The extraction step

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The extraction step is highlighted here from different points of view: raw material used (Section 19.3.1); chemical consumption (Section 19.3.2); and energy consumption (Section 19.3.3).

19.3.1 Raw materials

The successful introduction of natural dyes into commercial textile production is dependent on the formation of a standardised dye. A significant improvement in the overall consumption of energy, chemicals and water has to be reached in relation to the state of the art processes. Easy handling and storage of plant material with a minimum consumption of energy and formation of wastes has to be achieved. The material should contain a high content of extractable dyestuff. Production of commercial natural dyestuff can follow two different strategies:

• stabilised (e.g. dried) plant material, such as madder roots, green nuts and onion peels;37

• concentrated dyestuffs containing liquid or solid products from extraction processes.38

19.3.2 Chemical consumption during extraction step

In scientific investigations to identify the chemical nature of a natural dye, the use of solvents or chemicals is quite common, however for a full-scale process and commercial production of natural dyes the chemical consumption during the dyestuff extraction/stabilisation has to be thoroughly considered.39'40 Extraction with the addition of organic solvent or chemicals requires strict calculation of chemical consumption due to the losses of chemicals with the extracted plant residues.

Detailed calculations given in the literature demonstrate the major problem of natural dye extraction. Approximately 1-5% o.w. of the plant can be expected to be extractable components useful for dyeing purposes. Considerable amounts of added solvent or chemicals will remain in the extracted residue. Figure 19.3 gives a graphic summary of model calculations for chemical losses in natural dye extraction.18 When a rather high average dyestuff content of P = 0.02 is considered and 1 kg of solvent remains in 1 kg of extracted plant material, the value for the consumption of chemicals reaches 0.5 at c = 0.01 (10 g of chemicals added in per 1 kg of aqueous solvent) and 1.0 at c = 0.02. A value of 1 kg/kg for consumption indicates that 1 kg of chemicals will remain in the extracted plant wastes for each kilogram of natural dyestuff extracted. Thus great care has to be taken when chemicals or solvents other than water are used for dyestuff extraction.

19.3.3 Energy consumption

Direct use of the stabilised (e.g. dried) material in the dyehouse requires energy for stabilisation, handling and transport of large amounts of plant
















c n





— c = 0.02 kg/kg water -o - - c = 0.01 kg/kg water





Fig. 19.3 Consumption of chemicals during dyestuff extraction, defined as mass of chemicals per mass of extracted dyestuff (kg/kg), as a function of dyestuff content, P (kg dyestuff per kg plant material), in the plant source, calculated for different concentrations, c, of chemicals used in water.

material. Formation of extracts requires energy consumption for concentration but final volumes have to be transported. A remarkable difference between these two production strategies exists, when estimated energy consumption is compared (Table 19.6). In both cases wet plant material was considered as the raw material.

In the case of extract formation and production of a concentrated product, energy is required for extraction at the site of processing, reconcentration of the product and transport to the dyehouse. In addition, heating the dyebath to 95 °C consumes energy. Besides evaporation, membrane technology has been considered as an alternative technology for reconcentration (Table 19.6). A quite low value of 1 : 5 has been considered as the liquor ratio for the extraction step. This value is of particular importance with regard to the total consumption of energy, because about 50% of the energy consumption will be spent removing water during the production of the concentrated dye. The energy consumption for transportation remains low for short-distance transportation (<100 km). A considerable amount of energy will be required for heating the dyebath to 95 °C, as coupling between extract formation and the dyeing step is not possible. For concentrate formation and dyeing process together, the energy consumption is estimated to range from 600 to 900 MJ for 100 kg of goods.

In cases of direct use of the wet plant material, the material is transported to the dyehouse and an aqueous extraction step is performed in the dyeing apparatus before the dyeing step starts. In this case the 95 °C hot extract can be used directly as the dyebath, thereby saving considerable amounts of energy. The main energy consumption will be for transportation and hot extraction. For stabilisation by drying, additional energy is required. The total energy consumption ranges from 300 to 500 MJ.

Independent of the strategy used to produce the natural dyestuff, in both cases approximately 200 kg of wet plant material will be released after the extraction. In the case of aqueous extraction the extracted plant material can be processed as usual, e.g. as animal feed, for composting, for use as energy crop or disposal/landfill because no chemical load has to be removed before further use.

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