HLS testing can separate samples at a single density, resulting in a pair of sink-float products or into a range of density intervals with a minimum density increment as small as 0.1. Static separations are performed on coarser-grained fractions (greater than 0.5 mm) and for size fractions less than 0.5 mm, separations are usually performed in a centrifuge. This method is effective down to particle sizes as fine as 45μm and, with modifications, reasonable separation can be achieved on fractions as fine as 10μm.
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The resulting size fractions are then weighed and assayed. Calculated weight and element distribution data can then be used to indicate optimum density cut points or product weight splits to achieve the desired results in terms of grade and/or recovery. Once produced, the density fractions can be submitted for analysis at our in-house analytical and mineralogical laboratories.
ALS are able to provide testwork data on samples across a range of commodities including:
List of heavy liquids available for use together with SGs are tabulated below.
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).
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.
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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.
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.
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