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The Cook Islands Exclusive Economic Zone

The Cook Islands Exclusive Economic Zone (EEZ) covers 1. 8 million square kilometres. In this area, the most significant mineral resource is manganese nodules that are found on the seabed (see cover drawing of a nodule cross-section). These nodules are formed by metallic elements that slowly precipitate out of the ocean water. The most abundant element is manganese. Nodules may grow at a rate of about two millimetres per million years. Deep currents flowing from the Antarctic region towards the equator influence the formation of nodules in Cook Island waters, which are rich in cobalt, nickel and copper.

Manganese Nodules are small, dark, potato-shaped little balls where metals and other minerals have accumulated around a core over a few million years. The core can be a prehistoric shark tooth, a piece of prehistoric whale bone, a meteorite fragment or similar. They contain a relatively high percentage of useful metals, i. e. Nickel, Copper, Cobalt, Manganese and Iron, though the tailings (residues) are approximately 75% of the dry mass. They are mostly found in water depths of 4000-6000 metres in large fields often distant by a few thousend km from the closest continent shores.

There are so many of these nodules on the bottom of the ocean, that the metal reserves associated exceed for many metals the amount of known deposits on land. The difficulty to use them today is mainly of technical nature. Though it is not very difficult to pick them up in little amounts, e. g. with a grab sampler or with a dredge, it is very difficult to mine them in such quantities that they can be processed and the valuable metals extracted and still be competitive with land won metals. What could the substances obtained from manganese nod be used for?

Cobalt is the most valuable metal in these nodules, worth 10 to 30 times more than copper or nickel; Cook Islands nodules have among the highest cobalt content recorded in the Pacific. It is estimated that the cobalt resource is about 32 million tonnes, enough to sustain the current world demand for 520 years. Cobalt is utilised in diverse industrial applications, mostly in superalloys used by the aircraft industry. Demand is also increasing for use in rechargeable batteries for laptop computers and other applications.

How are they collected? Cook Islands nodules occur in very deep waters, on the order of 5,000 metres. These depths pose many challenges, not least of which includes finding an economically viable mining system. it’s hard to mine efficiently under 3000 meters of water. Development of Deep-sea Manganese Nodules It consists of four different process, i. e. exploration, mining, transportation and metallurgy. Among these, the mining process can be classified as three categories of collecting system, lifting system and mining ship.

The collecting system and lifting system are the most important parts in the commercial mining process. Lifting System This is the key system to control the success of total mining system in deep-sea mining project and it based on the lifting method is classified as the hydraulic pumping system and the air lifting system of the fluid dredging type, the continuous line buckets system of the mechanical type and the modular marine mining automation system.

The fluid dredging type has been extensively studied in U. S. A et al. ; the continuous line buckets system in Japan. The modular type is proposed as the next generation technology. Hydraulic pumping system In this type, using the submersible pump attached to the middle of one long lifting pipe, a slurry of sea water and manganese nodules is lifted to the mining ship from the deep-seabed where the manganese nodules are collected by the collecting system. The pressure difference within the lifting pipe is the driving force for lifting. This is called the air-lift.

In this system the manganese nodules collected by the collecting system are lifted to the mining ship by injecting the compressed air into the middle of the lifting pipe to make the density of mixtures at the upper part of the lifting pipe smaller than that of the lower part of the lifting pipe. Therefore the density difference causes the buoyance of seawater within the lifting pipe. The fluid flow is the two phases flow with solid and liquid in the pipe section below the air injection point, and the three phases flow with solid, liquid and air in the pipe section above the air injection point.

This system was invented by Yoshio Masuda in 1968, where a series of buckets are attached to a rope to mine and lift the manganese nodules. The system consists of buckets, rope and hoist. When this system is installed, several aspects such as rope tangle, mining efficiency, rope interval, mining direction and environmental problem should be taken into account since the loop rope with many buckets is circulated continuously between the seabed and the mining ship.

Who would think it is right to harvest manganese nodules and y? For twenty years, information on the Cook Islands manganese nodule resource has been collected. Institutions that have played an important role include: the South Pacific Applied Geoscience Commission, the Metal Mining Agency of Japan, the East-West Center, the US Trade and Development Agency and the Commonwealth Secretariat.

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