Chemical Processing Plant: A Virtual Tour

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Chemical Processing PlantA Virtual Tour
April 2003 Florida Institute of Phosphate Research
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Florida Phosphate Mining and Processing Overview
1. A dragline scoops away the overburden and
digs out the matrix that is equal portions of
sand, clay and phosphate. The overburden is later
used in reclamation. 2. The matrix is dumped
into a pit where high-pressured water guns create
a slurry that is pumped to the washer and the
beneficiation plant, often miles away. 3. The
washer and the beneficiation process separate the
phosphate from the sand and clay. The clay is
sent to a pond to settle. As it settles the top
clear water is recycled back to the plant. The
sand is used in reclamation. The phosphate is
sent by train or truck to a chemical plant.
4. At the chemical plant the phosphate is reacted
with sulfuric acid to create the phosphoric acid
that is used in fertilizer and animal feed. A
by-product of the chemical reaction is
phosphogypsum, which is stored in a stack near
the plant.
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At the chemical processing plant phosphate rock
is reacted with sulfuric acid and converted into
the phosphoric acid used to make fertilizer.
Phosphogypsum, a byproduct of the chemical
processing, is stored in stacks. It is pumped to
ponds at the top of the stacks to settle. Pond
systems include collection areas at the foot of a
stack for cooling the water coming out of the
plant. All water is recycled for use in the
plant.
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The sulfuric acid that is needed to convert the
phosphate rock into phosphoric acid is also
produced at the chemical processing plant, using
liquid (molten) sulfur most of which is shipped
to a Florida port and trucked to the processing
plants. Since the energy crisis in the 1970s,
most Florida phosphate companies capture the heat
released in the burning of sulfur and production
of sulfuric acid and use it to produce steam.
The steam is used to produce the heat required to
concentrate the phosphoric acid and also to
produce electricity to run the plant. Typically,
plants produce most of the energy they need and
some sell a portion to the area commercial energy
provider.
Sulfuric acid plant
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Processing Plant Flowsheet
Cogeneration Plant
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Trains and trucks bring the phosphate rock from
the beneficiation plant to the chemical
processing plant.
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The rock is then unloaded by conveyor belts and
stored on a rock storage pile.
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The grade of the rock determines where it is
deposited on the pile.
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This view of the rock being put on a storage pile
is a common sight for those who drive by a
chemical processing plant.
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Bulldozers move rock from various places on the
pile into chutes that feed the rock onto a
conveyor belt running through the tunnel
underneath the rock pile.
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The various types of rock are blended on the
tunnel conveyor belt. The blending operation is
becoming more important as the phosphate
companies are forced to use lower grades of rock.
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The conveyor belts carry the rock out of the
tunnel
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and up to the ball mill for grinding.
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The rock on the conveyor belt is fed into the
ball mill.
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Grinding the phosphate rock increases the surface
area of the rock so the acid can more easily
attack it. The smaller the phosphate particles,
the easier they are to dissolve.
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The mill may be fed either pebble or a blend of
pebble and concentrate.
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Water is added to the ball mill to create a
slurry.
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The slurry is held in storage tanks which provide
feed to the phosphoric acid plant.
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In the phosphoric acid plant, the slurry will be
reacted with sulfuric acid that is stored in
tanks like these.
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The slurry is pumped to the reactor.
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In the reactor it will be mixed with sulfuric
acid.
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The reactor is a series of mixing tanks.
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Attack tanks (reactors) use giant agitators to
produce a mixing action that promotes a
chemical reaction which forms a slurry
consisting of calcium sulfate and phosphoric acid.
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The calcium sulfate (gypsum) and phosphoric acid
slurry is then fed onto a large rotating filter
table.
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On this filter the waste calcium sulfate (gypsum)
is separated from the phosphoric acid.
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About 85 of this phosphoric acid (which at this
point is about 30 concentration) is pumped to
evaporators where it is evaporated to 54
concentration.
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The gypsum (commonly known as phosphogypsum) is
separated at the filter and pumped with wash
water to the top of a phosphogypsum stack where
the gypsum will settle out and the water will be
recovered for reprocessing in the plant.
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Belt filters are also commonly used to separate
the phosphoric acid and the phosphogypsum.
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A belt filter does the same job, but takes less
square feet of space than the traditional tilting
pan filter.
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Its a matter of opinion as to which filter a
company believes is more efficient and less of a
problem to maintain. Some use both.
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Radiation exposure connected to the chemical
processing of phosphate rock is a factor that is
monitored to protect worker health. The filters
are the highest radiation exposure areas in the
processing plant because radium scale collects on
and under the filter pans.
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Radiation connected to phosphate comes from
nature. Phosphate is in the ground with
radioactive uranium. In processing the rock, a
small amount of uranium goes with the phosphoric
acid and at one time was recovered as a product.
Most radium goes with the phosphogypsum.
Radium-226 decays to radon-222, making
phosphogypsums low level of radioactivity an
issue.
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Other radiation exposure areas in the plant
include storage tanks like these, pipes, or
belts that have quality control gauges on them
that use a radiation source to measure the
density of what is in or on them. These gauges
are shielded and structured so penetrating
radiation is directed through the vessel, but
some radiation can go through a
shield.Phosphate companies take extensive
safety measures to protect workers and they are
closely monitored by government health officials.
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After filtering and evaporation, concentrated
phosphoric acid can be enhanced with nitrogen by
reacting it with ammonia, then granulated and
used to make various fertilizer products. Also,
it can be processed into merchant grade
phosphoric acid, or it can be deflourinated and
used to make animal feed.
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Dried final products, such as diammonium
phosphate (DAP) or monoammonium phosphate (MAP),
and granulated triple superphosphate (GTSP) are
transferred to storage buildings like this.
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Products are carried by truck or train to the
seaport and shipped to markets around the world.
Floridas phosphate industry supplies 25 of the
world phosphate needs and 75 of the phosphate
used in the United States. About 95 of the
phosphate is used in fertilizer products.
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Meanwhile, the phosphogypsum is washed off the
filter and pumped to the top of the phosphogypsum
stack where it will settle and the water will be
reused in the plant.
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Phosphogypsum is stockpiled in stacks that can
cover up to 400 acres and rise as high as 200
feet into the air because the United State
Environmental Protection Agency (EPA) has said it
cannot be used because it contains a small amount
of radioactivity.
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Research shows that there are many economically
viable and safe ways to use phosphogypsum.
Phosphogypsum, for example, used as a daily cover
in landfills hastens decomposition thereby
extending the landfills capacity. It can also be
used as a roadbase material that is as effective,
more durable and less expensive than traditional
materials.
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Currently, however, it is not used and the stacks
play a key role in recycling process water as a
way to significantly reduce the amount of water
the industry pumps from the ground. This diagram
outlines the recycle process.
Fresh Water
Rain
Phosphoric Acid
Gypsum Slurry
Phosphate Rock
Phosphoric Acid Plant
Gypsum Stack
Other Water
Sulfuric Acid
Water From Stack
Pond Water
Pond System
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Water containment areas at the foot of the stacks
cool the water coming from the evaporators and
other parts of the process that generate heat.
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Ponds on top of the stacks also serve as a
cooling system and the water is drained off the
stack to the cooling ponds on the ground and then
recycled.
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Water in these ponds is an environmental concern
because of acid the phosphogypsum picks up during
processing. The water has a variety of dissolved
solids like sodium and it also contains some
ammonia.
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Concerns about the water leaching from the stack
into ground and surface waters has resulted in
regulations requiring that phosphogypsum stacks
be lined with heavy plastic to guard against
seepage from the stack. Rolls of the liner
material are pictured below. The regulations
also call for all unlined stacks to be closed.
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Pictured here are a closed phosphogypsum stack,
with a grass covering to control the quality and
quantity of water running off the stack (right),
and an active stack (left).
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The End