Titanium dioxide chlorine process

Two pigment production routes ate in commercial use. In the sulfate process, the ore is dissolved in sulfuric acid, the solution is hydrolyzed to precipitate a microcrystalline titanium dioxide, which in turn is grown by a process of calcination at temperatures of ca 900—1000°C. In the chloride process, titanium tetrachloride, formed by chlorinating the ore, is purified by distillation and is then oxidized at ca 1400—1600°C to form crystals of the required size. In both cases, the taw products are finished by coating with a layer of hydrous oxides, typically a mixture of siUca, alumina, etc. 

Titanium dioxide (E171, Cl white 6) is a white, opaque mineral occurring naturally in three main forms rutile, anatase, and brookite. More than 4 million tons of titanium dioxide are produced per year and it is widely used for industrial applications (paints, inks, plastics, textiles) and in small amounts as a food colorant. ° "° Production and properties — Titanium oxide is mainly produced from ilmenite, a titaniferous ore (FeTiOj). Rutile and anatase are relatively pure titanium dioxide (Ti02) forms. Titanium oxide pigment is produced via chloride or sulfate processes via the treatment of the titanium oxide ore with chlorine gas or sulfuric acid, followed by a series of purification steps. High-purity anatase is preferred for utilization in the food industry. It may be coated with small amounts of alumina or silica to improve technological properties. 

This process involves the reaction of dried rutile ore and coke with chlorine in the chlorinator to produce titanium tetrachloride. The liquefied titanium tetrachloride is sent to the distillation unit to remove impurities and then to a fluidized bed reactor to react with oxygen to form titanium dioxide and chlorine. Solid titanium dioxide formed at this stage is vacuum degassed before being treated... 

FIGURE 22.17 General production flow diagram of a typical plant production of titanium dioxide (the chlorine process). 

In the titanium dioxide production plant where the chlorine process is employed, the wastewater from the kiln, the distillation column, bottom residue, and those from other parts of the plant first settle in a pond. The overflow from this pond is neutralized with ground calcium carbonate in a particular reactor, while the scrubber wastewater is neutralized with lime in another reactor. The two streams are sent to a settling pond before being discharged. 

A typical wastewater treatment process flow diagram in a titanium dioxide production plant (the chlorine process) is shown in Figure 22.19. 

Chloride One of the two process used today for making titanium dioxide pigment. Mineral rutile, or another mineral rich in titanium, is chlorinated with coke to produce titanium tetrachloride ... 

ICON [Integrated chlorination and oxidation] An improved version of the Chloride Process for making titanium dioxide pigment. It operates at above atmospheric pressure and is claimed to be cheaper to build. Chlorine from the oxidation section, under pressure, is introduced directly to the chlorinator. Developed by Tioxide Group, and first operated at its plant at Greatham, UK, in 1990. 

In the chloride process, developed in about 1960, the titanium in the ore is converted to titanium(IV) chloride by heating it to 800 °C with chlorine in the presence of carbon, which combines with the released oxygen. The purified chloride is then oxidised to titanium dioxide at 1000 °C and the chlorine formed is recycled. Technical problems arise because the oxidation of titanium(IV) chloride is not sufficiently exothermic to make the reaction self-sustaining but these can be overcome by pre-heating the reactants and by burning carbon monoxide in the reactor to raise the temperature. By careful control of the conditions, it is possible to produce pure rutile particles of a mean size of 200 nm. 

Presently there are two main processes for manufacturing this important white pigment. The main one involves reaction of rutile ore (about 95% Ti02) with chlorine to give titanium tetrachloride. For this reason we have chosen to group this key chemical under chlorine and sodium chloride. The titanium tetrachloride is a liquid and can be purified by distillation, bp 136°C. It is then oxidized to pure titanium dioxide and the chlorine is regenerated. Approximately 94% of all titanium dioxide is made by this process. 

The photoelectrochemical production of chlorine at nanocrystalline titanium dioxide thin film electrodes exposed to U V light has been reported [96]. In this process, the energy from photons substantially reduces the overpotential required for the chlorine evolution process and therefore less harsh conditions are required. Metal doping of the Ti02 photoelectrocatalyst was explored but found to be not beneficial for this process. In future, this kind of process could be of practical value, in particular, for water treatment and disinfection applications requiring low levels of chlorine. 

The chloride process (Fig. 2) involves the reaction of rutile (an ore containing approximately 95% by weight titanium dioxide, Ti02) with chlorine to give titanium tetrachloride (T1C14), a liquid that can be purified by... 

The chloride process utilizes the treatment of rutile (natural or synthetic) with chlorine gas and coke to produce titanium tetrachloride (TiC14). The titanium tetrachloride is distilled to remove impurities and then reacted with oxygen or air in a flame at about 1500°C to produce chlorine and very fine particle titanium dioxide. The chlorine is recycled (Fig. 1). 

Chloride Process. A flow diagram for the chloride process is shown in Figure 1. The first stage in the process, carbothermal chlorination of the ore to produce titanium tetrachloride, is carried out in a fluid-bed chlorinator at ca 950°C. If mineral rutile is used as the feedstock, the dominant reaction is chlorination of titanium dioxide. 

The oxidation is conducted in a proprietary burner, with extra heat supplied because, although the reaction is exothermic, it is not self-sustaining. The chlorine is recycled. The process was developed by Du Pont in the 1940s, and its first plant started operating in 1958. The Chloride process has progressively replaced the older Sulfate process because it produces less effluent. In 2005, 54% of the world capacity for titanium dioxide production used the Chloride process. See also ICON. 

Wiedemann (143) determined the titanium content of a nonstoichiometric titanium carbide by a multistep, variable atmosphere, variable temperature program, TG method. The sample was first heated in a chlorine atmosphere to 975°C. This converted the titanium to volatile titanium chloride, leaving a residue of carbon in the sample container. The amount of carbon residue was then determined by heating from 475-600°C in an air atmosphere, resulting in the oxidation of carbon to carbon dioxide. These processes are illustrated in the TG curve in Figure 4.34. 

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