Desorption of Molybdenum from Purolite A-100 by Ammonium Hydroxide: Towards Technical-Grade MoO₃

Abstract

This study addresses the desorption of molybdenum from the Purolite A-100 macroporous weak-base anion-exchange resin using ammonium hydroxide solutions, followed by the recovery of technical-grade molybdenum trioxide from the concentrated eluate. Sorption from a model sulphuric acid solution (0.396–0.408 g/L Mo) was carried out at 80°C for 30 minutes, achieving a recovery of 48.7% with a static exchange capacity of 15.8–16 mg/mL. Under standard single-stage saturation, the degree of desorption with 12% NH₄OH did not exceed 27.2%, regardless of contact time (15–240 min) or temperature (20–40°C). A multi-cycle saturation strategy – 25 consecutive sorption cycles – produced a linear increase in resin capacity up to 182.27 mg/mL and brought subsequent molybdenum recovery into the eluate to 94%, at a final concentration of 3.63 g/L (120 g/L NH₄OH, 40°C, 120 min). Thermodynamic modelling of the Mo-H₂O system in HSC Chemistry 8.0 confirmed that pH 1.5 falls within the stability field of MoO₃·H₂O, justifying the chosen precipitation conditions. Acidification of the eluate to pH 1.5 with co-precipitation using ammonium sulphate, followed by calcination at 600°C, yielded 6.21 g of technical-grade molybdenum trioxide identified by XRD as molybdite MoO₃ (PDF 00-035-0609, 100%) with total impurity content below 0.14%.