THERMAL ACTIVATION OF TEBINBULAK TITANOMAGNETITE ORE AS A LOW-COST MAGNETIC SORBENT PRECURSOR: TEMPERATURE–TIME DEPENDENCE OF SPECIFIC SURFACE AREA, CRYSTALLITE GROWTH AND PHASE STABILITY

Natural titanomagnetite can serve as a precursor of magnetically recoverable iron-oxide sorbents, provided that its low initial surface area (≈4–7 m² g⁻¹) is developed without destroying the magnetite phase. Here the thermal activation of titanomagnetite ore from the Tebinbulak deposit (Karakalpakstan, Uzbekistan) was mapped over a full 6 × 5 factorial grid of temperature (500–1000 °C) and holding time (0.5–2.5 h), and the response of BET surface area, crystallite size, external surface and pore volume was described with empirical models. Surface area obeyed SBET = −83.1 + 15.8 ln T + 3.60 ln t (R² = 0.976), with first-order time saturation (τ = 0.31–0.45 h) and a limiting value S∞ that grows from 17.2 to 27.8 m² g⁻¹ between 500 and 1000 °C. Crystallite size followed a power law d ∝ T1.31 t0.12 (R² = 0.992), i.e. coarsening is governed by temperature far more than by time. The surface gained per nanometre of crystallite growth (ΔS/Δd) dropped eightfold, from 0.42 to 0.05 m² g⁻¹ nm⁻¹, across the 500–1000 °C range, with the steepest fall above 700 °C, where the maghemite-to-hematite transformation of magnetite is known to accelerate. Combining these trends, 700 °C for 2.0 h is identified as the upper practical limit of activation: it provides 84 % of the thermally attainable surface (23.67 m² g⁻¹, external 18.80 m² g⁻¹, pore volume 0.120 cm³ g⁻¹) at a crystallite size of 64.86 nm, with the product still predominantly in the spinel phase. The models provide design equations for a sorbent intended for removing Mo, Se, U, As and V oxyanions from saline drainage water.