| DENSE MEDIUM SEPARATION -> | METAL COAL PLASTIC AGRICULTURE |
ESR International LLC
patented a unique bi-directional dense medium barrel that
proved to be the most accurate dense medium separator ever
employed in the separation of a variety of materials
ranging from carrots and potatoes to plastics and
non-ferrous metals (US patent 5,373,946 of Dec 20, 1994).
This invention was further supported by two additional
patents: US patent 5,495,949 of March 5, 1996 and US patent
6,024,226 of Feb 15, 2000.
Here we find the dynamic of a quiescent bath where the density of water is changed by means of fine particles in suspension. At first glance, nothing could be simpler: one fraction floats, while the other fraction sinks. But at high tonnages even the best dense medium separators generate errors, and where should one go to find inexpensive suspension materials needed to change the density of water? |
ESR'S BI-DIRECTIONAL BARREL
Twenty years ago all dense
medium (DM) separators on the market were mono-directional,
with floats and sinks reporting on the same side of a
relatively short horizontal drum. This concept fails for
four basic reasons.
1) Correct
Injection
At the critical moment of
introducing solids into the drum, the floats of the typical
DM drum are easily buried with sinks and cannot find their
way to the surface of the bath. However in this new ESR
bi-directional design, the medium together with solids is
injected over a broad three dimensional plane, making it
almost impossible to bury floats with sinks.
2) The
Stability of the Medium
A typical dense medium bath
is relatively deep, and this makes it difficult for the
suspension particles to remain in suspension. If a medium
is not stable, we find water at the top of the bath and a
dense sludge at the bottom of the bath, and of course, in
this stratified liquid, no separation takes place. Instead
of a deep bath, ESR chose a shallow bath, and the gentle
action of the sinks scrolls pulling underneath the bath
provides just the right amount of agitation to keep the
suspension medium stable.
3) Correct
Floats Dynamic
The typical dense medium
drum is quite short (only 4 to 5 feet in length), and it
happens quite often that before a particle can float or
sink, it is already out of the bath. Therefore, ESR
extended the length of the separation zone, greatly
increasing the residence time of a particle in the bath,
thereby making it impossible to find sinks in floats. The
rule that ESR follows in this regard: the length of the
separation zone must be at least 2.2 times the diameter of
the drum.
4) Correct
Sinks Dynamic
In the conventional dense
medium drum, the curtains needed to prevent floats from
crossing over with sinks are located in the separation zone
and, due to turbulent fluid movement in the vicinity of
these curtains, floats are easily sucked under these
curtains and report with sinks.
In the new ESR design,
however, the curtain that prevents floats from crossing
over with sinks is situated completely outside the
separation zone, making it impossible to find floats in
sinks. In the classical design, sinks are continually
lifted out of the bath by means of a series of lifters
welded to the wall of the drum, and at each rotation of the
drum, these sinks are removed from the bath while still in
the separation zone. The action of these sinks evacuation
lifters, passing underneath the two curtains running the
full length of the separation zone, creates a great deal of
turbulence, and this turbulence completely destroys the
accuracy of separation. This severely limits the speed of
rotation of the drum as well as the tonnage of sinks
evacuated within a given period of time. Consequently the
typical DM drum cannot handle a large quantity of sinks.
However in the new ESR
design, sinks are lifted up and out of the bath only when
they are completely outside of the separation zone. Scrolls
welded to the bottom of the bi-directional drum gently move
the sinks in one direction, while the floats flow out on
the surface of the bath in the opposite direction. When the
sinks exit the separation zone, they drop down underneath a
curtain into an expanded drum and only at this point are
they screwed up and out of the bath. Since there are no
lifters and curtains within the separation zone, a
bi-directional drum can be rotated at relatively high
speeds without jeopardizing the accuracy of separation.
Since there are no lifters and curtains within the
separation zone, the entire surface of the bath is
available for separation and remains fully visible to the
operator at all times.
THE MOST ACCURATE SEPARATOR ON THE MARKET
ESR International sold throughout the world more than 25 bi-directional dense medium separators, and most of them were equipped with scrubber-rinser technology. These separators have truly revolutionized the root vegetable and recycling industries. From carrots and potatoes, to magnesium and aluminum, to brown coal and anthracite, to electronic waste and plastics, this technology is constrained by nothing. |
In correcting the four deficiencies of classical dense medium separators, ESR discovered that it stumbled upon something quite revolutionary. This separator could make distinctions between materials differing in density by only a few points to the third decimal place. The Ecart Probable (Ep) here situates at less than 0.003.
A New Way to Handle Sinks
Expanding the diameter of
the sinks cone relative to the diameter of the separation
zone allows for the insertion of a curtain that lies
completely outside the separation zone. But in situations
with a large amount of sinks relative to floats, a sinks
cone becomes a bottleneck, since all sinks must pass
through the smallest diameter of the cone. How then to
evacuate sinks without a cone? Keep in mind that it is by
means of a cone that the level within the bath is
maintained. In other words, how does one would remove sinks
from a bath without lifting the sinks out of the bath and
without draining the bath of liquid?
This problem was resolved by
US patent 5,495,949 of March 5, 1996, allowing for the free
passage of solids through the bottom of a drum, while
preventing the free flow of liquid out of the drum. This
sinks evacuation drum is a scrolled drum with two special
features: 1) it has a donut plate on its exit side that
maintains the level of the medium, and 2) the top of the
scroll at the exit side is sealed with metal plate for at
least one complete revolution of the scroll. The former
intersects and is welded to the latter.
Therefore, when the drum is
at rest, no medium flows out of the drum. Only when the
drum is set in motion and is in the process of screwing out
solids does medium exit the sink side of the separator.
Since the diameter of sinks evacuation drum is two feet
greater than that of the separating drum, the separator has
a very large capacity to evacuate sinks.
The Use of Fine Sand
On all density separations
below 1.6, ESR pioneered the use of fine sand in
suspension. ESR employs two stages of classifying cyclones
to isolate dense inorganic fines between 10 and 50 microns.
When separating root vegetables, this sand is obtained from
the dirt that naturally adheres to the incoming root
vegetables, and when separating automobile and industrial
shredder residue, the suspension fines for all separations
below 1.6 are obtained nowhere else but from the ultra-fine
metals and glass generated in abundance by hammer-mill
shredding. Those who process vegetables do not have to use
salt, and they do not have to worry about suspension
materials contaminated with dioxins and heavy metals. Those
who process automobile and industrial waste do not have to
buy expensive density-creating fines such as magnetite to
make these low-density separations.
The Scrubber-Rinser
After separation, the
dewatering and rinsing of the floats and sinks poses a
special problem. To dewater by means of vibratory
dewatering screens is far from ideal. All vibrating screens
are difficult to maintain, they have limited material
transfer capacity, their panels easily blind up with trash,
and they presuppose large volumes of rinse water emanating
from spray nozzles that only impact the surface of a bed of
floats or sinks. ESR International therefore designed a
unique dewatering, scrubbing and rinsing device called a
scrubber-rinser (US patent 6,024,226 of Feb 15, 2000).
A scrubber-rinser is a
counter-flow vessel consisting of one or more stages of
scrubbing and draining. Solids are scrolled in one
direction, while rinse water is pumped from scrub section
to scrub section in the opposite direction. The incoming
solids are scrubbed and rinsed with the dirtiest rinse
water, while the out-going solids are scrubber and rinsed
with clean water. Since in the scrub stage the material is
totally immersed in water, the rinsing efficiency is far
higher than what we typically see in the case of a
vibratory screen equipped with multiple banks of spray
nozzles.
The scrubber-rinser has
become an indispensable means of rinsing non-ferrous metals
from high density separations involving the use of
magnetite and ferrosilicon. With scrubber-rinsers replacing
vibratory screens, losses in ferrosilicon typically drop
from over 7 kgs per ton of metals to less than 500 grams.
Since atomized ferrosilcon easily sells for over $1,000 per
ton, the savings here are dramatic. Since the rinse water
in a scrubber-rinser is used multiple times, the amount of
clean water required to rinse the outgoing waste materials
often drops by a factor of four or five.
On materials that are fairly simple to rinse, ESR has designed scrubber-rinsers with only one scrub and drain section. On delicate materials such as carrots, ESR dispenses with the use of agitators in the scrub sections. Under no conditions does ESR ever employ classical vibratory screens.
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