Florida phosphate ore is almost like a placer deposit or an agglomerate,
compacted, and is typically referred to as "Phosphate Rock". The actual ore
consists of Fluoapatite, silica and clay. Generally, it breaks up into fairly
sand size particles and finer as soon as it is pumped through the centrifugal
pumps used to transport it to and through the mill. Like most processes, the
fines present the greatest difficulty, due to their high clay content, and
very often everything below 100 microns is discarded as refuse. Attrition
scrubbers are great for removing fine clays from the surface of fine ores.
The fines might typically have 30% phosphate content (P2O5) of the ore, but
70% is economically recoverable.
The ore is initially washed in a log washer, a long trough similar to a spiral
classifier, except instead of spirals, there are blades, that break up and
transport the solids from one end to the other end of the washer.
there it goes to a rotating trommel screen, to remove large particles greater
than 2 inches (50.8 mm) in diameter. Then the ore finer than 2 inches is
again washed to break up the larger particles. The ore is screened at ¾ inch
and at 0.04 inches to produce two size fractions, a pebble sized ore and
flotation feed (-0.04"). Ore larger than ¾" is sent to the refuse pile.
The ¾" x 18 mesh ore is called pebble ore and is sent to a stockpile. The
-18 mesh ore can be sent to a series of agitation tanks or attrition
scrubbers to separate the fine clays from the sandy material. I always
thought that attrition scrubbers did a much better job, myself. One plant
uses several banks of 22 feet diameter by 20 feet deep agitation tanks with a single 8 foot
diameter propeller to agitate the -8 mesh ore at 30% solids by weight, which
suitably sent the fine clays into the suspension with the coarser material.
-18 mesh ore is sent to hydrocyclones to remove the -150 mesh (0.004 in.)
particles, which are sent to refuse. Sometimes,
the 18 mesh x 35 mesh ore is separated from the feed, and either blended with
the pebble ore, sent to a coarse froth flotation circuit or chemically upgraded.
The -35 mesh material is sent to conditioning tanks, where reagents are added
prior to a series of froth flotation cells, where the phosphate ore is
recovered from the silica and remaining clays.
The rougher concentrate from the froth flotation is sent to attrition cells
and scrubbed with a dilute sulfuric acid solution at 50-70% solids, then
rinsed and sent to conditioner tanks. Then this feed is again floated in
cleaner froth cells and the final concentrate is produced. This concentrate
is reacted with a strong solution of sulfuric acid to make phosphoric acid,
which is used in the fertilizer industry and chemical industry.
Fine tailings disposal have been a problem with the phosphate industry in
Florida, and much has been written about having to dispose of large amounts
of fines, that are not too stable. They should take note of several projects
that I was involved with in the coal industry, where we stabilized fine clay
refuse, by mixing it with hydrated lime, and coarse refuse material, turning
it into a cement mixture, which when properly placed in a suitably designed
disposal, could support the weight of a 150 ton haul truck after 24 hours of
drying. Perhaps one day the phosphate industry will figure this out,
Information provided by Charles Kubach, Mining and Mineral Processing Engineer
Reference: SME Mineral Processing Handbook