ALTERNATION OF GEOMETRICAL AND FRACTAL DIMENSIONS OF PHOSPHATE ORE PARTICLES DURING GRINDING

A phosphate ore from the Drymona-Epirus area (northwest Greece) was gr ound and sieved(<40, 40-63, 63-125, 125-250, 250-500, and 500-1000 mu m). The particle fractions obtained were examined for their structure by XRD and for their specific surface area by N-2 adsorption (BET). Ch emical analysis and SEM photography were also performed. Crystallograp hically, the sieved particles consist mainly of apatite and quartz. Th e size of the crystallites of those two components, as determined via the Sherrer relationship, increases as the size of the particles decre ases, from 72 and 66 nm, respectively (500- to 1000-mu m fraction), to 99 and 80 nm (125- to 250-mu m fraction). Then between this and the n ext fraction (63-125 mu m), the sizes of the crystallites drop almost to the original values and thereafter increase again to 82 and 79 nm, respectively, in the last fraction (<40 mu m). The surface fractal dim ension D of the particles as determined from surface area measurements via N-2 adsorption (BET) is D = 2.00 +/- 0.01 for particles with diam eter d greater than or equal to 200 mu m and changes to D = 3.00 +/- 0 .01 for particles with d less than or equal to 200 mu m. This surface dimensionality remains unaltered when the samples are heated to 400 de grees C. The alteration of surface dimensionality and the abrupt drop in the crystallite size at d approximate to 200 mu m are accompanied b y some profound changes in the chemical composition of the particles. Thus larger particles appear rich in P2O5 and poor in M(2)O(3) (M = Al , Fe) and vice versa. SEM photography indicates a flake-like structure for larger particles while the smaller ones appear more robust. These results agree with the assumption that the larger phosphate ore parti cles possess a layered-type structure down almost to 200 mu m, while s maller particles appear invariant in three dimensions. (C) 1995 Academ ic Press, Inc.