The dissolution of phosphorus from Jamaican bauxites under low temperature Bayer conditions

Jamaica has about 7.1% of world bauxite reserves however the ore characteristics vary significantly depending on mining location and mineralogy. Traditionally, low phosphorus hematite-rich deposits from south central Jamaica are processed in a low temperature alumina refinery but these ores are becoming depleted and will be replaced by high phosphorus goethite-rich resources that are located north of the current mines. Depending on type, P-minerals may dissolve during bauxite digestion, accumulate in the liquor and negatively impact the process. This work compares the caustic extractable PO43− from the traditional and future bauxite mining areas and seeks to develop a model to predict soluble PO43− concentrations from their total phosphorus levels measured via XRF. Filtered liquors from the low temperature caustic digestion of 104 samples from a goethite-rich future-bauxite reserve (0.4-32.8% PO43−) and 30 hematite-rich samples (0.1-1.27% PO43−) from current mines were analyzed for soluble PO43− concentrations using the ascorbic acid molybdenum blue spectrophotometric procedure. The concentrations of PO43− in liquor increased dramatically with processing of the goethite-rich ores (5-21,000 mg PO43−/L) in comparison to digestion of the traditional hematite-rich bauxites (52-319 mg PO43−/L if one unusual sample is excluded). An empirical equation (mg PO43−/L = 0.085(mg PO4/kg) + 103; R2 = 0.93) is proposed that uses the total phosphate concentrations of goethite-rich bauxites to predict the soluble PO43− concentrations of their digestion liquors to within ±20% of the measured values. A comparable equation (mg PO43−/L = 0.082 mg PO43−/kg - 26; R2 = 0.67) to predict caustic soluble phosphorus from the hematite-rich ores was also developed however it has a larger margin of error. The correlations are applicable only to normal bauxite samples from the specific mining areas and with Ca:PO43− ratios of <0.40; they are less effective at higher ratios however as apatite or calcite is often present in such bauxites and results in lower than usual soluble PO43− concentrations. Evaluation of the Ca:PO43− ratios in the bauxites in conjunction with the measured soluble PO43− concentrations confirms that crandallite is the dominant P mineral in both the hematite-rich and goethite-rich bauxites.