Effectiveness of Secondary Liners in Reducing Phosphogypsum Stack Post-Closure Liabilities
Active phosphogypsum stacks are almost entirely saturated with acidic process water. In fact, approximately half of the pore water entrained within the gypsum stack is in “temporary storage” and will eventually drain once the stack is closed. Closure of a typical phosphogypsum stack requires consumption and/or management and treatment of approximately 4 billion gallons of drainable pore water. The objective of this research study was to evaluate the potential effectiveness of secondary liners in reducing costs and environmental liabilities related to treatment of post-closure phosphogypsum stack pore water drainage. In particular, a secondary liner could potentially enable the plant to consume drainable pore water from the lower “capped” portion of the stack during active operation of the upper stack above the secondary liner. Preliminary estimates had suggested that the reduction in process water treatment costs, via partial consumption of the stack pore water during plant operation, and the corresponding savings exceeded construction costs associated with incorporating a secondary liner at mid-height of the stack.
The use of secondary liners, installed when a gypsum stack achieves a height greater than mid-height of the stack and preferably when the stack rises to a higher elevation corresponding to two-thirds the design height, was determined to be a planning tool or a design feature that can effectively reduce costs and environmental liabilities at terminal closure of a given facility. In order for this design feature to be cost-effective, the plant must have a negative water balance and must be capable of consuming pore water that drains from the lower “capped” portion of the stack prior to terminal closure, i.e., prior to plant shut down.
Use of a secondary liner will increase the useful life of a stack by promoting consolidation of the in situ gypsum and will also provide an opportunity for the plant to recover P2O5 that would otherwise be lost. The quantity of post-closure drainable pore water requiring treatment will also be significantly reduced as a result. In order for the intermediate liner design feature to be cost-effective, the plant must be capable of consuming pore water that drains from beneath the secondary liner prior to terminal closure. The secondary liner, if installed when a gypsum stack rises to two-thirds the design height, is expected to yield significant savings in post-closure liabilities at unit treatment costs in the range of $15 to $25 per thousand gallons of acidic process water. At these unit rates, use of such a secondary liner on a typical stack is likely to net financial liability savings on the order of 20 to 32 million dollars (in 2006 dollars) as a direct result of the reduced quantity of process water requiring treatment after closure.
Considering P2O5 recovery benefits in the range of $150 to $175 per ton with 1 to 2% P2O5 in the pore water consumed prior to terminal closure, and using typical costs for liner system installation, cost/benefit analyses indicate that an intermediate liner installed at two-thirds the design height of the stack is likely to net cost savings on the order of 15 to 40 million dollars. A breakeven point wherein the intermediate liner will no longer be cost-effective would occur if emerging technologies lead to a reduction in the cost of treating process water to low-end range of $5 to 7 per thousand gallons.