In a closed loop recirculation system, any soluble material introduced with the gypsum or with the makeup water will increase in concentrations until equilibrium is reached whereby the material is removed from the system at the same rate it is introduced. In many systems, this means that a purge stream is required to prevent the concentrations from exceeding specified operating levels. In the case of wet stacking, purging of the cooling tower or cooling pond circulation water is unnecessary because, inherently, it continuously occurs by water being held in the pores of the gypsum as it is added to the stack.

Dry Stacking

Worldwide, dry stacking is being used much less frequently than wet stacking for phosphogypsum land disposal. In dry stacking, the gypsum off the filter is transported to the disposal area without adding water to the gypsum cake. Typically, the gypsum, which after filtration has a free moisture content of about 25% by dry weight, is transported to the disposal area by trucks or a belt conveyor. At the disposal site, dozers, with or without a mechanical stacker (Figure 5) and movable lateral conveyors (Figure 6), are used to spread the gypsum. In Jordan, disposal by dry stacking is being used at Aqaba for dihydrate and at Eshidiya for hemihydrate. It is also used in Tunisia, Senegal and several former USSR countries.

Transport Options

Two basic options, each with subsets, are used to convey the gypsum from the loading station at the phosphoric acid plant filters to the disposal site, namely, "dry" transportation and hydraulic or "wet" transportation. As with dry stacking, the term "dry transportation", as it applies to conveying or hauling, means that the gypsum is transported to the disposal site at the same moisture content as when it discharges from the filter. Wet or hydraulic transportation involves pumping through a pipeline using either fresh or sea water to pulp the gypsum cake.

Dry Transportation

Dry transportation can be achieved by one or a combination of the following methods: (i) mechanical conveyors; (ii) trucks; (iii) railroad cars; and (iv) barges and bottom dumping scows.

Mechanical Conveyors

At the loading station, the de-watered gypsum cake off the belt filters is transferred onto a belt conveyor transport system to the disposal site (Figure 7). Upon reaching the disposal site, the gypsum is spread and stacked dry using mechanical stackers or movable belt conveyors and dozers.

Trucking

For trucking, the de-watered gypsum cake off the filters is transferred onto a short belt conveyor. Loading the stacking trucks from the conveyor can be accomplished by several methods. Some alternatives may include feeding one or two bins which unload to the trucks, feeding a reversing cross conveyor which feeds the trucks, or loading the trucks from a diverter chute at the discharge of the conveyor. Truck drivers can control the changes in direction such that when one truck is full the driver actuates a control switch which reverses the conveyor to begin filling the next truck (Figure 8).

Railroad

If the phosphate rock is transported to the plant by rail from an open pit mine, only a portion of the gypsum generated can be returned to the mine and used to backfill the pits, because the volume of gypsum generated exceeds the volume of the phosphate rock feed. This would only be feasible if the hydrogeological conditions at the mine were such that the gypsum fill would not cause groundwater contamination or if measures are taken to prevent contamination. Loading the railroad car can be achieved in the same fashion as with trucks. To prevent spillage, bottom emptying rail cars should not be used as they complicate unloading and increase the cost.

Barging

If the chemical plant has access to a navigable waterway in which the phosphogypsum cannot be directly discharged, bottom dumping scows can be used to transport the gypsum to the open ocean for disposal. At the loading station, the de-watered gypsum cake off the belt filters is transferred onto a belt conveyor transport system, similar to that used with trucks, that discharges directly into the scows.

Wet Transportation

Wet or hydraulic transport is by far the most widely used alternative worldwide. It is used with wet disposal on land and with direct disposal in water bodies. The gypsum cake, directly off the filters, is pulped with either the process wash water or sea water to produce a slurry containing 5 to 30% gypsum. The gypsum slurry is usually gravity conveyed through a launder or pumped to an agitated surge tank from which it is pumped through a rubber-lined steel or HDPE pipeline to the on-land disposal site or to the receiving water body. With on-land disposal, the wet stacking methodology is then used to develop and operate the gypsum stack.

Two gypsum slurry pipelines (one being a spare) and one return water pipeline (or return water ditch) are usually used. To limit the hydraulic pressure in the slurry lines to about 10 bar, booster pump stations may be located at several locations along the pipeline route and one near the base of the gypsum stack to deliver the slurry to the top of the stack.

Air Emissions

Particulate Matter

When phosphogypsum cake is left undisturbed to dry, a thin fragile crust forms on its surface that is resistant to wind erosion. However, when the dried surface is mechanically disturbed by vehicle traffic and/or wind erosion, fugitive emissions of respirable particulate matter, i.e., particulate matter less than 10 micrometers in diameter or PM-10, can be generated from gypsum stacks and dried out spillage off conveyor belt systems, trucks or railroad cars.