material. With the number of longwall
miners in operation, the issue of how to
deal with the amount of spray water used
is increasing. This is especially true in
operations where there is a steep slope
belt on which the raw coal must travel
before reaching the preparation plant. In
this scenario, it is not feasible to run all of
the material across a dewatering screen
due to the production tonnage associated
with the longwall. Therefore, it must be
viable to take only the ‘worst’ part of the
feed, which in this case is the wet material,
and run this across the dewatering screen
to remove the majority of the water, while
retaining the solids. The need to capture
the fine coal from the excess water in a
longwall operation and send this material
to the preparation plant for processing is
an emerging concern. This has created an
interesting challenge for Tabor and its
engineering department, along with
various contractors, to design a system
that will work properly and efficiently,
with a justifiable return on their
investment. To date, the company has
been successful in working with several
underground operations in designing
dewatering systems that meet their needs.
Many customers require the use of
dewatering devices to meet the
specifications set forth by the end user of
their product. This could mean
dewatering coal, sand or a variety of other
products being produced throughout the
world before shipment. In the coal
market, with the competitive nature of
pricing, many coal users have a moisture
stipulation in their specification. Again,
the size and type of material being
screened, along with the required
moisture, would dictate the dewatering
technique used. For larger coarse material,
the producer may be able to use a
vibrating screen to meet the specifications;
whereas if the material is in the 2 in. to
1 mm range, a vibratory centrifuge could
be used. The typical coal preparation
plant would use a mixture of both
high-frequency dewatering screens, as
well as centrifuges.
Dewatering applications
Stationary screens
The use of stationary screens in the
dewatering process is dependent on the
material being screened along with the
amount of moisture that is considered
acceptable. While most coal preparation
plants use either curved sieves
and/or flat flume screens in their
process, most applications require
additional dewatering before the material
is considered ‘dry’. Stationary screens
must rely on gravity and capillary
attraction to dewater the material, which
is viewed by most as only the initial stage
of dewatering.
Aplant that is using heavy media
would use the sieve or flume before the
drain and rinse screen to capture as much
magnetite as possible within the free
drain section. If the feed is correct, the
majority of the water and magnetite
mixture will be removed before going to
the vibrating screen.
The material is generally seen
building up at the discharge of the sieve
or flume then ‘slough off’ into the feed
box of the vibrating screen. While the
sieve or flume is not complete
dewatering, it must be considered when
looking at the complete dewatering cycle
within a preparation plant. The vibrating
screen would then, after the water
application rinses off the remaining
magnetite, resume the dewatering task.
On a typical 16 ft (4.9 m) long horizontal
screen, the last 4 ft (1.2 m) would be
dedicated to removing as much of the
surface moisture as possible before
reporting to the next stage of dewatering.
If a multi-slope screen is used for the
drain and rinse application the same
holds true, except the last 8 ft (2.4 m) of
the screen would be expected to be used
for the dewatering process.
This process would be considered the
last stage of coarse refuse dewatering,
where the majority of the water was
removed and considered acceptable for
use in the refuse impoundment. The fine
refuse and clean coal would report to
their respective areas for additional
drying.
Centrifuge
The dewatering processes split after the
drain and rinse screens based on the
material. Very few coal plants use a
high-frequency dewatering screen on
clean coal, unless the material is coarse.
Almost all of the clean coal is dewatered
by centrifugal force. This is due to the
centrifuge’s ability to generate high
G forces in the
range of 70 – 80 G
for the vibratory
centrifuges and
175 – 225 G for
screen-scroll centrifuges, compared
to the high-frequency screen that
operates between 3.5 – 5 G. As was
mentioned previously, the customer’s
specification of the product drives the
decision, as most consumers are
unwilling to pay for water when
purchasing coal.
According to KevinWhisler of
Centrifugal &Mechanical Industries
(CMI), the vibratory centrifuge is the
common choice when dealing with clean
coal in the 2 in. x 1 mm size range while
the screen scroll centrifuge would be
chosen for the
3
/
8
in. x 100 mesh material.
The
1
/
2
in. x 1 mm size fraction is
generally dewatered in a screen scroll
centrifuge since this is generally the
highest volume size fraction. Generally
speaking, the screen scroll will achieve
1.5% – 3% lower surface moisture versus
the vibratory centrifuge on the
1
/
2
in. – 1 mm size fraction.
High‑frequency screen
Almost all fine refuse is dewatered using
a high-frequency dewatering screen. The
choice to use the screen versus the
centrifuge is twofold: initial cost and
operating expense. Generally, the initial
capital cost of the high-frequency screen
is less when compared to the centrifuge,
as are normal operating costs. Since the
refuse is the nuisance that every coal
preparation plant must deal with, there
is not a benefit to justify the extra
expenditure to achieve the lower surface
moisture when this material is just being
sent to the refuse area at no value. Also,
the normal operating costs would be
higher for the centrifuge due to the
abrasive nature of the refuse and the cost
of the replacement parts.
As mentioned previously, the refuse
has to be just dry enough so that there
are no problems associated with
conveyance and any excess water in the
refuse area.
Dewatering with the use of a
vibrating screen is based on retention
time, G force and capillary attraction.
The slurry of material is presented to the
screen, which is generally a reverse
December 2015
|
World Coal
|
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