Textiles Industry
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Textiles Industry

The textile industry is one of the longest and most complicated industrial chains in manufacturing industry. It is a fragmented and heterogeneous sector dominated by SMEs, with a demand mainly driven by three main end-uses: clothing, home furnishing and industrial use.

The textile chain begins with the production or harvest of raw fibre. The so-called “finishing processes” (i.e. pretreatment, dyeing, printing, finishing and coating, including washing and drying) represent the main processes and techniques in the textile industry. In addition upstream processes such as, for example, synthetic fibre manufacturing, spinning, weaving, knitting, etc. have a significant influence on the environmental impact of the subsequent wet processing activities. The “finishing processes” can take place at different stages of the production process, the sequence of treatments being very variable and dependent on the requirements of the final user.

The main environmental concern in the textile industry is about the amount of water discharged and the chemical load it carries. Other important issues are energy consumption, air emissions, solid wastes and odors, which can be a significant nuisance in certain treatments. The key findings about some particular processes in the textile industry you find below.

Wool scouring

Wool scouring with water leads to the discharge of an effluent with a high organic content (2 to 15 l/kg of greasy wool at about 150 - 500g COD/kg of wool) and variable amounts of micro- pollutants resulting from the pesticides applied on the sheep. The most common pesticides are organophosphorous (OP), synthetic pyrethroids (SP) and insect growth regulators (IGR).

Pre-treatment Process

A large percentage of the total emission load from textile industry activities is attributable to substances that are already on the raw material before it enters the finishing mill (e.g. impurities and associated materials for natural fibers, preparation agents, spinning lubricants, sizing agents, etc.). All these substances are usually removed from the fibre during the pretreatment process before coloring and finishing. The removal of auxiliaries such as, spinning lubricants, knitting oils and preparation agents by wet treatment may lead to the discharge not only of hard-to- biodegrade organic substances such as mineral oils, but also of hazardous compounds such as poly-aromatic hydrocarbons, APEO and biocides. Typical COD loads are in the order of 40 - 80 g/kg fibre. When the substrate is submitted to a dry process (heat-setting) before washing, the auxiliaries present on the substrate become airborne (emission factors of 10 - 16 g C/kg are typical of mineral oil-based compounds).

Desizing

The washing water from the desizing of cotton and cotton-blend fabrics may contain 70 % of the total COD load in the final effluent. The emission factor can well be in the order of 95 g COD/kg of fabric, with COD concentrations often above 20 000 mg COD/l.

Bleaching

Sodium hypochlorite bleaching gives rise to secondary reactions that form organic halogen compounds commonly measured as AOX (trichloromethane accounts for the bulk of the compounds formed). For the combined application of hypochlorite (1st step) and hydrogen peroxide (2nd step) values of 90 - 100 mg Cl/l of AOX have been observed from the exhausted NaClO-bleaching bath. Concentrations up to 6 mg Cl/l can still be found in the spent H2O2- bleaching bath, due to the carry over of the substrate from the previous bath.

Compared to sodium hypochlorite, the amount of AOX formed during chlorite bleaching is much lower. Recent investigations have shown that the formation of AOX is not caused by the sodium chlorite itself, but rather by the chlorine or hypochlorite present as impurities or are used as activating agents. The handling and storage of sodium chlorite needs particular attention because of toxicity, corrosion and explosion risks.

Dyeing Process

Apart from a few exceptions (e.g. the thermosol process, pigment dyeing, etc.), most of the emissions originating from the dyeing process are emissions to water. Water-polluting substances can originate from the dyes themselves (e.g. aquatic toxicity, metals, colour), auxiliaries contained in the dye formulation (e.g. dispersing agents, anti-foaming agents, etc.), basic chemicals and auxiliaries used in dyeing processes (e.g. alkali, salts, reducing and oxidizing agents, etc.) and residual contaminants present on the fibre (e.g. residues of pesticides on wool, spin finishes on synthetic fibers). Consumption and emission levels are strongly related to the type of fibre, the make-up, the dyeing technique and the machinery employed.

In batch dyeing, the concentration levels vary greatly in the dyeing sequence. Generally, spent dye baths have the highest concentration levels (values well above 5000 mg COD/l are common). The contribution of dyeing auxiliaries (e.g. dispersing and leveling agents) to the COD load is especially noticeable when dyeing with vat or disperse dyes. Operations like soaping, reductive aftertreatment and softening are also associated with high values of COD. Rinsing baths show concentrations 10 - 100 times lower than the exhausted dyeing bath and water consumption 2 to 5 times higher than for the dyeing process itself.

In continuous and semi-continuous dyeing, the water consumption is lower than in batch dyeing processes, but the discharge of highly concentrated residual dyeing-liquors can result in higher pollution load when short runs of material are processed (COD attributable to the dyestuffs may be in the order of 2 - 200 g/l). The padding technique is still the most commonly applied. The quantity of liquor in the padder can range from 10 - 15 liters for modern designs to 100 liters for conventional padders. The residual amount in the preparation tank can range from a few liters under optimized control conditions to up to 150 - 200 l. The total quantity of residual liquor increases with the number of batches per day.

Typical emission sources in printing processes include printing paste residues, waste water from wash-off and cleaning operations and volatile organic compounds from drying and fixing. Losses of printing pastes are particularly noticeable in rotary screen printing (losses of 6.5 - 8.5 kg per colour applied are common for textiles). With short runs (i.e. less than 250 m) the amount of losses may be higher than the quantity of paste printed on the textile substrate. Water consumption levels for cleaning of the equipment at the end of each run are in the order of about 500 l (excluding water for cleaning the printing belt). Printing pastes contain substances with high air emission potential (e.g. ammonia, formaldehyde, methanol and other alcohols, esters, aliphatic hydrocarbons, monomers such as, acrylates, vinylacetate, styrene, acrylonitrile, etc.).

Due to our experience deep knowledge of the inputs and outputs of the process is an essential part of good environmental management. This includes inputs of textile raw material, chemicals, heat, power and water, and outputs of product, waste water, air emissions, sludge, solid wastes and by- products. Monitoring process inputs and outputs is the starting point for identifying options and priorities for improving environmental and economic performance.

Optimizing water consumption by INNOCHEM in textile industry starts with controlling water consumption levels. The next step is reducing water consumption, through a number of often-complementary actions. These include improving working practices, reducing liquor ratio in batch processing, increasing washing efficiency, combining processes (e.g. scouring and desizing) and re- using/recycling water.

Benefits

Most of these measures proposed by INNOCHEM allow significant savings not only in water consumption, but also in energy consumption because energy is used to a great extent to heat up the process baths.