Phosphorus recovery and solid waste reutilization by synergistic treatment of BOF slag and sewage sludge incinerated ash

Basic oxygen furnace (BOF) slag generated from steelmaking process and sewage sludge incinerated ash (SSIA) are two kinds of P2O5-bearing solid wastes. To achieve the low-cost and efficient recovery of P from them, a novel process of synergistic treatment-selective leaching-precipitation was proposed. The BOF slag and SSIA were first melted to promote P2O5 enrichment, and then the P2O5-concentrating dicalcium silicate was selectively leached from the mixed slag, and finally phosphate precipitate was extracted from the leachate by precipitation. It showed that the dissolution ratio of P from the mixed slag was higher than that from the BOF slag or SSIA. The dissolution of P from the mixed slag was promoted by decreasing pH and S/L ratio while increasing temperature had little influence on it. At pH= 1, S/L ratio= 1:200, and T = 298 K, the dissolution ratio of P reached 88.2 % while that of Fe was only 2.5 %. The residues mainly consisted of Fe2O3, MgO, and Al2O3, and the P2O5 content was less than 0.2 wt%, which can be reused as a metallurgical feedstock. When the pH of leachate increased to 7.5, a precipitate containing 22.4 wt% P2O5 was extracted, which has the potential to be a fertilizer. The total recovery ratio of P from the mixed slag exceeded 83 %. In this process, the P2O5 in BOF slag and SSIA was both effectively extracted and the valuable metallic elements can be reused in steel plants.