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Home , Manganese nodule, Pyrolusite

Engineering 163 (2021) 106748

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Minerals Engineering

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Leaching nodules with -reducing agents – A critical review

Norman Toro a,b,*, Freddy Rodríguez b, Anyelo Rojas b, Pedro Robles c, Yousef Ghorbani d a Faculty of Engineering and Architecture, Universidad Arturo Prat, Almirante Juan Jos´e Latorre 2901, Antofagasta 1244260, Chile b Departamento de Ingeniería Metalúrgica y Minas, Universidad Catolica´ del Norte, Av. Angamos 0610, Antofagasta 1270709, Chile c Escuela De Ingeniería Química, Pontificia Universidad Catolica´ De Valparaíso, 2340000 Valparaíso, Chile d MiMeR—Minerals and Metallurgical Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden

ARTICLE INFO ABSTRACT

Keywords: The accelerated growth in global demand for Manganese coincides with the continuing depletion of high-grade Marine nodules on the earth’s surface. This element is essential for diverse markets such as steel production, dietary ad­ MnO2 dissolution ditives, fertilizers, cells, fine chemicals, and some chemical reagents. Interestingly, there are large re­ Iron reducer serves in the ocean’s depths, where marine nodules are an attractive option due to their high manganese content Submarine resources (between 16 and 24%). The dissolution of MnO2 from manganese nodules needs a reducing agent. Iron is a feasible alternative with a low cost and can even be recycled from scrap or industrial waste like smelting slag, tailings, or scrap steel. This paper provides a comprehensive review of Manganese acid-reducing processes from 2+ 0 0 marine nodules using different iron-reducing agents that include FeS2, Fe , Fe , Fe2O3, and Fe3O4. Fe has displayed the best performance in terms of dissolution kinetics and Mn extraction. This review further confers the chemistry and reactions involved in the reductive leaching and stresses the critical parameters that could be considered for optimization. In this sense, the concentration of reducing agent and temperature are highlighted as the most influential, making other parameters (e.g., particle size, stirring speed, acid concentration, and leaching time) almost irrelevant. Finally, it is concluded that the best way to extract Mn from marine nodules is 0 to reuse steel scraps, working with high Fe /MnO2 ratios (2/1), low concentrations of sulfuric acid (0.1 mol/L), and short times (5 min), achieving extractions of 90%.

1. Introduction Xie et al., 2013; Corathers, 2019; Haghshenas et al., 2007). Significant efforts have been made to extract Manganese from low- Nowadays, there is a constant depletion of available mineral deposits grade minerals1; however, this is insufficient to meet the on the earth’s crust (Biswas et al., 2009; Sen, 2010). Manganese (Mn) is required production levels (Su et al., 2010). It is estimated that the one of the twelve most abundant elements (it comprises approximately largest reserves of , , and Manganese are located in the 0.1% of the earth’s crust (Baba et al., 2014)), and can be associated in (COCHILCO, 2016; Toro et al., 2020a). Three mineral-rich re­ different ways, finding , , , and silicates with sources are found in the ocean’s depths: marine nodules, ferromanga­ greater abundance (Nordberg et al., 2007). However, these minerals are nese crusts, and polymetallic sulfides( Sharma, 2015), being the former widely disseminated, and high-grade deposits are scarce (Toro et al., an attractive option due to their high manganese content (between 16 2020b). Besides, Manganese is found in the ’s depths, in the form of and 24%) (Sharma, 2017). nodules, micro , coatings, and crusts (Post, 1999). Corathers Marine nodules, also known as manganese nodules, are rock con­ (2020) reported that global demand for Manganese is increasing. This is cretions formed by concentric layers of iron and manganese of great importance in various industrial application such as alloy for around a core (Senanayake, 2011). These minerals are mainly composed steel production, ferromanganese, non-ferrous alloys, preparation of of δ-MnO2 (hydrogenetic), diaganic 10 Å manganate, mostly containing dietary additives, fertilizers, dry cell batteries, and inorganic fine (formed by oxic ), and Vernardite (formed by chemicals (Astuti et al., 2019; El Barbary and Division, 2018; Das et al., hydrogenic precipitation) (Sharma, 2017). Its formation occurs in 2012; Hariprasad et al., 2013; Ismail et al., 2004; Yang Sun et al., 2017; abyssal plains covered with at water depths of 4000–6500 m,

* Corresponding author at: Faculty of Engineering and Architecture, Universidad Arturo Prat, Almirante Juan Jos´e Latorre 2901, Antofagasta 1244260, Chile. E-mail address: [email protected] (N. Toro). 1 Pyrolusite is a mineral consisting essentially of (MnO2) and is important as an of manganese https://doi.org/10.1016/j.mineng.2020.106748 Received 31 August 2020; Received in revised form 15 December 2020; Accepted 19 December 2020 0892-6875/© 2021 Elsevier Ltd. All rights reserved. N. Toro et al. Minerals Engineering 163 (2021) 106748

Table 1 Table 1 (continued ) The typical leaching processes of Mn(II) ores. Type of leaching Brief description References Type of leaching Brief description References process process o In the absence of , Reductive leaching This approach is mainly Das et al., 1982; Dundua Fe(II) can be used as a with a ferrous iron based on the treatment of and Agniashvili, 1999; Vu reductant in the solution Mn ore or Mn bearing et al., 2005 following leaching slimes with acidified process. ferrous sulfate or pickle Leaching with In a simultaneous leaching Jiang et al., 2003 liquors peroxide process for extraction of Mn Reductive leaching Reductive leaching of Mn Petrie, 1995; Das et al., and Ag by one-step leaching with sulfur dioxide nodules and low-grade Mn 1998; Grimanelis et al., using H₂SO₄ in hydrogen or sulfite ores using SO2 or 1992; Partenov et al., 2004; peroxide attendance, Solutions sulfite SO2- is an effective Ward et al., 2004 hydrogen peroxide plays reductant for higher Mn dual roles in the process: as oxide minerals such as an oxidizing agent for MnO2 and Mn nodules. native and a reducing Reductive leaching Reductive leaching of Sawdust (Sanigok and agent for Mn dioxide. with organic manganiferous ores Bayramoglu 1988), glucose, Leaching with a The process applies the Kholmogorov et al. 1998; Li reductants containing tetravalent Mn sucrose (Veglio’ and Toro, hydrochloric acid chloride as the 2000; Lu and Zou 2001; could be performed using 1994a, Veglio and Toro, solution reductant at high acidity Abdrashitov et al., 2001; organic reductants. 1994b), lactose (Ali et al., and the Cl2 product as the Kuh et al. 2001; Naik et al. 2002; Ismail et al., 2004), oxidant to precipitate the 2002; Yaozhong 2004 glycerine (Arsent’ev et al., reduced Mn as MnO2 in 1991), oxalic acid, citric alkaline conditions. acid, tartaric acid, formic Leaching with The process is involved Dean et al., 1942; Drinkard acid (Sahoo et al., 2001; nitrogen dioxide leaching of the Mn ore with and Woerner,1997 Rodriguez et al., 2004), and and nitric acid NO2 and decomposing the triethanolamine and solutions subsequent thiosulfate (Yavorskaya nitrates (Mn(NO3)2) et al., 1992). solution. Mn(NO3)2 could Bio-reductive Reductive biological Veglio` et al., 1997; be removed by bleeding and leaching treatment that commonly Elsherief, 2000; Acharya re-crystallization. uses heterotrophic et al., 2003; Das and Ghosh, Acid leaching of Mn Mn and silicate Comba et al., 1991; under 2018 (II) carbonate and minerals are soluble in Arsent’ev et al., 1992 direct (the bacteria are silicate ores and acids (e.g., in H2SO4 or adept at using the MnO2 as slags HCl). a finalacceptor of electrons in the respiratory chain of their metabolism, instead of where the accumulation rates are low (Gonzalez´ et al., 2012). oxygen) and indirect (the The coverage of marine nodules is about 50% in large areas of the reductive process is allied abyssal Pacific seabed and the central basin (Hein and with the formation of reductive compounds, Koschinsky, 2013). causing from their The dissolution of MnO2 from manganese nodules is possible by a metabolism) bioleaching leaching process that works at low potential values, using a reducing mechanism. The biological agent (Randhawa et al., 2016) such as coal (Kanungo and Das, 1988; process happens in the presence of organic carbon Zhang et al., 2017), H2SO3 (Han and Fuertenau, 1986; Khalafalla and and energy sources. The Pahlmann, 1981), (Feng et al., 2014a, 2014b; Kanungo, 1999a, bioleaching mechanism was 1999b), methanol (Momade and Momade, 1999), organic acids like primarily indirect through EDTA and oxalic acid (Pankratova et al., 2001; Sahoo et al., 2001), ’ organic acids production sulfur dioxide (Abbruzzese et al., 1990; Grimanelis et al., 1992; Miller (i.e., mainly oxalic acid and citric acid in the leaching and Wan, 1983; Naik et al., 2003; Gamini Senanayake, 2004), hydrogen medium), which reduced peroxide (Jiang et al., 2004; Nayl et al., 2011), cane molasses (Su et al., Mn oxides. 2008), glucose (Furlani et al., 2006; Muthalib et al., 2018), phytolacca Simultaneous In the leaching process of Kholmogorov et al., 1998; americana (Xue et al., 2016), biomass from residual tea (Tang et al., leaching Mn(IV) Mn oxides and Li, 2000; Lu and Zou, 2001; oxide and sulfide minerals (e.g., , Naik et al., 2002; Yaozhong, 2014), residual water from alcohol manufacture from molasses (Su minerals ; or matt, 2004 et al., 2009, 2010), the ear of corn (Tian et al., 2010), sponge iron pyrite; nickel matte, pyritic- (Bafghi et al., 2008), sucrose (Wang et al., 2017), biomass (sawdust and ferrous lignite) straw) (Sun et al., 2018), from flotation tailings (Toro et al., simultaneously using an 2019a), and tannic acid (Prasetyo et al., 2019) (Table 1 shows the acid medium, H2SO4 or HCl, the sulfide minerals typical leaching processes of Mn(II) ores). Among the reducing agents function as reductants mentioned, iron is a viable alternative for manganese extraction due to while the Mn oxides as its low cost and abundance (Toro et al., 2018). This article offers a oxidants. In comparison comprehensive review of Manganese acid-reducing processes from with other sulfides, pyrite concentrate offers some marine nodules using different iron-reducing agents. The review dis­ benefits: cusses the chemistry and reactions involved, emphases on critical pa­ o The iron introduced can rameters that could conceivably affect Manganese’s leaching from be simply removed as marine nodules. iron oxides or hydroxides in the presence of oxygen; and 2. Fundaments

The reductive leaching of MnO2 in the presence of iron is an efficient

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◦ Fig. 1. Potential–pH diagram for Mn–Fe–H2O system at 25 C and unit activity (based on HSC Chemistry 5.1). process, which may reach high Mn extractions in short periods (Toro on active sites existing on the surface of MnO2 particles and controlled + et al., 2020b; Toro et al., 2020c). Several studies have assessed the effect by Fe2 diffusion. Nonetheless, a study by Tekin and Bayramoǧlu of iron as a reducing agent on leaching in acidic media of marine nodules (1993) indicated that stirring speed had an insignificant effect, and the (Bafghi et al., 2008; Kanungo, 1999a, 1999b; Perez´ et al., 2020; Saldana˜ leaching reaction rate was controlled through a surface reaction. The et al., 2019; Toro et al., 2018; Toro et al., 2019a; Toro et al., 2019b; dissimilarity in the leaching process’s control can be ascribed to the Torres et al., 2019; Zakeri, 2007), reporting that the maximum man­ differences in the ore’s chemical composition and the conditions of ganese extraction occurs by increasing the amounts of Fe in the Mn/Fe leaching assays (Bafghi et al., 2008; Xueyi et al., 2003). ratio and at low acid concentrations (Bafghi et al., 2008; Toro et al., 2018). As shown in Fig. 1, for Mn’s dissolution from marine nodules, using a reducing agent (in this case, Fe) is necessary to decrease the 2.1. Studies with Fe reducing agents system’s potential value. Naturally, Manganese typically occurs in three oxidation states (II, 2.1.1. Pyrite III, IV). The comparative stability of each oxidation state in solution is Kanungo (1999a, 1999b) studied hydrochloric acid (HCL) leaching exceedingly affected by the Eh and the pH. Once Eh-pH conditions are at different temperatures with pyrite as a reducing agent. Kanungo changed, Manganese is transmuted into the phase utmost stable under 1999a indicated that by adding FeCl3 to the pyrite solution, H2SO4 is the new conditions if appropriate and thermodynamically approving consequently generated as expressed by Eq. (1). Reaction (1) suggests reaction pathways extant. Nevertheless, forecasting the conversion rates that pyrite’s oxidation rate should be inhibited due to decreased pH. entails a kinetic analysis (Amalia and Azhari, 2017; Baba et al., 2014; However, the increase in the initial concentration of acid to the Martin, 2005). Reductive leaching should be conducted within acid higher dissolution of ferric ions from the sample of manganese nodules. circumstances (lower pH value) in order to efficiently convert Mn(IV) Furthermore, the generated extra FeCl3 causes an increase in pyrite’s from manganese ore into Mn(II). Treatment of manganese ore with oxidation (see Eq. (2)). The MnO2 reduction rate is much faster than the 0 acidified ferrous sulfate or pickling solution was reported for the first pyrite oxidation rate. Eq. (3) shows the formation of S , which can occur time in 1947 and ever since has continuously been explored. A summary at low acidity (pH) and low oxidation potential (Eh), which fitsinto the of relevant studies was presented in a review paper by Sinha and Purcell appropriate pH and Eh ranges for acid-reducing leaching of MnO2. (2019). FeS2 + 14FeCl3 + 8H2O = 15FeCl2 + 12HCl + 2H2SO4 (1) In a study by Zakeri (2007), more than 95% Mn was extracted using + 2+ ore particle sizes of 60 100 μm, Fe /MnO2 molar ratio 1/3, H2SO4/ MnO2 + 2FeCl2 + 4HCl = MnCl2 + 2FeCl3 + 2H2O (2) MnO2 molar ratio of 1/2, and a reaction time of 20 min at room tem­ FeS = Fe2+ + 2S0 + 2e perature. The outcomes oriented that the dissolution of Manganese was 2 (3) + more affected by the concentration of Fe2 ion than the solution’s 2+ acidity. Thus, it was recommended that a surplus Fe /MnO2 ratio be + + used for manganese leaching. A drop of Fe2 and H concentration Table 2 3+ 2+ caused Fe and Mn products. That also led to a decline in mass Thermodynamic information of the reactions with FeS2 like reducing agent transfer and, accordingly, lessening the reaction rate. (based on HSC Chemistry 5.1). ◦ The kinetics of reductive manganese leaching is controlled by Reaction ΔG (kJ/ Equation diffusion through the insoluble layer of the associated minerals and mol) 2+ 2+ primarily limited by the diffusion of Fe and/or Mn through ash 3FeS2 + 4H2SO4 = 3FeSO4 + 4H2O + 7S 187 4 + = + + layers (Baba et al., 2014; Villinski et al., 2003; Dundua and Dobrokho­ 6FeSO4 4H2SO4 3Fe2(SO4)3 4H2O S 27.8 5 + = + + tov, 1982). Most previous studies (e.g., Bafghi et al., 2008; Dundua and 2FeS2 4H2SO4 Fe2(SO4)3 4 H2O 5S 133,9 6 + + = + Dobrokhotov, 1982; Majima et al., 1981) stated that the leaching ensued 15MnO2 2FeS2 14H2SO4 Fe2(SO4)3 15MnSO4 2987,7 7 + 14H2O

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2+ In an acidic medium (pH 1.5), the ferrous ions (Fe ) and ferric ions Table 4 + (Fe3 ) ratio remain constant until 50 min, above which the ratio tends to Comparison between the results obtained when working with ferrous ions and increase exponentially. From this, it is suggested that the reduction of sponge iron for the dissolution of manganese nodules (modified from Bafghi et al., 2008). MnO2 by ferrous ions occurs faster than the oxidation of pyrite gener­ ating ferric ions (Kanungo, 1999a). Experimental Condition Zakeri (2007) Bafghi et al. (2008) + Recently, Torres et al., 2019 carried out leaching tests of manganese Iron-reducing agent Fe2 Fe0 ◦ ◦ nodules using sulfuric acid (H2SO4), adding pyrite at 60 C. They pro­ Temperature ( C) 20 20 posed a series of chemical reactions presented in Table 2 that explains Particle Size of the solid (ore/Fe0) (µm) 250 + 150 250 + 150 H SO /MnO molar ratio 3 3 the dissolution of MnO2. 2 4 2 Fe/MnO molar ratio 2.4 0.8 Eq. (4) shows pyrite (II) dissolution in an acidic environment where 2 Mn extraction (10 min) (%) 80 98 ferrous sulfate (II) formation begins. This reaction is spontaneous ac­ cording to the Gibbs Free Energy calculated using HSC 5.1 software. Ferrous sulfate will fulfillthe function of reducing Manganese (IV) from Table 5 marine nodules. Ferrous sulfate oxidizes spontaneously but slowly in an Reactions for dissolution of MnO from marine nodules using Fe O and Fe O in acidic environment. The formation of ferric sulfate (III) is given in Eq. 2 2 3 2 4 an acidic medium (Modified from Toro et al., 2019b). (5). The direct solution of pyrite to ferric (III) sulfate in an acid medium Δ ◦ expressed by Eq. (6) is a spontaneous reaction slower than Eq. (1). Eq. Reaction G (kJ) Equation + = + (7) shows the overall reaction, where the reducing solution of manga­ Fe2O3(s) 3H2SO4(aq) Fe2(SO4)3(s) 3H2O(l) 163.37 17 + = + + nese (IV) oxide to manganese (II) sulfate and the oxidation of pyrite (II) Fe3O4(s) 4H2SO4(l) FeSO4(aq) Fe2(SO4)3(s) 4H2O 261.30 18 (l) to ferric (III) sulfate occurs in a swift and spontaneous reaction. 2FeSO4(aq) + 2H2SO4(aq) + MnO2(s) = Fe2(SO4)3(s) + 199.52 19 2H2O(l) + MnSO4(aq) 2.1.2. Ferrous ions For the use of ferrous ions, Zakeri (2007) indicated that the increase ◦ in temperature from 20 to 60 C together with a decrease in the man­ Table 6 ganese ’s particle size improves the dissolution rate of MnO2. The 2+ Comparison between foundry slag and tailings, as a reducing agent in the authors mentioned that it is necessary to work on molar ratios of Fe / dissolution of marine nodules. MnO2 and H2SO4/MnO2 in excess of stoichiometric, allowing high 2+ Experimental conditions Toro et al. Toro et al. extraction of Mn in short periods (5 min) and room temperature. (2018) (2019a) Zakeri (2007) proposed the following series of chemical reactions: ◦ Temperature ( C) 25 25 + 2+ + + MnO2 + 4H + 2e = Mn + 2H2O (8) The particle size of Mn nodules and slag/ 75 53 75 53 tailings (μm) 2+ 3+ 2Fe =2Fe + 2e (9) H2SO4 concentration (mol/L) 1 1 MnO2/Fe2O3 ratio 1/2 1/2 2+ + 2+ 3+ MnO2 + 2Fe + 4H = Mn + 2Fe + 2H2O (10) Mn dissolution rate at 5 min (%) 68 67 Mn dissolution rate at 40 min (%) 70 77 The overall reaction shows that the molar amounts of ferrous ions 2+ (Fe ) and acid protons in the solution, regarding the stoichiometric + Fe2 . The authors indicated that sponge Fe delivers better results than amounts represented in Eq. (10), could affect the MnO2 dissolution re­ action. The authors concluded that the optimal operating conditions ferrous ions under the same operating conditions, obtaining Mn ex­ 2+ tractions of 98% at room temperature and short leaching periods (see were a Fe /MnO2 molar ratio of 3.0, an H2SO4/MnO2 molar ratio of 2.0, and a mineral particle size of 60 + 100 Tyler mesh. Extractions of Table 4). It is because the Fe metal produces a high activity ratio through ◦ 95% Mn were obtained in less than 20 min at 20 C. the regeneration of ferrous ions. Bafghi et al. (2008) proposed the following chemical reactions in Table 4. 2.1.3. Sponge iron Bafghi et al. (2008) experimented similarly to Zakeri (2007) but 2.1.4. Slags using sponge Fe in an acid medium. In Table 3, Gibbs free energy values Slag from a smelting plant was used in the study of Toro et al. (2018), are presented, calculated from standard reduction potential values. The taking advantage of the high concentration of Fe2O3. In Table 5, the reactions 14, 15, and 16 involve the use of Fe0, which are thermody­ reactions for the dissolution of marine nodules with the use of sulfuric + namically favorable concerning those that consider Fe2 (Eq. (13)). acid and tailings are presented, where the primary reducing agent is Subsequently, Bafghi et al. (2008) compared their results when ferrous sulfate, generated from the leaching of (Eq. (17)) and working with Fe0 with respect to Zakeri (2007) when working with mainly magnetite (Eq. (18)) (majority component in tailings) when reacting with sulfuric acid. Then, this reducing agent (FeSO4) allows 4+ 2+ reducing MnO2 to (Mn ), obtaining a manganese sulfate (Mn ) (Eq. Table 3 (19)). Thermodynamic information of the reactions with Fe0 like reducing agent (based on HSC Chemistry 5.1). 2.1.5. Tailings ◦ Reaction ΔG (J/ Equation Toro et al. (2019a) used the tailings obtained by the flotation mol) from smelting slags with high Fe2O3 and Fe3O4 contents. These tailings + 2+ Fe(s) + 2H (aq) = Fe (aq) + H2(g) 84,900 11 samples contained a higher amount of magnetite and hematite than + + Fe(s) + 2Fe3 (aq) = 3Fe2 (aq) 234,000 12 typical slags. In the study, over 77% of extractions were achieved with a 2+ + 2+ MnO2(s) + 2Fe (aq) + 4H (aq) = Mn (aq) + 88,600 13 3+ MnO2/Fe2O3 ratio of 0.5, 1 mol/L of H2SO4 in 40 min, being higher than 2Fe (aq) + 2H2O(l) + 2+ 3+ that obtained with slags under the same operational conditions (see MnO2(s) + 2Fe(s) + 8H (aq) = Mn (aq) + 2Fe (aq) 258,000 14 + 2H2O(l) + 2H2(g) Table 6). The authors concluded that tailings from the flotationprocess + 2+ 2+ MnO2(s) + Fe(s) + 4H (aq) = Mn (aq) + Fe (aq) + –322,000 15 were a better additive for reducing MnO2 than foundry slags. This is 2H2O(l) probably because the tailings were previously exposed to chemical re­ + 2+ MnO2(s) + 2/3Fe(s) + 4H (aq) = Mn (aq) + 2/ 244,000 16 3+ agents during the flotation process. 3Fe (aq) + 2H2O(l)

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Table 7 dissolution of Mn from marine nodules using FeS2 is of first-order Effect of the addition of FeS2 on the maximum concentrations of Fe II, Fe III, and regarding the initial concentration of acid and the concentrations of + + Fe for acid-reducing leaching of in the presence of pyrite at Fe2 and Fe3 , and second-order to pyrite concentration. ◦ 75 C, Solid/Liquid 1/5, particle size 153, +105 mm (Modifiedfrom Kanungo Zakeri (2007) analyzed the effect of ferrous ions to dissolve MnO2 in 1999b). 2+ an acidic medium, working on Fe /MnO2 molar ratios of 2.0, 2.4, and + + + + Pyrite added (% [Fe3 ]m [Fe2 ]m [Fe]T [Fe2 ]m/ [Fe3 ] 3.0 (1, 1.2, and 1.5 times the stoichiometric amount, respectively), at + by wt. of mmol/L mmol/L mmol/L [Fe3 ]m m/[Fe]T ◦ 20 C, H2SO4/MnO2 = 2.0, and a mineral particle size between 60 and nodule) 100 Tyler mesh. The researchers observed that Mn extractions for molar 10.0 4.70 0.142 4.842 0.030 0.9707 ratios of 2.0 and 2.4 were similar, but when going from 2.0 to 3.0, the 15.0 5.40 0.250 5.650 0.046 0.9558 increase was close to 30%, achieving extractions greater than 90% in 10 20.0 6.20 0.300 6.500 0.048 0.9538 ’ 25.0 7.10 0.480 7.580 0.068 0.9367 min. The authors suggested that the gangue s presence around the larger MnO2 particles may hinder the diffusion speed. The loss of the reagents + + (Fe3 and Mn2 ) concentrations reduced the driving force for mass 3. Operational variables transfer, decreasing the reaction speed. Subsequently, Bafghi et al. (2008) evaluated the dissolution of ma­ 3.1. Effect on reducing agent concentration rine nodules at different concentrations of sponge Fe (molar ratio of iron to MnO2 of 0.67, 0.8, 1.0, and 1.2), H2SO4 concentrations (molar ratios There is a consensus by various researchers about the positive effect of acid to MnO2 of 2.0, 2.4 and 3.0), and particle size of mineral and ◦ of increasing the Fe/Mn ratio on the dissolution of manganese nodules sponge iron between 250 and 150 μm at 20 C. When working at Fe0/ (Kanungo 1999a, 1999b; Zakeri, 2007; Bafghi et al., 2008, Toro et al., MnO2 molar ratios of 0.67 and 0.8, Mn’s maximum extraction was 80%. 2018; 2019a; 2019b, Saldana˜ et al., 2019, Perez et al., 2020). Working at However, by increasing the reducing-agent beyond these levels, the high concentrations of an iron-reducing agent allows increasing the extraction increased significantlyby 97% when the H2SO4/MnO2 molar concentration and activity of ferrous ions. This favors the dissolution of ratio was 2.4. This may be because, with the sufficiency of H2SO4, the MnO2, decreasing the leaching times significantly, and avoiding Fe oxidation reaction of ferrous ions to ferric ions in the presence of MnO2 precipitates formation through oxidation–reduction reactions. occurs at a high rate. Bafghi et al. (2008) finally concluded that the ef­ 0 Kanungo 1999b evaluated the dissolution of manganese nodules at ficiencyin dissolving MnO2 is more sensitive to the amount of Fe than ◦ 75 C by adding FeS2, HCl at 1 mol/L, and particle size (nodule) of 153 the concentration of H2SO4. + μ ◦ 105 m. The author observed that the MnO2 dissolution increased as Toro et al. (2018) worked at 25 C with different slag/MnO2 ratios, FeS2 increased from 10 to 25 wt% concerning the marine nodule sample. particle size of 200 +270 Tyler mesh, and sulfuric acid concentration 2+ 3+ 3+ The Fe /Fe ratio increases, while the Fe /Total Fe ratio decreases of 1 mol/L. The authors concluded that Mn’s extraction times reduces at slightly at their maximum concentrations as the pyrite content increases high slag/MnO2 (2/1) ratios and 1 mol/L H2SO4, achieving extractions - (Table 7). By increasing the concentration of FeCl3, the amount of Cl close to 70% in 5 min. P´erez et al. (2020) continued this investigation by 3+ ions increases considerably. Therefore, stable Fe chloride complexes testing higher slag concentrations (slag/MnO2 ratio of 3/1) at different were formed, resulting in a decrease in pyrite’s oxidation rate. However, concentrations of sulfuric acid (0.1, 0.3, 0.5, and 1 mol/L). The authors ’ this did not happen when pyrite was incremented since HCl s chloride found that the optimal ratio of slag/MnO2 is 2/1 and indicates that the ions were limited. The resting potential of FeS2 was practically not acid concentration was irrelevant, achieving practically the same results altered since there were large amounts of pyrite. Kanungo 1999b when working at 1 mol/L or 0.3 mol/L H2SO4. For the use of tailings, the + 4+ concluded that pyrite’s order represents the Fe2 /Mn molar ratio studies carried out by Toro et al. (2018, 2019a; 2019b) indicated that an since MnO2 does not react directly with pyrite. Therefore, the optimal tailings/MnO2 ratio was 2/1; however, the authors mentioned

Fig. 2. Effect of the potential and pH in solution at different MnO2/Fe2O3 ratios (25 ◦C, 600 rpm, 75 + 53 µm, acid concentration of 0.1 mol/L) (Modified from: (Toro et al., 2019b)).

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◦ 2+ Fig. 3. Effect on MnO2/reducing agent ratio at 25 C, 0.1 mol/L H2SO4 and 600 rpm (Reducing agent: FeS2 (a), Fe (b), FeC (c) and Fe2O3 (d)). (Modified since: (Torres et al., 2019)). that a concentration of 0.1 mol/L sulfuric acid is enough with high stoichiometric amount, respectively). As the sulfuric acid concentration concentrations of reducing agents. This is because the FeSO4 reduces the increases, there was an increase in Mn extraction. The extraction effi­ system’s potential in values that favor the dissolution of MnO2; besides, ciency increased by 10% when rising the ratio from 2.0 to 2.4. Working low acid concentrations (0.1 mol/L) are enough to extract Mn (See at a ratio of 3.0, a significantly higher dissolution rate was achieved, Fig. 2). which was attributed to the increase in mass transfer driving force. Torres et al. (2019) analyzed Fe’s levels from different reducing Subsequently, Bafghi et al. (2008) studied an iron sponge under similar agents, working at a low acid concentration (0.1 mol/L H2SO4). As conditions to those used in the study by Zakeri (2007). The authors + shown in Fig. 3, when working with Fe0, Fe2 , and tailings, the optimal indicated that the concentration of Fe0 is more critical for the dissolution Fe/MnO2 was 2/1, where higher Fe concentrations do not obtain better of manganese dioxide than sulfuric acid. In the studies carried out by Mn extractions. The best results were obtained with Fe0 since it allows Toro et al. (2018, 2019a; 2019b) with the use of smelting slags and having a high activity ratio in the regeneration of ferrous ions, allowing tailings, both with high Fe2O3 and Fe3O4 contents, the authors + even better results compared to the direct use of Fe2 . The opposite concluded that the concentration of sulfuric acid is only relevant when occurs when working with pyrite, where a progressive rise occurs with the concentration of reducing agent in is low. While working at Fe2O3/ increasing the amounts of this reducing agent. The authors concluded MnO2 ratios of 2/1 or higher, the concentration of sulfuric acid is that it is possibly due to the low concentration of acid used that does not irrelevant to the dissolution of MnO2. Torres et al. (2019) evaluated the favor dissolution kinetics. This was consistent with Kanungo (1999a), H2SO4 concentration comparing different iron-reducing agents under ◦ who stated that a high initial concentration of acid is necessary to favor the same operational conditions (25 C, reducing agent/MnO2 ratio of the formation of ferrous ions. 2/1, 600 rpm and a particle size between 75 and 53 μm), except when ◦ working with pyrite where the temperature was 60 C (see Fig. 4). The + authors concluded that for the use of Fe2 , Fe0, and tailings in high 3.2. Acid concentration concentrations, the potential values were kept low (see Fig. 5). There­ fore, high Mn extractions were obtained despite working at a low acid Kanungo (1999a, 1999b) indicated that for the leaching of marine concentration (0.1 mol/L). This did not occur when working with FeS2, nodules using HCl and the pyrite, an accumulation of ferric and ferrous possibly due to the pyrite mineral’s refractoriness. ions was generated initially up to a particular time. This reduced as the medium’s acidity decreases, where the initial concentration of acid and the system’s temperature were essential. 3.3. Temperature For the use of ferrous ions, Zakeri (2007) highlighted the importance of sulfuric acid concentration. The authors worked with a 3/1 M ratio of The effect of temperature on acid leaching of marine nodules has H2SO4/MnO2, namely 2.0, 2.4, and 3.0 (1, 1.2, and 1.5 times the been investigated in many studies. The temperature positively affects

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2+ 0 Fig. 4. Effect on acid concentration (reducing agent: (a) FeS2, (b) Fe , (c) Fe and (d) tails). (Modified since: (Torres et al., 2019)).

Fig. 5. Effect of potential and pH on the dissolution of marine nodules with different reducing agents (Modifiaco since: (Torres et al., 2019)).

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Table 8 the recovery of elements of interest, selectivity, kinetics, and acid con­ Experimental values to statistical model (From: (Saldana˜ et al., 2019). sumption (Toro et al., 2020b). Parameter/Value Low Medium High Kanungo (1999a, 1999b) evaluated the effect of temperature ◦ (40–85 C) to dissolve marine nodules adding 15 wt% of pyrite and 1 Time (min) 5 10 20 Particle Size (µm) 150 + 106 75 + 53 47 + 38 mol/L of HCl. As the temperature increases, the author found a faster H2SO4 (mol/L) 0.1 0.3 0.5 reduction in MnO2, whereby the pH drops sharply, increasing the value 2+ 3+ F2O3/MnO2 ratio 1/2 1/1 2/1 in the Fe /Fe ratio. The dissolution kinetics of MnO2 increases Stirring Speed (rpm) 600 700 800 ◦ ◦ sharply with increasing temperature from 10 to 60 C, but when working Temperature ( C) 25 35 50 ◦ at 75 C, the increase was relatively slow. Zakeri (2007) evaluated the effect of temperature on the leaching of

Fig. 6. Impact of the variables in the regression model (From: (Saldana˜ et al., 2019)).

Fig. 7. Contour plot of the independent variables Fe2O3/MnO2 ratio, Time (a); Temperature, Time (b); and Temperature, Fe2O3/MnO2 ratio (c) in Mn recovery (%) (From: (Saldana˜ et al., 2019)).

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particle size of 600 + 250 µm and 250 + 150 µm to avoid dissolving so quickly that it prevents taking a sufficientnumber of samples before the reaction is complete. 100% Mn extractions were obtained in just 3 ◦ min at 60 C. They concluded that despite the beneficial effect of tem­ perature, the dissolution rate of MnO2 adding high Fe concentrations is fast, where at room temperature, complete dissolution was achieved in a time of 10 min. Saldana˜ et al. (2019) fitted an analytical model for manganese recovery, considering that first-orderequations can describe the system’s behavior. However, the proposed solution was an inverse exponential model. Then, as the exponent determines the leaching ki­ netics, a multiple regression model was developed based on the inde­ pendent variables. This describes its variability and consequently explains the recovery of Mn for the sampled domain. The operational variables studied were decided based on previous studies’ results to dissolve manganese nodules in acidic media and Fe reducing agents (See Table 8). Saldana˜ et al. (2019) discovered that, by interacting with all these operational variables, only the reducing agent concentration and tem­ Fig. 8. Effect of the ore particle size on dissolution efficiency of manganese perature have a significanteffect on the dissolution of MnO (see Fig. 6). ◦ + 2 (20 C, Fe2 /MnO = 2.4, H SO /MnO = 2.0). (Modified from: 2 2 4 2 At high concentrations of FeSO4 or high temperatures, a higher amount (Zakeri, 2007)). of MnO2 solutions can be obtained in brief periods (Fig. 7 a and b). Furthermore, the authors indicate no synergy between the reducing ◦ 2+ marine nodules at 20, 40, and 60 C, under fixed conditions of Fe / agent concentration and the temperature, showing Mn’s recovery as an = = + MnO2 2.4, H2SO4/MnO2 2.0, and mineral particle size of 16 20 approximately linear function (See Fig. 7 c). mesh. The authors found an increase in both speed and Mn extractions ◦ with increasing temperature, where at 60 C, it was possible to extract 96% of the Mn in 60 min. 3.4. Effect on particle size Bafghi et al. (2008) studied the same temperature ranges that Zakeri 0 0 (2007), but at lower molar concentrations of Fe (Fe /MnO2 = 2.0) and Researchers have no consensus regarding the effect of particle size in ’ a higher concentration of sulfuric acid (H2SO4/MnO2 = 4.0) to avoid manganese nodules solution using Fe reducing agents. Some research exhaustion of the reagents. Also, they worked at a reducing agent indicates that the particle size has a very positive effect on the dissolu­ tion kinetics of MnO2 (Zakeri 2007, Bafghi et al., 2008), while others mention that the particle size was very little relevant (Kanungo 1999b, Table 9 Toro et al., 2018, Saldana˜ et al. 2019). Chemical analysis of the different sieve fractions of the two manganese nodule Regarding the positive effect on particle size, Zakeri (2007) indicated samples (Modified from Kanungo 1999a). that the reaction rate increases considerably as the manganese nodule’s Sieve fractions (B.S.S.) Size (mm) Mn (%) Fe (%) particle size decreases. They attributed this to control the internal mass transfer speed, which was corroborated by kinetic analysis. In their Sample 1 25, +36 600, +420 11.00 11.80 ’ 36, +44 420, +355 11.50 12.00 studies, the mineral s MnO2 dissolution rate with a particle size of 100 60, +100 251, +153 10.10 11.46 + 170 Tyler mesh was more significant than the size fraction of 60 + 100, +150 153, +105 10.33 11.30 100 Tyler mesh (see Fig. 8). Bafghi et al. (2008) compared the effect of + + 150, 200 105, 76 9.37 11.43 particle size of manganese nodule ore and sponge iron under the same 200, +300 60, +53 9.63 11.37 Sample 2 60, +100 251, +153 20.20 9.49 operational conditions. The authors indicated that the sponge iron 100, +150 153, +105 20.10 10.02 particle size has a low effect on the dissolution efficiency of MnO2 150, +200 105, +60 19.56 10.03 compared to the particle size of the marine nodule, which does have a 240, +300 60, +53 19.85 10.22 positive effect on the dissolution rate. This may be explained because the

Fig. 9. Decrease in particle size over time from its initial size, until the leaching process is finished for samples 1 and 2 of manganese nodules (Modified from: (Kanungo, 1999b)).

9 N. Toro et al. Minerals Engineering 163 (2021) 106748

Table 10 surface or the edges. Reductive leaching of MnO2 also coincides with the Experimental conditions for studying the effect of manganese nodule particle tiny particles’ surface through a mixed process of chemical diffusion and size (Toro et al., 2018). dissolution. After a particular leaching time, the coarser grains’ di­ Parameters Values mensions no longer change due to silicate minerals within the ferro­

Sieve fraction (Tyler Mesh) 100 + 140, 200 + 270, 320 + 400 layers. For the finergrains, the degree of release of the Particle size (µm) 150 + 106, 75 + 53, 47 + 38 ferromanganese oxide grains with respect to the other minerals that Time (in min) 5, 10, 20, 30, 40 accompany the marine nodule helps Mn’s dissolution. H2SO4 (M) 0.1, 0.5, 0.75, 1 Toro et al. (2018) analyzed the effect of particle size at different acid MnO2/Fe (slag) 1/1 concentrations, keeping the other variables static (see Table 10). The authors indicated that particle size did not positively affect MnO2 iron’s porous structure facilitates the reagents’ mass transfer, making its dissolution, obtaining similar results for the three size fractions studied size irrelevant. (see Fig. 10). However, the tests were performed at high concentrations ˜ Regarding the low relevance of particle size, Kanungo (1999b) of Fe/MnO2, so according to what was stated by Saldana et al. (2019), studied the effect of particle size for two samples of manganese nodules this could have made the effect on particle size irrelevant. (see Table 9). The authors observed that for sample 1, Mn extractions increased slowly for coarse size fractions ( 600) mm to ( 251, +153) 4. Conclusions and outcomes mm, but with a slight increase in the finestsize fractions ( 153, +100) mm and ( 60, +53) mm. While in Sample 2, there was a uniform in­ The acid-reducing leaching of manganese from high MnO2 minerals crease as the particle size decreased, except for the finersize ( 60, +53) is attractive for its high Mn extractions and rapid dissolution kinetics. mm. However, it was concluded that this occurs due to the differences in This leaching type generates better results in Mn’s recovery than the the samples’ iron contents. Because the physical changes that samples 1 traditional process, which involves reduction at high temperatures fol­ and 2 obtain during the reaction did not allow gaining much information lowed by acid leaching. It is also a sustainable process with the envi­ regarding the particle size’s impact. Kanungo (1999b) made a log/log ronment, which meets future standards of the low-carbon economy. graph of time vs. initial average diameter for the different leachates Promising studies have been reported on various reducing agents for fractional (see Fig. 9). For sample 1, the slopes’ values decreased for the the dissolution of manganese nodules in acidic media, presenting Fe as a coarse size fractions. While in sample 2, the straight lines were almost possible alternative to be used widely in the extractive metallurgy in­ parallel to each other. The author concluded that this could be explained dustry. It has a low cost and can even be recycled from scrap or indus­ in the following way: First, when the samples are exposed to the reac­ trial waste smelting slag, tailings, or scrap steel. The relative stability of tant, the quick dissolution of iron occurs mainly from the particles’ three oxidation states (II, III, IV) for Manganese in solution is

◦ Fig. 10. Effect of particle size (manganese nodule) on Mn extraction (25 C, H2SO4: 0.1, 0.5, 0.75, 1 M). (A): 150 + 106 μm, (B): 75 + 53 μm, (C) 47 + 38 μm) (Modified from: (Toro et al., 2018)).

10 N. Toro et al. Minerals Engineering 163 (2021) 106748 significantly affected by the Eh and the pH. Reductive leaching should Miner. Process. Extr. Metall. Rev. 30 (2), 163–189. https://doi.org/10.1080/ be performed within acid circumstances to competently transform Mn 08827500802397284. COCHILCO. 2016. Situacion´ actual del mercado de tierras raras y su potencial en Chile. (IV) from manganese ore into Mn(II). The leaching advance on active 1–48. https://www.cochilco.cl/Listado Temtico/Tierras Raras final.pdf%0A. sites present on the surface of MnO2 particles and the kinetics of http://www.mch. reductive manganese leaching is controlled by diffusion through the cl/reportes/cartera-de-proyectos-2015-2024-inversion-en-la-mineria-chilena/#. Comba, P., Lei, K.P.V., Carnahan, T.G., 1991. fluoride enhanced hydrochloric insoluble layer of the associated minerals, and predominantly restricted acid leaching of a manganese-bearing silicate ore. U.S. Bureau of Mines Report of 2+ 2+ by the diffusion of Fe and/or Mn through ash. However, some Investigations. studies indicate that manganese leaching reaction rate is controlled Corathers, L., 2019. Manganese, Mineral Commodity Summaries (Issue 703). 2+ Corathers, L.A., 2020. Manganese Statistics and Information. National Minerals through the surface reaction rather than the F ion diffusion. The Information Center. https://www.usgs.gov/centers/nmic/manganese-statistics-an variation in the leaching process’s control can be attributed to the dif­ d-information. ferences in the chemical composition of ore and conditions that leaching Das, A.P., Ghosh, 2018. Bioleaching of Manganese from mining waste materials. Mater. Today:. Proc. 5 (1), 2381–2390. test is conducted. Das, A.P., Swain, S., Panda, S., Pradhan, N., Sukla, L.B., 2012. Reductive Acid Leaching 0 Among the Fe reducing agents, Fe presents the best outcomes since of Low Grade Manganese Ores. Geomaterials 02 (04), 70–72. https://doi.org/ it allows a high activity ratio, favoring the regeneration of ferrous ions. 10.4236/gm.2012.24011. On the other hand, reducing agents such as Fe O , Fe O , and FeS have Das, P.K., Anand, S., Das, R.P., 1998. Studies on reduction of manganese dioxide by 3 4 2 3 2 (NH4)2SO3 in ammoniacal medium. 50 (1), 39–49. lower dissolution kinetics and Mn extractions, even requiring the use of Das, S.C., Sahoo, P.K., Rao, P.K., 1982. Extraction of Manganese from low-grade temperature in the case of pyrite. Considering the operational variables, manganese ores by ferrous sulfate leaching. Hydrometallurgy 8 (1), 35–47. the concentration of reducing agent and temperature are identified as Dean, R.S., Fox, A.L., Beck, A.E., 1942. Nitrogen dioxide process for recovery of Manganese from ores. U.S. Bur. Mines Rep. Invest. 3626, 30 pp. the most influentialfactors, but no synergistic effects have been detected Drinkard, Jr., W.F., Woerner, H.J., 1997. Leaching of metallurgical dust wastes in nitric between them. Other parameters (e.g., particle size, stirring speed, acid acid for recovery of metal values. WO Patent No. 9716230. concentration, and leaching time) are almost irrelevant for the perfor­ Dundua, R.G., Dobrokhotov, G.N., 1982. Kinetics of Fe (II) oxidation by pyrolusite in sulfuric-acid-solutions. J. Appl. Chem. USSR 55 (8), 1685–1688. mance of the dissolution. Therefore, it is concluded that the best way to Dundua, R., Agniashvili, G., 1999. Manganese recovery from residual slimes in the process manganese nodules by acid-reducing processes adding iron is electrochemical manufacture of manganese dioxide. Izv. Akad. Nauk Gruz. SSR, Ser. with the use of Fe0 (can be by scrap steel) in high concentrations (Fe0/ Khim. 25 (1–2), 151–154. Elsherief, A., 2000. A study of the electroleaching of manganese ore. 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