Chloride vs Sulfate Process — Which TiO2 Should You Buy?
The two TiO2 production routes have different particle physics and different economics. Choose based on application and cost sensitivity.
Titanium dioxide is manufactured by two industrial routes: the chloride process and the sulfate process. The two routes produce TiO2 of slightly different physical and optical character, with significantly different cost structures. Choosing the right route for your application is the single most important grade-selection decision — it determines purity ceiling, color ceiling, and cost floor.
How the chloride process works
Chloride process starts with high-titanium-content feedstocks — upgraded ilmenite (synthetic rutile) or natural rutile sand, which typically contain 90%+ TiO2. The feedstock is chlorinated with petroleum coke at approximately 1000°C in a fluid-bed reactor to produce gaseous TiCl4. The TiCl4 stream is purified by fractional distillation, removing iron chlorides, vanadium oxychloride, and other metal chlorides that would degrade pigment quality. The purified TiCl4 is then oxidized with oxygen at 1400–1500°C in a jet-flame oxidizer; TiO2 vapor nucleates and condenses into very fine, narrowly-distributed particles. Byproduct chlorine is recycled back to the chlorinator, making the process relatively closed-loop.
The high-temperature vapor-phase crystallization produces particles with narrow particle size distribution (PSD), very low metallic impurity (Fe typically < 20 ppm, compared to 50–150 ppm in sulfate grades), and consistent crystal structure. These physical properties translate directly to higher tinting strength, brighter color undertone, and better consistency lot-to-lot. The iron purification step specifically is what gives chloride-process TiO2 its better b* value — less iron carry-through means less yellow tint.
How the sulfate process works
Sulfate process starts with lower-grade ilmenite ore — typically 45–65% TiO2 content, versus 90%+ for chloride feedstocks. This feedstock flexibility is the core economic advantage of the sulfate route. The ore is digested in concentrated sulfuric acid at 60–80°C to produce a mixed iron-titanium sulfate solution. Iron is removed as ferrous sulfate (green vitriol, a useful byproduct in some markets). The titanium-rich solution is hydrolyzed at 95–110°C, precipitating hydrated TiO2. The precipitate is washed, seeded with rutile or anatase seeds to control crystal form, and calcined at 800–900°C to produce the final TiO2 particles.
Sulfate process generates large volumes of dilute sulfuric acid and iron sulfate as byproducts that must be managed, and this environmental overhead is one reason that new capacity investment has moved toward chloride in recent decades. However, sulfate process remains competitive for economy grades and anatase production, which is exclusively sulfate-based (chloride anatase is not commercially produced at scale).
Key differences: particle physics and color
Particle size distribution (PSD): Chloride process produces narrower PSD — typical D50 0.24–0.28 μm with span ((D90−D10)/D50) of 0.7–0.9. Sulfate process produces broader PSD with higher span (1.0–1.3 typical). Narrower PSD means more particles near the peak-scattering diameter, better hiding efficiency, and less particle-to-particle variation in the finished film. This PSD difference explains approximately 5–10% of the hiding-power advantage chloride grades hold over sulfate grades at similar D50.
Color: Chloride process delivers CIE b* typically 1.5–2.0 (very slightly blue-toned white). Sulfate process delivers b* typically 2.0–2.5 (slightly more yellow). The difference is most visible in semi-gloss and gloss paints in neutral and cool tones; in deep-tint bases or flat/matte finishes the difference largely disappears because the colorant and texture dominate appearance.
Metallic impurity: Chloride's TiCl4 distillation step removes iron and other metals comprehensively. Sulfate process relies on selective precipitation and washing, which allows more metallic impurity carry-through. This matters in white and pastel paints where iron contamination causes yellowing, and in outdoor plastics where trace metal impurities catalyze photo-oxidative degradation.
Cost structure
| Factor | Chloride | Sulfate | |
|---|---|---|---|
| Feedstock grade required | High (synthetic rutile, natural rutile) | Moderate (ilmenite, leucoxene) | |
| Capital cost ($/ton capacity) | $4000+ | $1500–2000 | |
| Operating cost vs sulfate | +$300–500/ton | Reference | |
| Byproduct management | Chlorine recycled | Ferrous sulfate disposal | |
| Typical price premium | +20–35% | Reference |
The higher cost of chloride-process TiO2 is justified economically by the performance advantage in most premium applications. In premium architectural and industrial coatings, where 1–2% better hiding power can reduce TiO2 dosage enough to offset the price premium, chloride is often cost-neutral on a hiding-per-dollar basis.
Application fit
- Premium coatings (automotive OEM, coil, high-end architectural): chloride always. Color and purity margins are non-negotiable. OEM automotive suppliers specify Δb* ≤ 0.2 batch-to-batch — achievable only with chloride-process tier-1 producers.
- Premium plastics (engineering polymers, outdoor PVC, masterbatch): chloride. Thermal stability and color hold during extrusion at 230–300°C require low metallic impurity. Residual iron in sulfate-process TiO2 catalyzes polymer degradation at high temperatures.
- Economy interior paint, primers, ceiling white: sulfate often acceptable. Color tolerance is wider and cost dominates the purchase decision. A flat white ceiling paint at 15% TiO2 loading does not benefit measurably from chloride-process b* performance.
- Printing inks: usually chloride for process color precision; sulfate acceptable for some publication and packaging inks where color specification is less tight. For UV-cured digital inks and premium flexo, chloride is standard.
- Fiber delustering (nylon, polyester): anatase sulfate exclusively — rutile is too bright (creates harsh sheen), anatase is preferred for soft fiber sheen, and UV stability is not required for indoor fiber applications.
- Paper laminate and coating: either, depending on brightness specification and loading level. Decor paper at 30% loading typically uses chloride rutile for maximum brightness; general coated paper may use anatase sulfate.
- Rubber and cable: sulfate common — cost-driven, and the slightly broader PSD is not a limiting factor. Anatase sulfate is preferred for most rubber to minimize Banbury equipment wear.
Thermal stability and processing implications
One chloride-process advantage not always recognized is thermal stability during plastic processing. When TiO2 is incorporated into engineering plastics, masterbatch, or powder coatings at temperatures of 230–320°C, the purity difference between chloride and sulfate becomes significant.
Residual iron (Fe) in sulfate-process TiO2 (typically 50–150 ppm versus < 20 ppm in chloride) acts as a pro-oxidant at elevated temperatures, catalyzing thermo-oxidative degradation of the surrounding polymer. In white PVC profile extruded at 185–200°C, this manifests as yellowing within the first 3–6 months of outdoor exposure — even with adequate HALS stabilizer — because the iron impurity bypasses the stabilizer system. In polypropylene masterbatch processed at 240–260°C, iron catalysis accelerates MFI drift and reduces film tensile properties.
Chloride-process grades — with Fe typically < 15 ppm — avoid this iron-catalysis problem. This is why outdoor PVC and engineering plastics specifications almost universally require chloride-process TiO2, and why SEMITI grades for these applications (2160, 2190, 960) are exclusively chloride-based.
For powder coatings cured at 180–200°C for 10–20 minutes, sulfate-process TiO2 can cause yellowing during cure even with nominally adequate surface treatment, because sustained heat exposure allows iron impurities to diffuse to the surface and discolor the film. This is the primary reason powder coating formulators default to chloride rutile.
SEMITI grades by process
Chloride grades: SEMITI 996, 706, 706W, 902, 826D, 880, 960, 2310, 2160, 2190 — covering universal, premium, waterborne, durable, automotive, plastics, outdoor PVC, and high-performance applications. All chloride grades offer Fe < 20 ppm, b* ≤ 2.1, and narrow PSD (span ≤ 0.95).
Sulfate rutile grades: SEMITI 298 (general purpose coatings and rubber), SEMITI 218 (economy interior paint, ceiling white, primers), SEMITI 248 (coatings + plastics cost-performance balance). All sulfate rutile grades offer b* 2.0–2.5 and are priced 20–30% below equivalent chloride grades.
Sulfate anatase grades: SEMITI A100 (standard anatase for fiber delustering, rubber, and general paper), SEMITI A101 (food-grade low-heavy-metal anatase for paper and food contact applications), SEMITI A200 (high-brightness premium anatase for fiber and white rubber with maximum whiteness requirement). All anatase is sulfate-process — chloride anatase is not commercially produced at meaningful scale.
Switching between processes: what to expect
Switching from a sulfate-process grade to a chloride-process grade at equivalent D50 typically produces: - CIE b* improvement of 0.3–0.8 units (less yellow), visible in semi-gloss and gloss paints - Tinting strength increase of 3–8% (Reynolds number gain) - Improved batch-to-batch color consistency (Δb* tightens from ±0.4 to ±0.2) - Higher TiO2 cost, partially offset by the tinting-strength efficiency gain
Switching from chloride to sulfate at equivalent price-per-kg typically produces: - b* increase (more yellow) — acceptable in flat/matte paints, visible in gloss - Slight tinting strength reduction — may require 3–5% more TiO2 loading at iso-hiding - Wider batch-to-batch color variation — requires wider tolerance in finished product spec - Cost saving of 20–30% per kg TiO2
Always run a trial batch in your actual formulation before committing to a process switch. CoA comparison alone does not capture surface-treatment interaction effects that can shift dispersibility and in-formulation performance by 5–10% beyond what the headline specs predict.
If you are qualifying a new TiO2 grade and are unsure whether chloride or sulfate is the right starting point, send us your current CoA and application details — we will recommend the closest SEMITI grade for a trial sample and provide a side-by-side technical comparison for your specific use case.