**1. Introduction**

Copper mining is Chile's most important economic activity, accounting for 10% of the gross national product (GNP) [1]. According to the latest figures from the Chilean Copper Commission, 5.83 million metric tons of copper were produced in 2018, making Chile the leading copper producer, accounting for 27.7% of global copper production. Experts from the Chilean Association of Geologists have stated that Chile has the largest copper deposits in the world [2], with a total copper reserve of 170 million metric tons [3].

Porphyry minerals in deposits like pyrite oxidize when submitted to geological agents. When pyrite reacts with water, it generates sulfuric acid, promoting the mobility of metals like copper that can be transported under certain potential and pH conditions, precipitating downstream and forming what are termed exotic deposits [4–8].

These deposits are composed of different copper containing phases such as chrysocolla, atacamite, copper pitch, and copper wad [6,9]. The latter two are defined as mineraloids because they crystalize amorphously [2]. They are also termed silicates rich in Si-Fe-Cu-Mn [10].

Some examples of exotic deposits in Chile are Mina Sur in Chuquicamata [11], Damiana in El Salvador [7], Huanquintipa in Collahuasi [12], and La Cascada, Lomas Bayas Spence, El Tesoro [2], and Angélica in Tocopilla [13]. The copper and manganese of this type of deposit are often associated with oxidized minerals, mainly chrysocolla, which, in turn, are associated with gangue that can negatively affect leaching [11]. Silicates and aluminosilicates, like mica and clay minerals, have the capacity to consume some of the acid generated by oxidization [14]. Clay minerals, like montmorillonite, kaolinite, and smectite, easily absorb acid [15]. Other minerals, like chlorites and biotite, also consume large amounts of acid over the long term [15]. Helle et al. [11] studied the effect of gangue and clay minerals on the leaching of copper oxides such as atacamite, chrysocolla, and malachite. The copper oxides were treated with a strong solution of sulfuric acid (265 g/L) in small columns at ambient temperature (18 to 21 ◦C), with the addition of synthetic rocks composed of 57% quartz, 1% phase mineral, and 42% reactive gangue. The authors concluded that copper retention and acid consumption were the result of the presence of smectite, mordenite gangue, kaolinite, illite, and quartz.

Researchers have indicated that it is not possible to recover copper associated with these silicates using conventional hydrometallurgical methods for oxidized copper because of their non-crystalline or amorphous structure [16]. However, recent studies on techniques for extracting manganese have found that silicates can be recovered by treating them in a similar manner to treatment for manganese, owing to the similarity in their metallurgical behavior [17].

It has been demonstrated that a reducing agent is required to extract Mn from MnO2 in acid media [18,19]. Other studies have obtained good results dissolving MnO2 with different reducing agents like H2SO3 [20], SO2 [21], wastewater from producing molasses-based alcohol [22], and various iron-based reducing agents [20,23,24]. Iron, which is abundant and inexpensive, has proven to be a good alternative when working with MnO2 in acid media.

Zakeri et al. [25] obtained an Mn extraction rate of 90% in 20 min at ambient temperature with the addition of ferrous ions to the system, with an Fe2+/MnO2 molar ratio of 3.0 and H2SO4/MnO2 ratio of 2.0. They proposed the following series of reactions for MnO2 dissolution:

$$\rm{MnO\_2 + 4H^+ + 2e^- = Mn^{2+} + 2H\_2O} \tag{1}$$

$$\text{2Fe}^{2+} = \text{2Fe}^{3+} + \text{2e}^{-} \tag{2}$$

$$\text{MnO}\_2 + 2\text{Fe}^{2+} + 4\text{H}^+ = \text{Mn}^{2+} + 2\text{Fe}^{3+} + 2\text{H}\_2\text{O} \tag{3}$$

Toro et al. [26] leached Mn nodules using tailings with high Fe3O4 contents (58.52%) from slag flotation for the recovery of Cu from the Alto Norte Foundry Plant and optimized the working parameters (Fe2O3/MnO2 ratio and H2SO4 concentration). They found that for short periods of time (5 to 20 min), the optimal MnO2/Fe2O3 ratio is 1/3, with H2SO4 concentration of 0.1 mol/L, giving Mn extraction rates of approximately 70%. The authors proposed the following reactions to dissolve MnO2 with the addition of iron oxides:

$$\rm{Fe\_2O\_{3(s)} + 3\ H\_2SO\_{4(aq)} = Fe\_2(SO\_4)\_{3(s)} + 3\ H\_2O\_{(l)}}\tag{4}$$

$$\rm Fe\_3O\_{4(s)} + 4H\_2SO\_{4(l)} = FeSO\_{4(aq)} + Fe\_2(SO\_4)\_{3(s)} + 4\ H\_2O\_{(l)}\tag{5}$$

$$2\text{ FeSO}\_{4(aq)} + 2\text{ H}\_2\text{SO}\_{4(aq)} + \text{MnO}\_{2(s)} = \text{Fe}\_2(\text{SO}\_4)\_{3(s)} + 2\text{ H}\_2\text{O}\_{(l)} + \text{MnSO}\_{4(aq)}\tag{6}$$

The following reactions are proposed to dissolve manganese from black copper:

$$\text{(CuO} \times \text{MnO}\_2 \times \text{TH}\_2\text{O)}\_{\text{(s)}} + 3\text{ H}\_2\text{SO}\_{4\text{(aq)}} + 2\text{ FeSO}\_{4\text{(aq)}} = \text{Fe}\_2(\text{SO}\_4)\_{\text{(aq)}} + \text{MnSO}\_{4\text{(aq)}} + \text{CuSO}\_{4\text{(aq)}} + 10\text{ H}\_2\text{O}\_{(l)}\tag{7}$$

$$\text{H}\_2\text{(CuO} \times \text{MnO}\_2 \times 7\text{H}\_2\text{O)}\_{(8)} + 11\text{H}\_2\text{SO}\_{4(aq)} + 3\text{Fe}\_3\text{O}\_{(8)} = 3\text{ Fe}\_2\text{(SO}\_4)\_{3(aq)} + \text{MnSO}\_{4(aq)} + \text{CuSO}\_{4(aq)} + 18\text{ H}\_2\text{O}\_{(l)}\text{}^{-1}\text{(aq)}^{-1} \text{ (from equation (1))}$$

Equation (5) gives the reaction of magnetite with sulfuric acid forming ferrous sulfate, which is a good reducing agent for the leaching of MnO2. This is shown in Equation (6), where Mn4<sup>+</sup> is reduced to Mn2<sup>+</sup>. In Equation (7), the solution of manganese from black copper (copper wad) is proposed, using ferrous sulfate expressed in Equation (5). In general, Equation (8) represents the dissolution of manganese with iron oxide as a reducing agent, which demands high concentrations of sulfuric acid to first form FeSO4 from Fe3O4 and then continues to dissolve manganese until a manganese sulfate solution is obtained.

In Chile, big copper mining poses new challenges and needs. It seeks to diversify the extractions of other elements (besides the Cu) in order to boost the export of commodities and raise employment. Black copper ores are resources that are generally not incorporated into the extraction circuits or left untreated, whether in stock, leach pads, or waste [27]. These exotic minerals have considerable amounts of Mn (approximately 29%), which represent a commercial appeal. Besides, according to the study conducted by Benavente et al. [27], by dissolving black copper ores in a reducing condition, the decrease in redox potential favors the dissolution of manganese. This would allow the subsequent extraction of the Cu present in black copper, given the potential commercial value of these "wastes".

This work aimed to study the dissolution of MnO2 from black copper in acid media comparing the use of iron and iron oxide tailings as reducing agents.

#### **2. Methodology**

#### *2.1. Black Oxide Samples*

Two samples of black copper, obtained from different mines in northern Chile, were used in this investigation. One sample, black copper sample-1 (BCS-1), was from a high-grade vein and was almost 100% pure, while the other, black copper sample-2 (BCS-2), was low-grade and taken from the mine dumpsite. The black oxides ores were ground in a porcelain mortar to sizes ranging from −173 to +147 μm. Chemical composition was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Table 1 shows the chemical composition of the samples. A QEMSCAN analysis was applied, which is an electronic scanning microscope that was modified both in hardware and software. This performed the identification and automated quantification of ranges of elementary definitions that can be associated with inorganic solid phases (minerals, alloys, slags, etc.). To determine the mineralogical composition, the samples were mounted on briquettes and polished. The identification, mapping of 2-D distribution, and quantification of inorganic phases, was done by combining the emissions of retro-dispersed electrons (BSE) with a Zeiss EVO series, a Bruker AXS XFlash 4010 detector (Bruker, Billerica, MA, USA) and the iDiscover 5.3.2.501 software (FEI Company, Brisbane, Australia). The QEMSCAN analyses are based on the automated obtaining of EDS spectra (dispersed energy from X-rays) in hundreds of thousands or millions of collected analysis points, each in a time of milliseconds. The classification of mineralogical phases is done by classifying each EDS spectrum in a hierarchical and descending compositional list known as the "SIP List". The BSE image is used to discriminate between resin and graphite in the sample, to specify entries in the SIP list, and to establish thresholds for acceptance or rejection of particles. As a result, pixelated, 2-D and false color images of a specimen or a representative subsample of particles are obtained. Each pixel retains its elementary and BSE brightness information, which allows subsequent offline data processing. Through software, customized filters are generated that allow the quantification of ore and gangue species, mineral release, associations between inorganic phases, and the classification of particles

according to criteria of shape, size, texture, etc. Figure 1 shows the chemical species to black oxides using QEMSCAN.

**Table 1.** Chemical composition of black oxide samples.

Table 2 shows the mineralogical composition of the black copper samples. Copper wad refers to a subgroup of copper composed of manganese and copper hydroxides, as well as also traces of other elements such as Co, Ca, Fe, Al, Si, and Mg.


**Table 2.** The mineralogical composition of the black copper samples as determined by QEMSCAN.

#### *2.2. Ferrous Ions*

The ferrous ions used for this investigation (FeSO4 × 7H2O) were WINKLER brand, with a molecular weight of 278.01 g/mol.

**Figure 1.** Detailed modal mineralogy.

#### *2.3. Iron Oxide Tailings*

The iron oxide tailings used were from the Altonorte Smelting Plant. The particle sizes were in a range between −75 to +53 μm. The methods used to determine its chemical and mineralogical composition were the same as those used in black copper ores. Table 3 shows the minerals (and chemical formulas) from QEMSCAN analysis, noting that several iron-containing phases were present from which the Fe content was estimated at 41.9%. As the Fe was mainly in the form of magnetite, the most appropriate method of extraction was the same as that used by Toro et al. [26].


**Table 3.** Mineralogical composition of tailings, as determined by QEMSCAN.

### *2.4. Reagent and Leaching Test*

The sulfuric acid used for the leaching tests was grade P.A., with 95–97% purity, a density of 1.84 kg/L, and a molecular weight of 98.80 g/mol. The leaching tests were carried out in a 50 mL glass reactor with a 0.01 solid/liquid ratio. A total of 200 mg of black oxide ore was maintained in suspension with the use of a five-position magnetic stirrer (IKA ROS, CEP 13087-534, Campinas, Brazil) at a speed of 600 rpm. The tests were conducted at a room temperature of 25 ◦C, while variations were iron additives, particle size, and leaching time. The tests were performed in duplicate and measurements (or analyses) were carried out on 5 mL undiluted samples using atomic absorption spectrometry with a coefficient of variation ≤ 5% and a relative error between 5 to 10%. The measurements of pH and oxidation-reduction potential (ORP) of the leach solutions were made using a pH-ORP meter (HANNA HI-4222 (HANNA instruments, Woonsocket, Rhode Island, USA)). The solution ORP was measured in a combination ORP electrode cell composed of a platinum working electrode and a saturated Ag/AgCl reference electrode.

### *2.5. The E*ff*ect of the Fe*/*MnO2 Ratio*

Other investigations have shown that variables of particle size and stirring speed do not have significant effects when working with a high Fe/MnO2 ratio [26,28]. Given this result, we decided to work with the following parameters: Fe/MnO2 ratios of 1/1, 2/1 and 3/1, a particle size range of −75–+53 μm, a stirring speed of 600 rpm, 1 mol/L sulfuric acid, and room temperature (25 ◦C).

#### *2.6. The E*ff*ect of the Acid Concentration on the System*

The present research studied the effect of the sulfuric acid concentration on the system, working with H2SO4 concentrations of 0.5, 1, 2, and 3 mol/L under the following operating conditions: Reducing agent/black copper ratio of 1/2, particle size range of −75 + 53 μm, stirring speed of 600 rpm, and a temperature of 25 ◦C.

#### **3. Results**
