Gangue separation processes

Gangue. Undesired minerals associated with ore, mostly nonmetaUic. Gangue represents the portion of ore rejected as tailings in a separating process. It is usually valueless, but may have some secondary commercial use. 

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. 

Zinc and lead usually occur together in nature as sulfides. Earlier separation processes involved the fine grinding of the combined sulfides and then treating the particles with chemical reagents to cause one sulfide to be preferentially wetted and rhns the two sulfides separated by the froth flotation process. In a first stage, the lead sulfide is floated while the zinc sulfide sinks to the bottom of the tank. In the second stage, the process is reversed and the zinc sulfide is floated. Gangue and other nonmetals collect at the bottom of the tank. The separated sulfides are dewatered to a 6-7% moisture content and are referred to as the zinc concentrate and the lead concentrate. 

In a heavy-medium separation process, coal particles whose density is higher than the medium sink while the lower-density coal particles float the separation efficiency and the through put of the separation device depend on the velocity of coal particles in a dense medium. The viscosity of a medium has little effect on the low-density coal particles or the high-density gangue particles, but becomes critical in the separation of material of a density equal to or near that of the medium hence, a low viscosity must be maintained to separate near-density material at a high rate of feed. 

The heavy media separation process. The grains of the crushed ore are stirred into a suspension whose density is higher than the density of the gangue grains but lower than the density of the mineral. The gangue floats up and the valuable mineral is pressed down . The suspension is made of water with ferrosilicon or magnetite dispersed in it. 

The separation processes used for lead (and zinc) ores are based on either specific weight (or gravity) or floatability. In some instances, traditional gravity methods (shaking tables and jigging) are still used more frequent nowadays is heavy media separation, which is based on the differences in specific gravity between mineral and gangue material. It is often used prior to, or in association with, flotation methods. 

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. 

Gravity concentration, ie, the separation of ore from gangue based on the differences in specific gravities, using jigs, heavy—medium separators, or spiral concentrators for example, is appHcable for lead ores. However, the predominant beneficiation technique used in modem plants is the bubble or froth flotation (qv) process (4,5). 

Size reduction (qv) or comminution is the first and very important step in the processing of most minerals (2,6,10,20—24). It also involves large expenditures for heavy equipment, energy, operation, and maintenance. Size reduction is necessary because the value minerals are intimately associated with gangue and need to be Hberated, and/or because most minerals processing/separation methods require the ore mass to be of certain size and/or shape. Size reduction is also required in the case of quarry products to produce material of controlled particle size (see Size measurement of particles). In some instances, hberation of valuables or impurities from the ore matrix is achieved without any apparent size reduction. Scmbbers and attritors used in the industrial minerals plants, eg, phosphate, mtile, glass sands, or clay, ate examples. 

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. 

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. 

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