Placer deposit

Heavy mineral sands may differ in composition, depending on the location, and only generalized descriptions of the processing can be given. Monazite and 

4 Minca al Processing and Extractive Metallurgy of the Rare Earths 

In this subsection, the chemical treatment of the rare earths is addressed per ore mineral. The text is largely a summary of chapter three from Extractive Metallurgy of the Rare Earths by Gupta and Krishnamurthy (2005). 

Monazite is (Ce, La, Th)P04., which is the ideal formula. To extract the rare earths and to remove thorium, a variety of methods are applied after chemically attacking the mineral with either sulphuric acid, or sodium hydroxide. 

The sulphuric acid method has been used most extensively in the USA. With this method, depending on the ratio of acid to ore, temperature and concentration, the rare earths can either be solubilized, or the thorium can be extracted, or both can be brought into solution (Gupta and Krishnamurthy 2005). 

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Discovered in 1803 by Wollaston, Palladium is found with platinum and other metals of the platinum group in placer deposits of Russia, South America, North America, Ethiopia, and Australia. It is also found associated with the nickel-copper deposits of South Africa and Ontario. Palladium s separation from the platinum metals depends upon the type of ore in which it is found. 

In the United States, about 90% of gold production originates from ores and placer deposits. The remainder is recovered primarily as a by-product of the refining of base metals, chiefly copper. The principal gold producing states are Nevada (60%) and Califomia (10%) followed by Montana, Utah, S. Dakota, Washington, Colorado, Alaska, Idaho, Arizona, and New Mexico (7). 

Although estimates of their abundances vary considerably, Pd and Pt (approximately 0.015 and 0.01 ppm respectively) are much rarer than Ni. They are generally associated with the other platinum metals and occur either native in placer (i.e. alluvial) deposits or as sulfides or arsenides in Ni, Cu and Fe sulfide ores. Until the 1820s all platinum metals came from South America, but in 1819 the first of a series of rich placer deposits which were to make Russia the chief source of the metals for the next century, was discovered in the Urals. More recently however, the copper-nickel ores in South Africa and Russia (where the Noril sk-Talnakh deposits are well inside the Arctic Circle) have become the major sources, supplemented by supplies from Sudbury. 

Zinn-erschwerung, /. (Textiles) tin weighting, -erz, n. tin ore, cassrterite. -erzseife, /. tin placer deposit, -farbe, /. (t)yeihg) tin-mordant color, -feilicht, n., -feilspdne, m.pl. tin filings, -flammofen, m. reverberatory tin furnace, -folie, /. tin foil. 

Depending on the mode of origin, four main types of sedimentary ore deposits are recognized. These are (i) mechanical accumulations or placer deposits, (ii) residual deposits, (iii) syn-sedimentary chemical deposits, and (iv) volcano-sedimentary type deposits. 

The gravity preconcentration method, which is used mainly for recovery of gold from placer deposits that contain coarse native gold. Gravity is often used in combination with flotation and/or cyanidation. 

Placer deposits, in general conglomerates, which contain quartz, sericite, chlorite, tourmaline and sometimes rutile and graphite. Gold can be coarse. Some deposits contain up to 3% pyrite. Size ofthe gold contained in pyrite ranges from O.Olto 0.07 pm... 

According to the processing characteristics of PGM-dominated deposits, they can be divided into the following three groups (a) Morensky type, (b) hydrothermal deposits and (c) placer deposits. Each type of deposit is briefly described below. 

Ore types where gravity preconcentration is used include Alaskan-type deposits, alluvial and fossil placer deposits. 

The fossil placer deposits are in fact gold-bearing conglomerates that carry small amounts of PGM, together with gold, uranium and other heavy minerals. However, studies conducted revealed that some of the fossil placer deposits contain about 22 PGM species, including Ir-Os-Ru alloys, sperrylite and isoferroplatinum. 

There are several operating mines that recover PGM and gold from fossil placer deposits,... 

The world s major tin deposits are elongated zones over 2000 km, extending from Indonesia to Malaysia, Thailand and Burma and into China. Commercial production of tin is almost exclusively from placer deposits, and these are major producers in the world. Other placer deposits worth mentioning are Brazil, Nigeria and the Congo. For beneficiation of tin from these deposits, the physical concentration method is used exclusively. 

Hard rock deposits are richer in tin than in the placer deposits, ranging from 0.6% up to 5% Sn. Such known deposits are located in Brazil, Canada, Bolivia, Pem and the USA. Because the tin from these ores is disseminated, beneficiation processes include a combination of gravity preconcentration and flotation. 

The placer deposits are more efficiently mined by dredging. 

Free metallic gold is found in veins of rocks and in ores of other metals. Alluvial gold (placer deposits) is found in the sand and in the gravel at the bottom of streams where it has been deposited as a result of the movement of water over eons. 

This lode gold is the stuff worked by legendary dwellers below the earth, like the dwarf who forged the treasure of the Nibelungs. Recall, however, how the Rhinegold was first found, like a placer deposit, at the bottom of a river. 

Placer Ores, iron oxides, when compact, are rather resistant to weathering and erosion, and under favorable conditions may form placer deposits, which, in relatively lew instances, constitute iron ores. Generally they are of rather minor importance as sources of iron. 

These metals occur in both primary and secondary deposits. The primary deposits are generally associated with Ni-Cu sulfide ores. The Sudbury ores of Canada and the deposits of the Bushveld complex of South Africa are of this type. Native platinum occurs as a primary deposit in the Ural Mountains of the former U.S.S.R. and also in the Choco district of Colombia. Weathering and erosion of these deposits have resulted in the formation of secondary, or placer, deposits of native Pt in riverbeds and streams. One nugget of Pt found in Ihe Urals weighed over 25 pounds (11.3 kilograms). Most of the world s platinum comes from Canada, the former... 

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