*2.2. MnO2 (Manganese Nodules)*

The manganese nodules used in this study are the same as those used in the study by Toro et al. [29]. This sample was reduced in size with the use of a porcelain mortar until reaching a size range between −75 + 53 μm. This sample contains 15.96% of Mn. Table 2 shows the mineralogical composition. The sample was analyzed with a Bruker®tabletop M4-Tornado μ-FRX (Fremont, CA, USA). The interpretation of the μ-XRF data shows that the nodules were composed of fragments of preexisting nodules that formed their nucleus, with concentric layers that precipitated around the nucleus in later stages.

**Table 2.** Mineralogical Analysis of the Manganese Nodule.


#### 2.2.1. Reagent and Leaching Test

The sulfuric acid used for the leaching tests was P.A. grade (Merck, Darmstadt, Germany), purity 95–97%. We also work with the use of waste water from the Aguas Antofagasta Desalination Plant, which has a concentration of 39.16 g/L of chloride. Tables 3 and 4 shows the chemical composition of waste water and sea water.

**Table 3.** Chemical composition of waste water.


**Table 4.** Reference composition of seawater, with principal ions (Modified from Cisternas and Gálvez, [45]).

Leaching tests were carried out in a 50 mL glass reactor with a 0.01 S/L ratio. A total of 200 mg of chalcopyrite ore, with the addition of different concentrations of MnO2 (manganese nodules), was maintained in agitation and suspension with a 5 position magnetic stirrer (IKA ROS, CEP 13087-534, Campinas, Brazil) at a speed of 800 rpm. Temperature was controlled using an oil-heated circulator (Julabo). The temperature range tested in the experiments was 25 to 80 ◦C. The tests were performed in triplicate, and measurements (or analyzes) 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 pH levels and oxidation-reduction potential (ORP) of leaching solutions were measured with a pH-ORP meter (HANNA HI-4222 (HANNA instruments, Woonsocket, RI, USA)). The ORP solution was measured in a combination ORP electrode cell of a platinum working electrode and a saturated Ag/AgCl reference electrode.

In the previous study (Toro et al. [29]) the ratio of MnO2/CuFeS2, agitation rate, H2SO4 concentration, and chloride concentration were evaluated. Besides, the obtained residues were analyzed, but the formation of contaminating elements was not observed. However, no other fundamental variables were evaluated, and the performance in the extraction of Fe and Mn was not mentioned. For the reasons discussed above, the leaching of CuFeS2 with MnO2 and wastewater in the present investigation continues, evaluating the particle size and temperature.

## 2.2.2. Effect of Particle Size

In previous studies conducted by Devi et al. [27] and Havlik et al. [28], it was shown that high MnO2 concentrations favour the kinetics of chalcopyrite dissolution. Recently, Toro et al. [29] indicated that when working on MnO2/CuFeS2 ratios of 5/1, attractive results were obtained for short periods. Based on previous background, the effect of the chalcopyrite particle size was evaluated by adding MnO2 at different sulfuric acid concentrations over time under the conditions shown in Table 5.


**Table 5.** Experimental conditions for the study of the effect of chalcopyrite particle size.

#### 2.2.3. Effect of Temperature

In the study conducted by Toro et al. [29], positive results were obtained when working at high ratios of MnO2/CuFeS2 (5/1). However, the effect of temperature was not evaluated to shorten leaching times or decrease MnO2 concentrations.

This study investigated the effect of temperature (in which interval 25–80 ◦C) on the copper dissolution rate from chalcopyrite with the addition of MnO2, working with a particle size of −47 + 38 μm, MnO2/CuFeS2 ratios of 2/1 and 5/1, 1 mol/L of sulfuric acid, 39.16 g/L of chloride (wastewater) and at a stirring speed of 800 rpm.

#### **3. Results**
