Mining, Undersea

the working of mineral deposits beneath the waters of the world ocean.

Surface deposits on the shelf and ocean floor are worked through the water by the opencut method. Enormous mineral resources are concentrated on the surface of the shelf (19 percent of the land area of the earth) and the ocean floor (50 percent of the earth’s total area). In the ferromagnesian concretions of bottom deposits in the Pacific Ocean alone, there are predicted reserves of 2.4 × 10¹¹ tons of manganese, 2.8 × 10 tons of cobalt, 9.4 × 10⁹ tons of nickel, and 5.3 × 10⁹ tons of copper. The shelf has placer deposits of heavy minerals and metals.

The first attempts to develop the shelf date to the 11th century B.C., when the Phoenicians extracted raw material for purple dye from beds of seashells. In the sixth century B.C., the coral reefs of Polynesia were worked to obtain building materials. In the third century B.C., divers extracted copper ore from a depth of 4 m near the island of Khalki (now Heybeli Island) in the Bosporus. In the late 19th century, placers of gold, ilmenite, rutile, zircon, and monazite were developed off the coast of Australia (1870), Brazil (1884), and India (1909). Tin was first extracted from marine placers in Indonesia in the 1920’s, and in 1963 the mining of diamonds on the shelf of South West Africa was begun. In the early 1960’s iron ore was extracted from placers in Ariake Bay, Japan. Work to develop marine placers in the USSR was begun in 1966 on the shelf of the eastern Baltic, where titanium-zircon concentrates were extracted.

In 1973, more than 70 dredging enterprises extracted approximately 120–130 million cu m of rock from placers in the shelf. The extraction of tin concentrates from marine placers amounted to 10 percent of tin extraction worldwide, excluding the USSR. The value of diamonds extracted from beneath the surface of the water exceeded 3 percent of the total value of diamonds extracted in some years.

The choice of equipment and methods for undersea mining depends on mining-geological and hydrometeorological conditions, the depth of the work, and the type of mineral. Placers are worked primarily by multibucket, hydraulic, and grapple dredges. As of 1974, dredges using air lifts and buckets secured to an endless cable had been tested and were under construction for mining ferromagnesian concretions.

Opencut undersea mining on the shelf enjoys certain advantages over the mining of land deposits, and these advantages determine the prospects for the former. The construction of dredges and other specially equipped vessels at large shipyards eliminates the time spent on construction and installation work at land deposits. Work volume to open deposits of minerals is significantly reduced. It is not necessary to construct access routes, power lines, or worker communities, nor is it necessary to take agricultural land out of use and later restore it.

Mining work on the shelf is made more difficult by wave activity on the water surface, the tendency of excavations on the ocean bottom to become filled in, and the washing away of dump piles. It is also impeded by problems connected with the excavation and disposal of rocks in environment of marine fauna and flora and the need to maintain stable shorelines.

In the USSR, scientific research to develop the shelf seeks to find methods of geological prospecting and placer testing that allow for geological and economic assessments. It aims to provide the scientific basis for the technology needed to extract minerals in the regions of the continental shelf and ocean floor without harming water organisms. Research is aimed at developing machines to extract and concentrate minerals at all depths of the shelf.

Deposits beneath the floor of the world ocean are worked by underground mining excavations and boreholes.

Undersea mining in bedrock deposits differs little from land mining as regards the methods used to excavate mineral ores. In most undersea mines the shafts are laid on land, resulting in haulage excavations that are several kilometers long. Mines are sometimes opened by shafts from man-made islands, as with the Miike mine in Japan. Excavations are made deep enough beneath the bottom to guard against flooding; the depth depends on the characteristics of the overlying rock and is usually 65–80 m. The excavated space is flushed as the deposit is worked; underwater mines are ventilated by pipes through one shaft.

In 1974 there were 57 undersea coal mines operating in Japan, Great Britain, Turkey, and on Taiwan, two iron-ore mines in Finland and Canada, and two tin mines in Great Britain and the USSR.

The extraction of petroleum and gas from beneath the world ocean accounts for the largest part of undersea mining. The use of geotechnological methods to extract solid minerals is also promising. For example, the annual extraction of sulfur from deposits in the Gulf of Mexico by the melting method is more than 600,000 tons (1973).

Undersea mining also includes the extraction of minerals from seawater by means of physicochemical processes that separate the salts and the various chemical elements dissolved in the water. Total volume of the salts and elements is 48 million cu km, which includes approximately 2 × 10¹⁶ tons of sodium, 2 × 10¹⁵ tons of magnesium, and 1.3 × 10¹⁴ tons of bromine.

Bromine was first obtained from the mother liquors of common salt in France in the mid-19th century. In the 1930’s the industrial extraction of magnesium from seawater was begun. As of 1970, more than 100 enterprises designed to extract sodium chloride from seawater were operating in the USSR, the United States, Great Britain, and other countries. Production volume was more than 10 million tons of salt, 300,000 tons of magnesium, and 75,000 tons of bromine.

The technology for extracting chemical elements from seawater ordinarily provides the means for concentrating the elements and then receiving them in the form of compounds as the saturated solution interacts with other elements.

The concentration of chemical elements in seawater is low, except for sodium, magnesium, and bromine; as of 1974, therefore, extraction was unprofitable. Prospects for the future are linked to an increase in the volume of seawater desalinization. Chemical elements can be efficiently extracted from the byproduct brines obtained during this process; this is done by installations for adsorption exchange and extraction.