We wouldn’t exist if it weren’t for phosphorous. Phosphorous is a basic component, a building block of our DNA. It is also a vital part of plant life, making all our crops dependent upon it as well. The United Nations has estimated that without chemical fertilization, which includes phosphate, world agriculture could only support two-thirds of current world population. Other estimates suggest that without such fertilization one half of the people on the planet today would starve. Florida produces 75% of the phosphate used in the United States and supplies 25% of the world’s phosphate. While those deposits remain and are processed, Florida and its phosphate industry deal with unique challenges from the wastewater produced by this vital industry.
Phosphate mining started in the US in the 1870s, primarily in South Carolina. Bones were originally the main source of fertilizer phosphate in the country. At around this time, Capt. J. Francis Lebaron, chief engineer of the US Army Corps of Engineers, found calcium-phosphate-bearing pebbles along the Peace River in Florida. Evaluation of this rock about 10 years later yielded the discovery that this was a superior grade of phosphate, even better than that found in South Carolina. The Florida phosphate rock was more pebble-like than deposits to the north.
What took place over the next few years was something of a phosphate gold rush in Florida. Organizations from all over the world grew interested in phosphate mining for fertilizer. Mining activity eventually ended up in the pebbly deposits around present-day Tampa, growing exponentially into the 1920s.
In the early years of production, phosphate was shipped from Florida, both overseas and up the US coast. Small fertilizer processing plants resulted, each serving its own region. During the 1930s several plants began to produce phosphoric acid. U.S. Phosphoric at Tampa was the first phosphate mining and processing company. Today the plant is owned by the Mosaic Co.
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| Flotation cells use a combination of water, chemicals, air bubbles and turbulent water to separate the sand from the sand-sized phosphate. |
To create phosphoric acid, one ton of phosphate rock material is added to two and a half tons of sulfuric acid. When the phosphoric acid has ammonia added to it, the result is diammonium phosphate (DAP), a solid fertilizer. DAP fertilizer is water-soluble and will be available for the plants to take up through their roots. All of the DAP and all of the fertilizers made today are a product of phosphoric acid.
In mining operations, overburden is excavated using a dragline bucket to reach the matrix containing the phosphate rock, clay and sand. This matrix is dumped into a pit where water under high pressure creates a slurry that may be pumped up to a beneficiation plant 10 miles away.
At the beneficiation plant the sand and clay are removed from the phosphate through rotating drum screens or trommels, until all particles greater than 2 inches in diameter have been removed. After more breaking apart of particles, the goal is to produce a pebble product between 0.04 inches and .75 inches in diameter.
The sized sand and phosphate are then placed in a floatation cell into which a reagent is added. This reagent attaches itself to the phosphate. Air is bubbled through this solution. The reagent also attaches itself to the air bubbles so that the phosphate is on one end and the air bubble is on the other end before floating to the top of the tank, where the phosphate is separated and recovered.
The slurry of particles of that optimum size is then “de-slimed” by treatment with a hydrocyclone. This generates a waste clay slurry of about 3% solids of particles less then 0.004 inches and sand-sized particles between 0.04 and 0.004 inches that is the “floatation feed.”
In most Florida phosphate mines, over 100,000 gallons of water carrying tiny particles of clay are pumped every minute. This slurry is from 3%–5% solids when it is pumped to a pond. During settling, the clear top water is recycled for use in the beneficiation plant. Through research, methods have been developed to hasten the settling so that a top crust of 50%–60% solids forms in 3–5 years after the start of dewatering and reclamation. Beneath the crust, however, the clay is still puddinglike in consistency, limiting weight that can be place on the surface of the settling area. The phosphate then goes to the processing plant for conversion to DAP.
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| Trommel screens use water and force to capture the largest particles of phosphate and filter the smaller rock through the screen to the log washers. |
Sulfuric acid needed to convert the phosphate rock into phosphoric acid is also produced at the chemical processing plant using liquid (molten) sulfur. Since the energy crisis in the 1970s, most Florida phosphate companies capture the heat released in the burning of sulfur and production of sulfuric acid and use it to produce steam through cogeneration operations. The steam is used to produce the heat required to concentrate the phosphoric acid and also to produce electricity to run the plant. Plants produce most of the energy they need, as well as selling a portion to the area commercial energy provider.
When sulfuric acid is reacted with phosphate rock to produce phosphoric acid, calcium sulfate (gypsum) is also produced. This byproduct is called phosphogypsum. There are approximately 5 tons of phosphogypsum produced for every ton of phosphoric acid product produced. The phosphogypsum, however, contains a small amount of radioactivity, a result of the natural presence of radium in Florida’s phosphate rock. Because of this—as well as the presence of other impurities such as phosphate, fluorides, and heavy metals—the substance cannot be used for normal gypsum materials such as plaster, drywall, soil conditioner, or cement retarder. The USEPA has recently halted its use as a road base over worries that abandoned roadbeds, if ever used for future home sites, might prove hazardous. Because of this trace amount of radioactivity, a 1992 USEPA rule bans most uses of phosphogypsum. Since it cannot be used, the majority of all the 30 million tons produced annually in Florida is stockpiled in designated, sealed landfills.
The wet process manufacture of phosphoric acid, as practiced in Florida and many other parts of the world, requires a large volume of water. This water is commonly referred to as process water. It is used as a water source for the phosphoric acid, for gas scrubbing, to slurry the phosphogypsum produced and transport it to storage, to operate barometric condensers, and for a multitude of other uses in the chemical complex.
A major portion of the heat released in the process ends up in the process water and is lost to the atmosphere by evaporative cooling in ponds. Process water is stored both in ponds maintained on top of the phosphogypsum stacks, and in belowground ponds. These ponds provide the large surface area needed for evaporation and cooling of the water, sometimes covering more than a square mile.
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| Phosphogypsum stacks are lined at the bottom to prevent leachate runoff. |
If one of the operating plants is shut down, making it necessary to close the phosphogypsum stack and pond water system, the water in inventory must be treated before it can be discharged. The volume of water that would have to be treated could be as much as 2 to 3 billion gallons. The process water has a low pH of about 1 to 2 and contains a dilute mixture of phosphoric, sulfuric, and fluosilicic acids. It is saturated with calcium sulfate and contains numerous other ions found in the phosphate rock used as a raw material, as well as ammonia from the solid fertilizer manufacturing process.
“All of the phosphate plants are designed to run with a so-called ‘negative water balance,’ meaning they require no water to be discharged,” says Mike Lloyd, director of research programs and the director for chemical processing at the Florida Institute of Phosphate Research (FIPR). “But when you have large ponds covering extensive areas in a region prone to both hurricanes and occasional abnormally high rainfall, at times the ponds have more water than can be dealt with.” In cases such as years of excess water, the Florida Department of Environmental Protection may issue permits allowing the treatment of the pond water with lime as well as its discharge.
During a dry year, makeup or backup water for phosphate manufacturing comes from local groundwater sources. One particular plant in the Tampa Bay area has a large freshwater pond system as well as processing water. The freshwater ponds are there as a result of old mine cuts. During a drought, water is pumped from them. “But in large part, if they need makeup water it’s most likely going to come out of the ground,” says Lloyd.
When all is running smoothly with the wastewater streams for both the mining and manufacturing of phosphate, the water is being recycled; there is no discharge and there is no addition to the ponds. “These plants and operations all run as tightly as they can,” says Lloyd. “There is a clear advantage to having little freshwater going into their system, in effect going into that pond. Everything in the plant that might get spilled into the water is collected so that it never goes to the pond. They would always prefer to be running in a negative water balance and be adding water than to have too much water.”
Initially, during operations in the 1960s, the incentives for conserving water were scarce. “In those days there was little concern about water conservation anywhere,” says Lloyd. “They literally pumped all of their water out of the ground and had what might be termed ‘one pass’ water. This water went through the system and then went out through whatever river was nearby. Today, little or no water is pumped from the ground—unless there is a drought.
“Generally now they capture water in the mine cuts, save the rainwater for that very reason, and then if they have too much water, they discharge and they all have permits for discharging. The water that is discharged is good surface water. The only time they get into trouble is when they break a pipeline or when they are somehow pumping this clay slurry into the surface water system.”
Lloyd classifies the controls as more than adequate—until a mistake is made. Over the past 10 years there have been some spills from the ponds on top of the gypsum stacks. The Mulberry Phosphate Co. used a system where a pipe was placed approximately 15 feet below the top of the dam. The water was allowed to flow through that pipe and then down the side of the gypsum stack. “Of course, it is saturated with gypsum so it won’t dissolve anything,” says Lloyd. “It goes to the bottom, goes to a pond system where it is pumped back into the plant.
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| Phosphogypsum stacks can cover up to 400 acres and rise as high as 200 feet into the air. |
One Sunday morning the whole section that Mulberry workers had dug out to move the pipe had been washed out. In the process, over 50 million gallons of highly acidic water spilled into the Alafia River.
The Mulberry phosphate spill in 1997 resulted in a tremendous fish kill in the Alafia River, eventually affecting fish all the way to Tampa Bay. The fish kill was a result of a drop in the pH of the water bodies that the spill affected—not because the water in the spill itself was acidic. Freshwater fish were most severely affected as well as some of the saltwater species. “That failure was a human one,” says Lloyd. “It should never have happened. The rules were in fact changed after this.”
Fines for the spill were paid by the company’s insurance company. But by the time the fines were paid, Mulberry did not exist anymore. The company went bankrupt, walking away from the plant and leaving the state of Florida to pay for the cost of cleanup. The state is still paying the cost for curing the problems resulting from this spill and another at Piney Point.
FIPR is an organization funded by a portion of the money from the severance tax on phosphate. Its biggest challenge in working with phosphoric acid plants is reducing the volume of water involved or reducing its toxicity, meaning as well its acidity. FIPR was begun in 1979 with the goal of being beneficial to the industry, to the state, and to the people living in the vicinity of mining and manufacturing. Therefore, FIPR has an environmental interest in manufacturing and reclamation efforts. “We are looking for ways for phosphate mining and processing to have less environmental impact and to cost less,” says Lloyd.
FIPR has a committee that was formed a number of years ago at the request of the Department of Environmental Protection and the Phosphate Council. “We have been working to come up with better ways of treating the water,” says Lloyd. “This problem has been worked on for over 50 years now, so there is no simple solution to it. We have come up with some things that look like they are going to make a difference. But as with all things, we are in a preliminary stage of finding out if it’s practical to try to accomplish it. There are no quick fixes.”
Lloyd adds that there are new technologies present today that were not available in the past. The ultimate goal of wastewater efforts is the raising of the pH as well as completely eliminating the small quantities of dissolved sulfuric, phosphoric, fluorosilisic acids remaining in the water. “But by far our biggest problem is how our wastewater raises the pH of surrounding water in the event of a spill,” says Lloyd.
Another challenge for the phosphate industry is the fact that 40% of the area that is mined in Florida for phosphate is then used for clay-settling areas. The clay does not do a very good job of consolidating itself, according to Lloyd. It can be made for crops to be grown on it, and cattle can be run on it, but it cannot yet be made to reach the point where a house may be constructed on it. “It would be great if we could come up with a technique where we could mix the clay with the sand—or by itself—and be as good a land for homebuilding as what you had when you started mining,” says Lloyd. “But I feel that some of the things we developed about ten years ago are going to make a difference. We actually have a demonstration plant that is going to be running on it later this year. This will confirm whether the things we have learned in the laboratory will in fact work on a large scale.”
As long as 20 years ago, the mining of the phosphate rock involved some 30 feet of overburden and 15 feet of matrix, for a total of some 45 feet involved. Mining operations have moved south in the state of Florida as the phosphate rock declines in quality and increases in depth below the surface. At present, the overburden in Florida operations is closer to 50 feet, with 20 feet of matrix, for a total of 75 feet.
This presents a striking comparison to phosphate deposits in coastal North Carolina, which involve some 100 feet of overburden. All of these phosphate deposits are part of one vast deposit stretching from Gulf Coast Florida to northern Florida, coastal areas of Georgia, South Carolina and North Carolina all the way to the west bank of the Chesapeake Bay. It is only economically feasible to mine this huge deposit in certain areas, with much of it originally mined by hand.
Experts contend that phosphate deposits in Florida will last, at best, another 40 years. “When I first came to Florida in the 1960s they also said that there were only 40 years left of phosphate mining in the state,” says Lloyd. “They are getting closer to being correct, though. The material being mined now is decreasing in both quality and quantity from what it used to be. There are other phosphate deposits in the world. North Carolina contains a very large deposit and there is some being imported from Morocco. Morocco has perhaps the second-largest phosphate industry in the world. The western US contains phosphate deposits as well. These tend to be chunkier deposits, volcanic in origin, requiring more breakup than the pebbly Florida deposits.
“Phosphate rock itself is very plentiful. The economics come in, though, when you try to remove it. Peru has a spectacular phosphate deposit. The problem is that it is too far up in the Andes, with no railroads, so mining of this phosphate isn’t economically feasible. At some point though, even this phosphate will become a player in the world market.
“But the future for phosphate mining and processing in Florida and the US may involve the importation of phosphate after about 40 years.”
Pete Hildebrandt is a writer specializing in science and engineering topics.
OW - September/October 2006 |
