Heat and Acid Leach Treatments to Lower Phosphorus Levels in Goethitic Iron Ores

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Phosphorus associated with goethite in high-phosphorus (>0.10 mass% P) iron ores was lowered to below 0.075 mass% P with a heat treatment at 300 or 350°C for 1 h followed by a sulphuric acid (H2SO4) leach. This phosphorus removal was associated with a sample weight loss of 10–20 mass% due to dissolution of iron oxides. After heating at 900°C for 1 h, a sulphuric acid leach resulted in similar phosphorus removal but with dissolution of less than 3 mass% of the sample. The weight losses in the leach are associated with phase changes of the phosphorus-containing goethite phase during heating. Heating at 300 or 350°C resulted in conversion of the goethite into an intermediate hematite phase (protohematite), while heating at 900°C gave a dense hematite phase. Compared with goethite in the ore, the more porous protohematite phase was more soluble in the sulphuric acid resulting in dissolution of iron with the phosphorus, while the dense hematite phase was much less soluble and little iron was dissolved in the leach. Leaching at 25 mass% solids for 3 h at 60°C, at a pH of 0.5 or lower, gave significant lowering of phosphorus levels. Leaches were with 0.1–1M H2SO4; the concentration of acid required depended on the amount of phosphorus to be removed. Recycling of the acid leach liquor four times did not show evidence for precipitation of phosphorus and resulted in leach solutions with up to 1 g/L P and 134 g/L Fe.