For several years, city officials of Tyler, Texas have noticed a dramatic increase in the corrosion of their renowned marble statues by air pollutants. The leading cause for the increase is found to be the neighboring power plant that emits high concentrations of NO_x as byproduct. In order to abide by environmental regulations, the plant maintains that new measures taken to reduce the NO_x emissions will include the installation of titanium dioxide (TiO₂) scrubbers. As an external researcher, you are required to report to a panel of city officials and experts on the nature of the problem, the solution proposed by the plant, and the scope and effectiveness of the solution in reducing harmful NO_x air contaminants.

     While conducting your research, however, you discover that the diverse applications of titanium dioxide would make it profitable for the city to invest in a titanium dioxide manufacturing plant. In your proposal to city officials state the most efficient means of titanium dioxide production and the possible applications of the product.

     Titanium (IV) oxide (commonly known as titanium dioxide) is the most familiar compound of titanium. The highly opaque substance is widely used as white pigment in paint, paper, vinyl floor coverings, plastics, synthetic fibers and cosmetics. Titanium dioxide is a very large volume chemical product and, at more than 3.3 million metric tons consumed per year, one of the leading inorganics. The main consuming industries are paint, printing inks, plastics and ceramics, which altogether account for 60% to 70% of the total demand.

Titanium is common in nature. It exists in two crystalline forms, rutile and anatase. It is the ninth most abundant element in the earth's crust. The main ores are rutile (impure TiO₂) and ilmenite (FeTiO₃). Titanium occurs mainly in the form of rutile which is white when pure and is the preferred ore for most refinery operations of titanium metal. The mineral ilmenite (left), is a black, basic oxide.

     There are two methods to produce titanium dioxide, the sulfate or the chlorine process. Titanium ore, when treated with chlorine, forms volatile TiCl₄. This compound is separated from the impurities and burned to form TiO₂ in the rutile form:

TiCl_4(g) + O_2(g) ---> TiO_2(s) + 2 Cl_2(g)

     Another production reaction for titanium dioxide is dissolving the ilmenite in sulfuric acid to form a soluble sulfate:

FeTiO_3(s) + 2 H₂SO_4(aq) ---> Fe²⁺_(aq) + TiO²⁺_(aq) + 2 SO₄^2-_(aq) + 2 H₂O_(l)

     When this aqueous mixture is allowed to stand under vacuum, solid iron sulfate (FeSO₄ ^. 7 H₂O) forms and is removed. The mixture remaining is then heated, and the insoluble titanium dioxide hydrate (TiO₂ ^. H₂O) forms. Heating the hydrated compound yields pure TiO₂:

TiO₂ ^. H₂O_(s) + heat ---> TiO_2(s) + H₂O_(g)

     The acid-base properties and the ionic-covalent character of an oxide of an element depend on the element's position in the periodic table and on the oxidation state. Basic oxides are formed by metals on the left side of the periodic table and are usually ionically bonded. Acidic oxides are covalent and are formed by nonmetals on the right hand side of the periodic table. Amphoteric oxides possess acidic and basic properties. The elements that form amphoteric oxides have intermediate electronegativity, and their oxides have strongly polar covalent character. Both the acidic character and covalent character of an oxide increase across the periodic table, from the active metals on the left to the electronegative nonmetals on the right. Within a group in the periodic table both the basic character and the ionic character of an oxide increase from the more electronegative elements at the top to the less electronegative ones at the bottom. Amphoteric oxides exist in roughly the middle region of the periodic table. A diagonal band of amphoterism is formed where the horizontal and vertical trends combine. Both the acidic character and the covalent character of different oxides of the same element increase with the increasing oxidation state of the element. For example, chromium (VI) oxide (CrO₃) is acidic, chromium (III) oxide (Cr₂O₃) is amphoteric and chromium(II) oxide (CrO) is basic.

     Nitrogen dioxide is an odd-electron species which remains in equilibrium with dinitrogen tetroxide (N₂O₄) in its gaseous phase. In the presence of water and oxygen, it reacts to form nitric acid (HNO₃) and nitrous acid (HNO₂):

     These molecules are major air pollutants that contribute to environmental problems such as acid rain.

1)  Powdered limestone (CaCO₃) is blown into the combustion chamber and is broken down into lime and carbon dioxide:

CaCO_3(s) ---> CaO_(s) + CO_2(g)

2)  The lime then combines with the sulfur dioxide forming calcium sulfite:

CaO_(s) + SO_2(g) ---> CaSO_3(s)

     Afterward, an aqueous suspension of calcium carbonate (limestone) is injected into the combustion chamber in order to remove the calcium sulfite and any remaining sulfur dioxide. This produces a thick suspension known as a slurry that collects at the bottom of the combustion chamber. The general path followed by the exhaust gas SO₂ to form a solid calcium salt is as follows:

CaCO_3(s) + SO_2(g) ---> CaSO_3(s) + CO_2(g)

where the calcium carbonate base neutralizes the acidic SO₂ to form calcium sulfite:

2 SO_2(g) + O_2(g) + 2 CaCO_3(s) ---> 2 CaSO_4(s) + 2 CO_2(g)

S (in coal) + O_2(g) ---> SO_2(g)

2 SO_2(g) + O_2(g) ---> 2 SO_3(g)

SO_3(g) + H₂O_(l) ---> H₂SO_4(aq)

     Nitrogen dioxide reacts with water to produce a mixture of nitrous acid and nitric acid:

2 NO_2(g) + H₂O_(l) ---> HNO_2(aq) + HNO_3(aq)

CaCO_3(s) + H₂SO_4(aq) ---> CaSO_4(aq) + H₂O_(l) + CO_2(g)