Overview
That is, a metal sulfide in the sulfide form (a metal). Metal sulfides can be formed from sulfur and metals to form binary compounds, or from hydrogen sulfide (or hydrogen sulfuric acid) and metal oxide sulfides in acid solubility or hydroxide action.
E.g:
Cu (red hot) + S (steam) = (heated) CuS
H 2 S+CuO=CuS+H 2 O
H 2 S+2NaOH=Na 2 S+2H 2 O
Water solubility of metal sulfides: sodium sulfide, potassium sulfide, etc. are easily soluble in water, and other sulfides are insoluble in water;
Acid solubility of sulfide
There are sodium sulfide, potassium sulfide, zinc sulfide, magnesium sulfide, iron sulfide, manganese sulfide is not soluble in dilute acid solution, the other lead sulfide, cadmium sulfide, antimony sulfide, stannous sulfide, silver sulfide, copper sulfide, mercury sulfide In dilute acid. I.e. soluble alkali metal sulfides, alkaline earth metal sulfides; calcium sulfide, strontium sulfide, barium sulfide and other water-soluble.
Polymetallic sulfide
Discovery and formation
In 1979, at the 21st north latitude of the Pacific Rim in the East Pacific Ocean off the coast of California (Mexico), scientists discovered a chimney-like black rock structure on the sulphide mound while exploring the bottom of the ocean. The chimney spurts hydrothermal fluid and the surrounding animal species are unprecedented. see. Subsequent studies have shown that these black chimneys are produced when the new oceanic crust is formed, which is caused by the convergence or movement of tectonic plates beneath the surface of the earth and the expansion of the sea floor. In addition, this activity is closely related to the formation of seabed metal deposits.
Metal sulfide
At depths of up to 3,700 m, seawater seeps into the formation space from the ocean, is heated by the lava (magma) beneath the earth's crust, and is discharged from the black chimney, with a hydrothermal temperature of up to 400 °C. When these hydrothermal fluids are mixed with the surrounding cold seawater, metal sulfides in the water settle to the chimney and the nearby sea floor. These sulphides, including galena (lead), sphalerite (zinc) and chalcopyrite (copper), accumulate in the seabed or subsurface, forming massive deposits ranging from a few thousand tons to about 100 million tons. The fact that some massive sulphide deposits are rich in metals such as copper, zinc and lead, especially rich in precious metals (gold and silver), has attracted interest in the international mining industry in recent years. Many polymetallic sulphide deposits have also been discovered in places where there is no volcanic activity.
Distribution status
Most of the mines are located in the middle of the ocean and are distributed in the East Pacific Seamount, the Southeast Pacific Seamount and the Northeast Pacific Seamount. It is known that there are also some deposits in the mid-Atlantic Ridge, but only one place has been found in the Indian Ocean Ridge. There are fewer known sulphide deposits in the mid-Atlantic ridge and the mid-ocean ridge, mainly due to limited exploration activities in these areas. There are 60,000 kilometers of ridges in the world, and only about 5% of any surveys.
In the mid-1980s, some sulphide deposits were discovered in the southwestern Pacific Ocean, located at the edge of the ocean, between the continents and the volcanic island arcs, where the basins and ridges formed. At these so-called post-arc expansion centers, the magma rises to the surface near the edge of the converging plate (at the edge of the converging plate, a tectonic plate slides underneath another plate by a subduction process). These findings led to large-scale exploration of the basins of the western Pacific and southwestern Pacific Oceans, as well as island arcs and arcs. As a result, other deposits were discovered in the Laohai Basin in the eastern part of Australia and the northern Fiji Basin and the Okinawa Trough in southwestern Japan. In 1991, a large number of silt deposits associated with feldspar volcanic activity (the strongest volcanic activity, resulting in the largest volcanic ash flow) were found in the Manus Basin in northern New Caledonia. Hydrothermal deposits have also been discovered near the Woodlake Basin, where the expansion of the sea floor extends to the continental crust east of Papua New Guinea. Today, more than 100 hydrothermal mineralization points are known, including at least 25 high temperature black chimney spouts.
Metal content
After nearly 1,300 chemical analyses of seabed sulphides, it was found that deposits in different volcanic and tectonic environments have different metal ratios. Compared with the mid-ocean ridge samples lacking sediments, the higher average metal content of the massive sulphide (573 samples) produced in the basalt - andesite environment at the post-arc expansion center is: zinc (17%), lead ( 0.4%) and bismuth (13%), the iron content is not high. The polymetallic sulphides (40 samples) in the post-arc rift of the continental crust are also low in iron, but are usually rich in zinc (20%) and lead (12%) and contain high levels of silver (1.1%, or 2 304 g / ton). In general, the total composition of seabed sulphide deposits in various tectonic settings depends on the leaching of volcanic rocks from which the metals are derived.
Recently, gold is found to be very high in sulfide samples in the center of the post-arc expansion, while the average gold content in the mid-ocean ridge deposit is only 1.2 g/t (1,259 samples). After the arc, the gold content of the sulfide in the basin was as high as 29 g/t, with an average of 2.8 g/t (103 samples). In the Okinawa Trough, a sulphide deposit in a post-arc rift in the continental crust contains up to 14 g/t (average 3.1 g/t, 40 samples). A preliminary analysis of the sulphide in the East Manus Basin showed a gold content of 15 g/t and a maximum of 55 g/t (26 samples). A gold content of up to 21 g/t was found in the barite chimney of the Woodlake Basin. The most abundant gold-bearing deposits discovered to date are located in the conical seamounts near Nelhi Island, the territorial waters of Papua New Guinea. From the top of the seamount platform (base water depth of 1,600 meters, diameter 2.8 kilometers, peak water depth of 1,050 meters), the sample collected gold up to 230 grams / ton, an average of 26 grams / ton (40 samples), 10 times more than mining The average value of the value of the land gold mine.
Tonnage estimate
Estimates for several mid-ocean ridge deposits indicate that the tonnage is between 1 million and 100 million tons. However, the extension length of the sulfide outcrop is not easy to estimate, and there is a lack of information on the thickness of the deposit. The largest deposits that have been discovered are located in ancient ridges that are overlying a large amount of sediment but still have hydrothermal activity. The International Ocean Drilling Program has drilled 800 to 9 million tons of sulphide ore from the mid-valley deposits covered by sediments in the northern part of the Juan de Fuca Ridge off the northwest coast of the United States. After 125 meters of Trans-Atlantic Geotraverse (TAG) active hydrothermal mound at a depth of 3,650 meters in the mid-latitude latitude of the Pacific Ocean, it was found that there were about 2.7 million tons of sulfide ore on the surface of the seabed and about 120 deposits in the surface. Ten thousand tons (for reticular veins). The size of massive sulphide deposits discovered to date on the seabed is not comparable to that of Kidd Creek (135 million tons) in Canada or Neviscorvo (262 million tons) in Portugal.
The largest known sulphide deposit on the seabed is the Atlantis II sea of ​​the Red Sea, which was discovered more than a decade earlier than the first black chimney of the East Pacific Seamount. The mineralization of Atlantis II Haiyuan is mainly metal ooze, not massive sulphide. A detailed evaluation of a 40-square-kilometer deposit shows that the deposit has 94 million tons of lead-lean silver ore, including 2% zinc, 0.5% copper, 39 grams/ton silver and 0.5 g/ton gold. The total content is about 4,000 tons of silver and 50 tons of gold. Test mining at a depth of 2,000 meters proves that the deposit can be successfully mined.
Resource potential
Marine mining appears to be feasible under certain conditions. Ideal conditions include (1) high grade non-ferrous metals and/or gold; (2) mines are not too far from the land; and (3) shallow depths. Although deepwater mining technology is now available, it is more than 2,000 meters deep. In these cases, the exploitation of massive sulphide ore can be economically attractive. Considering that the entire range of mining equipment can be moved from one location to another, the investment in the mining system and vessel does not have to be fixed in one place like land mining. Mining on remote locations on land often requires large initial investments, including all infrastructure.
The exploitation of massive sulphide in the seabed may be concentrated in small submarine areas and is mainly limited to the surface layer (peeling) and shallow subsurface (excavation) in order to recover the sulphide mounds and chimneys of the seabed and the reticular veins beneath it. The confession of the ore body.
Research, exploration and mining prospects
Academic institutions and government agencies around the world are conducting scientific research on polymetallic sulphide deposits and their associated ecosystems. Leading countries in this field are Australia, Canada, France, Germany, Japan, the Russian Federation, the United Kingdom and the United States. Italy and Portugal have also developed research programmes.
Exploration requires sophisticated, multi-purpose research vessels using advanced technologies such as deep sea mapping equipment, manned submersibles or remotely controlled vessels, photography and video systems, sampling and drilling equipment. Drilling and core sampling equipment must be modified to allow drilling to a depth of 100 meters. Mining systems for the recovery of sulphide have not been specifically designed, but development efforts may focus on continuous recovery systems, using rotary cutting heads, with mining equipment, transporting the ore to the mining vessel and then to the processing plant.
surroundings
The hydrothermal vents associated with massive sulphide deposits provide a living environment for a variety of animals not previously known in science. Unlike other forms of life on land that rely directly or indirectly on sunlight and photosynthesis for energy, hydrothermal vents can multiply in hot fluids that are free of sunlight and hydrogen sulfide. Hydrogen sulfide is a deadly compound for most other animals. In this environment, worms of up to two meters live in self-made habitats, without digestive systems, and from microbes that oxidize methane and sulphur oxides. In these hydrothermal vent areas with diverse organisms, about 500 species of previously unknown animals have been discovered.
In planning the exploration and exploitation of minerals, the unique and fragile nature of this geographically indifferent ecosystem must be considered, as well as its value to the Institute of Basic Biology in metabolism, evolution and adaptation. Studies have shown that existing biological populations have strong resilience to adapt to the dramatic changes in the environment of volcanic activity areas. This resilience may be due to the existence of a certain “mother population†that has the ability to re-enter the disturbed area. If this basic “mother population†is destroyed by mining activities, it may lead to the extinction of rare species.
Many of the environmental impacts of sulphide mining are similar to the environmental problems caused by the mining of polymetallic nodules, including the destruction of the surface of the animal's habitat, the disturbed sediments burying the animals, and the underlying water undergoing chemical changes due to suspended plumes. On the other hand, the high density of sulphide particles can immediately redeposit any sulphide debris caused by the mining equipment. Due to the large contact surface with seawater, some of the released sulphide debris will oxidize, just like the oxidation of inactive massive sulphides in many seabed deposits. In the terrestrial sulfur mining, the discharge of acid sewage from mines, which usually causes major environmental problems, is not required to be disturbed on the seabed because of the desalination effect of the surrounding seawater. In addition, most submarine sulphide deposits usually do not have significant overlying deposits. Therefore, it should be possible to selectively open mining beds, especially those that do not have any spouts, because the environmental impact of mining in these areas may not be greater than the construction of a common port facility.
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