Abstract

Deep-sea mining is the mining process of mineral resources from ocean floor of over several hundred meters. These mineral resources include "polymetallic nodules", "seafloor massive sulfides" and "cobalt-rich ferromanganese crusts". Until now, no technology has been developed for environmentally friendly lifting of ores of these marine mineral resources economically from deep seafloor to sea surface. The authors recently proposed a new technical idea in which high density ores of several centimeters or larger in diameter can be transported by circulating carrier materials through a riser pipe from deep seafloor of several thousand meters. In this paper, the experimental apparatus is introduced that was developed for model tests to investigate the feasibility of this proposed technology. In this newly developed experimental apparatus, the lifting process can be observed in the section of 860 mm in length of the riser pipe with an internal diameter of about 50 mm. Since the carrier materials are not transparent, transportation processes of ore models through the riser pipe cannot be monitored by visual observation, a new measurement method using RFID (radio frequency identification) technology will be introduced.

1.
Introduction

Mineral resources such as manganese nodules and submarine hydrothermal deposits are reserved in and on the deep ocean floors around Japan. The development of economical and environmentally friendly technology for deep sea mining is one of the most important tasks to meet growing demand for mineral resources. Some attempts have been made for deep-sea mining of metalliferous sediments, massive sulphides and polymetallic nodules.

Fig.1 (a) shows one of the typical systems of deep-sea mining for the mineral resources on the seafloor or in the shallow depths (ECORYS, 2013; Kawano et al., 2015). Ores are collected from mining areas to the lift system by various techniques such as sediment suction, rock excavation or nodule collection. The collected ores are crushed into finer particles or even transformed into water-ore slurry, and then transported from the seafloor to the platform or the support vessel through the riser pipe by the subsea pump. Air-lifting technique may be used to enhance ability for ore lifting.

From the viewpoint of economy and ease of lifting ores by pumping, it is preferable to make the ores finer as much as possible. However, undersea execution of crushing and grinding works is rather troublesome. Moreover, it inevitably causes serious environmental impacts on the seafloor ecosystem. Due to low values of density and viscosity of seawater, the buoyancy and the dragging power are limited low. As a result, heavy coarse ore particles are difficult to lift by ordinary pumping even with air-lifting technique. So, plugging of the riser pipe is worried in case of accidents when the subsea pump stops operations. Moreover, it is necessary to pay attention to influences of sediment plumes on the marine environment caused by drainage of return water and discharge of tailing (Schriever and Thiel, 2013). Accordingly, from rock engineering point of view, enhancing the transportability of coarse ores is one of the most important issues.

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