Phosphogypsum Disposal  and  The Environment  (Page 4 of 4)

With closed-circuit wet stacking, all rain falling on top of the stack can be contained in the gypsum slurry settling ponds provided that the gypsum starter dikes around the ponds have sufficient freeboard. A ditch or channel usually surrounds the stack and retains rainfall runoff contaminated from contact with non-vegetated gypsum side slopes and any lateral seepage at the toe of the stack. In wet climates, excess water in the ditch or channel, which may also serve as a process water cooling pond, is either returned to the chemical plant as process makeup water, pumped up to the top of the stack for storage, and eventually either consumed or discharged to receiving water bodies after appropriate treatment.

Treatment of excess process water, not stored and/or consumed in the process, usually consists of two-stage lime neutralization and clarification. This treatment process is expensive and requires clarification ponds in which the lime sludges are stored. The treated water is also high in dissolved solids and sulphate and sometimes ammonia, where nitrogen-based fertilizers are manufactured and not air stripped. For non-contact water that is not highly impacted, reverse osmosis can be used in lieu of liming.

The quantity of excess process water needing to be treated and discharged can usually be reduced, and possibly even eliminated, by having adequate surge capacity and by the way the gypsum stack and chemical plant are operated16. For example, amending and vegetating the bare gypsum stack slopes, can, with time, prevent rainfall from coming into contact and getting contaminated by the gypsum, thus enabling side slope runoff to be segregated and discharged off-site without treatment. Florida recently expanded its rules, which became effective in July 1999, requiring all phosphoric acid producers in the State to annually prepare and submit to the FDEP process water balances and operating and water management plans for their phosphogypsum disposal facilities.

With dry stacking, runoff from the stack can be collected on-site and consumed in the chemical process or used to wet trafficked areas to abate dusting.

Gypsum Stack Closure

When the storage capacity of a gypsum stack is exhausted or when a phosphoric acid chemical plant is permanently closed, seepage out of the stack and rainfall runoff from the slopes have to be contained on-site and/or treated before discharge. This will continue as long as water infiltrates the stack and/or until the water in the pores of the gypsum in the stack has drained out. In addition, with time, rainfall seeping into the stack will produce a network of solution cavities that could make it more expensive to cap the top of the stack at a later date.

Regrading and vegetating the side slopes, after amending the surface with dolomitic limestone if needed, may often be sufficient to prevent rainfall (and snow melt) from seeping into the slopes and allow for slope runoff to be discharged off-site without treatment. Under favorable climatic conditions and hydrogeological settings, regrading the top of a gypsum stack to promote surface runoff in combination with good vegetation and/ or a relatively impervious clay cover may be sufficient to prevent rainfall infiltration. In other situations, an impervious geosynthetic liner may be needed.

Once closure has been completed (Figure 11) and infiltration into the stack has been eliminated, the stack will continue to drain until it stops settling due to self-weight consolidation and creep, and until the moisture in the gypsum pores reaches field capacity. The time required for a closed stack to stop draining depends on the dimensions and age of the stack, the properties of the gypsum and the hydrogeological setting17. A stack on a porous foundation will drain much more rapidly than one constructed atop a low permeability clay stratum.