Minerals separating from gangue

Pyrometallurgical Processes Such high temperature processes convert certain minerals into others for easier separation from gangue or for easier recovery of metal. They are accomplished in Idlns, hearth furnaces or fluidized bed reactors. 

Lead is produced commercially from its principal ore, galena (PbS). The ore is associated with sulfides of several metals including iron, copper, zinc, silver, bismuth, arsenic, antimony and tin. The ore is crushed and ground. It then is selectively separated from gangue and other valuable minerals by one or more processes that include gravity separation and flotation. Selective... 

In addition to rotary and vertical kilns, hearth furnaces or fluidized-bed reactors may be used. These high-temperature reactors convert minerals for easier separation from gangue or for easier recovery of metal. Fluidized beds are used for the combustion of solid fuels, and some 30 installations are listed in Encyclopedia of Chemical Technology (vol. 10, Wiley, 1980, p. 550). The roasting of iron sulfide in fluidized beds at 650 to 1100°C (1202 to 2012°F) is analogous. The pellets have 10-mm (0.39-in) diameter. There are numerous plants, but they are threatened with obsolescence because cheaper sources of sulfur are available for making sulfuric acid. 

Separation by Decrepitation Followed by Screening.—Minerals like calcite, fluorite and barite which decrepitate on heating may be separated from gangue by roasting followed by screening. 

Monazite sand occurs extensively in alluvial deposits, from which it is mined (along with other dense minerals) by placer techniques. It is separated from gangue and associated minerals, such as magnetite, ilmenite, rutile, zircon, garnet and xenotime, by ore beneficiation methods, including electrostatic and electromagnetic separations, prior to chemical processing. 

A very important but rather complex application of surface chemistry is to the separation of various types of solid particles from each other by what is known as flotation. The general method is of enormous importance to the mining industry it permits large-scale and economic processing of crushed ores whereby the desired mineral is separated from the gangue or non-mineral-containing material. Originally applied only to certain sulfide and oxide ores. 

Prior to about 1920, flotation procedures were rather crude and rested primarily on the observation that copper and lead-zinc ore pulps (crushed ore mixed with water) could be benefacted (improved in mineral content) by treatment with large amounts of fatty and oily materials. The mineral particles collected in the oily layer and thus could be separated from the gangue and the water. Since then, oil flotation has been largely replaced by froth or foam flotation. Here, only minor amounts of oil or surfactant are used and a froth is formed by agitating or bubbling air through the suspension. The oily froth or foam is concentrated in mineral particles and can be skimmed off as shown schematically in Fig. XIII-4. 

Ore Dressing. The principal lead mineral, galena, in most cmde ores, is separated from the valueless components, or gangue. Other valuable minerals that are present in the ore may be recovered either together with the lead, or in a separate step (2,3). Occasionally, the ores are sufftciendy rich in lead and low in impurities to be smelted direcdy. 

Although the size separation/classification methods are adequate in some cases to produce a final saleable mineral product, in a vast majority of cases these produce Httle separation of valuable minerals from gangue. Minerals can be separated from one another based on both physical and chemical properties (Fig. 8). Physical properties utilized in concentration include specific gravity, magnetic susceptibility, electrical conductivity, color, surface reflectance, and radioactivity level. Among the chemical properties, those of particle surfaces have been exploited in physico-chemical concentration methods such as flotation and flocculation. The main objective of concentration is to separate the valuable minerals into a small, concentrated mass which can be treated further to produce final mineral products. In some cases, these methods also produce a saleable product, especially in the case of industrial minerals. 

Sulfide collectors ia geaeral show Htfle affinity for nonsulfide minerals, thus separation of one sulfide from another becomes the main issue. The nonsulfide collectors are in general less selective and this is accentuated by the large similarities in surface properties between the various nonsulfide minerals (42). Some examples of sulfide flotation are copper sulfides flotation from siUceous gangue sequential flotation of sulfides of copper, lead, and zinc from complex and massive sulfide ores and flotation recovery of extremely small (a few ppm) amounts of precious metals. Examples of nonsulfide flotation include separation of sylvite, KCl, from haUte, NaCl, which are two soluble minerals having similar properties selective flocculation—flotation separation of iron oxides from siUca separation of feldspar from siUca, siUcates, and oxides phosphate rock separation from siUca and carbonates and coal flotation. 

Copper, Cu, is unreactive enough for some to be found as the metal, but most is produced from its sulfides, particularly the ore cbalcopyrite, CuFeS2 (Fig. 16.10). The crushed and ground ore is separated from excess rock by froth flotation, a process that depends on the ability of sulfide ores to be wetted by oils but not by water. In this process, the powdered ore is combined with oil, water, and detergents (Fig. 16.1 l). Then air is blown through the mixture the oil-coated sulfide mineral floats to the surface with the froth, and the unwanted copper-poor residue, which is called gangue, sinks to the bottom. 

A simple two-mineral separation, say of galena (specific gravity 7.5) from a siliceous gangue (specific gravity 2.65) can be taken as a good example to illustrate the process. The flotation operation comprises the following successive steps. 

On the basis of the function it performs, the flotation process can be divided into two categories (i) bulk and (ii) selective. The process is called bulk or collective flotation when it accomplishes the separation of several valuable components from the gangue minerals. In selective flotation, one valuable component is separated from several others. This selectivity could be accomplished by either using collectors selective with respect to a particular mineral or by differential flotation wherein two or more mineral concentrates are recovered consecutively from the same feed by using modifiers. 

Flotation is a solid-liquid separation process, that transfers solids to the liquid surface through attachment of gas bubbles to solid particles. Flotation processes are used in the processing of crushed ores, whereby a desired mineral is separated from the gangue or non-mineral containing material. Various applications in solid separation processes are also in use in waste treatment. 

In water the wetted solid is termed hydrophilic , whereas the non-wetted solid is hydrophobic . Naturally hydrophobic minerals, such as some types of coal, talc and molybdenite are easily separated from the unwanted hydrophilic quartz sand (referred to as gangue ). However, surfactants and oils are usually added as collectors . These compounds adsorb onto the hydrophilic mineral surface and make it hydrophobic. 

Finely divided solids or immiscible liquids can be made to adhere to gas bubbles and then can be removed from the main liquid. Affinity of a solid for an air bubble can be enhanced with surfactants which adhere to the surface of the solid and make it nonwetting. The main application of froth flotation is to the separation of valuable minerals from gangue. Ores of Cu, Zn, Mo, Pb, and Ni are among those commercially preconcentrated in this way. Reagent requirements of each ore are unique and are established by test. A... 

GANGUE. The minerals and rock mined with a metallic ore but valueless in themselves or used only as a by-product. They are separated from the ore in the milling and extraction processes, often as slag. Common gangue materials are quartz, calcite, limonite. feldspar, pyrite. etc. 

Froth flotation has been used to separate many raw-mineral ores, such as those for copper, lead, zinc, and tungsten. If two valuable minerals are very difficult to separate from each other then a sequential set of flotation stages may be needed. An illustration is provided in Figure 10.8. First, enough grinding is applied to permit the two minerals to be liberated from the non-valuable gangue minerals. A first,... 

Figure 10.8 Illustration of a selective flotation process for the separation of heavy metal sulfides such as Cu-Mo, Cu-Ni, or Ni-Co-Cu, from unwanted gangue minerals, and from each other. From Leja [91]. Copyright 1982, Plenum Press.

None of the effects brought about by these metal sulphides could therefore be taken at face value. However, lead and other sulphides do act as genuine heterogeneous catalysts in the important process of froth flotation by which these minerals are separated from sand and other "gangue materials [120— 122],... 

Before we examine the structures and properties of metallic classes in further detail, it is useful to consider the natural sources of the metals, generally as oxide and/or silicate-based mineral formations. If the mineral deposit contains an economically recoverable amount of a metal, it is referred to as an ore. The waste material of the rock formation is known as gangue, which must be separated from the desired portion of the ore through a variety of processing steps. 

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