Primary phases and natural weathering of old lead-zinc pyrometallurgical slag from Pribram, Czech Republic

Pyrometallurgical slag, produced 100-150 years ago from lead-zinc ores in the smelting region of Pribram, Czech Republic, contains elevated amounts of Zn and Pb. Knowledge of the distribution of these elements in the main phases and an investigation of natural weathering features represent the first step in the environmental assessment related to dumping of metallurgical slag. Optical microscopy, scanning electron microscopy (SEM/EDS), X-ray diffraction (XRD) and electron-beam microanalysis (EMPA) were used to identify the silicate, oxide, sulfide and metallic phases. The study focused on silicates and oxides, and the major constituents of the slag proved to be clinopyroxene, melilite, olivine, spinel and glass. A substantial a-mount of zinc, initially dissolved in the silicate melt, is held by zinc-rich end-members of the spinel (gahnite, up to 19.9 wt% ZnO) and silicates (e.g., hardystonite, up to 10.5 wt% ZnO). Lead, in contrast, behaves as an "incompatible element" and is likely to be concentrated in the residual matrix glass. Two distinct glasses were identified: (i) transparent (surface) glass from the quenched borders of slag fragments, and (ii) opaque (matrix) glass, which forms a black matrix in the center of the fragments. These glasses contain important amounts of lead and zinc, up to 3.72 wt% PbO and 9.80 wt% ZnO, partly in the form of droplets (generally <1 <mu>m) composed of galena, metallic lead, ZnS and other sulfide and metallic phases and alloys. Two features attributed to natural weathering, found mainly on the surface of chilled-glass borders, were identified in the slag: (i) deposition of a Fe- and Pb-rich veneer, and (ii) selective leaching of superficial glass revealing, with respect to unaltered glass, the mobilization of Ca, Fe, Na, K, and Zn.