Rapid reduction of copper sulfide (Cu2S) with elemental Fe and Mg using electrical discharge assisted mechanical milling (EDAMM)

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Abstract

The electric discharge assisted mechanochemical milling technique has been applied to the reduction of copper sulphide using either elemental magnesium or elemental iron as the reducing agent. X-ray diffraction was employed for phase identification of starting materials and reaction products. After only 5 min processing using the Electric discharge assisted mechanical milling (EDAMM) method, Cu2S + Mg can be fully reduced to Cu + MgS with small amount of CuO, and Cu2S + Fe can be partially reduced to Cu + Cu5FeS4 (bornite). Secondary electron imaging combined with X-ray mapping was used to investigate the distribution of elements within the processed powder particles. In the case of the Mg reduced sample the product comprised large Cu grains of purity >98%, surrounded by regions of agglomerated, small particles of Cu and MgS. In the case of the Fe partially reduced sample the product comprised a mixture of Cu plus Fe phases, frequently surrounded by regions containing the bornite phase.

The morphological evidence of the existence of deformed and some re-melted particles was consistent with arguments that high local temperatures can be achieved during EDAMM processing. However the SEM results also indicated that the primary reduction reaction mechanism involved the formation of agglomerates of small particles that did not exhibit melting. Further experiments under impulse plasma mode, both with and without conventional milling applied, demonstrated that a critical condition for the reduction reactions to occur is not based on temperature alone, but on how long the particles are processed within the in the gap between the milling rod and the base, under what may be described as Dusty Plasma conditions.

Introduction

Advanced materials manufacturing methods for the future are required to be clean, non-polluting, high speed and precision processes; producing highly reliable final products. Many materials are traditionally synthesized by slow chemical reaction processes that require capital and labour intensive facilities, and are both energy and time consuming. In the present world there is strong demand for the development of modern materials and materials processing methods that could offer rapid reaction rates, high energy efficiencies, and be environmentally safe. Electric discharge assisted mechanical milling (EDAMM) is a new and exciting materials processing technique which combines the attributes of conventional mechanical milling with a range of new effects associated with electric discharges [1], [2]. EDAMM involves mechanical milling and simultaneous application of an electrical discharge during processing of powder particles; one of the most important factors of this process being the formation of a dusty plasma environment between the powder particles which leads to formation of ions and free radicals. In addition, the powder particles are constantly being mixed and fractured by the vibrating electrode. All these factors contribute to reactivity enhancement and have the potential to drive the replacement reactions at conditions which are either not possible or require vigorous conditions in conventional processing routes. In this work, we study the application of EDAMM to generate replacement reactions on a laboratory scale. We have taken as a model reaction the displacement of Cu from Cu2S by metals belonging to higher positions in the electrochemical series; in the case of this investigation, Mg and Fe. The mechanism of chemical reactions between powder particles surfaces induced by an electric discharge is complex and poorly understood. It is therefore an additional aim of this work to present experimental results that shed light on factors related to EDAMM that induce chemical reactions.

Section snippets

Experimental

Cu2S powders were mixed with Fe and Mg in proportions required in order to obtain stoichiometrically the complete reduction of Cu2S to Cu + FeS2 or Cu + MgS, respectively. These starting powder mixtures were first mechanically pre-mixed in a conventional ball mill for 1 h under a high purity argon atmosphere. This pre-mixture was then subjected to 5 min of electric discharge assisted ball milling (EDAMM). Electric discharge milling was performed in a modified vibrational laboratory rod mill. The mill

Cu2S–Fe

X-ray diffraction of the pre-mixed Cu2S (chalcosine) and Fe powders, and the same mixture given a 5 min EDAMM treatment are shown in Fig. 2. After 1 h of pre-mixing in conventional ball mill there was no reaction detected between the Cu2S and the Fe (Fig. 2a). After 5 min of EDAMM reaction of Cu2S with elemental iron occurred and the following products were identified by XRD: pure Cu, the bornite phase (Cu5FeS4) and some unreacted iron (Fig. 2b, sample EDAMM processed in Ar for 5 min). SEM

Conclusions

XRD, SEM and EDS and X-ray mapping results demonstrated that Cu2S can be reduced to pure copper by EDAMM, using either Mg as the reducing agent, and partially reduced to and Cu2S + Cu5FeS4 using Fe as the reducing agent. More generally, it is demonstrated that EDAMM can be employed to generate replacement reactions in a matter of minutes, rather than hours or days. Morphological evidence of the existence of deformed and remelted particles was consistent with arguments based on thermodynamic

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