REGIONAL COMPARISON OF THE COMPOSITION OF CHARCOAL BLAST FURNACE SLAG AND PROSPECTIVE IRON ORE IN PENNSYLVANIA

In early 19^th-century Pennsylvania cast iron was produced by hundreds of charcoal blast furnaces. Furnaces were built near sources of iron ore, limestone flux, water power, and forests to provide charcoal for fuel. Slag, the waste product of smelted iron, often serves as the only remnant of this once active industry. The purpose of this study was to determine if properties found in slag materials at these furnace sites, especially the mineralogical and chemical signatures, reflect those found in samples of prospective, locally-collected iron ores. X-ray diffraction and X-ray fluorescence were used to analyze 100 different samples of slag from 36 furnace sites and 23 prospective iron ores. Furnace slag and ore samples were collected from the northwestern (C1), central (C2) and southeastern (C3) regions of the state. Mineralogical results showed that the predominant iron phases associated with collected ore samples from each cluster were siderite (C1), hematite (C2) and magnetite (C3), consistent with the regional geology. Minerals were reduced to elemental iron during the smelting process, leaving SiO₂ and CaCO₃ as the predominant mineral forms found in slag. Although, mineralogical analysis alone appears insufficient to predict the relationship between a given slag and collected ore samples, regional trends in some mineral species and chemical constituents were observed. Al₂O₃ concentrations decreased and Zr increased between C1 and C3; the concentrations of Fe₂O₃ and MgO were higher for C1 and C2, whereas Fe₂O₃ decreased and Mg increased in C3; and TiO₂ and V concentrations were higher in C2 than the other two regions. Such differences appear related to regional ore composition and furnace smelting efficiency. The refractory and desulfurization indices were calculated for each slag sample to determine furnace efficiency. The comparison of relict slag and ore samples for a given furnace is complicated by the fact that slag represents a single smelting operation, yet many different ore types may have been used at one furnace during its lifetime. Further investigation into the distribution of trace elements and their relative abundance in regional ore deposits and furnace slag is proposed.