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

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.