Mineralogy of Deep-Sea Manganese Nodules and Advances in Extraction Technology of Valuable Elements from Manganese Nodules

Abstract

The vast seabed holds tremendous resource potential that can provide necessary materials for future human societal development. This study focuses on the mineralogy of seafloor manganese nodules off the coast of China in the Western Pacific and the primary techniques for extracting valuable metal elements from manganese nodules. The research indicates that the main valuable metal elements in the manganese nodules from this region include Cu, Co, Ni, Mn, Fe, etc. The key to extracting these valuable metals lies in reducing Mn(IV) to Mn(II) to disrupt the structure of the nodules, thereby releasing the valuable elements. The extraction processes for the main valuable metal elements of manganese nodules are mainly divided into two categories: pyrometallurgical–hydrometallurgical and solely hydrometallurgical. In order to cope with the challenges of environmental change and improve utilization efficiency, bioleaching, hydrogen metallurgy, and co-extraction are gaining increasing attention. For promoting commercialization, the future development of manganese nodule resources can refer to the technical route of efficient short-process extraction technology, the comprehensive recovery of associated resources, and tail-free utilization.

1. Introduction

The vast depths of the ocean harbor rich reserves of energy resources, mineral resources, biological resources, and genetic resources. These resources offer potential solutions to humanity’s energy and food crises, providing opportunities for future survival and development. As terrestrial resources diminish day by day, humanity’s reliance on marine resources continues to grow. The utilization and development of deep-sea resources have become a crucial stage for countries worldwide to demonstrate their comprehensive national strength and international influence [1,2,3,4,5,6].

Manganese nodules, also known as polymetallic nodules, contain over 30 types of metal elements and represent a major form of deep-sea mineral resources. These nodules are extensively distributed across the seafloors of the Pacific, Atlantic, and Indian Oceans at depths ranging from 3500 to 6000 m [7]. The reserves of some metals in manganese nodules are dozens or even hundreds of times that of terrestrial resources. The most commercially valuable ones include copper, cobalt, nickel, and manganese. It happens that these four metals are energy metals that are currently in short supply [8]. Relevant data shows that the abundance of manganese nodules at the bottom of the ocean is mostly between 5~30 kg/m². The reserves of manganese nodules in the Pacific Ocean alone can reach 1.7 trillion tons, of which the metal content of manganese (Mn) is 400 billion tons, the metal content of nickel (Ni) is 16.4 billion tons, the metal amount of copper (Cu) is 8.8 billion tons, and the metal amount of cobalt (Co) is 5.8 billion tons. These reserves are equivalent to more than 400 times the current terrestrial Mn reserves, more than 1000 times the Ni reserves, 88 times the Cu reserves, and more than 5000 times the Co reserves. Therefore, manganese nodules can provide sufficient raw materials for new energy industries such as batteries, which is a beneficial supplement to the shortage of land-based energy metals. It can help humans cope with climate change and achieve the goal of “carbon peaking and carbon neutrality” [9,10,11,12,13,14,15]. Figure 1 presents the distribution of major mineral resources in the deep sea and their spatial overlay with the exclusive economic zones.

The mining of seabed mineral resources from water depths below thousands of meters faces huge technical challenges [17]. It involves multi-disciplinary fields such as exploration, mining, mineral processing, metallurgy, biology, environment, machinery, materials, and automation, which is a collection of cutting-edge technologies. Due to mining costs and marine environmental protection, manganese nodules have not yet been recovered and utilized on a large scale [18,19,20,21]. However, due to earlier awareness of the importance of seabed mineral resources, some countries and institutions have mastered key technologies and core equipment manufacturing capabilities for deep-sea mineral mining. For example, The Metals Company (TMC) of Canada has conducted an in-situ mining test at a sea depth of 4300 m in the Clarion–Clipperton zone (CCZ) in October 2022, and collected about 3600 t of deep-sea manganese nodules. At the same time, the corresponding environmental assessment is underway [22,23,24,25,26,27]. TMC plans to achieve commercial mining of manganese nodules on a scale of tens of millions by 2025, which means that the development and utilization of deep-sea mineral resources has entered a new stage.

With the advancement of mining technology, the extraction technology of valuable metal elements from manganese nodules has also attracted more and more attention. Unlike the occurrence state of terrestrial mineral resources, even though deep-sea manganese nodules contain higher concentrations of valuable metal elements, they are highly dispersed within the nodules in the form of fine particles, making it challenging to achieve effective enrichment through traditional ore beneficiation methods [28]. Detailed mineralogical studies play an important role in guiding the extraction process of valuable metal elements from manganese nodules. Building upon previous research, this paper conducts a detailed investigation of the mineralogy of manganese nodules along the southeastern coast of China, and reviews the existing extraction processes of valuable elements from manganese nodules. The objective is to serve as a reference for the subsequent improvement of low-cost, high-efficiency, and environmentally friendly extraction technologies for manganese nodules, thus promoting the commercial development and utilization of manganese nodules.

2. Mineralogy of Deep-Sea Manganese Nodules

The manganese nodules in this study were obtained from the southeastern coast of China in the Western Pacific Ocean during sea trials by the Changsha Institute of Mining and Metallurgy. As shown in Figure 2, the color of manganese nodules is often black or brown-black, and the shapes are mostly spherical, elliptical, potato-shaped, flat, slag-shaped, etc., with different sizes. It can be observed that the manganese nodules are composed of a loose core and a dense shell after cutting the manganese nodules in half, which is consistent with relevant reports. The main minerals of manganese nodules are cryptocrystalline iron-hydroxide manganese ore (δ-MnO₂·H₂O) and amorphous ferrihydrite (FeOOH), which are usually epitaxially intertwined [29,30,31]. The rest of the minerals are fine-grained clastic minerals, including quartz, feldspar, pyroxene, clay, zeolite, and authigenic mineral carbon fluoroapatite (CFA). The elements in manganese nodules in the Pacific can be roughly classified according to their mass fractions, as shown in

Table 1. Classification of elements in manganese nodule. Adapted from Ref. [35].

Classification	Element
Main element (>1%)	Mn (13~27%), Fe (6~18%), SiO2 (5%), Al2O3 (1.2%), MgO (1.5%), Na2O (1.8%)
Minor element (600 × 10−6~1%)	Co (0.3~1.2%), Fe (1670 × 10−6~7250 × 10−6%), Cu (573 × 10−6%), Ba, Sr, Pb
Trace element (<600 × 10−6%)	Mo, W, Pt, Pd, Nb, Ga, Te, Sc, Y

. The element contents indicate that the metal elements with greater economic value in deep-sea manganese nodules are mainly Mn, Fe, Ni, and Co, followed by Cu, etc. [32,33,34].

2.1. Composition and Content of Manganese Nodules

The representative manganese nodules were crushed and homogenized for X-ray fluorescence (XRF) analysis, multi-element chemical analysis, X-ray diffraction (XRD) analysis and chemical phase analysis. As present in results of XRF semi-quantitative analysis (

Table 2. Results of XRF analysis (mass fraction, %).

O	Mn	Si	Fe	Al	Ca
33	29.91	10.28	9.19	3.34	2.45
Na	Mg	Ni	K	Cu	Cl
2.45	2.17	1.44	1.28	1.18	0.94
Ti	Ba	Co	P	S	Zn
0.68	0.342	0.27	0.27	0.22	0.144
Sr	V	Mo	Pb	Ce	Zr
0.0765	0.0707	0.0573	0.559	0.051	0.0387
Tl	W	La	Y	Nb	Rb
0.0335	0.032	0.023	0.0129	0.003	0.002

), the element contents of Mn, Fe, Ni, Co, and Cu that are valuable for recovery in this manganese nodule sample are 29.92%, 9.92%, 1.44%, 0.27%, and 1.18%, respectively. The gangue elements with high content include Si, Al, Ca, Na, Mg, K, etc. The elements with lower content such as Ti, Ba, Pb, Zn, Mo, W, Ce, etc. can be considered for comprehensive recovery.

Further chemical analysis results (

Table 3. Results of multi-element chemical analysis (mass fraction, %).

Mn	Cu	Co	Ni	Na	Mg
24.68	0.92	0.23	1.05	1.80	2.19
K	Ca	Si	Fe	Al	W
1.01	1.86	7.76	7.85	2.65	0.0072
V	Mo	Pb	Zn	P	S
0.04	0.047	0.052	0.13	0.30	0.15

) indicate that the contents of Mn, Fe, Ni, Co, and Cu are 24.68%, 7.85%, 1.05%, 0.23%, and 0.92%, respectively. Coupled with the phase analysis results (

Table 4. Phase analysis results of manganese.

Phase of Mn	Manganous Silicate	Manganese Dioxide	Ferromanganese Oxide	Manganous Oxide	Total
Content/%	0.19	6.92	16.23	1.54	24.88
Distribution/%	0.76	27.81	65.23	6.19	100.00

,

Table 5. Phase analysis results of cobalt and nickel.

Phase	In Manganese Oxides	In Iron Oxides	Total
Content of Co/%	0.23	0.0005	0.2305
Distribution of Co/%	99.78	0.22	100.00
Content of Ni/%	1.04	0.0082	1.0482
Distribution of Ni/%	99.22	0.78	100.00

and

Table 6. Phase analysis results of copper.

Phase of Cu	Free Copper Oxide	Bound Copper Oxide	Primary Copper Sulfide	Secondary Copper Sulfide	Total
Content/%	0.89	0.02	0.005	0.005	0.92
Distribution/%	96.74	2.17	0.54	0.54	100.00

), it can be concluded that Mn mainly exists in the aggregate of manganese iron oxides, accounting for 65.23%, followed by 27.81% of manganese dioxide, 6.19% of manganese oxide and 0.76% of manganese silicate minerals. More than 99% of Co and Ni are distributed in manganese oxides, which is related to the formation mechanism of manganese nodules. During the formation of manganese nodules, Co²⁺ in seawater is adsorbed by the negatively charged hydrated MnO₂ colloid under the action of external coordination adsorption and oxidized to Co³⁺. Ni²⁺ is also fixed on MnO₂ through adsorption. Therefore, the distribution of Co and Ni is positively correlated with Mn. However, iron oxyhydroxide that is positively charged is easily adsorbed by certain hydrated metal ions that are negatively charged. The distribution of these metal elements is positively related to Fe and mainly includes Pb, V, As, Mo, W, Ti, Th, Zr, rare earth elements (REE), etc. [36,37,38,39,40]. The formation mechanism of manganese nodule is shown in Figure 3, which illustrates the co-occurrence relationship between various elements.

The Cu content in this nodule is 0.92%, which is far higher than the average Cu content of the Pacific manganese nodule (573 × 10⁻⁶%), and has great recovery value. Cu in seawater exists primarily in the form of hydrated divalent cations or carbonate complexes. Its concentration and distribution are influenced by geographical location, water depth, sediment types, and other environmental factors. It does not exhibit a strong correlation with the distribution patterns of Mn or Fe. A total of 96.02% of Cu in the manganese nodule in this area is in the form of free copper oxide.

As can be seen from Figure 4, the intensity of the XRD peak of the manganese nodule is very weak, and amorphous minerals such as vernadite are difficult to detect, which is consistent with the relevant research findings. However, hydromanganese mineral groups, such as todorokite and asbolane, and zeolite mineral groups, such as laumontite and leonhardite, can still be observed [41,42].

2.2. Occurrence of Main Elements of Manganese Nodules

The distribution status of the main elements in manganese nodules were investigated in detail using Multimode V scanning microscope. As shown in Figure 5, the dense shell and loose core of the cross-sectioned manganese nodule under the microscope show obvious boundaries. An area in the core and shell is randomly selected for surface scanning to analyze the content and distribution of the elements, respectively. The results show that the main element contents in the core from high to low are: O (34.28%), Si (27.36%), Fe (13.95%), Al (9.35%), Mn (5.60%), K (2.81%), Mg (2.53%), Ca (1.10%), Na (1.09%), Cu (0.72%), Ni (0.68%), Co (0.54%), the main elements in the shell are Mn (34.59%), O (27.81%), Fe (15.10%), Si (5.13%), Mo (4.71%), Ca (2.54%), Al (2.42%), Mg (2.19%), Ni (1.36%), Na (0.88%), Cu (0.86%), K (0.68%), Zn (0.61%), W (0.57%), Co (0.49%), and Pb (0.05%). The contents of Si and Al in the core significantly exceed those in the shell, and the distributions of Al and Si in the core are closely related (Figure 6), indicating that the main minerals in the core are aluminosilicate minerals such as feldspar, pyroxene, clay, zeolite, etc. The contents of Mn and Fe in the shell are much higher than those in the core, and the Mn and Fe in the shell are highly dispersed (Figure 7), indicating that the main mineral in the shell is the iron–manganese mineral phase. The main valuable metal elements Ni, Co, and Cu are scattered in the core and shell of nodules. Comparing the content of each element in the core and the shell, when the content of Mn and Fe is high, the content of elements related to them is also high, which is consistent with the research results on the formation mechanism of manganese nodules.

Line scans were performed on different areas of manganese nodules to further study the changes in the content of each element. As presented in Figure 8, the change pattern of element characteristic peak intensity also proves that Al and Si are closely associated, and Fe and Mn are closely associated. From the core to the shell, the contents of Al and Si gradually decrease, while the contents of Fe and Mn gradually increase.

3. Extraction of Main Valuable Elements from Manganese Nodules

Compared with the same metal minerals on land, the main metal content in manganese nodules is more abundant, which provides the possibility of direct metallurgical recovery. In the 1970s, four organizations, namely International Nickel Company (INCO), Kennecott Copper Corporation (KCC), Metallurgie Hoboken Overpelt Company (MHO), and Deep Sea Ventures (DSV), were among the pioneers to commence the metallurgical recovery of Cu, Co, Ni, and Mn from manganese nodules. Their efforts laid the foundation for the subsequent development of extraction technologies. At present, the mainstream extraction processes are mainly divided into two categories: pyrometallurgical–hydrometallurgical and hydrometallurgical [43,44,45]. In order to reduce costs or improve the comprehensive utilization value of manganese nodules, researchers have proposed new technologies such as microbial extraction, short-process for directly producing metal materials from manganese nodule alloys, and the preparation of environmentally friendly materials from metallurgical slag. The critical step in these extraction processes is to reduce Mn(IV) to Mn(II), destroying the structure of manganese nodules, so that the valuable elements such as Co, Ni, and Cu are released into the solution [46]. Figure 9 illustrates the chemical species distribution of Mn and Fe under varying redox potential (Eh) and pH conditions. At high redox potentials, manganese primarily exists in high oxidation states, such as MnO₂(s). In contrast, under low redox potentials or acidic conditions, manganese is reduced to its dissolved form, Mn²⁺. To convert MnO₂(s) or Mn₃O₄(s) into Mn²⁺, it is necessary to lower the redox potential to a reductive environment (e.g., negative Eh values) while maintaining acidic conditions (pH < 5). Therefore, using reductants to decrease the Eh value is a key strategy for achieving efficient manganese leaching. Iron primarily exists in dissolved forms (Fe²⁺ or Fe³⁺) under acidic conditions. In neutral or alkaline environments, iron tends to form hydroxide or oxide precipitates, such as Fe(OH)₃ and Fe₂O₃. At low Eh and acidic conditions, iron readily dissolves as Fe²⁺, which is critical for facilitating the reductive leaching of MnO₂. However, as pH increases or Eh rises, iron is more likely to transition into Fe³⁺, forming precipitates that hinder the leaching process. After leaching is completed, impurities are removed from the leachate and elements are separated using treatment processes similar to those used for terrestrial resources, such as solvent extraction, electrolytic deposition, and so on [47,48,49,50].

3.1. Pyrometallurgical–Hydrometallurgical Process

3.1.1. Reduction Roasting–Acid Leaching Process

(1) INCO process. This pyrometallurgical–hydrometallurgical process is developed by INCO. As shown in Figure 10, the dried and ground manganese nodules are roasted reductively, and then smelted in an electric furnace to produce slag rich in manganese and iron, as well as alloys containing copper, nickel, cobalt, and small amounts of iron and manganese. The slag is further utilized to produce ferromanganese. The alloy is sulfurized to produce matte, and then leached with sulfuric acid. Subsequently, the leach solution undergoes extraction, electrowinning, hydrogen reduction, and other processes to separate and produce copper, nickel, and cobalt [51,52].

(2) Improved INCO process. Based on the INCO process, China Ocean Mineral Resources Research & Development Association (COMRA) has made improvements to the subsequent hydrometallurgical leaching of polymetallic alloys. The hydrochloric acid corrosion method was employed to precipitate most of the iron, while copper, cobalt, nickel, and manganese enter the solution. The metals were then extracted and separated. In small-scale trials, the metal recovery rates were as follows: 94.21% of Cu, 95.23% of Co, 96.49% of Ni, 97.68% of Mn, and 96.16% of Fe [53]. Similarly, Interoceanmetal Joint Organization (IOM) employed H₂SO₄-H₂SO₃ instead of H₂SO₄ to dissolve the alloy of copper, nickel, and cobalt produced by electrosmelting. Nickel, cobalt, and a small amount of manganese and iron were selectively dissolved, while copper precipitated as CuS. The manganese-rich slag generated during electrosmelting was recovered in the form of Si-Mn alloy. Ultimately, the recovery rates of Cu, Ni, and Co can reach 92%, 93%, and 89%, respectively [54].

The related researchers has improved the INCO process by adding a second smelting step on the basis of optimizing the original smelting process. This enhancement further processed the manganese-rich slag to produce high-carbon ferromanganese, which is used in the production of manganese steel. In addition, this process reduced the heavy metal content in the waste slag, making it suitable for use in the construction industry. Simultaneously, the Fe-Ni-Cu-Co alloy was directly subjected to pressurized sulfuric acid leaching without requiring conversion to sulfide, followed by solvent extraction of copper, iron precipitation, and cobalt solvent extraction. Copper sulfate was added during leaching to prevent the formation of hydrogen gas. The waste generated in the hydrometallurgical process was recycled back into the pyrometallurgical process, achieving zero waste discharge, as shown in Figure 11 [55].

Xue et al. utilized residual carbon from coal gasification slag for the reduction roasting–sulfuric acid leaching treatment of manganese nodules, successfully extracting valuable metals such as manganese, copper, cobalt, and nickel. The leaching efficiency exceeded 98% for manganese and 96% for copper, cobalt, and nickel. The reduction and leaching processes were dominated by chemical interfacial reactions, with SO₂ generated from the residual carbon significantly enhancing manganese reduction. In addition to its high efficiency, this process offers substantial environmental and economic benefits by repurposing industrial by-products like coal gasification slag, minimizing waste, and reducing energy costs [56]. The selective carbothermal reduction and high-pressure sulfuric acid leaching has also been employed to recover cobalt, nickel, manganese, and copper from cobalt-rich deep-sea nodules. At 600 °C, 60 min and 3 wt% carbon, the method achieved leaching efficiencies of 98.47% cobalt, 95.72% nickel, 97.24% manganese, and 94.99% copper, with minimal extraction of iron and aluminum. This provides an efficient and sustainable method for recovering valuable metals [57].

3.1.2. Reduction Roasting–Ammonia Leaching Process

Inspired by the Caron process using ammonia solution to treat laterite nickel ores, the reduction roasting method has been introduced for processing manganese nodules. The products of reduction roasting of manganese nodules were subjected to ammonia leaching. Subsequently, copper, nickel, and cobalt formed soluble complexes with ammonia in the liquid phase. Finally, extraction and precipitation were used to achieve separation between different metals. Han et al. [56] performed reduction roasting of different manganese nodules at 600 °C in an atmosphere with a CO/CO₂ ratio of 60/40 for 2 h, followed by leaching with aqueous NH₃-(NH₄)₂CO₃ and NH₃-(NH₄)₂SO₄ at room temperature and atmospheric pressure. The extraction rate of nickel can reach more than 80%, and the extraction rate of cobalt was close to 50%. The mechanism study also confirmed that the leaching rate of cuprous oxide is faster than that of copper peroxide and metallic copper. Therefore, the excessive reduction at high temperature was not conducive to the ammonia leaching of copper. In the NH₃-(NH₄)₂SO₄ system, the extraction efficiency of metals is higher. However, since manganese, iron, and carbonate can form precipitates, manganese and iron are basically not extracted in the NH₃-(NH₄)₂CO₃ system, which is beneficial to the subsequent separation and purification of different metals. The dissolution reactions that may be involved are shown in

Table 7. Dissolution reactions of copper, nickel, cobalt, and their oxides in ammonia solution. Adapted from Ref. [58].

Reaction	∆G/kcal·mol−1
$\mathrm{Cu}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+2{\mathrm{NH}}_3+2{\mathrm{NH}}_4^{+}\iff \mathrm{Cu}{({\mathrm{NH}}_3)}_4^{2+}+{\mathrm{H}}_2\mathrm{O}$	−34.2
$\mathrm{CuO}+2{\mathrm{NH}}_3+2{\mathrm{NH}}_4^{+}\iff \mathrm{Cu}{({\mathrm{NH}}_3)}_4^{2+}+{\mathrm{H}}_2\mathrm{O}$	−3.8
${\mathrm{Cu}}_2\mathrm{O}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+4{\mathrm{NH}}_4^{+}\iff 2\mathrm{Cu}{({\mathrm{NH}}_3)}_2^{2+}+2{\mathrm{H}}_2\mathrm{O}$	−33.44
${\mathrm{Cu}}_2\mathrm{O}+2{\mathrm{NH}}_3+2{\mathrm{NH}}_4^{+}\iff 2\mathrm{Cu}{({\mathrm{NH}}_3)}_2^{+}+{\mathrm{H}}_2\mathrm{O}$	−2.05
$\mathrm{Ni}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+4{\mathrm{NH}}_3+2{\mathrm{NH}}_4^{+}\iff \mathrm{Ni}{({\mathrm{NH}}_3)}_6^{2+}+{\mathrm{H}}_2\mathrm{O}$	−54.53
$\mathrm{NiO}+4{\mathrm{NH}}_3+2{\mathrm{NH}}_4^{+}\iff \mathrm{Ni}{({\mathrm{NH}}_3)}_6^{2+}+{\mathrm{H}}_2\mathrm{O}$	−3.23
${\mathrm{Ni}}_3{\mathrm{O}}_4+10{\mathrm{NH}}_3+8{\mathrm{NH}}_4^{+}\iff 3\mathrm{Ni}{({\mathrm{NH}}_3)}_6^{2+}+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{H}}^{+}+{3\mathrm{H}}_2\mathrm{O}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$	+31.38
$\mathrm{Co}+{\displaystyle \frac{3}{4}}{\mathrm{O}}_2+3{\mathrm{NH}}_3+3{\mathrm{NH}}_4^{+}\iff \mathrm{Co}{({\mathrm{NH}}_3)}_6^{3+}+{\displaystyle \frac{3}{2}}{\mathrm{H}}_2\mathrm{O}$	−62.72
$\mathrm{CoO}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+3{\mathrm{NH}}_3+3{\mathrm{NH}}_4^{+}\iff \mathrm{Co}{({\mathrm{NH}}_3)}_6^{3+}+{\displaystyle \frac{3}{2}}{\mathrm{H}}_2\mathrm{O}$	−13.76
${\mathrm{Co}}_3{\mathrm{O}}_4+{\displaystyle \frac{1}{4}}{\mathrm{O}}_2+9{\mathrm{NH}}_3+9{\mathrm{NH}}_4^{+}\iff 3\mathrm{Co}{({\mathrm{NH}}_3)}_6^{3+}+{\displaystyle \frac{9}{2}}{\mathrm{H}}_2\mathrm{O}$	−20.47
${\mathrm{Co}}_2{\mathrm{O}}_3+6{\mathrm{NH}}_3+6{\mathrm{NH}}_4^{+}\iff 2\mathrm{Co}{({\mathrm{NH}}_3)}_6^{3+}+3{\mathrm{H}}_2\mathrm{O}$	−10.4

.

Kmetova [59] et al. separately investigated the performance of wood charcoal and natural gas in the reduction roasting–ammonia leaching process. The results indicated that the extraction efficiencies of nickel, copper, and molybdenum exceeded 90%, 70%, and 60%, respectively, when the roasting was at 1073 K of temperature, with a 6% addition ratio of wood charcoal for 120 min, and the leaching at 318 K of temperature, with a 1 M solution of (NH₄)₂CO₃ in 10% NH₃, 20 of L/S for 210 min. To achieve an extraction efficiency of over 90% for cobalt, the roasting temperature needed to be increased to 1123 K. However, the performance of natural gas was not as effective as wood charcoal. At a roasting temperature of 873~973 K, the extraction efficiencies of Cu and Mo were the highest, 60% and 85%, respectively. At a roasting temperature of 1173 K, the extraction efficiency of Ni and Co reached the maximum, more than 40%.

India’s National Metallurgical Laboratory (NML) has also been dedicated to improving the metal recovery rates of the reduction roasting–ammonia leaching process. The study revealed that excessive reduction roasting can lead to the formation of alloys between iron and nickel or cobalt. These alloys hindered the reaction between the leaching reagent and nickel or cobalt, resulting in a decrease in extraction efficiency. Therefore, it was essential to select appropriate roasting temperatures and durations. Another factor contributing to low leaching rates was that as the leaching time extends, the small amount of Cu (NH₃)₂⁺ that was released rapidly elevated the system’s redox potential, causing the precipitation of iron and manganese. The recovery rates of nickel and cobalt decreased due to co-precipitation/adsorption, especially cobalt [60,61]. Improvements in extraction efficiency have been achieved by employing a two-stage ammonium leaching process using high-concentration ammonium solution followed by low-concentration ammonium solution, while strictly controlling the redox potential in the first-stage leaching system. However, the high iron content in the leaching solution was not favorable for the subsequent extraction and electrowinning for purifying the various metals. Therefore, the recovery rate of Co needed to be sacrificed for the rationality of the entire process. After 16 leaching cycles, the recovery rates of Cu, Ni, and Co reached 92%, 90%, and 56% [62]. In order to improve the recovery rate of Co, Mishra et al. introduced anionic surfactants in the pretreatment process of roasted manganese nodules, which reduced the adsorption of Co on the ferromanganese precipitate and achieved an impressive average metal recovery rates of 92.5% of Cu, 91.5% of Ni, 71.35% of Co [63]. The optimized process is shown in Figure 12. In order to study the interactive influences of the leaching behavior of various metals in NH₃-CO₂ solution after the reduction roasting of manganese nodules, Jana et al. used metal powders of copper, nickel, cobalt, manganese, and iron to simulate ammonia leaching experiments. The results showed that, in the absence of iron and manganese, the combined leaching behavior of copper, nickel, and cobalt was the same as their individual leaching behaviors. However, when manganese was added, the leaching of each metal decreased. The reduced recovery rate was due to the reducing atmosphere caused by the hydrogen gas produced when metallic manganese reacted with the ammonia solution to form an aminomethanoate compound. The presence of iron alone had little effect on the leaching behavior of copper, nickel, and cobalt. In the actual leaching of manganese nodules after reduction roasting, the recovery of copper was still satisfactory (97%), while the recovery of nickel and cobalt were lower. The high recovery of copper may be due to the fact that the metallic manganese in the nodules after reduction roasting was still less than the oxidized manganese, which cannot create a reducing atmosphere by generating hydrogen. Additionally, when preconditioned in the presence of air, a passivation film formed on the surface of the iron–cobalt and iron–nickel alloys, inhibiting the dissolution of cobalt and nickel into the solution. By conducting the pretreatment in an air-free environment and then leaching in the presence of air, the formation of the passivation film can be avoided, and iron can quickly precipitate in the initial stage of leaching, improving the recovery of nickel and cobalt [64].

3.2. Hydrometallurgical Process

3.2.1. Ammonia Leaching

KCC initially employed the ammonia leaching process to extract valuable metals from manganese nodules, known as the Cuprion process. As presented in Figure 13, Cu(II) in the solution was reduced to Cu(I) by CO, and then Mn(IV) was reduced to Mn(II) by Cu(I). Copper, nickel, and cobalt formed soluble complexes with ammonia, while manganese and iron formed carbonates and precipitated into the leaching residue. Subsequently, a solvent extraction–electrowinning method was employed to produce copper and nickel, followed by the recovery of cobalt and molybdenum from the raffinate. The process had mild reaction conditions and good selectivity, but it also had disadvantages such as low cobalt recovery (50%), low pulp concentration, and the potential hazards associated with CO [65,66]. Other reductants have been introduced to improve the Cuprion process include glucose [67], manganese ions [68], ferrous sulfate [69], elemental sulfur [70], sulfur dioxide [71], ammonium thiosulfate, and ammonium sulfite [72]. The subsequent separation and purification of valuable metals in the leachate and leaching residue also adopted conventional metallurgical methods such as solvent extraction and precipitation [73,74].

The National Institute for Resources and Environmental Research (NIRE) in Japan combined the advantages of NH₃-CO and NH₃-SO₂ processes to leach manganese nodules and cobalt-rich crusts in solutions of (NH₄)₂CO₃ and (NH₄)₂SO₃. During the leaching process, the dissolved Mn, Fe, and CO₃²⁻ formed precipitates into the residue, and then Cu and Ni were simultaneously extracted from the leachate. Finally, methanol was added to the leachate containing Co, (NH₄)₂CO₃, and (NH₄)₂SO₄ to precipitate cobalt, and the remaining leachate was recycled [50,75].

India has perfected the NH₃-SO₂ process and built a 500kg/d industrial demonstration line. The average recovery of copper, nickel, and cobalt reached 85%, 90%, and 80%, respectively. As shown in Figure 14, the process mainly included: ① leaching manganese nodules in the presence of NH₃ and SO₂; ② solvent extraction and electrowinning of Cu; ③ sulfide co-precipitation of Ni and Co, and high-pressure acid leaching of precipitation; ④ extraction separation of Ni and Co; ⑤ electrowinning of Ni and Co. This process can selectively recover manganese from silicomanganese residue [76,77]. The leaching process involves reactions as Equations (1)–(6):

$$\mathrm{CuO}+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_3+{2\mathrm{NH}}_3=\mathrm{Cu}{({\mathrm{NH}}_3)}_4{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(1)

$$\mathrm{NiO}+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_3+{2\mathrm{NH}}_3=\mathrm{Ni}{({\mathrm{NH}}_3)}_4{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(2)

$$\mathrm{CoO}+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_3+{2\mathrm{NH}}_3=\mathrm{Co}{({\mathrm{NH}}_3)}_4{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(3)

$${\mathrm{MnO}}_2+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_3=\mathrm{MnO}+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_4$$

(4)

$${\mathrm{MnO}}_2+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_3+{2\mathrm{NH}}_3=\mathrm{Mn}{({\mathrm{NH}}_3)}_4{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(5)

$$2\mathrm{FeOOH}+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_3+{\mathrm{H}}_2\mathrm{O}=2\mathrm{Fe}{(\mathrm{OH})}_2+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_4$$

(6)

3.2.2. Hydrochloric Acid Leaching

DSV and MHO earlier developed extraction processes that used concentrated hydrochloric acid to dissolve all metals in manganese nodules [35,74]. The DSV process leached the ground nodules with concentrated hydrochloric acid, where Mn(IV) was reduced to Mn(II), as shown in Equation (7). Then, a series of solvent extraction and electrowinning operations were used to separate the various metals in the leachate: (1) extract iron and copper separately; (2) co-extract cobalt and nickel, with MnCl₂ entering the raffinate; (3) selectively strip nickel from the loaded organic phase, followed by cobalt; (4) electro-deposit copper, nickel, and cobalt from their respective chloride solutions. Meanwhile, HCl and Cl₂ were regenerated through high-temperature hydrolysis for recycling. This process achieved high metal recovery, but the chlorides were highly corrosive, requiring equipment that can withstand these conditions.

$${\mathrm{MnO}}_2+4\mathrm{HCl}={\mathrm{MnCl}}_2+{\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}$$

(7)

The MHO process is similar to the DSV process but uses the Cl₂ produced in the reaction to oxidize the dissolved Mn(II) back to Mn(IV), which then precipitates as MnO₂. In addition to copper, cobalt, nickel, and manganese, the MHO process also recovers lower concentrations of vanadium, molybdenum, and zinc from the chloride leachate.

To address the high energy consumption of the reduction roasting process, and the drawbacks of direct leaching with concentrated hydrochloric acid, the leaching of manganese nodules in dilute hydrochloric acid using iron minerals, sodium sulfite, and carbon as reducing agents has been investigated [78]. The results showed that using pyrite as a reducing agent, more than 80% of Mn and Co can be dissolved in 1.5 M HCl solution at temperatures of 80~90 °C over 3 h. With sodium sulfite (Na₂SO₃) as the reducing agent, similar extraction efficiencies can be achieved at lower leaching temperatures (50~60 °C) and a shorter leaching time (60 min). Furthermore, a combination of 10% carbon and 15% pyrite was more effective than using either carbon or pyrite alone.

The main advantage of the reductive leaching method using dilute hydrochloric acid was its ability to efficiently dissolve manganese while maintaining a low iron dissolution rate, which facilitated the subsequent separation process. In this process, the primary role of the reducing agents was to reduce dissolved FeCl₃ back to FeCl₂, which then promoted the reduction of MnO₂. For sodium sulfite, the SO₂ generated in the acidic solution also participated in the reduction of MnO₂. Therefore, lower temperature was sufficient for maximum metal extraction. The optimal leaching times for manganese nodules in 1.5 M HCl with different reducing agents were: sodium sulfite (60 min) < combination of carbon and pyrite (100 min) < pyrite (150 min) < carbon (180 min). The main reactions are shown in Equations (8)–(14).

Stepwise reactions with pyrite:

$${\mathrm{FeS}}_2+{14\mathrm{FeCl}}_3+8{\mathrm{H}}_2\mathrm{O}\to {15\mathrm{FeCl}}_2+12\mathrm{HCl}+{2\mathrm{H}}_2{\mathrm{SO}}_4$$

(8)

$${7\mathrm{MnO}}_2+{14\mathrm{FeCl}}_2+28\mathrm{HCl}\to 7{\mathrm{MnCl}}_2+{14\mathrm{FeCl}}_3+{14\mathrm{H}}_2\mathrm{O}$$

(9)

Overall reaction with pyrite:

$${\mathrm{FeS}}_2+7{\mathrm{MnO}}_2+16\mathrm{HCl}\to {\mathrm{FeCl}}_2+{7\mathrm{MnCl}}_2+{2\mathrm{H}}_2{\mathrm{SO}}_4+6{\mathrm{H}}_2\mathrm{O}$$

(10)

Stepwise reactions with sodium sulfite:

$${\mathrm{Na}}_2{\mathrm{SO}}_3+2\mathrm{HCl}\to 2\mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{SO}}_2$$

(11)

$$2{\mathrm{FeCl}}_3+{\mathrm{SO}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{FeCl}}_2+2\mathrm{HCl}+{\mathrm{H}}_2{\mathrm{SO}}_4$$

(12)

$${\mathrm{MnO}}_2+2{\mathrm{FeCl}}_2+4\mathrm{HCl}\to {\mathrm{MnCl}}_2+2{\mathrm{FeCl}}_3+2{\mathrm{H}}_2\mathrm{O}$$

(13)

Overall reaction with sodium sulfite:

$${\mathrm{MnO}}_2+{\mathrm{Na}}_2{\mathrm{SO}}_3+4\mathrm{HCl}\to {\mathrm{MnCl}}_2+2\mathrm{NaCl}+{\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(14)

Alcohols can also serve as reductants to enhance the leaching rate of Cu, Fe, Mn, Ni, and Co. In dilute hydrochloric acid (2.75 M), only 80% of Cu and 60% of Fe can be extracted from manganese nodules. However, by simply adding 9% propanol or 4.5% butanol to dilute the hydrochloric acid (2.75 M), the recovery rate of each metal can be increased to over 90%. Moreover, as the chain length of the alcohol increased, the required dosage of alcohol decreased [79]. The reactions involved are as follows:

Without alcohol:

$${\mathrm{MnO}}_2+4\mathrm{HCl}\to {\mathrm{MnCl}}_2+{\mathrm{Cl}}_2$$

(15)

With alcohol:

$${\mathrm{MnO}}_2+4{({\mathrm{CH}}_3)}_2{\mathrm{CHOH}}_2^{+}+{4\mathrm{Cl}}^{-}\to {\mathrm{MnCl}}_2+4{({\mathrm{CH}}_3)}_2\mathrm{CHOH}+{2\mathrm{H}}_2\mathrm{O}+{\mathrm{Cl}}_2$$

(16)

The commercial alkylphosphines, Cyanex, can be utilized for subsequently extracting pure Co(II), Ni(II), and Cu(II) from the concentrated hydrochloric acid leachate of manganese nodules. Co(II) and Cu(II) were extracted using Cyanex 923, and Ni(II) using Cyanex 301. These extractants exhibited excellent stability even after extended contact with hydrochloric acid and maintain their superior extraction capabilities after 20 cycles of reuse. Consequently, they were widely employed for the separation of Co(II), Ni(II), and Cu(II) from other metal ions, such as Ti(IV), Al(III), Fe(III), Mn(II), and Zn(II). In the organic phase, Co(II) and Cu(II) were present as H₂CoCl₄·2 Cyanex 923 and CuCl₂·2 Cyanex 923 complexes, respectively, while Ni(II) was extracted as NiR₂ (HR = Cyanex 301). The Stripping of Co(II) and Cu(II) was accomplished using a 0.001 M H₂SO₄, and Ni(II) was stripped with a 5% NH₄Cl in 75% NH₃. This method effectively recovered about 90% of Co(II), Ni(II), and Cu(II) from sea nodules, achieving an approximate purity of 99% for the metal ions [80].

3.2.3. Sulfuric Acid Leaching

Sulfuric acid is also widely used as an acidic medium for reduction leaching of manganese nodules. IOM has conducted extensive research on sulfuric acid leaching of manganese nodules from CCZ. As shown in Figure 15, the main operations were as follows: (I) preliminary grinding of wet raw materials in a ball mill to reach 86% particles of −0.2 mm; (Ⅱ) selective leaching of Cu, Ni, Co, and Mn under normal pressure containing SO₂, thickening of the leached pulp with flocculant, and filtration of the underflow of the thickener; (Ⅲ) selective precipitation of Cu from the leachate by introducing active sulfur powder and feeding of sulfuric anhydride to the reactor under normal pressure; (Ⅳ) precipitation of Ni and Co concentrate by introducing powdered sulfur and metallic manganese; (Ⅴ) precipitation of MnO₂ during the neutralization reaction with ammonia water, thickening, filtration, washing of filter with manganese hydroxide concentrate, drying in a rotating furnace, and briquetting [81,82].

The chemical reactions involved in the process of leaching manganese nodules with sulfuric acid using SO₂ as a reductant are as follows:

$${\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}={\mathrm{H}}_2{\mathrm{SO}}_3$$

(17)

$${\mathrm{MnO}}_2+{2\mathrm{H}}_2{\mathrm{SO}}_3={\mathrm{MnS}}_2{\mathrm{O}}_6+{2\mathrm{H}}_2\mathrm{O}$$

(18)

$$\mathrm{MeO}+{\mathrm{H}}_2{\mathrm{SO}}_3={\mathrm{MeSO}}_3+{\mathrm{H}}_2\mathrm{O}$$

(19)

$${\mathrm{Me}}_2\mathrm{O}+{\mathrm{H}}_2{\mathrm{SO}}_3={\mathrm{Me}}_2{\mathrm{SO}}_3+{\mathrm{H}}_2\mathrm{O}$$

(20)

$$\mathrm{MeO}+{\mathrm{H}}_2{\mathrm{SO}}_4={\mathrm{MeSO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(21)

$${\mathrm{Me}}_2\mathrm{O}+{\mathrm{H}}_2{\mathrm{SO}}_4={\mathrm{Me}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(22)

where Me is Cu, Ni, Co, Fe, Ca, and Mg.

$${\mathrm{Cu}}_2{\mathrm{SO}}_3+{\mathrm{CuSO}}_3+2{\mathrm{H}}_2\mathrm{O}={\mathrm{Cu}}_2{\mathrm{SO}}_3\cdot {\mathrm{CuSO}}_3\cdot 2{\mathrm{H}}_2\mathrm{O}$$

(23)

$${\mathrm{MnS}}_2{\mathrm{O}}_6+2{\mathrm{H}}_2{\mathrm{SO}}_4={\mathrm{MnSO}}_4+{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6$$

(24)

$${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6={\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{SO}}_2\uparrow$$

(25)

Other reactions may occur in the presence of dissolved oxygen:

$${\mathrm{H}}_2\mathrm{O}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+2{\mathrm{SO}}_2={\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6$$

(26)

$${\mathrm{H}}_2{\mathrm{SO}}_3+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2={\mathrm{H}}_2{\mathrm{SO}}_4$$

(27)

The chemical reactions during the selective precipitation of Cu with activated sulfur powder and SO₂ are as follows:

$${2\mathrm{CuSO}}_4+2{\mathrm{SO}}_2+\mathrm{S}+4{\mathrm{H}}_2\mathrm{O}={\mathrm{Cu}}_2\mathrm{S}\downarrow +4{\mathrm{H}}_2{\mathrm{SO}}_4$$

(28)

$${\mathrm{CuSO}}_4+{\mathrm{SO}}_2+\mathrm{S}+2{\mathrm{H}}_2\mathrm{O}={\mathrm{Cu}}_2\mathrm{S}\downarrow +2{\mathrm{H}}_2{\mathrm{SO}}_4$$

(29)

The chemical reactions during Ni-Co concentrate precipitation with activated sulfur powder and metallic manganese are followed:

$${\mathrm{MeSO}}_4+{\mathrm{Mn}}_{\mathrm{met}.}+{\mathrm{S}}^0=\mathrm{MeS}\downarrow +{\mathrm{MnSO}}_4$$

(30)

where Me is Ni and Co.

The chemical reactions during the precipitation of MnO₂ are:

$${\mathrm{MnSO}}_4+2\mathrm{NH}{}_4\mathrm{O}\mathrm{H}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2={\mathrm{MnO}}_2+{({\mathrm{NH}}_4)}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(31)

Finally, a copper concentrate with a Cu content of 40%, a nickel–cobalt concentrate with a Ni and Co content of 20.8% and 2.7% C, respectively, and a manganese concentrate with a Mn content of 62% can be obtained through this process. The recovery rates of each metal were 92% of Cu, 96% of Ni, 92% of Co, and 96% of Mn. Using a similar process for leaching manganese nodules from the Indian Ocean, over 85% of Mn, Ni, and Co, as well as more than 75% of Cu were successfully extracted. The target metal extraction process was generally completed within 5~10 min. However, extending the leaching duration to 25~30 min proved to be more beneficial for the precipitation and separation of Fe [83].

IOM has also developed a high-pressure sulfuric acid leaching process, using molasses and pyrite as reducing agents [81]. Ion exchange resin was used to extract and separate metal ions in the high-pressure sulfuric acid leachate, followed by sulfide precipitation to extract copper, solvent extraction to extract zinc, and finally sulfide precipitation of nickel and cobalt, and calcining to obtain manganese oxide. The optimal operating parameters and indicators for the high-pressure sulfuric acid leaching process with molasses and pyrite can be seen in Table 7.

The principal reactions that occur in the leaching process with molasses are:

$${24\mathrm{MnO}}_2+{\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{24\mathrm{H}}_2{\mathrm{SO}}_4={24\mathrm{MnSO}}_4+{12\mathrm{CO}}_2+{35\mathrm{H}}_2\mathrm{O}$$

(32)

$${24\mathrm{Me}}_2{\mathrm{O}}_3+{\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{48\mathrm{H}}_2{\mathrm{SO}}_4={48\mathrm{MeSO}}_4+{12\mathrm{CO}}_2+{59\mathrm{H}}_2\mathrm{O}$$

(33)

where Me are Co and Fe.

$${\mathrm{MnO}}_2+{\mathrm{FeSO}}_4+{\mathrm{H}}_2{\mathrm{SO}}_4={\mathrm{MnSO}}_4+{\mathrm{Fe}}_2{({\mathrm{SO}}_4)}_3+{\mathrm{H}}_2\mathrm{O}$$

(34)

$$\mathrm{MeO}+{\mathrm{H}}_2{\mathrm{SO}}_4={\mathrm{MeSO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(35)

where Me are Cu, Ni, Co, Zn, and Al.

The principal reactions that occur in the leaching process with pyrite are:

$${\mathrm{FeS}}_2+3{.5\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}={\mathrm{FeSO}}_4+{\mathrm{H}}_2{\mathrm{SO}}_4$$

(36)

$${2\mathrm{FeSO}}_4+{\mathrm{H}}_2{\mathrm{SO}}_4+0{.5\mathrm{O}}_2={\mathrm{Fe}}_2{({\mathrm{SO}}_4)}_3+{\mathrm{H}}_2\mathrm{O}$$

(37)

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

(38)

The research conducted by Anand et al. also explored the leaching of manganese nodules using dilute sulfuric acid under conditions of high temperature, high pressure, and oxygen. At the specific conditions of 423 K, a sulfuric acid concentration of 0.46 g/g of manganese nodules, an oxygen partial pressure of 0.55 MPa, and a leaching duration of 4 h, the process can extract nearly all the copper and nickel, as well as 88% of the cobalt. However, the leaching rates for manganese and iron were only 28% and 5.7%, respectively. To effectively extract all four metals—copper, nickel, cobalt, and manganese—charcoal can be utilized as a reducing agent to convert manganese dioxide into soluble manganese sulfate. When the leaching was conducted for 4 h at a temperature of 423 K, with a sulfuric acid concentration of 0.66 g/g of manganese nodules and an oxygen partial pressure of 0.55 MPa, the extraction for copper, nickel, cobalt, and manganese reached 77.8%, 99.8%, 88%, and 99.8%, respectively [84].

Barik et al. investigated the potential of using a micellar high-temperature acid dissolution process to selectively leach manganese from manganese nodules. It was found that cetyltrimethylammonium Bromide (CTAB) significantly enhanced the recovery of Cu, Ni, Co, Zn, and Mn. Moreover, the surfactant-assisted high-temperature sulfuric acid leaching method was employed for the effective removal of iron, aluminum, and silicon dioxide. Increasing the temperature of the medium had a notable impact on the selective leaching of Mn, Cu, Ni, and Co. The ideal conditions for maximum metal extraction were determined to be a pulp density of 10%, a leaching duration of 2 h, a leaching temperature of 160 °C, and a sulfuric acid concentration of 5.0% (v/v), with CTAB reaching its critical micelle concentration (CMC). Under these conditions, the recovery for Mn was 99%, and for Cu, Co, and Ni, it was also 99%. As presented in Figure 16, the addition of CTAB increased the rate of mass transfer during leaching, greatly enhancing the efficiency of the process. However, forming surfactant micelles in the leaching system beyond the CMC slightly reduced the mass transfer rate and leaching efficiency. Additionally, CTAB likely adsorbed effectively on iron oxide phases, inhibiting their dissolution, and thereby increasing the selectivity of the leaching system [85].

Torres et al. conducted a comparative study on the effectiveness of various iron-reducing agents in the sulfuric acid leaching process of manganese nodules. The research revealed that Fe⁰ (FeC) was the most efficient agent for this process. Its efficacy was attributed to the direct contact of Fe in the solution, which promoted the regeneration of ferrous ions due to the high concentrations of both ferrous and ferric ions. Remarkably, 97% of Mn was extracted using a MnO₂/FeC ratio of 1/2 and 0.1 mol/L H₂SO₄ in just 20 min. When employing Fe²⁺, FeC, and Fe₂O₃ as high-concentration reducing agents (with MnO₂ to reducing agent ratios of 1/2 or lower), it was possible to maintain low potential values, enabling effective leaching at a low acid concentration of 0.1 mol/L. However, better results were achieved with pyrite (FeS₂) at higher ratios of MnO₂/FeS₂ (1/3) and acid concentrations of 1 mol/L, likely due to the refractoriness of pyrite. Additionally, maintaining the potential and pH within the ranges of −0.4~1.4 V and −2~0.1, respectively, when using various iron-reducing agents, proved beneficial for dissolving manganese nodules and simultaneously prevented the formation of iron oxides [86]. Additionally, the studies confirmed that the leaching process of nodules in FeSO₄-H₂SO₄-H₂O solutions was remarkably swift [87]. The optimal conditions identified were using the stoichiometric amount of FeSO₄, 1.6 times the stoichiometric amount of H₂SO₄, a temperature of 90 °C, a liquid-to-solid ratio of 7:1, and a manganese nodule particle size of −1000 μm. Under these conditions, it was possible to extract more than 85% of cobalt and over 90% of nickel, copper, and manganese within 30 min. The concentration of cobalt in the leachate was about 146 mg/L, nickel was 1.63 g/L, copper was 1.69 g/L, and manganese was 30 g/L. The primary reaction involved in this process is represented by Equation (39):

$${\mathrm{MnO}}_2+2{\mathrm{FeSO}}_4+2{\mathrm{H}}_2{\mathrm{SO}}_4={\mathrm{MnSO}}_4+{\mathrm{Fe}}_2{{(\mathrm{SO}}_4)}_3+2{\mathrm{H}}_2\mathrm{O}$$

(39)

In subsequent research, (NH₄)₂S was also used for sulfide precipitation to precisely separate Cu, Ni, and Co from the leachate of manganese nodules in the FeSO₄-H₂SO₄-H₂O solution after iron removal. At a concentration of 5.5% (NH₄)₂S, room temperature, pH = 1.0 (for copper precipitation), and pH = 3.0 (for nickel-cobalt precipitation), and with the optimized flow rate of 0.15 g S²⁻/min per L of processed solution, ideal results were achieved with about 98% of Cu, Ni, and Co being effectively separated. This provided high-quality raw materials to produce the three metals [88].

Some commercial extractants can also be used directly for subsequent separation of metals. Shen et al. confirmed the effectiveness of sulfuric acid leaching of manganese nodules using pyrrhotite as the reducing agent and studied the subsequent separation and purification of manganese, nickel, and cobalt. The process involves: ① leaching manganese nodules at 90 °C for 4 h with a 1.25 mol/L sulfuric acid concentration, a 3:2 nodule-to-pyrrhotite ratio, a 2:1 liquid-to-solid ratio, stirring at 300 rpm; ② removing impurities such as iron and aluminum through oxidation precipitation; ③ adding sodium sulfide and stirring at 50 °C for 2 h to precipitate cobalt and nickel from the leachate, and the resulting solution is concentrated to obtain manganese sulfate; ④ redissolving cobalt and nickel precipitates and using an eight-stage process with di(2-ethylhexyl) phosphoric acid (D2EHPA) at an organic-to-aqueous ratio of 3:5 to eliminate impurities; ⑤ extracting cobalt using 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A or P507) in a three-stage counter-current operation with an O/A volume ratio of 2:3, followed by the washing and stripping of cobalt and nickel. The maximum recovery of manganese, cobalt, and nickel were 85%, 75%, and 78%, respectively [89]. A similar metallurgical process was used to separate valuable metals from a leachate produced by sulfuric acid leaching of manganese nodules, with starch as the reducing agent. The leachate contains 22.85 g/L of manganese, 6.38 g/L of iron, 1.01 g/L of copper, 0.023 g/L of zinc, 0.09 g/L of cobalt, and 1.44 g/L of nickel. Initially, iron was precipitated and removed from the solution using Ca(OH)₂ at a pH of 3.8. Subsequently, copper was extracted using the LIX 84I extractant. After the removal of iron and copper, zinc was removed using kerosene-based di-(2-ethylhexyl) phosphoric acid (D2EHPA). Following this, manganese was extracted using saponified D2EHPA (NaD2EHPA). Employing a two-stage extraction at A:O ratio of 3:4, highly efficient manganese extraction is achieved with an efficiency of 99.93%. During this process, cobalt was co-extracted with manganese at a concentration of 43 mg/L. To remove this cobalt, the solution was washed twice using a MnSO₄ solution. Furthermore, manganese is further extracted using 4% H₂SO₄ at an A:O ratio of 1:1 through a two-stage process. Nickel was extracted from the residual liquid after manganese extraction using NaD2EHPA at an A:O ratio of 3:2. This two-stage extraction results in a combined extraction quantity of cobalt and nickel of 47 mg/L. The nickel load was then washed with a NiSO₄ solution. Finally, nickel was efficiently leached at a rate of 99.93% using 0.5% H₂SO₄ at an A:O ratio of 1:1 in a two-stage process [90].

Other reductants like phenols [91], aromatic amines [92], and saw dust [93] employed for sulfuric acid leaching of manganese nodules have also been intensively studied. A summary of the sulfuric acid leaching process of manganese nodules under different reductants is shown in

Table 8. Summary of the sulfuric acid leaching process of manganese nodules under different reductants/auxiliaries.

Reductant/Auxiliary	Leaching Condition	Leaching Efficiency/%	Reference
SO2	Manganese nodules from CCZ, −0.2 mm particles accounted for 86%, temperature 343~353 k	In total, 92% of Cu, 96% of Ni, 92% of Co, 96% of Mn	[82]
SO2	Manganese nodules from Indian Ocean, liquid/solid 10:1, 10 mL 1 M H2SO4 added per 100 mL, SO2 concentration 5% (wt/v), leaching time 15 min, temperature 31 °C	Over 85% of Mn, Ni, and Co, over 75% of Cu	[83]
SO2	Manganese nodules from Cook Islands Exclusive Economic Zone, 30 °C, L/S ratio of 6:1 mL/g, H2SO4 dosage of 37.5 wt% and SO2 partial pressure of 200 kPa	In total, 98.7% of Mn, 91.4% of Fe, 95.3% of La, 99.2% of Ce, 99.3% of Ni, 95.9% of Co	[94]
Molasses	Manganese nodules from CCZ, concentration 31%, H2SO4 0.8 kg/kg nodule, molasses 0.12 kg/kg nodule, leaching time 60 min, temperature 140 °C	In total, 97.70% of Ni, 98.31% of Co, 91.77% of Cu, 97.99% of Zn, 97.07% of Mn, 24.54% of Fe	[81]
Pyrite	Manganese nodules from CCZ, concentration 35%, H2SO4 0.81 kg/kg nodule, pyrite 0.12 kg/kg nodule, leaching time 90 min, pressure 10 atm, temperature 160 °C	In total, 98.42% of Ni, 91.67% of Co, 95.98% of Cu, 96.75% of Zn, 27.52% of Mn, 27.22% of Fe	[81]
Water	Manganese nodules from Indian Ocean, −250 μm particles accounted for 100%, concentration 15% (wt/v), H2SO4 0.46 g/g nodule, leaching time 4 h, oxygen partial pressure 0.55 MPa, temperature 423 K	Nearly 100% of Cu and Ni, 88% of Co, 28% of Mn, 5.7% of Fe	[84]
CTAB	Manganese nodules from Indian Ocean, −100 μm particles accounted for 100%, concentration 10% (wt/v), H2SO4 5% (v/v), leaching time 2 h, CTAB at critical micelle concentration, temperature 160 °C	In total, 99% of Mn, Cu, Co, and Ni	[85]
FeC	Manganese nodules from the Blake Plateau in the Atlantic Ocean, particle size of −140 + 100 μm, liquid/solid 100:1, FeC/MnO2 2:1, H2SO4 0.1 mol/L, leaching time 20 min, room temperature	In total, 97% of Mn	[86]
FeSO4	Manganese nodules from CCZ, particle size of −1000 μm, liquid/solid 7:1, FeSO4 at stoichiometric amount, H2SO4 at 1.6 times stoichiometric amount, leaching time 30 min, temperature 90 °C	More than 85% of Co, over 90% of Ni, Co, Mn	[87]
Phenols	Manganese nodules from the central Pacific Basin, −74 μm particles accounted for 77%, phenol 0.25~0.4 g/g nodules, H2SO4 0.925g/g nodules, liquid/solid 4:1, leaching time 10~20 min, room temperature	Over or nearly 95% of Mn, Co, Ni, Cu	[91]
Aromatic amines	Manganese nodules from the central Pacific Basin, −74 μm particles accounted for 77%, aromatic amines 0.3 g/g nodules, H2SO4 0.925 g/g nodules, liquid/solid 4:1, leaching time 10~20 min, room temperature	Over 97% of Mn, Co, Ni, Cu	[92]
Sawdust	Manganese nodules from Indian Ocean, −100 μm particles accounted for 100%, pulp concentration 10% (wt/v), sawdust 0.5 g/g nodules, H2SO4 5% (v/v), leaching time 2 h, temperature 105 °C	In total, 99.5% of Mn, 99.1% of Cu, 99.6% of Ni, 93% of Co, 64.6% of Fe	[93]
Paper	Manganese nodules from Indian Ocean, −150 μm particles accounted for 100%, pulp concentration 20% (wt/v), paper 0.3 g/g nodules, H2SO4 7.56% (v/v), leaching time 2 h, temperature 90 °C	In total, 97.28% of Cu, 98.66% of Ni, 97.90% of Co and 99.00% of Mn	[95]
Glycerol	Manganese nodules from Indian Ocean, −150 μm particles accounted for 100%, pulp concentration 10% (wt/v), glycerol 1% (v/v), H2SO4 10% (v/v), leaching time 1 h, temperature 80 °C	Over 95% of Ni, over 98% of Cu, Co, Mn	[96]

.

3.3. Other Extraction Process

In order to enhance resource recovery efficiency and address the increasingly severe challenges of environmental protection, various new extraction technologies are continuously being developed.

(1) Co-extraction with other mineral resources. Tomas et al. investigated the leaching effect of chalcopyrite in hydrochloric acid when manganese nodules were used as oxidants. The results indicate that chalcopyrite is not dissolved independently, but is oxidatively leached by Cl₂ released by the reaction of MnO₂ with HCl, see DSV and MHO. Drawing inspiration from this research, it was possible to utilize terrestrial resources such as low-grade sulfide ore or tailings that are challenging to recover through conventional beneficiation methods as reducing agents to achieve the reduction leaching of valuable elements in manganese nodules. This can not only reduce the recovery cost of manganese nodules, but also maximize the resource recovery of low-grade sulfide ores on land [97]. Using manganese nodules as an oxidant and high chloride ion concentration wastewater for the sulfuric acid leaching of chalcopyrite, the high chloride ion content in the wastewater successfully addressed the passivation issue typically encountered in sulfuric acid leaching. This approach not only utilized the wastewater efficiently but also achieved synergistic leaching of manganese nodules and chalcopyrite. As a result, the leaching rate of copper from chalcopyrite can reach up to 77% [98].

The latest research involved co-leaching of manganese nodules with flotation tailings in sulfuric acid solution. The tailings primarily consist of iron minerals, with 58.52% magnetite and 4.47% limonite. The FeSO₄ produced during the leaching process acts as an effective reducing agent, shortening the dissolution time of MnO₂. High concentrations of FeSO₄ maintained the leaching system’s potential and pH within the ranges of −0.2~1.2V and −1.8~0.1, respectively, which was conducive to the formation of Fe²⁺ and Fe³⁺. Under these conditions, with a MnO₂/Fe₂O₃ ratio of 1:3 and an H₂SO₄ concentration of 0.1 M, the manganese nodules achieved rapid leaching within a short period (5~20 min), with a manganese leaching rate of 68% to 73%. The addition of tailings for the reductive leaching of marine nodules in an acidic medium presented an attractive and cost-efficient alternative [99,100]. The main reaction and its standard Gibbs free energy change (25 °C) are presented in

Table 9. Main reaction and thermodynamic information.

Reaction	ΔG° (kJ)
${\mathrm{Fe}}_2{{\mathrm{O}}_3}_{\left(\mathrm{s}\right)}+3{\mathrm{H}}_2{{\mathrm{SO}}_4}_{(\mathrm{aq})}={\mathrm{Fe}}_2{{(\mathrm{SO}}_4)}_{3(\mathrm{s})}+3{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}$	−163.37
${\mathrm{Fe}}_3{{\mathrm{O}}_4}_{\left(\mathrm{s}\right)}+4{\mathrm{H}}_2{{\mathrm{SO}}_4}_{(\mathrm{l})}={{\mathrm{FeSO}}_4}_{(\mathrm{aq})}+{\mathrm{Fe}}_2{({\mathrm{SO}}_4)}_{3(\mathrm{s})}+4{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}$	−261.30
${{2\mathrm{FeSO}}_4}_{\left(\mathrm{aq}\right)}+2{\mathrm{H}}_2{{\mathrm{SO}}_4}_{(\mathrm{aq})}+{\mathrm{MnO}}_{2(\mathrm{s})}={\mathrm{Fe}}_2{{{(\mathrm{SO}}_4)}_3}_{(\mathrm{s})}+{{\mathrm{MnSO}}_4}_{(\mathrm{aq})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}$	−199.52

.

(2) Solid-state metalized reduction technology. Zhao et al. proposed a process that combined solid-state metallized reduction treatment and magnetic separation to recover valuable metals in marine polymetallic nodules. Under the conditions of CaF₂ dosage of 4%, anthracite coal dosage of 7%, SiO₂ dosage of 5%, FeS dosage of 6%, temperature of 1100 °C, and time of 2.5 h, most of the valuable metals were selectively reduced to the metallic state. Magnetic separation was then used to recover the reduced metals as concentrate. The optimal recovery rates of Ni, Co, Cu, Mn, and Fe in the concentrate were 86.48%, 86.74%, 83.91%, 5.63%, and 91.46%, respectively [101]. The introduction of magnetic separation made the process of extracting valuable metals from manganese nodules more economical and environmentally friendly.

(3) Segregation roasting. In 1974, the University of California pioneered the application of segregation roasting for extracting metals from manganese nodules. The results indicated that using CaCl₂ as the chlorinating agent and petroleum coke as the reductant, the optimal conditions for metal separation were achieved with a roasting time of about 2 h. When the roasting temperature was set to 850 °C, the recovery of copper reached a maximum of 75%, while the recovery of nickel and cobalt only reached around 25%. At 1050 °C, the recovery of nickel and cobalt increased to about 60%, while copper dropped to 35%. The metal oxides in the nodules mainly went through chlorination and reduction. Based on reaction energies, the metal response order was Cu > Ni > Co > Fe > Mn.

Table 10. Standard free energy for the chloritization reduction cycle in metal oxide–hydrogen–chloride systems.

Reaction	∆Gθ (kcal)	Temp (°K)
${\mathrm{Cu}}_2\mathrm{O}+2\mathrm{HCl}=2/3{\mathrm{Cu}}_3{\mathrm{Cl}}_3+{\mathrm{H}}_2\mathrm{O}$	−14.9	1100
${2/3\mathrm{Cu}}_3{\mathrm{Cl}}_3+{\mathrm{H}}_2=2\mathrm{Cu}+2\mathrm{HCl}$	−8.3
${\mathrm{Cu}}_2\mathrm{O}+{\mathrm{H}}_2=2\mathrm{Cu}+{\mathrm{H}}_2\mathrm{O}$	−23.2
$\mathrm{NiO}+2\mathrm{HCl}={\mathrm{NiCl}}_2+{\mathrm{H}}_2\mathrm{O}$	+8.2	1200
${\mathrm{NiCl}}_2+{\mathrm{H}}_2=\mathrm{Ni}+2\mathrm{HCl}$	−20.0
$\mathrm{NiO}+{\mathrm{H}}_2=\mathrm{Ni}+{\mathrm{H}}_2\mathrm{O}$	−11.8
$\mathrm{CoO}+2\mathrm{HCl}={\mathrm{CoCl}}_2+{\mathrm{H}}_2\mathrm{O}$	+3.0	1200
${\mathrm{CoCl}}_2+{\mathrm{H}}_2=\mathrm{Co}+2\mathrm{HCl}$	−10.7
$\mathrm{CoO}+{\mathrm{H}}_2=\mathrm{Co}+{\mathrm{H}}_2\mathrm{O}$	+7.7
$\mathrm{FeO}+2\mathrm{HCl}={\mathrm{FeCl}}_2+{\mathrm{H}}_2\mathrm{O}$	+4.1	1200
${\mathrm{FeCl}}_2+{\mathrm{H}}_2=\mathrm{Fe}+2\mathrm{HCl}$	−2.9
$\mathrm{FeO}+{\mathrm{H}}_2=\mathrm{Fe}+{\mathrm{H}}_2\mathrm{O}$	+1.2
$\mathrm{MnO}+2\mathrm{HCl}={\mathrm{MnCl}}_2+{\mathrm{H}}_2\mathrm{O}$	+3.16	1200
${\mathrm{MnCl}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2(\mathrm{g})=\mathrm{Mn}+2\mathrm{HCl}$	+50.62
$\mathrm{MnO}+{\mathrm{H}}_2=\mathrm{Mn}+{\mathrm{H}}_2\mathrm{O}$	+53.78

depicts the energy for this reaction in the metal oxide–hydrogen–chloride system. Despite current limited studies on this method, it is effective for producing high-purity metals from refractory materials, making it a top choice for deriving ultra-pure metals from manganese nodules.

(4) Hydrogen metallurgy. Hydrogen metallurgy is an attractive method for reducing carbon emissions and is equally applicable to the reduction roasting of deep-sea manganese nodules. The result has revealed that the presence of H₂ can disrupt the lattice structure of manganese minerals, thereby releasing NiO, CoO, and CuO, and facilitating the formation of Fe-Ni-Co-Cu alloys. However, the reducing capacity of H₂ was limited, leading to the proposal of a gas–solid synergistic reduction process. Here, H₂ and C acted together as reducing agents, effectively separating Ni, Co, and Cu from the manganese minerals. The maximum contents of Ni, Co, and Cu in the alloy can reach 29.72%, 4.35%, and 15.44%, respectively. Moreover, the reduction temperature played a vital role in the reduction process of pellets. As the temperature increased, the mass loss rate of the pellets raised, SiO₂ in the nodules gradually disappeared, and a large amount of FeMn(SiO₄) and MnO formed. Particularly at 1100 °C, nearly all Ni, Co, and Cu in the manganese minerals were reduced and entered the alloy. Below 1000 °C, the reduction of pellets was still in its initial stage, primarily driven by H₂, with excess C not participating in the reaction. When the temperature exceeded 1100 °C, the carbon gasification reaction intensified, weakening the effect of H₂ on the reduction of carbon-containing multi-metallic nodule pellets. Further increasing the temperature only promoted the sintering and aggregation of the pellets, but had little effect on enhancing the reduction reaction itself. Since the reduction at low temperatures was mainly accomplished by H₂ with minimal participation of C, increasing the carbon-to-oxygen ratio had a limited impact on the efficiency of pellet reduction, thus helping to reduce carbon consumption. At higher temperatures, the enhanced carbon gasification reaction released a significant amount of heat, aiding the separation of Fe and Mn. At this stage, manganese primarily existed in the form of FeMn(SiO₄) and MnO [102].

(5) Bioleaching. Bioleaching is well-suited for processing low-grade, complex ores and has achieved commercial success in extracting base metals from low-grade sulfide ores and in the pre-treatment of refractory sulfide gold ores. Additionally, various bacteria play a crucial role in the formation of manganese nodules [103]. These nodules host manganese-oxidizing and manganese-reducing bacteria, as well as bacteria that do not interact with manganese. The bacteria are key in oxidizing Mn(Ⅱ) to Mn(IV) oxides, a critical component of the nodules. Furthermore, under specific conditions, some bacteria can directly or indirectly reduce Mn(IV) to Mn(Ⅱ), disrupting the structure of the nodules and releasing elements like Cu, Co, and Ni. Developing biohydrometallurgical methods to extract metals from manganese nodules holds great potential. Compared to traditional methods, this approach is more sustainable, consumes less energy, helps reduce carbon emissions, and supports the development of a circular economy [104,105].

The bioleaching of manganese nodules from CCZ was conducted using anaerobic Mn-reducing bacteria. The experimental results demonstrated that the leaching efficiencies of Mn, Co, and Ni were significantly increased from 18%, 7%, and 10% to 77%, 70%, and 75%, respectively, following the inoculation of the Mn-reducing bacterial enrichment culture. The research identified that the optimal temperature range for effective leaching was between 30 °C to 45 °C, with the most suitable pH being slightly acidic. Moreover, the particle size of the manganese nodules significantly impacted the leaching efficiency, with smaller sizes yielding higher efficiencies. Notably, the experiment did not require the addition of mineral salts, indicating that the bioleaching process only necessitated the addition of a carbon source. After 48 h of anaerobic treatment, the leaching efficiencies of Mn, Co, and Ni were greatly enhanced. This suggests that anaerobic Mn-reducing microorganisms, using metal ions as electron acceptors and organic compounds as electron donors, can effectively leach these metals from manganese nodules [106].

Mehta et al. [107] studied the biodissolution kinetics of precious metals such as copper, nickel, and cobalt from Indian Ocean nodules using Thiobacillus ferrooxidans and Thiobacillus thiooxidans in the presence of pyrite and sulfur. Ferrous sulfate and sulfuric acid produced, respectively, when pyrite and sulfur were oxidized by bacteria reduce Mn(IV) in nodules to Mn(II), destroying the structure of manganese nodules and indirectly dissolving metals. The bacterial oxidation and Mn(Ⅳ) reduction reactions involved are shown in Equations (40)–(45).

$${\mathrm{FeS}}_2\to {\mathrm{Fe}}^{2+}+{2\mathrm{S}}^0+{2\mathrm{e}}^{-}$$

(40)

$${\mathrm{S}}^0+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{SO}}_3$$

(41)

$${\mathrm{S}}^0+{3/2 \mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{SO}}_4$$

(42)

$${\mathrm{MnO}}_2+2{\mathrm{Fe}}^{2+}+4{\mathrm{H}}^{+}\to {\mathrm{Mn}}^{2+}+2{\mathrm{Fe}}^{3+}+2{\mathrm{H}}_2\mathrm{O}$$

(43)

$${\mathrm{MnO}}_2+2{\mathrm{e}}^{-}+4{\mathrm{H}}^{+}\to {\mathrm{Mn}}^{2+}+2{\mathrm{H}}_2\mathrm{O}$$

(44)

$${\mathrm{MnO}}_2+{\mathrm{H}}_2{\mathrm{SO}}_3+2{\mathrm{H}}^{+}\to {\mathrm{Mn}}^{2+}+{\mathrm{H}}_2{\mathrm{SO}}_4+2{\mathrm{H}}_2\mathrm{O}$$

(45)

The dissolution of metals followed the shrinking core kinetic model, with leaching reactions controlled by internal diffusion. Pyrite was more effective than sulfur for the biorecovery of the metals. At a pH of 2, with a stirring speed of 100 rpm, a slurry concentration of 5%, a feed particle size ranging from 75 to 300 μm, a 10% addition of pyrite, and a temperature of 308K, the recovery of various metals were as follows: 96% for Cu, 79% for Ni, 49.5% for Mn, 48% for Co, and 13% for Fe. In a sulfuric acid solution, pyrite/sulfur undergoes redox reactions via a galvanic action [FeS₂/S (anode): MnO₂ (cathode)] to reduce MnO₂, as seen in Equations (16) and (17). Additionally, pyrite can also reduce Fe³⁺ to Fe²⁺, forming a Mn⁴⁺—Fe²⁺ redox system, as seen in Equation (48). In the pyrite system, bacteria will also slowly oxidize Fe²⁺, as seen in Equation (49). Nakazawa and Sato also confirmed the effectiveness of sulfur and pyrite as reducing agents in their experiments on the bioleaching of cobalt-rich crusts [108].

$${15\mathrm{MnO}}_2+{\mathrm{O}}_2+{2\mathrm{FeS}}_2+{14\mathrm{H}}_2{\mathrm{SO}}_4\to {15\mathrm{MnSO}}_4+{\mathrm{Fe}}_2{{(\mathrm{SO}}_4)}_3+14{\mathrm{H}}_2\mathrm{O}$$

(46)

$${2\mathrm{MnO}}_2+{\mathrm{S}}^0+{1/2\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{SO}}_4\to {2\mathrm{MnSO}}_4+{\mathrm{H}}_2\mathrm{O}$$

(47)

$${\mathrm{Fe}}_2{({\mathrm{SO}}_4)}_3+{\mathrm{FeS}}_2\to {3\mathrm{FeSO}}_4+{2\mathrm{S}}^0$$

(48)

$${\mathrm{Fe}}^{2+}\to {\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}$$

(49)

Beolchini et al. conducted a study on the fungal leaching of metals from manganese nodules located approximately 5000 m deep in CCZ. The research thoroughly investigated the bioleaching efficiency of Aspergillus and a mixed cultures of A. niger and Trichoderma sp. under various growth conditions. These fungi primarily dissolve metals from the manganese nodules by forming complexes with organic acids produced during metabolism, such as oxalic acid and citric acid. Under optimal culture conditions, after 11 days of growth, Penicillium achieved extraction exceeding 80% for Mn, Cu, and Ni, while the extraction for Co and Fe were approximately 70% and 30%, respectively. Compared to chemical strategies, the biotechnological process offers a more sustainable method for reducing carbon footprint [10].

The National Metallurgical Laboratory in India conducted a study on leaching copper, nickel, and cobalt from polymetallic manganese nodules using the fungus Aspergillus niger. The findings revealed that under conditions of an initial pH of 4.5, a temperature of 35 °C, and a pulp density of 5% (w/v), Aspergillus niger can extract 97% of copper, 98% of nickel, 86% of cobalt, 91% of manganese, and 36% of iron within 30 days. This extraction rate was significantly higher compared to control experiments that directly used oxalic and citric acids. The dissolution process of metals from the nodules by Aspergillus niger was indirect, predominantly driven by the secretion of oxalic and citric acids. These organic acids disrupted the main structure of manganese (IV) and iron (III) oxides, effectively liberating valuable metals, particularly copper, nickel, and cobalt [109]. The involved reactions are as follows:

$${\mathrm{MnO}}_2+\mathrm{HOOC}\text{-}\mathrm{COOH}+2{\mathrm{H}}^{+}\to {\mathrm{Mn}}^{2+}+2{\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(50)

$$2\mathrm{Fe}\text{-}\mathrm{O}\text{-}\mathrm{OH}+\mathrm{HOOC}\text{-}\mathrm{COOH}+4{\mathrm{H}}^{+}\to {2\mathrm{Fe}}^{2+}+2{\mathrm{CO}}_2+4{\mathrm{H}}_2\mathrm{O}$$

(51)

The mechano-chemical activation’s effect on the bioleaching of metals such as copper, cobalt, and nickel has been further studied. After 10 min of treatment with a high-energy mill, the surface potential of manganese nodules decreased from −18 mV to −34 mV, and the duration to leach out over 95% of copper, nickel, and cobalt from the nodules was reduced to 15 days [110].

Kang et al. investigated the role of A. ferrooxidans in the reductive dissolution of polymetallic nodules. Using Fe²⁺ ions as reductants, the extraction rates of manganese, nickel, copper, and cobalt exceeded 95%, with an apparent activation energy of 13.9 kJ·mol⁻¹ for manganese oxide dissolution. The role of A. ferrooxidans in these processes can be summarized in three main aspects. I. Electron-proton carrier or buffer: facilitates the transfer of interfacial species, enhancing reaction efficiency. II. Catalyst: accelerates the transformation of Fe³⁺/Fe²⁺, increasing the reduction rate of manganese oxides. III. Product manager: optimizes the migration and transformation of intermediates and final products. This study provides valuable insights into the mechanisms underlying the bio-assisted reductive dissolution of polymetallic nodules [111].

(6) Electrochemical leaching and electrobioleaching. Kumari and Natarajan achieved almost complete dissolution of copper, nickel, cobalt, and other metals in sulfuric acid medium or the action of microorganisms by applying an external electric field and a negative direct current of −600 mV or lower. Under the condition of an external electric field, the reduction of iron and manganese oxides was the main mechanism for the release of other non-ferrous metals. Such electrochemical or electrobioleaching processes developed under laboratory conditions may be cost-effective, energy-saving, and environmentally friendly [112,113,114,115]. The electroleaching system is shown in Figure 17, the unmarked A, B, and C in the figure are three pyrex glass compartments.

4. Other Utilization Potentials of Manganese Nodules

In order to maximize the utilization value of manganese nodules, other possible utilization potentials of manganese nodules have also been widely explored.

(1) Used as catalyst or adsorbent. The fact that manganese nodules have a porous structure and are composed of a variety of transition metal oxides has long attracted attention for their potential as catalysts or adsorbent. Relevant studies indicate that manganese nodules can be used to catalytically decompose isopropyl alcohol at a roasting temperature of 200~300 °C. At this time, the specific surface area of manganese nodules can reach 300 m²/g, which is comparable to commercial aluminum-based or silicon-aluminum base catalyst. The large amount of surface excess oxygen contained in manganese oxide played a major role in the catalytic process, and trace amounts of metal oxides in manganese nodules can enhance the reactivity [116].

The adsorption and oxidation capabilities of manganese nodules towards Cr(III) at varying concentrations were also investigated. When the initial concentration of Cr(III) was set at 200 mg/L, the adsorption concentration and capacity of Cr(III) reached their maximums at 154 mg/L and 30.7 mg/g, respectively. In different gaseous environments, the catalytic oxidation capability of manganese followed the sequence of oxygen < air < nitrogen. As the pH increased, the oxidation capability of manganese nodules for Cr(III) diminished. The total concentration of Cr(VI) decreased from 8.2 mg/L at pH 2.0 to 4.0 mg/L at pH 6.0. The presence of Ca²⁺ and Mg²⁺ slightly reduced the oxidation capacity to Cr(III), while the presence of silicates significantly lowered it. As presented in Figure 18, manganese oxides primarily facilitate the oxidation of Cr(III), while iron oxides predominantly aid in the adsorption and stabilization of Cr(VI). Most of the Cr(III) was adsorbed by manganese nodules, with only a minor proportion being oxidized during the reaction, after which the newly formed Cr(VI) was released into the solution. Additionally, the released Mn(II) was re-oxidized to form Mn(III) intermediates or Mn(IV) oxides under the catalysis of manganese nodules [117].

The washed manganese nodule leached residue (WMNLR) had a larger specific surface area than manganese nodules, exhibited good affinity for phosphate, and can be used as an adsorbent to remove phosphate contaminants in aqueous phases [118]. It can also be used to adsorb aqueous selenite (SeO₃²⁻), with a removal efficiency of over 90% [119,120]. The molecular sieve (OMN-C), obtained by heating 5 g of manganese nodules with 0.02 mol of potassium permanganate at 1000 °C for 6 h, exhibited excellent selective adsorption capabilities for K⁺ in seawater. The adsorption capacity of OMN-C for K⁺ reached 22.1 mg/g, while its adsorption capacity for Na⁺ was only 0.4 mg/g. The primary composition of OMN-C was birnessite-type potassium manganese oxides, characterized by a high micropore volume, which was the key factor contributing to its efficiency and high selectivity [121]. In addition, the manganese nodule extraction residue had good adsorption capacity for Cr(Ⅵ) [122], Cd(Ⅱ) [123].

A summary and generalization of the adsorption mechanisms of manganese nodules can be divided into two main types. As illustrated by Equations (52) and (53), one type involved the exchange between ligands in the aqueous solution and surface hydroxyl groups, forming inner-sphere complexes in a process driven by not only electrostatic forces but also surface coordination interactions. This mechanism primarily occurred during the adsorption of strongly binding anions, such as phosphates on manganese nodules. The other type involved the formation of outer-sphere complexes, mainly occurring during the adsorption of weakly binding anions like chlorides and nitrate ions on manganese nodules. Water molecules remained retained between surface sites and the adsorbed ligands, and the adsorption forces are predominantly electrostatic [120].

$$\mathrm{SOH}+{\mathrm{L}}^{2-}+{\mathrm{H}}^{+}\leftrightarrow {\mathrm{SL}}^{-}+{\mathrm{H}}_2\mathrm{O}$$

(52)

$$\mathrm{SOH}+{\mathrm{L}}^{2-}+{\mathrm{H}}^{+}\leftrightarrow {{\mathrm{SOH}}_2}^{+}{-\mathrm{L}}^{2-}$$

(53)

where S-OH is a surface hydroxyl group and L is the ligand-adsorbed species.

Transforming leach residues into environmentally friendly materials not only addresses the disposal challenges of manganese nodule metallurgical residues but also further increases the utilization value of manganese nodules.

(2) Preparing material precursors. To minimize the potential reagent consumption and wastewater generation during the separation process of valuable metals from manganese nodule leachate, a mixture of pyridine carboxylate and naphthalene sulfonic acid was employed as the extracting agent to selectively co-extract Ni, Co, and Mn and directly prepare battery-grade Ni/Co/Mn sulfate solutions (Ca < 0.02 mg/L and Mg < 0.05 mg/L). Through counter-current extraction and washing with varied sulfuric acid concentrations, the final recovery rates in the battery-grade solution were 96.84% for Ni, 95.11% for Co, and 5.30% for Mn. This innovative method not only maximized metal recovery but also significantly reduced operational costs, holding vast potential for industrial applications, which offered a new approach for the short-process, high-value utilization of manganese nodules [124]. Alena et al. explored the development of natural alloys from deep-sea manganese nodules using aluminothermic reduction and rapid solidification. The resulting alloy was processed into thin ribbons via melt spinning, followed by rapid cooling, which significantly altered its phase composition. The alloy, primarily composed of β-Mn, Mn₂FeSi, and (Cu,Mn)₃(Al,Si), demonstrated excellent thermal stability during annealing at 500–750 °C for up to 100 h. Rapid cooling effectively prevented the formation of brittle α-Mn, and the alloy retained outstanding hardness exceeding 800 HV0.1 even after prolonged annealing. This study highlights the potential of deep-sea nodules as a resource for high-performance structural materials while supporting sustainable resource utilization [125].

(3) Recovery of other co-associated elements. Beyond manganese, iron, copper, cobalt, and nickel, the comprehensive recovery of other minor non-ferrous and rare earth metal elements from manganese nodules can further enhance their economic value. Parhi et al. separated and recovered molybdenum from a leachate of sea nodules containing Mo(VI) at 0.505 kg/m³, Fe(III) at 0.212 kg/m³, Cu(II) at 12.08 kg/m³, Co(II) at 2.012 kg/m³, and Ni(II) at 15.16 kg/m³. Alamine 304-1, dissolved in kerosene, was used to extract Mo(VI) from the leachate through solvent extraction, followed by the preparation of ammonium molybdate through crystallization. The ammonium molybdate crystals were then thermally decomposed to successfully produce MoO₃ with a purity of 99.9% [126]. The specific process flow and related parameters can be seen in Figure 19.

In the experiment of leaching rare earth metals from deep-sea nodules using sulfuric acid solution, it was found that the extraction efficiency of rare earth metals from sea nodules using a combination of H₂SO₄ and H₂SO₃ was almost similar to that using only H₂SO₄. This suggested that most of the rare earth (RE) metals were likely to be present in the iron oxide phase rather than the manganese oxide phase. The concentration of H₂SO₄ and the leaching temperature significantly affected the leaching efficiency. The maximum leaching efficiency (total of 68.0%) was achieved in 3 M H₂SO₄, with most RE leaching rates around 90%, and the remaining metals being Ce 44.6%, La 55.3%, Nd 84.7%, Eu 85.8%, Pr 76.4%, and Th 78.7%, determining an average RE leaching efficiency of 83.03%. Subsequently, dilute H₂SO₄ (0.2 M) was chosen as the optimal lixiviant concentration for selective leaching of REE, thereby minimizing the co-extraction of base metals Mn, Cu, Co, and Ni. Under the optimal conditions of 0.2 M H₂SO₄ and a temperature of 45 °C, a total REE extraction of approximately 58% was achieved, along with the co-extraction of Mn 0.3%, Fe 4.63%, Cu 23.7%, Co 0.2%, and Ni 31.8% [127].

5. Conclusions

With the depletion of terrestrial mineral resources, the development of seabed mineral resources will inevitably become a critical support for human society’s advancement. This study focuses on the mineralogy of seabed manganese nodules off the coast of China in the Western Pacific and the main technical methods for extracting valuable metal elements from manganese nodules (as shown in Figure 20). It reviews the primary techniques for extracting valuable metal elements from manganese nodules and, based on current research, summarizes new directions for the extraction and comprehensive utilization of valuable metal elements from manganese nodules in the future. The main conclusions are as follows:

(1)

The mineralogy study results indicate that the manganese nodules off the coast of China in the Western Pacific are composed of a shell of dense ferromanganese phase and a core of loose silicate phase. The primary valuable metal elements mainly include Cu, Co, Ni, Mn, Fe, etc. Among them, Co and Ni are mainly distributed in the manganese oxide phase, and Cu mainly exists in the form of free copper oxide. To extract valuable metals, the key is to reduce Mn(IV) to Mn(II), which disrupts the crystal structure of the nodules and releases the valuable metal elements. During the recovery process of the primary valuable metals, the recovery of a small amount of associated non-ferrous metals and rare earth elements can also be considered to enhance the recovery and utilization value of manganese nodules.

(2)

The extraction processes for the main valuable metal elements of manganese nodules are mainly divided into two categories: pyrometallurgical–hydrometallurgical and sole hydrometallurgical. The pyrometallurgical–hydrometallurgical process combines the high reduction efficiency of pyrometallurgy with the high recovery rates of hydrometallurgy, but it is associated with high energy consumption and carbon emissions. By introducing hydrogen metallurgy, partially replacing carbon with hydrogen can reduce carbon emissions and improve reduction efficiency. Additionally, utilizing residual heat from the pyrometallurgical process to support the hydrometallurgical stage can further reduce energy consumption. The hydrometallurgical process avoids high-temperature operations but faces challenges such as high acid consumption, complex wastewater treatment, difficulties in handling leaching residues, and complicated downstream separation processes. These issues can be addressed by developing highly selective leaching reagents, optimizing wastewater treatment technologies, and achieving resource utilization of leaching residues to enhance economic and environmental performance.

(3)

Bioleaching, co-extraction technology, short-process extraction technology, etc. have attracted widespread attention due to their environmentally friendly and efficient characteristics. In the future, manganese nodule extraction technology should focus on integrated development, promote the application of technologies such as the comprehensive recovery of associated resources and tailings-free utilization, improve process stability and efficiency through intelligent control systems, increase the utilization value of manganese nodules, reduce development costs, and accelerate its commercialization process, thereby achieving more efficient and sustainable resource development.