Abstract
Vanadium is a ‘critical mineral’ that is a key component in the production of high strength steel and high-performance titanium alloys. Natural vanadium ores are typically low grade, with ~85% of global vanadium produced from titanomagnetite ores containing less than 0.5 wt% vanadium. As such, vanadium is usually considered a “companion metal”, with ~70% of vanadium extracted from vanadium-rich slags formed during the smelting of titanomagnetite ores into steel. New Zealand currently accounts for ~1% of world supply of vanadium-rich slag through the Glenbrook steel plant operated by NZ Steel, which utilises a feedstock of vanadium-bearing titanomagnetite ironsand that is mined locally.
In steel-making slags, vanadium is usually present in an insoluble V(III) state as a spinel such as FeV2O4 or (Mn,Fe)(V,Cr)2O4, with typical recovery methods comprising of an oxidative sodium roast to produce V(V) species, before leaching with an aqueous lixiviant. The resulting leachate is then treated with ammonium salt compounds to precipitate ammonium vanadate at low pH. While effective, sodium oxidative roasting typically releases polluting and corrosive gases. Treatment of the leachate with ammonium salts also requires a significant excess of reagents, resulting in the discharge of large volumes of contaminated saline wastewater. This presents significant obstacles to the environmental consenting of any proposed new plant. As such, there is a clear need for alternative cleaner Zero-Liquid-Discharge (ZLD) processes for vanadium recovery.
Here, we describe an integrated ZLD process for vanadium recovery, from vanadium-bearing slag through to a solid vanadium-oxide product. We show that air-roasted steelmaking slags obtained from the NZ Steel plant can be successfully leached with an ammonium bicarbonate solution to achieve up to 99% vanadium extraction to the leachate, and without leaching hazardous hexavalent chrome. Treating the resulting leachate with a solution of hydrazine hydrate then reduces the vanadium species in solution to a lower valence state, decreasing their solubility at near neutral pH and hence driving precipitation. Solid vanadium-bearing oxides of up to 99.5% purity have been produced in this manner, at precipitation efficiencies of up to 94%. Precipitation via hydrazine reduction requires near-stoichiometric quantities of hydrazine which swiftly decomposes to inert nitrogen, thus ensuring no further soluble contaminants are added during the precipitation step. In addition, excess ammonium bicarbonate from the leaching step can be decomposed to CO2 and NH3 gases which can then be recycled, through recombining with residual clean water obtained after precipitation. As such, the combined process of ammonium bicarbonate leaching followed by hydrazine-reduction precipitation forms a ZLD closed-loop process for vanadium-oxide extraction from steel slags.