Raw Materials and Wastes for Dyeing and Printing Processes

Table 9 shows the auxiliaries required for different dyes and Table 8 shows the amount of water used and liquor rations of dyeing machines.

Wastewater generation from a typical dyeing facility is estimated at 3800 to 7500 m3 per day. Including the dyeing, post scouring and rinsing processes, approximately 100 to 150 m3 of wastewater per tonne of product are produced for disperse dyeing, while 125 to 170 m3 per tonne is more typical for direct and reactive dyeing.

The primary source of wastewater is spent dyebath and washwater, which contain by-products (hydrolyzed dye), some intact dye, and auxiliary chemicals. In addition to process water and chemicals, a major source of toxic pollutants in wastewater is cleaning solvents used in dyeing and printing machine cleaning, such as oxalic acid, hydrochloric acid and carbon tetrachloride.

With the abundance of individual dyes and the wide range of dyeing equipment in use today, it is difficult to summarize waste stream characteristics. In general, wastewater from batch dyeing is high in volume and pollutant load, and tends to contain heavy metals, aromatics, and halogenated hydrocarbons from the dyebath makeup. All are toxic to aquatic life. In addition to the dyestuff itself, many auxiliary chemicals are used to aid in dye transfer; the majority of these chemicals, including unreacted color, are discharged with the spent bath [2].

Table 8: Water Use and Liquor Rations of Dyeing Machines [26] [30]
Dyeing Machine Water consumption (m3/tonne) Liquor/goods ratio
Continuous 137 1:1
Beck 234 17:1
Jet 200 12:1
Jig 100 5:1
Beam 167 10:1
Package 184 10:1
Paddle 292 40:1
Stock 167 12:1
Skein 250 17:1

As mentioned earlier, dyes and auxiliaries used in printing are similar to those used in fabric dyeing. However, the colour application techniques are quite different. In the most commonly used technique, pigment printing, the main source of waste is from the cleanup, during which unused printing paste is removed from the screen. Consequently, proper planning of paste use and housekeeping are major issues in minimizing waste in printing operations [2]. Volatile organic compounds from the paste composition are generated. Table 11 gives, according to reference 33, the characteristics of a typical textile fabric printing run, characteristics for different printing equipment and sources of mineral spirit emissions.

Table 9: Auxiliaries Required for Satisfactory Dyeing [34]
Chemical Auxiliary Acid Direct Basic Disperse Mordant Premetalized Reactive Sulfur VAT
Acetic acid x   x x x x x (1) x
Ammonium acetate x                
Ammonium phosphate x                
Ammonium sulphate x       x x      
Aromatic amines   x              
Buffer             x    
Defoamer       x          
Dispersing agents       x          
Formic acid x   x   x x      
Gelatin                 x
Hydrochloric acid   x              
Hydrogen peroxide               x x
Leveling or retarder agents x   x x         x
Oxalic acid     x x          
Penetrating agents         x x      
Potassium dichromate         x x      
Sequestering agents   x   x          
Sodium acetate     x            
Sodium carbonate   x         x x  
Sodium chloride   x         x x  
Sodium dichromate         x x x    
Sodium hydrosulfite             x   x
Sodium hydroxide                 x
Sodium nitrate   x              
Sodium Sulfate(2) x   x   x x      
Sodium sulfide             x    
Soluble oil               x  
Sulfuric acid x     x x        
Urea   x       x      
Wetting agent   x              

Table 10: Application Calls and Suitability for Natural and Synthetic Fibers [20]
Dye Class Wool Cotton Cellulose Derivatives Polyamide Polyester Acrylics
Basic x x x x x x
Direct   x        
Sulfur   x        
Azoic   x x      
Ingrain         x  
Vat   x x      
Acid Leveling x     x    
Acid milling x     x    
Mordant x (x)   x    
Metal Complex x     x   x
Disperse       x   x
Reactive (x) x x x x  
Pigment       x x x

X=suitable; (X)=of secondary importance. Pigments may be applied to any substrate by the use of adhesives.

Table 11: Typical Textile Fabric Printing Run Characteristics (1) and Sources of Mineral Spirit Emissions from a Typical Textile Fabric Printing Run [33]
Roller Rotary Screen Flat Screen
Units Range Avg. Range Avg. Range Avg.
Characteristics of the run
Wet Pickup Rate(1) Kg/Kg 0.51-0.58 0.56 0.10-1.89 0.58 0.22-0.83 0.35
Fabric Weight (2) Kg/m2   0.116   0.116   0.314
Mineral Spirits added to print paste weight % 0-60 26 0-50 3   23
Print Paste used in run (3) Kg 673-764 741 137-2497 764 787-2975 1254
Mineral Spirits used in run (4) Kg 0-458 193 0-1249 23 181-684 288
Source of emissions
Wasted mineral spirits (potential water emissions) (5) Kg 0-28 12 0-77 1 11-42 18
Overprinted mineral spirit fugitives (6) Kg 0-16 7 0-44 1 6-24 10
Tray and barrel fugitives (7) Kg 0-1 1 0-4 0 1-2 1
Flashoff fugitives (7) Kg 0-7 3 0-19 0 3-10 4
Dryer emissions (7) Kg 0-405 170 0-1105 21 160-606 255

Length of run=10,000 m, fabric width=1.14 m, total fabric area=11,400 m2, line speed=40 m/min, distance, printer to oven = 5 m.

(1) Wet pickup ratio is a method of yield calculation in which mass of print paste consumed is divided by mass of fabric used.

(2) Only average fabric weight is presented.

(3) Print paste used per fabric area multiplied by area of fabric printed.

(4) Print paste used in run multiplied by mineral spirits added to print paste, weight percent.

(5) Estimate provided by industry contact.

(6) Estimated on the basis of 2.5cm of overprint on each side of fabric.

(7) Emission splits calculated from percentages provided by evaporation computations.