Kier Boiling Process in Textile

What is Kier Boiling?
Kier boiling is often referred as scouring which is the hot alkaline process necessary to remove the non-cellulosic impurities. Kier boiling is an important step in the pretreatment of textiles, particularly for cotton fabrics. The main effects of this treatment are a 5–10% loss in mass and an improvement in wettability and absorbency. It is primarily used to remove natural impurities, such as waxes, pectins, and other non-cellulosic materials, to ensure the fabric is clean, white, and ready for further processing, like dyeing or printing.

A kier or keeve (or similar spellings) is a large circular boiler or vat used in bleaching or scouring cotton fabric. They are also used for processing paper pulp. When in use, they are continuously rotated by an engine, steam being supplied through a rotating joint in the axle. They are usually spherical, sometimes cylindrical, and some were recycled from old boiler shells.

The essential feature of the kiering process is the treatment of a large quantity of fabric, typically several tonnes, over a period of some hours in a steam-heated pressure vessel.

Purposes of Kier Boiling:
Kier boiling is done for the below purposes,

1. Removes natural impurities (waxes, fats, oils, pectins) from cotton fabrics.
2. Improves fabric whiteness and absorbency.
3. Prepares fabric for uniform dyeing, printing, and bleaching.
4. Enhances fabric softness and reduces yarn imperfections.
5. Minimizes microbial growth, increasing durability.
6. Contributes to producing high-quality, aesthetically pleasing textiles.

Parameters of Kier Boiling:
Kier boiling parameters such as-

• Temperature
• Pressure
• Time
• Liquor ratio
• Wetting agents to be used
• Concentration of caustic solution
• Neutralizer
• Number of washes after kiering
• Draining out the water

Working Procedure of Kier Boiling Machine:
Kier is a cylindrical vessel designed to carry out scouring of cotton fabrics in rope form. The kiers may either be open where scouring is carried out at boil at atmosphere pressure or be closed to carry out scouring at high temperature and pressure. The material used for the construction is generally steel or cast iron in which the inner side is rendered passive by a coat of sodium silicate, cement and magnesium sulphate. The kier has a perforated false bottom, located about 18 in above the base. The fabric is piled on it either manually or by a mechanical piler. It is important to lay the fabric in an even manner or else the channels of least resistance may lead to form an uneven scouring.

The movement of fabric during boiling can be avoided by placing heavy weights over the fabric pile. The capacity of a kier normally varies from 0.5 to 5 tonnes, whereas 2 tonnes capacity kiers are more common.

The circulation of scouring liquor is maintained by passing a stream in the kier by a central piper (puffer) through a hole in the perforated bottom. The steam draws liquor with it which hits a curved baffle plate above the goods and is distributed over the surface of the fabric. The liquor gradually percolates down the pile below the perforated bottom from where it is again forced through the buffer pipe by the steam pressure. The use of steam to circulate liquor as well as heat it is not very efficient and economical. Moreover, steam condensation may result in an increase in material to liquor ratio (M:L ratio) which is generally maintained at around 1:3. Many modern kiers use external heat exchangers and the circulation is maintained by a powerful centrifugal pump.

For scouring of cotton in kiers, it is important that all the air is forced out from the system as cotton can be oxidized into oxycellulose in the presence of air in hot alkaline liquors. This is ensured by heating the liquor to 90°C and then heating and circulation is stopped for 10–15 min. The steam and air is allowed to escape from a valve. The liquor is heated again and air is allowed to escape periodically till only steam emerges out from the valve. After this, the valve can be closed and scouring is continued for desired time.

In a typical open kier boiling process–10–20 g/l sodium hydroxide for 4–6 h at 95–98°C. After completion of the boil it is still important to avoid contact with the air and it is therefore advisable to run in hot rinsing water as the alkaline liquor is drained-off.

Whereas in pressure boiling alkali requirements are lower than open boiling–5–10 g/l sodium hydroxide at a pressure of 200 kPa (30 lbf/in²) and a temperature of 130°C for 2–4 h. Little or no degradation of the cotton fibre. After a pressure boil the liquor continues to circulate whilst cooling to 90°C, before dropping the liquor gradually and giving several rinses, hot and then cold.

Recipe for Kier Boiling:

Examples of recipe for kier boiling are given below.

Normal Problems in Kier Boiling:
This method is very seldom practiced, other than special cases. A good scouring process, this method is not much followed due to its high utility costs. Their disadvantages are their high cost, the need for stainless steel or other specially fabricated processing equipment due to their corrosive behavior, and special safety precautions that need to be taken because of their toxic and explosive characteristics.

Following are the normal problems experienced in kier boiling:

1. Improper loading of materials with more height at some place, and hence chemicals not reaching to that level
2. Inadequate wetting leading to improper scouring
3. Low temperature and pressure due to shortage or leakage of steam
4. Improper neutralization leading to tendering of cellulosic fibers
5. Insufficient washing leaving traces of acids on material gradually damaging them
6. Improper closing of lid leading to blast and injury to persons operating
7. Worn-out gasket leading to steam leakage

Precautions to be Taken for Kier Boiling:

• Kier should be cleaned by washing with water before loading new lot.
• Material should be packed evenly so that chemicals can move freely and uniformly and the materials can have uniform treatment at all places.
• The material such as cotton, yarn or fabric should be immersed in liquor completely.
• Kier works with high pressure and high temperature. It is very essential to ensure that the joints are in good condition and there are no leaks or else there are chances of explosion and accidents.
• Before scouring, the fabric should be starch free.
• The kiers need to be maintained well and periodically get inspected by a competent authority for its safety systems. It is mandatory to display the inspection certificate issued by the competent authority.

References:

1. Handbook of Value Addition Processes for Fabrics By B. Purushothama
2. Pretreatment of Textile Substrates by Mathews Kolanjikombil
3. Textile Processing and Properties: Preparation, Dyeing, Finishing and Performance by Tyrone L. Vigo
4. Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar