Definition of “Beneficiation”: This is the second step in the mining process, after removal of the ore from the ground. Beneficiation is the technical term describing the industrial process of mechanically separating minerals from each other. No chemical changes to the minerals are made at this point in the mining process.
The phosphate ore, or “matrix” as it is called locally, contains three different mineral groups which have to be separated from each other. They are:
- ● Phosphatic minerals
● Clay minerals
● Quartz, or sand.
The Florida mineral composition is unique in that these minerals have different size distributions. It is this difference in particle size that makes the Florida separation process easier to perform than at many other phosphate mines around the world. Note the following:
- ● The individual phosphate particles range from a micron, up to ~20 mm in size.
● The clay particles are all smaller than 0.1 mm, with the average particle size only ~10 microns.
● The sand particles range from a micron, up to 1 mm in size.
This particle size distribution leads to a separation strategy that is unique to the Florida phosphate beneficiation process.
The conventional phosphate beneficiation process
As mined, the phosphate and sand particles are embedded in compacted mud, or “clay-balls”. Before separation can begin, all the particles must be liberated from the matrix of mud. The very first unit operation in the beneficiation process is to disaggregate the various particles; this actually starts while the matrix is flowing through the multi-mile pipeline from the mine to the beneficiation plant (see the “phosphate mining primer”). While in the pipeline, the matrix is exposed to shear forces as it passes thru the various centrifugal pumps along the pipeline. These intense shear forces cause a significant percentage of the sand and phosphate particles to be liberated from the clay-balls by the time they arrive at the plant. Once reaching the plant, the first goal is to finish disaggregating the clay, and follow that by making a size separation at 1 mm. This processing is conducted in the “washer”. In all the currently operating beneficiation plants, the washer is a large structure that receives the matrix, screens it, then discharges a +1.0 mm “pebble” phosphate product and a -1.0 mm slurry of liberated clay, sand, and phosphate particles. This first phosphate product (the “pebble”) can be as little as 5%, and as large as 70%, of the mine’s total production depending on the nature of the matrix being mined. [See notes at the end of this discussion about the “Future of Phosphate Beneficiation” describing additional processing that is sometimes required.]
The next process objective is to remove the clays. Remember that the clays are finer than 0.1 mm. To reject the clay, all that is required is to size at 0.1 mm and discard the fine fraction. The beneficiation plant does this with equipment called “hydro-cyclones”. Slurry from the washer is fed tangentially into the cyclone (a conical chamber) at a high G force. The slurry swirls around inside the cyclone until fines overflow the top of the chamber. Coarse sand and phosphate particles swirl to the bottom of the cyclone and exit. The fine clays are collected and pumped to large impoundment ponds which are discussed in another section (the “clay pond primer”). The +0.1 mm sand and phosphate move on to the next process operation.
Depending upon which company designed the plant, some existing Florida plants will size the slurry leaving the cyclone underflow into various tight size fractions before further processing. This is done to enhance efficiency in the next operation, but some plants skip sizing and just send the cyclone underflow on to the next treatment. Sizing is typically done in equipment called “hydrosizers”. Feed and upward flowing water are injected into large tanks which force the fine particles to rise and overflow the tank, while the coarse particles gently fall and flow out the sizer’s underflow.
The next step, “flotation”, is a separation process that is used in mineral beneficiation plants around the world. Flotation was discovered early in the 20th century, and today it is the most commonly used separation technology in the mining industry. Flotation separates valuable minerals (copper, lead, zinc, iron, and phosphate too) from the contaminating minerals in the ore (sand in this case). In the direct flotation process the valuable mineral is coated with a special hydrocarbon (fatty acid) that makes coated particles behave the same as a waxed car. Once the phosphate surfaces are coated, they repel water just like a freshly waxed car during a rainstorm. The slurry of waxed-phosphate and un-waxed sand is diluted and put in agitated tanks. Tiny air bubbles are injected into the tanks (called flotation cells) which attach to the waxed phosphate particles (the water-repelling particles are pushed out of the water into the bubbles). The air bubbles rise with the phosphate to the top of the flotation cell where the valuable froth is skimmed from the surface and collected. Although it seems amazing, this unique chemical technology can make denser than water particles rise to the top and float on the surface of a slurry.
In order to upgrade the initial (“rougher”) phosphate concentrate to a salable product, a second cleaning flotation process is used to remove the last of the residual sand. The original hydrocarbons are stripped from the phosphate surfaces, and then a different hydrocarbon is applied to the rougher concentrate. This second hydrocarbon is an amine based reagent that coats sand, but not phosphate. Once again, the slurry is fed into flotation cells, agitated and exposed to tiny air bubbles. The air carries the remaining sand to the surface where it is skimmed off and discarded. The remaining phosphate mineral (“concentrate”) is collected, blended with the pebble product and shipped via rail or truck to the chemical plant for the third step in making phosphate fertilizer.
The sand from both the rougher and cleaner flotation process are collected, and pumped back to the mine cuts for use in land reclamation.
The Future of Phosphate Beneficiation – the MgO Problem
The central Florida mines dig matrix from the “Bone Valley” geological district. Matrix in the north end of the district has only the three minerals phosphate, clay and sand present. In the southern extension of the district, the incidence of the mineral dolomite (MgO) increases. Historically, draglines avoided digging any matrix with elevated dolomite, but over the years mining has progressed south and this option is not always possible now. Today, the southern Bone Valley mines have to take additional steps that the northern mines did not in the past. Fortunately, most of the dolomite in the matrix being mined today is very coarse (greater than ~30 mm in size), which makes removal easy to perform.
The southern district mines generally screen out and discard the coarser pebble fractions > 30 mm in size, but some locations with higher dolomite content in the ore need to discard pebble > 10 mm. As mining continues to move south, screening alone will not remove enough dolomite to produce an acceptable quality product. New dolomite removal technologies have been developed (and even used industrially in one case) and will become a requirement in future beneficiation plants.