Process Water

The wet process manufacture of phosphoric acid, as practiced in Florida and many other parts of the world, requires a large volume of water. The 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 a below-ground level pond (cooling pond). These ponds provide the large surface area needed for evaporation and cooling of the water.

In Florida, the average yearly rainfall and the evaporation rate are approximately equal and it is possible by strict control of the water inputs to the ponds to operate the chemical complex with a negative water balance. However, in a year where rainfall is significantly above average due to multiple tropical storms, El Niño weather effects, etc., it may become necessary to treat the surplus water and release it to the surface waters in order to avoid an uncontrolled discharge of the untreated process water.

If one of the operating plants is shut down and it is 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 need 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. (See table below “Typical Pond Water Analysis”)

Present treatment practice is to lime to a pH of approximately 4.5, remove the solids formed, lime the water to a pH of approximately 11, remove the solids formed, air strip the water to remove ammonia, and add acid to reduce the pH to approximately 6.5. While the treated water still has dissolved solids and conductivity above discharge standards, it can still be discharged to the surface waters under an emergency permit from the Florida Department of Environmental Protection. Any treatment procedure proposed should eliminate the dissolved solids problem.

Process water became a public topic of concern in Florida after the Florida Department of Environmental Protection (DEP) had to take on responsibility for three phosphogypsum stacks with full ponds when Mulberry Corporation (a chemical processing company) declared bankruptcy in January 2001. The company had two stacks in Mulberry in Polk County and one at Piney Point in Manatee County. A spill from the Mulberry stack would endanger the Alafia River and a spill from Piney Point would endanger Bishops Harbor, a prized estuary.

Other process water incidents include a 1997 spill into the Alafia River as the result of an improperly installed overflow pipe and two spills in late 2004 due to high winds and heavy rain associated with hurricanes that year.

Over the years, regulations on how phosphogypsum is stacked have strengthened to protect groundwater seepage. There are also strict standards companies must meet before they can release any of the process water into the environment, but human error and natural occurrences such as heavy rains can cause the acidic water to spill. Even if there is never another spill, however, there is still a need to handle the billions of gallons of water stored in and around a stack during the rainy season and when the stack is eventually closed.

Typical Pond Water Analysis

Parameter
Untreated
 Process 
Water
Lab pH
2.1
Conductivity (µmhos/cm)
22,100
Turbidity (NTU), 24 hours
Turbidity (NTU), 72 hours
Lab Turbidity (NTU)
0.9
Color (Pt/Co units)
300
 
 
Calcium, Ca (mg/l)
538
Magnesium, Mg (mg/l)
223
Sodium, Na (mg/l)
2260
Potassium, K (mg/l)
210
Iron, Fe (mg/l)
59
Manganese, Mn (mg/l)
15
 
 
Chloride, Cl (mg/l)
140
Fluoride, F (mg/l)
4120
Sulfate, SO4 (mg/l)
6200
 
 
Total Phosphorus, P (mg/l)
6600
Ammonia Nitrogen, N (mg/l)
1240
 
 
Acidity, CaCO3 (mg/l)
Alkalinity, CaCO3 (mg/l)
Solids, Total Dissolved (mg/l)
39,800
Solids, Total Suspended (mg/l)
22