Simultaneous recovery of copper and manganese from exotic copper ore in acid chloride media using cane molasses as the reducing agent

Highlights

• The presence of molasses provides a suitable reductant to leach the Mn phase.

• Addition of chloride ions enhances the dissolution of exotic-Cu ore.

• The use of cane molasses increases the Mn dissolution.

• Chloride ions enable the Cu(II)/Cu(I) couple to be a redox shuttle/catalyst.

Abstract

Chile is host to one of the most considerable Cu resources in the world. The primary source of Cu is from porphyry deposits. However, associated “exotic-Cu” deposits are also significant in the Atacama Desert of northern Chile. The critical processing issue for these ores is that they are refractory to dissolution under acidic and oxidative conditions, and consequently their dissolution kinetics are relatively slow. This paper reports a study on the extraction of copper and manganese from exotic-Cu ores using cane molasses as a reducing agent in the presence of chloride ions and diluted sulfuric acid media. The effects of concentrations of cane molasses, chloride ion, and sulfuric acid are discussed. Leaching tests on ground ore in shake flasks were studied, as well as the agglomeration and curing stages of a coarse ore sample. The dissolution of the exotic copper ore is enhanced when 50 g/L of NaCl and 60 g/L of cane molasses are added to the leaching solution. The dissolution of copper associated with copper wad appears independent of the Mn dissolution. Tests with the ore agglomerated with acid, chloride ions, and cane molasses showed enhanced dissolution of manganese contained in the copper wad, but not copper. It is thus confirmed that cane molasses is a suitable reductant in the leaching of Mn, especially in the presence of chloride ions. The behavior and fate of the wad-associated copper requires further investigation.

Introduction

Chile has one of the largest Cu resources in the world. The major source of Cu is from porphyry deposits, but associated “exotic-Cu” deposits are also significant. Such deposits represent supergene mineralization and comprise different species of copper minerals and mineraloids broadly defined as “green‑copper” and “black‑copper” ores (Chavez, 2000). The species that commonly comprise the “green‑copper” ore are chrysocolla, atacamite, antlerite, chalcantite and brochantite, whereas the “black‑copper” ores include copper-pitch (hydrated Cu, Mn, and Fe silicate), copper wad (Mn oxides and Cu hydroxides) and neotocite (hydrated oxides of Mn and Fe with scattered Cu silicates). Copper wad is commonly amorphous, showing characteristics similar to cryptomelan e (K_1-2(Mn³⁺ Mn⁴⁺)₈ O16 XH₂O), birnessite (K_0.33 Mn^3.9+₇O₁₄.7H₂O), and crednerite CuMnO₂ (Mote et al., 2001; Cuadra and Rojas, 2001). The colloform textures and different colours on the banded textures (see Figure1) are due to different mineral species precipitating at different stages during the formation of the exotic‑copper deposit. Copper pitch or copper wad is normally the first mineral species to precipitate and then fluid composition and environmental condition changes to allow the precipitation of chrysocolla (Menzies et al., 2015; Campos et al., 2015) .

Exotic‑copper deposits form when topographically driven groundwater flow promotes the lateral migration component of Cu-bearing solution, where copper oxide minerals will be deposited in the surrounding paleo drainage network as interstitial cement in permeable gravel sequences or impregnation along fractures and pores in older basement rocks (Münchmeyer, 1994, Münchmeyer, 1996). The best-known exotic-Cu deposits are found in the Atacama Desert of northern Chile. Radomiro Tomic, El Tesoro, Spence, Mina Sur, and Lomas Bayas are some key examples (Münchmeyer, 1996). Numerous porphyry copper deposits have areas of supergene enrichment, and in turn, are associated with bodies of exotic-Cu mineralization (Sillitoe, 2005). These exotic-Cu deposits cover large areas and are an economically important copper reserve that can extend the mine life of porphyric copper deposits or be a potential option for recovering manganese in Chile. Manganese plays an important role in fields as steel production, fertilizers, fine chemicals, dry batteries and medicines (Sahoo et al., 2001; El-Hazek et al., 2006). As rich Mn ores are exhausted gradually, many studies have paid attention to developing an economical commercial hydrometallurgy process from low-grade Mn ores to satisfy the increasing demand for this element (Yan and Gao, 2009; Zhu, 2016).

Dissolution of copper from exotic‑copper deposits has slow kinetics, mainly due to the Mn compounds that are impregnated along fractures and pores being stable in acid or alkaline under oxidizing conditions. Therefore, the dissolution of Mn must be carried out under reducing conditions (Tian et al., 2010a; Hariprasad et al., 2007).

To overcome the refractoriness of these exotic-Cu ores, the focus has to be to first get rid of the manganese to liberate the associated copper contained in the ore followed by acid leaching. Manganese ores have been treated by reduction roasting followed by acid leaching (Sahoo and Srinivasa, 1989). However, according to Ismail et al. (2004), this method requires over 800 °C as reaction temperature, which is high and has an associated high-energy consumption and carbon footprint.

Manganese ores can be leached directly using different acidic solution, which include iron (II) sulfate (Das et al., 1982), sulfuric acid with oxalic acid (Sahoo et al., 2001), aqueous sulfur dioxide (Naik et al., 2003), and acidic hydrogen peroxide media (Jiang et al., 2004). Various studies have investigated reductive acid leaching of manganese using reducing agents such as metallic iron (Mohammad et al., 2008), lactose (Ismail et al., 2004) and tea waste (Tang et al., 2014). Also cane molasses (Su et al., 2008), wastewater from cane molasses alcohol (Su et al., 2009), cane molasses with nitric acid (Lasheen et al., 2009), sawdust (Hariprasad et al., 2007) and corn cob (Tian et al., 2010a) have been successfully employed. According to Su and co-workers (Su et al., 2008; Su et al., 2009) cane molasses, a by-product of sugar manufacture generally contains between 25 and 35% of cane sugar, between 15 and 25% of invert sugar and 11% of colloidal materials. Molasses is the byproduct from a sugarcane factory or raw sugar refinery; it is the heavy, dark viscous liquid remaining after the final stage of sugar crystallization from which no further sugar can be crystallized economically by the usual methods. It is a low-cost rich resource, containing renewable and non-hazardous reducing agents compared to other available raw materials that can be used for manganese leaching under mild acidic conditions.

The use of acid-chloride media to leach copper ores is well established and is suitable to dissolve refractory minerals such as chalcopyrite (Velásquez-Yévenes et al., 2010a; Velásquez-Yévenes et al., 2010b; Velásquez-Yévenes and David Torres, 2018; Velásquez-Yévenes and Quezada-Reyes, 2018; Nicol et al., 2010). The leaching of copper ore in chloride media presents many advantages in hydrometallurgical processes over the conventional sulfate systems. In chloride systems the copper(I) ion is stable (Watling, 2013), and this allows the use of copper(II) ions as an oxidant in addition to iron(III) (Velásquez-Yévenes et al., 2010a). In some cases, greater leach kinetics in a chloride system can also be caused by the enhanced proton activity (Senanayake, 2007), in that the addition of chloride solvates free water, causing an increase in the activity of protons (Muir, 2002).

In a heap leaching process, agglomeration and curing time can be crucial for the success of the subsequent leaching. The addition of a leaching solution in the agglomerating stage can facilitate the attachment of fine to coarse particles to increase the permeability of the heap. Curing leads to a homogeneous distribution of the acid in the ore bed along with a greater porosity (Cruz et al., 1980; Jansen and Taylor, 2003). Curing also can inhibit the dissolution of some silicate impurities and accelerates copper extraction.

This work uses the experience of agglomeration, curing and leaching of a refractory copper ore in acid-chloride media to investigate a new leaching process for exotic-Cu ores from the Atacama Desert, but now with the addition of an organic reducing agent as cane molasses.