# Development of an Analytical Model for the Extraction of Manganese from Marine Nodules

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## Abstract

**:**

_{2}SO

_{4}) in the presence of iron-containing tailings, which are both by-products of conventional copper extraction. The experiments are configured to address the effect of time, particle size, acid concentration, Fe

_{2}O

_{3}/MnO

_{2}ratio, stirring speed and temperature, under typical industrial conditions. The recovery of manganese has been modeled using a first order differential equation that accurately fits experimental results, noting that Fe

_{2}O

_{3}/MnO

_{2}and temperature are the most critical independent variables, while the particle size is the least influential (under typical conditions). This study obtains representative fitting parameters, that can be used to explore the incorporation of Mn recovery from marine nodules, as part of the extended value chain of copper sulfide processing.

## 1. Introduction

_{2}in acidic media, it is necessary to maintain the system in potential and pH ranges of −0.4 to 1.4 V and −2 to 0.1, respectively [9]. This indicates that the use of a reducing agent is necessary to extract Mn from marine nodules [11]. Due to its low cost, abundance and good results in previous studies [4,12,13,14,15,16,17], Fe is an attractive additive for the reductive leaching of manganese nodules. Toro et al. [4] performed leaching tests of marine nodules at the laboratory level by adding smelting slags with high Fe

_{2}O

_{3}contents, where it was discovered that Fe

_{2}O

_{3}, when reacting with H

_{2}SO

_{4}, forms FeSO

_{4}, which is a good reducing agent of MnO

_{2}, achieving extractions of 68% of the manganese in 5 min. In addition, in the same study previously mentioned, it was concluded that an MnO

_{2}/Fe

_{2}O

_{3}ratio of 1/2 is suitable for dissolving MnO

_{2}in an acidic medium. Later, Toro et al. [16] conducted tests using the Fe

_{2}O

_{3}present in tailings, obtaining even better results than with the slag, as the tailings were observed to be even more reactive. When exposed to H

_{2}SO

_{4}, Fe

_{2}O

_{3}generates ferrous sulfate (FeSO

_{4}), that acts as a reducing agent for MnO

_{2}, as described by the following reactions:

_{2}O

_{3(s)}+ 3 H

_{2}SO

_{4(aq)}= Fe

_{2}(SO

_{4})

_{3(s)}+ 3 H

_{2}O

_{(l)}

_{3}O

_{4(s)}+ 4H

_{2}SO

_{4(l)}= FeSO

_{4(aq)}+ Fe

_{2}(SO

_{4})

_{3(s)}+ 4 H

_{2}O

_{(l)}

_{2}(SO

_{4})

_{3(s)}+ 6 H

_{2}O

_{(l)}= 2 Fe(OH)

_{3(s)}+ 3 H

_{2}SO

_{4(l)}

_{4(aq)}+ 2 H

_{2}O

_{(l)}= 2 Fe

_{(s)}+ 2 H

_{2}SO

_{4(l)}+ O

_{2(g)}

_{4(aq)}+ 2 H

_{2}SO

_{4(aq)}+ MnO

_{2(s)}= Fe

_{2}(SO

_{4})

_{3(s)}+ 2 H

_{2}O

_{(l)}+ MnSO

_{4(aq)}

_{2}/Fe

_{2}O

_{3}ratio and the sulfuric acid concentration, but only under time-independent (static) conditions. This approach involved a three-level factorial design, ignoring the potential impact of particle size distribution and agitation velocity, which are likely to have a dynamic effect upon Mn recovery. Interestingly, iron-containing tailings have been observed to be more reactive than smelter slags within the context of leaching [30]. Moreover, the previous work of Toro et al. [4,16,17] considered neither temperature, nor its interactions with other critical variables that control the effect of iron-containing reducing agents on the leaching of Mn from MnO

_{2}[12,13,14,15].

## 2. Materials and Methods

#### 2.1. Manganese Nodule Samples

_{2}.

#### 2.2. Tailings Samples

#### 2.3. Reagents and Leaching Parameters

_{2}O

_{3}/MnO

_{2}ratio, stirring speed and temperature.

#### 2.4. Experimental Design

_{2}SO

_{4}) concentration, Fe

_{2}O

_{3}/MnO

_{2}ratio, agitation speed, and temperature on Mn recovery. The operational parameters considered in the factorial design of six factors and three levels by factor are presented in the Table 2.

_{1}corresponds to time, x

_{2}to the size of the particle, x

_{3}to the concentration of sulfuric acid, x

_{4}to the ratio Fe

_{2}O

_{3}/MnO

_{2}, x

_{5}to the speed of agitation and x

_{6}to the temperature.

#### 2.5. Adjustment of an Analytical Model

_{2}O

_{3}/MnO

_{2}ratio. The analytical model presented in Equation (3) can be expressed as:

_{2}SO

_{4}concentration μ and stirring speed v are proportional to Mn recovery, and that the square of particle size r is inversely linear to Mn recovery [32,34], the following model is proposed:

## 3. Results and Discussion

#### 3.1. Multilinear Regression of Experimental Data

_{2}O

_{3}/MnO

_{2}ratio.

_{1}), to the following quadratic regression considers only ${x}_{4}$ and ${x}_{6}$, which are the Fe

_{2}O

_{3}/MnO

_{2}ratio and the temperature, respectively. This, response variable y is hence approximated by:

^{2}value of 85.93% (Figure 2); this implies that 85.93% of the total variation is represented by Equation (12). The ANOVA analysis further confirms the significance of the model, as the computed F score greatly exceeds the 95% level, 539.87 > 1.9512. Equivalently, the p-value (Figure 3) of the model represented by the equation also indicates that the model is statistically significant.

^{2}is 85.35%, indicating that the model has a good capacity for predicting responses to new observations. The small difference between the value of R

^{2}and the predictive R

^{2}is an indicator that the model is not over fitted. Moreover, the residuals fall relatively close to the adjusted normal distribution line, and it is not possible to reject the normality assumption with α = 0.05. Equation (12) will be further developed in the following section. Figure 4 describes the full quadratic behavior, in which all three critical variables are maintained: Time, MnO

_{2}/Fe

_{3}O

_{4}and temperature. As expected, the manganese recovery increases with the passage of time. Figure 4a,b show that Fe

_{2}O

_{3}/MnO

_{2}and temperature have a qualitatively similar effect over time, although the former is more pronounced. Indeed, Figure 4c confirms the scalable equivalence between Fe

_{2}O

_{3}/MnO

_{2}and temperature, showing recovery as an approximately linear function, increasing in both temperature and Fe

_{2}O

_{3}/MnO

_{2}. Nonetheless, the full dynamic behavior is not well-represented by such a function, as it does not capture the asymptotic tendency of reaction kinetics [30,31].

#### 3.2. Fitting of the Exponential Function

## 4. Conclusions

_{2}/Fe

_{2}O

_{3}ratio and temperature. This work demonstrates the use of laboratory-level testing for the extraction of manganese from marine nodules in an acid medium at different temperatures, and with the use of iron-containing tailings, as a potential step toward industrialization of the process. This source of iron is indeed an effective reducing agent. Future experimental work will be carried out to characterize the constants in Equation (15) better through more batch tests, and to represent the effects of different time scales [30,32]. From the modelling perspective, future work will be to simulate an industrial implementation, and test potential operational responses to feed variations and related risks [38].

_{2}SO

_{4}concentration, Fe

_{2}O

_{3}/MnO

_{2}ratio, stirring speed and temperature.

## Author Contributions

## Funding

## Acknowledgments

^{®}, and for facilitating the chemical analysis of the solutions. We are also grateful to the Altonorte Mining Company for supporting this research and providing slag for this study, and we thank Marina Vargas Aleuy, María Barraza Bustos and Carolina Ossandón Cortés of the Universidad Católica del Norte for supporting the experimental tests.

## Conflicts of Interest

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**Figure 4.**Contour plot of the independent variables Fe

_{2}O

_{3}/MnO

_{2}ratio, Time (

**a**); Temperature, Time (

**b**); and Temperature, Fe

_{2}O

_{3}/MnO

_{2}ratio (

**c**) in Mn recovery (%) (Figures created with Minitab 18).

Mineral | Amount% (w/w) |
---|---|

Chalcopyrite/Bornite CuFeS_{2}/Cu_{5}FeS_{4} | 0.47 |

Tennantite/Tetrahedrite (Cu_{12}As_{4}S_{13}/Cu_{12}Sb_{4}S_{13}) | 0.03 |

Other Cu Minerals | 0.63 |

Cu–Fe Hydroxides | 0.94 |

Pyrite (FeS_{2}) | 0.12 |

Magnetite (Fe_{3}O_{4}) | 58.52 |

Specular Hematite (Fe_{2}O_{3}) | 0.89 |

Hematite (Fe_{2}O_{3}) | 4.47 |

Ilmenite/Titanite/Rutile (FeTiO_{3}/CaTiSiO_{3}/TiO_{2}) | 0.04 |

Siderite (FeCO_{3}) | 0.22 |

Chlorite/Biotite (Mg_{3}Si_{4}O_{10}(OH)_{2}(Mg)_{3}(OH)_{6}/K(Mg)_{3}AlSi_{3}O_{10}(OH)_{2}) | 3.13 |

Other Phyllosilicates | 11.61 |

Fayalite (Fe_{2}SiO_{4}) | 4.59 |

Dicalcium Silicate (Ca_{2}SiO_{4} | 8.30 |

Kirschsteinite (CaFeSiO_{4}) | 3.40 |

Forsterita (Mg_{2}SiO_{4}) | 2.30 |

Baritine (BaSO_{4}) | 0.08 |

Zinc Oxide (ZnO) | 0.02 |

Lead Oxide (PbO) | 0.01 |

Sulfate (SO_{4}) | 0.20 |

Others | 0.03 |

Total | 100.00 |

Parameter/Value | Low | Medium | High |
---|---|---|---|

Time (min) | 5 | 10 | 20 |

Particle Size (µm) | −150 + 106 | −75 + 53 | −47 + 38 |

Sulfuric Acid (H_{2}SO_{4}) | 0.1 | 0.3 | 0.5 |

Fe_{2}O_{3}/MnO_{2} ratio | 1/2 | 1/1 | 2/1 |

Stirring Speed (rpm) | 600 | 700 | 800 |

Temperature (°C) | 25 | 35 | 50 |

Model/Statistic | MAD | MSE | MAPE |
---|---|---|---|

R(t) | 6.19 × 10^{−5} | 3.57 × 10^{−7} | 3.88 × 10^{−4} |

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**MDPI and ACS Style**

Saldaña, M.; Toro, N.; Castillo, J.; Hernández, P.; Trigueros, E.; Navarra, A.
Development of an Analytical Model for the Extraction of Manganese from Marine Nodules. *Metals* **2019**, *9*, 903.
https://doi.org/10.3390/met9080903

**AMA Style**

Saldaña M, Toro N, Castillo J, Hernández P, Trigueros E, Navarra A.
Development of an Analytical Model for the Extraction of Manganese from Marine Nodules. *Metals*. 2019; 9(8):903.
https://doi.org/10.3390/met9080903

**Chicago/Turabian Style**

Saldaña, Manuel, Norman Toro, Jonathan Castillo, Pía Hernández, Emilio Trigueros, and Alessandro Navarra.
2019. "Development of an Analytical Model for the Extraction of Manganese from Marine Nodules" *Metals* 9, no. 8: 903.
https://doi.org/10.3390/met9080903