Introduction

For every tonne of phosphoric acid as P2O5 produced using the wet process, from 4.5 to more than 5.5 tonnes of phosphogypsum are generated, depending on the quality of the phosphate rock. It is estimated that more than 22 million tonnes of phosphoric acid (as P2O5) are produced annually worldwide*, generating in excess of 110 million tonnes of gypsum by-product. The free water in the gypsum cake off the filters is highly acidic, having a pH as low as 1.0. While commercial uses, in agriculture and in manufacturing gypsum board and Portland cement, consume less than a few percent of this by-product, the vast majority is disposed of on land in gypsum stacks or is discharged into water bodies.

Properties of Phosphogypsum

Physical Properties

Depending on the reaction temperature used to produce phosphoric acid, calcium sulfate in either the dihydrate (CaSO4.2H20) or the hemihydrate (CaSO4.½H20) form is generated as a by-product filter cake. The gypsum cake, after filtration, usually has a free moisture content between 25 and 30 percent. Hemihydrate, in the presence of free water will, fairly rapidly, convert to dihydrate (gypsum) and in the process, if left undisturbed, will set up into a relatively hard cemented mass.

Dihydrate consists of relatively soft, principally silt-size (<0.075mm) aggregates of crystals, the morphology of which depends on the source of the phosphate rock and the reactor conditions.

Typical engineering properties of phosphogypsum can be found in Wissa , . Properties, such as density, strength, compressibility and permeability (hydraulic conductivity), are not only controlled by the rock source and reaction process, but also by the method of deposition, age, location and depth within the landfill or stack in which the gypsum is placed. The deeper the gypsum is within a stack and the older the stack, the higher its density and strength and the lower its compressibility and permeability, provided solution channels and cavities have not developed in the stack as a result of rainfall infiltration**.

Chemical Properties

Phosphogypsum consists primarily of calcium sulphate dihydrate with small amounts of silica, usually as quartz, and unreacted phosphate rock. Radium and uranium, as well as minor amounts of USEPA toxic metals, namely, arsenic, barium, cadmium, chromium, lead, mercury, selenium and silver and phytotoxic fluoride and aluminum are also present in phosphogypsum and its pore water. The concentrations of the heavy metals and radionuclides depend on the composition of the phosphate rock feed. The concentrations of constituents in phosphate rock, phosphogypsum and process water ( i.e., free water in gypsum stacks and cooling ponds) from Central Florida have been reported by Garlanger .

Environmental concerns, more perceived than real, have developed in the last ten years because of the presence of the trace toxic metals and radionuclides in phosphogypsum and its pore water. As a result, its usage, transportation and storage, as well as that of the associated ponds which contain process water at pH values below 2, are now in the U.S.A. regulated by state agencies such as the Florida Department of Environmental Protection (FDEP) and/or the U.S. Environmental Protection Agency (USEPA) .

Disposal Options

Worldwide, four methods are being used by the phosphate industry to dispose of surplus phosphogypsum, namely: (i) discharging to water bodies; (ii) backfilling in mine pits; (iii) dry stacking; and (iv) wet stacking.

The term "dry stacking" (as it applies to conveying or hauling and stacking) means that the gypsum is transported and stacked at the same moisture content as when it discharges from the filter. The term is somewhat misleading since the actual moisture content is typically between 20 and 25%*. "Wet stacking" involves pulping the cake with additional water to form a slurry that can be pumped or hydraulically transported to a disposal area. For the sake of completeness, all four of the methods are described in the following sections even though only environmental aspects of the dry and wet stacking options will be addressed.

Discharging to Water Bodies

A relatively small number of phosphoric acid chemical plants discharge their phosphogypsum directly to a receiving water body. These include major producers in Morocco, where the gypsum cake is slurried with sea water and discharged into the Atlantic Ocean. While discharging waste gypsum as a dilute slurry into very large open water bodies with strong currents and tides off uninhabited coasts is the most economical disposal option, and may be politically and environmentally acceptable, discharging gypsum into rivers and smaller bodies of water, such as the Mediterranean Sea, is no longer politically acceptable.

Most existing phosphoric acid plants, and those currently under construction or in the planning stage, are not located near suitable water bodies to allow for disposal of their gypsum by-product in the sea, and disposal on land is the only economically viable option. Therefore, this paper will be limited to the environmental aspects of land disposal.

Gypsum-Tailings Blending

The pits created from mining phosphates at PCS Phosphates in North Carolina, USA, are being reclaimed by backfilling with a blend of phosphatic clay tailing from the rock beneficiation process and phosphogypsum obtained directly off the phosphoric acid plant filters. This is environmentally acceptable because the high calcareous content of the clay in the ore at that mine neutralizes the acid remaining in the gypsum cake after filtration. PCS Phosphates has a unique set of hydrogeologic conditions that are not likely to be present anywhere else.

Phosphogypsum, because of its relatively high solubility, high compressibility and radioactivity, is generally not recommended as structural fill for land reclamation, especially if the land will be used for housing.

Wet Stacking

Wet stacking is by far the most popular phosphogypsum land disposal method. This method involves pulping the gypsum cake as it comes off the filter with either fresh or sea water and pumping the slurry through a pipeline to a gypsum disposal area where the solids are allowed to settle out (Figure 1). Clarified excess water is decanted, and in the case of a fresh water system, pumped back to the plant for re-slurrying gypsum and reuse in the process. With time, the recirculated process water usually reaches an equilibrium pH between 1.3 and 2.0, depending on the process and climatic conditions. The soluble P2O5 content of the acidic process water is typically between 10,000 and 15,000 mg/liter (mg/l), but can be as high as 20,000 mg/l.

When sea water is used to slurry the gypsum, the clarified sea water is not returned to the plant for reuse, but discharged back to the sea. When sea water is the carrier, the slurry is usually very dilute (about 5% solids by weight), and the pH of the water being discharged is above 2.0. Several plants, namely in Greece, Spain, Mexico and Tunisia, have considered or are considering eliminating the use of sea water and converting to closed-circuit fresh water systems. Slurry concentrations between 20% and 30% by weight are commonly used in closed-circuit water systems. It should be noted that when a gypsum stack is converted from a sea water to a fresh water system, unless special provisions are taken to reduce contamination, the return water to the plant from the stack will be, at least initially, high in chlorides due to leaching of the old gypsum. This can cause corrosion problems.

The gypsum stack is typically operated using elevated rim ditching (Figure 2) and/or spigots (Figure 3) and the upstream method of construction to raise the perimeter starter dikes with gypsum1. Draglines are being phased out and are being replaced with backhoes and hydraulic excavators to construct and raise the gypsum starter dikes and rim ditches, as shown in Figure 4. A small dozer is needed, usually part-time, to level the gypsum excavated from the rim ditches to raise the starter dikes.
Where fresh water is available at a reasonable cost, a wet stack is much easier and more reliable and economical to operate than a dry stack. It also has the additional economic benefit of improving process P2O5 recovery. Unlike dry stacking, wet stacking does not involve construction around the clock and can be operated with much less equipment and personnel. Dusting is generally not a problem because construction traffic is limited to the stack access road and perimeter and interior partition dikes which cover relatively limited and defined areas that can easily be kept damp.

Most plants that produce dihydrate gypsum use the wet stacking disposal method. Moreover, several plants, including the Swift Creek Plant of PCS Fertilizer in Florida and the PCS Nitrogen Plant in Louisiana, that produce hemihydrate, also use wet stacking. At these plants, conversion to dihydrate is not complete by the time the slurry reaches the stack; nevertheless, the stacks are managed and raised using the same methods and equipment used for gypsum from dihydrate plants.