Effects of CaCO₃ as Additive on Coal-based Reduction of High-phosphorus Oolitic Hematite Ore

Abstract

Recently, additives such as CaCO₃ and Na₂CO₃ have been proven to be capable of enhancing the reduction of high-phosphorus oolitic hematite ore and obtaining high-grade direct reduction iron (DRI) with low-P by magnetic separation. In this study, the mechanisms of adding CaCO₃ during direct reduction were further studied by gas analyzer, X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that CaCO₃ can inhibit the reduction of fluorapatite, and more P still remained as fluorapatite which can be removed by magnetic separation. In addition, CaO and CO₂ decomposed from CaCO₃ promoted reduction atmosphere, but the contribution of CO₂ to reduction of iron oxides was very limited, in fact CaO played a central role. An appropriate CaCO₃ dosage can enhance reduction of oolitic hematite. Addition of CaCO₃ led to the consumption of quartz and inhibited the generation of fayalite, which reduced FeO content in the slag, and as a result more FeO was reduced to metallic iron. A DRI with 94.51 mass% Fe, and 0.17 mass% P can be obtained at a iron recovery of 89.3 mass% by adding 10 mass% CaCO₃. However, excessive CaCO₃ reacted with gangue and FeO to generate a large amount of slag (such as kirschsteinite), which hindered the diffusion and growth of the iron grain and increased the iron loss.