Leaching Manganese Nodules in an Acid Medium and Room Temperature Comparing the Use of Different Fe Reducing Agents

Abstract

The deposits of Fe-Mn, in the seabed of the planet, are a good alternative source for the extraction of elements of interest. Among these are marine nodules, which have approximately 24% manganese and may be a solution to the shortage of high-grade ores on the surface. In this investigation, an ANOVA analysis was performed to evaluate the time independent variables and MnO₂/reducing agent in the leaching of manganese nodules with the use of different Fe reducing agents (FeS₂, Fe²⁺, Fe⁰ and Fe₂O₃). Tests were also carried out for the different reducing agents evaluating the MnO₂/Fe ratio, in which the Fe⁰ (FeC) proved to be the best reducing agent for the dissolution of Mn from marine nodules, achieving solutions of 97% in 20 min. In addition, it was discovered that at low MnO₂/Fe ratios the acid concentration in the system is not very relevant and the potential and pH were in ranges of −0.4–1.4 V and −2–0.1 favoring the dissolution of Mn from MnO₂.

1. Introduction

Deposits of ferromanganese (Fe-Mn) are found in the oceans around the world [1,2,3,4]. These deposits contain ferromanganese crusts, as well as cobalt-rich crusts and manganese nodules [5,6,7]. 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 [12,13,14].

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]:

3FeS₂ + 4H₂SO₄ = 3FeSO₄ + 4H₂O + 7S

(1)

6FeSO₄ + 4H₂SO₄ = 3Fe₂(SO₄)₃ + 4H₂O + S

(2)

2FeS₂ + 4H₂SO₄ = Fe₂(SO₄)₃ + 4H₂O + 5S

(3)

15MnO₂ + 2FeS₂ + 14H₂SO₄ = Fe₂(SO₄)₃ + 15MnSO₄ + 14H₂O

(4)

For the use of ferrous ions, Zakeri et al. [24] indicated that when working in a molar ratio of Fe²⁺/MnO₂ of 3/1, a molar ratio of H₂SO₄/MnO₂ of 2/1 and a mineral particle size of −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:

MnO₂ + 4H⁺ + 2e⁻ = Mn²⁺ + 2H₂O

(5)

2Fe²⁺ =2Fe³⁺ + 2e⁻

(6)

MnO₂ + 2Fe²⁺ + 4H⁺ = Mn²⁺ + 2Fe³⁺ + 2H₂O

(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]:

Fe (s) + 2H⁺ = Fe + H₂ (g)

(8)

Fe (s) + 2Fe³⁺ = 3Fe²⁺

(9)

MnO₂ (s) + 2Fe²⁺ + 4H⁺ = Mn²⁺ + 2Fe³⁺ + 2H₂O

(10)

MnO₂ (s) + 2Fe (s) + 8H⁺ = Mn²⁺+ 2Fe³⁺+ 2H₂O + 2 H₂ (g)

(11)

MnO₂ (s) + Fe (s) + 4H⁺ = Mn²⁺ + Fe²⁺ + 2H₂O

(12)

MnO₂ (s) + 2/3Fe (s)+ 4H⁺ = Mn²⁺ + 2/3Fe³⁺ + 2H₂O

(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:

Fe₂O₃ (s) + 3H₂SO₄ = Fe₂(SO₄)₃ (s) + 3H₂O

(14)

Fe₃O₄ (s) + 4H₂SO₄ = FeSO₄ + Fe₂(SO₄)₃ (s) + 4H₂O

(15)

Fe₂(SO₄)₃ (s) + 6H₂O = 2Fe(OH)₃ (s) + 3H₂SO₄

(16)

2FeSO₄ + 2H₂O = 2Fe (s) + 2H₂SO₄ + O₂ (g)

(17)

2FeSO₄ + 2H₂SO₄ + MnO₂ (s) = Fe₂(SO₄)₃ (s) + 2H₂O + MnSO₄

(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. Mineralogical analysis of the manganese nodule.

Component	MgO	Al2O3	SiO2	P2O5	SO3	K2O	CaO	TiO2	MnO2	Fe2O3
Mass (%)	3.54	3.69	2.97	7.20	1.17	0.33	22.48	1.07	25.24	26.02

. 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.

2.2. FeS₂

For this study, a cubic pyrite crystal obtained from the Navajún Mine (La Rioja, Spain) was used. This sample was reduced in size with the use of a cone crusher at laboratory level and later a sprayer. It was then classified through meshes sieves until reaching a size range of −75 + 53 μm. It was then 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.

Table 2. Chemical composition.

Component	Fe	S2
Mass (%)	46.63	53.37

shows the chemical composition of the samples.

X-ray diffraction analyses (XRD) of the pyrite were performed on a Bruker D8 ADVANCE diffractometer (Billerica, MA, US) with Cu λ = 1.5406 Å radiation generated at 40 kV and 30 mA. The analysis and identification of the crystalline phases were obtained using the DIFFRAC.EVA V4.2.1 program, with the Powder Diffraction File of ICDD database (PDF-2 (2004)) (Billerica, MA, US). According to the initial qualitative analysis of XRD, the primary mineral phase in the samples was pyrite, whose main peaks are at 33.153°, 37.121° and 40.797°. These peaks correspond to those given in the reference pattern PDF 01-1295 (ICDD, 2004). As seen in Figure 1, the analysis showed the sample has a purity of 99.40%.

2.3. Fe₂O₃

The Fe₂O₃ used is found in tailings from the Altonorte Smelting Plant. Its size is in a range of −75 + 53 μm. The methods used to determine its chemical and mineralogical composition are the same as those used in marine nodules. Figure 2 and

Table 3. Shows the mineralogical composition of the tailings. The Fe in it was mainly in the form of magnetite.

Mineral	Amount % (w/w)
Chalcopyrite/Bornite CuFeS2/Cu5FeS4	0.47
Tennantite/Tetrahedrite (Cu12As4S13/Cu12Sb4S13)	0.03
Other Cu Minerals	0.63
Cu–Fe Hydroxides	0.94
Pyrite (FeS2)	0.12
Magnetite (Fe3O4)	58.52
Specular Hematite (Fe2O3)	0.89
Hematite (Fe2O3)	4.47
Ilmenite/Titanite/Rutile (FeTiO3/CaTiSiO3/TiO2)	0.04
Siderite (FeCO3)	0.22
Chlorite/Biotite (Mg3Si4O10(OH)2(Mg)3(OH)6/K(Mg)3AlSi3O10(OH)2)	3.13
Other Phyllosilicates	11.61
Fayalite (Fe2SiO4)	4.59
Dicalcium Silicate (Ca2SiO4)	8.3
Kirschsteinite (CaFeSiO4)	3.4
Forsterita (Mg2SiO4)	2.3
Baritine (BaSO4)	0.08
Zinc Oxide (ZnO)	0.02
Lead Oxide (PbO)	0.01
Sulfate (SO4)	0.2
Others	0.03
Total	100

shows the chemical species that use QEMSCAN (QEMSCAN has a database, which has the elemental composition, and density of the minerals that are detected. With this information, it is possible to obtain the elementary contribution of the measured sample), and several iron-containing phases are presented, while the Fe content is estimated at 41.9%.

2.4. Steel (FeC)

A low carbon steel sheet (FeC; 0.25% C) from the steel supplier company Salomon Sack was used. This sample was reduced in size with the use of a cone crusher at laboratory level and later a pulverizer until reaching a size range between −75 + 53 μm.

2.5. Ferrous Ions

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

2.6. Reactor and Leaching Tests

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 in leaching solution. A total of 200 mg of Mn nodules were maintained in agitation and 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, with variations in additives, particle size and leaching time. The tests performed in duplicate, measurements (or analyses) carried on 5 mL undiluted samples using atomic absorption spectrometry with a coefficient of variation ≤5% and a relative error between 5% and 10%. Measurements of pH and oxidation-reduction potential (ORP) of leach solutions were made using a pH-ORP meter (HANNA HI-4222, St. Louis, MO, 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.

2.7. Estimation of Linear and Interaction Coefficients for Factorial Designs of Experiments of 2³

Two independent variables were chosen for the factorial design of 36 experiments, where: time and ratio MnO₂/reducing agent represent the independent variables that explain the extraction of Mn for a certain type of reducing agent. The analysis through a factorial design allowed us to study the effect of the factors and their levels in a response variable, helping to understand which factors are the most relevant [30,31]. Four factorial designs were carried out that involved two factors with three levels each, with a total of 36 experimental tests (

Table 4. Experimental conditions.

Parameters/Values	Low	Medium	High
Time (min)	10	20	30
MnO2/Reducing agent	2/1	1/1	1/2
Codifications	−1	0	1

). The Minitab 18 software (version 18, Pennsylvania State University, State College, PA, USA) was used for modeling, experimental design and adjustment of a multiple regression [32].

The expression of the response variable according to the linear effect of the variables of interest and considering the effects of interaction and curvature, is shown in Equation (19).

$$Cu Recovery(\%)=\alpha +{\displaystyle {{\displaystyle \sum }}_{i = 1}^n}{\beta }_i\times x_i+{\displaystyle {{\displaystyle \sum }}_{i=1}^n}{\beta }_i^2\times x_i^2+{\beta }_{1,2}\times x_1\times x_2,$$

(19)

where α is the overall constant, $x_i$ is the value of the level “i” of the factor, ${\beta }_i$ is the coefficient of the linear factor $x_i$, ${\beta }_i^2$ is the coefficient of the quadratic factors, ${\beta }_{1,2}$ is the coefficient of the interaction, n are the levels of the factors and Mn recovery is the dependent variable.

Table 4 shows the values of the levels for each factor, while

Table 5. Experimental configuration and Mn extraction data.

Exp. No.	Time (min)	MnO2/Reducing Agent Ratio	Mn Recovery (%; Reducing Agent)
			FeS2	Fe2+	FeC	Fe2O3
1	10	2/1	4.12	20.52	22.31	33.33
2	10	1/1	8.51	40.69	44.00	50.23
3	10	1/2	10.66	80.27	87.13	71.00
4	20	2/1	8.34	27.80	30.22	39.22
5	20	1/1	12.69	63.11	67.43	57.32
6	20	1/2	19.21	90.18	97.00	73.21
7	30	2/1	15.84	40.32	41.99	42.55
8	30	1/1	19.11	70.00	74.33	72.96
9	30	1/2	26.32	93.50	97.34	75.14

shows the recovery obtained for each configuration.

3. Results

3.1. Statistical Analysis

From the analysis of the main components, the time and ratio factors MnO₂/Reducing agent showed a main effect, since the variation between the different levels affected the response in a different way, as shown in Figure 3.

By developing the ANOVA test and the multiple linear regression adjustment for each of the configurations, it is necessary to recover the Mn as a function of the time predictor variables, and MnO₂/reducing agent, which is given by:

Mn Extraction (%) [FeS₂] = 13.867 + 6.330 × Time + 4.648MnO₂/FeS₂ × ratio.

(20)

Mn Extraction (%) [Fe²⁺] = 58.49 + 10.39 × Time + 29.22 MnO₂/Fe²⁺ × ratio.

(21)

Mn Extraction (%) [FeC] = 62.42 + 10.04 × Time + 31.16 MnO₂/FeC × ratio.

(22)

Mn Extraction (%) [Fe₂O₃] = 57.22 + 6.01 × Time + 17.37 MnO₂/Fe₂O₃ × ratio.

(23)

The time and ratio MnO₂/reducing agents were coded according to low and medium high levels. From the adjustment of multiple regression models, the interactions of the factors together with the curvature of the time factor and MnO₂/reducing agent did not contribute to explain the variability in any of the adjusted models.

From Equations (20)–(23) and from the main effect graphs in Figure 3, the factor that had showed a higher marginal contribution in Mn recovery was the MnO₂/Reducing agent ratio for the experimental design whose reducing agent was Fe²⁺, FeC and Fe₂O₃, while in case of using FeS₂ as a reducing agent, the factor that has a greater impact on recovery is time.

The ANOVA test indicates that the models adequately represent Mn extraction for the set of sampled values. The model does not require additional adjustments and is validated by the goodness-of-fit statistics shown in

Table 6. Goodness of fit statistics.

Response	F-Value	p-Value	S	R2	R2 (Pred)
Mn Extraction (%) [FeS2]	102.13	0.000	1.34602	97.15%	92.65%
Mn Extraction (%) [Fe2+]	145.76	0.000	4.44890	97.98%	95.63%
Mn Extraction (%) [FeC]	116.48	0.000	5.25341	97.49%	94.35%
Mn Extraction (%) [Fe2O3]	42.02	0.000	4.91305	93.34%	84.37%

. The p value (p < 0.05) and the significance tests F (${\mathrm{F}}_{\mathrm{Regression}}{ >> (\mathrm{F}}_{\mathrm{Table}}{=\mathrm{F}}_{2,6}(5.1432\left)\right)$ for a level of significance of α = 0.05 (95% confidence level) indicate that all models generated for the representation of the experimental tests were statistically significant. The normality tests indicate that the assumption of normality of the residuals was met. The low values of the S statistic indicate that there were no large deviations between the experimental data and the values of the adjusted model.

The value of the $R^2$ statistics was greater than 90%, which indicates that a large part of the total variability was explained by the models, while the similarity between the R² and R² predictive statistics indicates that the model could adequately predict the response to new observations.

3.2. Effect on MnO₂/Reducing Agent Ratio

In Figure 4, results are presented for the dissolution of Mn with the use of different Fe reducing agents at different ratios of Mn/Fe. For all the cases presented (Figure 4a–d), when working at low Mn/Fe ratios the highest recoveries of Mn were obtained. Ratios of 1/2 proved to be an optimum in Figure 4b–d. While for the Figure 4a in ratios of 1/3, the increase in the dissolution of Mn continued. The best results were obtained in Figure 4c when working with FeC because it allowed a high activity ratio through the regeneration of ferrous ions, favoring the dissolution of Mn and allowing better results to the use of Fe²⁺ in a direct way that is presented in Figure 4b. Using Fe₂O₃ shows good results when working with MnO₂/Fe₂O₃ ratios of 1/2, although it is lower than those presented when using Fe²⁺ and FeC. However, this may be an attractive proposal due to the reuse of tailings that are an environmental responsibility. For the use of pyrite, the lowest Mn solutions could be observed in this study. In previous studies [2,22,23,33], it has been indicated that it is not necessary to work at high concentrations of H₂SO₄ in the system to obtain high Mn solutions from marine nodules, but that if it is important to have low Mn/Fe ratios. The results presented in Figure 4a show a progressive increase in the Mn dissolution when increasing the amounts of FeS₂ in the system, however, it may be necessary to increase the acid concentration or temperature because of the kinetics of dissolution of ferrous ions from the pyrite ore.

For the performed tests, the values of potential and pH for the different reducing agents used for Mn/Fe ratios of 1/2 are presented in Figure 5. Senanayake [13] indicated that dissolving Mn from marine nodules requires to work in potential ranges between −0.4 and 1.4 V and pH between −2 and 0.1. With this, it is possible to avoid the precipitation of the Mn through the oxidation-reduction reaction, due to the presence of ferrous and ferric ions [34]. The outcomes met the operational condition mentioned above, which is due to the high concentrations of reducing agent. The lowest potential values were obtained with Fe²⁺ and FeC, wherein the iron (FeC) favored the regeneration of ferrous ions, which allows maintaining low potential ranges [22].

3.3. Effect on the Concentration of H₂SO₄

Figure 6 shows the effect of sulfuric acid concentration when working at Mn/Fe ratios of 1/2 with the use of different Fe reducing agents. Figure 6b,c shows that the concentration of H₂SO₄ was irrelevant in the extraction of Mn when working at low ratios of Mn/Fe with the use of Fe²⁺ and FeC. This is compatible with previous studies conducted by Zakeri et al. [24] and Bafghi et al. [22]. The researchers indicated that working at high concentrations of ferrous ions, variables like acid concentration and particle size were irrelevant. For the case shown in Figure 6d, it was observed that when working with the use of Fe₂O₃ there was a slight increase in Mn solutions when working above 0.1 mol/L, although it was observed that there were no differences between 0.5 and 1 mol/L, which reaffirms what was raised by Saldaña et al. [2], where they indicated that when working on acid-reducing leaching of MnO₂ using tailings, the acid concentration only influenced the extractions of Mn when it was not operated in high levels of Fe or no temperature increase. Finally, it can be seen in Figure 6a that when working with pyrite, the concentration of acid in the system was important. This was consistent with the results obtained by Kanungo et al. [21], which states that in an acid solution of marine nodules with the use of pyrite as the acidity of the medium decreases, the rate of reduction of MnO₂ decreases.

4. Conclusions

The Fe presented in the different additives proved to be a good reducing agent, increasing the dissolution of MnO₂. The main findings of this study were the following:

(1) Fe⁰ (FeC) proved to be the best reducing agent for the dissolution of Mn from marine nodules since the direct contact of Fe in the liquid solution kept the regeneration of ferrous ions, due to high levels of ferrous and ferric ions.

(2) When working with Fe²⁺, FeC and Fe₂O₃, and having high concentrations of reducing agent (MnO₂ ratios/reducing agent 1/2 or lower), low potential values were maintained, which allowed working at low acid concentrations (0.1 mol/L). However, for FeS₂, better results were achieved at higher ratios of MnO₂/FeS₂ (1/3) and acid levels of 1 mol/L, which was possibly due to the refractoriness of pyrite.

(3) For the tests carried out in this study with the different Fe reducing agents, the potential and pH ranges were from −0.4 to 1.4 V and −2 to 0.1, favoring the dissolution of Mn from marine nodules, and avoiding the formation of precipitates of the Fe.

(4) The best results of this research (97% of Mn) were obtained at MnO₂/FeC ratios of 1/2, 0.1 mol/L of H₂SO₄, in a time of 20 min.

In future work, other industrial iron wastes, generated in large industries, should be evaluated to create novel acid-reducing processes of MnO₂. Subsequently, to recover the manganese present in the solution, zero-valent iron (ZVl) is a good alternative. Zero valence iron can be reused, from scraps of the metal finishing industry.