Titanium dioxide (TiO₂) is widely used across industries due to its exceptional pigment properties, excellent whiteness, non-toxicity, and high opacity. It is a staple material in the manufacturing of coatings, plastics, inks, rubber, and paper. Driven by industrial demand, TiO₂ production in China has grown significantly over recent decades, positioning the country as the leading producer globally. In 2024 alone, China produced 5.5 million tons of TiO₂, accounting for over 56% of the global output. However, this massive production has brought substantial environmental concerns, particularly the generation of titanium dioxide waste acids (TDWA), a hazardous byproduct.
TDWA is a highly acidic effluent produced during TiO₂ manufacturing, especially from the sulfuric acid and chlorination methods. As the industry amplifies, the amount of TDWA being generated also increases, posing environmental hazards such as soil and water acidification, heavy metal contamination, and biological toxicity. Consequently, recovering valuable metals from TDWA has become crucial for mitigating environmental damage and achieving sustainable resource utilization.
TDWA differs in chemical composition and environmental impact based on the TiO₂ production method. The sulfuric acid process, currently the dominant method in China, involves the acid leaching of ilmenite ores and subsequently produces large volumes of sulfuric acid-based waste. On average, 1 ton of TiO₂ made using this method results in 8 tons of waste sulfuric acid and 120 tons of hydrolysis waste acid.
In contrast, the chlorination process, more common globally, yields waste acids primarily composed of hydrochloric acid and chlorinated metal salts. While it produces less waste by volume, the acidic nature and high metal ion content of the effluent remain significant concerns. Both types of TDWA contain high concentrations of iron (Fe) and titanium (Ti), along with trace but valuable amounts of vanadium (V), scandium (Sc), manganese (Mn), and aluminum (Al). Their high acidity (pH < 0) and complex composition make TDWA challenging to treat, but also a potential secondary resource for critical metals.
Titanium recovery from TDWA is more complex due to its variable speciation under acidic conditions. Acidic organophosphorus extractants like P204 and P507 have shown moderate success in extracting Ti(IV), though these also tend to co-extract iron and require multistage processing. Neutral extractants such as TBP, TOPO, or TRPO offer better selectivity and minimize third-phase formation when used with alcohol modifiers. These systems are more effective in sulfate-rich media.
Amine extractants such as N1923 have gained attention for their high Ti selectivity and extraction efficiency. Used in a two-phase solvent system, N1923 can recover over 97% of Ti and separate it from Fe, Mg, Mn, and Al efficiently. However, handling issues such as emulsion stability and phase separation must be addressed for large-scale applications.
Liquid membrane methods offer high enrichment rates and selectivity but face industrial challenges due to membrane instability and operational complexity.
Integrated approaches for recovering multiple metals from TDWA include both sequential and co-extraction techniques. Multistep solvent extraction leverages the different affinities of extractants to selectively remove Sc, Ti, and Fe in sequence. Co-extraction followed by gradient stripping can also separate Ti, Fe, and V based on stability differences in their complexes.
Combining precipitation with extraction helps improve overall process efficiency. For instance, Fe is first removed by jarosite precipitation before V is extracted by weak acid extractants. Similarly, gradient pH adjustments can facilitate the sequential precipitation of Al, Fe, and Mn after Sc and V extraction. Such comprehensive recovery schemes are not only resource-efficient but also reduce waste generation and operational complexity.
Despite promising lab-scale developments, several challenges hinder industrial implementation. Many current methods rely on costly oxidants, generate secondary waste streams, and lack economic and environmental sustainability assessments. Moreover, rare strategic elements like niobium (Nb) and yttrium (Y), present in trace amounts, are often overlooked. Future efforts must focus on developing selective, regenerable extractants; reducing chemical consumption; and integrating processes into pilot-scale and industrial-scale systems. A unified evaluation methodology for techno-economic and environmental performance is essential to guide adoption.
Recovering critical metals from TDWA is crucial for the sustainable development of the TiO₂ industry. Established techniques such as chemical precipitation, solvent extraction, ion exchange, and electrochemical methods demonstrate varying degrees of success in extracting Fe, Ti, V, and Sc. Among these, solvent extraction offers the most excellent versatility and scalability for all four metals. Ammonia-free and oxidant-efficient processes and novel extractants broaden the potential for cleaner production. To fully capitalize on TDWA as a secondary resource, research must extend beyond laboratory studies to full-scale implementations, ensuring economic viability, environmental compliance, and integration into a circular economy.