Titanium Dioxide Manufacture: When Pollutant is a good Feed Stock

On a cool autumn day in 1964, I was monitoring the performance of a reactor in the centre of a vast industrial complex. A chlorine gas stream was transforming black rutile sand (‘black-sand’) feed into titanium tetrachloride before being oxidized into titanium dioxide by Oxygen injection in a closely guarded and commercially sensitive plant. Although the end-product was chemically analogous with the black-sand feed, it was a polymorph with enhanced properties which continues to impact lives in modern-day world including its use in cosmetics, sunscreen, food colouring, paper, plastics, and paint pigment amongst others. It was a safe alternative to lead-oxide. 

The black-sand feed was from the coast of western Australia, thousands of miles away where an 'eco' conscious company was remediating a polluted site by carefully removing the black-sand with a dredger. 

However, the story starts much earlier.  Dating back to Roman times, lead oxide (‘white lead’) was in cosmetics, paint pigment, and anywhere an artificial white colour was required. Over time, white lead was deemed as poisonous, hence an alternative was sought. This alternative was titanium dioxide. It is thought to be benign but with incredible optical properties which after treatment give an unsurpassed whiteness. Several processes have, to date, been considered for producing titanium oxide. 

The first process, called the ‘sulphate route’ involved dissolving mineral sand (usually in a form called ilmenite) in sulphuric acid to produce pure white pigment after a sequence of treatment stages. Unfortunately, this process resulted in considerable amount of polluting waste by-product (ferrous sulphate complexes). So, an alternative process called the ‘chloride route’ was developed. Chlorine was used to produce titanium tetrachloride, which is then converted to the white pigment end-product in specialised equipment. The chlorine is recovered and recycled leading to much less pollution. 

The chloride process is environmentally friendly and exemplifies the applications of Chemical Engineering unit operations and techniques and how Chemical Engineers can transform seemingly useless waste into vital end-products. It is an ideal process for the development of young chemical engineers. 

Many companies have deployed similar concepts in other areas. This includes the harvesting and use of the rare microbes and micro-organisms contained in the soda encrusted ‘polluted’ water of Lake Magadi in Kenya’s rift valley. Valuable enzymes have been obtained from the microbes and micro-organisms to produce antibiotics, detergents and to dye fabrics. 

Reference:

UN Environment Program, Sharing the benefits of Kenya’s soda lakes, 2020.

https://www.unenvironment.org/news-and-stories/story/sharing-benefits-kenyas-soda-lakes