Dye Manufacturing
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Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998 Dye Manufacturing Industry Description and Practices • Acid dyes: used for coloring animal fibers via acidified solution (containing sulfuric acid, This document discusses the synthesis of dyes acetic acid, sodium sulfate, and surfactants) in and pigments used in textiles and other indus- combination with amphoteric protein tries. Dyes are soluble at some stage of the appli- • Azoic dyes: contain the azo group (and formic cation process, whereas pigments, in general, acid, caustic soda, metallic compounds, and retain essentially their particulate or crystalline sodium nitrate); especially for application to form during application. A dye is used to impart cotton color to materials of which it becomes an inte- • Basic dyes: amino derivatives (and acetic acid gral part. An aromatic ring structure coupled and softening agents); used mainly for appli- with a side chain is usually required for resonance cation on paper and thus to impart color. (Resonance structures • Direct dyes: azo dyes, and sodium salts, fixing that cause displacement or appearance of absorp- agents, and metallic (chrome and copper) com- tion bands in the visible spectrum of light are pounds; used generally on cotton-wool, or cot- responsible for color.) Correlation of chemical ton-silk combinations structure with color has been accomplished in • Mordant or chrome dyes: metallic salt or lake the synthesis of dye using a chromogen-chro- formed directly on the fiber by the use of alu- mophore with auxochrome. Chromogen is the minum, chromium, or iron salts that cause aromatic structure containing benzene, naphtha- precipitation in situ lene, or anthracene rings. A chromophore group • Lake or pigment dyes: form insoluble compounds is a color giver and is represented by the follow- with aluminum, barium, or chromium on mo- ing radicals, which form a basis for the chemical lybdenum salts; the precipitates are ground to classification of dyes when coupled with the chro- form pigments used in paint and inks mogen: azo (–N=N–); carbonyl (=C=O); carbon • Sulfur or sulfide dyes: contain sulfur or are pre- (=C=C=); carbon-nitrogen (>C=NH or –CH=N–); cipitated from sodium sulfide bath; furnish nitroso (–NO or N–OH); nitro (–NO or =NO–OH); 2 dull shades with good fastness to light, wash- and sulfur (>C=S, and other carbon-sulfur ing, and acids but susceptible to chlorine and groups). The chromogen-chromophore structure light is often not sufficient to impart solubility and • Vat dyes: impregnated into fiber under reduc- cause adherence of dye to fiber. The auxochrome ing conditions and reoxidized to an insoluble or bonding affinity groups are amine, hydroxyl, color. carboxyl, and sulfonic radicals, or their deriva- tives. These auxochromes are important in the Chemical classification is based on chromogen. use classification of dyes. A listing of dyes by use classification comprises the following: For example, nitro dyes have the chromophore –NO2. The Color Index (C.I.), published by the So- • Acetate rayon dyes: developed for cellulose ac- ciety of Dyers and Colourists (United Kingdom) etate and some synthetic fibers in cooperation with the American Association of 298 Dye Manufacturing 299 Textile Chemists and Colorists (AATC), provides sludges, spent acids, and process residues from a detailed classification of commercial dyes and the manufacture of chrome yellow and orange pigments by generic name and chemical consti- pigments, molybdate orange pigments, zinc yel- tution. This sourcebook also gives useful infor- low pigments, chrome and chrome oxide green mation on technical performance, physical pigments, iron blue pigments, and azo dyes. properties, and application areas. Dyes are synthesized in a reactor, filtered, Pollution Prevention and Control dried, and blended with other additives to produce the final product. The synthesis step Every effort should be made to substitute degrad- involves reactions such as sulfonation, haloge- able and less toxic ingredients for highly toxic nation, amination, diazotization, and coupling, and persistent ingredients. Recommended pol- followed by separation processes that may in- lution prevention measures are to: clude distillation, precipitation, and crystalliza- • Avoid the manufacture of toxic azo dyes and tion. In general, organic compounds such as provide alternative dyestuffs to users such as naphthalene are reacted with an acid or an alkali textile manufacturers. along with an intermediate (such as a nitrating • Meter and control the quantities of toxic in- or a sulfonating compound) and a solvent to form gredients to minimize wastage. a dye mixture. The dye is then separated from • Reuse by-products from the process as raw the mixture and purified. On completion of the materials or as raw material substitutes in manufacture of actual color, finishing operations, other processes. including drying, grinding, and standardization, • Use automated filling to minimize spillage. are performed; these are important for maintain- • Use equipment washdown waters as makeup ing consistent product quality. solutions for subsequent batches. • Return toxic materials packaging to supplier Waste Characteristics for reuse, where feasible. • Find productive uses for off-specification prod- The principal air pollutants from dye manufac- ucts to avoid disposal problems. turing are volatile organic compounds (VOCs), • Use high-pressure hoses for equipment clean- nitrogen oxides (NO ), hydrogen chloride (HCl), x ing to reduce the amount of wastewater gen- and sulfur oxides (SO ). x erated. Liquid effluents resulting from equipment • Label and store toxic and hazardous materi- cleaning after batch operation can contain toxic als in secure, bunded areas. organic residues. Cooling waters are normally recirculated. Wastewater generation rates are of A dye and pigment manufacturing plant the order of 1–700 liters per kg (l/kg) of product should prepare and implement an emergency except for vat dyes. The wastewater generation plan that takes into account neighboring land rate for vat dyes can be of the order of 8,000 l/kg uses and the potential consequences of an emer- of product. Biochemical oxygen demand (BOD) gency. Measures to avoid the release of harmful and chemical oxygen demand (COD) levels of substances should be incorporated in the design, reactive and azo dyes can be of the order of 25 operation, maintenance, and management of the kg/kg of product and 80 kg/ kg of product, re- plant. spectively. Values for other dyes are, for example, BOD5, 6 kg/kg; COD, 25 kg/kg; suspending sol- Target Pollution Loads ids, 6 kg/kg; and oil and grease, 30 kg/kg of product. Implementation of cleaner production processes Major solid wastes of concern include filtra- and pollution prevention measures can yield both tion sludges, process and effluent treatment slud- economic and environmental benefits. ges, and container residues. Examples of wastes Specific reduction targets for the different pro- considered toxic include wastewater treatment cesses have not been determined. In the absence 300 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES of specific pollution reduction targets, new plants from these levels must be described in the World should always achieve better than the industry Bank Group project documentation. The emis- averages cited in “Waste Characteristics,” above. sions levels given here can be consistently achieved by well-designed, well-operated, and Treatment Technologies well-maintained pollution control systems. The guidelines are expressed as concentrations Air Emissions to facilitate monitoring. Dilution of air emissions or effluents to achieve these guidelines is unac- Stack gas scrubbing and/or carbon adsorption ceptable. (for toxic organics) are applicable and effective All of the maximum levels should be achieved technologies for minimizing the release of sig- for at least 95% of the time that the plant or unit nificant pollutants to air. Combustion is used to is operating, to be calculated as a proportion of destroy toxic organics. Combustion devices annual operating hours. should be operated at temperatures above 1,100° C (when required for the effective destruction of Air Emissions toxic organics), with a residence time of at least 0.5 second. The emissions levels presented in Table 1 should be achieved. Liquid Effluents Liquid Effluents Effluent treatment normally includes neutraliza- tion, flocculation, coagulation, settling, carbon ad- The effluent levels presented in Table 2 should sorption, detoxification of organics by oxidation be achieved. (using ultraviolet systems or peroxide solutions), and biological treatment. Exhausted carbon from adsorption processes may be sent for regenera- Table 1. Emissions from Dye Manufacturing tion or combustion. Reverse osmosis, ultrafiltra- (milligrams per normal cubic meter) tion, and other filtration techniques are used to Parameter Maximum value recover and concentrate process intermediates. Chlorine (or chloride) 10 VOCs 20 Solid Hazardous Wastes Contaminated solid wastes are generally incin- erated, and the flue gases, when acidic, are Table 2. Effluents from Dye Manufacturing scrubbed. (milligrams per liter, except for pH) Parameter Maximum value Emissions Guidelines pH 6–9 Emissions levels for the design and operation of BOD 30 each project must be established through the en- COD 150 vironmental assessment (EA) process on the ba- TSS 50 Oil and grease 10 sis of country legislation and the Pollution