Optimum conditions for phosphorus recovery from steelmaking slag with high P2O5 content by selective leaching

Abstract

Selective leaching of P-concentrated solid solution is considered an effective method for recovering P from slag with high P2O5 content. This plays a significant role in stimulating the utilization of high-P iron ores. To determine the optimum conditions for selective leaching of P, we investigated the effects of the cooling rate of molten slag, Na2O content in slag, and pH on the dissolution behavior of the modified slag in the aqueous solution. Following leaching, a precipitation method was studied to recover P from the leachate. Compared to the quenched slag, the furnace-cooled slag exhibited a higher P dissolution ratio and lower Fe dissolution ratio, indicating that slow cooling was necessary to realize selective leaching. The addition of 2.5–4.0 mass% of Na2O to the slag was sufficient to cause most of the solid solution to dissolve at pH 6, Fe being difficult to dissolve. The dissolution ratio of P from the modified slag increased significantly when the pH decreased from 7 to 5. A further decrease in the pH promoted Fe dissolution. Therefore, the pH of the aqueous solution should be controlled between 5 and 6. After leaching, with an increase in the pH of the leachate, the precipitation ratio of P from the leachate increased, while the P2O5 content in the obtained phosphate product decreased. In this process, approximately 70% of P in the slag was recovered in the form of the phosphate product, which can be used as a phosphate fertilizer.