1. Introduction
Ferromanganese (Fe–Mn) deposits are present in the oceans across the world, marine ridges, and plateaus where the currents have delivered sediments for millions of years [
1]. These deposits form through the accumulation of iron and manganese oxides in seawater, within either volcanic or sedimentary rocks that act as substrates, as observed in the central and northeastern ocean beds of the Pacific [
2]. They may have economic potential [
3], due to the high concentrations of Co, Ni, Te, Ti, Pt, and rare earth elements [
4]. These Fe–Mn oceanic deposits include ferromanganese crusts, as well as cobalt-rich crusts, polymetallic nodules, and hydrothermal infusions [
5]. Polymetallic nodules have a particular importance for the steel industry as an they may eventually become an alternate source of manganese [
6].
In order to extract manganese and other metals from marine nodules, the use of a reducing agent is necessary [
7]. Acid leaching of marine nodules, with the use of iron as a reducing agent, has shown good results [
8,
9,
10]. In a previous study carried out by Toro et al. [
11], several parameters were evaluated for dissolving Mn from marine nodules using slag at room temperature in an acid medium. This study established that high MnO
2/Fe
2O
3 ratios significantly shorten the manganese dissolution time from 30 to 5 min. They also conclude that MnO
2 particle size does not significantly affect the Mn extraction rate in an acid medium in the presence of Fe contained in ferrous slag.
The positive effect of Fe as a reducing agent for dissolving Mn from marine nodules was noted when lower Mn/Fe ratios were used [
8,
9,
10,
11]. Bafghi et al. [
12] and Toro et al. [
11] determined that sulfuric acid concentration is less important than Fe concentration in dissolving Mn.
The Mn extraction rate increases with a higher agitation speed [
13,
14,
15]. Jiang et al. [
13] evaluated the kinetic aspects of manganese and silver extraction during leaching of pyrolusite in sulfuric acid solutions in the presence of H
2O
2, and concluded that agitation speed was one of the most important variables affecting the Mn extraction rate. Su et al. [
14] indicated that the Mn extraction rate increases significantly when the agitation speed increases from 100 to 700 rpm because high speed improves mixing and allows better contact between reagents and reactants. Jiang et al. [
13] also reported that the extraction rate decreases slightly at 1000 rpm because excessive agitation can cause material to adhere to the walls of the reactor and prevent it from being leached. Velásquez et al. [
16] indicated that it is only necessary to keep particles in suspension and prevent agglomeration.
The addition of Fe as a reducing agent in temperature-controlled acid media has already been studied [
8,
10,
12]. In particular, Zakeri et al. [
10] used ferrous ions with a Fe
2+/MnO
2 ratio of 2.4 and sulfuric acid as a leaching agent with a H
2SO
4/MnO
2 ratio of 2.0 over a temperature range of 20 to 60 °C, and found out that Mn extraction was notably higher at 60 °C and reached 96% after 60 min. Bafghi et al. [
12] used Fe sponge with a molar ratio of 2, and H
2SO
4 with a molar ratio of 4 (both ratios with respect to MnO
2), under the same temperatures as Zakeri et al. [
10]; at 60 °C, 100% of the Mn present within the nodules was dissolved in 3 min. Both cases demonstrate the positive impact of higher temperature on the extraction rate; however, the positive impact of the presence of iron indicated that effective processing may take place even at ambient temperatures. Furthermore, both studies demonstrate that the acid concentration is less significant than the Fe/MnO
2 ratio.
The present work investigates the effect of using of tailings, obtained after flotation of slag at the Altonorte Foundry Plant, on the dissolution of Mn from marine nodules. A report by SERNAGEOMIN [
17] indicates that the production of copper concentrate in Chile has been increasing steadily, and is expected to almost double by 2026 from its 2014 level, from 3.9 to 5.4 million tons. For every ton of Cu concentrate obtained by flotation, 151 tons of tailings are generated [
18], which are disposed of in tailing dams and have significant impacts on the environment [
19]. Consequently, it is necessary to find new uses for tailings with the application of more environmentally friendly hydrometallurgical techniques [
20]. This results in an attractive proposal given the quantities of waste generated in the country by flotation, providing an added value for this material while introducing a new initiative in the context of the need to overcome stagnation in the mining sector [
21].
2. Materials and Methods
2.1. Manganese Nodule Sample
The marine nodules used in this work were the same as those used in Toro et al. [
11]. They were composed of 15.96% Mn and 0.45% Fe.
Table 1 shows the chemical composition. The sample material was analyzed with a Bruker® M4-Tornado μ-FRX tabletop device (Fremont, CA, USA). The μ-XRF data shows that the nodules were composed of fragments of preexisting nodules that formed their nuclei, with concentric layers that precipitated around the nuclei in later stages.
2.2. Tailings
The sample of tailings used in this study was obtained after flotation of slag during the production of copper concentrate at the Altonorte Smelting Plant. The methods used to determine the chemical and mineralogical composition of the tailings were the same as those used to determine marine nodule content. Chemical species were determined by QEMSCAN. Several iron-containing phases were present, while the Fe content was estimated at 41.9%.
Table 2 shows the mineralogical composition of the tailings. As the Fe was mainly in the form of magnetite, the most appropriate method of extraction was the same as that used in Toro et al. [
11].
2.3. Reagents Used—Leaching Parameters
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 98.8 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 Mn nodules were maintained in suspension with the use of a 5-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 the parameters studied were additives, particle size, and leaching time. Also, the tests were performed in duplicate, measurements (or analyses) were carried on 5 mL of undiluted samples using atomic absorption spectrometry with a coefficient of variation ≤5% and a relative error between 5% to 10%.
2.4. Experimental Design
The effect of the independent variables on the extraction rate of Mn from manganese nodules was studied using the response surface method [
22,
23], which helped in understanding and optimizing the response by refining the determinations of relevant factors using the model. An experiment was designed involving three factors that could influence the response variable, and with three levels for each factor for a total of 27 experimental tests (
Table 3), the purpose of which was to study the effects of H
2SO
4 concentration, particle size, and time on the dependent variable. Minitab 18 software was used for modeling and experimental design, providing the same analytical approach as used in Toro et al. [
11].
The response variable can be expressed as showed in Equation (1):
Table 4 shows the ranges for values of the parameters used for the experimental design.
The levels of the factors are coded as (−1, 0, 1), where each number represents a particular value of the factor, with (−1) as the lowest value, (0) as the intermediate, and (1) as the highest. Equation (2) is used to transform a real value (
) into a coded value (
) according to the experimental design:
where
and
are, respectively, the highest and lowest values of a variable [
22].
The statistics used to determine whether the model can adequately describe the extraction of Mn from marine nodules are similar with those used in the study of Toro et al. [
11].
2.5. Effect of Stirring Speed
The effect of particle size was evaluated by Toro et al. [
11]. It was concluded that this variable did not significantly influence the manganese solutions. Consequently, the present work assessed the effect of agitation speed on Mn dissolution kinetics.
This investigation determined the effect of increasing agitation speed (200, 400, 600, 800, and 1000 rpm) on leaching manganese nodules, using a particle size of −75 + 53 μm, MnO2/Fe2O3 ratio of 1, leaching solution volume of 20 mL, 1 mol/L sulfuric acid, and room temperature (25 °C).
2.6. Effect of the MnO2/Fe2O3 Ratio
The present study evaluated the effect of the MnO2/Fe2O3 ratio on leaching time with the use of tailings, using a particle size of −75 + 53 μm, agitation speed of 600 rpm, leaching solution volume of 20 mL, 1 mol/L sulfuric acid, and room temperature (25 °C).