Research Overview
Research Overview
Research on Mineral Processing of Seabed Mineral Resources on the Seabed
• Introduction
A mining system has been under research and development to allow the development of seafloor massive sulfides since Japan’s first Basic Plan on Ocean Policy was established in 2008. The mining system consists of a Subsea Mining Tool, a Pump Unit, Riser Pipes, and a Mining Support Vessel. In the mining process employing the system, the Subsea Mining Tool excavates the minerals on the seafloor. Next, the pump lifts the slurry (a mixture of solids and seawater) through the Riser Pipe up to the Mining Support Vessel on the sea surface. The slurry is separated on board, the remaining water is conveyed through another riser pipe for disposal, and the ore is transferred to a shuttle to transport to a port (see Figure 1).
Mineral processing and smelting (*1) are necessary for extracting valuable metals from ores (see Figure 2 (left)). All the excavated ore, including invaluable minerals (tailings or gangues), is transported to a mineral processing facility onshore in the aforementioned mining system. Thus, if only high-concentration ores were lifted to the sea surface and shipped to shore, we could reduce the costs of the mining system. (see Figure 2 (right)).
Therefore, NMRI named this idea "Seafloor(Marine) Mineral Processing" and carried out the development of fundamental technology [1]. The proposed system consists of units for grinding and separation (see Figure 3). In the grinding unit, the ore is ground to a size of several hundred micrometers. The separation unit divides the crushed ore into concentrate and tailings. In this study, we appraised the conventional grinding and separation technologies used in onshore mines. Among these technologies, we selected candidates for elemental technologies for the grinding and separation units and tested their applicability to deep sea conditions.
Here, the results of our study on grinding technology are presented.
Figure 1 Schematic of a mining system for seafloor massive sulfides
Figure 2 Process charts for ores from seafloor massive sulfides
(left: conventional process; right: our proposed process introducing the seafloor mineral processing)
Figure 3 Schematic of Seafloor (Marine) Mineral Processing System
• About grinding technology
Various types of crushing and grinding machines are used in onshore mines according to the size of ore and other factors, e.g., jaw crusher and gyratory crusher for relatively large ores, roll mill for medium-size ores, and ball mill for fine grinding. This study focused on ball mills and examined their applicability for seafloor mineral processing.
A ball mill consists of a cylindrical container (mill pot), in which the material to be ground (coarse particles such as ore) and the grinding media (typically steel balls) are placed, and rotating mechanism. Then, the mill pot rotates, crushing the ore by collisions and friction with the steel balls (see Figure 4). Due to its simple structure and low failure rate, it has been selected as a candidate for the grinding unit.
In ball mill grinding, a space is left within the mill pot for the movement of ore and steel balls, which is occupied by air. If the ore and steel balls are packed without spaces, they cannot move and thus cannot be crushed. A small amount of water is often added to promote grinding, which is called wet grinding. To perform wet grinding in seafloor mineral processing, as done onshore, it is necessary to keep air in the mill pot at high pressures. However, a large amount of compressed air to be supplied from the sea surface to the seafloor (*2) potentially complicates the system and increases the total cost (see Figure 5).
In this study, we manufactured a small grinding test apparatus simulating a ball mill to confirm the necessity of air injection. We conducted grinding tests under high pressure corresponding to deep sea conditions. We tested the grinding process assuming two experimental cases: (i) the mill pot was filled with water, and (ii) air was kept in the mill pot. The grinding performances in these cases were also compared with those at the atmospheric pressure. Hereafter, grinding in the mill pot filled with water is called water-filled grinding. We used silica sand as a material for grinding tests.
Figure 4 Mechanism of ball mill grinding
Figure 5 Early image of grinding unit for seafloor mineral processing system
(The study showed that the air injection tube is not necessary.)
• Grinding tests in high-pressure water and results
To conduct grinding tests in high-pressure water, we manufactured a small grinding test apparatus consisting of a stainless-steel mill pot and a stand to rotate the pot (see Figure 6). A porous filter plate that allows fluids (water and air) to pass through but not silica sand was attached to the lid of the mill pot, ensuring that the internal pressure of the mill pot is always equal to the external pressure. After silica sand and steel balls were set inside the mill pot, the test apparatus was set in the High Pressure Tank (*3), and pressure in the tank was increased to 10MPa, which corresponds to water depth of 1,000 m. Water was allowed to enter the mill pot through the filter in water-filled grinding. In wet grinding at high pressure, the air was injected from a cylinder outside the High Pressure Tank into the mill pot through a buffer and filter; a relatively constant volume of air was kept in the pot. Additionally, enough amount of water to perform wet grinding was poured in the pot when the sand was enclosed. During the grinding tests, a hydraulic power unit settled in the tank was used to rotate the mill pot.
Tests were conducted for water-filled and wet grinding under atmospheric and high-pressure conditions. As experimental parameters, the rotational speed of the mill pot was varied, and tests were conducted at 70%, 100%, and 130% of the critical speed (*4). The particle size of the crushed silica sand was measured using a method called laser diffraction. Figure 7 shows the particle size distribution of ground silica sand. The particle size distribution obtained under high-pressure conditions was generally similar to that obtained under atmospheric pressure. Figure 8 shows the mean particle size of ground silica sand. Under atmospheric wet grinding conditions, the smallest average particle size was obtained near the critical speed. Still, under high-pressure wet grinding conditions, smaller particle sizes were obtained at speeds higher than the critical speed. By adjusting the rotational speed, water-filled grinding under high-pressure conditions could achieve approximately the same grinding performance as wet grinding onshore.
Figure 6 Photo of grinding test apparatus
(The mill pot mounted on the stand)
Figure 7 Particle size distribution of ground silica sand
(upper: atmospheric pressure; lower: 10MPa)
Figure 8 Mean particle size of ground silica sand
• Summary
To contribute to the development of seafloor massive sulfides, we studied fundamental technology for Seafloor Mineral Processing Systems. We investigated the applicability of ball mill grinding to high-pressure submerged conditions and conducted grinding tests. The results showed that water-filled grinding under high-pressure conditions could provide performance comparable to wet grinding onshore. We will continue to conduct further research and contribute to the realization of the seafloor massive sulfide development through technological development.
- The process of grinding ores and separating them into concentrate (rich in useful metal minerals) and tailings (invaluable minerals) is called mineral processing or ore dressing. The process of extracting metals from concentrate by processing with chemicals and/or at high temperatures is called smelting.
- The mill pot for seafloor mineral processing system is an open type vessel (non-sealed) to allow ore loading underwater. The pressure inside the mill pot is the same as outside, so there is great water pressure in the deep sea. Since the volume of gas decreases inversely proportional to its pressure, a large amount of gas is required to fill the same volume at high pressures.
- For details on the specifications of the High Pressure Tank, please refer to the main research facilities of the Deep Sea Technology Research Group ( link )
- The critical rotating speed is the rotating speed at which the centrifugal force acting on particles on the inner wall of the mill pot balances with gravity.
Acknowledgments
Part of this research was supported by JSPS KAKENHI Grant Number 22360373. We would also like to thank Professor Toyohisa Fujita, Assistant Professor Katsunori Okaya, and Assistant Professor Seiji Matsuo of the Department of Systems Innovation, Graduate School of Engineering, The University of Tokyo, and other related individuals for cooperation in this research.
(Reference)
[1] Nakajima, Y., Uto, S., Kanada, S., Yamamoto, J., Takahashi, I., Otabe, S., Sadaki, J., Okaya, K., Matsuo, S., and Fujita, T. : CONCEPT OF SEAFLOOR MINERAL PROCESSING FOR DEVELOPMENT OF SEAFLOOR MASSIVE SULFIDES, Proceedings of the ASME2011 30th International Conference on Ocean, Offshore and Arctic Engineering (2011), OMAE2011-49981.