Chlorination

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chlorination

[‚klȯr·ə′nā·shən]
(chemistry)
Introduction of chlorine into a compound. Also known as chloridization.
Water sterilization by chlorine gas.
(textiles)
A process in which wool is treated with a solution of hypochlorite and an acid or similar mixture to reduce the tendency of the fiber to shrink by matting.

Chlorination

 

in nonferrous metallurgy, a technological process involving heating materials containing nonferrous metals in the presence of chlorine gas, chlorine-containing gases, or chlorine salts in order to recover and separate the nonferrous metals.

The process is based on the interaction of metal oxides or sulfides with chlorine or hydrogen chloride in reversible reactions. The oxides for which the Gibbs free energy of the reactions has large negative values, for example, PbO, ZnO, and Ag2O, are chlorinated at low concentrations of chlorine in a gaseous medium containing oxygen. The oxides with large positive values of the Gibbs free energy, such as SiO2, TiO2, and A12O3, exhibit virtually no reaction with chlorine gas, since the presence of even minute amounts of oxygen in the gaseous medium inhibits the formation of chlorides. The chlorination of oxides is facilitated in the presence of substances that take up free oxygen and reduce its concentration in the gas phase, such as carbon, hydrogen, and sulfur dioxide. Thus, by changing the composition of the gas phase and the temperature of the process, it is possible to choose the conditions for selective chlorination; specifically, in the presence of oxygen and water vapor it is possible to chlorinate a number of nonferrous metals while keeping iron in the form of an oxide, whereas in a reducing atmosphere the iron oxides are converted into chlorides.

In addition to elemental chlorine and HCl, the chlorinating agents used include inexpensive salts, such as rock salt (NaCl), sylvinite (KCl · 2NaCl), and calcium chloride (CaCl2). In this case, chlorination, especially when using the low-volatile CaCl2, proceeds through the decomposition of the salt by water vapor with the formation of HCl. The presence of SO2 or SiO2, which form CaSO4, CaSiO3, and other compounds, assists the decomposition of the salt chlorinating agent.

Chlorination can be carried out by roasting, chloridation, or segregation. Chlorination by roasting is conducted at relatively low temperatures, leading to the formation of nonvolatile chlorides. It is achieved in electric furnaces, fluidized-bed furnaces, tube furnaces, or multiple-hearth roasting furnaces. It is used in the production of magnesium to convert magnesium oxide to the chloride, which is then subjected to electrolysis. It is also used to recover cobalt and copper from low-grade materials, mostly from pyrite cinders and cobalt-nickel mattes. In this case, the cobalt, copper, and zinc are converted to chlorides and leached out with water or a weak acid, while the iron is not chlorinated and remains in the form of oxides in the solid residue.

Chlorination by chloridation is conducted at higher temperatures than chlorination by roasting, which ensures the volatilization of the metal chlorides. It is a more versatile method, making possible the recovery of numerous nonferrous and rare metals, as well as gold and silver.

Segregation requires smaller amounts of chlorinating agents than the chloridation method and is conducted at lower temperatures. However, in order to obtain a concentrate, an additional operation is needed, such as flotation or magnetic separation.

Chlorination is also used to remove impurities from molten metals, for example, sodium and calcium from aluminum, zinc from lead, and lead from tin. Work is under way to develop methods of recovering copper and cobalt from nickel converter matte by means of chloride melts.

REFERENCES

Smirnov, V. I., and A. I. Tikhonov. Obzhig mednykh rud i kontsentratov, 2nd ed. Moscow, 1966.
Morozov, I. S. Primenenie khlora v metallurgii redkikh i tsvetnykh metallov. Moscow, 1966.
Gudima, N. V., and Ia. P. Shein. Kratkii spravochnik po metallurgii tsvetnykh metallov. Moscow, 1975.

I. L. REZNIK


Chlorination

 

in organic chemistry, the direct substitution of chlorine atoms for hydrogen atoms in organic compounds. Chlorination may be accomplished by the action of free chlorine or compounds that generate chlorine, for example, sulfuryl chloride, SO2Cl2. Its mechanism is determined by the nature of the organic compound and the reaction conditions. Thus, saturated hydrocarbons react with chlorine upon irradiation by ultraviolet light according to a radical-chain mechanism:

Cl2 → Cl + Cl; Cl + CH4 → CH3 + HCl;

CH3 + Cl2 → CH3Cl + Cl; and so forth

The above reaction is the basis of the industrial production of methyl chloride, methylene chloride, chloroform, and carbon tetrachloride from methane and the production of amyl chlorides from the pentane fractions of gasoline. The chlorination of aromatic compounds proceeds according to an ionic mechanism in the presence of an acid catalyst, such as A1Cl3 or FeCl3. For example, chlorobenzene is produced in industry this way:

Cl2 + FeCl3 → + [FeCl4]; C6H6 + Cl+ → C6H5Cl + H+;

[FeCl4] + H+ → FeCl3 + HCl

By taking into account the differences in the mechanisms of the chlorination of aliphatic and aromatic compounds, the chlorination of aliphatic-aromatic hydrocarbons may be controlled: the addition of FeCl3 leads to the replacement of hydrogen atoms in the aromatic ring, while ultraviolet irradiation and higher temperatures facilitate the chlorination of the aliphatic side chains. Therefore, in industry, the chlorination of toluene yields chlorotoluenes (in the presence of FeCl3) or benzyl chloride, C6H5CH2Cl (by the action of ultraviolet irradiation). At high temperatures, the direct substitution of chlorine atoms for hydrogen atoms may also be accomplished in the alkyl groups of olefins (with retention of the multiple bond); for example,

This reaction is used in industry for the production of allyl chloride, the starting substance in the production of glycerine.

Sometimes chlorination is taken in a broader sense to mean the creation of the C—Cl bond by any means, such as the attachment of chlorine, hydrogen chloride, hypochlorous acid, or nitrosyl chloride through multiple bonds or the substitution of chlorine for other functional groups, such as hydroxyl groups in alcohols, carboxylic groups in acids, and amino groups in aromatic amines after prior diazotization. Thus, in industry, dichloroethane, which is a starting material in one method for the production of vinyl chloride, is produced by the addition of chlorine to ethylene. Tetrachloroethane, which is used in the production of trichloroethylene, is produced by the chlorination of acetylene, and chlorinated rubbers are produced by the chlorination of some rubbers. Vinyl chloride, ethyl chloride, and chloroprene are produced in industry by the reaction of unsaturated compounds with hydrogen chloride.

Chlorination is also used in the production of a number of important products, such as insecticides (hexachloran, polychloropinene, polychlorocamphene), herbicides (esters of 2,4-dichlorophenoxyacetic acid), and hexachloroethane (a camphor substitute).