Processing Florida Phosphate Ores

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. From 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 inch and at 0.04 inches to produce two size fractions, a pebble sized ore and flotation feed (-0.04"). Ore larger than " is sent to the refuse pile.

The " 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.

The -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, perhaps not.

Information provided by Charles Kubach, Mining and Mineral Processing Engineer
Reference: SME Mineral Processing Handbook  

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