1.2. Description of mining techniques
The phosphate reserves in west-central
Florida are extracted through area strip-mining process due its relatively
shallow and flat deposit in the bedrock.
Initial site preparation involves the clearing all vegetation. A deep trench is cut along the perimeter of
the planned mine site to drain the surficial aquifer system and lower the water
table. Next, the overburden layer of soil
and rock is removed in a series of parallel strips, using a dragline. Bulldozers collect the overburden in stockpiles
for reuse in reclamation area and construct a ditch and berm system. The dragline begins extracting the mineral reserve
(matrix) at around twenty to fifty feet (ft) below the surface. The matrix is removed and placed into the
containment pit where high-pressure hoses cannon in water form a slurry.
The slurry is then pumped offsite to the beneficiation
plant to separate the matrix into its phosphate, clay, and sand components. First, a series of one millimeter (mm)
screens extract and collect the pebble-sized phosphate particles, while the
sand, clay, and remaining phosphate particles move to the hydrocyclone. The hydrocyclone further sorts the matrix by
settling the sand and 0.1 to 1 mm phosphate particles and overflowing the clay
and smaller phosphate particles. The
sand mixture is sent to a water-based floatation chamber where a fatty acid is
added to coat the particles. Air is
injected in the chamber and attaches to the fatty acid coat, causing the phosphate
particles to float to the top of the chamber.
The coated particles are collected, remixed with the pebbled-sized
particles, and sent to the refinery to produce the fertilizer. The sand-tailings from the floatation chamber
is drained and returned to the mine to refill mine pits or cap reclamation
areas as needed. Meanwhile, the clay waste
is collected from the hydrocyclone and transported to a clay settling ponds within
the ditch and berm system at the mine site.
The clay waste is mostly water with three to five percent solids. The phosphate particles which were too small for
extraction in the beneficiation plant and settle with the clay particles in the
settling ponds. As the waste dewaters
over a period of several months, the water is gradually decanted from the surface
and recirculated through the system. The
remaining clay-phosphate mixture dries and consolidates in an extremely slow
process. Yao and Znidarcic (1997) determined
that a waste with 12 to 15 percent solids requires anywhere from three to
thirty months of drying, causing the land to remain “in operation” decades
after mining activities have stopped to achieve a final average solids content
of 20 to 25 percent.
1.2. Reclamation methods
The topography of post-mined land is
characterized by long, uneven rows of overburden spoil piles, interspersed with
mine pits and elongated waste ponds. The
invasive mining activities impair the landscape and water resources and impede urban
development and natural ecosystem growth. Reclamation methods are needed to return the
land back to beneficial use. Typically,
reclamation does not restore the area to its previous ecosystem trajectory
since the pre-mining soil stratification and vegetation is difficult to
replicate and requires detailed consideration before extraction. Instead, reclamation builds the land to suit
the community’s need. Some reclaimed
land uses include agricultural operations, forestry, recreation, residential
and commercial development, wildlife habitat, or wetlands.
first step of reclamation is contouring.
Mine pits are backfilled with overburden spoil piles to smooth the ground
surface; however, the removal of matrix prevents a full elevation recovery. The sand-tailings from the beneficiation plant
are transported back to the site to define uplands, gently rolling hills, and
shallow lakes which drain internally.