Evaluation of Lime Treatment Sludge Alternative Disposal Methodologies Including Utilization in Closure of Phosphogypsum Stacks
A characterization of the engineering properties of lime sludges derived from doublelime treatment of phosphogypsum stack system process waters is presented along with an evaluation of lime treatment sludge production quantities and CaO utilization (i.e., lime requirements) as a function of chemical characteristics of the process waters. Utilization of lime-treatment sludges as an agronomic amendment in gypsum stack side slope final covers in lieu of dolomitic limestone was investigated. Greenhouse plant growth and irrigation studies were performed to determine whether various turfgrass species (bermudagrass, bahiagrass and seashore paspalum) can be successfully grown on leached and unleached phosphogypsum amended with either dolomitic limestone and/or different-type process water lime treatment sludges, and to evaluate performance of grass species grown in these media and in natural sandy soils when irrigated with fluids derived from lime treatment of process water.
Evaluations of conventional lime sludge disposal and alternative disposal methods for lime sludge and supernatant to minimize on-land disposal within sedimentation ponds and discharge of Stage II effluent to surface water are presented. Lime-treatment sludge could potentially be mixed and added to process water in a cooling pond system during closure. Some lime sludge constituents (including free lime present in the sludge) will dissolve and/or react with the acidic process water such that the quantities of lime needed for ultimate treatment of the process water are reduced, and the pond system could be simultaneously filled with sludge in preparation for closure construction and capping.
Co-disposal of lime-treatment sludge with phosphogypsum slurry atop an active phosphogypsum stack may be feasible when adding: (i) up to 2.5% of Stage I sludge to the gypsum (dry weight basis); and (ii) up to 1 percent Stage II sludge to the gypsum (dry weight basis), with the slurried mix discharged at initial solids contents on the order of 20-30%. Codisposal needs to be controlled so as not to adversely impact stack stability, and the handling, dewatering and compaction characteristics of the gypsum. If the lime sludge contains some free lime (which it often does), application rates of 1% and up to about 6% of Stage II sludge (dry weight basis) appear to be suitable for amending leached and unleached gypsum, respectively, prior to grassing a side slope as part of phosphogypsum stack closure.
Bermudagrass and bahiagrass grown in sandy soil can be irrigated with effluent from either pH 7 single-stage treatment or conventional Stage II double-lime treatment (diluted or undiluted). Bermudagrass can successfully grow in both leached or unleached phosphogypsum media, properly amended with dolomitic limestone or with single-stage (pH 7.5) or Stage II lime-treatment sludges, and irrigated with effluents from diluted single-stage and Stage II lime treatment of process water. Seashore paspalum turfgrass can grow reasonably well in sandy soil when irrigated even with the more acidic Stage I (pH 5) effluent.
Barring restrictions imposed by surface water runoff and groundwater quality requirements, recycling of effluents generated by lime-treatment of the process water to irrigate the grass cover on a closed phosphogypsum stack or grass pasture nearby appears to be technically viable. Spray irrigation would provide a substantial benefit in that reliance on valuable fresh water resources for dilution of the treated effluent for the sole purpose of achieving surface water discharge standards could be substantially reduced or eliminated.
If the land areas available for spray irrigation of turfgrass are limited and/or the quality of surface water runoff from the irrigated areas is of concern, water consumption by spray evaporation could be a viable option for disposing of lime-treatment effluents.