Application of Industrial Waste CaF2 for Vegetative Covering of Phosphogypsum Disposal Site

Phosphogypsum, i.e. calcium sulphate dihydrate is generated as a by-product in the phosphoric acid production during reaction between phosphate rock and sulphuric acid. It is stored as nonhazardous waste in a disposal site. Since 1983, when the phosphoric acid plant started operation, about 8 140 000 t of phosphogypsum have been disposed there. The disposal site consists of four separate ponds (compartments) which are bounded by 6 meter high embankments of earth. According to a special design, it is possible to build layers upon the existing disposal site using phosphogypsum for making embankments. So far, the first 6-meter high level has been built with a 1:3 side slope of phosphogypsum embankments. Formation of the second level with 1:5 side slope is currently in progress. Another byproduct of phosphoric acid production is fluoride- and phosphorus-polluted wastewater. Before being discharged into the natural recipient, this wastewater is treated with calcium hydroxide. The product of neutralisation is calcium fluoride (CaF2) which is deposited in separate lagoons as nonhazardous waste. The application of calcium fluoride as a substrate for plants in the process of vegetative covering of the phosphogypsum disposal site is a new method of its usage. This way, a significant financial benefit is achieved because it is not necessary to build a new lagoon for calcium fluoride disposal. Regarding the environmental aspect, usage for vegetative covering is far better than the standard process of calcium fluoride disposal because residual phosphorus from CaF2 is utilised for enhanced growth of the plants. At the same time, the necessity for natural soil covering of the disposal site is reduced by 500 000 m3. Apart from the natural grass species, alfalfa (Medicago sativa L.), Bermuda grass (Cynodon dactylon (L.) Pers.), grass mixtures with high content of red fescue (Festuca rubra L.) and false indigo bush (Amorpha fruticosa L.) displayed the best growth on the CaF2 substrate. These plant species have strong roots and can grow deeply and thickly through the surface layer of the substrate by forming a thick and compact vegetative cover.