Resource amount estimation

After the KR13-02, MR13-E02, and KR14-02 cruises, three additional research cruises (MR14-E02, MR15-E01 Leg 2, and MR15-02) were conducted to reveal the detailed distribution of highly to extremely REY-rich mud in the southern part of the Minamitorishima EEZ (Fig. 1)8,9. During these cruises, REY-rich mud having a maximum of almost 8,000 ppm of total REY content (ΣREY) was confirmed. We estimated the resource amount of REY in the region bounded by 21°48′N to 22°15′N and 153°30′E to 154°07′E (about 2,500 km2) by using whole sediment chemical data of newly analysed 573 samples and previously reported 104 samples (KR13-02 PC05: 82 samples8 and KR13-02 PC06: 22 samples)9 from 25 sampling points (Supplementary Tables S1–S2, Fig. 1). Geographical Information System software (ArcGIS) was used to visualise the REY-rich mud distribution and evaluate its resource potential. ΣREY maps of the average concentration values from the seafloor to 10 meters below the seafloor (mbsf) and of the values for each 1 m depth interval are shown in Fig. 2. The ΣREY map was further divided into 24 grid squares (rows A to D and columns 1 to 6, A1–D6). The calculated ΣREY values and resource potential of each grid are listed in Table 1. In addition, Supplementary Table S3 shows the average ΣREY and the total resource amount from the seafloor to each target depth. There is a vast (over 400 km2) area high in REY in the northwest part of the research area (see the panels for 5–6 and 6–7 mbsf in Fig. 2), which continues loosely to the southeast (see the average panel map in Fig. 2). ΣREY is relatively low in the basin in the middle of the southern area and in the topographical high area in the northeast. The calculated ΣREY for the entire research area is more than 16 million tons of rare-earth oxides (Mt-REO) (average ΣREY = 964 ppm). In addition, the mud is especially enriched in Y and HREE, which accounted for 44% (Y: 4.4 Mt-REO; HREE: 2.6 Mt-REO) of the total amount of REY in this region. The research area was estimated to be able to supply Y, Eu, Tb, and Dy for 780, 620, 420, and 730 years, respectively, and has the potential to supply these metals on a semi-infinite basis to the world11. Of the divided areas, B1 (9.9 km × 10.6 km = 105 km2) shows the highest REY resource potential, with an average ΣREY of more than 1,700 ppm. This area includes MR15-02 PC01, where extremely REY-rich mud was confirmed, and the ΣREY of the 5–6 mbsf interval exceeded 5,600 ppm. The resource amount of area B1 was estimated to be 1.2 Mt-REO, which would account for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively (Supplementary Table S4)11.

Figure 2 Concentration maps of average ΣREY of mud from the seafloor to 10 mbsf and of each 1-m depth interval. The target area (Fig. 1) is divided into 24 areas (A1–D6). The maps were generated by ArcGIS and are shown with 2,400 grids (60 × 40). Coring sites are shown as white circles. Full size image

Table 1 Average ΣREY and resource amount of each interval for each grid (A1–D6). Full size table

Determination of the host mineral of REY

In addition to the huge resource amount, it will be possible to enhance the economic value of the REY-rich mud by selectively recovering the host mineral of REY in the mud and thereby significantly improving the ore grade. Kashiwabara et al.12 determined that the host mineral of REY in the REY-rich mud distributed in the eastern South Pacific Ocean is an apatitic mineral phase (composed of calcium phosphates) by using X-ray absorption fine structure and micro-focused X-ray fluorescence analyses. In addition, it has long been held that some trace elements, including REY, are substantially adsorbed by biogenic calcium phosphates (BCP) in marine sediment after their deposition13,14. Here, we conducted electron probe microanalyser (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of 32 BCP (KR13-02 PC04: 7 samples, PC05: 25 samples) and 9 phillipsite samples (KR13-02 PC05), which are ubiquitous in highly to extremely REY-rich mud (Supplementary Table S5). Kon et al.15 also conducted in-situ chemical analyses of BCP grains in REY-rich mud, although their sampling locations and stratigraphic positions were not clarified. In contrast, spatial information is well documented for our samples (Supplementary Table S5). Our measurements show that the average ΣREY in BCP exceeds 15,000 ppm (up to 22,000 ppm), and that BCP can generally account for the total REY in highly/extremely REY-rich mud (Fig. 3). A negative correlation was observed between the ΣREY of BCP and the total value of elemental contents, determined by EPMA (major elements) and LA-ICP-MS (minor elements) analyses (Fig. 3). In the EPMA analysis, H 2 O and CO 2 were the main constituent elements of total deficit (low total from 100%). Therefore, Fig. 3 indicates that H 2 O and CO 2 in BCP grains increase with progressing of REY uptake, owing to the successive interaction between BCP and seawater and/or pore water. The potential of BCP to uptake REY could be variable depending on the diffusive coefficient and susceptibility to diagenesis of each grain16. Difference in these factors may be attributable to mineralogical features of the grains, such as lattice defects or crystallinity17. They can differ among body regions in each organism (e.g., dentin versus enamel of fish teeth)16,17 or species of organisms. The broad negative correlation probably reflects such variability (Fig. 3). On the other hand, most of the phillipsite grains contained less than 100 ppm of ΣREY. In a pelagic environment, phillipsite appears to be generated by the alteration of volcanic glasses18,19. The requisite environmental condition for the formation of phillipsite in marine sediment (i.e., a sufficiently low sedimentation rate) is similar to that of pelagic clay enriched in BCP grains having a high REY content20,21,22. Hence, phillipsite merely co-occurs with BCP in marine sediment, resulting in a spurious correlation between the amount of phillipsite and ΣREY in the mud.

Figure 3 ΣREY in BCP grains determined by EPMA and LA-ICP-MS. The vertical axis shows the total value [%] of the analysis (left axis) and the frequency of the samples (right axis). A moderate negative correlation (gray shaded area) can be observed between ΣREY in BCP and the total value. Full size image

Grain size separation with test sieves

The BCP grains in marine sediment and in highly to extremely REY-rich mud generally are larger than the grains of other constituent minerals23. Toyoda et al.14 conducted grain-size separation of marine sediment from the central Pacific (17°06′N, 146°12′W) and divided it into 6 grain-size categories (<1 µm, <2 µm, 2–10 µm, 10–38 µm, 38–100 µm, and >100 µm). They concluded that the larger fractions (10–38 µm and 38–100 µm) have remarkably high Ca, P, and REY contents as compared with the bulk composition and the other fractions, primarily because of an abundance of BCP grains in these larger fractions. This result indicates that it is possible to selectively collect REY-enriched BCP grains by using size separation techniques. We, therefore, conducted grain-size separation experiments to elucidate the weight, ΣREY, and REY distribution in each grain-size fraction and to demonstrate the effectiveness of grain-size separation. Three REY-rich mud samples from core KR13-02 PC05 were used in the experiments: a “normally” REY-rich mud (ΣREY = 795 ppm, 1.94–2.10 mbsf), a “highly” REY-rich mud (ΣREY = 3,950 ppm, 2.62–2.78 mbsf), and an “extremely” REY-rich mud (ΣREY = 7,226 ppm, 3.08–3.24 mbsf). The experiments were conducted with polypropylene test sieves (new Perlon Sieves, Ito-Seisakusho) with openings of 20, 37, 75, and 125 µm. The ΣREY was the lowest for the smallest grain-size fraction (<20 µm) in all of the REY-rich mud samples (normal: 589 ppm; high: 1,651 ppm; extreme: 2,586 ppm) (Supplementary Table S6). ΣREY greatly increased with increased grain-size for all samples, and the highest ΣREY was observed at 37–75 µm in normally REY-rich mud and at 75–125 µm in highly and extremely REY-rich mud (Fig. 4). The ΣREY of the mud decreased in the >125 µm size fraction (normal: 1,020 ppm; high: 2,404 ppm; extreme: 4,897 ppm), most likely because of an increase in manganese oxide components such as micro Mn-nodules. Moreover, the weight distribution was greatest in the smallest size fraction (<20 µm) for all samples (normal: 79%; high: 64%; extreme: 55%) (Fig. 4). The ΣREY of each size fraction indicates that most of BCP grains are distributed in the >20 µm size fractions. When the >20 µm fraction was selectively collected, the grade of normally, highly, and extremely REY-rich mud increased from 795 ppm to 1,496 ppm (recovery ratio of REY: 40.2%), from 3,950 ppm to 8,235 ppm (73.6%), and from 7,226 ppm to 10,360 ppm (77.4%), respectively. Our experimental data strongly suggest that the selective recovery of BCP by grain-size separation is an easily applicable method to increase REY content of marine sediment that could be used in any ocean area.

Figure 4 ΣREY, weight distribution, and REY amount distribution [%] of each fraction obtained from the grain-size separation experiment with test sieves. Full size image

Mineral processing by a hydrocyclone separator

We conducted additional separation experiments of REY-rich mud with a hydrocyclone separator, which would be an applicable technology on an industrial scale24. A hydrocyclone separator is a device that can separate heavy/large and light/small components from a flowing solid-liquid mixture (slurry) by centrifugal force, and a Super-30-Cyclone (Nihon Bunri Co.) was used in our experiments. Based on the grain-size separation experiments with test sieves, the experimental conditions of the hydrocyclone separator were coordinated to separate the >15–20 µm grains from the slurry. Three REY-rich mud samples from MR15-02 PC01 cored at 22°02′N and 153°34′E were used in the experiments (Supplementary Table S7). The ΣREY of the original slurry samples were 722 ppm (normally REY-rich mud: MR15-02 PC01, 2.28–4.28 mbsf), 2,315 ppm (highly REY-rich mud: 6.27–7.05 mbsf), and 4,802 ppm (although this was less than 5,000 ppm, it was considered as an extremely REY-rich mud: 4.28–6.28 mbsf). Supplementary Fig. S1(A) shows the particle size distribution of the over-flow (OF, waste flow) and under-flow (UF, collection flow) components. The nodes of the fraction curves are roughly 15–20 μm in all cases, and separation by the hydrocyclone was generally well performed; the imperfection is calculated to be 0.74 for normally REY-rich mud, 0.38 for highly REY-rich mud, and 0.38 for extremely REY-rich mud, respectively (Supplementary Fig. S1). The ΣREY values of the OF and UF components were 429 ppm and 1,401 ppm for normally REY-rich mud, 997 ppm and 6,031 ppm for highly REY-rich mud, and 994 ppm and 8,902 ppm for extremely REY-rich mud, respectively (Supplementary Table S7). The recovery ratio of REY in UF were 70.7%, 75.0%, and 93.0% for the normally, highly, and extremely REY-rich muds, respectively (Fig. 5). The concentration factor of ΣREY through the mineral processing (the ratio of ΣREY in UF to ΣREY in the original slurry) had the highest value at a ΣREY of about 2,000–3,000 ppm in the original mud sample (Fig. 5). The improved ore grade gained via grain-size separation with test sieves and hydrocyclone treatment indicates that the separation is both an effective and predictable method for preliminary mineral processing of REY-rich mud. The ΣREY of the UF for highly REY-rich mud increased to 260% of the value of the original sample. In addition, the hydrocyclone can reduce the slurry volume in UF to less than one-fifth of the original value and mud weight to 42.5%, 33.1%, and 59.8% of the original sample weights of the normally, highly, and extremely REY-rich mud, respectively. Because the amount of the resource is enormous, improving the ore grade will greatly enhance the economic value of the mud even if the recovery yield is somewhat lower than that we observed. A decrease in mud weight and volume will directly lead to reductions in smelting costs. Moreover, if a hydrocyclone can be operated in-situ on the deep-sea floor, it would be possible to reduce lifting costs, which would further contribute to improving the economic efficiency of any development project.

Figure 5 Concentration factor of ΣREY through the grain-size separation experiments with test sieves (ΣREY of the >20 µm component/ΣREY of the original sample) and the hydrocyclone separator (ΣREY of the under-flow component/ΣREY of the original slurry sample). Data of the original sample, under-flow component, and over-flow component are shown in gray, red, and blue, respectively. The pink shaded area shows the expected concentration factor of grain-size separation with respect to ΣREY of the original sample/slurry. Full size image

In summary, our results demonstrate the enormous resource amount of REY-rich in the western North Pacific Ocean. In addition, a hydrocyclone separator can greatly enhance the economic value of REY-rich mud by taking the advantage of mineralogical features of the mud, which could stimulate future exploitation of this new deep-sea mineral resource. Given the huge resource amount, its high grade (notably Y and HREEs), and the effectiveness of simple grain-size separation with a hydrocyclone, we believe that the REY-rich mud has great potential as ore deposits for some of the most critically important elements in the modern society.