Investigations into N.Z. Steel slags

Abstract

A study has been made of the petrography, mineralogy, and chemistry of N.Z. Steel slags. Methods for extracting vanadium and titanium from slag have been investigated. Slags may be classified into flushing, refining, melting, vanadium, steel, and scrap slags. Twenty five phases have been identified in the range of slags studied. Slag mineralogy is a function of bulk composition and may be predicted from the FeO and CaO content. Slag fluidity, the Ti³⁺ content of slag, and formation of titanium carbide, are functions of bulk slag FeO content. Slags have not solidified in a state of equilibrium. Flushing slags typically contain 1.2% V₂O₃. Concentrated sulphuric or hydrochloric acid is capable of dissolving 60-80% of vanadium into solution. Roasting slag in NaCl followed by leaching in dilute acid extracts 30-50% vanadium. Roasting slag in Na₂CO₃ followed by leaching in water extracts>80% vanadium when slag SiO₂≤14%, but extracts<35% when SiO₂≥16%. Roasting slag simultaneously in NaCl + Na₃CO₃ yields poor vanadium extractions. Solvent extraction techniques are capable of recovering vanadium from water or dilute acid leach solutions. Production of vanadium slag will enable recovery of vanadium using a Na₂CO₃-roast/water-leach/solvent-extraction process. A two stage carburisation-chlorination process has been developed to recover titanium as TiCl4 from melting slag. This process involves in the first stage sintering of slag powder with charcoal at 1300-1350°C for 1-2 hours to reduce titanium oxide to titanium carbide, and in the second stage chlorinating the powdered carburised slag at 200-500°C to yield TiCl4. Laboratory scale simulation of this process indicates that>90% titanium is recoverable from slag.