What unit operations are used in dye manufacturing?
All synthetic organic dyes and pigments contain a ring structure of atoms. The same formation is present in dye intermediates. It is usual also for the dye intermediate to have one or more of the groups of atoms that react chemically to form salts.A simple example of an intermediate is -naphthol, which is obtained from naphthalene by heating it with 96 percent sulfuric acid at 160 C (320 F), adding sodium sulfate, and volatizing the remaining naphthalene with steam. Sodium naphthalene-2-sulfonate, left behind after this steam treatment, is then fused with caustic soda to yield -naphthol. The -naphthol is separated from the mixture by diluting it with water and treating it with sulfuric acid. These reactions, known respectively as sulfonation and alkali fusion, are given the name unit processes.A number of such unit processes are available to the chemical manufacturers, and by means of various permutations, it is possible to synthesize the several hundred intermediates needed for the manufacture of the modern range of colorants.Nitration.In nitration, a nitro group is substituted for a hydrogen atom in an aromatic hydrocarbon molecule by the action of nitric acid. Benzene yields nitrobenzene by reaction with a mixture of nitric and sulfuric acids at a temperature not exceeding 50 C (122 F). Toluene, with mixed acid at 30-35 C (86-95 F), yields orthonitrotoluene, metanitrotoluene, and paranitrotoluene. Technical quality nitrotoluenes are obtained from the crude nitration product by physical separation methods including fractional distillation; 1-nitronaphthalene is obtained when naphthalene is nitrated. Nitration is a strongly exothermic, or heat-releasing, reaction; in large-scale nitrations the temperature must be maintained between established limits and efficient means of cooling provided. Cast-iron, mild steel, or stainless-steel vessels are used, arranged to permit agitating the mixture. The addition of nitric acid is carefully controlled by instruments that detect incipient rises in temperature. Other safeguards stop the addition in case the stirring mechanism is halted by a power or mechanical failure.Reduction.Amines, chemical compounds formed by substituting organic radicals for the hydrogen atoms of ammonia, may be obtained from nitro compounds by reduction; that is, replacement of oxygen by hydrogen. This is accomplished by mixing the nitro compound with iron borings and a minimum quantity of aqueous hydrochloric acid in a cast-iron reducer with a powerful agitator. The end products are aniline, or other amines, and ferric oxide. The aniline, which separates as an upper layer above the aqueous suspension of iron and oxide, is removed mechanically and purified by steam distillation followed by fractionation. Analogous processes are employed in making other amines from their corresponding nitro compounds. Halogenation.Halogenation is the introduction, by direct or indirect means, of a halogen (fluorine, chlorine, or bromine) into molecules of dye intermediates. In many cases direct substitution can be effected using elemental chlorine or bromine with or without a catalyst. In other cases a diazotized amine is treated with cuprous chloride or bromide. Indirect means must be employed for introducing fluorine. Hydrogen fluoride (HF), for example, is used to displace chlorine in chloro compounds. Amination.The amination, or conversion of chloro compounds to amines, can be accomplished by ammonolysis, a process that involves heating the chloro compound with aqueous ammonia in a steel autoclave (equipment for working at pressures above atmospheric). Chloronitrobenzene, for example, is transformed into nitroaniline by this process. Several industrial amination processes exist. In the Dow Process monochlorobenzene is converted into aniline by heating it with aqueous ammonia at 240 C (464 F) in the presence of a copper oxide catalyst. In the Halcon Process phenol undergoes ammonolysis to yield aniline. Hydroxylation.Hydroxylation is the introduction of a hydroxyl, or OH, group into the dye-intermediate molecule. Among the methods used for this are fusion by sodium hydroxide, Bucherer reaction (conversion of aromatic amines into phenolic compounds by aqueous sulfite or bisulfite), hydrolysis of chloro compounds under various reaction conditions according to the reactivity of the chloro compound, decomposition of diazonium salts by hot aqueous sulfuric acid, hydrolysis of sulfo groups, and oxidation (described below); for example, the compound cumene yields phenol and acetone. Oxidation.Oxidation, the combination of a substance with oxygen, or, generally, any reaction in which an atom loses electrons, is a ubiquitous chemical reaction in dye chemistry; only a few examples can be given here. The conversion of methyl groups into carboxylic acids, and more especially into styryl compounds, is effected by various oxidants such as sodium hypochlorite, potassium permanganate, and sodium dichromate. The catalyzed oxidation of naphthalene with air leads to phthalic anhydride. Other important reactions include oxidation of leuco compounds to dyes and the formation of complex polycyclic substances from simpler molecules. Benzidine rearrangement.The benzidine rearrangement consists of the conversion of nitrobenzene and its derivatives into derivatives of biphenyl in a two-stage process. The first stage consists of alkaline reduction with zinc dust to the hydrazobenzene; and the second of treatment with hydrochloric acid. The above and other unit processes are used in various combinations in order to produce the important intermediate chemicals that are, in turn, used to manufacture the dyes themselves.H Acid.H Acid, which is 1-amino-8-hydroxynaphthalene-3,6-disulfonic acid, an important dye intermediate, is produced from naphthalene by a combination of the unit processes of sulfonation, nitration, reduction, and hydrolysis. H Acid is used in the manufacture of a large number of azo dyes. Diaminostilbene disulfonic acid.4,4'-diaminostilbene-2,2'-disulfonic acid is produced from p-nitrotoluene by a combination of the unit processes of sulfonation, oxidation, and reduction. This product is used in large quantities for manufacture of fluorescent brightening agents.
