WEATHERING OF CLAY-MINERALS INDUCED BY FLUORIDE-CONTAINING SOLUTIONS FROM PHOSPHOGYPSUM BY-PRODUCT

The production of phosphate fertilizer from phosphate rock results in the formation of phosphogypsum (PG) and HF. Soluble F- ends up in proc ess water and in the pore space of the PG by-product. This study deter mined changes in the amount, properties and mineral composition of soi l clay upon reaction with acidic PG solutions. Calcareous and non calc areous soils were reacted with PG leachates (PG-L, 30 mg F- L(-1)) and process water (PG-PW, 6070 mg F- L(-1)) using a sequential batch equi libration method. Phosphogypsum-L did not alter the clay fraction as m uch as PG-PW, which dissolved as much as two-thirds of the clay fracti on. Phosphogypsum-PW reduced surface area of clay fractions from about 500 to 150 m(2) g(-1); Cation exchange capacity CEC was decreased fro m about 60 to 17 cmol, kg(-1) clay. Reduction in surface area and CEC in PG-PW treated soil corresponded to near complete dissolution of sme ctite and a negative enrichment of mica. Kaolinite in coarse clay of t he PG-PW treatment was reduced to 20% of the control and was completel y destroyed in fine clay fractions. Amorphous silica was produced from the F--induced dissolution of clay. The results indicate there is lik ely to be considerable alteration of minerals in subsoils beneath PG w aste repositories and of clay liners used at some storage facilities.