Mechanochemical reduction of natural pyrite by aluminum and magnesium
Introduction
In mechanochemistry, mechanical energy was applied for reactivity enhancement, performance of alloying and synthesis processes. The processes have widely used in various fields such as hydrometallurgy and preparation of advanced and nano structured materials [1]. There are different methods for preparation of nano structured materials and among them, mechanochemical synthesis of nano materials are often faced intrinsic problems due to its top-down nature. A part of the problem arises from agglomeration phenomenon when ultrafine particles with high surface energy aggregate and stick to each other, hindering formation of nano particles. Also, due the exerted impact during intensive milling, the solid composite phases produced during mechanochemical reactions are not well separable when needed. Although many efforts were conducted to solve the problems by using additives and selection of proper mechanochemical reactions, the agglomeration and separation problems are essential and create general challenges in mechanochemical preparation of nano structured materials [1], [2], [3]. The complete reaction between reactants during mechanochemical reaction is first criterion to find a solution for the problems, especially the separation problem. The mechanically-induced self-propagating reaction (MSR) is a mechanochemical reactions type with high potential capability to proceed completion. In the MSR reactions, the ratio of standard reaction enthalpy to product heat capacity (ΔH/ΔCP) is higher than about 2000 K [4], [5]. The displacement type of MSR reactions are known as a powerful tool for reduction of sulfide compounds during metal-sulfide mechanochemical reaction. The reactive metals such as Al and Mg are supposed to reduce the less reactive metals in sulfide compounds [4]. Nevertheless, over the last years, many investigations were developed for mechanochemical reactions wherein advanced nano structured materials were prepared [2], [3], [4], [5], [6], [7], [8].
Preparation of nano zero-valent Iron (NZVI) is classified in nano scale environmental technology due to appreciable capability of NZVI for removing of various contaminants such as chlorinated organic solvent, organochlorine pesticides, organic dyes, various inorganic compounds and metal ions such as Pb (II), As(III), Cu(II) [9]. Various chemical methods have been used for preparation of NZVI generally based on reduction of ferric or ferrous iron from aqueous solutions by using related reducing agents such as sodium borohydride [10]. Due to the chemical procedure performed in aqueous media, the NZVI surface were oxidized converting to iron oxide nano powders [9], [10], [11]. Although in most cases, it has been tried to eliminate oxygen during preparation operation and maintenance condition, the NZVI powders have oxidized. Therefore, the mechanochemical synthesis methods having tougher controlled atmosphere (reaction media) can be employed to suppress oxidization process during NZVI synthesis. Regarding to existed solid and natural sources of iron, iron sulfides such as pyrite are suggested as one of the best nominates for preparation of NZVI by using mechanochemical methods.
It should be noted that natural pyrite (FeS2) with two main constituents of iron and sulfur is the most common gangue mineral associated with valuable minerals in ore deposits and it can be processed easily by conventional flotation methods. The pyrite concentrate is a refractory concentrate due to difficulty in its treatment [12], [13], [14], [15]. The gangue pyrite disposal is unavoidable in mineral processing plants raising environmental concerns. Previous investigations indicated that pyrite can be used as an adsorbent [15], [16], [17], precursor for superconductor preparation, high-energy density batteries [18] and photocatalyst [19], opening new opportunities for the gangue mineral of pyrite to be applied. The applications of pyrite have advantages e.g. it is cost-efficient, abundant and easier to handle due to its simple chemical composition.
Regarding to the facts that the application of pyrite as an initial source for preparation of NZVI is not reported, the present study aimed to investigate mechanochemical reduction of pyrite by reactive metals of aluminum and magnesium metal powders as first step for preparation of NZVI from natural pyrite by using mechanochemical methods. The progress of mentioned MSR reactions are evaluated using X-ray diffraction analysis and an attempt is exerted to complete at least one of them.
Section snippets
Materials
A high-purity pyrite concentrate was obtained using conventional beneficiation methods. XRD analysis of the pyrite concentrate disclosed that the pyrite reflections solely corresponded with JCPDS No. 1-1295 (Fig. 1).
The aluminum (Merck 1.01056.0250) and magnesium (Merck art 5815) metal powders were used for mechanochemical reaction of pyrite as will be described in the following section.
Mechanochemical reactions
Mechanical reactions of pyrite was conducted in a planetary ball mill (Pulverisette 6, FRITSCH, Germany) in a
Mechanochemical reaction of pyrite with Mg metal powders
To face disadvantageous impact of high-energy ball milling process such as high energy consumption and low industrial capacity, the milling times were considered less than 5 h in the primary mechanochemical reaction test of pyrite. The XRD patterns of the samples obtained during mechanochemical reaction of pyrite with magnesium are displayed in Fig. 2, Fig. 3, Fig. 4, Fig. 5. To provide clear information about mechanochemical reaction of pyrite and Mg, first and final samples of the
Conclusion
As a main achievement, the ZVI-aluminum sulfide compound was prepared through co-milling of pyrite with aluminum for 120 min during a mechanochemical reaction. In addition, the mechanochemical treatment of natural pyrite with metal reactants of aluminum and magnesium proved:
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The mechanochemical reaction of pyrite and magnesium could not be complete in short period of milling. The result confirmed the refractory structure of pyrite resisting even in intensive milling conditions in the MSR
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