Additional information to Scouring of wool

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1. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT

Wool is processed to yarn mainly via two systems: woollen and worsted. Scourers tend to specialise in wools for one system or the other. Woollen system scourers normally only scour the wool, though some may blend it before despatch to the customer. Worsted system scourers (referred to as combers) scour, card and comb the wool and their product is called top.

Within Europe, significant quantities of wool are obtained from the skins of slaughtered animals by fellmongering.

Cleaning and Washing with Water A conventional wool scouring set is shown in Figure 2.3. The process is carried out by passing the wool through a series of 4 – 8 wash bowls, each followed by a mangle or squeeze press which removes excess scouring liquor from the wool and returns it to the bowl. Clean water is added at the last bowl and passes via a counter-flow system from bowl to bowl with final discharge from the first bowl in a controlled manner to drain.

In the scouring bowls, suint is removed fro the wool by dissolution, grease by emulsification and dirt by suspension. For merino wool, the first bowl may be charged with water only and, in that case, its purpose is the removal of water-soluble suint before the wool enters the scouring process proper (this bowl is usually called “de-suint”).

In order to achieve grease emulsification, the scouring bowls are charged with detergent and often with sodium carbonate, or other alkali, which acts as a detergent builder. Concentrations of detergent and builder are usually highest in the first scour bowl and they decrease in subsequent bowls.

Detergents used by scourers are mainly synthetic non-ionic surfactants, namely alcohol ethoxylates and alkylphenol ethoxylates. Some scourers also report the use of “solvent-assisted detergents” for the removal of marking fluids from fleeces. Finally, the wool is rinsed by passing it through bolws containing water only.

In coarse wool scouring plants the final bowl of the scouring train is sometimes used for chemical treatments. In this case, it is isolated from the countercurrent liquor flow system and may also be isolated from the drain if the chemical treatment uses ecotoxic chemicals. The most commonly used treatment is bleaching, in which hydrogen peroxide and formic or acetic acid are added to the bowl. Other treatments sometimes applied include mothproofing, using a synthetic pyrethroid insecticide and acetic or formic acid, and sterilisation (of goat hairs) using formaldehyde.

Wool grease has a melting point around 40°C. Since removal of solid grease from wool by detergents is slow and difficult, 40 °C is the lowest temperature at which aqueous scouring liquors are effective for removing grease. In addition, non-ionic detergents lose efficiency rather rapidly below 60°C, which means that scour and rinse bowls are typically operated at 55 – 70°C.

After leaving the final squeeze roller the wool will contain 40 to 60% moisture. It is therefore dried by convection in a hot-air drier. The drier is usually heated either by closed steam pipes or by direct gas firing. The heat supply to the drier may be controlled by a signal from a device which senses the humidity of the drier atmosphere or the moisture content of the wool at the output end, thus saving energy and avoiding overdrying the wool.

The mechanical design of wool scours and the arrangements for circulating the scour and rinse liquors vary widely. Since these matters have a direct influence on energy and water usage, as well as the partial removal of contaminants from the effluent, it is important to illustrate them in more detail.

New generation scouring plants like the one illustrated in Figure 2.4 have an integrated system for grease and dirt recovery.

The dirt tends to settle at the bottom of the bowl and modern scour bowls usually have hopper-shaped bottoms from which the sludge is removed by gravity through a valve. Opening of the valve may be under the control of a timer or may respond to a signal from a turbidity meter which senses the thickness of the dirt suspension in the hopper bottom. The discharge from the scour bowl hopper bottom is led to a heavy solids settling tank where it is gravity-settled and the settled liquor partly recycled to scour bowl 1 and partly discharged. Flocculant may be added to the heavy solids settling tank to assist the separation of dirt, or a decanter centrifuge or hydrocyclone may be used in preference to gravity settling for dirt removal.

For grease recovery, modern scour bowls have a side tank in which the grease-rich liquors removed from the wool by the squeeze press are collected. From here, part of the flow may be pumped to the previous bowl or, in the case of bowl 1, to a primary grease centrifuge. The centrifuge separates the liquor into three phases. The top phase, referred to as the cream, is rich in grease and passes to secondary and possibly tertiary centrifuges for further dewatering, finally producing anhydrous grease; the bottom phase is rich in dirt and goes to the heavy solids settling tank; the middle phase is impoverished in both grease and dirt compared with the input and this is split, part being recycled to scour bowl 1 and part being discharged.

In a commonly used variation of the above recycled arrangements, the dirt and grease removal and recycling loops may be combined. In this case, scouring liquor may be removed from the bottoms of the bowls only, or from top and bottom, and passed first through the dir removal equipment, then through the primary grease centrifuge.

Some scourers recycle rinse water (see Figure 2.4). The flowdown from the first rinse bowl can be treated to make it suitable for addition to the feed to the final rinse bowl. Normally, this is accomplished by removing dirt in a hydrocyclone and processing the water through a membrane filtration plant to remove other impurities.

It is normally necessary to purge dirty liquors which collect at the bottoms of the rinse bowls, but this is not always the case.

Purging of rinse bowls will depend upon the efficiency of the bowls. Some modern scours habe rinse bowls discharge controlled by solid detectors, but genrally rinse bowls merely habe a timed discharge of bottom liquor which operates automatically whatever the state of the liquor [208, ENco, 2001].

The dirt removal and grease recovery loops described above serve several purposes. They save water, by recycling effluent to the scour, and they act as a process-integrated partial effluent treatment. The recovered wool grease can be sold, although the market for this by-product has been variable in most recent years. Finally, since the discharges from the loops are the only points at which heavily contaminated scour liquors are discharged, valves and meters at these points can be used to control the rate of water usage in the scouring section.

Cleaning and washing with solvent

Various solvent processes exist that use a non-aqueous solvent for scouring wool.

The Wooltech wool cleaning system involves the use of trichloroethylene and does not use any water in the washing process. A schematic layout is shown in Figure 2.5.

The following information was submitted by [201, Wooltech, 2001].

Wash bowls

Wool is received in bales, unpacked and then fed into the reception area. This wool is lightly broken up and fed through a series of solvent wash bowls (typically 3 or 4) and washed with a countercurrent flow of solvent. Up to 10 kg of solvent is added for the production of 500 kg of clean wool, however this is a function of plant management and maintenance, the exact plant arrangement, and the quality of wool being processed.

Clean, solvent saturated wool is taken from the last wash bowl to a centrifuge where the solvent concentration is reduced to around 4 wt%. A centrifuge has been found to be particularly effective in this duty owing to the low surface tension and significant density of trichloroethylene. The wool with a small quantity of solvent is taken to a dryer where warm air is used to evaporate the last quantity of solvent. The processing area from the wash bowls through to the centrifuge and the dryer is all fully enclosed and is kept under a slight negative pressure by evacuating air to a vapour recovery system.

The solvent from the first washing bowl is processed through high-speed centrifuge equipment to remove solid particles and recycled back to bowl 1. A proportion of the fluid is drawn off for grease removal and upgrading for recycling.

Dirt separation

The dir slurry from the Dirt Separation stage is sent to an indirect heated Dir Dryer, where the solvent is evaporated off (and recovered), leaving a warm, dry, and solvent free dirt stream.

Expected pesticide analysis of dirt will result in no organochlorines (OC), less than 1 ppm organophosphates (OP) and less than 0,1 ppm synthetic pyrethroids (SP), Further reducing these levels requires the relatively simple modification of fitting a small Solid Bowl Centrifuge, such that the dirt slurry, in its way to the dirt dryer, is rinsed with fresh solvent. This will remove the grease-associated pesticides from the dirt and send these back to the evaporator where they will leave with the grease stream.

Solvent evaporation system

Solvent is recycled by various stages of evaporation in the Solvent Evaporation System. The first stage of evaporation is a multiple effect evaporator, which does the bulk of the solvent recovery work. It boils the spent solvent from a concentration of typically 2 wt% grease up to 20 wt% grease (i.e. 90 % recovery of solvent). To recover all possible solvent from the grease, it has been found necessary to use three stages of evaporation – each at progressively lower pressure and higher temperature. It has been found practical to evaporate the grease down to containing < 1 wt% solvent, corresponding to a 99,98 % recovery of solvent through the evaporation stage.

Vapour recovery unit

Other areas where solvent is recovered include when the vapour leaves the dirt dryer, when the solvent laden air leaves the wool dryer, and when the air saturated with solvent is extracted from the wash bowl area/ wool centrifuge/ wool dryer area. The combined stream from these areas is followed by activated carbon adsorption recovery system.

Solvent destruction

As discussed, the Wooltech wool cleaning system does not use any water in the washing process. There is, however, a small flow of water into the solvent system, This is due to moisture in the wool, moisture in the air and some input from vacuum raising equipment (steam ejectors). This water, referred to as fleece and steam moisture, is separated from the clean solvent in the solvent recovery section by gravity. Whilst the solubility of the solvent in this water is low, it is nevertheless saturated in solvent, which is thus removed in a two step process.

In the first step, most of the small proportion of solvent in fleece and steam moisture is removed by heating the water and stripping it with air in the Solvent Air Stripping Unit. The small flow of solvent is recovered by condensing it and then by passing it through the Vapour Recovery Unit.

In the second step, a free radical process based on Fenton’s reaction (a redox reaction between iron and hydrogen peroxide) is used to remove the traces of solvent left after stripping in fleece and steam moisture. Using an improved Fenton effect, the Residual Solvent Destruction Unit is effective in eliminating all traces of solvent from water by oxidizing/de-halogenating, thereby destroying the solvent molecules. The solvent is broken down into chloride ions, carbon dioxide and water upon treatment with hydrogen peroxide. Provision is allowed to ensure the water is neutralized prior to discharge. The fine detail of the destruction process is confidential; however processes that use the improved Fenton Effect are well established.

Another source of waste in the Wooltech plant is contaminated liquids from maintenance activities or as a result of recovered spills. These fluis are treated in a very similar manner to process water, The first step of Maintenance/Spill Recovery and Recycle is the recovery of the bulk of the solvent, which is performed by boiling most of the solvent from the water. Finally, the mildly contaminated water is treated in the Residual Solvent Destruction Unit.

It is expected that the Enhanced Fenton Process Residual Solvent Destruction Unit will reduce hazardous substances in water (including solvents, breakdown products and water-solubilized pesticides) to near zero. This is consistent with the long-term objectives (2020) set by OSPAR (protection of the marine environment) and the European Water Framework Directive (for surface waters).

Scouring of wool yarn and fabric before dyeing

Both yarn and fabric contain, besides accidental impurities, a certain amount of spinning oils and in some case also sizing agents such as CMC and PVA. All these substances are usually removed before dyeing in order to make the fibre more hydrophilic and allow the penetration of the fibre by dyestuffs. However, this operation is not always necessary. In some cases, if the preparation agents are applied in low amounts and they do not interfere with the dyeing process, a separate scouring/washing step can be omitted.

As pointed out before, the percentage of spinning oils on woollen wool is quite relevant and it is always above 5 %, while on worsted wool it never reaches 2 %.

Typical substances that have to be removed by scouring can be classified as:

• soluble in water

• insoluble in water, but emulsifiable thanks to the action of surfactants

• insoluble in water and non-emulsifiable (or difficult to emulsify) with surfactants. These substances can be removed only by using organic solvents (in general, halogenated solvents like perchloroethylene).

As a result the material can be washed (scoured):

• with water or

• with solvent (dry cleaning)

Water washing is carried out in neutral or weakly alkaline conditions (by sodium carbonate or bicarbonate) in the presence of detergents. Commonly used detergents are mixtures of anionic and non-ionic surfactants such as alkyl sulphates, fatty alcohols and alkylphenol ethoxylates. In wool carpet yarn production the scouring process can include simultaneous chemical setting of yarn twist reductive agents (sodium metabisulphite) and/or application of insect-resist agents.

Water scouring is normally a batch operation which is carried out in the equipment in which the textile material will be subsequently dyed. This means that an autoclave is the commonly used equipment for yarn, while jets and overflows are the machines typically applied for fabric. In this respect, the carpet sector is an exception. Wool yarn for carpet is scoured on continuous or semi-continuous basis in tape scouring machines (hanks) or in package-to-package scouring machines (package yarn), where the yarn is passed through a series of interconnected bowls.

Dry cleaning is less common and is applied when the fabric is heavily soiled and stained with oils from the weaving or knitting process. The most widely used solvent is perchloroethylene. In some cases water and surfactants are added to the solvent to provide a softening effect.

Solvent washing can be carried out either in discontinuous mode in a tumbler (generally for knitted fabric) or in continuous mode in open-width (for woven and knitted fabric). Impurities are carried away by the solvent, which is continuously purified and recycled in a closed loop.

2. NEW TECHNOLOGIES:

a) Changes in the process

1.) Use of integrated dirt removal/grease recovery loops

Main achieved energy benefits

• reduction in water consumption from a minimum of 25% to a maximum of more than 50%, taking as reference point the consumption of water of a conventional plant operating in countercurrent (between 5 and 10 l/kg of greasy wool)

• a reduction in energy consumption equivalent to the amount of thermal energy carried by the recycled liquor (the liquor temperature is generally about 60°C)

• and other environmental benefits (see BAT in textile industry, 2003)

A wool scouring plant operating in countercurrent mode normally produces three liquid waste streams:

• a dirt-rich flow, from the bottoms of the scouring bowls

• a less concentrated dirty flow, from the bottoms of the rinse bowls

• a grease-rich flow, from the top of the first scour bowl, or from the side tank of the first scour bowl, which receives the liquor removed from the wool as it exits the bowl through the squeeze press

For grease recovery, plate-type centrifuges are employed. They are usually protected from the abrasive effect of dirt by hydro cyclones in cases where separate rather than sequential grease recovery and dirt removal is practiced. The centrifuge produces a top phase, known as “cream”, which is grease containing a small amount of water. This “cream” is usually passed to a secondary centrifuge, which produces an upper, a lower and a middle phase. The upper phase consists of anhydrous grease, which can be sold as a by-product. The bottom phase is high in dirt and may be passed to the input side of the dirt recovery loop, or to the effluent treatment plant. The middle phase is impoverished in both grease and dirt and may be completely or partially recycled to the scour, by addition to the first scouring bowl. A portion of the middle phase may flow to effluent treatment.

In mills with more than one scouring line, the lines normally share dirt removal/grease recovery facilities.

For fine and extra-fine wool, when carried out using machinery that has a separate continuous sludge flow output, the wool grease recovery loop also allows the elimination of the very fine fraction without the need for a separate loop for dirt removal.

Operational data

Medium-to-large scouring mills (say 15 000 – 25 000 tons greasy wool per year) employing dirt removal/grease recovery loops should be able to achieve net specific water consumption figures to 2 – 4 l/kg of greasy wool for most types of wool.

Source: BAT of textile industry, 2003

2.) Minimizing energy consumption in wool scouring installations

Wool scouring is an energy-intensive process. Big energy savings come from reducing effluent flowdown (and consequent heat losses) to drain or to on-site effluent treatment plant, by the installation of a dirt/grease recovery loop (as described above). Techniques include fitting a heat exchanger to recover heat from the dirt/grease loop flowdown.

Further savings arise from each of the following measures:

• Fitting of covers on scour bowls to prevent heat loss by convection or evaporation; Retrofitting, however, is sometimes difficult on existing installations.

• Optimizing the performance of the final squeeze press in order to improve mechanical removal of water from the wool before it enters the evaporation dryer; The presses used for squeezing wool usually have steel bottom rollers and a porous top roller. Traditionally, the top roller was a steel roller wound with crossbred (coarse) wool top (a sliver of parallel fibres). More recently, this has been replaced with a blended top of wool and nylon (polyamide), a nylon top, or a square section rope, usually of wool and nylon blend. The last option combines durability with good performance. Porous composition rollers are offered commercially, but no information is available on their performance in this application.

• Running the last bowl at relatively high temperature in order to improve squeezing efficiency. Many scours are set to run with bowl temperatures decreasing from first or second to last bowl. Last bowl temperatures in the survey ranged from ambient (say 20°C) to 65°C, with an average of 48°C. Since heat losses from the last bowl will increase as its temperatures increases and heat consumption in the dryer will correspondingly decrease as the squeezing efficiency improves, it follows that there is an optimum temperature for the last bowl. It has been shown that this temperature is 60 – 65°C for wool throughout rates above about 500 kg/h

• Retrofitting heat recovery units to dryers. However, this is expensive and the heat saving available is only about 0.2 MJ/kg. Scourers’ practical experience with heat recovery units on wool dryers is also negative; the units quickly become blocked with fibre and may even cause increases in energy consumption.

• Direct gas firing of scour bowls and driers in order to avoid losses which occur in the generation and distribution of steam for use in direct or indirect steam heating. Retrofitting is not always possible in existing plant and the cost is relatively high. Energy saving is 0.3 MJ/kg.

Main achieved environmental benefits

Reduction in energy consumption will have the effect of reducing emissions of CO2, SOX and NOX, either from the scouring plant itself or off-site.

Energy saving from a dirt/grease recovery loop can be estimated as about 2 MJ/kg of greasy wool if a scour with loop and heat exchangers is used. It is assumed that a conventional scourer discharging 10 liters of water per kg greasy wool needs 2.09 MJ to heat 10 liters of fresh water from 10°C to 60°C (209 kJ/l). A scouring installation with loop and heat exchangers discharges only 2 l/kg (see BAT 2003 – Section 4.4.1) and recovers 80% of the heat contained in the effluent 8the energy input needed becomes 0.084 MJ/kg greasy wool).

It is also interesting to show the energy savings achievable in the dryer by operating the last bowl at optimum temperature (65°C) as discussed earlier.

In conclusion, medium-to-large wool scouring plants can be operated with energy consumptions of 4 – 4.5 MJ/kg electrical energy.

Source: BAT in textile energy, 2003

b) Changes in the heat supply system

No information is available.

c) Changes in the energy distribution system

No information is available.

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