Chloride vs Sulfate Process TiO₂: Practical Differences

Introduction

TiO₂ pigments are manufactured using two primary processes: the chloride process and the sulfate process. While both produce high-quality titanium dioxide, there are practical differences in product characteristics that affect grade selection for specific applications.

Process Overview

Chloride Process

The chloride process involves:

1. Chlorination of titanium ore (rutile or synthetic rutile) at high temperature

2. Purification of titanium tetrachloride (TiCl₄) by distillation

3. Oxidation of purified TiCl₄ to produce TiO₂

4. Surface treatment and finishing

Key characteristics:

• Produces only rutile TiO₂
• Higher capital investment required
• Lower waste generation per ton of product
• Requires higher-grade ore feedstock

Sulfate Process

The sulfate process involves:

1. Digestion of ilmenite ore in sulfuric acid

2. Separation and purification of titanyl sulfate solution

3. Hydrolysis to precipitate TiO₂ hydrate

4. Calcination and surface treatment

Key characteristics:

• Can produce both rutile and anatase grades
• Lower capital requirements
• Higher waste volumes (iron sulfate, spent acid)
• Can utilize lower-grade ore feedstocks

Product Quality Differences

Purity

Chloride process TiO₂ typically achieves higher purity levels due to the distillation purification step. Trace impurities that affect optical properties (iron, chromium, vanadium) are more effectively removed.

Typical impurity levels:

• Chloride process: <0.01% total trace metals
• Sulfate process: 0.01-0.05% total trace metals

Particle Size Distribution

Chloride process grades generally exhibit narrower particle size distributions due to better control in the vapor-phase oxidation step. This translates to:

• More consistent optical properties
• Better reproducibility batch-to-batch
• Improved dispersibility in some systems

Photochemical Stability

The higher purity of chloride process grades typically results in better photochemical stability and reduced chalking tendency in exterior applications. For demanding exterior durability applications (automotive OEM, premium architectural), chloride process grades are generally specified.

Application Considerations

Exterior Coatings

For exterior durability requirements, chloride process grades are typically preferred due to superior photochemical stability. However, well-formulated sulfate process rutile grades with appropriate surface treatments can meet many exterior specifications.

Interior Applications

Both processes produce grades suitable for interior coatings. Selection often comes down to cost and availability rather than technical requirements.

Plastics

Both chloride and sulfate process grades are used successfully in plastics. Lacing resistance and dispersion characteristics are more dependent on surface treatment than base pigment process.

Paper and Laminates

Sulfate process grades are widely used and often preferred for cost-sensitive paper applications. Anatase grades (sulfate process only) remain important for certain paper applications.

Environmental Considerations

The chloride process generates significantly less waste per ton of TiO₂ produced:

• No spent sulfuric acid requiring regeneration
• Lower iron sulfate waste volumes
• Smaller overall environmental footprint

However, modern sulfate process plants have improved waste handling and recycling to minimize environmental impact.

Regional Availability

Chloride process capacity is concentrated in North America, Europe, and Australia. Sulfate process capacity is more geographically distributed, with significant production in Asia (particularly China).

This affects supply chain considerations and logistics costs depending on your location.

Conclusion

Both processes produce high-quality TiO₂ suitable for demanding applications. Chloride process grades offer advantages in purity and consistency that benefit the most demanding applications, while sulfate process grades provide cost-effective solutions for a wide range of uses. Grade selection should consider specific application requirements, not just manufacturing process.