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The reaction of enargite with alkaline NaClO solutions: an AFM and flow-through study

¹ Dipartimento di Scienze della Terra, Università di Cagliari, Via Trentino 51, 09127 Cagliari, Italy
² Institut de Ciències de la Terra "Jaume Almera" CSIC, Lluís Solé i Sabarís s/n, Barcelona 08028, Catalunya, Spain

^* Corresponding author, e-mail: elodiamusu{at}yahoo.it

The reaction of enargite, Cu₃AsS₄, with alkaline NaClO solutions is of interest for hydrometallurgical applications. In this study, the surface of cleavage fragments was observed ex situ by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) after sequential interaction (up to 240 min) with a 0.27 M NaClO/0.03 M NaOH solution (pH 12.2) at room temperature (295 ± 2 K). Likewise, the evolution of the reaction of enargite powder (25 < size fraction < 53 µm) interacting with the same solution was investigated in a flow-through experiment.

AFM images show the progressive formation of protrusions, preferentially along surface features such as steps and cracks. XPS analyses of the surface after 240 min reaction indicate the establishment of Cu(II)–O, As(III)–O and As(V)–O bonds. SEM images and EDS analyses show that a newly formed, highly porous phase is composed of a copper oxide, identified as tenorite (CuO) by X-ray diffraction.

In the flow-through experiment, arsenic and sulphur are released in nearly stoichiometric proportions (As/S molar ratio = 0.27 ± 0.01). Release of As and S decreases slightly with time, whereas Cu depletion in the output solution occurs, because of the formation of CuO. Based on sulphur and arsenic release, the steady-state dissolution rate (normalized to initial surface area) is estimated to be on the order of 10^–8 mol m^–2 s^–1.

Key-words: enargite, oxidative dissolution, alkaline solution, AFM, flow-through experiment, sulphur, arsenic.