Concentration, Separation, Flotation - Mineral processing

Concentration involves the separation of valuable minerals from the other raw materials received from the grinding mill. In large-scale operations this is accomplished by taking advantage of the different properties of the minerals to be separated. These properties can be colour (optical sorting), density (gravity separation), magnetic or electric (magnetic and electrostatic separation), and physicochemical (flotation separation).

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This process is used for the concentration of particles that have sufficiently different colours (the best contrast being black and white) to be detected by the naked eye. In addition, electro-optic detectors collect data on the responses of minerals when exposed to infrared, visible, and ultraviolet light . The same principle, only using gamma radiation , is called radiometric separation.

Gravity methods use the difference in the density of minerals as the concentrating agent.

In heavy-media separation (also called sink-and-float separation), the medium used is a suspension in water of a finely ground heavy mineral (such as magnetite or arsenopyrite) or technical product (such as ferrosilicon). Such a suspension can simulate a fluid with a higher density than water. When ground ores are fed into the suspension, the gangue particles, having a lower density, tend to float and are removed as tailings, whereas the particles of valuable minerals, having higher density, sink and are also removed. The magnetite or ferrosilicon can be removed from the tailings by magnetic separation and recycled.

In the process called jigging, a water stream is pulsed, or moved by pistons upward and downward, through the material bed. Under the influence of this oscillating motion, the bed is separated into layers of different densities, the heaviest concentrate forming the lowest layer and the lightest product the highest. Important to this process is a thorough classification of the feed, since particles less than one millimetre in size cannot be separated by jigging.

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Finer-grained particles (from 1 millimetre to 50 micrometres) can be effectively separated in a flowing stream of water on horizontal or inclined planes. Most systems employ additional forces&#;for example, centrifugal force on spirals or impact forces on shaking tables. Spirals consist of a vertical spiral channel with an oval cross section. As the pulp flows from the top to the bottom of the channel, heavier particles concentrate on the inner side of the stream, where they can be removed through special openings. Owing to their low energy costs and simplicity of operation, the use of spirals has increased rapidly. They are especially effective at concentrating heavy mineral sands and gold ores.

Gravity concentration on inclined planes is carried out on shaking tables, which can be smoothed or grooved and which are vibrated back and forth at right angles to the flow of water. As the pulp flows down the incline, the ground material is stratified into heavy and light layers in the water; in addition, under the influence of the vibration, the particles are separated in the impact direction. Shaking tables are often used for concentrating finely grained ores of tin, tungsten, niobium, and tantalum.

Crushing and grinding is the process of reducing large chunks of ore to small sand-sized particles. Crushing and grinding are the most expensive steps in mineral processing, but they are necessary in order to liberate and seperate the valuable minerals from the waste rock.

Imagine that a chocolate chip cookie represents a chunk of ore. The chocolate chips represent the valuable mineral we want, and the cookie represents the waste rock.

Looking at the cookie, we can easily pick out the chips we see on the surface, but how do we get to the ones further inside? 

If we break up the cookie, it becomes easier to get at the chocolate chips without taking along any cookie. We can keep breaking the cookie until any single piece is either all chocolate chip, or all cookie. In mining, we would say that we have liberated the chocolate chips. 

The same principle applies to recovering grains of minerals from ore. The particle size required to seperate grains of minerals from the waste rock is called liberation size. Ore usually goes through several crushers before being transported to the mill building, where it enters a series of grinding mills that pulverize it into fine sand-sized pieces. Once liberated, there are a number of methods for collecting the minerals, while leaving the waste rock behind. 

Caveat: Some methods of mineral extraction, like heap leaching, do not require crushing and grinding. 

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