Sulfuric Acid in Titanium Dioxide Production: Sulfate Route Demand is concentrated in the digestion section, where technical-grade H₂SO₄ functions as a decomposition reagent for ilmenite and sulfated titania slag. The sulfate route consumes fresh sulfuric acid mainly through iron oxide reactions, weak-acid bleed, washing losses, and neutralization of purge streams. Industrial consumption ranges from 2.5 t to 4.0 t H₂SO₄ per tonne of finished TiO₂ for rock ilmenite, and from 1.6 t to 2.2 t per tonne when sulfate slag is the titanium source.

In digestion, technical-grade sulfuric acid at 85–92 wt% is preheated to 60–80 °C and charged into brick-lined reactors. Direct steam injection raises the reaction mass to 150–200 °C. Ilmenite decomposes according to FeTiO₃ + 2H₂SO₄ → TiOSO₄ + FeSO₄ + 2H₂O. The acid-to-ore mass ratio is maintained between 1.5:1 and 1.8:1 in ilmenite operations; lower ratios depress titanium extraction, while higher ratios increase free acid load in hydrolysis.

Global sulfate-route demand for sulfuric acid is tied to ilmenite and slag supply decisions. Published capacity estimates place the sulfate route at approximately 40% of global TiO₂ capacity, with the remainder using the chloride route. Because each sulfate-route tonne of pigment consumes 1.6–4.0 t H₂SO₄, regional shifts from chloride to sulfate-route capacity directly alter merchant acid balances and storage logistics.

Sulfate-route demand for acid is also linked to anatase production. The chloride route predominantly yields rutile, whereas the sulfate route can produce both anatase and rutile. Anatase is required in certain pigment, catalyst, and low-hardness applications, and its production via sulfate route keeps sulfuric acid demand structurally stable even when chloride-route rutile capacity expands.

What Controls Specific Acid Consumption in Ilmenite Digestion?

Specific acid consumption is controlled mainly by the FeO content of the feedstock. Iron in the ilmenite lattice consumes acid stoichiometrically rather than catalytically. Each tonne of FeO reacts with 1.365 t H₂SO₄ to form FeSO₄, while each tonne of Fe₂O₃ reacts with 1.843 t H₂SO₄ to form Fe₂(SO₄)₃. For a rock ilmenite containing 35 wt% FeO and 45 wt% TiO₂, the theoretical iron-related acid demand is approximately 0.478 t H₂SO₄ per tonne of ore, equivalent to 1.06 t H₂SO₄ per tonne of contained TiO₂. This stoichiometric base is amplified by free acid retained in filter cake, acid mist loss, and purge neutralization.

Carbonate gangue also consumes acid. Calcite in the ore reacts with H₂SO₄ to form calcium sulfate, consuming 0.98 t H₂SO₄ per tonne CaCO₃. This gangue-related demand is frequently underestimated in acid-sourcing models because ore blends change between shipments. Batch-to-batch variance in acid demand can be observed when the carbonate content of the feed shifts by a few percentage points.

Because hydrolysis produces weak acid, purchased acid demand is not equal to fresh acid demand. Clarified titanyl sulfate liquor at 180–230 g/L TiO₂ equivalent is hydrolyzed at 95–110 °C after seeding. The filtered weak acid contains 18–25 wt% H₂SO₄ and is returned to digestion after clarification. Recycle efficiency is bounded by impurity buildup, especially magnesium, aluminum, and colored transition metals. A continuous liquor bleed is required. Weak-acid return can reduce fresh 96 wt% acid consumption by 20–30% in operations designed for high water recovery, although published data for specific plants is limited by proprietary water balance.

Water balance governs the lower limit of acid concentration in recycle. Hydrolysis requires controlled dilution of titanyl sulfate liquor; if recycled acid concentration falls below 18 wt%, evaporation load increases and the titanium hydrolysis rate can shift. Conversely, recycle above 25 wt% may increase liquor viscosity and salt precipitation in transfer lines. These boundaries are operational, not thermodynamic phase limits, and are managed by controlling steam addition and evaporator duty.

When Sulfate Slag Replaces Rock Ilmenite, Fresh Acid Demand Declines

Switching from rock ilmenite to sulfate slag changes the consumption profile because slag contains 8–14 wt% FeO rather than 30–40 wt% FeO. The acid-to-ore ratio in slag-based digestion is typically reduced to 1.2:1–1.5:1, and fresh acid demand falls to 1.6–2.2 t H₂SO₄ per tonne TiO₂. Lower iron content reduces self-heating from iron sulfate formation, so the digestion batch requires more direct steam to hold 160–200 °C. The sulfate-route demand therefore shifts from acid volume toward energy and water balance.

Ferrous sulfate heptahydrate crystallization is the main acid-retaining by-product. In ilmenite-based operations, clarified liquor is cooled in forced-circulation crystallizers to below 15 °C to precipitate FeSO₄·7H₂O. Interstitial acid in the crystal cake must be washed, and washing losses add to total H₂SO₄ demand. Slag-based operations generate less ferrous sulfate, which reduces crystallizer throughput but removes a natural acid sink from the recovery loop.

Digestion vessels in sulfate-route plants are typically brick-lined carbon steel with acid-resistant mortar, while weak-acid recycle lines use rubber-lined steel or high-density polyethylene. Agitator shafts and impellers are specified from alloys resistant to dilute boiling sulfuric acid and abrasive ore solids. Manual charging lines have shown acid-to-ore ratio variance of ±0.05 t/t; automated weigh hoppers and mass-flow acid totalizers reduce this variance. Digestion blow tanks and air sparges control foam generated by carbonate minerals, and off-gas scrubbers remove acid mist before atmospheric discharge.

Acid Quality and Handling Standards for Sulfate-Route Procurement

Acid purity constraints for sulfate-route titanium dioxide are set by corrosion and pigment brightness rather than digestion yield alone. Iron in purchased acid adds to the sulfate load, but its concentration is normally small relative to iron in the ore. Chloride, nitrate, and organic impurities can affect heat exchanger corrosion and off-gas composition. Published threshold values vary by plant metallurgy and emission permit, so qualification programs rely on corrosion coupon testing and full acid assay rather than single-element limits. Fresh makeup acid is commonly specified at ≥96.0 wt% H₂SO₄, with lower-strength streams used only as recycle.

Storage and unloading systems for sulfate-route acid demand are designed around the corrosive and hygroscopic properties of concentrated H₂SO₄. Dilution is performed by adding acid to water, never water to acid, to avoid localized boiling at the interface. Bulk tanks are vented through scrubbers or drying media because sulfuric acid absorbs atmospheric moisture, which reduces strength and increases corrosion risk. The material is incompatible with reducing agents, chlorates, carbides, and alkaline materials; segregation under UN 1830 and local fire codes is mandatory in tank farms serving TiO₂ plants.