During separation, the slurry enters the spiral through a feed box and then flows on to the spiral surface. As the slurry travels down the spiral, mineral grains settle and sort according to size, shape, and specific gravity.
Spiral separation uses a flowing film principle based on the size and specific gravity difference present in a mineral suite. When fed a dilute pulp mixture of minerals of different specific gravities, the lighter minerals are more readily suspended by the water and attain relatively high tangential velocities so that they climb toward the outer rim of the spiral trough. At the same time, the heavier, non-suspended grains migrate by saltation (non-linear motion that is a combination of rolling and bouncing) along the spiral surface at the lowest portion of the spiral cross section. In a spiral, heavy mineral concentrate is selectively directed into the spiral trough near the inside of the spiral surface through the use of adjustable product splitters. The concentrate then flows through product discharge outlet ports down the length of the helix.
Like most concentrating devices that utilize physical separation principles, the spiral works best on a closely sized feed, though in select cases the unit has some tolerance for wide size ranges. In general, a coarse size limit would be about 1 mm (20 mesh) and a fine limit of approximately 45μm (325 mesh). For best performance, feed should be relatively free of slimes as a high slime content may act like a heavy medium and decrease the effective differences in specific gravity between the minerals to be separated. If slime is in excess of 10% by weight, desliming prior to spiral concentration will almost certainly result in improved spiral performance.
In general, a specific gravity difference of at least 1 is required between light and heavy particles for successful gravity separation.
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