Quantification of Iron Oxidation States and Carbon in Direct Reduced and Hot Briquetted Iron by Full-Pattern X-ray Diffraction and Partial Least Squares Regression: Reduction Coordinate, Analyte Independence and Rietveld Benchmark

Direct reduced iron (DRI) and hot briquetted iron (HBI) are controlled by five quantities: total iron, metallic iron (Fe0), Fe2+, Fe3+ and carbon. Today these are measured with several wet-chemical and spectroscopic methods. We predict all five from one X-ray diffraction (XRD) pattern. The method uses partial least squares regression (PLSR) on the full pattern. We tested it on 79 industrial DRI/HBI samples with leave-one-out cross-validation. Reference chemistry and Rietveld phase analysis show that the samples vary along three axes. The first is the degree of reduction, where ferric oxides convert into metallic iron. It carries Fe0, Fe3+ and total iron. The second is carburization. The third is residual Fe2+, mainly in wüstite. PLSR predicts metallic iron, total iron and \Fe3+ with R² >= 0.975. It predicts Fe2+ and carbon with R² = 0.83 and 0.71. A residual test shows that Fe2+ and carbon are predicted independently of the degree of reduction. Fe3+ is not: its prediction mostly follows metallic iron. Rietveld refinement of the same patterns gives similar accuracy. It handles unusual samples better. The two routes therefore complement each other. We discuss what more data, and more diverse data, could add.