Kinetics of chalcopyrite leaching by sodium nitrate in sulphuric acid

Abstract

Hydrometallurgical processes offer a great potential for treating chalcopyrite ores and concentrates, and it results in increased metal recoveries and reduced air pollution hazards. In recent years interest for application of various reagents in a hydrometallurgical processing of sulphide concentrates has increased. The objective of this work was to examine the leaching of the chalcopyrite concentrate, from “Rudnik” flotation plant, by sodium nitrate in sulphuric acid media. The probabilities of various chemical reactions occurring was based upon literature data, thermodynamic analysis, and the products formed during the leaching process. The influence of various parameters was studied to optimize the conditions and to determine the kinetics of the reaction. An increase in temperature, sulphuric acid and sodium nitrate concentration, and decrease in stirring speed and particle size, enhanced the leaching rate of copper. The experimental data were best fitted by a mixed control kinetic model. An activation energy of 83 kJ/mol was found. Elemental sulphur was formed as the main leaching product and tended to inhibit the leaching rate. The leaching mechanism was confirmed by characterizing the concentrate and the leach residue using XRD and SEM/EDX analysis.

Introduction

Non-ferrous metals and iron usually appear in a form of sulphide complex ores. Large deposits of complex ores may contain chalcopyrite, sphalerite, galena and pyrite in disseminated form with complex mineralogical composition and fine grained structures. These minerals are generally separated from each other by flotation and treated by conventional pyrometallurgical processes. Sometimes, when it is difficult to prepare flotation concentrates of the individual minerals, then it is easier to prepare bulk concentrates (Vračar et al., 2003a). Although treatments by pyrometallurgical processes are not attractive, because large amounts of SO₂ are produced, approximately 80–85% of the world's total copper is produced pyrometallurgically (Antonijević et al., 2004).

Hydrometallurgical processes offer great potential for treating complex sulphide concentrates, especially chalcopyrite concentrate, resulting in increased metal recoveries and reduced air pollution hazards. The main outcomes are the production of elemental sulphur or sulphuric acid and soluble copper sulphate. Ferric and cupric ions, bacteria, oxygen, and other oxidants have been used as leaching agents of chalcopyrite in sulphate and chloride media under atmospheric or pressure leaching conditions (Prasad and Pandey, 1998, Majima et al., 1985, Dutrizac, 1981, Dutrizac, 1989, Dutrizac, 1990, Arslan et al., 2004, Havlik et al., 1995, Havlik et al., 2005, Mikhlin et al., 2004, Lu et al., 2000, Saxena and Mandre, 1992, Tchoumou and Roynette, 2007, McDonald and Muir, 2007, Akcil and Ciftci, 2003, Dreisinger, 2006, Hackl et al., 1995, Aydogan et al., 2006, Misra and Fuerstenau, 2005, Mahajan et al., 2007, Antonijević et al., 1994, Antonijević et al., 2004, Padilla et al., 2007, Al-Harahsheh et al., 2005).

Much attention has been given to the development of nitric acid based processes for complex concentrates; e.g. Habashi (1999) indicates that metal sulphide oxidation by nitric acid can be achieved in two ways. In the first case, NO₃⁻ ion is the oxidant, and during the reaction it is reduced to NO or NO₂. In the second case, oxygen that arises from nitric acid decomposition is the oxidant.

Some industrial operations use nitric or nitrous acid added in a small concentration to the sulphuric acid, e.g. in oxygen extended pressure leach processes. At Sunshine Precious Metals, silver and copper were recovered from a complex sulphide concentrate at the temperatures between 145° and 155 °C and at total pressure of 709 kPa (Ackerman et al., 1993, Anderson et al., 1992, Anderson et al., 1996). In this case, nitrous acid was found to enhance the solubilization of minerals at lower temperatures and pressures, and the nitrous/sulphuric acid leach process was used with success.

Following successful operation in the Sunshine Pressure Leach plant, the catalysis under the extended pressure oxidation, using nitrogen-containing species, has been promoted as a nitrogen species catalysed (NSC) technology (Anderson, 2003, Anderson et al., 1996). It was demonstrated that the addition of nitrite ion in small amounts catalyses the oxidation of sulphides in the presence of oxygen; recent data show that it is a fast reaction, typically taking less than 30 min, for slurry containing 100 g/L of solids. The leaching of metal sulphides using nitric acid as the oxidant is more efficient in the presence of NO⁺ ions. The addition of NO₂⁻ ions instead of NO₃⁻ ions accelerates the formation of NO⁺ ions, which further oxidizes sulphide minerals at lower temperatures to form elemental sulphur (Anderson et al., 1996, Fleming et al., 2001).

Oxidative dissolution of a sulphide concentrate using nitrate as the leaching agent in an acid medium takes place with formation of elemental sulphur, and it can be represented by one of the following chemical reactions (Bredenhann and Van Vuuren, 1999, Vračar et al., 2003b, Sokić et al., 2008):

3MeS + 2NO₃⁻ + 8H⁺ = 3Me²⁺ + 3S⁰ + 2NO + 4H₂O

or

MeS + 2NO₃⁻ + 4H⁺ = Me²⁺ + S⁰ + 2NO₂ + 2H₂O

Initially the rate of reaction is controlled by a surface chemical reaction and later, becomes diffusion controlled.

The dissolution mechanism of chalcopyrite is based on three main kinetic models. These kinetic models are diffusion controlled, surface reaction-controlled and a mixed kinetic model containing diffusion and surface reaction components, which simultaneously take place.

This study examines the leaching process of chalcopyrite concentrate from Serbian deposit using sodium nitrate in sulphuric acid as it has not been studied sufficiently either from the theoretical or practical point of view. Dutrizac (1982) observed significant differences ∼ 50% in leaching rates of eleven chalcopyrites from different localities under various leaching conditions. The scattering results can be mainly attributed to admixtures and impurities as well as to the influence of real structure of the chalcopyrite. For this purpose, the effects of different variables, such as particle size, stirring speed, temperature and acid and nitrate concentration, on the reaction rate were investigated to optimize the conditions and to determine the kinetics of the reaction. The kinetic data showed good fit to the mixed control model, and the rate was controlled by the surface reaction in the initial stage of reaction, and changed into a diffusion controlled mechanism in the latter stages of reaction.