<p>The shortage of phosphorus ore necessitates phosphorus recycling from secondary resources such as steelmaking slag, which is one of the most important phosphorus-containing secondary resources. The presence of phosphorus hampers the inner loop process during steel production; therefore, the recovery of phosphorus from steelmaking slag is favorable for relieving phosphorus scarcity and recycling the slag. However, inorganic acid treatments result in the massive dissolution of iron oxide in the slag, while organic acid treatments are costly. To overcome these shortcomings, this study investigated separating phosphorus from steelmaking slag using composite acid solutions composed of inorganic and organic acids. The experimental results showed that in the citric-hydrochloric/nitric composite acid system, the leaching percentage of phosphorus increased rapidly to over 85% at 0.0156&#xa0;mol/L citric acid and 0.05&#xa0;mol/L hydrochloric/nitric acid and then slowly decreased. Similar results were observed as the citric acid concentration increased. The iron leaching percentage sharply increased as both inorganic acid concentrations increased, whereas it only slightly increased as the citric acid concentration increased. In addition, more iron was extracted from the steelmaking slag in the sulfuric/citric acid system. The experimental and thermodynamic analysis results indicated that dilute citric acid can suppress Ca²⁺ formation, which inhibits Ca₅(PO₄)₃(OH) precipitation and improves FeH₂PO₄²⁺ generation; however, it inhibits Fe(OH)₃(s) precipitation, increases FeH₂PO₄²⁺, FeOH²⁺, and Fe(OH)₂⁺, and only slightly increases the iron leaching percentage. These findings may promote phosphorus recycling for fertilizer or cathode material production and the reuse of residue in the steel industry.</p> Graphical Abstract <p></p>