Phosphorus Removal from Goethitic Iron Ore with a Low Temperature Heat Treatment and a Caustic Leach

Abstract

The phosphorus in high-phosphorus (>0.1% P) iron ores from the Pilbara area of Western Australia is mainly associated with the goethite fraction of the ore. Physical separation methods and simple leaching processes do not remove sufficient phosphorus from the ores to meet market specifications of 0.075% P. Processing to disrupt the goethite structure to make the phosphorus amenable to leaching is necessary.Phosphorus associated with the goethite in high-phosphorus iron ores can be removed to 0.075% P using a heat treatment at 300–350°C for 1 h with 10 wt% NaOH, followed by a water leach. Heating at higher temperatures, up to 500°C, with heating times of 0.5 h to 4 h, gave no improvement in phosphorus removal.Similar phosphorus removal was achieved by heating the ore at 300–350°C for more than 0.5 h and leaching with 1–5 M NaOH at the boiling point for 3 h. The concentration of sodium hydroxide required depended on the amount of phosphorus to be removed. Heating for up to 2 h or at higher temperatures up to 750°C did not improve the amount of phosphorus removed in the caustic leach. The temperature of the leach had a significant effect on the amount of phosphorus removed with less phosphorus being removed below the boiling point of the leach liquor.The heat treatment at 300–350°C is considered to dehydroxylate the goethite to form a hematite intermediate phase, ‘protohematite’, from which the phosphorus is dissolved during the leach step.