Selective dissolution of copper and iron from molybdenite concentrate using acidic sodium nitrate solution

Highlights

• Using the chemical purification for upgrading content of molybdenite concentrate.

• Solid: liquid phase ratio significantly affects Cu and Fe dissolution.

• Cu and Fe in molybdenite concentrate were dissolved with acidic NaNO₃ solution.

• Other metals extraction in acidic NaNO₃ solution does not affect the oxidation of MoS_2.

• The copper dissolution from the molybdenite is controlled by a mixed reaction model.

Abstract

In this study, the selective dissolution of copper and iron from the molybdenite (MoS₂) concentrate consisting of chalcopyrite and pyrite by acidic sodium nitrate leaching has been investigated to improve the quality of molybdenite. The influence of various leaching parameters, namely sodium nitrate (NaNO₃) and sulfuric acid (H₂SO₄) concentration, temperature, contact time, and solid: liquid phase ratio, was examined on the dissolution of copper (Cu) and iron (Fe) from the concentrate. Under the optimized conditions, the extraction of Cu and Fe can achieve 81.4% and 74.1%, respectively, along with a minor amount of molybdenum (Mo) loss in the solution from the concentrate in 240 min leaching. The kinetic study indicated that the impurities removal process was controlled by a mixed mechanism with a corresponding activation energy of 35.77 kJ/mol at the temperature range of 70–97 °C. The XRD and SEM-EDS analysis of the solid residues from the leaching revealed the insolubility of molybdenite in the leaching media. As a result, the molybdenite grade reached 90.73 wt% from an initial value of 81.33 wt%.

Introduction

The global demand for molybdenum and its compounds is growing due to their applications in the chemical, metallurgical, engineering, and petroleum industries. Molybdenum and its marketable products, which are critical raw materials for manufacturing alloys, steels, pigments, smoke suppressants, semiconductors, biosensors, catalysts, and nanomaterials, are suitably produced from only molybdenite (MoS₂) by pyro- and hydrometallurgical approaches (Gupta, 2003, Gupta et al., 2020). Although molybdenum is present in various minerals, particularly molybdenite, wulfenite, powellite, and ferrimolybdite, its most crucial ore source is the mineral molybdenite. Molybdenite is separated from porphyry molybdenum ores and copper-molybdenum sulfide ores by froth flotation. Hence, molybdenite concentrate contains copper minerals, which include chalcopyrite (CuFeS₂), chalcocite (Cu₂S), covellite (CuS), and other minerals such as pyrite (FeS₂) and quartz (SiO₂) (Finch, 2015, Gupta, 1992).

The market value of molybdenite concentrate depends on its purity and amount of impurity constituents, including copper, iron, and other metals in the concentrate. The highest acceptable levels of copper and iron in the molybdenite concentrate are 0.5% and 0.25%, respectively. However, it is challenging to achieve such acceptable levels by employing the conventional froth flotation process. On the other hand, high-quality MoS₂ can be produced using numerous synthesis approaches, including the reaction of sulfur vapor with molybdenum, heating sulfide in the absence of air, and melting soda (Na₂CO₃) and sulfur with MoO₃. However, the lubricity of synthesized MoS₂ is lower than the purified MoS₂ from molybdenite concentrate using leaching processes (Kirk-Othmer, 1993, Lansdown, 1999). Therefore, using hydrometallurgical techniques to diminish copper impurities, a high-grade marketable level of MoS₂ can be obtained from the molybdenite concentrate (Braithwaite and Haber, 1994). Significantly, the highest purity (>98%) of MoS₂ is the leading solid lubricant, which is widely used in metallurgical applications to reduce wear and friction coefficient, particularly for high-temperature and high-pressure conditions in the industries (Lansdown, 1999); Sebenik et al., 2000).

Many studies have been performed to dissolve impurity components from the molybdenite concentrate using various lixiviants. Jennings et al. (1973) established a practicable mixed chloride (30% CaCl₂ − 1% CuCl₂ − 10% FeCl₃) leaching process for preparing high-grade MoS₂ from molybdenite concentrate. Their study resulted in 98% copper and 98% lead, and 79% calcium impurities dissolution from the molybdenite with the mixed chloride solution. Zhang et al. (2010) investigated the copper leaching kinetics from chalcopyrite in the molybdenite concentrate under acidic FeCl₃ solution in the presence of FeCl₂. Meanwhile, they concluded that at 100 °C, about 80% of Cu was dissolved from the concentrate, and a chemical surface reaction was proposed to control the leaching process due to an activation energy of 79 kJ/mol. Ruiz and Padilla (1998) presented 95% Cu dissolution from molybdenite concentrate by leaching with 0.3 M H₂SO₄ and 0.12 M Na₂Cr₂O₇ solution at 373 K. Padilla et al., 2013a, Padilla et al., 2013b investigated the kinetics of copper dissolution from molybdenite concentrate with a combined sulfidation and leaching process. They used the leaching by mixed solution (H₂SO₄-NaCl-O₂) at 100 °C and found that Cu removal efficiency from sulfidized concentrate at 380 °C with gaseous sulfur was 96%. In addition, the authors (Padilla et al., 2014) also studied the removal of copper from sulfidized molybdenite concentrates by pressure leaching in sulfuric acid-oxygen media. Results showed that copper dissolution was achieved at 97% at 140 °C, oxygen pressure of 1013.25 kPa, and 0.2 M H₂SO₄ concentration by controlling a surface chemical reaction.

Numerous bioleaching studies (Abdollahi et al., 2014, Abdollahi et al., 2013, Askari Zamani et al., 2006, Romano et al., 2001) have indicated that about 50–100% of copper was dissolved from molybdenite concentrate through 0.5–3 months and at 35–68 °C by using different acidophilic types of bacteria in Erlenmeyer flask/equipment. In addition, the leaching process of molybdenite concentrate containing 55.1% Mo has demonstrated the possibility of refining concentrate via NaNO₃-HCl-HNO₃ solution to dissolve metal impurities (Guo et al., 2018). Under optimum conditions, Mo content has enhanced up to 59% after 10 hrs of leaching. The two processes above-mentioned typically demand a prolonged time to dissolve copper and other metal sulfides from molybdenite concentrate.

In particular, sodium nitrate is a strong oxidant that can easily leach copper sulfide minerals, including chalcopyrite, due to its high oxidative potential between +0.79 V and +1.25 V (Anderson, 2013, Habashi, 1999, Prasad and Pandey, 1998). While during the dissolution of sulfide minerals by using nitrate ion in an acidic medium as the leaching agent, the formation of elementary sulfur occurs. (Sokić et al., 2009, Vračar et al., 2003). Nevertheless, Cu removal from molybdenite concentrates by sodium nitrate leaching has not been sufficiently investigated. MoS₂ is difficult to react with NaNO₃-H₂SO₄ solution because of its chemically inert and resistance properties to attack by most common acids (Lansdown, 1999). This investigation focused on a method of removing Cu and Fe from the molybdenite concentrates without breaking the layer-lattice structure of MoS₂.

This study aims to improve the selective dissolution of Cu and Fe from chalcopyrite in the molybdenite concentrate and enhance MoS₂ grade using acidic sodium nitrate leaching. The influential leaching parameters, including leaching temperature, NaNO₃ concentration, H₂SO₄ concentration, and solid to liquid (S:L) phase ratio, are investigated under various intervals. The leaching kinetics of copper from molybdenite concentrate is also discussed in this paper.