a passivating agent, and they demonstrated that the oxidation rates of pyrite could be reduced remarkably. In recent years, our laboratory has also developed a new passivating agent: triethylenetetramine (TETA) [25]. Compared to the coating agents mentioned above, TETA is currently used in the floatation process of sulfide minerals in Inco Limited as depressant; therefore, it does not represent any extra cost. On the other hand, TETA is a base which can neutralize protons produced in the oxidation of the sulphide minerals. TETA has already been proved that it could retard the oxidation of pyrrhotite and need not initial oxidation [25, 26]. However, there is not date on the capability of TETA to passivate pyrite. In addition, all the studies cited above were carried out by the method of extraction using hydrogen peroxide or atmospheric oxygen as oxidant to test the coating effectiveness of different passivating agents. These processes usually require long times, and, moreover, the operation is complicated as the quantity of dissolved metal ions need to be monitored by techniques such as spectrophotometer [23]. As the simplicity, efficacy, and low cost of these methods, electrochemical techniques are used extensively to investigate the corrosion of steel [27–30]. Nowadays, electrochemical techniques have been becoming essential measurements to evaluate the effect of inhibitors on the corrosion inhibition of steel. Although pyrite is not a very good electrical conductor, its oxidation is usually described in terms of electrochemical corrosion mechanisms developed for metals [31, 32]. Therefore, electrochemical methods can be chosen to study the corrosion inhibition behavior of passivating agents on pyrite. The main aim of this study is to test the coating effectiveness of triethylenetetramine (TETA) on pyrite using the open-circuit potential (OCP), cyclic voltammetry (CV), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). 2. Experimental Methods 2.1. Mineral Samples Preparation. Natural pyrite was obtained from the Dabaoshan sulfur-polymetallic mines in the north of Guangdong Province, China. Its chemical composition analysis by X-ray fluorescence (XRF) is listed in Table 1. The XRD pattern (Figure 1) of the crushed sample is typical that was expected for pyrite and showed that it was including trace of quartz. The material was ground with an agate mortar and then sieved to isolate particles with a diameter of less than 75 𝜇m an (...truncated)