**1. Introduction**

Deposits of ferromanganese (Fe-Mn) are found in the oceans around the world [1–4]. These deposits contain ferromanganese crusts, as well as cobalt-rich crusts and manganese nodules [5–7]. These marine resources are found mainly in the Pacific, Atlantic and Indian Ocean [8], and are formed by precipitation processes of Mn and Fe oxides around a nucleus, which is commonly composed of a fragment of an older nodule [9]. Manganese nodules also called polymetallic nodules because they are associated with large reserves of metals, such as Cu, Ni, Co, Fe and Mn, the latter being the most abundant, with an average content of around 24% [10]. In addition to the aforementioned elements, considerable quantities of Te, Ti, Pt and rare earths can also be found [11]. These nodules might be good source of manganese in the industry for high demand in steel production [12–14].

To extract Mn and other metals of interest from marine nodules, the use of a reducing agent is necessary [15,16]. Studies have used different reducing agents, such as, wastewater from the manufacture of alcohol from molasses [17], coal [18], H2SO3 [19,20], pyrite [21], sponge iron [22] and cast iron slag magnetite [23]. Iron has shown to be a good reducing agent for manganese extraction, from those, due to its low cost and abundance [23]. Several studies have been carried out to evaluate the effect of iron as a reducing agent in leaching in acid media of marine nodules [21,24]. For studies in acidic media and iron, it has been reported that the best results for extracting manganese are obtained by increasing the amounts of Fe in the Mn/Fe ratio and working at low acid concentrations [22,23].

In the studies by Kanungo [21,25], an acid leaching (HCl) was conducted at different temperatures with the addition of pyrite as a reducing agent achieving 50% manganese extractions. The author concluded that, in a moderately acidic medium, pH of 1.5, the Fe (II) and Fe (III) ratio in the system remains essentially constant up to 50 min above, which the ratio tends to increase exponentially. From this, it is suggested that the reduction of MnO2 by ferrous ions occurs at a faster rate than the oxidation of pyrite generating ferric ions. For the dissolution of Mn with the use of pyrite in acidic media, the following series of reactions is proposed [21]:

$$\rm{3FeS}\_2 + 4H\_2SO\_4 = 3FeSO\_4 + 4H\_2O + 7S \tag{1}$$

$$\text{3FeSO}\_4 + 4\text{H}\_2\text{SO}\_4 = 3\text{Fe}\_2(\text{SO}\_4)\_3 + 4\text{H}\_2\text{O} + \text{S} \tag{2}$$

$$\text{-2FeS}\_2 + 4\text{H}\_2\text{SO}\_4 = \text{Fe}\_2(\text{SO}\_4)\_3 + 4\text{H}\_2\text{O} + 5\text{S} \tag{3}$$

$$1\text{5MnO}\_2 + 2\text{FeS}\_2 + 14\text{H}\_2\text{SO}\_4 = \text{Fe}\_2(\text{SO}\_4)\_3 + 15\text{MnSO}\_4 + 14\text{H}\_2\text{O} \tag{4}$$

For the use of ferrous ions, Zakeri et al. [24] indicated that when working in a molar ratio of Fe<sup>2</sup>+/MnO2 of 3/1, a molar ratio of H2SO4/MnO2 of 2/1 and a mineral particle size of <sup>−</sup>60 + 100 Tyler mesh, 90% extractions of Mn can be obtained in less than 20 min at a temperature of 20 ◦C. In their work they proposed the following series of reactions:

$$\text{MnO}\_2 + 4\text{H}^+ + 2\text{e}^- = \text{Mn}^{2+} + 2\text{H}\_2\text{O} \tag{5}$$

$$2\text{Fe}^{2+} = 2\text{Fe}^{3+} + 2\text{e}^- \tag{6}$$

$$\text{MnO}\_2 + 2\text{Fe}^{2+} + 4\text{H}^+ = \text{Mn}^{2+} + 2\text{Fe}^{3+} + 2\text{H}\_2\text{O} \tag{7}$$

Subsequently, Bafghi et al. [22] conducted a similar experiment but with the use of Fe sponge, where he compared the results reported by Zakeri et al. [24] and indicated that under the same operating conditions, sponge Fe delivers better results than the addition of ferrous ions, because the metal of Fe allows us to have a high activity ratio through the regeneration of ferrous ions. For the dissolution of Mn with the use of Fe (s), the following reactions are presented [22]:

$$\text{Fe (s)} + 2\text{H}^+ = \text{Fe} + \text{H}\_2 \text{ (g)}\tag{8}$$

$$\text{Fe (s)} + 2\text{Fe}^{3+} = 3\text{Fe}^{2+} \tag{9}$$

$$\text{MnO}\_2\text{ (s)} + 2\text{Fe}^{2+} + 4\text{H}^+ = \text{Mn}^{2+} + 2\text{Fe}^{3+} + 2\text{H}\_2\text{O} \tag{10}$$

$$\text{MnO}\_2\text{ (s)} + 2\text{Fe (s)} + 8\text{H}^+ = \text{Mn}^{2+} + 2\text{Fe}^{3+} + 2\text{H}\_2\text{O} + 2\text{ H}\_2\text{ (g)}\tag{11}$$

$$\text{MnO}\_2\text{ (s)} + \text{Fe (s)} + 4\text{H}^+ = \text{Mn}^{2+} + \text{Fe}^{2+} + 2\text{H}\_2\text{O} \tag{12}$$

$$2\text{MnO}\_2\text{ (s)} + 2\beta \text{Fe (s)} + 4\text{H}^+ = \text{Mn}^{2+} + 2\beta \text{Fe}^{3+} + 2\text{H}\_2\text{O} \tag{13}$$

In the studies carried out by Toro et al. [23,26] smelting slag was used, taking advantage of the Fe2O3 presented in these to reduce MnO2 in an acid medium. It was concluded that the ratios of MnO2/Fe = 1/2 and 1 M H2SO4 significantly shorten the dissolution time of manganese (from 30 to 5 min). In addition, the authors indicated that the particle size is not as significant in Mn solutions as in the concentration of H2SO4. For the dissolution of Mn with the use of Fe2O3 in acid media, the following series of reactions is presented:

$$\rm Fe\_2O\_3\ (s) + 3H\_2SO\_4 = Fe\_2(SO\_4)\_3\ (s) + 3H\_2O\tag{14}$$

$$\text{Fe}\_3\text{O}\_4\text{ (s)} + 4\text{H}\_2\text{SO}\_4 = \text{FeSO}\_4 + \text{Fe}\_2\text{(SO}\_4)\_3\text{ (s)} + 4\text{H}\_2\text{O} \tag{15}$$

$$\rm Fe\_2(SO\_4)\_3\ (s) + 6H\_2O = 2Fe(OH)\_3\ (s) + 3H\_2SO\_4\tag{16}$$

$$2\text{FeSO}\_4 + 2\text{H}\_2\text{O} = 2\text{Fe (s)} + 2\text{H}\_2\text{SO}\_4 + \text{O}\_2\text{ (g)}\tag{17}$$

$$\text{-2FeSO}\_4 + 2\text{H}\_2\text{SO}\_4 + \text{MnO}\_2\text{ (s)} = \text{Fe}\_2(\text{SO}\_4)\_3\text{ (s)} + 2\text{H}\_2\text{O} + \text{MnSO}\_4\tag{18}$$

It is imperative to create innovative methods for the treatment of minerals that involve industrial waste reusing. Big mining companies are promoting recycling to generate a more sustainable sector. An example is the iron industry in China, where it is sought to reduce pollution by adding scrap in steelmaking [27]. Another example is mining in Chile, where companies like Collahuasi have recycling programs, in which they annually recover 3000 tons of scrap metal, 4 thousand kilos of electronic waste, 182 thousand units of plastic bottles and 680 kg of paper and cardboard [28]. Regarding steel scrap, the copper mining industry generates large amounts of this waste in the milling processes, but the steel balls or bars are discarded [29].

In this research, the leaching of MnO2 to recover manganese with the use of different types of Fe reducing agents (pyrite, ferric ions, steel and magnetite) working under the same operating conditions was studied. The objective of this work is to find the most suitable iron reducing agent to extract manganese when working in an acidic environment and room temperature, with the novelty of testing the use of steel. A statistical analysis was conducted performed to evaluate the performance of the different selected reducers. Finally, the obtained results were compared in leaching tests over time, indicating which allow obtaining the best results.

#### **2. Methodology**

### *2.1. Manganese Nodule*

The marine nodules used in this research were collected in the 1970s from the Blake Plateau in the Atlantic Ocean. The sample was reduced in size using a porcelain mortar and classified by mesh sieves until reaching a range between −140 + 100 μm. Later, it was analyzed chemically by atomic emission spectrometry via induction-coupled plasma (ICP-AES), developed in the Applied Geochemistry Laboratory of the Department of Geological Sciences of the Catholic University of the North, and its chemical composition was 0.12% of Cu, 0.29% Co and 15.96% Mn. Its mineralogical composition is presented in Table 1. Micro X-ray fluorescence spectrometry (Micro-XRF) is a method for elementary analysis of non-homogeneous or irregularly shaped samples, as well as small samples or even inclusions. The sample material was analyzed in a Bruker® M4-Tornado μ-FRX table (Fremont, CA, USA). This spectrometer consists of an X-ray tube (Rh-anode), and the system features a polycapillary X-ray optic, which concentrates the radiation of the tube in minimal areas, allowing a point size of 20 μm for Mo-K. The elementary maps created with the built-in software of the M4 Tornado ™ (Fremont, CA, USA), ESPRIT, indicate that the nodules were composed of fragments of pre-existing nodules that formed its nucleus, with concentric layers that precipitated around the core in later stages.

**Table 1.** Mineralogical analysis of the manganese nodule.

