FLORIDA PHOSPHATE BENEFICIATION PRACTICE IN Glen Oswald Minerals Process Eng. Supt. Mosaic

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1 FLORIDA PHOSPHATE BENEFICIATION PRACTICE IN 2012 Glen Oswald Minerals Process Eng. Supt. Mosaic Presentation at AICHE Conference Clearwater, Fl June 9,

2 Abstract The Florida 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 process used in central Florida to separate these minerals has not changed much since Arthur Crago developed the Crago process in This presentation will explain the unique beneficiation process that is used in the local phosphate mining industry that once produced >40 million tons a year of phosphate rock. 2

3 The Florida Phosphate Beneficiation process has been well refined since the last major process innovation in 1942 Arthur Crago s creation of the double-flot process. Many engineers have sought to improve upon this robust process, but to date none have been successful at the industrial level. This 70 year duration stands as a testament to either the industry s stubbornness, or the appropriateness of the process. We will let history decide on that issue, but the matter at hand is a review of the current beneficiation practice. Florida Phosphate Beneficiation Process Description The development of any mineral beneficiation processes must begin with an analysis of the ore characteristics seeking differences between the mineral properties. The Florida phosphate ore consists of three mineral types, and each of these minerals has unique properties that can be used to facilitate separation. These properties are as follows: When the ore is removed from the ground, it is a mass of phosphate and sand particles agglomerated in a matrix of clay. Once this mass is disaggregated, it is observed that each particle consists of a single mineral. There are no locked multi-mineral particles. Crushing and grinding is not necessary to liberate the ore, which leads to lower beneficiation costs when compared with phosphate processes in other locations. The phosphate minerals range in size from one micron to ~20 mm. The sand particles range in size from one micron to one mm. The clay particles (after disaggregation) are all smaller than 0.1 mm, averaging 10 microns in size. Armed with this knowledge, the initial sequence of processing should be obvious. The first step in beneficiation is to disaggregate the clay matrix; there should be no mudconsolidated particles remaining after scrubbing the ore. This scrubbing begins in the long pipelines transporting ore from the mine site to the beneficiation plant (pictures in the appendix). High shear forces from the large centrifugal pumps and friction along the pipeline result in the clayballs breaking apart. Once the ore arrives at the plant, log washers are used to break up any remaining mud. Once the ore is fully liberated, several size separations are performed. The first is to screen the ore at one mm; the sand and clay are all finer than one mm so the oversize product is a pure phosphate product called pebble. The log washing and screening of pebble is performed in a structure called the washer (see a picture in the appendix). It is a series of trommels, static screens, vibrating screens and log washers. Next, a 0.1 mm separation is made using hydrocyclones, to remove the clay minerals. Unfortunately, some fine phosphate is rejected in this step because there is not currently an economical process to recover this material. Phosphate losses in this operation can range between 15-30% of the total mineral in the ore. The rejected fines are discarded and deposited in large retention ponds. The nature of these clay fines are such that they require many years to consolidate to their ultimate %solids. Much work has been performed trying to improve the clay consolidation process, but few innovations have 3

4 made it to the field, mostly for economic reasons (large capital or operating costs). While that subject is of interest, it is out of scope for this discussion, however. Once the clays and coarse phosphate are removed, the remaining sand and phosphate particles must be separated from each other. This is where the beneficiation process gets more costly and complex. The industry uses a common mineral beneficiation practice called flotation to accomplish this goal. The remaining ore consists of the particles between 16 mesh and 150 mesh (991 microns, to 104 microns) which the flotation process is capable of processing. Flotation is a process developed over a century ago, but was not adapted for phosphatesand separation until The first Florida flotation plants appeared in the late 1920 s, and revolutionized the business. Previously, only the +16 mesh pebble could be produced; after this innovation companies were able to double production by recovering fine phosphate as well. The process is based upon the property that certain hydrocarbons tend to selectively coat phosphate and not the sand mineral. When properly applied, a hydrocarbon coating on the phosphate will result in the creation of a hydrophobic (water hating) surface. When this surface is exposed to water, it repels the water the same way a freshly waxed car will repel rain. Water beads and rolls off any waxed surface. The chemical principal applied in the flotation process is non-polar surfaces will repel polar substance such as water. Non-polar compounds in air, such as Oxygen and Nitrogen, will be neutral when in contact with the phosphate particles. Once a thorough coating is placed on the phosphate particles, it and the uncoated sand are immersed in a series of water filled tanks. The tanks (called flotation cells see a picture in the appendix) are agitated, then fine air bubbles are injected into the slurry. The air bubbles provide a non-polar destination for the hydrophobic phosphate particles to exit the water phase. The bubbles rise due to buoyancy and carry the phosphate along to the surface (a picture of a phosphate froth is shown in the appendix), leaving the sand behind. A surfactant is added to create a stable froth on top of the flotation cell where the bubbles and phosphate mineral collect. That froth is mechanically skimmed away from the tank, and the sand exits out tailing ports at the bottom of the flotation cell. In the early days of phosphate flotation, feed grades were almost always higher than 45 BPL. It was simple to produce an acceptable grade phosphate flot product from feed this rich. Over the years, the high grade ore was mined out. It became difficult to meet the needs of the chemical plants as the flotation feed had decreasing quantities of phosphate. Mr. Crago (the general manager of Brewster Phosphates) had his metallurgists develop a new process to address this problem. In the process named after Mr. Crago, the initial phosphate concentrate (now called rougher concentrate) was stripped of its organic surface coating, and then subjected to a second flotation process. This second flot required development of a completely new coating agent which would be selectively attracted to the silica mineral. They tested many potential chemicals, but found a certain kind of amine based reagent was best suited for this task. Under the right solution conditions, it will coat sand, but not phosphate. Once the sand is coated, the rougher concentrate can be subjected to the same flotation processing as the original rougher 4

5 flotation feed. This time, sand rises to the froth phase and is removed. Phosphate exits from the tailing port and will meet the grade requirements of chemical plant customers. The Chemistry of Flotation Coating a mineral surface prior to flotation is accomplished in one of several manners. The most common method is to create aqueous conditions that result in a highly charged surface on one mineral, and an opposite polarity charge on any undesirable minerals (called the gangue minerals). A chemical must be found (called the collector ) with the general chemical structure of a long hydrocarbon chain attached to a charged ionic head with a charge opposite to the mineral of interest. The collector s ionic head is electrostatically attracted to the mineral, creating a hydrophobic surface due to the long hydrocarbon chains pointing out from the mineral. The mineral is fed into a flotation cell, agitation and air introduced and the separation made. This simple electrostatic attraction is how the second (amine) flotation process works. The chemistry of the rougher flotation operation is quite different. Until publication of the FIPR study Understanding the Basics of Anionic Conditioning in Phosphate Flotation 1 in May 1995, there was widely divergent speculation about rougher flotation chemistry. Many respected scientists and engineers thought the attachment mechanism was simple physical smearing, others thought it was the traditional electrostatic attraction. As it was shown by Lead Principal Investigator Glenn Gruber, the principal mechanism is chemisorption. The chemical structure of the Florida phosphate mineral carbonate-fluorapatite is: Ca 5 (PO 4,CO 3 ) 3 F An ionized carbonic acid molecule will precipitate when exposed to calcium ions (the well known soap-scum found in bathtubs when hard water and soap are mixed). In the same manner, ionized fatty acid molecules will precipitate at calcium sites on the surface of phosphate mineral particles. This leaves a long hydrocarbon tail pointing out from the surface, resulting in a strongly attached hydrophobic coating on the phosphate: 5

6 This leaves a coating on phosphate that can only be removed via chemical means; it cannot be physically scrubbed from the surface as many previously thought. Rougher flotation is a tricky process to do well. The now famous quote from the renown phosphate metallurgist Ed Finch sums it up well: There are three important requirements for good phosphate flotation. Number one is condition at high %solids, number two is condition at high %solids, and number three is condition at high %solids 2. Rougher conditioning (coating the phosphate mineral) is a complex industrial process that has been locally refined and perfected over the years, but outside Florida is still not well known. In order to be successful (high recovery, low cost, selective) conditioning requires strict adherence to the following requirements: Conditioning at >72% solids is essential (certain applications up to 80% solids have shown even greater benefits). Kinetics in plant scale equipment requires a minimum of 4 minutes reaction time. Conditioning equipment designed to minimize short-circuiting (4 tanks in series, plug reactors such as drum conditioners). Design of conditioning tanks to avoid choking (no baffles, cruciform agitator blades at >55% of tank diameter pumping downward, large sloping chutes from one tank to the next, proper RPM, etc.) The chemical basis for these requirements can be easily understood by looking at the equilibrium and kinetic rate relationships. High concentration of the ionized fatty acid reagent (COOH - ) will prompt a more complete reaction, and at a higher rate (all good things): Reaction: R-COO - + Phosphate Reagentized Phosphate 6

7 The equilibrium equation is: K eq = [Reagentized Phosphate] / ( [R-COO - ] * [Phosphate] ) Obviously, a high concentration of ionized fatty acid will push the reaction further to completion; lowering the consumption of costly chemicals, and utilizing a higher percentage of the chemical applied. As to the kinetics, the rate equation is: RATE = K k * [ R-COO - ] Once again, a high concentration of the ionized fatty acid will increase the rate of reaction, allowing the flotation operator to minimize the use and cost of the reagent, and insuring completion of the reaction in the time permitted (based on the feed rate and size of the conditioning tanks). The rougher coating must be removed prior to the amine flotation process. The conditioning reaction is reversed in the deoiling step. This is accomplished by adding acid that indirectly result in reversing the rougher conditioning process: H 2 SO 4 H + + HSO 4 - H + + R-COO - R-COOH Reagentized Phosphate R-COO - + Phosphate Once the phosphate surfaces are deoiled via this series of reactions, they can be subjected to the amine flotation process. The pre-1995 theory (misconception) that deoiling is primarily a mechanical process is not correct; it is a chemical reaction. Summary The Phosphate beneficiation flotation process has been reviewed and some of the basic chemistry involved in the process shown. There is clearly more to the chemistry than what has been presented here; this has been a short introduction to industry practices. 7

8 Appendix A dragline depositing ore in the pit: PICTURES: A matrix (ore) pipeline from the mine to the beneficiation plant, and a lift pump: 8

9 A washer: Banks of Flotation cells and phosphate rougher froth: 9

10 REFERENCES: 1 Gruber, Glenn A, Moudgil, Brij M, Somasundaran, P, in May 1995 FIPR Publication # , Understanding the basics of Anionic Conditioning in Phosphate Flotation. 2 Ambrose, David, in Oct 2009 presentation to SME annual Phosphate Conference (Lakeland Fl), Back to Basics. Quote from a video interview of Ed Finch. 10