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Industrial
Wastewater Program
Phosphate Industry
Along
with tourism and agriculture, the phosphate industry is one of Florida's
oldest and largest industries. With its vast phosphate deposits, the majority
of which are located in the west-central part of the state, Florida has
become one of the world's largest producers of phosphate and accounts for
75% of the nation's and 25% of the world's phosphate production. Greater
than 90% of the phosphate is used for its phosphorus to produce inorganic
fertilizer. Wastewater associated with the phosphate industry, especially
that generated by the manufacture of fertilizer, contains various pollutants
that can exceed water quality standards and, if left untreated, could cause
adverse impacts to the environment.
Phosphate production
begins with the mining of calcium phosphate (phosphate rock). Typically,
phosphate rock is found at an average depth of 25 feet beneath the ground's
surface. The surface sediment overlying the phosphate, known as overburden,
consists mostly of sandy soil. The overburden is removed and stockpiled
for future use in reclamation of the site. Once the overburden is
removed, a matrix of phosphate pebbles, sand and clay is exposed. The matrix
is mined and deposited in a shallow containment area known as a well. While
in the well, the matrix is sprayed by high-pressure water guns that liquefy
the material into a mixture called a slurry. The material is then transported
through pipelines to a beneficiation plant, where the clay and sand are
separated from from the phosphate rock. The first step in separating phosphate
from the clay and sand is to put the slurry in a washer where large balls
of clay are mechanically disintegrated. The slurry then moves through a
series of vibrating screens where fine clay particles pass through and
the phosphate pebbles remain. The clay is then pumped to settling ponds
while the pebbles are moved to dewatering tanks and stockpiled by conveyors
for further processing. Also separated in the process is a mixture of sand
and finer particles of phosphate (concentrate) that is then put through
a process called flotation. In the flotation process, the mixture is put
in a vessel of water where reagents such as fuel oil, soap or fatty acids
are mixed in to coat the concentrate and attach air bubbles, allowing it
to float, separating it from the sand. The sand is transported back to
the mine and stockpiled for reclamation while the concentrate is transported
to dewatering tanks and stockpiled for further processing.
Once
the phosphate has been separated from the matrix and dewatered, it is ready
to be processed into components used in inorganic fertilizers. In
order to make the phosphorus in phosphate rock more readily available to
plants, the phosphorus must be in a soluble form. The process begins with
the phosphate rock being ground into a fine uniform grain size. It
is then reacted with sulfuric acid to release the phosphorus from its chemical
bond with calcium and other elements. The reaction of the phosphate rock
with the sulfuric acid produces phosphoric acid and hydrated calcium sulfate
(phosphogypsum), a by-product. The phosphoric acid is then separated
from the phosphogypsum and concentrated. The concentrated phosphoric acid
is finally used to manufacture ingredients for inorganic fertilizer. Examples
of ingredients are diammonium phosphate and monoammonium phosphate which
are produced when phosphoric acid is reacted with anhydrous ammonia (for
its plant-available nitrogen). Another fertilizer ingredient, produced
by mixing phosphoric acid with finely ground phosphate rock, is granular
triple superphosphate. Meanwhile, water is added to the phosphogypsum
by-product to create a slurry that is hydraulically pumped to a lagoon.
As phosphogypsum
lagoons fill, the solids are scooped out to build up the sides, forming
another lagoon on the resulting phosphogypsum stack that increases in height
(up to 200 feet high) as the process continues. The process
water that remains after the solids settle out is returned to be reused
in processing the phosphoric acid. The acidic nature of the process
water is of major concern to DEP. Without proper treatment aquatic
life can be seriously affected when this highly acidic water enters state
waterbodies from careless spills or failed structures
The
phosphogypsum by-product presents a serious disposal problem for the phosphate
industry. Normally, gypsum is a material with uses for such things
as plaster, dry wall, soil conditioner, cement retarder and road base,
but because of impurities such as phosphate, fluorides and trace amounts
of heavy metals, including radium, the material is undesirable. Until
recently, phosphogypsum was being used in the state as a road base. Due
to the presence of trace amounts of radium in the material, the United
States Environmental Protection Agency banned its use for fear that
future houses may be built on the sites of abandoned roads that used phosphogypsum
as a base. Without any means of disposing of phosphogypsum, the industry
is forced to stack the by-product in designated landfill areas and seal
it off to prevent leaching to ground and surface water. The Florida
Institute of Phosphate Research is currently working on research to
find uses for phosphogypsum, and improve environmental quality of wastes
and land reclamation, as well as improving the efficiency of the industry's
processes.
A specialized group
at DEP, the Phosphogypsum Management Group, was created to deal with permitting,
compliance and enforcement for both wastewater and solid waste produced
by the phosphate industry. This group reports to the Division's
Bureau of Mine Reclamation. Since the majority of the phosphate industry
is located in DEP's Southwest Regulatory District, the group works out
of the district's office in Tampa.
