PHOSPHATE ROCK

The matrix as mined, contains three basic mineral types. These are phosphates, silica and clays. The objective in washing and flotation is to utilize the chemical and physical differences between the three minerals to effect a separation.

If we could break the agglomerated mud balls of clay/silica/phosphate into individual particles, we could then wash this slurry over 16 mesh screens, and have an oversize product consisting of phosphate. This is in fact the first step of beneficiation. It is done in the washer, which consists of trommels, flume screens, disintegration mills, log washers, and series of 16 mesh screens. The washer oversize product is called pebble, and is generally a salable product after this simple washing process. The washer undersize (or -16 mesh) product contains the remaining phosphate not removed as pebble, all the silica and all the clays.

The next step in beneficiation, is to remove all the clays. Once again we make a size split, this time at 150 mesh. The separation is conducted in a hydrocyclone, which by design of its feed pressure, diameter, overflow, and underflow openings will reject the -150 mesh particles to the overflow, and all +150 mesh particles to the underflow. This underflow product (referred to as "feed") is then ready for further processing at the flotation plant.

The flotation plant design varies from one company to another. Basically, the feed is processed in a "rougher" float, to upgrade the phosphate content, then cleaned in an "amine" float to reach salable product requirements. Variations include pre-flotation sizing (for size segregated float feeds), bulk unsized feed flotation, scalping out a coarse "almost pebble" fraction which bypasses flotation (a non-salable product for internal company use - due to low grade) and some plants process the coarsest feed in special devices with applications unique to the Florida and North Carolina Phosphate Industry (belt separators, spirals) and in column float devices.

Rougher flotation, spirals, belts and column floats are all variations of the same process, differing in the mechanical application. The process is one in which organic acids (R-COOH compounds) or fatty acids are saponified by addition of ammonia (sometimes caustic is used) and then reacted with the phosphate/silica feed. The R-COO ion will form a bond with open calcium sites on the surface of the phosphate mineral, but will not react with the silica surface. This ionic head, attached to the surface of the phosphate with a long chain hydrocarbon extending out from the surface, creates what is called hydrophobic, or "water-hating" surface. The non-polar hydrocarbons repel water molecules. This reagentized surface is further treated by application of fuel oil, to "extend" the hydrocarbon tails. The oil also serves as a froth reduction reagent. This reagentized slurry is then diluted, and put in agitated cells, where air bubbles are introduced. The hydrophobic phosphate particles are attracted to air bubbles, (air is of course non-polar like hydrocarbon) and after attachment to the bubbles, rise to the top of the float cell. The froth of phosphate is removed by rotating paddles.

The rougher flotation process is not a perfect one, and if good recovery is sought, a fair quantity of silica also reports to the froth product (called "rougher concentrate"). In order to produce a salable product, a second flotation process is conducted to remove the residual silica. In this float, a different reagent is introduced, an amine based compound (also with the long hydrocarbon tail) and silica is floated away from the phosphate. Before this can be done, the feed is prepared for amine flotation by passing thru acid scrubbers (to remove the fatty acid coating on the phosphate) and a counter-current rinse to separate the now de-oiled rougher concentrate solids from the oily water. The concentrate (non-floating solids) from the amine flotation process is the final salable product. In addition to the amine reagent, some operations use small amounts of diesel oil, and some use small amounts of ammonia to adjust the flotation pH after acid scrubbing.

All products (pebble and concentrate) are either stored in stockpiles near the wet rock producing plant or shipped by rail or truck to the customer.