Production
Industrially, neptunium dioxide is formed from the precipitation of neptunium(IV) oxalate from a neptunium feed solution with oxalic acid, followed by calcination to neptunium dioxide. The neptunium feed solution (which includes varying oxidation states of neptunium) is reduced to a predominately neptunium(IV) solution via ascorbic acid prior to the addition of oxalic acid. A hydrazine inhibitor is initially added to the neptunium feed solution to protect the neptunium and ascorbic acid from decay.
Extrapolated and balanced from "The Production of Neptunium Dioxide" by J. A. Porter
Np4+ + Np5+ + Np6+ + HNO3 + C6H8O6 → 3 Np4+ + C6H6O6 + H2 + HNO3
Np4+ + C2O4H2 → Np(C2O4) • 6H2O + 2H−
Np(C2O4) • 6H2O + Δ → Np(C2O4)
Np(C2O4) + Δ → NpO2 + 2CO2
Neptunium dioxide can also be formed effectively from precipitation of neptunium(IV) peroxide, but the oxalate reduction has been found to be more industrially efficient.
Read more about this topic: Neptunium(IV) Oxide
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