edited by DARRELL H. BEACH
chemical of the month
The Culver Academies Culver, IN 4651 1
Titanium Dioxide Kathleen A. Davfs Delaware State College Dover. OE 19901
The formula for titanium dioxide or titanium(1V) oxide is TiO2. Structures
Titanium dioxide has three different crystal structures: rutile, anatase, and hrookite. In all three structures there are three titanium atoms around each oxygen atom and six oxygen atoms around each titanium atom. The oxygen atoms around each titanium atom form a distorted octahedron. The distortion is greatest in brookite, the least stable and least common crystal structure. Rutile crystals are tetragonal, anatase crystals are slender tetragonal prisms, and brookite crystals are flat orthorhomhic plates.
101
Table 1.
Some Reactions 01 Titanium Dloxide
+ 2 ZnO AneatZ n n T i O a neat TiO* + 2 MgO Mg2Ti04
Properties
TiOp
Pure titanium dioxide is white or colorless; impurities (iron, niobium, chromium, tantalum, and vanadium) sometimes cause i t to he yellow, red, brown, or black. The anatase and hrookite forms change to the mtile form when they are heated to approximately 800°C. The rutile form melts hetween 1830°C and 1850°C. I t is always difficult to get titanium dioxide to dissolve or react; it is especiallydifficult when the sample has been heated for a long time ahove 1000°C. Some samples dissolve in concentrated sulfuric acid, hydrofluoric acid, and hot mixtures of sulfuric acid and ammonium sulfate or alkali metals. In the few situations where titanium dioxide does react, the product is usually a complex oxide. Titanium dioxide is not only relatively inert chemically, but also it is relatively inert physiologically. In 1929, N. Heaton used a straightforward and rather foolhardy method to prove that titanium dioxide is nontoxic. He sweetened one pound of a titanium dioxide-harium sulfate mixture with glucose and ate it. Heaton suffered no ill effects. X-ray and fluorescence studies showed that the titanium dioxide was eliminated from his alimentary canal in twenty-four hours. Later, more conventional studies have also shown that ingestion of titanium dioxide and skin and eye contact with titanium dioxide have no harmful effects. However, inhalation of titanium dioxide dust (and all other dusts) can irritate the lungs. OSHA has set a workday exposure limit of fifteen milligrams of titanium dioxide dust per cubic meter. Unlike many other dusts, titanium dioxide dust is not chemically irritating and does not cause much scarring of the lungs. All three forms of titanium dioxide have very high refractive indices and are moderately hard and dense. Because the refractive index of titanium dioxide is so much higher than that of other materials, mixtures which contain small titanium dioxide particles reflect almost all light and are opaque. uses
Because it is nontoxic, relatively nonreactive, and very opaque and in spite of its higher cost, titanium dioxide is the most widely used white pigment. I t is used in white and colored oaint. oaner. linoleum. ravon and nvlon fibers, nlastics, , wr:mit s. \vhi~eu311 rircs, rtding &r,inuIe-,printing ink. : ~ n d < ~ ) w ~ r t i cAi .l n ~ all ~ ft l f t h : ~15.0t1bflmj t , i ~ i ~ , ~ n i(I18nide um
.
158
lbi
Figure 1. Distorted octahedra in (a) Rutile Form and (b) Anatase Form.
Journal of Chemical Education
A
haat
TiOz
Table 2.
+ BaCOs -+BaTiOs + other products
Some Properties of the Different Crystal Structures Hardness
Crystal Structure
Refractive Index
Oensity
(Moh's scale)
Rutiie Anatase Brookite
2.7 2.5 2.6
4.26 g/cm3 3.84 4.17
5.5-6.0 5.5-6.0 6.0-6.5
produced in the United States in 1980 were used to make materials opaque and/or white. The remainder of the titanium dioxide produced was used in a variety of products, catalysts, welding fluxes, condenser dielectrics, and synthetic gem stones. Preparation
Manufactures of titanium dioxide use two processes. 1. The Chlorine Process
The chlorine process is used to remove impurities (iron, niobium, tantalum) which give rutile (TiO2) ore its red or reddish brown color. In the first step, ground rutile ore, chlorine, and coke are heated together. Titanium tetrachloride, carbon dioxide, and chlorides of impurities are formed. 3TiOz+4C+6Clz-3TiCL+ZCOz+ZCO
Then the reaction products, which are all gases or vapors, move to a washer which contains liquid titanium tetrachloride.
The chlorides of iron and other impurities solidify in the liquid titanium tetrachloride. After the remaing gases and vapors (TiCL. CO?. HCI) leave the washer. thev are cooled until the
Bv 1939 titanium dioxide had renlaced toxic lead carhonate as the most common white pigment. DuPont, American Cyanamid. and National Lead have been imuortant U S . uroducers since the 1930's.
oxizzed in a special burner. The product is the ritile form of titanium dioxide.
Occurrence
TiCI,
+0 2
-
Ti02 + 2 Clz
2. The Sulfuric Acid Process
The first step in the sulfuric acid process is an acid-base reaction. Ground ilmenite (FeTi03) ore is placed in large tanks of sulfuric acid. FeTiO3 + 2 H2S04
-
FeSOd
+ TiOz. SO4 + 2 H 2 0 + Hz
When this step is finished, the tanks contain a dry, solid mass. Water is added to dissolve the iron and titanium sulfates in the mass. The solution is then transferred to a vacuum crystallizer. After the vacuum pumps have been turned on and the solution has become more concentrated, iron sulfate crystals (FeSOr7 HzO)form. These iron sulfate crystals are removed. The vacuum pumps are again turned on, the solution is further concentrated, seed crystals and steam are added, and the mixture is heated for several hours. Eventually insoluble titanium hydrate forms.
+
T 1 0 . SOn 2 HzO
-
TiOz. H z 0 + HzSOa
111 the Ins1 jlep. the hydratt, is hv.~t,11 11n1iIit d e w n ~ p ~ ~inlo ses : i;,r~n,,i titnni.~mdi,~xidr.r l t i s water \.spur and I ~ I ;unnt:lic possible do modify this process so that the product is the rutile form.)
There are larae deuosits of rutile, which is simply . . impure . titanium dioxide, in Virginia, Quebec, Sierra Leone, Brazil, and Australia. In addition, most of the soil and rocks in the Earth's crust contain some titanium dioxide. It has been found in volcanic ash, deep-sea horings, meteorites (0.080.22%), clays (0.06-27%), almost all soils (usually 0.50-1.50%), and almost all crystalline rocks. Human and animal tissues also contain a small amount of titanium dioxide; typical results of analyses are 0.001 mg to 0.02 mg per 100 g of tissue.
orw way,
Cost
As expected, the higher the purity of the titanium dioxide, the higher the price. Chemical and Engineering News (April 27, 1981) lists prices of $0.64 per pound for the anatase form and $0.69 per pound for the rutile form when they are purchased hv industries throueh a hulk contract. Two chemical wpply hmse+list prices 01 $12.:~ and .$1;1.1: ior a me-pound io~icainerUI reaecnt eradv titanium d:oxidt.. A third chem~cal supply house &es :price of $126 for 25 g of 99.999% pure titanium dioxide. Conclusion Titanium dioxide is probably in the page you are now reading, the paint on your house, and the carpet or linoleum under ;our feet. It is dontoxic, it has the best covering power of all the white pigments, and its ore is abundant. Titanium dioxide is nature's hest cover-up. General References
History
When William Gregor, an English clergyman and mineralogist, analyzed a black magnetic sand found on a Cornwall beach, he found that almost 50%of the sand was a previously unknown metal oxide. Four years later, in 1795, M. H. Klaproth analyzed a red ore found in Hungary, obtained a white oxide, and realized that this was the same white oxide which Gregor had separated from the black sand. Klaproth borrowed his name for the oxide, titanium oxide, from the Titans of Greek mythology. It was many years before titanium dioxide was used as a pigment. In 1870 Overton received a U.S. patent for an anti fouling coating which contained ground rutile ore and tar or pitch. (It was intended for ships' bottoms.) A. J. Rossi prenared nure titanium dioxide and mixed it with salad oil to
Rarksdale, Jelks, "Titanium: Its Occurrence, Chemistry, and Technology: Ronald Pmss. New York. 1949. pp. 401404.452-456.472-478. Barbdale, delks, "Titanium: Its Occurrence, Chomistry,and Technology: 2nd Ed.,Ronald Press, NewYork, 1966, pp. 3-11.67-73,134-135.56&575. Chemical and Enginaenng News, 59[17], 11 (April 27,1981l. Chtm~colond Endiinetring News, 391231.33 (June 8.19811. C1mk.R. J. H.. "The ChemistryofTitanium and Vanadium," Elsevier, New York, 1968. pp. "ac 3.7,,,. 2
Cotton, F. A. and Wiikinsun, G.,"Aduanced Inorganic Chemistry," 3rd Ed., lnterscipnce, New York, 1972,pp. 810-911. Heslop. R. 8. and Jones, K.,"lnorganie Chemistry: Elsevier, New York. 1976, pp. 631699
..
Kepert, D. I. "The Early Transition Metals: Academic Press, New York, 1972, pp. 6166
. ".
"Titanium Dioxide:
Hygienic Guide Se1ies, Amnican Indmtrial Hygiene knanciation,
Airnn nhin 1978 . ~~
"Toxic and Hazardous Indvstrial Chemicals Safety Manual."International Technical Information Institute, Japan, 1977, p. 623. WeaL. RobertC. (Editor). "Handbook of Chemistry and Physics." 54th Ed., The Chemical Rubber Company, 1973, pp. 8-149-150,
Volume 59
Number 2
February 1982
159
