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

摘要

Phosphogypsum, i.e. calcium sulphate dihydrate is generated as a by-product in the phosphoricacid production during reaction between phosphate rock and sulphuric acid. It is stored as nonhazardouswaste in a disposal site.Since 1983, when the phosphoric acid plant started operation, about 8 140 000 t of phosphogypsumhave 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 makingembankments. So far, the first 6-meter high level has been built with a 1:3 side slope ofphosphogypsum embankments. Formation of the second level with 1:5 side slope is currently inprogress.Another byproduct of phosphoric acid production is fluoride- and phosphorus-polluted wastewater.Before being discharged into the natural recipient, this wastewater is treated with calciumhydroxide. The product of neutralisation is calcium fluoride (CaF2) which is deposited in separatelagoons as nonhazardous waste. The application of calcium fluoride as a substrate for plants in theprocess 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 newlagoon for calcium fluoride disposal. Regarding the environmental aspect, usage for vegetativecovering is far better than the standard process of calcium fluoride disposal because residualphosphorus from CaF2 is utilised for enhanced growth of the plants. At the same time, the necessityfor 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 indigobush (Amorpha fruticosa L.) displayed the best growth on the CaF2 substrate. These plant specieshave strong roots and can grow deeply and thickly through the surface layer of the substrate byforming a thick and compact vegetative cover.