Phosphate mining sites don't necessarily need to be reclaimed with clay settling areas, say two retired mining engineers who have researched alternatives.
If their ideas are feasible in Central Florida, that would be good news for Charlotte County, which fears the clay settling areas disrupt the ability of ground water to seep into the Peace River, and Hardee County, which is concerned that clay settling areas will impede economic growth.
Clay settling areas are impoundments for waste clay slimes that are a byproduct of phosphate ore processing. They take up to 40 percent of mine sites -- and Hardee County is faced with proposals for 35,000 acres in new mines.
Mike Dibble of Michigan, a retired metals engineer who worked for IMC-Agrico (now IMC Phosphates) until the early 1990s, said he pioneered a phosphate mining method in the early 1980s called "bore hole mining."
The method calls for a pipe mounted with a drill bit to be drilled as much as 300 feet down to a phosphate ore deposit. The apparatus then uses a rotating drill bit and jets of air and water to chew out the mix of phosphate, clay and sand and pump it to the surface.
The cavity is later filled with waste clay slimes, a byproduct of ore purification processes.
That eliminates the need for clay settling areas and the land surface remains virtually unchanged.
Also, Jack Hird of North Carolina, a retired mine engineer for Texas Gulf Mining, pioneered a method in which the clay and sand left over from ore processing is mixed with phosphogypsum, a byproduct of fertilizer production, to create what he called "a good soil."
Phospho-gypsum is a slightly radioactive waste left over after phosphate is turned into phosphoric acid for fertilizer. Florida stores the material in giant stacks totaling 1 billion tons.
The mix is then used to fill in the mine pits.
About 40 percent of the area to be mined in Central Florida will be reclaimed as clay settling areas.
That troubles Charlotte County leaders, who fear the nearly impervious clay settling areas will increase stormwater runoff and block the underground movement of water to the Peace River and Horse Creek.
The bore hole method was also "shown to be both technically and economically feasible ... with only a negligible effect on the site environment," according to the former U.S. Bureau of Mining, which in 1981 simultaneously studied bore hole mining in Florida.
Despite the successful tests, IMC-Agrico abandoned its plans to use the method on the Northeast Florida phosphate reserve, which was located near St. Augustine.
"The method proved profitable in tests at a large phosphate reserve in Northeast Florida," said Dibble. "But the Northeast Florida people were just absolutely not going to have anything to do with mining no matter what the impact to the resource."
Dibble said the feasibility of bore hole mining in Central Florida will depend upon whether the geology provides a strong "rock cap" over the ore zone, such as a layer of limestone or hardpan. That's to keep the cavity underground from caving in before waste clay can be injected back to where it came from.
Stephen Partney, a Florida Department of Environmental Protection mining bureau administrator, said there is no limestone cap in Central Florida and what hardpan exists is "weak." So he doubts the method would be feasible in the area.
Earlier this year, Dibble conducted a demonstration project he believes could reduce the amount of space needed for clay settling areas. However, the primary purpose of the study was to find a way to reduce the amount of energy needed to transport ore from the strip mines to the processing plants.
Funded by the Florida Institute of Phosphate Research, the project called for a conveyor-like rail-car system to be built at strip mines. That would allow the ore to be transported to the plant in a dry condition.
Traditionally, Florida phosphate mines blast the ore into a wet slurry with a water cannon and pump the material by pipeline. That breaks the clay particles to very small sizes. Water clings to the particles, increasing the volume of clay slime that later must be deposited in clay settling areas, according to Dibble.
If the clay was less diluted, it would remain in larger particles through processing, Dibble believes.
Partney, however, said more research is needed to determine whether the conveyor transport system will reduce the need to dilute clay. It still must be diluted in order for chemicals in the processing to work, he said.
Partney also pointed out that North Carolina's water table is far below the groundwater level of Florida. So, the risks of aquifer contamination may be lower at PSC's North Carolina mine.
Partney said Florida first required mining companies to store clay wastes on their own properties in 1930. Before that, the miners merely "washed the clays into rivers" to dispose of them, he said.
Both Dibble and Hird feel the phosphate industry in Florida should find a better solution to clay settling ponds and gyp stacks for the storage of waste products.
"I'm 67 years old and I've wanted to do something good for mining and I hate mine inefficiencies," said Dibble.
"The problem is: Mining companies are like all companies," said Hird. "They want to make maximum profit. To do that, you spend as little as you can. The way to do that is cut back on reclamation.
"I want to say: Look fellows, you don't own the world," he added. "Your property is part of the United States, part of the state of Florida and part of Hardee County."
You can e-mail Greg Martin at gmartin@sun-herald.com
By GREG MARTIN
Staff Writer