A potential resource for REEs: Deep-sea mud in the Pacific Ocean
There has been a rapid increase in world demand for rare-earth elements (REE) because they are essential to modern green-energy technologies, technologies in the communications, novel electronic equipment, and military weapon markets. At present, China dominates the REE market, especially in heavy rare-earth elements (HREE; from Gd to Lu, including Eu) which are very important metals for high-technology industries. The recent statistics show that China produces a remarkable 97.4 percent of the world REEs and has one-third of the world’s REE reserves1 although light REE (LREE) can be mined from carbonatite/alkaline igneous complexes in other countries. However, most HREE reserves are in ion-absorption type ore deposits in southern China.2 China's rare earth reserve has been steadily decreasing from 75% in 1970 to 23% in 2011.3 Therefore, it seems likely that new HREE reserves are needed.
Seafloor muds have not been regarded as a REE resource yet, because data on the spatial distribution of these deposits are insufficient. Kato, Y. et al. 2011 measured the elemental composition of over 2,000 seafloor sediments and showed that the deep-sea mud contains high concentrations of REE at numerous sites throughout the eastern South and central North Pacific. In the eastern South Pacific (5°-20°S, 90°-150°W), the REE-rich mud has high REE contents, 1,000-2,230 ppm total REE and 200-430 ppm total HREE. REE contents of the mud are comparable to or greater than those of the southern China ion-absorption-type deposits, (∑REE = 500-2,000 ppm; ∑HREE = 50-200ppm); notably, the HREE contents are nearly twice as abundant as those in typical inland REE deposits in China. The REY-rich mud in the North Pacific east and west of the Hawaiian Islands (3-20°N, 130°W-170°E) has moderate REE contents (∑REE = 400-1,000ppm, ∑HREE = 70-180ppm). Also, REE-rich mud with moderate REE content is also found in the northeast Pacific, west of the Juan de Fuca Ridge.
The distribution map of average ∑REE contents for the seafloor sediment is roughly consistent with the helium-3 anomaly map of mid-depth seawater in the Pacific, which reflects broad spreading of hydrothermal plumes from the East Pacific Rise and Juan de Fuca Ridge. This coincidence shows that the REE deposition is related to mid-ocean ridge hydrothermal activity. Their bulk-sediment data also indicate that one of the main hosts for the REE is a Fe-oxyhydroxide precipitate from hydrothermal plumes that has taken up rare earths from ambient seawater. Phillipsite is regarded as the second main hosts for the REE because of its major element composition.
Assuming an average ∑REE content of 1,180 189 ppm and dry bulk density of 0.66g/cm-3, the 10-m thick bed of REE-rich mud in an area of 1 km2 at Site 76, in the eastern South Pacific, has the potential to yield 9,110 1,460 tonnes of REE oxides, or one-fifteenth of the global annual consumption of REE in 2010. Similarly, assuming an average ∑REE content of 640 102 ppm and dry bulk density of 0.477 g/cm-3, the 70-m thick REE-rich mud layer in an area of 5km2 at Site 1222 in the central North Pacific holds nearly 125,000 20,000 total REY-oxides. This amounts of REY could possibly supply the majority of current annual REY consumption in the world. Furthermore, experiments using dilute acids reveal that almost all of the REY, except Ce, are readily leached from the mud in 1 to 3 hours and pulverization and beneficiation are not needed before metal leaching. These characteristics improve the potential of the REY-rich mud as a REY resource. The REY-rich mud in the Pacific Ocean may be able to be regarded as a highly promising REY resource in the future.
Figure 1. Distribution of average 6REY contents for surface sediments (<2m in depth) in the Pacific Ocean.
Reference
US Department of the Interior and US Geological Survey. Mineral Commodity Summaries 2011 (US Government Printing Office, 2011).
Roskill Information Services Ltd. The Economics of Rare Earths & Yttrium 13 edn (Roskill, 2007).
Chen, Z.,2011.Global rare earth resources and scenarios of future rare earth industry. Journal of Rare Earths 29 (1), 1–6.
