Improved Fe(II) Determination in Ilmenite: Complete Matrix Dissolution via HCl + HF Digestion

Central Laboratory, Mitra S. K. Pvt. Ltd.

Ilmenite (FeTiO₃) is the primary titanium-bearing mineral used in the production of TiO₂ pigments, welding fluxes, and titanium metal. Accurate determination of ferrous iron (Fe²⁺) in ilmenite is essential for evaluating reducibility, process control, and feed quality in metallurgical operations. Traditionally, Fe(II) is determined titrimetrically after dissolving the sample in hydrochloric acid (HCl) [ISO 9035:1989] under an inert atmosphere. However, ilmenite ores—especially those containing silicate and secondary titanate impurities—are only partially soluble in HCl. This incomplete dissolution leads to underestimation of Fe(II) and inconsistent analytical results. To overcome this limitation, a modified digestion process combining HCl + HF has been developed. The inclusion of hydrofluoric acid (HF) enables complete matrix breakdown and ensures quantitative recovery of Fe(II), making this method both chemically superior and analytically reliable.

Chemical Mechanism Behind the Difference

1. HCl Digestion Reaction:

〖"FeTiO" 〗_3+6HCl→FeCl_2+TiCl_4+3H_2 O
However, ilmenite associated with silicate or iron-titanate gangue does not dissolve completely. Residual TiO₂ and Fe₂SiO₄ phases can entrap Fe²⁺, leading to underestimation.

2. HCl + HF Digestion Reaction:

〖"FeTiO" 〗_3+6HCl+6HF→FeCl_2+H_2 [TiF_6]+3H_2 O
〖"SiO" 〗_2+6HF→H_2 [SiF_6]+2H_2 O
Hydrofluoric acid forms hexafluorotitanate (H₂[TiF₆]) and hexafluorosilicate (H₂[SiF₆]), which are highly soluble. This ensures complete mineral decomposition and full Fe²⁺ liberation for titration.

Procedure Summary (Modified Digestion)

1. Weigh 0.2 g ilmenite sample into a 500 mL conical flask.
2. Add 3–5 mL HF and 60 mL HCl, along with marble chips to generate CO₂.
3. Seal with a slit rubber stopper and heat gently at ~90°C until the sample dissolves completely.
4. After cooling under CO₂ flow, add 100 mL oxygen-free water and 15 mL H₃PO₄.
5. Add Ba-diphenylamine sulphonate indicator and titrate with 0.1 N K₂Cr₂O₇ to a violet-blue endpoint.
This process yields accurate and reproducible Fe(II) values, even for complex ore samples.

Analytical Advantages of the HCl + HF Digestion

1. Total matrix dissolution — No residue, even with silicate-rich ilmenite.
2. Accurate Fe(II) estimation — Eliminates underreporting caused by incomplete digestion.
3. Enhanced reproducibility — Consistent results across replicate analyses and CRMs.
4. Faster analysis — Improved reaction kinetics reduce overall test time.

Conclusion

The conventional HCl digestion method for ilmenite often leaves behind undissolved silicate and titanate residues, leading to low and inconsistent Fe(II) results. By integrating HF with HCl, the modified digestion achieves complete mineral breakdown, yielding true FeO values and superior analytical reliability. This approach has become a benchmark for accurate Fe(II) estimation in complex and industrial ilmenite samples, aligning classical wet chemistry with modern process demands.

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