«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
In conical pan machines (Figure 10.1) the fibre is packed into a removable fibre carrier, which is located onto a central spigot in the base of the vessel. Liquor circulation is provided, via this connection, by an external pump. Associated pipework allows liquor to be circulated either from the base of the pack or from the top.
Pear-shapes machines have a removable perforated base plate through which liquor is circulated via an impeller, returning to the dyeing vessel via a weir. Fibre is loaded directly into these machines and a further perforated plate is positioned on top. Liquor circulation packs the fibre into the base of the machine between the two plates. To unload the machine both plates are removed by crane and the fibre manually removed.
Radial flow machines are characterised by a fibre carrier, equipped with a central perforated column from which liquor flows across the pack to the perforated walls of the carrier.
Loose fibre is typically packed into these machines manually. Capacity varies between 200 - 300 kg, with a working volume equivalent to between 7 and 10 litres per kg fibre. The low packing density in these machines allows liquor to circulate freely through the fibre pack at modest pressures, thus minimising mechanical damage to the fibre while ensuring level dyeing.
The bath is heated by closed steam coils in the base of the machine. In many cases the level of automation on these machines is low and temperature may be controlled manually with a simple steam valve. In other cases electro/pneumatic programmers or logic controllers may be installed to regulate time / temperature and to control the direction of liquor circulation.
Autoclaves can be equipped for operation at higher pressure (this is not the case when they are used for dyeing wool fibres).
Autoclaves of all designs may be fitted with an external holding tank, capable of accommodating at least the volume of the dyeing vessel. Such tanks are used to facilitate re-use of liquors in more than one dyeing. Occasionally more than one dyeing vessel may be connected to a common tank, allowing liquor to be shared between vessels.
The liquor ratio for loose fibre can vary between 1:4 to 1:12, depending on the type of machine, level of loading, type of fibre, etc. [32, ENco, 2001]
Figure 10.1: Schematic layout of a conical pan loose fibre dyeing machine [32, ENco, 2001]
10.2 Yarn Yarn can be processed either in hank form or in package. Different machines are used depending on the method chosen. They are used for all wet operations, that is, pretreatment, dyeing, application of finishing agents and washing.
10.2.1 Hank dyeing machines Hank dyeing machines are mostly of the single stick (Hussong) design, in which hanks are hung from the underside of the dyeing vessel lid on removable sticks. (Figure 10.2) The lid is lowered vertically onto the dyeing vessel, which consists of a simple box with a perforated false bottom.
Liquor is circulated by a reversible impeller, located vertically in a weir chamber at one end of the machine. Heating is typically by closed steam coils beneath the false bottom and on smaller machines by live steam injection. Temperature control is provided by electro-mechanical or programmable logic controllers. These devices may also control/time the timing of chemical and dye additions and any required cooling cycles. Machine capacities vary from 10 kg sample machines to 1000 kg machines. These larger machines may be coupled together in pairs with interconnecting pipework in such a way that yarn loads of up to 4000 kg can be dyed while still retaining the flexibility to dye individual 1000 kg lots.
Variations of this design may utilise a horizontal circulation impeller passing through a sealing gland at the base of the weir chamber. Such machines invariably have a concave bottom, which is said to improve circulation and reduce fibre to liquor ratio by a small margin. [32, ENco, 2001] Liquor ratios from 1:15 to 1:25 are typical for these machines.
Figure 10.2: Schematic diagram of a Hussong type hank dying machine [32, ENco, 2001] 10.
2.2 Package dyeing machines Three basic types of machine may be used for package dyeing wool yarns: horizontal or vertical spindle machines or tube type machines.
Horizontal spindle machines may be rectangular in design, similar to hank dyeing machines, but modified to take frames, onto which yarn packages are inserted horizontally, or alternatively may be a horizontal autoclave into which is wheeled the carrier containing the yarn packages.
Both types operate with high flow rate pumps, which are necessary to give good circulation of the dye liquor. These machines are usually used for bulky yarns, which are wound onto soft packages, again to increase dye liquor penetration.
Vertical spindle machines are the most commonly used (see Figure 10.3). The packages may be press packed onto vertical carrier spindles to increase payload, assist in dye liquor circulation and minimise liquor to fibre ratio.
Tube type machines consist of a number of vertical or horizontal tubes into which package carriers are inserted, the tubes forming individual dyeing vessels linked by common pipework and circulation pumps. These machines are more flexible than the above types because individual tubes can be blanked off to vary the overall load capacity of the machine.
Liquor ratios employed in package dyeing are close to 1:12 (typically from 1:8 to 1:15).
Machines with capacities of up to 500 kg are used in dyeing carpet yarns, with the facility to link two or more machine together when dyeing larger single batches. [32, ENco, 2001]
Figure 10.3: Schematic layout of a package dyeing machine [186, Ullmann's, 2000]
10.3 Fabric in rope form Wet treatments on fabrics in rope form can be carried out both in batch and in continuous processes.
10.3.1 Batch processes 10.3.1.1 Winch beck The common element in all winch beck machines is the winch mechanism used to move the fabric. The winch draws the fabric via a guide roller out of the bath and returns it in folds into the bath. In the conventional winch beck (see Figure 10.4), the bath stands still, while the fabric is kept in circulation by a reel positioned in the upper part of the machine. In modern winches both the bath and the fabric are kept in circulation, which improves homogenisation and exchange of the liquor with the fabric.
Greige goods are loaded into the winch either in rope form or in open width, which means that the winches must be 5 - 6 meters in width. The ends of the fabric piece to be dyed are sewn together to form endless loops over the rotating boom (winch).
Winch becks are primarily machines for dyeing, but for practical reasons both preparation and dyeing are normally carried out in the same machine.
These machines are essentially operated at atmospheric pressure although the development of synthetic fibres has led to the production of pressurised machines (HT machines may reach 130 – 140 ºC).
Winch beck is a very versatile machine and it can be used for all types of fabric. It is a very common technique for dyeing carpets (they are usually dyed in full width).
Typical bath ratios vary from 1:15 to 1:40 (typically 1:30 in the carpet sector) [171, GuT, 2001], which makes this technique fairly expensive due to high water and energy consumption.
However, recent technological developments have been made to improve its environmental performance (see Section 4.6.20)
Figure 10.4: Schematic representation of a winch beck dyeing machine [186, Ullmann's, 2000] 10.
3.1.2 Jet Jet machines (see Figure 10.5) have been designed with the aim of eliminating some of the problems associated with the use of winch machines.
The reel is eliminated and the fabric is placed in a closed tubular system. A jet of dye liquor is supplied through a venturi to transport the fabric through the tube. Turbulence created by the jet aids in dye penetration and prevents the fabric from touching the walls of the tube.
Since the fabric is frequently exposed to high liquor concentrations within the transport tube, relatively little dye bath is needed in the bottom of the vessel: just enough to ensure smooth movement from rear to front. Advantages of this machine are therefore low consumption of
water and short treatment time (e.g. short dyeing time). Typical liquor ratios vary between 1:4
and 1:20, ranging from 1:4 to 1:10 for fabric and from 1:6 to 1:20 for carpet (lower values apply to synthetic fibres while higher values are typical for cotton) [171, GuT, 2001].
Jets can usually be operated at high temperatures, which makes them very suitable for dyeing polyester fibres. A disadvantage, however, is the high mechanical stress on the textile, caused by the speed difference between the bath and the fabric. For this reason jets are not suitable for some delicate fabrics.
Depending on the shape of the fabric storage area (long shape machine or J-box compact machine), the type of nozzle and its position (above or below the level of the bath) various types of jets exist. Overflow, soft-flow and airflow dyeing machines can be regarded as developments of the conventional jet. The main features of these machines are reported in the following sections, while the latest developments in this dyeing technology are further described in Section 4.6.21.
Figure 10.5: Schematic representation of a jet dyeing machine [186, Ullmann's, 2000] 10.
3.1.3 Overflow Overflows (see Figure 10.6) have been designed for delicate knitted and woven fabrics made of natural and synthetic fibres. They are also found in the carpet sector.
The main difference between jet and overflows machines remains in the fact that in jet machines the fabric is transported by the bath flowing at high speed through the nozzle, while with overflows the fabric is transported by the gravitational force of the liquor overflow.
A winch (usually not motor driven) is located in the upper part of the machine and the fabric hangs over it. A longer length of textile hangs from the exit side of the winch than from the inlet side. Gravitational forces pull the longer length of textile downward more strongly than the shorter. The fabric is therefore soaked in the bath without any tension (transportation is very gentle).
Different designs are available on the market and some of them can operate under pressure and consequently at higher temperatures.
Typical liquor ratios for overflows range between 1:12 and 1:20.
Figure 10.6: Schematic representation of an overflow dyeing machine [69, Corbani, 1994] 10.
3.1.4 Soft-flow The so-called "soft-flow" machines use the same transport tube principle as overflow machines where the fabric is transported in a stream of dye liquor. However, while in overflow machines the reel is not motor driven, in soft-flow equipment the reel and the jet work in constant harmony to remove the fabric from the front of the storage area, expose it briefly to a high concentration of liquor within the transport tube, then return it to the rear of the vessel. The soft flow machines are more gentle on the fabric than conventional jet overflow machines.
The difference between air jet (see Figure 10.7) and jet machines is that in the former an air jet instead of a water jet keeps the fabric in circulation. The fabric passes into the storage area which contains a very small amount of free liquor. As a result, a reduction in water, energy and chemicals consumption can be achieved.
Because of the short liquor ratios achievable (from 1:2 to 1:5) the dye must be highly watersoluble.
Figure 10.7: Schematic representation of jet dyeing machine [186, Ullmann's, 2000] 10.
3.2 Continuous processes Machines for treatment in continuous processes of fabrics in rope form are essentially composed
of the following parts:
· a padding device for impregnating the fabric in rope form · a storage area for the fixation of the chemicals applied · a washing machine for fabric in rope form.
The padding device is composed of a long basin equipped with two or three rotating rollers at the inlet and another two at the outlet. The basin contains a concentrated solution of the chemicals and auxiliaries that have to be applied (desizing agents, bleaching agents, etc.). The fabric is pressed by the rotating rollers at the inlet in order to obtain a homogeneous absorption of the chemicals and the elimination of air. The other two rollers only squeeze the fabric. After squeezing, the fabric retains only a relatively low amount of bath. As a result high concentrations of the chemicals are needed, moreover the solutions must be adequately stabilised to avoid unwanted oxidation reactions, etc.
The storage area (also called reaction chamber) can have different shapes: one typical model is the J-Box. The J-Box is filled to 1/3 of its capacity with the treatment bath.
The main advantage of this technique is the high production capacity. On the other hand there is the risk of longitudinal creases, which can negatively affect the dyeing process. As a result this technique is mainly applied for white end-products for pretreatment operations (e.g. bleaching).
Figure 10.8: Example of continuous process for knitted fabric in rope form [69, Corbani, 1994]
10.4 Fabric in open-width 10.4.1 Batch processes 10.4.1.1 Beam The fabric is wound in open-width on a perforated cylinder called a beam (see Figure 10.9). The fabric is stationary and the bath is pumped through the beam. The direction of the flow is normally from the inside to the outside of the roll of textile.
The ends of the beam are covered with metal sheets before the fabric is wound on, to prevent a short circuit of the liquor.
Beams can operate both under pressure and at atmospheric pressure.
These machines are suitable for pretreatment operations like scouring and bleaching and also for dyeing of lightweight, wide and delicate goods. One disadvantage is represented by the risk of non-homogeneous penetration of chemicals and auxiliaries used for the treatment.
Figure 10.9: Schematic representation of a beam dyeing machine [18, VITO, 1998]
10.4.1.2 Jigger A jigger (see Figure 10.10) is composed of a trapezoidal tub containing the bath and two rolls on which the fabric is alternatively wound. In this type of machine the bath is stationary while the fabric is in motion. The fabric, initially wound on the first roll, flows through the bath and then is wound on the other one; the rotation is then reversed and the cycle continues. The fabric is led in its path by a few small guide rollers.
HT jiggers are also produced, allowing treatments at temperatures above 100 ºC. This type of machine is used not only for dyeing, but also for various wet treatments on fabrics in full width.
The main disadvantage of this system is the risk of non-homogeneity between the initial and final part of the roll. This is due to variations in feed speed and tension of the fabric, or temperature and chemicals concentration in the bath during the period of the treatment.
However, in modern jiggers tension of the fabric is kept constant during the whole process, thanks to special devices.
Figure 10.10: Schematic representation of a jigger [18, VITO, 1998] 10.
4.2 Semi-continuous and continuous processes
Some of the most widely used continuous and semi-continuous processes are:
· pad-batch · pad-roll · pad-jig · pad-steam · pad-dry · thermosol.
They are briefly described in the following sections.
Typical treatment steps in both semi-continuous and continuous processes are:
· application of the dye or finishing agent either by impregnation (using a padding device) or by means of other types of application systems (see Figure 10.12) · storage/fixation, which can be carried out in different ways depending on the process applied (e.g. dry heat, steam) · washing in continuous mode in open width.