Inorganic Chemicals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 11

Titanium dioxide exists in two major crystal forms, rutile and anatase.
Rutile is more closely packed and therefore denser than anatase. Compared
with anatase, it has a higher refractive index and higher opacity. Because of its
brilliant whiteness, excellent covering power, and resistance to color change,
rutile is a valuable pigment for a broad range of applications in paints, plastics,
inks, and paper. Anatase has a bluer undertone and is less abrasive than rutile.
It is often preferred for applications in paper, ceramics, rubber, and fibers.
Titanium dioxide has higher opacity than other white pigments. Rutile is
about 15% higher than anatase but more than double that of zinc sulfide, more
than three times that of antimony oxide, and more than ten times that of cal-
cium carbonate.
There are two major processes: the sulfate process and the chloride process.
In the sulfate process, titanium-bearing ore, typically ilmenite, is treated with
sulfuric acid. The titanium dissolves as titanyl sulfate and is then hydrolyzed
to titanium dioxide which forms as a precipitate. Filtration, washing and calci-
nation give titanium dioxide crystals of the correct pigment size [20]. Crystal
structure (rutile or anatase), size, and distribution are determined by process
conditions and additives [21]. The sulfate process was the dominant process
in the 1970s and 1980s and continues to be used today, but it is a lengthy pro-
cess. Today, the chloride process dominates because of advantages in waste
generation, energy usage, and quality improvements. There has not been a
sulfate process plant built in the United States since 1971.
In the sulfate process, titanium ore is digested with sulfuric acid to form
titanium sulfate (TiOSO 4 ). The titanium sulfate is then hydrolyzed, filtered,
washed, and calcined to give titanium dioxide.
In the chloride process, dried ore reacts with chlorine gas to produce
titanium tetrachloride. The titanium tetrachloride is purified chemically
and by distillation and then reacted with oxygen to form titanium dioxide
and chlorine. The chlorine is recycled. Prior to the oxidation, a minor amount
of aluminum chloride is added to promote rutile formation [22]. The rutile
is preferred because it is more durable and has a higher refractive index.
Untreated titanium dioxide is used, but more commonly a surface treatment
is done to improve specific end-use property requirements such as ability to
disperse or weather resistance.
The chlorine used for titanium dioxide production is prepared by electrol-
ysis of sodium chloride. Like titanium ore, sodium chloride is mined.
2.6 SODIUM CHLORIDE AND CHLORALKALI
Sodium chloride (NaCl), also called salt or halite, is widely available in the
United States. In the United States, 28 companies with a total of 60 plants