Clay Ponds: Can Technology Increase Their Value?

Clay is a word that surfaces routinely these days in discussions about phosphate mining.

Why?

A typical Florida phosphate mining operation pumps more than 100,000 gallons of water carrying tiny particles of clay every minute. This is known as clay slurry that is slightly viscous (less than 5% solids). This clay is part of the matrix that is mined. That matrix contains clay, sand and fluorapatite, a phosphate mineral. To do anything productive with the clay that is separated from the sand and the phosphate during beneficiation, the water content must be significantly lowered.

Dewatering and consolidating the clay, which retains water, is easier said than done.

Research and technology has hastened the rate at which the clay dries in the settling ponds where it is stored. Currently a crust that is 50-60% solids will form in three to five years. A few feet below the crust, however, the clay is pudding-like for many more years, which limits the amount of weight the settling area can support. About 30-40% of the land that is mined for phosphate is left behind in clay settling areas.

Clay has been a subject of FIPR research since the Institute was founded in 1978. Much of what is known about phosphatic clay and the settling ponds result from FIPR research projects. In light of current industry and public interest to find a better way to handle the clay that phosphate mining leaves behind, FIPR has gathered county officials, researchers, phosphate industry experts and others to discuss what is known and what needs to be studied to find ways to decrease the size of clay settling areas and use the clay.

These technical workshop discussions have resulted in the state legislature providing FIPR with special funding in 2003 to test the possibility of using a process FIPR developed and patented in 1993 to speed up the time it takes clay to thicken. The process is known as the FIPR-DIPR, an acronym for Dewatering Instantaneously with Pulp Recycle. In short, the process treats phosphatic clay slurry with a flocculant and a fibrous material, such as recycled newspaper, to achieve rapid dewatering.

FIPR-DIPR worked in a pilot plant but the process was never adopted by the phosphate industry. It was deemed too expensive at the time. While logistics and economics are still the mitigating factors for new technologies, the public and corporate costs attached to phosphatic clay today have renewed the interest in the FIPR-DIPR as a way to thicken the clay.

FIPR’s special $800,000 appropriation fron the Florida Legislature to continue developing the FIPR-DIPR may change its economic feasibility. After several months of working with a special FIPR steering committee, the researcher who developed the FIPR-DIPR process received a grant to continue its development. Dr. Hassan El-Shall, Associate professor of Materials Science and Engineering and Associate Director of Research for the Particle Engineering Research Center at the University of Florida, is testing different kinds of flocculants and fibrous materials such as recycled newspaper pulp and yard waste. The goal is to find a low-cost flocculant and a fibrous material that is plentiful, easy to acquire and inexpensive.

In a recent project update, he said that he had found a source of paper pulp at the Polk County landfill that could meet the fiber needs for 3,000,000 tons of clay (one plant’s yearly clay production). It is a mixed paper that cannot be recycled and the landfill would be happy to give it away if it didn’t cost anymore than burying it, Dr. El-Shall said. Yard waste, he said, also looks promising, but must be tested on a larger scale.

Researchers are running a small, 100 gallons/minute, lab test and they hope to move to a larger pilot plant by the end of the year that would process 1,100 gallons of slurry per minute.

“It moves so fast it is hard to make adjustments with small off-site batch tests,” he said. To add flocculant and fiber and run the mix through a hydrocyclone to spin out the water takes about 30 seconds, he said, adding that in that time the FIPR-DIPR process recovers 80% of the water.

If the larger plant is successful, then researchers will dump the thickened clay into a pit to estimate how long it will take the mixture to consolidate to a thickness that will make the clay useable for different purposes, including construction.

Currently the clay and the settling areas that can cover up to 800 acres have limited use.

Settling areas can be developed if support systems such as lime columns are used. Such support needs, however, increase the cost of developing the land. A ten-year study FIPR conducted in conjunction with the Polk County Cooperative Extension Service showed clay settling areas provide fertile farmland that produces high yields of most crops. They likely won’t be used much for farming, however, until traditional farmland is in short demand because rains can make the settling areas almost impossible to farm without equipment designed for slick, muddy, soft fields.

As phosphate companies seek to permit new mines in Hardee, Manatee and DeSoto counties the public has voiced much concern about the amount of land that will be left behind in future clay settling areas. At the same time, Hardee County residents and public officials have raised questions about what can be developed on the land in their county that has already been mined and reclaimed with a soil that is a mix of sand and clay.

Phosphate companies continuously look for new ways to handle the clay that mining leaves behind said John Keating, a process engineer with IMC (now Mosaic), who studied the feasibility of the FIPR-DIPR process when it was first developed.

“It had potential,” he said. “We cannot afford not to investigate technology that has potential.”

The investigation, however, led Keating and others to the conclusion that putting the FIPR-DIPR process to use at the time would be a logistical nightmare and much more expensive than using settling ponds. How much clay a plant produces varies with the size of the plant. Keating was looking at one of the larger plants which was producing 1000 tons of clay per hour.

“It would have taken a tractor trailer load of newsprint every couple of hours to have made it work,” he said, adding that the industry would have destroyed the used newsprint market at that time.

Keating also looked at putting the FIPR-DIPR clay product through a belt filter press and said it worked well. The clay came off at 45% solids.

“It looked like a piece of damp cardboard,” he said. It would have, however, taken 8 football fields of belt filters to process the amount of clay his plant produced. He added that the price of the flocculant ran in excess of $1 per pound at the time, in the early 1990’s, further escalating the cost.

“The logistics and economics did not work,” he said. “Logistics are critical when you deal with the volumes we deal with at low percent solids.”

A decade later new polymers, using a hydrocyclone instead of belt filters and researching a variety of fiber options, may make the FIPR-DIPR process more appealing, especially when mine sites to the south have more clay to deal with and less public support. Building settling pond dams is more expensive due to regulations to make them less likely to breach and spill. There are also reclamation costs and a growing public dissatisfaction with the amount of land mining leaves behind as clay settling areas with limited high-income use due to low load bearing capabilities.

In the end, however, the FIPR-DIPR process or any other new technology will have to compete with the current industry cost to deal with the clay.

December 2004