Phosphorus Extraction from Synthesized Steelmaking Slag by Hydrothermal Hydroxyapatite Precipitation

Abstract

During the steelmaking process, phosphorus is removed from molten iron by oxidization and distribution into molten slag, which is discharged as dephosphorization slag. This slag is expected to become a secondary resource for phosphorus. Phosphorus needs to be selectively extracted to enhance its purity and value as a resource. We propose a hydrothermal treatment using subcritical acid solution as a simple, low-energy means to extract phosphorus from steelmaking slag. Hydroxyapatite was spontaneously precipitated by hydrothermal treatment of synthesized steelmaking slag (in the glassy or crystalized state). This is attributed to leaching of phosphorus and calcium ions from the slag into the subcritical water during the initial stage, which increased the pH, finally achieving optimum conditions for hydroxyapatite formation. Purity of the hydroxyapatite precipitates was quite high. Chemical analysis of the aqueous solution revealed that phosphorus did not remain in solution after the hydrothermal treatment. This suggested that most phosphorus removed from the slag precipitated as hydroxyapatite. The extent of phosphorus recovery, evaluated from the amount of hydroxyapatite precipitate and initial phosphorus concentration in the slag, was 73% for glassy slag and 58% for crystalized slag when the hydrothermal treatment was performed at 613 K for 48 h under the present experimental condition. Enhancement of phosphorus leaching to the acid solution in the initial stage of hydrothermal treatment may aid in increasing phosphorus recovery from the crystalized slag. Graphical Abstract: [Figure not available: see fulltext.].