Yutaro Takaya, et al., report that the potential risks of supply shortages for critical metals including rare-earth elements and yttrium (REY) have spurred great interest in commercial mining of deep-sea mineral resources. Deep-sea sediments containing over 5,000 ppm total REY content was discovered in the western North Pacific Ocean near Minamitorishima Island, Japan, in 2013. These REY-rich sediments have great potential as a rare-earth metal resource because of the enormous amount available and its advantageous mineralogical features. In this paper, they estimated the resource amount in REY-rich sediments with Geographical Information System software and established a mineral processing procedure to greatly enhance its economic value.
The resource amount was estimated to be 1.2 Mt of rare-earth oxide for the most promising area (105 km 2×0–10 mbsf), which accounts for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively. Moreover, using a hydrocyclone separator enabled us to recover selectively biogenic calcium phosphate grains, which have high REY content (up to 22,000 ppm) and constitute the coarser domain in the grain-size distribution. The enormous resource amount and the effectiveness of the mineral processing are strong indicators that this new REY resource could be exploited in the near future.
In addition to the huge resource amount, it will be possible to enhance the economic value of the REY-rich sediments by selectively recovering the host mineral of REY in the sediments and thereby significantly improving the ore grade. Kashiwabara, et al., determined that the host mineral of REY in the REY-rich sediments 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 deposition. They concluded conducted electron probe microanalyzer (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 sediments. Kon et al., also conducted in-situ chemical analyses of BCP grains in REY-rich sediments, although their sampling locations and stratigraphic positions were not clarified. In contrast, spatial information is well documented for our samples.
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. 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. In the EPMA analysis, H2O and CO2 were the main constituent elements of total deficit (low total from 100%). Therefore, H2O and CO2 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 grain. Difference in these factors may be attributable to mineralogical features of the grains, such as lattice defects or crystallinity. They differentiated among body regions in each organism (e.g., dentin versus enamel of fish teeth) or species of organisms. Te broad negative correlation probably reflects such variability. 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 glasses.
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 content. Hence, phillipsite merely co-occurs with BCP in marine sediment, resulting in a spurious correlation between the
amount of phillipsite and ΣREY in the sediments.
See: https://www.nature.com/articles/s41598-018-23948-5.pdf
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