There are a number of factors that affect scouring performance.

Opening before Scouring

Before scouring the greasy wool needs to be blended, opened and presented to the scour in a condition that maximises scouring effectiveness without being over opened. Excessive opening of fine wools, in particular, can lead to entanglement in the scour that becomes manifest as a shorter top or a lower top yield.

Pre-scour opening, whilst preparing the wool for the scour, can remove some dirt and vegetable matter (VM). This removal is more relevant for crossbred and coarse dirty wools.

A number of machines, having different levels of severity, are available.

For Australian wools a typical pre-scour operation would consist of a bale breaker or hopper feeder, double drum opener, hopper feeder and weighbelt. A weighbelt is used on all modern scouring lines to ensure an even feed of wool to the scour. A signal from the weighbelt usually controls the speed of the brattice in the feed hopper and in a well run operation a level control in the final hopper controls the feed of wool through the bale breaker to the drum opener.

Bowl design

In a modern wool scouring line hopper bottomed bowls are used. This type of design leads to better dirt removal from the scouring liquor which is a major contributing factor towards the continuous operation of the plant. Older machines would have long bowls with flat bottoms or with so called “self cleaning” actions which amounted to a simple screw device for moving the dirt to a central discharge point. Operation of these types of scour was by necessity a batchwise process with bowl drops being required every 8 to 12 hours in order to clean the bowls.

Number of bowls

Older scouring lines consisted of four to five long bowls whereas modern scouring lines have at least six bowls. The cumulative length of the scouring bowls is about 35-50 metres and 16-28 metres for the respective systems. The longer the cumulative length, the higher is the potential for fibre entanglement.

Types of detergent and builders used

Two types of non-ionic detergent are mostly used for scouring. These are alkylphenol ethoxylates that are highly effective but are being phased out due to environmental concerns and fatty alcohol ethoxylates that are more environmentally friendly. Detergent use for Australian wools is about 0.6 to 1.0% on the greasy weight.

Sodium carbonate (soda ash) is the most common builder used although sodium chloride and sodium sulphate are sometimes used. The purpose of the builder is to facilitate contaminant removal especially when the immersion time is short.

Water quality

The presence of multi-valent cations, such as calcium and magnesium causes redeposition of fibre contaminants. Ideally the total hardness should be zero.

Production rate

If the production rate is too low then the wool has much more freedom to move around in the scour and become entangled. If the production rate is too high then the wool can also become entangled as it is jammed into the squeezing rollers. In addition, contaminant removal could be compromised.

Immersion time

The overall immersion time must be long enough to allow the contaminants to swell so that they can be removed. The immersion time is determined by both the speeds of the mechanical systems for transporting the wool through the scour and the length of the bowls.

Temperature profile

The temperature profile in the scouring bowls is very important. The temperature in a desuint bowl should not exceed 30oC. High temperatures encourage entanglement and can lead to fibre damage especially under alkaline conditions.

Mechanical action

Mechanical action is needed to facilitate the removal of contaminants. However, the same mechanical action leads to fibre entanglement. The method of feeding the wool to each bowl, dunkers, the transport mechanism in the bowls, the way in which wool flows out of each bowl, the means used to assist this, the squeeze rollers, the conveyors between the bowls and the wet opening process, if any, prior to drying all have the potential to entangle the wool.

Performance of squeeze presses

An effective squeezing action is very important for contaminant removal. Therefore the squeeze press should be kept in good order with an even layer of roller lapping on the top roller. The speed of the rollers should maintain an even mat of wool through the scouring line.

Drying

After final squeezing at the last bowl the scoured wool passes to the dryer. Sometimes a wet opener may be placed prior to the dryer to present the wool in a more open condition to facilitate better drying particularly if a brattice dryer is used. Suction drum type dryers are mostly used for Australian wool because they are shorter and more efficient. If the dryer temperature is too hot then damage is possible especially if an alkaline builder is used.