materials feature | Titanium dioxide How production processes compare
Titanium dioxide exists naturally in various crystalline forms, including anatase and rutile. However, for indus- trial purposes, TiO2
pigments are
synthesised from raw titanium ore, by one of two processes, sulphate and chloride.
Synthetic rutile TiO2 pigment, which is
generally preferred for plastics com- pounds because of its better light scatter, can be made by either process; anatase pigment can only be made by the
sulphate process. The processes differ in the feedstock, and the methods used to refi ne the ore, remove impurities, and to form the pigment particle. In the sulphate process,
introduced 1916 by Titanium Pigment in the USA and Titan in Norway, either ilmenite (iron titanate, FeTiO3
obtain titanium tetrachloride and metal chloride impurities, which are subse- quently separated from the process. Austin Reid, research fellow at DuPont Titanium Technologies, says that there are now various versions of the chloride process – DuPont developed and commercialised the original in 1948 – but claims the DuPont version is the only one that can use low-grade ores. Highly purifi ed titanium chloride, TiCl4, is then oxidised at a high temperature to produce
Different methods are used
to refi ne and purify TiO2 pigments
intermediate TiO2 . The oxidation step
permits close control of particle size distribution and crystal type to make grades with high hiding power and tinting strength.
At the moment, a third process, CTL ) or titanium-rich
(around 80%) slag is dissolved in sulphuric acid, and then TiO2 is precipitated out by hydrolysis.
In the chloride process, enriched slag (85-95% TiO2) or synthetic rutile (upgraded ilmenite containing 88-95% TiO2) is reacted with chlorine gas to
China, it is carrying out a large development pro- gramme to produce such grades and it is confi dent it can make them to the required standards. “We are planning to produce a grade especially developed for masterbatch applications as well as a grade for use in outside applications,” said Yong-ai, adding that the company expects to start marketing these products in 2014.
Henan Billions is in the process of starting up a
production facility using the chloride process. “We feel confi dent that, with this process, we can start to supply the market outside China with quality products,” Yong-ai said.
New kid on the block While some established TiO2
suppliers in the West seem
keen to quit the scene, Canadian mining exploration company Argex Titanium has plans to move down- stream into the production of TiO2
and vanadium pentoxide (V2O5). 32 PIPE & PROFILE EXTRUSION | March/April 2014 as well as also iron
from Argex, operates only on pilot scale, but full-scale production is scheduled for 2015 – just a year short of the 100th anniversary of the sulphate process. The CTL process is said to be characterised by high energy effi ciency, low emissions and a closed-loop design that uses relatively low levels of hydrochloric acid, which is regenerated after use. The CTL process uses only ilmenite as the feedstock and Argex says it is the only process that can produce high quality pigment from ores containing common contaminants, such as Cr, V, Mg, and Ca, which prevents use in the other process- es. In the CTL process, the ilmenite is attacked by acids, followed by a fi rst iron solvent extraction to remove
iron, and a second solvent extraction to remove Ti, giving a clear solution containing Ti. Hydrolysis yields pure TiO2
. Argex says that it has adopted a “simple and low risk
strategy for the scale-up of its proprietary process that allows it to produce high purity TiO2
directly from its
100% owned deposit.” It collaborated with Process Research Ortech in Mississauga, Ontario to develop the CTL mineral extraction process, which allows for the production of high purity (or 99.8% pure) pigment grade TiO2
for the paint, plastics and coating industries. “The process is unique because of its ability to
produce high purity TiO2 , Iron and V2O5 through a single
process directly from the ore material,” Argex said. “The equipment used in all parts of the process is known and readily available. The innovation of the process lies on how the equipment is used and on how the solvents and temperatures are properly balanced. The process operates at atmospheric pressure, thus avoiding the need to handle chemicals at very high temperatures.” (See box story, for Argex’s comparison of how its process compares with the chloride and sulphate processes.)
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