New look at titanium dioxide extraction
A simpler, cheaper and greener method of extracting higher yields of white titanium dioxide powder from mineral ore has been developed as a result of research conducted at the University of Leeds, UK.
This could have wide implications as the mineral is used as a white pigment in paint, plastics, food, medicines, ceramics and cosmetics, while its UV adsorption properties are exploited in sunblocks. The powder is also a precursor material to titanium metal production for use in aerospace, electronics and medical implants.
Titanium dioxide is derived from titani-ferrous minerals such as illminites, found in the dark sand beaches in Australia, India, South Africa and the USA.
These minerals are bound to contaminant metals in the ore including iron, aluminium and radioactive elements. The new extraction process involves roasting the mineral ore in alkali, usually biocarbonate of soda or potassium, at temperatures of 600-800ºC to produce alkali complexes of iron and titanium.
‘The alkali iron complexes are water soluble, so after roasting, you dissolve the [mixture],’ explains Professor Animesh Jha from the University’s Faculty of Engineering. ‘And because the impurities are finely dispersed inside the mineral matrix, during this leaching they are mechanically liberated. So you produce soluble iron-rich alkali ferrites, insoluble titanium rich particles and a colloidal medium that contains the majority of the impurities.'
The coarse titanium residue is leached again in chlorine to produce an average yield of about 97% pure titanium dioxide. The industry average is 85%.
Unlike the conventional extraction method, which involves smelting of the mineral ore and slag treating of impurities at over 1,000ºC using chlorine or sulphuric acid, the new technique reduces the use of chlorine, as well as increasingly expensive coke for chlorination.
Instead, rare and pre-lantanites are recovered from the colloidal medium as feedstock for lantanite oxide. This limits the level of hazardous chlorine waste and greenhouse gas and hydrocarbon emissions from burning coke, as well as subsequent waste disposal costs.
Carbon dioxide generated during decomposition of the bicarbonates and carbonates is recaptured to make bicarbonates, while heat is also recycled.
Jha adds, ‘Once you recover the lantanites, tiny amounts of weak radioactive elements – uranium and chlorium – remain in the complex mineral form. They can be safety returned to the mining site from where they came, as they will remain immobilised [as long as] the material lattice is not disturbed.’
The team at Leeds believes the market potential for its technology is vast. ‘All rich varieties of titani-ferrous minerals have been largely depleted around the world. It is important to start looking at low-grade minerals with high concentrations of impurities,’ says Jha. This makes conventional techniques with high costs and waste unsustainable.
‘Our aim was to develop technology for low-grade minerals that are readily available, but can’t yet be extracted economically,’ adds Jha.
Researchers have demonstrated the process at 50-100g scale, and are working with international company Millenium Inorganic Chemicals, the world’s second largest producer of titanium dioxide, to develop a pilot plant.