Extraction of Mn from Black Copper Using Iron Oxides from Tailings and Fe2+ as Reducing Agents in Acid Medium

Exotic type deposits include several species of minerals, such as atacamite, chrysocolla, copper pitch, and copper wad. Among these, copper pitch and copper wad have considerable concentrations of manganese. However, their non-crystalline and amorphous structure makes it challenging to recover the elements of interest (like Cu or Mn) by conventional hydrometallurgical methods. For this reason, black copper ores are generally not incorporated into the extraction circuits or left unprocessed, whether in stock, leach pads, or waste. Therefore, to dilute MnO2, the use of reducing agents is essential. In the present research, agitated leaching was performed to dissolve Mn of black copper in an acidic medium, comparing the use of ferrous ions and tailings as reducing agents. Two samples of black copper were studied, of high and low grade of Mn, respectively, the latter with a high content of clays. The effect on the reducing agent/black copper ratio and the concentration of sulfuric acid in the system were evaluated. Better results in removing Mn were achieved using the highest-grade black copper sample when working with ferrous ions at a ratio of Fe2+/black copper of 2/1 and 1 mol/L of sulfuric acid. Besides, the low-grade sample induced a significant consumption of H2SO4 due to the high presence of gangue and clays.


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].
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 MnO 2 from black copper in acid media comparing the use of iron and iron oxide tailings as reducing agents.

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 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.

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

Iron Oxide Tailings
The iron oxide tailings used were from the Altonorte Smelting Plant. The particle sizes were in  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.

Ferrous Ions
The ferrous ions used for this investigation (FeSO 4 × 7H 2 O) were WINKLER brand, with a molecular weight of 278.01 g/mol.

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].

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.

The Effect of the Fe/MnO 2 Ratio
Other investigations have shown that variables of particle size and stirring speed do not have significant effects when working with a high Fe/MnO 2 ratio [26,28]. Given this result, we decided to work with the following parameters: Fe/MnO 2 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).

The Effect of the Acid Concentration on the System
The present research studied the effect of the sulfuric acid concentration on the system, working with H 2 SO 4 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.

Results
3.1. The Effect of the Fe 2+ /MnO 2 Ratio Figure 2a,b show the results for the dissolution of two black copper samples using Fe 2+ in acid media. As can be seen, better results were achieved with the sample BCS-1, which was due to the high presence of clay in sample BSC-2. It was observed that high Mn extraction rates can be achieved in short periods of time using MnO 2 /Fe 2+ ratios of 1/2 or less, achieving dissolution rates of over 78% in 5 min with sample BCS-1, and 65% in 5 min with sample BCS-2. The results shown in Figure 2a are similar to the 90% recovery obtained by Zakeri et al. [25] in 20 min leaching MnO 2 from manganese nodules with an Fe 2+ /MnO 2 ratio of 3, and an H 2 SO 4 /MnO 2 molar ratio of 2/1. A 1/1 Fe 2+ /MnO 2 ratio resulted in a lower MnO 2 dissolution kinetics, with an extraction of 40% in 5 min with sample A and 31% in 5 min with sample B. In general, Mn dissolution rates were similar with a longer period (30 min). However, the dissolution kinetics were slower for the sample BCS-2.

The Effect 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. Figure 2a and 2b show the results for the dissolution of two black copper samples using Fe 2+ in acid media. As can be seen, better results were achieved with the sample BCS-1, which was due to the high presence of clay in sample BSC-2. It was observed that high Mn extraction rates can be achieved in short periods of time using MnO2/Fe 2+ ratios of 1/2 or less, achieving dissolution rates of over 78% in 5 min with sample BCS-1, and 65% in 5 min with sample BCS-2. The results shown in Figure 2a are similar to the 90% recovery obtained by Zakeri et al. [25] in 20 min leaching MnO2 from manganese nodules with an Fe 2+ /MnO2 ratio of 3, and an H2SO4/MnO2 molar ratio of 2/1. A 1/1 Fe 2+ /MnO2 ratio resulted in a lower MnO2 dissolution kinetics, with an extraction of 40% in 5 min with sample A and 31% in 5 min with sample B. In general, Mn dissolution rates were similar with a longer period (30 min). However, the dissolution kinetics were slower for the sample BCS-2. (b) Black copper sample-2 (BCS-2), 25 °C, particle size range of −75-+53 µm, 1 mol/L H2SO4. Figure 3a and 3b show Mn dissolution with two black copper samples using Fe2O3 in acid media. As in earlier investigations by Toro et al. [26,28], working with an Fe2O3/MnO2 ratio of 2/1 or higher significantly increased MnO2 dissolution kinetics. There was little difference in the Mn extraction rates working with Fe2O3/MnO2 ratios of either 2/1 or 3/1, while the Mn extraction fell significantly when the quantity of Fe2O3 was reduced. Potential and pH levels were respectively in the ranges of −0.5 to 1.3 V and −1.5 to 0.4 in all the tests in this study.  Figure 3a,b show Mn dissolution with two black copper samples using Fe 2 O 3 in acid media. As in earlier investigations by Toro et al. [26,28], working with an Fe 2 O 3 /MnO 2 ratio of 2/1 or higher significantly increased MnO 2 dissolution kinetics. There was little difference in the Mn extraction rates working with Fe 2 O 3 /MnO 2 ratios of either 2/1 or 3/1, while the Mn extraction fell significantly when the quantity of Fe 2 O 3 was reduced. Potential and pH levels were respectively in the ranges of −0.5 to 1.3 V and −1.5 to 0.4 in all the tests in this study.  Figure 4 shows the effect of the acid concentration on dissolving Mn dissolution from the two black copper samples with the addition of high concentrations of iron oxides from tailings or ferrous ions. It can be seen from Figures 4a and 4c that for sample BCS-1, the sulfuric acid concentration was not significant in either case when working with high concentrations of the reducing agent. Differences in the effect of the acid concentration could only be noted with very low concentrations of iron oxide tailings (0.5 mol/L). The above concurs with findings of previous studies by Toro et al. [24,26] on extraction of MnO2 from manganese nodules. The Mn extraction rate from the BCS-2 sample increased with higher concentrations of H2SO4, possibly owing to the high consumption of acid generated by the presence of mineral impurities in this sample, mainly montmorillonite, kaolinite, and chlorite. This is consistent with what was previously found by Helle and Kelm [29], where the leaching of exotic Cu minerals (atacamite, chrysocolla, and malachite) required higher acid consumption by incorporating reactive bargains into the system. This was driven by smectites, mordenite bargain, and the presence of kaolinite, illite, and quartz.  Figure 4 shows the effect of the acid concentration on dissolving Mn dissolution from the two black copper samples with the addition of high concentrations of iron oxides from tailings or ferrous ions. It can be seen from Figure 4a,c that for sample BCS-1, the sulfuric acid concentration was not significant in either case when working with high concentrations of the reducing agent. Differences in the effect of the acid concentration could only be noted with very low concentrations of iron oxide tailings (0.5 mol/L). The above concurs with findings of previous studies by Toro et al. [24,26] on extraction of MnO 2 from manganese nodules. The Mn extraction rate from the BCS-2 sample increased with higher concentrations of H 2 SO 4 , possibly owing to the high consumption of acid generated by the presence of mineral impurities in this sample, mainly montmorillonite, kaolinite, and chlorite. This is consistent with what was previously found by Helle and Kelm [29], where the leaching of exotic Cu minerals (atacamite, chrysocolla, and malachite) required higher acid consumption by incorporating reactive bargains into the system. This was driven by smectites, mordenite bargain, and the presence of kaolinite, illite, and quartz.  Figure 4 shows the effect of the acid concentration on dissolving Mn dissolution from the two black copper samples with the addition of high concentrations of iron oxides from tailings or ferrous ions. It can be seen from Figures 4a and 4c that for sample BCS-1, the sulfuric acid concentration was not significant in either case when working with high concentrations of the reducing agent. Differences in the effect of the acid concentration could only be noted with very low concentrations of iron oxide tailings (0.5 mol/L). The above concurs with findings of previous studies by Toro et al. [24,26] on extraction of MnO2 from manganese nodules. The Mn extraction rate from the BCS-2 sample increased with higher concentrations of H2SO4, possibly owing to the high consumption of acid generated by the presence of mineral impurities in this sample, mainly montmorillonite, kaolinite, and chlorite. This is consistent with what was previously found by Helle and Kelm [29], where the leaching of exotic Cu minerals (atacamite, chrysocolla, and malachite) required higher acid consumption by incorporating reactive bargains into the system. This was driven by smectites, mordenite bargain, and the presence of kaolinite, illite, and quartz.

Conclusions
This study presents the results obtained for dissolving Mn from black copper using iron oxides (and specifically magnetite) from tailings and Fe 2+ as reducing agents in acid media. Both reducing agents yielded good results with the two samples studied. Similar behavior was observed with the two samples in relation to Mn extraction, with the best results obtained in all the experiments with the BCS-1 sample. These encouraging results give new options to extract the Cu present in these exotic minerals, which are considered as industrial waste today. The main findings are the following: 1) The ferrous ions were a better reducing agent than iron oxides to dissolve MnO2 in black copper.
2) The optimal reducing agent/black copper ratio was 2:1 for the studied reducing agents studied. 3) High concentrations of H2SO4 had a positive effect on the dissolution of Mn with the BCS-2 sample owing to the high content of clay (montmorillonite and kaolinite) and gangue (chlorite), which consume significant amounts of acid. The acid concentration was not significant with the BCS-1 sample. 4) The best results in this study were obtained working with the sample with fewer impurities (BCS-1), with an Fe 2+ /black copper ratio of 2:1, and 1 mol/L of sulfuric acid.
Despite the good results obtained with BCS-1, BCS-2 was more like the mineralogy found at the industrial scale. It should be noted that although lower Mn extraction rates are obtained using tailings instead of ferrous ions, tailings can be a more attractive additive for leaching black copper because they are an industrial waste with no economic value. Given the above results, future investigations should aim to optimize operational parameters for leaching black copper minerals with high gangue content using industrial waste or wastewater as reducing agents, with the aim of taking this process to the industrial scale.