Hard Rock Deposits

An intimate knowledge of the mineralogical assembly of the ore is necessary if efficient processing is to take place. Not only the nature of the valuable minerals and gangue must be known, but also knowledge of the texture of the ore is required. The texture refers to the size, dissemination, association and shape of the minerals within the ore. The processing of the ore should always be considered in relation to the mineralogy (Wills and Napier-Munn 2006). 

After comminution, sieving operations separate large-, mainly poorly-ground particles from smaller wellground particles. Particles that are too large are reground. 

After comminution, mineral separation will have to take place, to separate the valuable particles from the non-valuable gangue particles. As the ore minerals of the rare earths are hardly magnetic, useful separation techniques would be gravity separation or flotation. For bastnaesite, which does not have an extremely high density, flotation is applied. The principle of flotation is depicted in Fig. 4.3. 

Bastnaesite from Mountain Pass ore (hard-rock ore) is processed as follows. After comminution, the ground ore is subjected to hot-froth flotation. However, gangue minerals like calcite, barite or celestite may cause problems in flotation, because they have flotation properties similar to bastnaesite. For this, special treatment has been devised, consisting of six different (steam) conditioning steps 

4 Mineral Processing and Extractive Metallurgy of the Rare Earths 

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The hard rock deposits are mined mainly for feldspar with mica and quartz being accessory minerals. These deposits are extensive, often covering hundreds of square meters and are recognized by the light-colored, granite-like appearance with shiny mica flakes being a prominent feature. The mica content of these deposits ranges from approximately 6—10 wt %. 

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 Guadalajara titanium-bearing ore comes from a hard rock deposit consisting principally rutile and ilmenite. Over 85% of the mtile and ilmenite are liberated at relatively... 

United States.—The principal deposits are in South Carolina, Florida, Tennessee, Arkansas, Utah and Wyoming. The South Carolina deposits are of Miocene age and occur both as land and river rock. They contain 25 to 28 per cent, of phosphoric oxide and 35 to 42 per cent, of lime. They were the earliest to be exploited, namely, from 1868 onwards, and in 1893 they furnished about one-fifth of the world s supply. Since this date the production has declined, while that of Florida has greatly increased. In 1913 Florida and Tennessee together produced some 96 per cent, of the total output from the United States of America. The hard-rock deposits of Florida are of Tertiary age, and they run parallel to the coast for 144 miles. After concentration by mechanical means they contain usually from 77 to 79 per cent, of calcium phosphate (more rarely up to 82 per cent.) with 3 per cent, of oxides of iron and alumina, some calcium fluoride and other constituents, and 3 per cent, of moisture. The best grades of Tennessee rock were guaranteed to contain 72 per cent, of calcium phosphate, and 65 per cent, is common. The soft rock is a phosphatic clay. The river pebbles are dark grey to black and are very cheaply obtained by dredging. 

Bastnasite is mined from hard rock deposits. Production in China is a by-product of iron ore mining while U.S, production is solely for rare-earths. Ore is recovered by drilling and blasting. The ore Is crushed, ground and subjected to flotation. The bastnasite fraction is floated off and thereby seperated from other minerals to produce a concentrate. Bastnasite can be converted directly, without separating individual rare-earths, to other derivatives such as sulphate or chloride by dissolution in acid. The following step to crack the concentrate for further processing used in the U.S. is to roast in air and then to leach with HCl. This produces an insoluble cerium rich... 

Tin is a relatively scarce element with an average abundance in the Earth s crust of about two parts per million (ppm) compared with 94 ppm for zinc, 63 ppm for copper, and 12 ppm for lead. Tin is produced from lode (hard-rock) deposits and placer deposits derived from the lodes. The tin mineral cassiterite (Sn02) is the source of most tin production. A notable exception is the complex tin sulfide minerals in the subvolcanic or tin-silver lode deposits in Bolivia. Cassiterite has a high specific gravity (6.8 to 7.1), a Moh s scale hardness of 6 to 7, and is usually a dark brown or black color with an adamantine luster. 

In this section, it is explained how hard-rock deposits of rare earths are processed. Ores that consist of hard rocks, have to undergo several procedures before the concentration of the valuable material and the extraction of the valuable material from this concentrate can begin. Figure 4.1 gives a basic flow sheet of the process. 

At present, the only dedicated scandium mining operation in the world is the Zhovti Vody mine in Ukraine. The ore is mined underground from a hard-rock deposit at depths of 1000 m or more at a grade of 105 mg/kg scandium. Proven mineable scandium reserves are 7.38 million tonnes of raw ore, corresponding to ca. 775 tonnes of proven reserves of scandium. 

Mining of the ore deposit constitutes a significant cost, especially in hard rock mining. Mining costs vary considerably from ore to ore and from a few cents to well over 100/t mined. Underground mining is the most expensive hydrauHc mining of sedimentary deposits is the least expensive. 

Vinegar is recommended for cleaning a variety of appliances and other items that may be stained by hard water deposits. Automatic coffee makers, steam irons, dishwashers, teapots, faucet heads, and shower heads — over time, all accumulate calcium deposits from hard water. Groundwater, that is, water that travels through soil and rocks, accumulates dissolved calcium ions as a consequence of the natural weathering of minerals that contain calcium such as limestone and calcite, shells, and coral. At the same time, carbon dioxide in the air dissolves in water to form carbonate ions that combine with calcium ions to form a white solid, calcium... 

The physical concentration is primarily used in beneficiation of alluvial and some coarser grained vein deposits. The combination of gravity and flotation is normally used for beneficiation of hard rock ores. 

The cassiterite ore from underground, hard rock veins is finer grained than alluvial deposits. From this ore, good concentrate grade is produced but at relatively low tin recoveries. Fines from the gravity tailing assay between 0.4% and 0.8% Sn. 

Titanium minerals have been recovered from both hard rock and sand deposits. Until 1945, most of the ilmenite and rutile produced commercially came from sand deposits, but nowadays, the production of ilmenite from rock deposits exceeds that of sand deposits. Rutile, however, is exclusively produced from sand deposits, although a new technology exists that recovers rutile from rock deposits. 

From a beneficiation point of view, formation of hard rock and sand deposits, and their mineral composition, determines the beneficiation method. 

Extensive research has been carried out mainly on ilmenite and, to a lesser degree, on flotation of rutile and perovskite. Flotation studies have been performed on titanium minerals from both hard rock and fine-grained sand deposits. 

Extensive research work has been carried out on ilmenite flotation from different ores [1-3], including hard rock and sand deposits. Because the chemical composition of ilmenite is unstable, flotation processing characteristics of ilmenite varies from one ore type to another. Figure 25.1 shows the flotation of ilmenite from different ore types at different pH levels using 200 g/t of oleic acid. 

A large hard rock rutile deposit was discovered in central Chile. This ore is relatively complex with variable head grade of rutile ranging from 2% to 4% Ti02. The liberation of rutile occurs at about 100 mesh nominal size. The major gangue minerals present in this ore include feldspars, calcite and some silicates. 

Polkin, S.I., Concentration of Ores from Sand Deposits and Hard Rock, Izdatelstro Nedra 1987, pp. 1180-23. 

There are two types of caverns used for storing liquids. Hard rock (mined) caverns are constructed by mining rock formations such as shale, granite, limestone, and many other types of rock. Solution-mined caverns are constructed by dissolution processes, i.e., solution mining or leaching a mineral deposit, most often salt (sodium chloride). The salt deposit may take the form of a massive salt dome or thinner layers of bedded salt that are stratified between layers of rock. Hard rock and solution-mined caverns have been constructed in the United States and many other parts of the world. 

Diagenesis processes that alter the structure, texture, and mineralogy of a sediment, turning it progressively into solid hard rock early diagenesis occurs immediately after deposition or burial of the sediment. 

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