Single Start Spiral
Splitter and discharge port of spiral
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Spiral concentrators are a gravity
based concentrating device, that separates
light density granular and sandy (10 mesh to 200 mesh (2 mm to 0.075 mm))
consistency material from heavier density material. In order to have a good
separation, there should be a difference in SG's of at least 1.0. One main
benefit of spiral concentrators is they have no moving parts. The feed range,
in percent solids, to a spiral ranges from 20% solids up to 40% solids.
Depending upon the material characteristics, a maximum efficiency will usually
be reached somewhere in this range. All that is required are some slurry
pumps, the slurry to be separated and the banks of spirals with a feed
distributor.
Slurry is pumped to the top of the spiral (typically 13' to 15' from the floor),
and it enters a feed distributor that evenly distributes the feed to each
spiral concentrator. The design and shape of the spiral make it work, when
combined with gravitational acceleration. As the slurry travels the spiraling
path down the spiral, mineral grains settle and start sorting according to size,
density and to a lesser extend shape. Low density particles are carried with
the bulk of the water towards the outside of the spiral (perimeter), while
particles with the greatest density migrate towards the inside of the spiral
A cross section of a spiral concentrator can be divided into various regions,
with each region describing the effect it has on the slurry traveling through
it. On the outer most region (1) (perimeter), will have mostly water, with
fine particles, trapped by the high velocity of the moving water. Moving
inward towards the center of the spiral, the next region(2) would consist of
a very small area where the maximum water velocity exists, and prevents any
separation to occur. This region is defined since it separates the next
region (3) from the first region.
Region 3 is a very active region where the velocity begins to slow down and most
of the separation occurs, as more dense particles settle to the bottom and
the water velocity keeps the light density particles in the stream near the
surface, where they eventually wind up in the outer regions (2 and 1). The
next region is actually where two regions overlap (region 3 and 5), and is a
very narrow region (like region 2). Next to the last region (region 5) is
where the heavy density concentrates collect. The remaining low density
particles in this region find their way to the top of the slurry surface
and are carried off by the fast flowing water to the perimeter of the spiral,
with the bulk of the water and the low density solids. Some spirals have a
wash water section, where additional water is added to free any trapped light
density material in the concentrates, and on a wash water spiral, the
innermost portion is where this water is added, and it is called region 6.
From the innermost region of the spiral, the concentrates then flow to the
bottom section of the spiral, where splitter "bars" actually make a cut of
the material, channeling the inner most material to the heavy concentrate
port, a 'middlings' splitter can be used to channel the intermediate to a
separate discharge port, and the majority of the water and the light density
material is cut to a low density port. Some spiral concentrators, especially
those used in coal cleaning, have the capability of removing the high density
material at multiple positions in the vertical spiral. Coal can consist of
as much as 50% high density material, compared to a typical heavy mineral
with only 5% to 10% high density content. These cutter bars are adjustable,
and are usually set up during the start up and rarely moved, unless differing
material is processed. They can be changed easily, to accommodate differing
feed material.
Spiral concentrators can be made with multiple starts (multiple spirals
interwound) to save floor space. I have seen up to 3 start spirals, which
would essentially give 3 spirals in about the same space that one would take up.
Spiral concentrators are normally used in banks of multiple spirals. Typical
capacities for spirals run from 1-3 tons per hour of feed for minerals and 3-5
tons per hour for coal. Typical construction of a spiral concentrator is
fiberglass and urethane to reduce wear from abrasion.
Information provided by Charles Kubach, Mining and Mineral Processing Engineer
Reference material, photos from Outokumpu
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